Dictionary of Chemistry [6Th Ed.]

A Dictionary of Chemistry

SIXTH EDITION

Edited by JOHN DAINTITH

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10987654321 Contents

Preface vii Credits viii

Dictionary 1 Atomic Theory Chronology 49 Biochemistry Chronology 70 Crystal Defects (Feature) 152 Explosives Chronology 217 Plastics Chronology 422 Polymers (Feature) 430

Appendices The Greek alphabet 569 Fundamental constants 569 SI units 570 The electromagnetic spectrum 572 The periodic table 573 The chemical elements 574 Nobel prizes in chemistry 576 Useful websites 583 Preface

This dictionary was originally derived from the Concise Science Dictionary, first published by Oxford University Press in 1984 (fifth edition, retitled Dictionary of Science, 2005). It consisted of all the entries relating to chemistry in this dictionary, including physical chemistry, as well as many of the terms used in biochemistry. Subsequent editions included special feature articles on important topics as well as several chronologies tracing the history of some topics and short biographical entries on the chemists and other scientists who have been responsible for the development of the subject. For this sixth edition the text has been fully revised and some entries have been substantially expanded. In addition over 350 new entries have been added covering all branches of the subject. The coverage of certain fields, in particular biochemistry, forensic chemistry, and chemoinformatics, has been expanded. A further improvement has been the inclusion of about 90 additional chemical structures. An asterisk placed before a word used in an entry indicates that this word can be looked up in the dictionary and will provide further explanation or clarification. However, not every word that appears in the dictionary has an asterisk placed before it. Some entries simply refer the reader to another entry, indicating either that they are synonyms or abbreviations or that they are most conveniently explained in one of the dictionary’s longer articles or features. Synonyms and abbreviations are usually placed within brackets immediately after the headword. Terms that are explained within an entry are highlighted by being printed in boldface type. The more physical aspects of physical chemistry and the physics itself will be found in A Dictionary of Physics, which is a companion volume to this dictionary. A Dictionary of Biology contains a more thorough coverage of the biophysical and biochemical entries from the Dictionary of Science together with the entries relating to biology. SI units are used throughout this book and its companion volumes. J.D. 2007

AAR See amino acid racemization. heit-size degrees has been used: this is the Rankine scale. AAS See atomic absorption spectroscopy. absolute alcohol See ethanol. abherent See release agent. absolute conÜguration A way of denoting the absolute structure of an ab-initio calculation A method of optical isomer (see optical activity). calculating atomic and molecular Two conventions are in use: The d–l structure directly from the Ürst prin- convention relates the structure of ciples of quantum mechanics, with- the molecule to some reference mol- out using quantities derived from ecule. In the case of sugars and simi- experiment (such as ionization ener- lar compounds, the dextrorotatory gies found by spectroscopy) as para- form of glyceraldehyde meters. Ab-initio calculations require (HOCH CH(OH)CHO), 2,3-dihydroxy- a large amount of numerical compu- 2 propanal) was used. The rule is as tation; the amount of computing follows. Write the structure of this time required increases rapidly as molecule down with the asymmetric the size of the atom or molecule in- carbon in the centre, the –CHO creases. The development of comput- group at the top, the –OH on the ing power has enabled the properties right, the –CH OH at the bottom, and of both small and large molecules to 2 the –H on the left. Now imagine that be calculated accurately, so that this the central carbon atom is at the cen- form of calculation can now replace tre of a tetrahedron with the four *semi-empirical calculations. Ab- groups at the corners and that the –H initio calculations can, for example, and –OH come out of the paper and be used to determine the bond the –CHO and –CH OH groups go lengths and bond angles of molecules 2 into the paper. The resulting three- by calculating the total energy of the dimensional structure was taken to molecule for a variety of molecular be that of d-glyceraldehyde and geometries and Ünding which confor- called d-glyceraldehyde. Any com- mation has the lowest energy. pound that contains an asymmetric absolute 1. Not dependent on or carbon atom having this conÜgura- relative to anything else, e.g. *ab- tion belongs to the d-series. One hav- solute zero. 2. Denoting a tempera- ing the opposite conÜguration ture measured on an absolute scale, belongs to the l-series. It is important a scale of temperature based on ab- to note that the preÜxes d- and l- do solute zero. The usual absolute scale not stand for dextrorotatory and now is that of thermodynamic *tem- laevorotatory (i.e. they are not the perature; its unit, the kelvin, was for- same as d- and l-). In fact the arbitrary merly called the degree absolute (°A) conÜguration assigned to d-glycer- and is the same size as the degree aldehyde is now known to be the cor- Celsius. In British engineering prac- rect one for the dextrorotatory form, tice an absolute scale with Fahren- although this was not known at the absolute configuration 2

CHO CHO CHO a H C OH C HCOH HOH CH OH CH OH CH OH 2 2 2

planar formula structure in 3 Fischer projection dimensions

D-(+)-glyceraldehyde (2,3-dihydroxypropanal)

H COOH

C H CH COOH CH NH 3 3 2 NH 2

D-alanine (R is CH2 in the CORN rule). The molecule is viewed with H on top

1 1

C C

3 2 2 3

Rconfiguration Sconfiguration

RS system. The lowest priority group is behind the chiral carbon atom

Absolute configuration

time. However, all d-compounds are being viewed along the H–C bond be- not dextrorotatory. For instance, the tween the hydrogen and the asym- acid obtained by oxidizing the –CHO metric carbon atom. If the clockwise group of glyceraldehyde is glyceric order of the other three groups is

acid (1,2-dihydroxypropanoic acid). –COOH, –R, –NH2, the amino acid be- By convention, this belongs to the d- longs to the d-series; otherwise it be- series, but it is in fact laevorotatory; longs to the l-series. This is known as i.e. its name can be written as d- the CORN rule. glyceric acid or l-glyceric acid. To The r–s convention is a convention avoid confusion it is better to use + based on priority of groups attached (for dextrorotatory) and – (for laevo- to the chiral carbon atom. The order d rotatory), as in -(+)-glyceraldehyde of priority is I, Br, Cl, SO3H, OCOCH3, d and -(–)-glyceric acid. OCH3, OH, NO2, NH2, COOCH3, d l The – convention can also be CONH2, COCH3, CHO, CH2OH, C6H5, used with alpha amino acids (com- C2H5, CH3, H, with hydrogen lowest. pounds with the –NH2 group on the The molecule is viewed with the same carbon as the –COOH group). In group of lowest priority behind the this case the molecule is imagined as chiral atom. If the clockwise arrange- 3 abundance ment of the other three groups is in spread over a range of values of the descending priority, the compound frequency ν it is useful to deÜne a a belongs to the r-series; if the de- quantity called the integrated ab- scending order is anticlockwise it is sorption coefÜcient, A, which is the in the s-series. d-(+)-glyceraldehyde is integral of all the absorption coefÜ- r-(+)-glyceraldehyde. See illustration. cients in the band, i.e. A = ∫ε(ν)dν. This quantity characterizes the inten- absolute temperature See ab- sity of a transition. It was formerly solute; temperature. called the extinction coefÜcient. See absolute zero Zero of thermody- also beer–lambert law. 2. The vol- namic *temperature (0 kelvin) and ume of a given gas, measured at stan- the lowest temperature theoretically dard temperature and pressure, that attainable. It is the temperature at will dissolve in unit volume of a which the kinetic energy of atoms given liquid. and molecules is minimal. It is equiv- adsorp- ° ° absorption indicator See alent to –273.15 C or –459.67 F. See tion indicator. also zero-point energy. absorption spectrum See spec- absorption 1. (in chemistry) The trum. take up of a gas by a solid or liquid, or the take up of a liquid by a solid. absorption tower A long vertical Absorption differs from adsorption column used in industry for absorb- in that the absorbed substance per- ing gases. The gas is introduced at meates the bulk of the absorbing the bottom of the column and the substance. 2. (in physics) The conver- absorbing liquid, often water, passes sion of the energy of electromagnetic in at the top and falls down against radiation, sound, streams of particles, the countercurrent of gas. The tow- etc., into other forms of energy on ers are also known as scrubbers. passing through a medium. A beam ABS plastic Any of a class of of light, for instance, passing plastics based on acrylonitrile– through a medium, may lose inten- butadiene–styrene copolymers. sity because of two effects: scattering of light out of the beam, and absorp- abstraction A chemical reaction tion of photons by atoms or mol- that involves bimolecular removal of ecules in the medium. When a an atom or ion from a molecule. An photon is absorbed, there is a transi- example is the abstraction of hydro- tion to an excited state. gen from methane by reaction with a radical: Ü absorption coef cient 1. (in CH + X. → H C. + HX. spectroscopy) The molar absorption 4 3 coefÜcient (symbol ε) is a quantity abundance 1. The ratio of the total that characterizes the absorption of mass of a speciÜed element in the light (or any other type of electro- earth’s crust to the total mass of the magnetic radiation) as it passes earth’s crust, often expressed as a through a sample of the absorbing percentage. For example, the abun- material. It has the dimensions of dance of aluminium in the earth’s 1/(concentration × length). ε is de- crust is about 8%. 2. The ratio of the pendent on the frequency of the inci- number of atoms of a particular iso- dent light; its highest value occurs tope of an element to the total num- where the absorption is most in- ber of atoms of all the isotopes tense. Since absorption bands usually present, often expressed as a percent- ac 4

age. For example, the abundance of common types are the *lead–acid ac- a uranium-235 in natural uranium is cumulator and the *nickel–iron and 0.71%. This is the natural abundance, nickel–cadmium accumulators. See i.e. the abundance as found in nature also sodium–sulphur cell. before any enrichment has taken acenaphthene A colourless crys- place. talline aromatic compound, C12H10; ac Anticlinal. See torsion angle. m.p. 95°C; b.p. 278°C. It is an intermediate in the production of some acac The symbol for the *acetyl- dyes. acetonato ligand, used in formulae.

accelerant A Ûammable material 12 used to start and spread a Üre in cases of arson. Petrol and parafÜn are the substances commonly used. Traces of accelerant are detectable by gas chromatography in forensic work. Acenaphthene accelerator A substance that increases the rate of a chemical reac- acetaldehyde See ethanal. tion, i.e. a catalyst. acetaldol See aldol reaction. acceptor 1. (in chemistry and bio- acetals Organic compounds formed chemistry) A compound, molecule, by addition of alcohol molecules to ion, etc., to which electrons are aldehyde molecules. If one molecule donated in the formation of a co- of aldehyde (RCHO) reacts with one ordinate bond. 2. (in physics) A sub- molecule of alcohol (R1OH) a hemiac- stance that is added as an impurity to etal is formed (RCH(OH)OR1). The a *semiconductor because of its abil- rings of aldose sugars are hemiac- ity to accept electrons from the va- etals. Further reaction with a second lence bands, causing p-type alcohol molecule produces a full ac- conduction by the mobile positive etal (RCH(OR1) ). It is common to holes left. Compare donor. 2 refer to both types of compound sim- accessory pigment A *photosyn- ply as ‘acetals’. The formation of ac- thetic pigment that traps light en- etals is reversible; acetals can be ergy and channels it to chlorophyll a, hydrolysed back to aldehydes in the primary pigment, which initiates acidic solutions. In synthetic organic the reactions of photosynthesis. Ac- chemistry aldehyde groups are often cessory pigments include the carotenes and chlorophylls b, c, and d. accumulator (secondary cell; storage battery) A type of *voltaic cell or battery that can be recharged by passing a current through it from an external d.c. supply. The charging current, which is passed in the opposite direction to that in which the cell supplies current, reverses the chemical reactions in the cell. The Acetals 5 Acheson process converted into acetal groups to pro- through the two oxygen atoms. In tect them before performing other formulae, the symbol acac is used. a reactions on different groups in the ethanoy- ketals acetylating agent See Amolecule. See also . lating agent. acylation • Information about IUPAC nomenclature acetylation See . acetyl chloride ethanoyl acetamide See ethanamide. See chloride. acetanilide A white crystalline pri- acetylcholine A substance that is mary amide of ethanoic acid, released at some (cholinergic) nerve CH CONHC H ; r.d. 1.2; m.p. 114.3°C; 3 6 5 endings. Its function is to pass on a b.p. 304°C. It is made by reacting nerve impulse to the next nerve (i.e. phenylamine (aniline) with excess at a synapse) or to initiate muscular ethanoic acid or ethanoic anhydride contraction. Once acetylcholine has and is used in the manufacture of been released, it has only a transitory dyestuffs and rubber. The full sys- effect because it is rapidly broken tematic name is N-phenyl- down by the enzyme cholinesterase. ethanamide. acetate See ethanoate. O 3CH CH3 H + acetate process See rayon. 2 C N ethanoic acid H acetic acid See . CH O C CH3 3 H acetic anhydride See ethanoic 2 anhydride. Acetylcholine acetoacetic acid See 3-oxobu- acetyl coenzyme A (acetyl CoA) A tanoic acid. compound formed in the mitochon- ethyl 3- acetoacetic ester See dria when an acetyl group (CH3CO–), oxobutanoate. derived from the breakdown of fats, * acetone See propanone. proteins, or carbohydrates (via glycolysis), combines with the thiol acetone–chlor–haemin test group (–SH) of *coenzyme A. Acetyl (Wagenaar test) A *presumptive test CoA feeds into the energy generating for blood in which a small amount of *Kreb’s cycle and also plays a role in acetone (propenal) is added to the the synthesis and oxidation of fatty bloodstain, followed by a drop of hy- acids. drochloric acid. Haemoglobin pro- acetylene See ethyne. duces derivatives such as haematin and haemin, forming small charac- acetylenes See alkynes. teristic crystals that can be identiÜed acetyl group See ethanoyl group. under a microscope. acetylide See carbide. acetonitrile See ethanenitrile. Acheson process An industrial acetophenone See phenyl methyl process for the manufacture of ketone. graphite by heating coke mixed with acetylacetonato The ion clay. The reaction involves the pro- – (CH3COCHCOCH3) , functioning as a duction of silicon carbide, which bidentate ligand coordinating loses silicon at 4150°C to leave achiral 6

graphite. The process was patented tions, two species related by loss or a in 1896 by the US inventor Edward gain of a proton are said to be conju- Goodrich Acheson (1856–1931). gate. Thus, in the reaction above HCN is the conjugate acid of the base achiral Describing a molecule that – – * CN , and CN is the conjugate base of does not contain a chirality el- + ement. the acid HCN. Similarly, H3O is the conjugate acid of the base H2O. An acid 1. A type of compound that equilibrium, such as that above, is a contains hydrogen and dissociates in competition for protons between an water to produce positive hydrogen acid and its conjugate base. A strong ions. The reaction, for an acid HX, is acid has a weak conjugate base, and commonly written: vice versa. Under this deÜnition HX ˆ H+ + X– water can act as both acid and base. In fact, the hydrogen ion (the proton) Thus in ˆ + – is solvated, and the complete reac- NH3 + H2O NH4 + OH tion is: – the H2O is the conjugate acid of OH . ˆ + – HX + H2O H3O + X The deÜnition also extends the idea + The ion H3O is the oxonium ion (or of acid–base reaction to solvents hydroxonium ion or hydronium ion). other than water. For instance, liquid This deÜnition of acids comes from ammonia, like water, has a high di- the Arrhenius theory. Such acids tend electric constant and is a good ioniz- to be corrosive substances with a ing solvent. Equilibria of the type sharp taste, which turn litmus red + – ˆ + – NH3 + Na Cl Na NH2 + HCl and give colour changes with other can be studied, in which NH3 and *indicators. They are referred to as – – protonic acids and are classiÜed into HCl are acids and NH2 and Cl are strong acids, which are almost com- their conjugate bases. pletely dissociated in water (e.g. sul- 3. A further extension of the idea of phuric acid and hydrochloric acid), acids and bases was made in the and weak acids, which are only par- Lewis theory (G. N. Lewis, 1923). In tially dissociated (e.g. ethanoic acid this, a Lewis acid is a compound or and hydrogen sulphide). The strength atom that can accept a pair of elec- of an acid depends on the extent to trons and a Lewis base is one that which it dissociates, and is measured can donate an electron pair. This by its *dissociation constant. See also deÜnition encompasses ‘traditional’ base. acid–base reactions. In → 2. In the Lowry–Brønsted theory of HCl + NaOH NaCl + H2O Ü acids and bases (1923), the de nition the reaction is essentially was extended to one in which an + – → acid is a proton donor (a Brønsted H + :OH H:OH acid), and a base is a proton acceptor i.e. donation of an electron pair by (a Brønsted base). For example, in OH–. But it also includes reactions ˆ + – that do not involve ions, e.g. HCN + H2O H3O + CN → the HCN is an acid, in that it donates H3N: + BCl3 H3NBCl3 a proton to H2O. The H2O is acting as in which NH3 is the base (donor) and a base in accepting a proton. Simi- BCl3 the acid (acceptor). The Lewis + larly, in the reverse reaction H3O is theory establishes a relationship bean acid and CN– a base. In such reac- tween acid–base reactions and *oxi- 7 acid rain dation–reduction reactions. See hsab that is an acid. 2. Describing a solu- principle. tion that has an excess of hydrogen a See also aqua acid; hydroxoacid; ions. 3. Describing a compound that oxoacid. forms an acid when dissolved in acid anhydrides (acyl anhydrides) water. Carbon dioxide, for example, Compounds that react with water to is an acidic oxide. form an acid. For example, carbon acidic hydrogen (acid hydrogen) A dioxide reacts with water to give car- hydrogen atom in an *acid that bonic acid: forms a positive ion when the acid ˆ CO2(g) + H2O(aq) H2CO3(aq) dissociates. For instance, in A particular group of acid anhydrides methanoic acid are anhydrides of carboxylic acids. HCOOH ˆ H+ + HCOO– They have a general formula of the the hydrogen atom on the carboxy- ′ ′ type R.CO.O.CO.R , where R and R late group is the acidic hydrogen (the are alkyl or aryl groups. For example, one bound directly to the carbon the compound ethanoic anhydride atom does not dissociate). (CH3.CO.O.CO.CH3) is the acid anhydride of ethanoic (acetic) acid. Or- acidimetry Volumetric analysis ganic acid anhydrides can be using standard solutions of acids to produced by dehydrating acids (or determine the amount of base pre- mixtures of acids). They are usually sent. made by reacting an acyl halide with acidity constant See dissociation. the sodium salt of the acid. They react readily with water, alcohols, acid rain Precipitation having a pH phenols, and amines and are used in value of less than about 5.0, which *acylation reactions. has adverse effects on the fauna and A Ûora on which it falls. Rainwater typ- • Information about IUPAC nomenclature ically has a pH value of 5.6, due to the presence of dissolved carbon R dioxide (forming carbonic acid). Acid R rain results from the emission into O O the atmosphere of various pollutant OH gases, in particular sulphur dioxide O OH and various oxides of nitrogen, O O which originate from the burning of fossil fuels and from car exhaust R R fumes, respectively. These gases dis- Acid anhydride solve in atmospheric water to form sulphuric and nitric acids in rain, acid–base indicator See snow, or hail (wet deposition). Alter- natively, the pollutants are deposited indicator. as gases or minute particles (dry de- acid dissociation constant See position). Both types of acid deposi- dissociation. tion affect plant growth – by damaging the leaves and impairing acid dye See dyes. photosynthesis and by increasing the acid halides See acyl halides. acidity of the soil, which results in the leaching of essential nutrients. acidic 1. Describing a compound This acid pollution of the soil also acid salt 8

leads to acidiÜcation of water drain- acrolein See propenal. a ing from the soil into lakes and acrylamide rivers, which become unable to sup- An inert gel (polyacry- port Üsh life. Lichens are particularly lamide) employed as a medium in sensitive to changes in pH and can be *electrophoresis. It is used particu- used as indicators of acid pollution. larly in the separation of macromolecules, such as nucleic acids and acid salt A salt of a polybasic acid proteins. (i.e. an acid having two or more acidic hydrogens) in which not all acrylate See propenoate. the hydrogen atoms have been re- acrylic acid See propenoic acid. placed by positive ions. For example, the dibasic acid carbonic acid (H2CO3) acrylic resins Synthetic resins forms acid salts (hydrogencarbonates) made by polymerizing esters or other – containing the ion HCO3 . Some salts derivatives of acrylic acid (propenoic of monobasic acids are also known as acid). Examples are poly(propenoni- acid salts. For instance, the com- trile) (e.g. Acrilan), and poly(methyl 2- pound potassium hydrogendiÛuoride, methylpropenoate) (polymethyl – KHF2, contains the ion [F...H–F] , in methacrylate, e.g. Perspex). which there is hydrogen bonding be- propenonitrile tween the Ûuoride ion F– and a hy- acrylonitrile See . drogen Ûuoride molecule. ACT See activated-complex theory. acid value A measure of the actinic radiation Electromagnetic amount of free acid present in a fat, radiation that is capable of initiating equal to the number of milligrams of a chemical reaction. The term is used potassium hydroxide needed to neu- especially of ultraviolet radiation and tralize this acid. Fresh fats contain also to denote radiation that will af- glycerides of fatty acids and very lit- fect a photographic emulsion. tle free acid, but the glycerides decompose slowly with time and the actinides See actinoids. acid value increases. actinium Symbol Ac. A silvery acridine A colourless crystalline radioactive metallic element belong- heterocyclic compound, C12H9N; m.p. ing to group 3 (formerly IIIA) of the 110°C. The ring structure is similar periodic table; a.n. 89; mass number to that of anthracene, with three of most stable isotope 227 (half-life fused rings, the centre ring contain- 21.7 years); m.p. 1050 ± 50°C; b.p. ing a nitrogen heteroatom. Several 3200°C (estimated). Actinium–227 oc- derivatives of acridine (such as acri- curs in natural uranium to an extent dine orange) are used as dyes or bio- of about 0.715%. Actinium–228 (half- logical stains. life 6.13 hours) also occurs in nature. There are 22 other artiÜcial isotopes, N all radioactive and all with very short half-lives. Its chemistry is similar to that of lanthanum. Its main use is as a source of alpha particles. The el- Acridine ement was discovered by A. Debierne in 1899. Acrilan A tradename for a synthetic A Übre. See acrylic resins. • Information from the WebElements site 9 action potential actinium series See radioactive transition metals in forming coordi- series. nation complexes and displaying a variable valency. As a result of in- actinoid contraction A smooth creased nuclear charge, the heavier decrease in atomic or ionic radius members (curium to lawrencium) with increasing proton number tend not to use their inner f-electrons found in the *actinoids. in forming bonds and resemble the actinoids (actinides) A series of el- lanthanoids in forming compounds ements in the *periodic table, gener- containing the M3+ ion. The reason ally considered to range in atomic for this is pulling of these inner elec- number from thorium (90) to lawren- trons towards the centre of the atom cium (103) inclusive. The actinoids all by the increased nuclear charge. have two outer s-electrons (a 7s2 Note that actinium itself does not conÜguration), follow actinium, and have a 5f electron, but it is usually are classiÜed together by the fact that classiÜed with the actinoids because increasing proton number corre- of its chemical similarities. See also sponds to Ülling of the 5f level. In transition elements. fact, because the 5f and 6d levels are actinometry close in energy the Ülling of the 5f or- actinometer See . bitals is not smooth. The outer elec- actinometry The measurement of tron conÜgurations are as follows: the intensity of electromagnetic radi- 89 actinium (Ac) 6d17s2 ation. An instrument that measures 90 thorium (Th) 6d27s2 this quantity is called an actinometer. 91 protactinium (Pa) 5f26d17s2 Recent actinometers use the *photo- 3 2 92 uranium (Ur) 5f 6d7s electric effect but earlier instruments 5 2 93 neptunium (Np) 5f 7s (or depended either on the Ûuorescence 4 1 2 5f 6d 7s ) produced by the radiation on a 6 2 94 plutonium (Pu) 5f 7s screen or on the amount of chemical 7 2 95 americium (Am) 5f 7s change induced in some suitable sub- 7 1 2 96 curium (Cm) 5f 6d s stance. Different types of actinome- 8 2 9 2 97 berkelium (Bk) 5f 6d7s (or 5f 7s ) ter have different names according to 10 2 98 californium (Cf) 5f 7s the type of radiation they measure. A 11 2 99 einsteinium (Es) 5f 7s pyroheliometer measures the inten- 12 2 100 fermium (Fm) 5f 7s sity of radiation from the sun. A 13 2 101 mendelevium (Md) 5f 7s pyranometer 14 2 measures the intensity 102 nobelium (Nb) 5f 7s of radiation that reaches the surface 103 lawrencium (Lw) 5f146d1s2 Ü of the earth after being scattered by The rst four members (Ac to Ur) molecules or objects suspended in occur naturally. All are radioactive Ü the atmosphere. A pyrogeometer and this makes investigation dif cult measures the difference between the because of self-heating, short life- outgoing infrared radiation from the times, safety precautions, etc. Like earth and the incoming radiation the *lanthanoids, the actinoids show from the sun that penetrates the a smooth decrease in atomic and earth’s atmosphere. ionic radius with increasing proton number. The lighter members of the action potential The change in series (up to americium) have f-elec- electrical potential that occurs across trons that can participate in bonding, a cell membrane during the passage unlike the lanthanoids. Conse- of a nerve impulse. As an impulse quently, these elements resemble the travels in a wavelike manner along action spectrum 10

the axon of a nerve, it causes a local- tential energy is called the transition a ized and transient switch in electric state of the reaction, as reactants potential across the cell membrane passing through this state become from –60 mV (the resting potential) products. In ACT, it is assumed that to +45 mV. The change in electric po- the reactants are in equilibrium with tential is caused by an inÛux of the activated complex, and that this sodium ions. Nervous stimulation of decomposes along the reaction coor- a muscle Übre has a similar effect. dinate to give the products. ACT was action spectrum A graphical plot developed by the US chemist Henry Ü Eyring and colleagues in the 1930s. of the ef ciency of electromagnetic eyring equation radiation in producing a photochemi- See also . cal reaction against the wavelength activated sludge process A of the radiation used. For example, sewage and waste-water treatment. the action spectrum for photosynthe- The sludge produced after primary sis using light shows a peak in the re- treatment is pumped into aeration gion 670–700 nm. This corresponds tanks, where it is continuously to a maximum absorption in the ab- stirred and aerated, resulting in the sorption spectrum of chlorophylls in formation of small aggregates of sus- this region. pended colloidal organic matter Û activated adsorption *Adsorp- called oc. Floc contains numerous tion that involves an activation en- slime-forming and nitrifying bac- ergy. This occurs in certain cases of teria, as well as protozoans, which chemisorption. decompose organic substances in the sludge. Agitation or air injection activated alumina See aluminium maintains high levels of dissolved hydroxide. oxygen, which helps to reduce the activated charcoal See charcoal. *biochemical oxygen demand. Roughly half the sewage in Britain is activated- activated complex See treated using this method. complex theory. activation analysis An analytical activated-complex theory (ACT) technique that can be used to detect A theory enabling the rate constants most elements when present in a in chemical reactions to be calcu- sample in milligram quantities (or lated using statistical thermodynam- less). In neutron activation analysis ics. The events assumed to be taking the sample is exposed to a Ûux of place can be shown in a diagram thermal neutrons in a nuclear reac- with the potential energy as the ver- tor. Some of these neutrons are cap- tical axis, while the horizontal axis, tured by nuclides in the sample to called the reaction coordinate, repre- form nuclides of the same atomic sents the course of the reaction. As number but a higher mass number. two reactants A and B approach each These newly formed nuclides emit other, the potential energy rises to a gamma radiation, which can be used maximum. The collection of atoms to identify the element present by near the maximum is called the acti- means of a gamma-ray spectrometer. vated complex. After the atoms have Activation analysis has also been em- rearranged in the chemical reaction, ployed using charged particles, such the value of the potential energy falls as protons or alpha particles. as the products of the reaction are formed. The point of maximum po- activation energy Symbol Ea. The 11 activity minimum energy required for a binds the substrate molecule. The chemical reaction to take place. In a properties of an active site are deter- a reaction, the reactant molecules mined by the three-dimensional come together and chemical bonds arrangement of the polypeptide are stretched, broken, and formed in chains of the enzyme and their con- producing the products. During this stituent amino acids. These govern process the energy of the system in- the nature of the interaction that creases to a maximum, then decreases takes place and hence the degree of to the energy of the products (see il- substrate speciÜcity and susceptibil- lustration). The activation energy is ity to *inhibition. the difference between the maximum energy and the energy of the reac- activity 1. Symbol a. A thermody- tants; i.e. it is the energy barrier that namic function used in place of con- has to be overcome for the reaction centration in equilibrium constants to proceed. The activation energy defor reactions involving nonideal termines the way in which the rate gases and solutions. For example, in of the reaction varies with tempera- a reaction ture (see arrhenius equation). It is A ˆ B + C usual to express activation energies the true equilibrium constant is in joules per mole of reactants. An given by activation energy greater than 200 KJ K = a a /a mol-1 suggests that a bond has been B C A where aA, aB, and aC are the activities completely broken in forming the of the components, which function transition state (as in the SN1 reac- as concentrations (or pressures) cor- tion). A lower Ügure suggests incom- rected for nonideal behaviour. Activ- plete breakage (as in the S 2 reaction). N ity coefÜcients (symbol γ) are deÜned See also activated-complex theory. for gases by γ = a/p (where p is pressure) and for solutions by γ = aX (where X is the mole fraction). Thus, products Ea the equilibrium constant of a gas re- ∆H action has the form energy γ γ γ reactants Kp = BpB CpC/ ApA The equilibrium constant of a reac- Activation energy tion in solution is γ γ γ activator 1. A substance that in- Kc = BXB CXC/ AXA creases the activity of a catalyst; for The activity coefÜcients thus act as example, a substance that – by bind- correction factors for the pressures ing to an *allosteric site on an en- or concentrations. The activity is zyme – enables the active site of the given by an equation enzyme to bind to the substrate. 2. µ µŠ + Any compound that potentiates the = RT ln a activity of a drug or other foreign where µ is chemical potential See also substance in the body. fugacity. mass action 2. Symbol A. The number of atoms of active mass See . a radioactive substance that disinte- active site (active centre) 1. A site grate per unit time. The speciÜc ac- on the surface of a catalyst at which tivity (a) is the activity per unit mass activity occurs. 2. The site on the of a pure radioisotope. See radiation surface of an *enzyme molecule that units. activity series 12 a activity series See electromotive adamantane A colourless crys- series. talline hydrocarbon C10H16; m.p. 269°C. It is found in certain petro- acyclic Describing a compound leum fractions. The structure con- that does not have a ring in its mol- tains three symmetrically fused ecules. cyclohexane rings. acid anhy- acyl anhydrides See H drides. acylation The process of introducing an acyl group (RCO–) into a com- H pound. The usual method is to react H an alcohol with an acyl halide or a carboxylic acid anhydride; e.g. H RCOCl + R′OH → RCOOR′ + HCl Adamantane The introduction of an acetyl group (CH3CO–) is acetylation, a process Adams catalyst A dark brown used for protecting –OH groups in or- powder, a hydrated form of platinum ganic synthesis. (IV) oxide (PtO2), produced by heating acyl Üssion The breaking of the chloroplatinic acid (H2PtCl6) with carbon–oxygen bond in an acyl sodium nitrate (NaNO3). Platinum ni- group. It occurs in the hydrolysis of trate is produced, and this decom- an *ester to produce an alcohol and a poses to Platinum (IV) oxide with carboxylic acid. evolution of NO2 and oxygen. It is used in hydrogenations of alkenes to acylglycerols See glycerides. alkanes, nitro compounds to aminos, and ketones to alcohols. The actual acyl group A group of the type Ü RCO–, where R is an organic group. catalyst is not the oxide but nely di- * An example is the acetyl group vided platinum black, which forms during the hydrogenation reaction. CH3CO–. addition polymerization See acyl halides (acid halides) Organic polymerization. compounds containing the group –CO.X, where X is a halogen atom addition reaction A chemical re- (see formula). Acyl chlorides, for in- action in which one molecule adds to stance, have the general formula another. Addition reactions occur RCOCl. The group RCO– is the acyl with unsaturated compounds con- group. In systematic chemical nomen- taining double or triple bonds, and clature acyl-halide names end in the may be *electrophilic or *nucle- sufÜx -oyl; for example, ethanoyl ophilic. An example of electrophilic chloride, CH3COCl. Acyl halides react addition is the reaction of hydrogen readily with water, alcohols, phenols, chloride with an alkene, e.g. → and amines and are used in *acyla- HCl + CH2:CH2 CH3CH2Cl tion reactions. They are made by An example of nucleophilic addition replacing the –OH group in a car- is the addition of hydrogen cyanide boxylic acid by a halogen using a across the carbonyl bond in aldehy- halogenating agent such as PCl5. des to form *cyanohydrins. Addi- A tion–elimination reactions are ones in • Information about IUPAC nomenclature which the addition is followed by 13 adiabatic demagnetization elimination of another molecule (see phate-ester derivatives of adenosine, condensation reaction). AMP, ADP, and *ATP, are of funda- a mental biological importance as car- additive A substance added to an- riers of chemical energy. other substance or material to improve its properties in some way. adenosine diphosphate (ADP) See Additives are often present in small atp. amounts and are used for a variety of adenosine monophosphate purposes, as in preventing corrosion, (AMP) atp stabilizing polymers, and preserving See . and improving foods (see food addi- adenosine triphosphate See atp. tive). adhesive A substance used for join- adduct A compound formed by an ing surfaces together. Adhesives are addition reaction. The term is used generally colloidal solutions, which particularly for compounds formed set to gels. There are many types in- by coordination between a Lewis acid cluding animal glues (based on colla- (acceptor) and a Lewis base (donor). gen), vegetable mucilages, and See acid. synthetic resins (e.g. *epoxy resins). adenine A *purine derivative. It is adiabatic approximation An ap- one of the major component bases of proximation used in *quantum me- *nucleotides and the nucleic acids chanics when the time dependence *DNA and *RNA. of parameters, such as the internuclear distance between atoms in a NH 2 molecule, is slowly varying. This approximation means that the solution N N of the *Schrödinger equation at one CH time goes continuously over to the CH solution at a later time. It was formu- N H N lated by Max *Born and the Soviet physicist Vladimir Alexandrovich Adenine Fock (1898–1974) in 1928. The *Born–Oppenheimer approximation adenosine A nucleoside compris- is an example of the adiabatic ap- ing one adenine molecule linked to a proximation. d -ribose sugar molecule. The phos- adiabatic demagnetization A technique for cooling a paramagnetic NH 2 salt, such as potassium chrome alum, to a temperature near *absolute zero. N N The salt is placed between the poles of an electromagnet and the heat produced during magnetization is re- N N moved by liquid helium. The salt is then isolated thermally from the sur- O roundings and the Üeld is switched OH off; the salt is demagnetized adiabati- OH cally and its temperature falls. This is because the demagnetized state, OH being less ordered, involves more en- Adenosine ergy than the magnetized state. The adiabatic process 14

extra energy can come only from the adsorption The formation of a a internal, or thermal, energy of the layer of gas, liquid, or solid on the substance. surface of a solid or, less frequently, adiabatic process Any process of a liquid. There are two types de- that occurs without heat entering or pending on the nature of the forces leaving a system. In general, an adia- involved. In chemisorption a single batic change involves a fall or rise in layer of molecules, atoms, or ions is temperature of the system. For exam- attached to the adsorbent surface by ple, if a gas expands under adiabatic chemical bonds. In physisorption ad- conditions, its temperature falls sorbed molecules are held by the (work is done against the retreating weaker *van der Waals’ forces. Ad- walls of the container). The adiabatic sorption is an important feature of equation describes the relationship surface reactions, such as corrosion, between the pressure (p) of an ideal and heterogeneous catalysis. The gas and its volume (V), i.e. pVγ = K, property is also utilized in adsorption where γ is the ratio of the principal *chromatography. Ü speci c *heat capacities of the gas adsorption indicator (absorption and K is a constant. indicator) A type of indicator used adipic acid See hexanedioic acid. in reactions that involve precipitation. The yellow dye Ûuorescein is a ADP See atp. common example, used for the reac- adrenaline (epinephrine) A hor- tion mone, produced by the medulla of → NaCl(aq) + AgNO3(aq) AgCl(s) + the adrenal glands, that increases NaNO3(aq) heart activity, improves the power As silver nitrate solution is added and prolongs the action of muscles, to the sodium chloride, silver chlo- and increases the rate and depth of – breathing to prepare the body for ride precipitates. As long as Cl ions ‘fright, Ûight, or Üght’. At the same are in excess, they adsorb on the precipitate particles. At the end time it inhibits digestion and excre- – tion. point, no Cl ions are left in solution and negative Ûuorescein ions are OH CH then adsorbed, giving a pink colour 3 to the precipitate. The technique N H is sometimes known as Fajan’s method. adsorption isotherm An equation that describes how the amount of a OH substance adsorbed onto a surface de- OH pends on its pressure (if a gas) or its Adrenaline concentration (if in a solution), at a constant temperature. Several adsorption isotherms are used in sur- adsorbate A substance that is ad- face chemistry including the *BET sorbed on a surface. isotherm and the *Langmuir adsorption isotherm. The different adsorbent A substance on the sur- isotherms correspond to different as- face of which a substance is ad- sumptions about the surface and the sorbed. adsorbed molecules. 15 air adulterant See cutting agent. red seaweeds that is used as a gelling agent in microbiological culture a aerogel A low-density porous trans- media, foodstuffs, medicines, and parent material that consists of more cosmetic creams and jellies. Nutrient than 90% air. Usually based on metal agar consists of a broth made from oxides or silica, aerogels are used as beef extract or blood that is gelled drying agents and insulators. with agar and used for the cultiva- aerosol A colloidal dispersion of a tion of bacteria, fungi, and some solid or liquid in a gas. The com- algae. monly used aerosol sprays contain an Ü agarose A carbohydrate polymer inert propellant lique ed under pres- that is a component of agar. It is used sure. *ChloroÛuorocarbons, such as Û in chromatography and electrophore- dichlorodi uoromethane, are com- sis. monly used in aerosol cans. This use has been criticized on the grounds agate A variety of *chalcedony that that these compounds persist in the forms in rock cavities and has a pat- atmosphere and may lead to deple- tern of concentrically arranged bands tion of the *ozone layer. or layers that lie parallel to the cavity walls. These layers are frequently al- atomic emission spec- AES See ternating tones of brownish-red. troscopy . Moss agate does not show the same A-factor See arrhenius equation. banding and is a milky chalcedony containing mosslike or dendritic pat- Ü af nity chromatography A bio- terns formed by inclusions of man- chemical technique for purifying nat- ganese and iron oxides. Agates are ural polymers, especially proteins. It used in jewellery and for ornamental Ü functions by attaching a speci c lig- purposes. and by covalent bonding to an insoluble inert support. The ligand has to agitator A bladelike instrument have a speciÜc afÜnity for the poly- used in fermenters and *bioreactors mer, so that when a solution contain- to mix the medium continuously in ing the ligand is passed down a order to maintain the rate of oxygen column of the material it is speciÜ- transfer and to help keep the cells in cally retarded and thus separated suspension. from any contaminating molecules. air See earth’s atmosphere. An example of a suitable ligand is the substrate of an enzyme, provided that it does not change irreversibly NH during the chromatography. 2 aÛatoxin A poisonous compound, N N C15H12O6, produced by the fungus As- pergillus Ûavus. It is extremely toxic to N farm animals and can cause liver can- N cer in humans. It may occur as a con- taminant of stored cereal crops, O cotton seed, and, especially, peanuts. There are four isomeric forms. OH OH atomic force microscope AFM See . OH agar An extract of certain species of Aflatoxin air pollution 16

air pollution (atmospheric pollu- contain the group –CH2–OH); sec- a tion) The release into the atmos- ondary alcohols have one hydrogen phere of substances that cause a on this carbon (the other two bonds variety of harmful effects to the nat- being to carbon atoms, as in ural environment. Most air pollu- (CH3)2CHOH); tertiary alcohols have tants are gases that are released into no hydrogen on this carbon (as in the troposphere, which extends (CH3)3COH): see formulae. The differ- about 8 km above the surface of the ent types of alcohols may differ in earth. The burning of fossil fuels, for the way they react chemically. For example in power stations, is a major example, with potassium dichro- source of air pollution as this process mate(VI) in sulphuric acid the follow- produces such gases as sulphur diox- ing reactions occur: → → ide and carbon dioxide. Released into primary alcohol aldehyde car- the atmosphere, both these gases are boxylic acid → thought to contribute to the green- secondary alcohol ketone house effect. Sulphur dioxide and ni- tertiary alcohol – no reaction trogen oxides, released in car Other characteristics of alcohols exhaust fumes, are air pollutants that are reaction with acids to give *esters are responsible for the formation of and dehydration to give *alkenes or *acid rain; nitrogen oxides also con- *ethers. Alcohols that have two –OH diols tribute to the formation of *photo- groups in their molecules are (or dihydric alcohols), those with chemical smog. See also ozone layer; three are triols (or trihydric alcohols), pollution. etc. alabaster See gypsum. A alanine See amino acid. • Information about IUPAC nomenclature _ albumin (albumen) One of a group H _ OH _ C _ of globular proteins that are soluble H H in water but form insoluble coagu- primary alcohol (methanol) lates when heated. Albumins occur H _ _ OH in egg white, blood, milk, and plants. _ C _ CH CH Serum albumins, which constitute 3 3 secondary alcohol (propan-2-ol) about 55% of blood plasma protein, _ help regulate the osmotic pressure CH _ OH 3 C _ and hence plasma volume. They also CH _ CH 3 3 bind and transport fatty acids. α-lac- tertiary alcohol (2-methylpropan-2-ol) talbumin is one of the proteins in milk. Alcohols fer- alcoholic fermentation See aldehydes Organic compounds mentation . that contain the group –CHO (the alcohols Organic compounds that aldehyde group; i.e. a carbonyl group contain the –OH group. In systematic (C=O) with a hydrogen atom bound chemical nomenclature alcohol to the carbon atom). In systematic names end in the sufÜx -ol. Examples chemical nomenclature, aldehyde Ü are methanol, CH3OH, and ethanol, names end with the suf x -al. Exam- C2H5OH. Primary alcohols have two ples of aldehydes are methanal hydrogen atoms on the carbon (formaldehyde), HCOH, and ethanal joined to the –OH group (i.e. they (acetaldehyde), CH3CHO. Aldehydes 17 alicyclic compound are formed by oxidation of primary atom on the carbon next to the car- *alcohols; further oxidation yields bonyl group. a carboxylic acids. They are reducing Aldols can be further converted to agents and tests for aldehydes in- other products; in particular, they clude *Fehling’s test and *Tollens are a source of unsaturated aldehy- reagent. Aldehydes have certain char- des. For example, the reaction of acteristic addition and condensation ethanal gives 3-hydroxybutenal (ac- reactions. With sodium hydrogensul- etaldol): phate(IV) they form addition com- 2CH CHO ˆ CH CH(OH)CH CHO – 3 3 2 pounds of the type [RCOH(SO3)H] Na+. Formerly these were known as This can be further dehydrated to 2- bisulphite addition compounds. They butenal (crotonaldehyde): → also form addition compounds with CH3CH(OH)CH2CHO H2O + hydrogen cyanide to give *cyanohy- CH3CH:CHCHO drins and with alcohols to give *ac- monosaccharide etals and undergo condensation aldose See . reactions to yield *oximes, *hydra- aldosterone A hormone produced zones, and *semicarbazones. Aldehy- by the adrenal glands that controls des readily polymerize. See also excretion of sodium by the kidneys ketones. and thereby maintains the balance of A salt and water in the body Ûuids. • Information about IUPAC nomenclature aldoximes Compounds formed by reaction between hydroxylamine and O aldehyde group an aldehyde RC → RCOH + H2NOH RCH:NOH + H2O H If R is an aliphatic group, the aldoxime is generally a liquid or low- Aldehyde melting solid. If R is aromatic, the monosaccharide aldoxime is a crystalline solid. Al- aldohexose See . doximes have a planar structure and aldol See aldol reaction. can exist in two isomeric forms. In the syn form, the OH group is on the aldol reaction A reaction of alde- same side of the double bond as the hydes of the type H. In the anti form the OH and H are ˆ 2RCH2CHO on opposite sides. Typically, aliphatic RCH2CH(OH)CHRCHO aldehydes give anti aldoximes; aro- where R is a hydrocarbon group. The matic aldehydes give syn aldoximes. resulting compound is a hydroxy- algin (alginic acid) A complex poly- aldehyde, i.e. an aldehyde–alcohol or saccharide occurring in the cell walls aldol, containing alcohol (–OH) and of the brown algae (Phaeophyta). aldehyde (–CHO) groups on adjacent Algin strongly absorbs water to form carbon atoms. The reaction is base- a viscous gel. It is produced commer- catalysed, the Ürst step being the for- cially from a variety of species of mation of a carbanion of the type Laminaria and from Macrocystis pyrifera RHC–CHO, which adds to the car- in the form of alginates, which are bonyl group of the other aldehyde used mainly as a stabilizer and tex- molecule. For the carbanion to form, turing agent in the food industry. the aldehyde must have a hydrogen alicyclic compound A compound aliphatic compounds 18 a that contains a ring of atoms and is and ionic radius, an increase in den- aliphatic. Cyclohexane, C6H12, is an sity, and a decrease in melting and example. boiling point. The standard electrode potentials are low and negative, al- aliphatic compounds Organic though they do not show a regular compounds that are *alkanes, trend because they depend both on *alkenes, or *alkynes or their deriva- ionization energy (which decreases tives. The term is used to denote down the group) and the hydration compounds that do not have the spe- energy of the ions (which increases). cial stability of *aromatic com- All the elements react with water pounds. All noncyclic organic (lithium slowly; the others violently) compounds are aliphatic. Cyclic and tarnish rapidly in air. They can aliphatic compounds are said to be all be made to react with chlorine, alicyclic. bromine, sulphur, and hydrogen. The alizarin An orange-red compound, hydroxides of the alkali metals are C14H8O4. The compound is a deriva- strongly alkaline (hence the name) tive of *anthraquinone, with hy- and do not decompose on heating. droxyl groups substituted at the 1 The salts are generally soluble. The and 2 positions. It is an important carbonates do not decompose on dyestuff producing red or violet heating, except at very high tempera- *lakes with metal hydroxide. tures. The nitrates (except for Alizarin occurs naturally as the glu- lithium) decompose to give the ni- coside in madder. It can be synthe- trite and oxygen: sized by heating anthraquinone with → 2MNO3(s) 2MNO2(s) + O2(g) sodium hydroxide. Lithium nitrate decomposes to the alkali A *base that dissolves in oxide. In fact lithium shows a num- water to give hydroxide ions. ber of dissimilarities to the other alkali metals (group 1 elements) members of group 1 and in many ways resembles magnesium (see diag- The elements of group 1 (formerly onal relationship). In general, the IA) of the *periodic table: lithium stability of salts of oxo acids in- (Li), sodium (Na), potassium (K), creases down the group (i.e. with in- rubidium (Rb), caesium (Cs), and creasing size of the M+ ion). This francium (Fr). All have a characteris- trend occurs because the smaller tic electron conÜguration that is a cations (at the top of the group) tend noble gas structure with one outer to polarize the oxo anion more effec- s-electron. They are typical metals (in tively than the larger cations at the the chemical sense) and readily lose bottom of the group. their outer electron to form stable M+ ions with noble-gas conÜgura- alkalimetry Volumetric analysis tions. All are highly reactive, with using standard solutions of alkali to the reactivity (i.e. metallic character) determine the amount of acid pre- increasing down the group. There is sent. a decrease in ionization energy from alkaline 1. Describing an alkali. lithium (520 kJ mol–1) to caesium 2. Describing a solution that has an (380 kJ mol–1). The second ionization excess of hydroxide ions (i.e. a pH energies are much higher and diva- greater than 7). lent ions are not formed. Other properties also change down the group. alkaline-earth metals (group 2 Thus, there is an increase in atomic elements) The elements of group 2 19 alkaloid

(formerly IIA) of the *periodic table: trode potentials are negative and beryllium (Be), magnesium (Mg), cal- show a regular small decrease from a cium (Ca), strontium (Sr), and barium magnesium to barium. In some ways (Ba). The elements are sometimes re- beryllium resembles aluminium (see ferred to as the ‘alkaline earths’, al- diagonal relationship). though strictly the ‘earths’ are the All the metals are rather less reac- oxides of the elements. All have a tive than the alkali metals. They characteristic electron conÜguration react with water and oxygen (beryl- that is a noble-gas structure with two lium and magnesium form a protec- outer s-electrons. They are typical tive surface Ülm) and can be made to metals (in the chemical sense) and react with chlorine, bromine, sul- readily lose both outer electrons to phur, and hydrogen. The oxides and form stable M2+ ions; i.e. they are hydroxides of the metals show the strong reducing agents. All are reac- increasing ionic character in moving tive, with the reactivity increasing down the group: beryllium hydrox- down the group. There is a decrease ide is amphoteric, magnesium hy- in both Ürst and second ionization droxide is only very slightly soluble energies down the group. Although in water and is weakly basic, calcium there is a signiÜcant difference be- hydroxide is sparingly soluble and tween the Ürst and second ionization distinctly basic, strontium and bar- energies of each element, com- ium hydroxides are quite soluble and pounds containing univalent ions are basic. The hydroxides decompose on not known. This is because the diva- heating to give the oxide and water: lent ions have a smaller size and → M(OH)2(s) MO(s) + H2O(g) larger charge, leading to higher The carbonates also decompose on hydration energies (in solution) or heating to the oxide and carbon diox- lattice energies (in solids). Conse- ide: quently, the overall energy change → favours the formation of divalent MCO3(s) MO(s) + CO2(g) compounds. The third ionization The nitrates decompose to give the energies are much higher than the oxide: second ionization energies, and tri- → 2M(NO3)2(s) 2MO(s) + 4NO2(g) + valent compounds (containing M3+) O2(g) are unknown. As with the *alkali metals, the stabil- Beryllium, the Ürst member of the ity of salts of oxo acids increases group, has anomalous properties be- down the group. In general, salts of cause of the small size of the ion; its the alkaline-earth elements are solu- atomic radius (0.112 nm) is much less ble if the anion has a single charge than that of magnesium (0.16 nm). (e.g. nitrates, chlorides). Most salts From magnesium to radium there is with a doubly charged anion (e.g. car- a fairly regular increase in atomic bonates, sulphates) are insoluble. The and ionic radius. Other regular solubilities of salts of a particular changes take place in moving down acid tend to decrease down the the group from magnesium. Thus, group. (Solubilities of hydroxides in- the density and melting and boiling crease for larger cations.) points all increase. Beryllium, on the other hand, has higher boiling and alkaloid One of a group of nitroge- melting points than calcium and its nous organic compounds, mostly de- density lies between those of calcium rived from plants, and having diverse and strontium. The standard elec- pharmacological properties. They are alkanal 20

biosynthesized from amino acids and alkenes (oleÜnes; oleÜns) Unsatu- a classiÜed according to some struc- rated hydrocarbons that contain one tural feature. A simple classiÜcation or more double carbon–carbon bonds is into: in their molecules. In systematic the pyridine group (e.g. coniine, nico- chemical nomenclature alkene tine) names end in the sufÜx -ene. Alkenes the tropine group (e.g. atropine, co- that have only one double bond form caine) a homologous series (the alkene se- the quinoline group (e.g. quinine, ries) starting ethene (ethylene), strychnine, brucine) CH2:CH2, propene, CH3CH:CH2, etc. the isoquinoline group (e.g. mor- The general formula is CnH2n. Higher phine, codeine) members of the series show iso- the phenylethylamine group (e.g. merism depending on position of the methamphetamine, mescaline, double bond; for example, butene ephedrine) (C4H8) has two isomers, which are (1) the indole group (e.g. tryptamine, ly- but-1-ene (C2H5CH:CH2) and (2) but-2- sergic acid) ene (CH3CH:CHCH3): see formulae. the purine group (e.g. caffeine, theo- Alkenes can be made by dehydration bromine, theophylline) of alcohols (passing the vapour over hot pumice): alkanal An aliphatic aldehyde. → RCH2CH2OH – H2O RCH:CH2 alkanes (parafÜns) Saturated hydrocarbons with the general formula H C H . In systematic chemical C CH n 2n+2 3 but-1-ene nomenclature alkane names end in CH C Ü 2 the suf x -ane. They form a *homolo- H2 gous series (the alkane series) methane (CH4), ethane (C2H6), H propane (C3H8), butane (C4H10), pen- C CH3 but-2-ene tane (C H ), etc. The lower members 5 12 3CH C of the series are gases; the high-mo- H lecular weight alkanes are waxy Alkenes solids. Alkanes are present in natural gas and petroleum. They can be An alternative method is the removal made by heating the sodium salt of a of a hydrogen atom and halogen carboxylic acid with soda lime: atom from a haloalkane by potas- – + + – → sium hydroxide in hot alcoholic solu- RCOO Na + Na OH Na2CO3 + RH tion: → Other methods include the *Wurtz RCH2CH2Cl + KOH KCl + H2O + reaction and *Kolbe’s method. Gener- RCH:CH2 ally the alkanes are fairly unreactive. Alkenes typically undergo *addi- They form haloalkanes with halo- tion reactions to the double bond. gens when irradiated with ultraviolet They can be tested for by the *Baeyer radiation. test. See also hydrogenation; oxo A process; ozonolysis; ziegler process • Information about IUPAC nomenclature . • Further details about IUPAC A nomenclature • Information about IUPAC nomenclature alkanol An aliphatic alcohol. alkoxides Compounds formed by 21 allosteric site reaction of alcohols with sodium or Like *alkenes, alkynes undergo addi- potassium metal. Alkoxides are salt- tion reactions. a like compounds containing the ion A – R–O . • Information about IUPAC nomenclature alkyd resin A type of *polyester allenes Compounds that contain resin used in paints and other sur- the group >C=C=C<, in which three face coating. The original alkyd carbon atoms are linked by two adja- resins were made by copolymerizing cent double bonds. The outer carbon phthalic anhydride with glycerol, to atoms are each linked to two other give a brittle cross-linked polymer. atoms or groups by single bonds. The The properties of such resins can be simplest example is 1,2-propadiene, modiÜed by adding monobasic acids CH2CCH2. Allenes are *dienes with or alcohols during the polymeriza- typical reactions of alkenes. Under tion. basic conditions, they often convert alkylation A chemical reaction to alkynes. In an allene, the two that introduces an *alkyl group into double bonds lie in planes that are an organic molecule. The *Friedel– perpendicular to each other. Con- Crafts reaction results in alkylation sequently, in an allene of the type of aromatic compounds. R1R2C:C:CR3R4, the groups R1 and R lie in a plane perpendicular to alkylbenzenes Organic com- 2 the plane containing R and R . pounds that have an alkyl group 3 4 bound to a benzene ring. The sim- Under these circumstances, the mol- plest example is methylbenzene ecule is chiral and can show optical activity. (toluene), CH3C6H5. Alkyl benzenes can be made by the *Friedel–Crafts allosteric enzyme An enzyme reaction. that has two structurally distinct alkyl group A group obtained by forms, one of which is active and the removing a hydrogen atom from an other inactive. In the active form, the quaternary structure (see protein) of alkane, e.g. methyl group, CH3–, derived from methane. the enzyme is such that a substrate can interact with the enzyme at the alkyl halides See haloalkanes. active site (see enzyme–substrate complex alkynes (acetylenes) Unsaturated ). The conformation of the hydrocarbons that contain one or substrate-binding site becomes al- more triple carbon–carbon bonds in tered in the inactive form and inter- their molecules. In systematic chemi- action with the substrate is not cal nomenclature alkyne names end possible. Allosteric enzymes tend to in the sufÜx -yne. Alkynes that have catalyse the initial step in a pathway only one triple bond form a *ho- leading to the synthesis of molecules. mologous series: ethyne (acetylene), The end product of this synthesis can ≡ ≡ act as a feedback inhibitor (see inhibi- CH CH, propyne, CH3CH CH, etc. They can be made by the action of tion) and the enzyme is converted to potassium hydroxide in alcohol solu- the inactive form, thereby control- tion on haloalkanes containing halo- ling the amount of product synthe- gen atoms on adjacent carbon atoms; sized. for example: allosteric site A binding site on → RCHClCH2Cl + 2KOH 2KCl + the surface of an enzyme other than ≡ 2H2O + RCH CH the *active site. In noncompetitive allotropy 22

*inhibition, binding of the inhibitor dipole moment. If this quantity is not a to an allosteric site inhibits the activ- zero, the transition is an allowed ity of the enzyme. In an *allosteric transition; if it is zero the transition enzyme, the binding of a regulatory is a *forbidden transition as a dipole molecule to the allosteric site transition. It may, however, be an al- changes the overall shape of the en- lowed transition for magnetic dipole zyme, either enabling the substrate or quadrupole-moment transitions, to bind to the active site or prevent- which have much smaller transition ing the binding of the substrate. probabilities and consequently give much weaker lines in the spectrum. allotropy The existence of elements in two or more different alloy A material consisting of two forms (allotropes). In the case of oxy- or more metals (e.g. brass is an alloy gen, there are two forms: ‘normal’ of copper and zinc) or a metal and a nonmetal (e.g. steel is an alloy of iron dioxygen (O2) and ozone, or trioxy- and carbon, sometimes with other gen (O3). These two allotropes have different molecular conÜgurations. metals included). Alloys may be com- More commonly, allotropy occurs be- pounds, *solid solutions, or mixtures cause of different crystal structures of the components. in the solid, and is particularly preva- alloy steels See steel. lent in groups 14, 15, and 16 of the allyl alcohol propenol periodic table. In some cases, the al- See . lotropes are stable over a tempera- allyl group See propenyl group. ture range, with a deÜnite transition Alnico A tradename for a series of point at which one changes into the alloys, containing iron, aluminium, other. For instance, tin has two al- nickel, cobalt, and copper, used to lotropes: white (metallic) tin stable make permanent magnets. above 13.2°C and grey (nonmetallic) tin stable below 13.2°C. This form of alpha helix The most common allotropy is called enantiotropy. Car- form of secondary structure in *pro- bon also has two allotropes – dia- teins, in which the polypeptide chain mond and graphite – although is coiled into a helix. The helical graphite is the stable form at all tem- structure is held in place by weak hy- peratures. This form of allotropy, in drogen bonds between the N–H and which there is no transition tempera- C=O groups in successive turns of the beta ture at which the two are in equilib- helix (see illustration). Compare sheet rium, is called monotropy. See also . polymorphism. alpha-iron See iron. allowed bands See energy bands. alphamethyltryptamine (AMT) A allowed transition A transition synthetic derivative of *tyrptamine between two electronic states allowed according to *selection rules associated with group theory. The

probability of a transition between H2 states m and n produced by the inter- C NH2 action of electromagnetic radiation with an atomic system is propor- NH CH tional to the square of the magnitude 3 of the matrix elements of the electric Alphamethyltryptamine 23 aluminate with stimulant and hallucinogenic alpha particles is known as an alpha- properties. It is used illegally as a ray or alpha-radiation. a club drug. alternant Describing a conjugated alpha-naphthol test A biochemi- molecule in which the atoms can be cal test to detect the presence of car- divided into two sets of alternate bohydrates in solution, also known atoms such that no atom has a direct as Molisch’s test (after the Austrian link to another atom in the same set. chemist H. Molisch (1856–1937), who Naphthalene, for example, has an alternant conjugated system. devised it). A small amount of alcoholic alpha-naphthol is mixed with alum See aluminium potassium the test solution and concentrated sulphate; alums. sulphuric acid is poured slowly down alumina See aluminium oxide; alu- the side of the test tube. A positive minium hydroxide. reaction is indicated by the forma- aluminate A salt formed when alu- tion of a violet ring at the junction of γ the liquids. minium hydroxide or -alumina is dissolved in solutions of strong bases, alpha particle A helium nucleus such as sodium hydroxide. Alumi- emitted by a larger nucleus during nates exist in solutions containing the course of the type of radioactive the aluminate ion, commonly writ- – decay known as alpha decay. As a he- ten [Al(OH)4] . In fact the ion proba- lium nucleus consists of two protons bly is a complex hydrated ion and and two neutrons bound together as can be regarded as formed from a hy- 3+ a stable entity the loss of an alpha drated Al ion by removal of four hy- particle involves a decrease in *nu- drogen ions: 3+ – → cleon number of 4 and decrease of 2 [Al(H2O)6] + 4OH 4H2O + – in the *atomic number, e.g. the [Al(OH)4(H2O)2] decay of a uranium–238 nucleus into Other aluminates and polyalumi- 3– a thorium–234 nucleus. A stream of nates, such as [Al(OH)6] and

O H N C C H N • R • H O • H • C O C N C R H O • • O H N

C • • • hydrogen bond C H R = amino-acid side chain R

Alpha helix aluminium 24

2– Cl Cl [(HO)3AlOAl(OH)3] , are also present. Cl a See also aluminium hydroxide. Al Al aluminium Symbol Al. A silvery- Cl Cl Cl white lustrous metallic element be- Aluminium chloride longing to *group 3 (formerly IIIB) of the periodic table; a.n. 13; r.a.m. 2.398; loses water at 100°C), both of 26.98; r.d. 2.7; m.p. 660°C; b.p. ° which are deliquescent. Aluminium 2467 C. The metal itself is highly re- chloride may be prepared by passing active but is protected by a thin hydrogen chloride or chlorine over transparent layer of the oxide, which hot aluminium or (industrially) by forms quickly in air. Aluminium and passing chlorine over heated alu- its oxide are amphoteric. The metal Ü minium oxide and carbon. The chlo- is extracted from puri ed bauxite ride ion is polarized by the small (Al O ) by electrolysis; the main 2 3 positive aluminium ion and the process uses a *Hall–Heroult cell but bonding in the solid is intermediate other electrolytic methods are under between covalent and ionic. In the development, including conversion liquid and vapour phases dimer mol- of bauxite with chlorine and electrol- ecules exist, Al Cl , in which there ysis of the molten chloride. Pure alu- 2 6 are chlorine bridges making coordi- minium is soft and ductile but its nate bonds to aluminium atoms (see strength can be increased by work- formula). The AlCl molecule can also hardening. A large number of alloys 3 form compounds with other mol- are manufactured; alloying elements include copper, manganese, silicon, ecules that donate pairs of electrons zinc, and magnesium. Its lightness, (e.g. amines or hydrogen sulphide); strength (when alloyed), corrosion re- i.e. it acts as a Lewis *acid. At high sistance, and electrical conductivity temperatures the Al2Cl6 molecules in (62% of that of copper) make it suit- the vapour dissociate to (planar) able for a variety of uses, including AlCl3 molecules. Aluminium chloride vehicle and aircraft construction, is used commercially as a catalyst in building (window and door frames), the cracking of oils. It is also a cata- and overhead power cables. Al- lyst in certain other organic reac- though it is the third most abundant tions, especially the Friedel–Crafts element in the earth’s crust (8.1% by reaction. weight) it was not isolated until 1825 aluminium ethanoate (aluminium by H. C. *Oersted. acetate) A white solid, Al(OOCCH3)3, A which decomposes on heating, is • Information from the WebElements site very slightly soluble in cold water, and decomposes in warm water. The aluminium acetate See alu- normal salt, Al(OOCCH ) , can only minium ethanoate. 3 3 be made in the absence of water (e.g. aluminium chloride A whitish ethanoic anhydride and aluminium solid, AlCl3, which fumes in moist air chloride at 180°C); in water it forms and reacts violently with water (to the basic salts Al(OH)(OOCCH3)2 and give hydrogen chloride). It is known Al2(OH)2(OOCCH3)4. The reaction of as the anhydrous salt (hexagonal; r.d. aluminium hydroxide with ethanoic 2.44 (fused solid); m.p. 190°C (2.5 acid gives these basic salts directly. atm.); sublimes at 178°C) or the hexa- The compound is used extensively in hydrate AlCl3.6H2O (rhombic; r.d. dyeing as a mordant, particularly in 25 aluminium oxide combination with aluminium sul- drated alumina. Heating the hydrated phate (known as red liquor); in the hydroxide causes loss of water, and a paper and board industry for sizing produces various activated aluminas, and hardening; and in tanning. It which differ in porosity, number of was previously used as an antiseptic remaining –OH groups, and particle and astringent. size. These are used as catalysts (par- aluminium hydroxide A white ticularly for organic dehydration reactions), as catalyst supports, and in crystalline compound, Al(OH)3; r.d. 2.42–2.52. The compound occurs nat- chromatography. Gelatinous freshly urally as the mineral gibbsite (mono- precipitated aluminium hydroxide clinic). In the laboratory it can be was formerly widely used as a mor- prepared by precipitation from dant for dyeing and calico printing solutions of aluminium salts. Such because of its ability to form insolu- solutions contain the hexaquo- ble coloured *lakes with vegetable aluminium oxide aluminium(III) ion with six water dyes. See also . 3+ molecules coordinated, [Al(H2O)6] . aluminium oxide (alumina) A In neutral solution this ionizes: white or colourless oxide of alu- 3+ ˆ + 2+ [Al(H2O)6] H + [Al(H2O)5OH] minium occurring in two main α The presence of a weak base such as forms. The stable form -alumina ° ± S2– or CO 2– (by bubbling hydrogen (r.d. 3.97; m.p. 2015 C; b.p. 2980 3 ° sulphide or carbon dioxide through 60 C) has colourless hexagonal or γ the solution) causes further ioniza- rhombic crystals; -alumina (r.d. α tion with precipitation of aluminium 3.5–3.9) transforms to the -form on hydroxide heating and is a white microcrystalline solid. The compound occurs [Al(H O) ]3+(aq) → Al(H O) (OH) (s) + 2 6 2 3 3 naturally as corundum or emery in 3H+(aq) the α-form with a hexagonal-close- The substance contains coordinated packed structure of oxide ions with water molecules and is more cor- aluminium ions in the octahedral in- rectly termed hydrated aluminium terstices. The gemstones ruby and hydroxide. In addition, the precipi- sapphire are aluminium oxide tate has water molecules trapped in coloured by minute traces of it and has a characteristic gelatinous chromium and cobalt respectively. A form. The substance is amphoteric. number of other forms of aluminium In strong bases the *aluminate ion is oxide have been described (β-, δ-, and produced by loss of a further proton: ζ-alumina) but these contain alkali- – ˆ Al(H2O)3(OH)3(s) + OH (aq) metal ions. There is also a short-lived – [Al(H2O)2(OH)4] (aq) + H2O(l) spectroscopic suboxide AlO. The On heating, the hydroxide trans- highly protective Ülm of oxide forms to a mixed oxide hydroxide, formed on the surface of aluminium AlO.OH (rhombic; r.d. 3.01). This sub- metal is yet another structural varia- stance occurs naturally as diaspore tion, being a defective rock-salt form and boehmite. Above 450°C it trans- (every third Al missing). forms to γ-alumina. Pure aluminium oxide is obtained In practice various substances can by dissolving the ore bauxite in be produced that are mixed crys- sodium hydroxide solution; impuri- talline forms of Al(OH)3, AlO.OH, and ties such as iron oxides remain in- aluminium oxide (Al2O3) with water soluble because they are not molecules. These are known as hy- amphoteric. The hydrated oxide is aluminium potassium sulphate 26

° a precipitated by seeding with material 770 C) or as the hydrate Al2(SO)3. from a previous batch and this is 18H2O (monoclinic; r.d. 1.69; loses then roasted at 1150–1200°C to give water at 86.5°C). The anhydrous salt pure α-alumina, or at 500–800°C to is soluble in water and slightly solu- give γ-alumina. The bonding in alu- ble in ethanol; the hydrate is very minium hydroxide is not purely ionic soluble in water and insoluble in due to polarization of the oxide ion. ethanol. The compound occurs natu- Although the compound might be rally in the rare mineral alunogenite α expected to be amphoteric, -alu- (Al2(SO)3.18H2O). It may be prepared mina is weakly acidic, dissolving in by dissolving aluminium hydroxide alkalis to give solutions containing or china clays (aluminosilicates) in aluminate ions; it is resistant to acid sulphuric acid. It decomposes on attack. In contrast γ-alumina is typi- heating to sulphur dioxide, sulphur cally amphoteric dissolving both in trioxide, and aluminium oxide. Its so- acids to give aluminium salts and in lutions are acidic because of hydroly- bases to give aluminates. α-alumina sis. is one of the hardest materials Aluminium sulphate is commer- known (silicon carbide and diamond cially one of the most important alu- are harder) and is widely used as an minium compounds; it is used in abrasive in both natural (corundum) sewage treatment (as a Ûocculating and synthetic forms. Its refractory agent) and in the puriÜcation of nature makes alumina brick an ideal drinking water, the paper industry, material for furnace linings and alu- and in the preparation of mordants. mina is also used in cements for It is also a Üre-prooÜng agent. Alu- high-temperature conditions. See also minium sulphate is often wrongly aluminium hydroxide. called alum in these industries. aluminium potassium sulphate aluminium trimethyl See (potash alum; alum) A white or trimethylaluminium. colourless crystalline compound, alums A group of double salts with Al2(SO4)3.K2SO4.24H2O; r.d. 1.757; ° the formula A SO .B (SO ) .24H O, loses 18H2O at 92.5 C; becomes anhy- 2 4 2 4 3 2 drous at 200°C. It forms cubic or oc- where A is a monovalent metal and B tahedral crystals that are soluble in a trivalent metal. The original exam- cold water, very soluble in hot water, ple contains potassium and alu- and insoluble in ethanol and acetone. minium (called potash alum or The compound occurs naturally as simply alum); its formula is often the mineral kalinite. It is a double written AlK(SO4)2.12H2O (aluminium salt and can be prepared by recrystal- potassium sulphate-12-water). Ammo- lization from a solution containing nium alum is AlNH4(SO4)2.12H2O, equimolar quantities of potassium chrome alum is KCr(SO4)2.12H2O (see sulphate and aluminium sulphate. It potassium chromium sulphate), etc. is used as a mordant for dyeing and The alums are isomorphous and can in the tanning and Ünishing of be made by dissolving equivalent leather goods (for white leather). See amounts of the two salts in water also alums. and recrystallizing. See also aluminium sulphate. aluminium sulphate A white or colourless crystalline compound, alunogenite A mineral form of Al2(SO4)3, known as the anhydrous hydrated *aluminium sulphate, compound (r.d. 2.71; decomposes at Al2(SO4)3.18H2O. 27 amides amalgam An alloy of mercury with scribed as l-alanyl-ambo-leucine, to one or more other metals. Most met- indicate the mixture. a als form amalgams (iron and plat- americium Symbol Am. A radio- inum are exceptions), which may be active metallic transuranic element Ü liquid or solid. Some contain de nite belonging to the *actinoids; a.n. 95; intermetallic compounds, such as mass number of most stable isotope NaHg2. 243 (half-life 7.95 × 103 years); r.d. amatol A high explosive consisting 13.67 (20°C); m.p. 994 ± 4°C; b.p. ° of a mixture of ammonium nitrate 2607 C. Ten isotopes are known. The and trinitrotoluene. element was discovered by G. T. Seaborg and associates in 1945, who ambident Describing a chemical obtained it by bombarding ura- species that has two alternative reac- nium–238 with alpha particles. tive centres such that reaction at one A centre stops or inhibits reaction at • Information from the WebElements site the other. An example is the *enolate ion in which electrophilic attack amethyst The purple variety of the can occur at either the oxygen atom mineral *quartz. It is found chieÛy in or at the beta-carbon atom. Brazil, the Urals (Soviet Union), Ari- zona (USA), and Uruguay. The colour ambidentate Describing a ligand is due to impurities, especially iron that can coordinate at two different oxide. It is used as a gemstone. sites. For example, the NO2 molecule can coordinate through the N atom amides 1. Organic compounds con- (the nitro ligand) or through an O taining the group –CO.NH2 (the atom (the nitrido ligand). Complexes amide group). Compounds contain- primary amides that differ only in the way the ligand ing this group are . Secondary and tertiary amides can coordinates display linkage isomerism. also exist, in which the hydrogen ambo- A preÜx used to indicate that atoms on the nitrogen are replaced a substance is present as a mixture by one or two other organic groups of racemic diastereoisomers in un- respectively. Simple examples of pri- speciÜed proportions. For example, if mary amides are ethanamide, l-alanine is reacted with dl-leucine, CH3CONH2, and propanamide, the resulting dipeptide can be de- C2H5CONH2. They are made by heat-

H H 3CH CH3 N N

H H imino group ammonia secondary amine (dimethylamine) CH CH 3CH H 3 3 N N

amino group H CH3

primary amine tertiary amine (methylamine) (trimethylamine) Amines amidol 28

O amine salts Salts similar to ammo- a nium salts in which the hydrogen R C amide group atoms attached to the nitrogen are NH replaced by one or more organic 2 groups. Amines readily form salts by Amides reaction with acids, gaining a proton to form a positive ammonium ion, ing the ammonium salt of the corre- They are named as if they were sub- sponding carboxylic acid. Amides can stituted derivatives of ammonium also be made by reaction of ammonia compounds; for example, dimethy- (or an amine) with an acyl halide. See lamine ((CH3)2NH) will react with also hofmann’s reaction. 2. Inor- hydrogen chloride to give dimethyl- ganic compounds containing the ion ammonium chloride, which is an NH –, e.g. KNH and Cd(NH ) . They + – 2 2 2 2 ionic compound [(CH3)2NH2] Cl . are formed by the reaction of ammo- When the amine has a common non- nia with electropositive metals. systematic name the sufÜx -ium can A be used; for example, phenylamine + – • Information about IUPAC nomenclature (aniline) would give [C6H5NH3] Cl , known as anilinium chloride. For- amidol See aminophenol. merly, such compounds were some- amination A chemical reaction in times called hydrochlorides, e.g. which an amino group (–NH2) is in- aniline hydrochloride with the for- troduced into a molecule. Examples mula C6H5NH2.HCl. of amination reaction include the re- Salts formed by amines are crys- action of halogenated hydrocarbons talline substances that are readily with ammonia (high pressure and soluble in water. Many insoluble temperature) and the reduction of *alkaloids (e.g. quinine and atropine) nitro compounds and nitriles. are used medicinally in the form of amines Organic compounds de- soluble salts (‘hydrochlorides’). If al- rived by replacing one or more of the kali (sodium hydroxide) is added to hydrogen atoms in ammonia by or- solutions of such salts the free amine ganic groups (see illustration). Pri- is liberated. mary amines have one hydrogen If all four hydrogen atoms of an ammonium salt are replaced by or- replaced, e.g. methylamine, CH3NH2. They contain the functional group ganic groups a quaternary ammonium compound is formed. Such –NH2 (the amino group). Secondary amines have two hydrogens replaced, compounds are made by reacting tertiary amines with halogen com- e.g. methylethylamine, CH3(C2H5)NH. Tertiary amines have all three hydro- pounds; for example, trimethylamine gens replaced, e.g. trimethylamine, ((CH3)3N) with chloromethane (CH ) N. Amines are produced by the (CH3Cl) gives tetramethylammonium 3 3 + – decomposition of organic matter. chloride, (CH3)4N Cl . Salts of this They can be made by reducing nitro type do not liberate the free amine compounds or amides. See also imines. when alkali is added, and quaternary + – A hydroxides (such as (CH3)4N OH ) can be isolated. Such compounds are • Information about IUPAC nomenclature strong alkalis, comparable to sodium of primary amines hydroxide. • Information about IUPAC nomenclature of secondary and tertiary amines amino acid Any of a group of 29 ammonia water-soluble organic compounds mate of the time since death. Not all that possess both a carboxyl (–COOH) amino acids racemize at the same a and an amino (–NH2) group attached rate, and the rate of the process de- to the same carbon atom, called the pends on other factors such as mois- α-carbon atom. Amino acids can be ture and temperature. Most work has represented by the general formula been done using leucine or aspartic R–CH(NH2)COOH. R may be hydrogen acid. or an organic group and determines A particular application in forensic the properties of any particular science involves measuring the d/l amino acid. Through the formation ratio of aspartic acid in the dentine of peptide bonds, amino acids join to- of teeth. Once a tooth has fully gether to form short chains (*pep- formed, the dentine is isolated by the tides) or much longer chains enamel and then racemization takes (*polypeptides). Proteins are com- place in the living subject at a fairly posed of various proportions of about constant temperature and moisture 20 commonly occurring amino acids level. Measuring the ratio gives a (see table on pp 30–31). The sequence fairly good estimate of the age of the of these amino acids in the protein subject (rather than the time since polypeptides determines the shape, death). properties, and hence biological role aminobenzene See phenylamine. of the protein. Some amino acids that never occur in proteins are nev- amino group See amines. ertheless important, e.g. ornithine aminophenol Any of various or- and citrulline, which are intermedi- ganic compounds used as reducing ates in the urea cycle. agents, especially as photographic Plants and many microorganisms developers, and for making dyes. Ex- can synthesize amino acids from sim- amples include amidol (the dihydro- ple inorganic compounds, but ani- chloride of 2,4-diaminophenol), mals rely on adequate supplies in metol (the hemisulphate of 4-methyl- their diet. The *essential amino acids aminophenol) and rhodinol (4-methyl- must be present in the diet whereas aminophenol). others can be manufactured from them. α-aminotoluene See benzylamine. A ammine A coordination *complex • Information about IUPAC nomenclature in which the ligands are ammonia molecules. An example of an am- amino acid racemization (AAR) mine is the tetraamminecopper(II) A dating technique used in archaeol- ion [Cu(NH ) ]2+. ogy based on the relative amounts of 3 4 the optical isomers of an amino acid ammonia A colourless gas, NH3, in a sample. In most organisms, the with a strong pungent odour; r.d. l-isomer of the amino acid is the one 0.59 (relative to air); m.p. –77.7°C; produced by metabolism. When the b.p. –33.35°C. It is very soluble in organism dies, this isomer slowly water and soluble in alcohol. The converts into the d-form, and eventu- compound may be prepared in the ally an equilibrium is reached in laboratory by reacting ammonium which the two forms are present in salts with bases such as calcium hy- equal amounts. Measuring the pro- droxide, or by the hydrolysis of a ni- portions of the l- and d-forms in a tride. Industrially it is made by the sample can, in principle, give an esti- *Haber process and over 80 million 30

amino acid abbreviation formula a 3-letter 1-letter

CH C COOH alanine Ala A 3 NH 2 H

arginine Arg R H NCNH CH CH CH C COOH 2 2 2 2 NH NH 2 H

H N C CH C COOH asparagine Asn N 2 2 O NH 2 H

HOOC CH C COOH aspartic acid Asp D 2 NH 2 H

HS CH C COOH cysteine Cys C 2 NH 2 H

HOOC CH CH C COOH glutamic acid Glu E 2 2 NH 2 H H N 2 glutamine Gln Q C CH CH C COOH O 2 2 NH 2 H

glycine Gly G H C COOH

NH 2 H

*histidine His H HC C CH C COOH 2 NHN NH 2 C H H

*isoleucine Ile I CH CH CH C COOH 3 2 CH NH 3 2 H H C *leucine Leu L 3 CH CH C COOH H C 2 3 NH 2 H

*lysine Lys K H N CH CH CH CH C COOH 2 2 2 2 2 NH 2 31 ammonia H a *methionine Met M CH S CH CH C COOH 3 2 2 NH 2 H

*phenylalanine Phe F CH C COOH 2 NH 2

H proline Pro P H C CH 2 2 C CH OH 2 H C CH COOH 2 H C CH COOH 2 N H N H 4hydroxyproline H

serine Ser S HO CH C COOH 2 NH 2 H

CH CH C COOH *threonine Thr T 3 NH OH 2 H

*tryptophan Trp W C CH C COOH 2 CH NH N 2 H H

*tyrosine Tyr Y HO CH C COOH 2 NH 2 H H C *valine Val V 3 CH C COOH H C 3 NH 2

*an essential amino acid

The amino acids occurring in proteins tonnes per year are used either distant, which permits it to act as an rectly or in combination. Major uses ionizing solvent. It is weakly self- are the manufacture of nitric acid, ionized to give ammonium ions, + – ammonium nitrate, ammonium NH4 and amide ions, NH2 . It also phosphate, and urea (the last three as dissolves electropositive metals to fertilizers), explosives, dyestuffs and give blue solutions, which are be- resins. lieved to contain solvated electrons. Liquid ammonia has some similar- Ammonia is extremely soluble in ity to water as it is hydrogen bonded water giving basic solutions that con- and has a moderate dielectric contain solvated NH3 molecules and ammoniacal 32

+ small amounts of the ions NH4 and ammonia–soda process See a OH–. The combustion of ammonia in solvay process. air yields nitrogen and water. In the ammonium alum See alums. presence of catalysts NO, NO2, and water are formed; this last reaction is ammonium bicarbonate See am- the basis for the industrial produc- monium hydrogencarbonate. tion of nitric acid. Ammonia is a ammonium carbonate A colour- good proton acceptor (i.e. it is a base) less or white crystalline solid, and gives rise to a series of ammo- (NH4)2CO3, usually encountered as nium salts, e.g. the monohydrate. It is very soluble in → + – NH3 + HCl NH4 + Cl . cold water. The compound decom- It is also a reducing agent. poses slowly to give ammonia, water, The participation of ammonia in and carbon dioxide. Commercial ‘am- the *nitrogen cycle is a most impor- monium carbonate’ is a double salt tant natural process. Nitrogen-Üxing of ammonium hydrogencarbonate bacteria are able to achieve similar and ammonium aminomethanoate reactions to those of the Haber (carbamate), NH4HCO3.NH2COONH4. process, but under normal conditions This material is manufactured by of temperature and pressure. These heating a mixture of ammonium release ammonium ions, which are chloride and calcium carbonate and converted by nitrifying bacteria into recovering the product as a sublimed nitrite and nitrate ions. solid. It readily releases ammonia and is the basis of sal volatile. It is ammoniacal Describing a solution also used in dyeing and wool prepa- in which the solvent is aqueous am- ration and in baking powders. monia. ammonium chloride (sal ammo- ammonia clock A form of atomic niac) A white or colourless cubic clock in which the frequency of a solid, NH4Cl; r.d. 1.53; sublimes at quartz oscillator is controlled by 340°C. It is very soluble in water and the vibrations of excited ammonia slightly soluble in ethanol but insolu- molecules. The ammonia molecule ble in ether. It may be prepared by (NH3) consists of a pyramid with a ni- fractional crystallization from a solu- trogen atom at the apex and one hy- tion containing ammonium sulphate drogen atom at each corner of the and sodium chloride or ammonium triangular base. When the molecule carbonate and calcium chloride. Pure is excited, once every 20.9 microsec- samples may be made directly by the onds the nitrogen atom passes gas-phase reaction of ammonia and through the base and forms a pyra- hydrogen chloride. Because of its mid the other side: 20.9 microsec- ease of preparation it can be manu- onds later it returns to its original factured industrially alongside any position. This vibration back and plant that uses or produces ammo- forth has a frequency of 23 870 hertz nia. The compound is used in dry and ammonia gas will only absorb cells, metal Ünishing, and in the excitation energy at exactly this fre- preparation of cotton for dyeing and quency. By using a crystal oscillator printing. to feed energy to the gas and a suitable feedback mechanism, the oscil- ammonium hydrogencarbonate lator can be locked to exactly this (ammonium bicarbonate) A white frequency. crystalline compound, NH4HCO3. It is 33 ampere formed naturally as a decay product acid. It is decomposed by heating to of nitrogenous matter and is made release ammonia (and ammonium a commercially by various methods: hydrogensulphate) and eventually the action of carbon dioxide and water, sulphur dioxide, and ammo- steam on a solution of ammonium nia. Vast quantities of ammonium carbonate; heating commercial am- sulphate are used as fertilizers. monium carbonate (which always ammonium thiocyanate A contains some hydrogencarbonate); colourless, soluble crystalline com- and the interaction of ammonia, car- pound, NH NCS. It is made by the bon dioxide, and water vapour. It is 4 action of hydrogen cyanide on am- used in some *baking powders and monium sulphide or from ammonia medicines. and carbon disulphide in ethanol. On ammonium ion The monovalent heating, it turns into its isomer + cation NH4 . It may be regarded as thiourea, SC(NH2)2. Its solutions give the product of the reaction of ammo- a characteristic blood-red colour with nia (a Lewis base) with a hydrogen iron(III) compounds and so are em- ion. The ion has tetrahedral symme- ployed as a test for ferric iron. Am- try. The chemical properties of ammonium thiocyanate is used as a monium salts are frequently very rapid Üxative in photography and as similar to those of equivalent alkali- an ingredient in making explosives. metal salts. amorphous Describing a solid that ammonium nitrate A colourless is not crystalline; i.e. one that has no crystalline solid, NH4NO3; r.d. 1.72; long-range order in its lattice. Many m.p. 169.6°C; b.p. 210°C. It is very powders that are described as ‘amor- soluble in water and soluble in phous’ in fact are composed of ethanol. The crystals are rhombic microscopic crystals, as can be when obtained below 32°C and demonstrated by X-ray diffraction. monoclinic above 32°C. It may be *Glasses are examples of true amor- readily prepared in the laboratory by phous solids. the reaction of nitric acid with aque- amount of substance Symbol n. ous ammonia. Industrially, it is man- A measure of the number of entities ufactured by the same reaction using present in a substance. The speciÜed ammonia gas. Vast quantities of am- entity may be an atom, molecule, monium nitrate are used as fertiliz- ion, electron, photon, etc., or any ers (over 20 million tonnes per year) speciÜed group of such entities. The and it is also a component of some amount of substance of an element, explosives. for example, is proportional to the ammonium sulphate A white number of atoms present. For all en- rhombic solid, (NH4)2SO4; r.d. 1.77; tities, the constant of proportionality decomposes at 235°C. It is very solu- is the *Avogadro constant. The SI unit ble in water and insoluble in ethanol. of amount of substance is the *mole. It occurs naturally as the mineral AMP See atp; cyclic amp. mascagnite. Ammonium sulphate was formerly manufactured from the ampere Symbol A. The SI unit of ‘ammoniacal liquors’ produced dur- electric current. The constant current ing coal-gas manufacture but is now that, maintained in two straight par- produced by the direct reaction be- allel inÜnite conductors of negligible tween ammonia gas and sulphuric cross section placed one metre apart ampere-hour 34

in a vacuum, would produce a force Methamphetamine has a methyl a between the conductors of 2 × 10–7 group on the amino group, i.e. Nm–1. This deÜnition replaced the C6H5CH2CH(NHCH3)CH3. earlier international ampere deÜned The hydrochloride of methampheta- as the current required to deposit mine can be crystallized giving a par- 0.001 118 00 gram of silver from a so- ticularly powerful form known as lution of silver nitrate in one second. ‘ice’ or crystal meth. Other examples The unit is named after the French of amphetamines are *ecstasy and physicist André Marie Ampère (1775– *mescaline. In the UK, ampheta- 1836). mines are class B controlled sub- ampere-hour A practical unit of stances (or class A if prepared for electric charge equal to the charge injection). Ûowing in one hour through a con- amphiboles A large group of rock- ductor passing one ampere. It is equal to 3600 coulombs. forming metasilicate minerals. They have a structure of silicate tetrahedra ampere-turn The SI unit of magne- linked to form double endless chains, tomotive force equal to the magneto- in contrast to the single chains of the motive force produced when a *pyroxenes, to which they are current of one ampere Ûows through closely related. They are present in one turn of a magnetizing coil. many igneous and metamorphic amperometric titration A rocks. The amphiboles show a wide method of determining the chemical range of compositional variation but composition of a solution by measur- conform to the general formula: ing the current passing through a X2–3Y5Z8O22(OH)2, where X = Ca, Na, 2+ 2+ 3+ cell containing the solution; the po- K, Mg, or Fe ; Y = Mg, Fe , Fe , Al, tential is held constant during the Ti, or Mn; and Z = Si or Al. The hy- titration for both the indicator and droxyl ions may be replaced by F, Cl, reference electrodes, with changes in or O. Most amphiboles are mono- the current being measured. The cur- clinic, including: 2+ rent Ûowing through the cell is meas- cummingtonite, (Mg,Fe )7(Si8O22)- ured as a function of the amount of (OH)2; tremolite, Ca2Mg5(Si8O22)- 2+ substance being titrated. (OH,F)2; actinolite, Ca2(Mg,Fe )5- (Si8O22)(OH,F)2; *hornblende, amphetamine A drug, 1-phenyl-2- 2+ 3+ NaCa2(Mg,Fe ,Fe ,Al)5((Si,Al)8O22)- aminopropane (or a derivative of this 2+ (OH,F)2; edenite, NaCa2(Mg,Fe )5- compound), that stimulates the cen- (Si7AlO22)(OH,F)2; and riebeckite, tral nervous system by causing the 2+ Na2,Fe3 (Si8O22)(OH,F)2. Anthophyl- release of the transmitters noradren- 2+ lite, (Mg,Fe )7(Si8O22)(OH,F)2, and aline and dopamine from nerve end- 2+ gedrite, (Mg,Fe )6Al(Si,Al)8O22)- ings. It inhibits sleep, suppresses the (OH,F)2, are orthorhombic amphi- appetite, and has variable effects on boles. mood; prolonged use can lead to addiction. amphibolic pathway A biochemical pathway that serves both anabolic amphetamines A group of syn- and catabolic processes. An impor- thetic stimulants, with structures tant example of an amphibolic path- based in phenyl amines. Ampheta- way is the *Krebs cycle, which mine itself has the formula involves both the catabolism of car- C6H5CH2CH(NH2)CH3. bohydrates and fatty acids and the 35 Analar reagent synthesis of anabolic precursors for saliva (as salivary amylase or ptyalin). amino-acid synthesis. Amylases cleave the long polysaccha- a amphiphilic Describing a molecule ride chains, producing a mixture of that has both hydrophilic and hy- glucose and maltose. drophobic parts, as in *detergents. amyl group Formerly, any of sev- amphiprotic See amphoteric; sol- eral isomeric groups with the for- vent. mula C5H11–. ampholyte A substance that can amylopectin A *polysaccharide act as either an acid, in the presence comprising highly branched chains of a strong base, or a base, when in of glucose molecules. It is one of the the presence of a strong acid. constituents (the other being amylose) of *starch. ampholyte ion See zwitterion. amylose A *polysaccharide consist- amphoteric Describing a coming of linear chains of between 100 pound that can act as both an acid and 1000 linked glucose molecules. and a base (in the traditional sense of Amylose is a constituent of *starch. the term). For instance, aluminium In water, amylose reacts with iodine hydroxide is amphoteric: as a base to give a characteristic blue colour. Al(OH)3 it reacts with acids to form aluminium salts; as an acid H3AlO3 it anabolic steroids Steroid hor- reacts with alkalis to give *alumi- mones related to the male sex hor- nates. Oxides of metals are typically mone testosterone. They promote basic and oxides of nonmetals tend the development of masculine char- to be acidic. The existence of ampho- acteristics and increase muscle teric oxides is sometimes regarded as growth. Anabolic steroids have been evidence that an element is a *metal- used medically for various conditions loid. Compounds such as the amino but are also used illegally by sports- acids, which contain both acidic and men and women and by body- basic groups in their molecules, can builders. They have a number of also be described as amphoteric. Sol- deleterious side effects and are a con- vents, such as water, that can both trolled drug in the UK and many donate and accept protons are usu- other countries. Their use is banned ally described as amphiprotic (see sol- by nearly all major sports regulating vent). bodies. They can be detected in blood AMT See alphamethyltryptamine. and urine by gas chromatography– mass spectroscopy. a.m.u. See atomic mass unit. anabolism The metabolic synthesis amylase Any of a group of closely of proteins, fats, and other con- related enzymes that degrade starch, stituents of living organisms from glycogen, and other polysaccharides. α β molecules or simple precursors. This Plants contain both - and - process requires energy in the form amylases; the name diastase is given of ATP. Drugs that promote such to the component of malt containing metabolic activity are described as β-amylase, important in the brewing anabolic. See metabolism. Compare ca- industry. Animals possess only α- tabolism. amylases, found in pancreatic juice (as pancreatic amylase) and also (in Analar reagent A chemical humans and some other species) in reagent of high purity with known analyser 36

contaminants for use in chemical anchimeric assistance See neigh- a analyses. bouring-group participation. analyser A device, used in the *po- Andrews titration A titration larization of light, that is placed in used to estimate amounts of reduc- the eyepiece of a *polarimeter to ob- ing agents. The reducing agent being serve plane-polarized light. The estimated is dissolved in concen- analyser, which may be a *Nicol trated hydrochloric acid and titrated prism or *Polaroid, can be oriented with a solution of potassium iodate. in different directions to investigate A drop of tetrachloromethane is in which plane an incoming wave is added to the solution. The end point polarized or if the light is plane po- of the titration is reached when the larized. If there is one direction from iodine colour disappears from this which light does not emerge from layer. This is due to the reducing the analyser when it is rotated, the agent being oxidized and the iodate incoming wave is plane polarized. If being reduced to ICl. This reaction involves a four-electron change. the analyser is horizontal when extinction of light takes place, the po- ANFO Ammonium nitrate–fuel oil. larization of light must have been in A mixture used extensively as a blast- the vertical plane. The intensity of a ing agent in mining and quarrying. beam of light transmitted through an The proportions are approximately analyser is proportional to cos2θ, 94% ammonium nitrate and 6% fuel where θ is the angle between the oil. ANFO has been used in terrorist plane of polarization and the plane attacks (e.g. an attack on the Murrah of the analyser. Extinction is said to Federal Building, Oklahoma City, in be produced by ‘crossing’ the *polar- 1995). izer and analyser. angle-resolved photoelectron analysis The determination of the spectroscopy (ARPES) A technique components in a chemical sample. for studying the composition and Qualitative analysis involves deter- structure of surfaces by measuring mining the nature of a pure un- both the kinetic energy and angular distribution of photoelectrons known compound or the compounds ejected from a surface by electromag- present in a mixture. Various chemi- netic radiation. See also photoelec- cal tests exist for different elements tron spectroscopy. or types of compound, and systematic analytical procedures can be anglesite A mineral form of used for mixtures. Quantitative *lead(II) sulphate, PbSO4. analysis involves measuring the pro- angle strain The departure of a portions of known components in a bond angle from its normal value. mixture. Chemical techniques for The effects of angle strain are often this fall into two main classes: *volu- apparent in aliphatic ring com- metric analysis and *gravimetric pounds. For example, in cyclopen- analysis. In addition, there are numer- tane, C5H10, the angles between the ous physical methods of qualitative bonds of the ring differ from the nor- and quantitative analysis, including mal tetrahedral angle (109° 28′), spectroscopic techniques, mass spec- which is found in cyclohexane. This trometry, polarography, chromatog- ring form of angle strain is often raphy, activation analysis, etc. called Baeyer strain. 37 anilinium ion angstrom Symbol Å. A unit of atomic distance, re is the equilibrium length equal to 10–10 metre. It was interatomic distance, and f is a con- a formerly used to measure wave- stant. Anharmonicity is taken into lengths and intermolecular distances account by adding a cubic term g(r – 3 but has now been replaced by the re) to the quadratic term, where g is nanometre. 1 Å = 0.1 nanometre. The much smaller than f. Higher terms in unit is named after A. J. *Ångström. (r – re) can be added to improve the description of anharmonicity. Angström, Anders Jonas (1814– 74) Swedish astronomer and physi- anharmonic oscillator An oscil- cist who became professor of physics lating system (in either classical meat the University of Uppsala from chanics or *quantum mechanics) 1858 until his death. He worked that is not oscillating in simple har- mainly with emission *spectra, monic motion. In general, the prob- demonstrating the presence of hydro- lem of an anharmonic oscillator is gen in the sun. He also worked out not exactly soluble, although many the wavelengths of *Fraunhofer lines. systems approximate to harmonic os- Since 1905 spectral wavelengths cillators and for such systems the have been expressed in *angstroms. *anharmonicity can be calculated using *perturbation theory. angular momentum A property of a rotating body. In the case of a anhydride A compound that pro- rigid rotating body the angular mo- duces a given compound on reaction mentum is given by Iω, where I is the with water. For instance, sulphur tri- *moment of inertia of the body and oxide is the (acid) anhydride of sul- → ω is its angular velocity. The quan- phuric acid SO3 + H2O H2SO4. tum theory of angular momentum is See also acid anhydrides. closely associated with the *rotation anhydrite An important rock- group and has important applica- forming anhydrous mineral form of tions in the electronic structure of calcium sulphate, CaSO4. It is chemi- atoms and diatomic molecules and cally similar to *gypsum but is *rotational spectroscopy of mol- harder and heavier and crystallizes in ecules. Electron spin is a type of an- the rhombic form (gypsum is mono- gular momentum. clinic). Under natural conditions an- anharmonicity The extent to hydrite slowly hydrates to form Û which the oscillation of an oscillator gypsum. It occurs chie y in white differs from simple harmonic mo- and greyish granular masses and is tion. In molecular vibrations the an- often found in the caprock of certain harmonicity is very small near the salt domes. It is used as a raw ma- equilibrium position, becomes large terial in the chemical industry and in as the vibration moves away from the manufacture of cement and fer- the equilibrium position, and is very tilizers. large as dissociation is approached. anhydrous Denoting a chemical Anharmonicity is taken into account compound lacking water: applied in molecular vibrations by adding an particularly to salts lacking their anharmonicity term to the potential water of crystallization. energy function of the molecule. phenylamine For a harmonic oscillator the poten- aniline See . + tial energy function U is given by anilinium ion The ion C6H5NH3 , 2 U = f(r – re) where r is the inter- derived from *phenylamine. animal charcoal 38

animal charcoal See charcoal. ment applied to a metal to soften it, a relieve internal stresses and instabili- animal starch See glycogen. ties, and make it easier to work or anion A negatively charged *ion, machine. It consists of heating the i.e. an ion that is attracted to the metal to a speciÜed temperature for a *anode in *electrolysis. Compare speciÜed time, both of which depend cation. on the metal involved, and then al- anionic detergent See detergent. lowing it to cool slowly. It is applied to both ferrous and nonferrous met- ion exchange anionic resin See . als and a similar process can be ap- anisotropic Denoting a medium in plied to other materials, such as which certain physical properties are glass. different in different directions. annelation See annulation. Wood, for instance, is an anisotropic material: its strength along the grain annulation A type of chemical re- differs from that perpendicular to action in which a ring is fused to a the grain. Single crystals that are not molecule by formation of two new cubic are anisotropic with respect to bonds. Sometimes the term annela- some physical properties, such as the tion is used. See also cyclization. transmission of electromagnetic radi- annulenes Organic hydrocarbons ation. Compare isotropic. that have molecules containing sim- annealing A form of heat treat- ple single rings of carbon atoms

H H

H H HH H H H H H H H

H H HH H H H H HH [14]-Annulene H H H H

H H H H H H HH H H H H H H

H H H HH H H HH HHH HH H [30]-Annulene HH

HH [18]-Annulene

Annulenes 39 anoxic reactor linked by alternating single and dou- anodizing A method of coating ob- ble bonds. Such compounds have jects made of aluminium with a pro- a even numbers of carbon atoms. tective oxide Ülm, by making them *Cyclo-octatetraene, C8H8, is the next the anode in an electrolytic bath con- in the series following benzene. taining an oxidizing electrolyte. An- Higher annulenes are usually re- odizing can also be used to produce ferred to by the number of carbon a decorative Ünish by formation of atoms in the ring, as in [10]-annu- an oxide layer that can absorb a lene, C10H10, [12]-annulene, C12H12, coloured dye. etc. The lower members are not sta- anomeric effect If a pyranose ring ble as a result of the interactions be- has an electronegative substituent at tween hydrogen atoms inside the the anomeric carbon then this sub- ring. This is true even for molecules stituent is more likely to take the that have the necessary number of pi axial conÜguration than the equator- electrons to be *aromatic com- ial conÜguration. This is a conse- pounds. Thus, [10]-annulene has 4n + quence of a general effect (the 2 pi electrons with n = 2, but is not generalized anomeric effect) that in a aromatic because it is not planar. chain of atoms X–C–Y–C, in which Y [14]-annulene also has a suitable and X are atoms with nonbonding number of pi electrons to be aro- electron pairs (e.g. F, O, N), the syn- matic (n = 3) but is not planar be- clinal conformation is more likely. cause of interaction between the For example, in the compound inner hydrogens. CH3–O–CH2Cl, rotation can occur The compound [18]-annulene is along the O–C bond. The conforma- large enough to be planar and obeys tion in which the chlorine atom is the Hückel rule (4n + 2 = 18, with n = closer to the methyl group is pre- 4). It is a brownish red fairly stable ferred (the lone pairs on the oxygen reactive solid. NMR evidence shows atom act as groups in determining that it has aromatic character. The the conformation). annulene with n = 7, [30]-annulene, can also exist in a planar form but is anomers Diastereoisomers of cyclic highly unstable. See also pseudoaro- forms of sugars or similar molecules Ü matic. differing in the con guration at the C1 atom of an aldose or the C2 atom anode A positive electrode. In of a ketose. This atom is called the *electrolysis anions are attracted to anomeric carbon. Anomers are desig- the anode. In an electronic vacuum nated α if the conÜguration at the tube it attracts electrons from the anomeric carbon is the same as that *cathode and it is therefore from the at the reference asymmetric carbon anode that electrons Ûow out of the in a Fischer projection. If the conÜgu- device. In these instances the anode ration differs the anomer is desig- is made positive by external means; nated β. The α- and β-forms of however in a *voltaic cell the anode glucose are examples of anomers (see is the electrode that spontaneously monosaccharide). becomes positive and therefore at- anoxic reactor A *bioreactor in tracts electrons to it from the exter- which the organisms being cultured nal circuit. are anaerobes or in which the reac- anode sludge See electrolytic re- tion being exploited does not require fining. oxygen. anthocyanin 40

anthocyanin One of a group of pound is the basis of a range of a *Ûavonoid pigments. Anthocyanins dyestuffs. *Alizarin is an example. occur in various plant organs and are anti 1. See torsion angle. 2. See responsible for many of the blue, e–z convention. red, and purple colours in plants (particularly in Ûowers). antiaromatic See pseudoaromatic. R antibiotics Substances that destroy or inhibit the growth of microorgan- R isms, particularly disease-producing + bacteria. Antibiotics are obtained from microorganisms (especially R O R moulds) or synthesized. Common antibiotics include the penicillins, streptomycin, and the tetracyclines. R R They are used to treat various infections but tend to weaken the body’s R natural defence mechanisms and can Anthocyanin cause allergies. Overuse of antibiotics can lead to the development of resis- anthracene A white crystalline tant strains of microorganisms. solid, C H ; r.d. 1.28; m.p. 215.8°C; 14 10 antibonding orbital See orbital. b.p. 341.4°C. It is an aromatic hydrocarbon with three fused rings (see anticlinal See torsion angle. formula), and is obtained by the dis- antiferromagnetism See magnet- tillation of crude oils. The main use ism. is in the manufacture of dyes. antiÛuorite structure A crystal structure for ionic compounds of the type M2X, which is the same as the Û uorite (CaF2) structure except that the positions of the anions and cations are reversed, i.e. the cations Anthracene occupy the F– sites and the anions occupy the Ca2+ sites. Examples of the anthracite See coal. antiÛuorite structure are given by fluorite structure anthraquinone A colourless crys- K2O and K2S. See . talling *quinone; m.p. 154°C. It may antifoaming agent A substance be prepared by reacting benzene that prevents a foam from forming, with phthalic anhydride. The com- employed in such processes as electroplating and paper-making, and in O water for boilers. They are com- 8 1 pounds that are absorbed strongly by 7 the liquid (usually water) but lack the 2 properties that allow foaming. Sub- stances used include polyamides, 6 3 polysiloxanes and silicones. 5 4 antifreeze A substance added to O the liquid (usually water) in the cool- Anthraquinone ing systems of internal-combustion 41 anti-Stokes radiation engines to lower its freezing point so air but is unaffected by water or di- that it does not solidify at sub-zero lute acids. It is attacked by oxidizing a temperatures. The commonest an- acids and by halogens. It was Ürst re- tifreeze is *ethane-1,2-diol (ethylene ported by Tholden in 1450. glycol). A antigorite See serpentine. • Information from the WebElements site anti-isomer See isomerism. antioxidants Substances that slow the rate of oxidation reactions. Vari- antiknock agent A petrol additive ous antioxidants are used to preserve that inhibits preignition (‘knocking’) foodstuffs and to prevent the deterio- in internal-combustion engines. Anti- ration of rubber, synthetic plastics, knock agents work by retarding and many other materials. Some an- combustion chain reactions. The tioxidants act as chelating agents to commonest, lead(IV) tetraethyl, sequester the metal ions that catalyse causes environmental pollution, and oxidation reactions. Others inhibit its use is being discouraged. the oxidation reaction by removing antimonic compounds Com- oxygen free radicals. Naturally occur- pounds of antimony in its +5 oxida- ring antioxidants include *vitamin E β tion state; e.g. antimonic chloride is and -carotene; they limit the cell and tissue damage caused by foreign antimony(V) chloride (SbCl5). substances, such as toxins and pollu- antimonous compounds Com- tants, in the body. pounds of antimony in its +3 oxidation state; e.g. antimonous chloride is antiparallel spins Neighbouring antimony(III) chloride (SbCl ). spinning electrons in which the 3 *spins, and hence the magnetic mo- antimony Symbol Sb. An element ments, of the electrons are aligned in belonging to *group 15 (formerly VB) the opposite direction. Under some of the periodic table; a.n. 51; r.a.m. circumstances the interactions be- 121.75; r.d. 6.68; m.p. 630.5°C; b.p. tween magnetic moments in atoms 1750°C. Antimony has several al- favour *parallel spins, while under lotropes. The stable form is a bluish- other conditions they favour antipar- white metal. Yellow antimony and allel spins. The case of antiferromag- black antimony are unstable non- netism (see magnetism) is an example metallic allotropes made at low tem- of a system with antiparallel spins. peratures. The main source is antiperiplanar See torsion angle. stibnite (Sb2S3), from which antimony is extracted by reduction with anti-Stokes radiation Electro- iron metal or by roasting (to give the magnetic radiation occurring in the oxide) followed by reduction with *Raman effect, which is of much carbon and sodium carbonate. The lower intensity than the Rayleigh main use of the metal is as an alloy- scattering in which the frequency of ing agent in lead-accumulator plates, the radiation is higher than that of type metals, bearing alloys, solders, the incident light, i.e. displaced to- Britannia metal, and pewter. It is also wards shorter wavelengths. If the fre- an agent for producing pearlitic cast quency of the original light is ν, the iron. Its compounds are used in frequency of the anti-Stokes radia- Û Ü ν ν ν ame-proo ng, paints, ceramics, tion is + k, where k is the fre- enamels, glass dyestuffs, and rubber quency of the rotation or vibration of technology. The element will burn in the molecule. The spectral lines asso- ap 42

ciated with anti-Stokes radiation are dizing mixture and will dissolve all a called anti-Stokes lines. The quantum metals (except silver, which forms an theory of the Raman effect provides insoluble chloride) including such an explanation for the intensity of noble metals as gold and platinum, the anti-Stokes line being much less hence its name (‘royal water’). Nitro- than the intensity of the Stokes line. syl chloride (NOCl) is believed to be ap Antiperiplanar. See torsion one of the active constituents. angle. aquation The process in which apatite A complex mineral form of water molecules solvate or form coordination complexes with ions. *calcium phosphate, Ca5(PO4)3- (OH,F,Cl); the commonest of the aqueous Describing a solution in phosphate minerals. It has a hexago- water. nal structure and occurs widely as an accessory mineral in igneous rocks aquo ion A hydrated positive ion (e.g. pegmatite) and often in regional present in a crystal or in solution. and contact metamorphic rocks, es- arachidonic acid An unsaturated pecially limestone. Large deposits fatty acid, CH (CH ) (CH CH:CH) - occur in the Kola Peninsula, Russia. It 3 2 3 2 4 (CH ) COOH, that is essential for is used in the production of fertiliz- 2 3 growth in mammals. It can be syn- ers and is a major source of phospho- * rus. The enamel of teeth is composed thesized from linoleic acid. Arachi- chieÛy of apatite. donic acid acts as a precursor to several biologically active com- apical Designating certain bonds pounds, including prostaglandins, and positions in a bipyramidal or and plays an important role in mem- pyramidal structure. For example, in brane production and fat metabo- a trigonal bipyramid the two bonds lism. The release of arachidonic acid aligned through the central atom are from membrane phospholipids is the apical bonds and the groups at- triggered by certain hormones. tached to these bonds are in apical positions. The three other bonds and arachno structure See borane. positions are equatorial. Similarly, in aragonite A rock-forming anhy- a square pyramidal structure the drous mineral form of calcium car- bond perpendicular to the base of bonate, CaCO . It is much less stable the pyramid is the apical bond. The 3 than *calcite, the commoner form of four other coplanar bonds (and posi- calcium carbonate, from which it tions) are said to be basal. may be distinguished by its greater aprotic See solvent. hardness and speciÜc gravity. Over aqua acid A type of acid in which time aragonite undergoes recrystal- the acidic hydrogen is on a water lization to calcite. Aragonite occurs molecule coordinated to a metal ion. in cavities in limestone, as a deposit For example, in limestone caverns, as a precipitate 3+ → around hot springs and geysers, and Al(OH2)6 + H2O Al(OH ) (OH)2+ + H O+ in high-pressure low-temperature 2 5 3 metamorphic rocks; it is also found aqua regia A mixture of concen- in the shells of a number of molluscs trated nitric acid and concentrated and corals and is the main con- hydrochloric acid in the ratio 1:3 re- stituent of pearls. It is white or spectively. It is a very powerful oxi- colourless when pure but the pres- 43 Arrhenius, Svante (August) ence of impurities may tint it grey, Arndt–Eisert systhesis A method blue, green, or pink. of converting a carboxylic acid into a the next higher homologue acid (or arene complex A complex in one of its derivatives). Diazomethane which an aromatic ring is bound to a is used to insert a CH group. metal atom by its pi-electrons. Exam- 2 ples of arene complexes are the aromatic compound An organic *sandwich compounds (C6H6)2Cr and compound that contains a benzene (C5H5)2Fe. ring in its molecules or that has chemical properties similar to ben- arenes Aromatic hydrocarbons, zene. Aromatic compounds are un- such as benzene, toluene, and naph- saturated compounds, yet they do thalene. not easily partake in addition reac- argentic compounds Compounds tions. Instead they undergo elec- of silver in its higher (+2) oxidation trophilic substitution. state; e.g. argentic oxide is silver(II) Benzene, the archetypal aromatic oxide (AgO). compound, has an hexagonal ring of carbon atoms and the classical for- argentite A sulphide ore of silver, mula (the Kekulé structure) would Ag2S. It crystallizes in the cubic sys- have alternating double and single tem but most commonly occurs in bonds. In fact all the bonds in ben- massive form. It is dull grey-black in zene are the same length intermedi- Ü colour but bright when rst cut and ate between double and single C–C occurs in veins associated with other bonds. The properties arise because silver minerals. Important deposits the electrons in the π-orbitals are de- occur in Mexico, Peru, Chile, Bolivia, localized over the ring, giving an and Norway. extra stabilization energy of argentous compounds Com- 150 kJ mol–1 over the energy of a pounds of silver in its lower (+1) oxi- Kekulé structure. The condition for dation state; e.g. argentous chloride such delocalization is that a com- is silver(I) chloride. pound should have a planar ring with (4n + 2) pi electrons – this is arginine See amino acid. known as the Hückel rule. Aromatic argon Symbol Ar. A monatomic behaviour is also found in hetero- noble gas present in air (0.93%); a.n. cyclic compounds such as pyridine. 18; r.a.m. 39.948; d. 0.00178 g cm–3; Aromatic character can be detected m.p. –189°C; b.p. –185°C. Argon is by the presence of a ring current annulenes non- separated from liquid air by frac- using NMR. See also ; benzenoid aromatic pseudoaro- tional distillation. It is slightly solu- ; matic ble in water, colourless, and has no . smell. Its uses include inert atmos- A pheres in welding and special-metal • Information about IUPAC nomenclature manufacture (Ti and Zr), and (when Ü aromaticity The property charac- mixed with 20% nitrogen) in gas- lled teristic of *aromatic compounds. electric-light bulbs. The element is inert and has no true compounds. ARPES See angle-resolved photo- Lord *Rayleigh and Sir William electron spectroscopy. Ü *Ramsey identi ed argon in 1894. Arrhenius, Svante (August) A (1859–1927) Swedish physical • Information from the WebElements site chemist who Ürst demonstrated that Arrhenius equation 44

electrolytes conduct because of the ement will react with halogens, con- a presence of ions. He also worked on centrated oxidizing acids, and hot al- kinetics and proposed the *Arrhe- kalis. Albertus Magnus is believed to nius equation. He was the Ürst to pre- have been the Ürst to isolate the el- dict the greenhouse effect resulting ement in 1250. from carbon dioxide. Arrhenius was A awarded the 1903 Nobel Prize for • Information from the WebElements site chemistry. arsenic acid See arsenic(v) oxide. Arrhenius equation An equation of the form arsenic(III) acid See arsenic(iii) oxide. k = Aexp(–Ea/RT) where k is the rate constant of a arsenic hydride See arsine. given reaction and Ea the *activation arsenic(III) oxide (arsenic trioxide; energy. A is a constant for a given re- arsenious oxide; white arsenic) A action, called the pre-exponential white or colourless compound, factor (or A-factor). Often the equa- As4O6, existing in three solid forms. tion is written in logarithmic form The commonest has cubic or octahe- lnk = lnA – Ea/RT dral crystals (r.d. 3.87; sublimes at A graph of lnk against 1/T is a straight 193°C) and is soluble in water, line with a gradient –Ea/R and an in- ethanol, and alkali solutions. It oc- tercept on the lnk axis of lnA. It is curs naturally as arsenolite. A vitre- named after Svante *Arrhenius. ous form can be prepared by slow Arrhenius theory See acid. condensation of the vapour (r.d. arsenic(iii) oxide 3.74); its solubility in cold water is arsenate(III) See . more than double that of the cubic arsenate(V) See arsenic(v) oxide. form. The third modiÜcation, which occurs naturally as claudetite, has arsenic Symbol As. A metalloid el- monoclinic crystals (r.d. 4.15). Ar- ement of *group 15 (formerly VB) of senic(III) oxide is obtained commer- the periodic table; a.n. 33; r.a.m. cially as a byproduct from the 74.92; r.d. 5.7; sublimes at 613°C. It smelting of nonferrous sulphide ores; has three allotropes – yellow, black, it may be produced in the laboratory and grey. The grey metallic form is by burning elemental arsenic in air. the stable and most common one. The structure of the molecule is simi- Over 150 minerals contain arsenic lar to that of P O , with a tetrahedral but the main sources are as impuri- 4 6 arrangement of As atoms edge linked ties in sulphide ores and in the min- by oxygen bridges. Arsenic(III) oxide erals orpiment (As S ) and realgar 2 3 is acidic; its solutions were formerly (As S ). Ores are roasted in air to 4 4 called arsenious acid (technically, ar- form arsenic oxide and then reduced senic(III) acid). It forms arsenate(III) by hydrogen or carbon to metallic ar- salts (formerly called arsenites). Ar- senic. Arsenic compounds are used in senic(III) oxide is extremely toxic and insecticides and as doping agents in is used as a poison for vermin; trace semiconductors. The element is in- doses are used for a variety of medici- cluded in some lead-based alloys to nal purposes. It is also used for pro- promote hardening. Confusion can ducing opalescent glasses and arise because As O is often sold as 4 6 enamels. white arsenic. Arsenic compounds are accumulative poisons. The el- arsenic(V) oxide (arsenic oxide) A 45 aspartic acid white amorphous deliquescent solid, components. It is used in a dilute gas a As2O5; r.d. 4.32; decomposes at mixture with an inert gas and its 315°C. It is soluble in water and ready thermal decomposition is ex- ethanol. Arsenic(V) oxide cannot be ploited to enable other growing crys- obtained by direct combination of ar- tals to be doped with minute traces senic and oxygen; it is usually pre- of arsenic to give n-type semiconduc- pared by the reaction of arsenic with tors. nitric acid followed by dehydration artinite of the arsenic acid thus formed. It A mineral form of basic * readily loses oxygen on heating to magnesium carbonate, give arsenic(III) oxide. Arsenic(V) MgCO3.Mg(OH)2.3H2O. oxide is acidic, dissolving in water to aryl group A group obtained by re- give arsenic(V) acid (formerly called moving a hydrogen atom from an arsenic acid), H3AsO4; the acid is trib- aromatic compound, e.g. phenyl asic and slightly weaker than phos- group, C6H5–, derived from benzene. phoric acid and should be visualized aryne A compound that can be re- as (HO)3AsO. It gives arsenate(V) salts (formerly called arsenates). garded as formed from an arene by removing two adjacent hydrogen arsenic trioxide See arsenic(iii) atoms to convert a double bond into oxide. a triple bond. Arynes are transient in- arsenious acid See arsenic(iii) termediates in a number of reactions. oxide. The simplest example is *benzyne. arsenious oxide See arsenic(iii) asbestos Any one of a group of Ü oxide. brous amphibole minerals (amosite, crocidolite (blue asbestos), tremolite, arsenic(iii) oxide arsenite See . anthophyllite, and actinolite) or the arsenolite A mineral form of *ar- Übrous serpentine mineral chrysotile. senic(III) oxide, As4O6. Asbestos has widespread commercial uses because of its resistance to heat, arsine (arsenic hydride) A colourless ° ° chemical inertness, and high electri- gas, AsH3; m.p. –116.3 C; b.p. –55 C. cal resistance. The Übres may be It is soluble in water, chloroform, spun and woven into Üreproof cloth and benzene. Liquid arsine has a rela- for use in protective clothing, cur- tive density of 1.69. Arsine is pro- tains, brake linings, etc., or moulded duced by the reaction of mineral into blocks. Since the 1970s short as- acids with arsenides of electroposi- bestos Übres have been recognized as tive metals or by the reduction of a cause of asbestosis, a serious lung many arsenic compounds using disorder, and mesothelioma, a fatal nascent hydrogen. It is extremely form of lung cancer. These concerns poisonous and, like the hydrides of have limited its use and imposed the heavier members of group 15 many safety procedures when it is (formerly VB), is readily decomposed used. Canada is the largest producer at elevated temperatures (around of asbestos; others include Russia, 260–300°C). Like ammonia and phos- South Africa, Zimbabwe, and China. phine, arsine has a pyramidal structure. ascorbic acid See vitamin c. Arsine gas has a very important asparagine See amino acid. commercial application in the production of modern microelectronic aspartic acid See amino acid. asphalt 46

asphalt Bitumen. See petroleum. bonds. For example, ethanol and a water form a mixture (an associated aspirin (acetylsalicylic acid) an liquid) in which hydrogen bonding acetylated form of *salicylic acid (1- holds the different molecules to- hydroxybenzoic acid), used exten- gether. sively as a medicinal drug; r.d. 1.4; m.p. 138–140°C; b.p. 140°C (with de- astatine Symbol At. A radioactive composition). It was Ürst marketed in *halogen element; a.n. 85; r.a.m. 211; 1899 as a analgesic. The acid can be m.p. 302°C; b.p. 337°C. It occurs nat- obtained from willow bark and it has urally by radioactive decay from ura- long been known that the bark could nium and thorium isotopes. Astatine be used for pain relief and for the re- forms at least 20 isotopes, the most duction of fever. The name salicylic stable astatine–210 has a half-life of acid comes from the botanical name 8.3 hours. It can also be produced by of the willow (Salix alba). Aspirin acts alpha bombardment of bismuth–200. by suppressing the production of cer- Astatine is stated to be more metallic tain hormones (prostaglandins and than iodine; at least 5 oxidation thromboxanes) by inhibiting the en- states are known in aqueous solu- zyme cyclooxygenase (COX). Conse- tions. It will form interhalogen com- quently, it is known as a ‘COX pounds, such as AtI and AtCl. The inhibitor’. It is used for the treatment existence of At2 has not yet been es- of arthritis and to reduce body tem- tablished. The element was synthe- perature. It also acts as an anticoagu- sized by nuclear bombardment in lant in the blood and small doses are 1940 by D. R. Corson, K. R. MacKen- taken regularly to reduce the risk of zie, and E. Segré at the University of heart attack. A common side effect of California. high doses is stomach bleeding and A stomach ulcers. Aspirin is made in- • Information from the WebElements site dustrially from phenol, which with Aston, Francis William concentrated sodium hydroxide and (1877–1945) British chemist and carbon dioxide gives sodium phenox- physicist, who until 1910 worked at ide: Mason College (later Birmingham → – + C6H5OH + NaOH C6H5O Na + University) and then with J. J. *Thom- H2O. son at Cambridge University. In 1919 The phenoxide ion undergoes elec- Aston designed the mass spectro- trophilic substitution to give sodium graph (see mass spectroscopy), for salicylate: which he was awarded the Nobel – + → Prize for chemistry in 1922. With it C6H5O + CO2 + Na C6H4 (OH)COO– Na+ he discovered the *isotopes of neon, and was thus able to explain noninte- With acid, this forms salicylic acid, gral atomic weights. which can be acetylated in the ortho position with ethanoic anhydride. astrochemistry The study of molecules in interstellar space. Interstel- associated liquid See association. lar molecules are usually detected association The combination of by their spectra in the radio, micro- molecules of one substance with wave, or infrared regions of the elec- those of another to form chemical tromagnetic spectrum. To date, over species that are held together by 140 different molecules have been forces weaker than normal chemical detected. Of special interest in astro- 47 atom chemistry is the way in which these about 760 mm high. This decreases molecules are formed and the way with increasing altitude. The stan- a in which they interact with clouds of dard value for the atmospheric pres- interstellar dust. sure at sea level in SI units is 101 325 A pascals. • The website of the astrochemistry work atom The smallest part of an el- group of the International Astronomical ement that can exist chemically. Union Atoms consist of a small dense nu- asymmetric atom See optical ac- cleus of protons and neutrons sur- tivity. rounded by moving electrons. The number of electrons equals the num- asymmetric induction The pref- ber of protons so the overall charge erential formation of one particular is zero. The electrons are considered enantiomer or diastereoisomer in a to move in circular or elliptical orbits reaction as a result of a chiral el- (see bohr theory) or, more accu- ement in one of the reactants or in rately, in regions of space around the the catalyst used. nucleus (see orbital). asymmetric top See moment of The electronic structure of an atom inertia. refers to the way in which the electrons are arranged about the nucleus, atactic polymer See polymer. and in particular the *energy levels ATLEED (automated tensor low- that they occupy. Each electron can energy electron diffraction) A form be characterized by a set of four of *LEED (low-energy electron dif- quantum numbers, as follows: fraction) in which the information (1) The principal quantum number n obtained can readily be stored and gives the main energy level and has analysed using a computer. values 1, 2, 3, etc. (the higher the atmolysis The separation of a mix- number, the further the electron ture of gases by means of their differ- from the nucleus). Traditionally, ent rates of diffusion. Usually, these levels, or the orbits correspond- separation is effected by allowing the ing to them, are referred to as shells gases to diffuse through the walls of and given letters K, L, M, etc. The K- a porous partition or membrane. shell is the one nearest the nucleus. The maximum number of electrons atmosphere 1. (atm.) A unit of in a given shell is 2n2. pressure equal to 101 325 pascals. (2) The orbital quantum number l, This is equal to 760.0 mmHg. The ac- which governs the angular momen- Û tual *atmospheric pressure uctuates tum of the electron. The possible val- around this value. The unit is usually ues of l are (n – 1), (n – 2), … 1, 0. used for expressing pressures well in Thus, in the Ürst shell (n = 1) the elec- excess of standard atmospheric pres- trons can only have angular momen- sure, e.g. in high-pressure chemical tum zero (l = 0). In the second shell (n processes. 2. See earth’s atmosphere = 2), the values of l can be 1 or 0, giv- . ing rise to two subshells of slightly atmospheric pressure The pres- different energy. In the third shell (n sure exerted by the weight of the air = 3) there are three subshells, with l above it at any point on the earth’s = 2, 1, or 0. The subshells are de- surface. At sea level the atmosphere noted by letters s(l = 0), p(l = 1), d(l = will support a column of mercury 2), f(l = 3). The number of electrons in atomic absorption spectroscopy 48

each subshell is written as a super- atomic force microscope (AFM) a script numeral to the subshell sym- A type of microscope in which a bol, and the maximum number of small probe, consisting of a tiny chip electrons in each subshell is s2, p6, of diamond, is held on a spring- d10, and f14. The orbital quantum loaded cantilever in contact with the number is sometimes called the az- surface of the sample. The probe is imuthal quantum number. moved slowly across the surface and (3) The magnetic quantum number m, the tracking force between the tip which governs the energies of elec- and the surface is monitored. The Ü trons in an external magnetic eld. probe is raised and lowered so as to This can take values of +l, +(l – 1), … keep this force constant, and a 1, 0, –1, … –(l – 1), –l. In an s-subshell proÜle of the surface is produced. (i.e. l = 0) the value of m = 0. In a p- Scanning the probe over the sample subshell (l = 1), m can have values gives a computer-generated contour +1, 0, and –1; i.e. there are three map of the surface. The instrument p-orbitals in the p-subshell, usually is similar to the scanning tunnelling designated px, py, and pz. Under nor- microscope, but uses mechanical mal circumstances, these all have forces rather than electrical signals. the same energy level. It can resolve individual molecules (4) The spin quantum number m , s and, unlike the scanning tunnelling which gives the spin of the individ- microscope, can be used with non- ual electrons and can have the values conducting samples, such as biologi- +½ or –½. cal specimens. According to the *Pauli exclusion principle, no two electrons in the atomicity The number of atoms in atom can have the same set of quan- a given molecule. For example, oxy- Ü tum numbers. The numbers de ne gen (O2) has an atomicity of 2, ozone the quantum state of the electron, (O3) an atomicity of 3, benzene (C6H6) and explain how the electronic struc- an atomicity of 12, etc. tures of atoms occur. See Chronol- ogy: Atomic Theory. atomic mass unit (a.m.u.) A unit of mass used to express *relative atomic absorption spectroscopy atomic masses. It is equal to 1/12 of (AAS) An analytical technique in the mass of an atom of the isotope which a sample is vaporized and the carbon–12 and is equal to 1.660 33 × nonexcited atoms absorb electromag- 10–27 kg. This unit superseded both netic radiation at characteristic wave- the physical and chemical mass units lengths. based on oxygen–16 and is some- atomic clock An apparatus for times called the uniÜed mass unit or standardizing time based on periodic the dalton. phenomena within atoms or mol- atomic number (proton number) ecules. See ammonia clock; caesium clock Symbol Z. The number of protons in . the nucleus of an atom. The atomic atomic emission spectroscopy number is equal to the number of (AES) An analytical technique in electrons orbiting the nucleus in a which a sample is vaporized and the neutral atom. atoms present are detected by their atomic orbital See orbital. emission of electromagnetic radiation at characteristic wavelengths. atomic volume The relative 49 atom-probe Üeld-ion microscopy

ATOMIC THEORY a

c.430 BC Greek natural philosopher Empedocles (d. c. 430 BC) proposes that all matter consists of four elements: earth, air, fire, and water. c.400 BC Greek natural philosopher Democritus of Abdera (c. 460–370 BC) proposes that all matter consists of atoms. c.306 BC Greek philosopher Epicurus (c. 342–270 BC) champions Democritus’ atomic theory. 1649 French philosopher Pierre Gassendi (1592–1655) proposes an atomic theory (having read Epicurus). 1803 John Dalton proposes Dalton’s atomic theory. 1897 J. J. Thomson discovers the electron. 1904 J. J. Thomson proposes his ‘plum pudding’ model of the atom, with electrons embedded in a nucleus of positive charges. Japanese physicist Hantaro Nagaoka (1865–1950) proposes a ‘Saturn’ model of the atom with a central nucleus having a ring of many electrons. 1911 New Zealand-born physicist Ernest Rutherford (1871–1937) discovers the atomic nucleus. 1913 Niels Bohr proposes model of the atom with a central nucleus surrounded by orbiting electrons. British physicist Henry Moseley (1887–1915) equates the positive charge on the nucleus with its atomic number. Frederick Soddy discovers isotopes. 1916 German physicist Arnold Sommerfield (1868–1951) modifies Bohr’s model of the atom specifying elliptical orbits for the electrons. 1919 Ernest Rutherford discovers the proton. 1920 Ernest Rutherford postulates the existence of the neutron. 1926 Erwin Schrödinger proposes a wave-mechanical model of the atom (with electrons represented as wave trains). 1932 British physicist James Chadwick (1891–1974) discovers the neutron. Werner Heisenberg proposes a model of the atomic nucleus in which protons and neutrons exchange electrons to achieve stability. 1939 Niels Bohr proposes a ‘liquid drop’ model of the atomic nucleus. 1948 German-born US physicist Marie Goeppert-Meyer (1906–72) and German physicist Hans Jensen (1907–73) independently propose the ‘shell’ structure of the nucleus. 1950 US physicist Leo Rainwater (1917–86) combines the ‘liquid-drop’ and ‘shell’ models of the nucleus into a single theory. atomic mass of an element divided dividual atoms on surfaces. In the by its density. atom-probe *Üeld-ionization microscope (FIM) there is a hole in the atomic weight See relative Ûuorescent screen, with which the atomic mass. FIM image of an adsorbed atom is atom-probe Üeld-ion micros- brought into coincidence. The gas copy A technique for identifying in- causing the imaging is removed. The ATP 50

adsorbed atom is removed as an ion group to intermediate substrates. a by a pulse of potential difference. Most ATP-mediated reactions require The atom then passes in the same di- Mg2+ ions as *cofactors. rection as the gas ions, through the ATP is regenerated by the rephos- hole in the screen. This enables the phorylation of AMP and ADP using atom to be identiÜed by a mass spec- the chemical energy obtained from trometer behind the screen. Atom- the oxidation of food. This takes probe FIM identiÜes both the type place during *glycolysis and the and the position of the atom and can *Krebs cycle but, most signiÜcantly, be used to observe atomic processes, is also a result of the reduction– such as evaporation, with the pulse oxidation reactions of the *electron used for analysis lasting about 2 transport chain, which ultimately re- nanoseconds. duces molecular oxygen to water (ox- ATP (adenosine triphosphate) A nu- idative phosphorylation). cleotide that is of fundamental im- atropine A poisonous crystalline al- portance as a carrier of chemical ° kaloid, C17H23NO3; m.p. 118–119 C. It energy in all living organisms. It con- can be extracted from deadly night- d sists of adenine linked to -ribose (i.e. shade and other solanaceous plants d adenosine); the -ribose component and is used in medicine to treat colic, bears three phosphate groups, lin- to reduce secretions, and to dilate early linked together by covalent the pupil of the eye. bonds (see formula). These bonds can undergo hydrolysis to yield either a atropisomers Conformers that molecule of ADP (adenosine diphos- have highly restricted rotation about phate) and inorganic phosphate or a a single bond and can consequently molecule of AMP (adenosine mono- be separated as distinct species. This phosphate) and pyrophosphate. Both can occur, for example, in the case of these reactions yield a large amount substituted biphenyls with large of energy (about 30.6 kJ mol–1) that is groups at the orthopositions of the used to bring about such biological rings. processes as muscle contraction, the ATRS See attenuated total reactive transport of ions and mol- flectance spectroscopy. ecules across cell membranes, and the synthesis of biomolecules. The attenuated total reÛectance reactions bringing about these pro- spectroscopy (ATRS) A variation of cesses often involve the enzyme- infrared spectroscopy in which the IR catalysed transfer of the phosphate source is reÛected from the sample

NH 2 N O– O– O– N adenine

–OOP P O P O CH N N 2 O

O O O ribose

OH OH ATP 51 autoradiography and absorption occurs only in the discovered by the French physicist surface layer. ATRS is used in foren- Pierre Auger (1899–1994) in 1925. a sic science for analysis of thin layers Auger electron See auger effect. (e.g. paint). Ü auric compounds Compounds of atto- Symbol a. A pre x used in the gold in its higher (+3) oxidation state; metric system to denote 10–18. For ex- –18 e.g. auric chloride is gold(III) chloride ample, 10 second = 1 attosecond (AuCl ). (as). 3 aurous compounds Compounds attractor The set of points in phase of gold in its lower (+1) oxidation space to which the representative state; e.g. aurous chloride is gold(I) point of a dissipative system (i.e. one chloride (AuCl). with internal friction) tends as the steel system evolves. The attractor can be: austenite See . a single point; a closed curve (a limit autocatalysis *Catalysis in which cycle), which describes a system with one of the products of the reaction is periodic behaviour; or a strange at- a catalyst for the reaction. Reactions tractor, in which case the system ex- in which autocatalysis occurs have a hibits *chaos. characteristic S-shaped curve for re- Aufbau principle A principle that action rate against time – the reac- gives the order in which orbitals are tion starts slowly and increases as Ülled in successive elements in the the amount of catalyst builds up, falling off again as the products are periodic table. The order of Ülling is used up. 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d. See atom. autoclave A strong steel vessel used for carrying out chemical reac- Auger effect The ejection of an tions, sterilizations, etc., at high tem- electron from an atom as a result of perature and pressure. the de-excitation of an excited electron within the atom. An electron is automated tensor low-energy Ürst ejected from an atom by a pho- electron diffraction See atleed. ton, electron impact, ion impact, or autoprotolysis A transfer of a hy- some other process, thus creating a drogen ion (H+) between molecules of vacancy. In the subsequent re- an amphiprotic *solvent, one mol- arrangement of the electronic struc- ecule acting as a Brønsted acid and ture of the atom, an electron from a the other as a Brønsted base. It oc- higher energy level falls into the va- curs in the autoionization of water. cancy. This process is associated with excess energy, which is released by autoprotolysis constant See ionic product the ejection of a second electron . (rather than by emission of a pho- autoradiography An experimen- ton). This second electron is called tal technique in which a radioactive the Auger electron. Auger spectros- specimen is placed in contact with copy is a form of electron spectros- (or close to) a photographic plate, so copy using this effect to study the as to produce a record of the distribu- energy levels of ions. It is also a form tion of radioactivity in the specimen. of analysis and can be used to iden- The Ülm is darkened by the ionizing tify the presence of elements in sur- radiation from radioactive parts of face layers of solids. The effect was the sample. Autoradiography has a auxochrome 52

number of applications, particularly At low temperatures, the Axilrod– a in the study of living tissues and Teller terms can have a comparable cells. size to two-body interactions, making them of importance to such proper- auxochrome A group in a dye mol- ties of liquids as pressure and surface ecule that inÛuences the colour due to the *chromophore. Auxochromes tension. The Axilrod–Teller formula for the dispersion energy V of three are groups, such as –OH and –NH2, containing lone pairs of electrons closed-shell atoms A, B, and C is that can be delocalized along with given by 6 6 6 the delocalized electrons of the chro- V = – C6/rAB – C6/rBC – C6/rCA + ′ 3 mophore. The auxochrome inten- C /(rABrBCrCA) , Ü Ü si es the colour of the dye. Formerly, where C6 is a coef cient depending the term was also used of such on the identity of the atoms, C′ is a – groups as –SO2O , which make the coefÜcient depending on another molecule soluble and affect its appli- constant and the angles between the cation. atoms, and the r terms give distances Avogadro, Amedeo (1776–1856) between the indicated atoms. Italian chemist and physicist. In 1811 AX spectrum A general pattern of he published his hypothesis (see avo- the nuclear magnetic resonance spec- gadro’s law), which provided a trum of a molecule AX, where A and method of calculating molecular X are both spin-½ nuclei. If X is spin weights from vapour densities. The α, the spin–spin interaction between importance of the work remained the nuclei of A and X results in one unrecognized, however, until cham- line in the spectrum of A being pioned by Stanislao Cannizzaro shifted by J/2 from the frequency of (1826–1910) in 1860. precession it has in the absence of coupling. If X is spin β, the change in Avogadro constant Symbol NA or L. The number of atoms or molecules frequency of precession of A is –J/2. in one *mole of substance. It has the Thus, single lines in the resonances value 6.022 1367(36) × 1023. Formerly of both A and X are split into dou- it was called Avogadro’s number. blets, with splittings J. The A resonance in molecules of the type AnX is Avogadro’s law Equal volumes of also a doublet with the splitting J. all gases contain equal numbers of molecules at the same pressure and azeotrope (azeotropic mixture; con- temperature. The law, often called stant-boiling mixture) A mixture of Avogadro’s hypothesis, is true only two liquids that boils at constant for ideal gases. It was Ürst proposed composition; i.e. the composition of in 1811 by Amedeo *Avogadro. the vapour is the same as that of the ring conformations liquid. Azeotropes occur because of axial See . deviations in Raoult’s law leading to Axilrod–Teller formula An ex- a maximum or minimum in the pression for three-body interactions *boiling-point–composition diagram. in intermolecular forces. These di- When the mixture is boiled, the pole dispersion terms, called vapour initially has a higher propor- Axilrod–Teller terms (named after tion of one component than is pre- B. M. Axilrod and E. Teller, who dis- sent in the liquid, so the proportion covered them in 1943) are of impor- of this in the liquid falls with time. tance in the third virial coefÜcient. Eventually, the maximum and mini- 53 azurite mum point is reached, at which the be formed by reaction of a diazo- two liquids distil together without nium ion with a benzene ring. a change in composition. The composi- A tion of an azeotrope depends on the • Information about IUPAC nomenclature pressure. azo dye See dyes. azeotropic distillation A tech- azoimide See hydrogen azide. nique for separating components of an azeotrope by adding a third liquid azulene 1. A blue crystalline com- ° to form a new azeotrope with one of pound, C10H8; m.p. 99 C. It contains a the original components. It is most Üve-membered ring fused to a seven- commonly used to separate ethanol membered ring and has aromatic from water, adding benzene to asso- properties. When heated it is con- ciate with the ethanol. verted into naphthalene. 2. Any of a number of blue oils that can be pro- azides Compounds containing the duced by distilling or heating essen- – ion N3 or the group –N3. tial oils from plants. azimuthal quantum number See 8 atom. 1 7 azine An organic heterocyclic com- 2 6 pound containing a six-membered 3 5 ring formed from carbon and nitro- 4 gen atoms. Pyridine is an example containing one nitrogen atom Azulene (C5H5N). Diazines have two nitrogen atoms in the ring (e.g. C4H4N2), and azurite A secondary mineral con- isomers exist depending on the rela- sisting of hydrated basic copper car- tive positions of the nitrogen atoms. bonate, Cu3(OH)2(CO3)2, in Triazines contain three nitrogen monoclinic crystalline form. It is atoms. generally formed in the upper zone A of copper ore deposits and often oc- * • Information about IUPAC nomenclature curs with malachite. Its intense azure-blue colour made it formerly azo compounds Organic com- important as a pigment. It is a minor pounds containing the group –N=N– ore of copper and is used as a gem- linking two other groups. They can stone. B

Babbit metal Any of a group of re- may change into lated alloys used for making bear- . RCH2CH CH2CH2CH2CH3 ings. They consist of tin containing The rearrangement is equivalent to antimony (about 10%) and copper a hydrogen atom being transferred (1–2%), and often lead. The original within the molecule. The new un- alloy was invented in 1839 by the US paired electron initiates further inventor Isaac Babbit (1799–1862). polymerization, with the produc- Babo’s law The vapour pressure of tion of polymers with butyl a liquid is decreased when a solute is (CH3CH2CH2CH2–) side chains. added, the amount of the decrease back donation A form of chemical being proportional to the amount of bonding in which a *ligand forms a solute dissolved. The law was discov- sigma bond to an atom or ion by do- ered in 1847 by the German chemist nating a pair of electrons, and the Lambert Babo (1818–99). See also central atom donates electrons back raoult’s law. by overlap of its d-orbitals with backbiting A rearrangement that empty p- or d-orbitals on the ligand. can occur in some *polymerization It occurs, for example, in metal car- reactions involving free radicals. A bonyls. radical that has an unpaired electron back e.m.f. An electromotive force at the end of the chain changes into that opposes the main current Ûow a radical with the unpaired electron in a circuit. For example, in an elec- elsewhere along the chain, the new tric cell, *polarization causes a back radical being more stable than the e.m.f. to be set up by chemical one from which it originates. For ex- means. ample, the radical . background radiation Low inten- RCH2CH2CH2CH2CH2CH2 sity *ionizing radiation present on

H H H H2 2 2 2 R C C CH2 R C C CH3 C C C CH C C H2 H2 H2 H2 H2

H H2 2 R C R C

C CH2 CH CH2 H2

H2C CH2 3CH CH2 C C H2 H2

Backbiting 55 baking powder the surface of the earth and in the awarded the 1905 Nobel Prize for atmosphere as a result of cosmic chemistry. radiation and the presence of radio- Baeyer strain See angle strain. isotopes in the earth’s rocks, soil, b and atmosphere. The radioisotopes Baeyer test A test for unsaturated are either natural or the result of nu- compounds in which potassium per- clear fallout or waste gas from power manganate is used. Alkenes, for ex- stations. Background counts must be ample, are oxidised to glycols, and taken into account when measuring the permanganate loses its colour: Ü → the radiation produced by a speci ed 3R2C=CR2 + 2KMnO4 + 4H2O source. 2MnO2 + 2KOH + 3R2COHR2COH back titration A technique in *vol- Baeyer–Villiger reaction A re- umetric analysis in which a known arrangement reaction, sometimes excess amount of a reagent is added known as the Dakin reaction, com- to the solution to be estimated. The monly used in organic synthesis in unreacted amount of the added which a ketone reacts with a peroxy reagent is then determined by titra- acid to form an ester. For example, tion, allowing the amount of sub- R–CO–R → R–CO–O–R stance in the original test solution to be calculated. The reaction is equivalent to the insertion of an oxygen atom next to bacteriocidal Capable of killing the ketone’s carbonyl (>C=O) group. bacteria. Common bacteriocides are Meta-chloroperbenzoic acid (m-CPBA; some antibiotics, antiseptics, and dis- Û ClC6H4.CO.O.OH) and tri uoro- infectants. perethanoic acid (CF3.CO.O.OH) are bacteriorhodopsin A membrane- typical peroxy acids employed in the bound protein of the halophilic (salt- reaction. It was discovered by Adolf resistant) bacterium Halobacterium von *Baeyer and the German halobium. When activated by light, it chemist V. Villiger in 1899. pumps protons out of the cell; this Bakelite A trade name for certain creates a concentration gradient, *phenol–formaldehyde resins, Ürst which enables ATP to be synthesized. introduced in 1909 by the Belgian– Bacteriorhodopsin is composed of US chemist Leo Hendrik Baekeland seven α-helix segments, which span (1863–1944). the membrane and are joined together by short amino-acid chains. It baking powder A mixture of pow- contains the prosthetic group retinal, dered compounds added to dough or which is also found in the pigment cake mixture to make it rise in cooking. It is used as a substitute for yeast rhodopsin in the rod cells of verte- in bread-making. Baking powders brates. consist of a source of carbon dioxide, Baeyer, Adolf von (1835–1917) such as sodium hydrogencarbonate German organic chemist noted for or ammonium hydrogencarbonate, his work on organic synthesis. He and an acidic substance such as cal- synthesized and determined the cium hydrogenphosphate, potassium structure of indigo and also synthe- hydrogentartrate (cream of tartar), or sized some of the Ürst barbiturates. sodium hydrogenphosphate. In the In his work on carbon rings he for- hot wet mixture, the acid releases mulated his strain theory. Bayer was bubbles of carbon dioxide gas from baking soda 56

the hydrogencarbonate, which make centre bond holding the B–H–B the mixture rise. bridges in *boranes and similar compounds. b baking soda See sodium hydrogencarbonate. band spectrum See spectrum. balance An accurate weighing de- band theory See energy bands. beam balance vice. The simple con- bar A c.g.s. unit of pressure equal sists of two pans suspended from a to 106 dynes per square centimetre centrally pivoted beam. Known or 105 pascals (approximately masses are placed on one pan and 750 mmHg or 0.987 atmosphere). the substance or body to be weighed The millibar (100 Pa) is commonly is placed in the other. When the used in meteorology. beam is exactly horizontal the two masses are equal. An accurate labora- Barbier–Wieland degradation tory balance weighs to the nearest The stepwise degradation of a car- hundredth of a milligram. Specially boxylic acid to the next lower homo- designed balances can be accurate to logue. First the ester is converted a millionth of a milligram. More into a tertiary alcohol using a Grig- modern substitution balances use the nard reagent (PhMgX) and acid (HX): → substitution principle. In this cali- RCH2COOCH3 RCH2C(OH)Ph2. brated weights are removed from the The secondary alcohol is then dehy- single lever arm to bring the single drated using ethanoic anhydride pan suspended from it into equilib- (CH3COOCOCH3) to give an alkene: rium with a Üxed counter weight. RCH C(OH)Ph → RCH=CPh . The substitution balance is more ac- 2 2 2 curate than the two-pan device and The alkene is oxidized with chromic enables weighing to be carried out acid: → more rapidly. In automatic electronic RCH=CPh2 RCOOH + Ph2CO. balances, mass is determined not by The result is conversion of an acid Û mechanical de ection but by elec- RCH2COOH to the lower acid tronically controlled compensation RCOOH. of an electric force. A scanner monitors the displacement of the pan barbiturate Any one of a group of support generating a current propor- drugs derived from barbituric acid tional to the displacement. This cur- that have a depressant effect on the rent Ûows through a coil forcing the central nervous system. Barbiturates pan support to return to its original were originally used as sedatives and position by means of a magnetic sleeping pills but their clinical use is force. The signal generated enables now limited due to their toxic side- effects; prolonged use can lead to ad- the mass to be read from a digital Ü display. The mass of the empty con- diction. Speci c barbiturates in clinical use include phenobarbitone, tainer can be stored in the balance’s for treating epilepsy, and methohexi- computer memory and automatically tane sodium, used as an anaesthetic. deducted from the mass of the container plus its contents. Barfoed’s test A biochemical test to detect monosaccharide (reducing) Balmer series See hydrogen spec- sugars in solution, devised by the trum. Swedish physician Christen Barfoed banana bond Informal name for (1815–99). Barfoed’s reagent, a mix- the type of electron-deÜcient three- ture of ethanoic (acetic) acid and cop- 57 barium peroxide per(II) acetate, is added to the test so- α form (monoclinic; r.d. 3.856), lution and boiled. If any reducing which transforms at 962°C to a β sugars are present a red precipitate form (cubic; r.d. 3.917; m.p. 963°C; of copper(II) oxide is formed. The re- b.p. 1560°C). There is also a dihy- b action will be negative in the pres- drate, BaCl2.2H2O (cubic; r.d. 3.1), ence of disaccharide sugars as they which loses water at 113°C. It is pre- are weaker reducing agents. pared by dissolving barium carbonate barite See barytes. (witherite) in hydrochloric acid and crystallizing out the dihydrate. The barium Symbol Ba. A silvery-white compound is used in the extraction reactive element belonging to *group of barium by electrolysis. 2 (formerly IIA) of the periodic table; a.n. 56; r.a.m. 137.34; r.d. 3.51; m.p. barium hydrogencarbonate (bar- 725°C; b.p. 1640°C. It occurs as the ium bicarbonate) A compound, Ba(HCO ) , which is only stable in so- minerals barytes (BaSO4) and 3 2 lution. It can be formed by the action witherite (BaCO3). Extraction is by high-temperature reduction of bar- of carbon dioxide on a suspension of ium oxide with aluminium or silicon barium carbonate in cold water: → in a vacuum, or by electrolysis of BaCO3(s) + CO2(g) + H2O(l) fused barium chloride. The metal is Ba(HCO3)2(aq) used as a getter in vacuum systems. On heating, this reaction is reversed. It oxidizes readily in air and reacts with ethanol and water. Soluble bar- barium hydroxide (baryta) A ium compounds are extremely poiso- white solid, Ba(OH)2, sparingly solu- nous. It was Ürst identiÜed in 1774 by ble in water. The common form is Karl *Scheele, and was extracted by the octahydrate, Ba(OH)2.8H2O; mon- Humphry *Davy in 1808. oclinic; r.d. 2.18; m.p. 78°C. It can be A produced by adding water to barium • Information from the WebElements site monoxide or by the action of sodium hydroxide on soluble barium com- barium bicarbonate See barium pounds and is used as a weak alkali hydrogencarbonate. in volumetric analysis. barium carbonate A white insolu- barium oxide A white or yellowish ble compound, BaCO3; r.d. 4.43. It de- solid, BaO, obtained by heating bar- composes on heating to give barium ium in oxygen or by the thermal de- oxide and carbon dioxide: composition of barium carbonate or → BaCO3(s) BaO(s) + CO2(g) nitrate; cubic; r.d. 5.72; m.p. 1923°C; The compound occurs naturally as b.p. 2000°C. When barium oxide is the mineral witherite and can be preheated in oxygen the peroxide, BaO2, pared by adding an alkaline solution is formed in a reversible reaction of a carbonate to a solution of a bar- that was once used as a method for ium salt. It is used as a raw material obtaining oxygen (the Brin process). for making other barium salts, as a Barium oxide is now used in the Ûux for ceramics, and as a raw ma- manufacture of lubricating-oil addi- terial in the manufacture of certain tives. types of optical glass. barium peroxide A dense off- barium chloride A white com- white solid, BaO2, prepared by care- pound, BaCl2. The anhydrous com- fully heating *barium oxide in pound has two crystalline forms: an oxygen; r.d. 4.96; m.p. 450°C. It is barium sulphate 58

used as a bleaching agent. With barytes (barite) An orthorhombic acids, hydrogen peroxide is formed mineral form of *barium sulphate, and the reaction is used in the labo- BaSO4; the chief ore of barium. It is b ratory preparation of hydrogen per- usually white but may also be yellow, oxide. grey, or brown. Large deposits occur in Andalusia, Spain, and in the USA. barium sulphate An insoluble white solid, BaSO4, that occurs natu- basal See apical. rally as the mineral *barytes (or basalt A Üne-grained basic igneous heavy spar) and can be prepared as a rock. It is composed chieÛy of cal- precipitate by adding sulphuric acid cium-rich plagioclase feldspar and to barium chloride solution; r.d. 4.50; pyroxene; other minerals present m.p. 1580°C. The rhombic form may be olivine, magnetite, and ap- changes to a monoclinic form at atite. Basalt is the commonest type of 1149°C. It is used as a raw material lava. for making other barium salts, as a pigment extender in surface coating base A compound that reacts with a materials (called blanc Üxe), and protonic acid to give water (and a in the glass and rubber industries. salt). The deÜnition comes from the Barium compounds are opaque to Arrhenius theory of acids and bases. X-rays, and a suspension of the sul- Typically, bases are metal oxides, hy- phate in water is used in medicine to droxides, or compounds (such as am- provide a contrast medium for X-rays monia) that give hydroxide ions in of the stomach and intestine. Al- aqueous solution. Thus, a base may though barium compounds are ex- be either: (1) An insoluble oxide or tremely poisonous, the sulphate is hydroxide that reacts with an acid, safe to use because it is very insolu- e.g. ble. → CuO(s) + 2HCl(aq) CuCl2(aq) + barrel A measurement of volume, H2O(l) widely used in the chemical industry, Here the reaction involves hydrogen equal to 35 UK gallons (approxi- ions from the acid mately 159 litres). + → CuO(s) + 2H (aq) H2O(l) + 2+ Bartlett, Neil (1932– ) British in- Cu (aq) organic chemist who prepared oxy- (2) A soluble hydroxide, in which gen hexachloroplatinate in 1961 and, case the solution contains hydroxide in 1962, xenon hexachloroplatine – ions. The reaction with acids is a re- the Ürst compound of a noble gas. action between hydrogen ions and hydroxide ions: Barton, Sir Derek (Harold + – → Richard) (1918–98) British organic H + OH H2O chemist who worked on steroids and (3) A compound that dissolves in other natural products. He is noted water to produce hydroxide ions. for his investigations into the confor- For example, ammonia reacts as fol- mation of organic compounds and its lows: effect on chemical properties. Barton ˆ + – NH3(g) + H2O(l) NH4 (aq) + OH was awarded the 1969 Nobel Prize Similar reactions occur with organic for chemistry (with O. Hassell *amines (see also nitrogenous base; (1897–1981)). amine salts). A base that dissolves in baryta See barium hydroxide. water to give hydroxide ions is called 59 battery an alkali. Ammonia and sodium hy- charged into a tilting furnace, similar droxide are common examples. to the Bessemer furnace except that The original Arrhenius deÜnition of it has no tuyeres. The charge is con- a base has been extended by the verted to steel by blowing high- b Lowry–Brønsted theory and by the pressure oxygen onto the surface of Lewis theory. See acid. the metal through a water-cooled base dissociation constant See lance. The excess heat produced en- dissociation. ables up to 30% of scrap to be incorporated into the charge. The process base metal A common relatively has largely replaced the Bessemer inexpensive metal, such as iron or and open-hearth processes. lead, that corrodes, oxidizes, or tarnishes on exposure to air, moisture, basic salt A compound that can be or heat, as distinguished from pre- regarded as being formed by replac- cious metals, such as gold and silver. ing some of the oxide or hydroxide ions in a base by other negative ions. base pairing Pairing of nucleotide Basic salts are thus mixed salt–oxides bases by hydrogen bonding. Base (e.g. bismuth(III) chloride oxide, pairing holds together the two BiOCl) or salt–hydroxides (e.g. lead(II) strands in the double helix structure chloride hydroxide, Pb(OH)Cl). of *DNA. The purine adenine pairs with the pyrimidine thymine and the basic slag *Slag formed from a purine guanine pairs with the pyrim- basic Ûux (e.g. calcium oxide) in a idine cytosine. See also chargaff’s blast furnace. The basic Ûux is used rule. to remove acid impurities in the ore and contains calcium silicate, phos- base unit A unit that is deÜned ar- phate, and sulphide. If the phospho- bitrarily rather than being deÜned by rus content is high the slag can be simple combinations of other units. For example, the ampere is a base used as a fertilizer. unit in the SI system deÜned in terms bathochromic shift A shift of a of the force produced between two spectral band to longer wavelengths current-carrying conductors, whereas as a result of substitution in a mol- the coulomb is a derived unit, ecule or a change in the conditions. deÜned as the quantity of charge Compare hypsochromic shift. transferred by one ampere in one second. battery A number of electric cells joined together. The common car basic 1. Describing a compound battery, or *accumulator, usually that is a base. 2. Describing a solu- consists of six secondary cells con- tion containing an excess of hydrox- nected in series to give a total e.m.f. ide ions; alkaline. of 12 volts. A torch battery is usually basic dye See dyes. a dry version of the *Leclanché cell, two of which are often connected in basicity constant See dissociation series. Batteries may also have cells . connected in parallel, in which case basic-oxygen process (BOP they have the same e.m.f. as a single process) A high-speed method of cell, but their capacity is increased, making high-grade steel. It originated i.e. they will provide more total in the Linz–Donawitz (L–D) process. charge. The capacity of a battery is Molten pig iron and scrap are usually speciÜed in ampere-hours, bauxite 60

the ability to supply 1 A for 1 hr, or cury bulbs, the range of temperature the equivalent. to be measured is varied by running mercury from the upper bulb into bauxite The chief ore of alu- b the larger lower bulb. It is used par- minium, consisting of hydrous alu- ticularly for measuring *depression minium oxides and aluminous of freezing point or *elevation of laterite. It is a claylike amorphous boiling point of liquids when solute material formed by the weathering is added, in order to Ünd relative mo- of silicate rocks under tropical condi- lecular masses. tions. The chief producers are Aus- tralia, Guinea, Jamaica, Russia, Brazil, and Surinam.

b.c.c. Body-centred cubic. See cubic reservoir for crystal. adjusting range beam balance See balance. 5 Beattie–Bridgman equation An *equation of state that relates the 4 pressure, volume, and temperature 3 scale for measuring of a gas and the gas constant. The temperature change Beattie–Bridgman equation uses em- 2 pirical constants to take into account 1 the reduction in the effective number of molecules due to various types 0 of molecular aggregation. It is given by stem P = RT(1 – ε)(V + B)/V2 – A/V2, where P is the pressure, T is the thermodynamic temperature, V is the volume, R is the gas constant, and A, B, and ε are constants related to Üve Beckmann thermometer empirical constants A0, B0, a, b, and c by: A = A0(1 – a/V), B = B0(1 – b/V), and becquerel Symbol Bq. The SI unit ε = c/VT3. of activity (see radiation units). The unit is named after the discoverer of Beckmann rearrangement The radioactivity A. H. *Becquerel. chemical conversion of a ketone *oxime into an *amide, usually using Becquerel, Antoine Henri sulphuric acid as a catalyst. The reac- (1852–1908) French physicist. His tion, used in the manufacture of early researches were in optics, then nylon and other polyamides, is in 1896 he accidentally discovered named after the German chemist *radioactivity in Ûuorescent salts of Ernst Beckmann (1853–1923). uranium. Three years later he showed that it consists of charged Beckmann thermometer A ther- particles that are deÛected by a mag- mometer for measuring small netic Üeld. For this work he was changes of temperature (see illustra- awarded the 1903 Nobel Prize for tion). It consists of a mercury-in-glass physics, which he shared with Pierre thermometer with a scale covering and Marie *Curie. only 5 or 6°C calibrated in hun- dredths of a degree. It has two mer- Beer–Lambert law A law relating 61 benzene the reduction in luminous intensity much larger than the width of the of light passing through a material to layer. Before heating the liquid is ho- the length of the light’s path through mogeneous. However, if after heating the material: i.e. from below the temperatures of the b log(I/I ) = –ε[J]l, plates are T1 and T2, at a critical value 0 ∆ where ε is the molar *absorption of the temperature gradient T = T1 – coefÜcient, I is the intensity after T2 the liquid abruptly starts to con- passing through a sample of length l, vect. The liquid spontaneously orga- nizes itself into a set of convection I0 is the incident intensity, and [J] is the molar concentration of species J. rolls, i.e. the liquid goes round in a The Beer–Lambert law was formed series of ‘cells’, called Bénard cells. empirically; however, it can be de- Benedict’s test A biochemical test rived on the basis that the loss in in- to detect reducing sugars in solution, tensity dI is proportional to the devised by the US chemist S. R. Bene- thickness dl of the sample, the con- dict (1884–1936). Benedict’s reagent – centration [J], and the intensity I a mixture of copper(II) sulphate and a (since the rate of absorption is pro- Ültered mixture of hydrated sodium portional to the intensity). The citrate and hydrated sodium carbon- Beer–Lambert law means that the in- ate – is added to the test solution and tensity of light (or any other form of boiled. A high concentration of re- electromagnetic radiation) passing ducing sugars induces the formation through a sample diminishes expo- of a red precipitate; a lower concen- nentially with the concentration and tration produces a yellow precipitate. the thickness of the sample (for a Benedict’s test is a more sensitive al- given wave number). ternative to *Fehling’s test. sucrose beet sugar See . beneÜciation (ore dressing) The Beilstein’s test A test for the pres- separation of an ore into the valuable ence of a halogen (chlorine, bromine, components and the waste material or iodine) in an organic compound. A (gangue). This may be achieved by a piece of copper wire or gauze is pre- number of processes, including heated strongly in the oxidizing crushing, grinding, magnetic separa- Ûame of a Bunsen burner (until the tion, froth Ûotation, etc. The dressed Ûame is no longer green) and the test ore, consisting of a high proportion substance placed on the wire or of valuable components, is then gauze, which is re-heated. A green ready for smelting or some other Ûame indicates the presence of a reÜning process. halogen. bent sandwich See sandwich com- bell metal A type of *bronze used pound. in casting bells. It consists of 60–85% benzaldehyde See benzenecar- copper alloyed with tin, often with baldehyde. some zinc and lead included. benzene A colourless liquid hydro- Belousov–Zhabotinskii reaction ° b z reaction carbon, C6H6; r.d. 0.88; m.p. 5.5 C; See – . b.p. 80.1°C. It is now made from Bénard cell A structure associated gasoline from petroleum by catalytic with a layer of liquid that is conÜned reforming (formerly obtained from by two horizontal parallel plates, in coal tar). Benzene is the archetypal which the lateral dimensions are *aromatic compound. It has an un- benzenecarbaldehyde 62 b

Kekulé structures Dewar structures Benzene

saturated molecule, yet will not read- acid) A white crystalline compound, ° ily undergo addition reactions. On C6H5COOH; r.d. 1.27; m.p. 122.4 C; the other hand, it does undergo sub- b.p. 249°C. It occurs naturally in stitution reactions in which hydro- some plants and is used as a food gen atoms are replaced by other preservative. Benzenecarboxylic acid atoms or groups. This behaviour oc- has a carboxyl group bound directly curs because of delocalization of p- to a benzene ring. It is a weak car- × –5 ° electrons over the benzene ring, and boxylic acid (Ka = 6.4 10 at 25 C), all the C–C bonds in benzene are which is slightly soluble in water. It equivalent and intermediate in also undergoes substitution reactions length between single and double on the benzene ring. bonds. It can be regarded as a resonance hybrid of Kekulé and Dewar benzene-1,4-diol (hydroquinone; structures (see formulae). In formu- quinol) A white crystalline solid, ° lae it can be represented by a hexa- C6H4(OH)2; r.d. 1.33; m.p. 170 C; b.p. ° gon with a ring inside it. 285 C. It is used in making dyes. See also quinhydrone electrode. benzenecarbaldehyde (benzaldehyde) A yellowish volatile oily liq- benzene hexacarboxylic acid See ° mellitic acid. uid, C6H5CHO; r.d. 1.04; m.p. –26 C; b.p. 178.1°C. The compound occurs benzene hexachloride (BHC) A in almond kernels and has an al- crystalline substance, C6H6Cl6, made mond-like smell. It is made from by adding chlorine to benzene. It is methylbenzene (by conversion to used as a pesticide and, like *DDT, dichloromethyl benzene, C6H5CHCl2, concern has been expressed at its en- followed by hydrolysis). Benzenecar- vironmental effects. baldehyde is used in Ûavourings, perfumery, and the dyestuffs industry. benzenesulphonic acid A colourless deliquescent solid, C6H5SO2OH, benzenecarbonyl chloride (ben- m.p. 43–44°C, usually found as an zoyl chloride) A colourless liquid, oily liquid. It is made by treating ben- ° C6H5COCl; r.d. 1.21; m.p. 0 C; b.p. zene with concentrated sulphuric ° 197.2 C. It is an *acyl halide, used to acid. Its alkyl derivatives are used as introduce benzenecarbonyl groups *detergents. into molecules. See acylation. benzfuran (coumarone) A crys- benzenecarbonyl group (benzoyl talline aromatic compound, C8H6O. group) The organic group C6H5CO–. It is a heterocyclic compound hav- benzenecarboxylate (benzoate) A ing a benzene ring fused to a Üve- salt or ester of benzenecarboxylic membered *furan ring. acid. benzil 1,2-diphenylethan-1,2-dione. benzenecarboxylic acid (benzoic See benzilic acid rearrangement. 63 benzoylation

Benzfuran isomers benzilic acid rearrangement An 137°C. It is a condensation product of organic rearrangement reaction in *benzenecarbaldehyde (benzalde- which benzil (1,2-diphenylethan-1,2- hyde), made by the action of sodium dione) is treated with hydroxide and cyanide on benzenecarbaldehyde in then acid to give benzilic acid (2-hy- alcoholic solution. It also occurs natu- droxy-2,2-diphenylethanoic acid): rally as the resin of a tropical tree. It → is both a secondary alcohol and a ke- C6H5.CO.CO.C6H5 (C H ) C(OH).COOH tone, and gives reactions characteris- 6 5 2 tic of both types of compound. In the reaction a phenyl group (C6H5–) migrates from one carbon benzopyrene A crystalline aro- atom to another. The reaction was matic hydrocarbon, C20H12; m.p. discovered in 1828 by Justus von 179°C. It is found in coal tar and is *Liebig; it was the Ürst rearrange- highly carcinogenic. ment reaction to be described. 2 3 benzenecarboxylate 2a benzoate See . 1 4 benzodiazepines A group of re- 12 5 lated psychotic drugs that affect the 11 12a central nervous system. They are 5a used medically in the treatment of 10 6 anxiety, insomnia, convulsions, and 8a 7a alcohol withdrawal. All are addictive 9 8 7 and available only as prescription Benzopyrene drugs in the UK. Common examples Û are *diazepam (Valium) and * uni- benzoquinone See cyclohexadi- trazepam (Rohypnol). ene-1,4-dione. benzenecar- benzoic acid See benzoylation A chemical reaction boxylic acid. in which a benzoyl group (benzene- benzoin A colourless crystalline carbonyl group, C6H5CO) is intro- compound, C6H5CHOHCOC6H5; m.p. duced into a molecule. See acylation.

Benzilic acid rearrangement benzoyl chloride 64

benzoyl chloride See benzenecar- *Haber in Karlsruhe, he become in- bonyl chloride. terested in reactions at high pressures. In 1912 he devised an benzoylecgonine (BZ) A primary b industrial process for making light metabolite of cocaine, used in drug hydrocarbons by the high-pressure testing. It can be detected in the hydrogenation of coal or heavy oil. urine up to 48 hours after taking co- The work earned him a share of the caine. BZ is tested for by immunoas- 1931 Nobel Prize for chemistry with say or by gas chromatography/mass Carl Bosch (1874–1940). The Bergius spectrometry. process proved important for supply- benzoyl group See benzenecar- ing petrol for the German war effort bonyl group. in World War II. benzpyrene A pale yellow solid, Bergius process A process for ° C20H12, m.p. 179 C, whose molecules making hydrocarbon mixtures (for consist of Üve fused benzene rings. It fuels) from coal by heating powdered occurs in tars from coal and tobacco coal mixed with tar and iron(III) smoke and is a *carcinogen. oxide catalyst at 450°C under hydro- benzvalene A valence isomer of gen at a pressure of about 200 atmospheres. In later developments of the benzene, C6H6, with a bridged structure. process, the coal was suspended in liquid hydrocarbons and other cata- benzyl alcohol See phenyl- lysts were used. The process was de- methanol. veloped by Friedrich *Bergius during benzylamine (α-aminotoluene, World War I as a source of motor fuel. phenylmethylamine) A colourless liq- berkelium Symbol Bk. A radio- uid, C6H5CH2NH2; r.d. 0.981; b.p. active metallic transuranic element 185°C. It behaves in the same way as belonging to the *actinoids; a.n. 97; primary aliphatic amines. mass number of the most stable iso- × 3 benzyne (1,2-didehydrobenzene) A tope 247 (half-life 1.4 10 years); r.d. highly reactive short-lived com- (calculated) 14. There are eight known isotopes. It was Ürst produced pound, C6H4, having a hexagonal ring of carbon atoms containing two by G. T. Seborg and associates in double bonds and one triple bond. 1949 by bombarding americium–241 Benzyne, which is the simplest exam- with alpha particles. ple of an *aryne, is thought to be an A intermediate in a number of reac- • Information from the WebElements site tions. Berry mechanism A mechanism H by which ligands in trigonal bipyra- C midal complexes can interchange be- CH tween axial and equatorial positions. It involves intermediate formation of CH a square pyramidal conformation. It C H is also known as the Berry pseudo- Benzyne rotation. Berthelot, Marcellin (Pierre Eu- Bergius, Friedrich Karl Rudolf gène) (1827–1907) French chemist (1884–1949) German organic who pioneered organic synthesis, chemist. While working with Fritz producing many organic compounds 65 beryllium bronze from simple starting materials. strong alkali. The reaction (for the Berthelot was also one of the Ürst to metal) is often written investigate thermochemistry. – → 2– Be + 2OH (aq) BeO2 (aq) + H2(g) b 2– Berthelot equation An *equation The ion BeO2 is the beryllate ion. In of state that relates the pressure, vol- fact, as with the *aluminates, the ume, and temperature of a gas and ions present are probably hydroxy 2– the gas constant. It is given by: ions of the type Be(OH)4 (the 2 2 tetrahydroxoberyllate(II) ion) to- PV = RT[1 + 9PTc(1 – 6Tc /T )/128PcT], gether with polymeric ions. where P is the pressure, V is the volume, R is the gas constant, T is the beryllia See beryllium oxide. thermodynamic temperature, and T c beryllium Symbol Be. A grey metal- and P are the critical temperature c lic element of *group 2 (formerly IIA) and pressure of the gas. The Berth- of the periodic table; a.n. 4; r.a.m. elot equation can be derived from 9.012; r.d. 1.85; m.p. 1278°C; b.p. the *Clapeyron–Clausius equation. 2970°C. Beryllium occurs as beryl Berthollide compound A solid (3BeO.Al2O3.6SiO2) and chrysoberyl compound with slight variations in (BeO.Al2O3). The metal is extracted nonstoi- chemical composition (see from a fused mixture of BeF2/NaF by chiometric compound). Berthollide electrolysis or by magnesium reduc- compounds are named after the tion of BeF2. It is used to manufac- French inorganic chemist Claude ture Be–Cu alloys, which are used in Louis Berthollet (1748–1822), who at- nuclear reactors as reÛectors and tacked the law of constant composi- moderators because of their low ab- tion. sorption cross section. Beryllium beryl A hexagonal mineral form of oxide is used in ceramics and in nu- beryllium aluminium silicate, clear reactors. Beryllium and its compounds are toxic and can cause Be3Al2Si6O18; the chief ore of beryllium. It may be green, blue, yellow, serious lung diseases and dermatitis. or white and has long been used as a The metal is resistant to oxidation by gemstone. Beryl occurs throughout air because of the formation of an the world in granite and pegmatites. oxide layer, but will react with dilute *Emerald, the green gem variety, oc- hydrochloric and sulphuric acids. curs more rarely and is of great Beryllium compounds show high co- value. Important sources of beryl are valent character. The element was found in Brazil, Madagascar, and the isolated independently by F. *Wöhler USA. Aand A. A. Bussy in 1828. beryllate A compound formed in • Information from the WebElements site solution when beryllium metal, or the oxide or hydroxide, dissolves in beryllium bronze A hard, strong

Berry mechanism beryllium hydroxide 66

type of *bronze containing about 2% the furnace is tilted so that the beryllium, in addition to copper and molten steel can be poured off. In tin. the modern VLN (very low nitrogen) b version of this process, oxygen and beryllium hydroxide A white steam are blown into the furnace in crystalline compound, Be(OH) , pre- 2 place of air to minimize the absorp- cipitated from solutions of beryllium tion of nitrogen from the air by the salts by adding alkali. Like the oxide, steel. The process is named after the it is amphoteric and dissolves in ex- British engineer Sir Henry Bessemer cess alkali to give *beryllates. (1813–98), who announced it in 1856. beryllium oxide (beryllia) An in- See also basic-oxygen process. soluble solid compound, BeO; hexag- beta decay A type of radioactive onal; r.d. 3.01; m.p. 2530°C; b.p. decay in which an unstable atomic 3900°C. It occurs naturally as bromel- nucleus changes into a nucleus of the lite, and can be made by burning same mass number but different pro- beryllium in oxygen or by the de- ton number. The change involves the composition of beryllium carbonate conversion of a neutron into a pro- or hydroxide. It is an important am- ton with the emission of an electron photeric oxide, reacting with acids to _ and an antineutrino (n → p + e– + ν) form salts and with alkalis to form or of a proton into a neutron with compounds known as *beryllates. the emission of a positron and a neu- Beryllium oxide is used in the trino (p → n + e+ + ν). An example is production of beryllium and the decay of carbon–14: beryllium–copper refractories, tran- _ 14 → 14 – ν sistors, and integrated circuits. 6C 7 N + e + The electrons or positrons emitted Berzelius, Jöns Jacob (1779–1848) are called beta particles and streams Swedish chemist. After moving to of beta particles are known as beta Stockholm he worked with mining radiation. chemists and, with them, discovered several elements, including cerium beta-iron A nonmagnetic allotrope (1803), selenium (1817), lithium of iron that exists between 768°C (1818), thorium (1828), and vanadium and 900°C. (1830). He produced the Ürst accurate beta particle See beta decay. table of atomic weights and was extremely inÛuential in the general beta sheet (β-pleated sheet) A development of 19th-century form of secondary structure in *pro- chemistry. teins in which extended polypeptide chains lie parallel to each other and Bessemer process A process for are linked by hydrogen bonds be- converting *pig iron from a *blast tween the N–H and C=O groups (see furnace into *steel. The molten pig illustration overleaf). Beta sheets iron is loaded into a refractory-lined occur in many globular proteins and tilting furnace (Bessemer converter) link polypeptides of the same type in at about 1250°C. Air is blown into certain Übrous proteins, including the furnace from the base and Übroin (the protein of silk). *spiegel is added to introduce the correct amount of carbon. Impurities BET isotherm An isotherm that (especially silicon, phosphorus, and takes account of the possibility that manganese) are removed by the con- the monolayer in the *Langmuir ad- verter lining to form a slag. Finally sorption isotherm can act as a sub- 67 binary acid

R • • • hydrogen bond R = amino-acid side chain H O R C H H O b N C N C H C H O C N H N C H C • H • R O N C • • C H O R O • C • •• R • H • •• C O H H C R C N H C R N H C C C N N H R C H O H C • • R • O H C N • • • H O C • • R • R • O H • N O R H C H • N H C C H •• O N C N H O H C N C H C C H O C R R R

Beta sheet strate for further adsorption. The bicarbonate of soda See sodium BET isotherm (named after S. hydrogencarbonate. Brunauer, P. Emmett, and E. Teller) bifurcation A phenomenon in dy- has the form: namical systems in which the num- V/Vmon = cz/{(1 – z)[1 – (1 – c)z]}, ber of solutions for a type of where z = p/p* (p* is the vapour pres- behaviour suddenly changes when Ü sure above a macroscopically thick one of the parameters de ning the layer of liquid on the surface), V is system reaches a critical value. Bifur- mon cations can be depicted in a bifurca- the volume that corresponds to the tion diagram in which one axis of a surface being covered by a mono- graph is a variable of the system and layer, V and p are the volume and the other axis is a parameter that can pressure of the gas respectively, and c bring about bifurcations. In many is a constant. In the BET isotherm, systems a whole series of bifurca- the isotherm rises indeÜnitely at tions occur, called bifurcation cas- high pressures (in contrast to the cades. In certain cases these Langmuir isotherm). It provides a bifurcation cascades can lead to useful approximation over some chaotic reactions. ranges of pressure but underesti- bimolecular reaction A step in a mates adsorption for low pressures chemical reaction that involves two and overestimates adsorption for molecules. See molecularity. high pressures. binary Describing a compound or BHC See benzene hexachloride. alloy formed from two elements. bicarbonate See hydrogencarbon- binary acid An *acid in which the ate. acidic hydrogen atom(s) are bound di- binding site 68

rectly to an atom other than oxygen. pose organic waste matter in water. Examples are hydrogen chloride It is therefore used as a measure of (HCl) and hydrogen sulphide (H2S). the amount of certain types of or- b Such compounds are sometimes ganic pollutant in water. BOD is cal- called hydracids. Compare oxoacid. culated by keeping a sample of water containing a known amount of oxy- binding site An area on the sur- gen for Üve days at 20°C. The oxygen face of a molecule that combines content is measured again after this with another molecule. Binding sites time. A high BOD indicates the pres- * on enzymes can be active sites or ence of a large number of microor- *allosteric sites. ganisms, which suggests a high level bioaccumulation An increase in of pollution. the concentration of chemicals, such biochemistry The study of the as pesticides, in organisms that live chemistry of living organisms, espe- in environments contaminated by a cially the structure and function of wide variety of organic compounds. their chemical components (princi- These compounds are not usually de- pally proteins, carbohydrates, lipids, composed in the environment (i.e. and nucleic acids). Biochemistry has they are not biodegradable) or metab- advanced rapidly with the develop- olized by the organisms, so that their ment, from the mid-20th century, of rate of absorption and storage is such techniques as chromatography, greater than their rate of excretion. X-ray diffraction, radioisotopic la- The chemicals are normally stored in belling, and electron microscopy. fatty tissues. *DDT is known as a per- Using these techniques to separate sistent pesticide, as it is not easily and analyse biologically important broken down and bioaccumulates molecules, the steps of the metabolic along food chains, so that increasing pathways in which they are involved concentrations occur in individual or- (e.g. glycolysis) have been deter- ganisms at each trophic level. mined. This has provided some bioactivation A metabolic process knowledge of how organisms obtain in which a product that is chemically and store energy, how they manufac- reactive is produced from a relatively ture and degrade their biomolecules, inactive precursor. and how they sense and respond to their environment. See Chronology. biochemical fuel cell A system biofuel that exploits biological reactions for biodiesel See . the conversion of biomass (chemical bioelement Any chemical element energy) to electricity (electrical en- that is found in the molecules and ergy). One potential application is the compounds that make up a living or- generation of electricity from indus- ganism. In the human body the most trial waste and sewage. Methyl- common bioelements (in decreasing trophic organisms (i.e. organisms that order of occurrence) are oxygen, car- use methane or methanol as their bon, hydrogen, nitrogen, calcium, sole carbon sources) are being inves- and phosphorus. Other bioelements tigated for their potential use in bio- include sodium, potassium, magne- chemical fuel cells. sium, and copper. See essential element biochemical oxygen demand . (BOD) The amount of oxygen taken bioenergetics The study of the up by microorganisms that decom- Ûow and the transformations of en- 69 biopolymer ergy that occur in living organisms. bioinorganic chemistry Biochem- Typically, the amount of energy that istry involving compounds that con- an organism takes in (from food or tain metal atoms or ions. Two sunlight) is measured and divided common examples of bioinorganic b into the amount used for growth of compounds are haemoglobin (which new tissues; that lost through death, contains iron) and chlorophyll wastes, and (in plants) transpiration; (which contains magnesium). Many and that lost to the environment as enzymes contain metal atoms and heat (through respiration). bioinorganics are important in a number of biochemical processes, in- biofuel A gaseous, liquid, or solid cluding oxygen transport, electron fuel that contains an energy content transfer, and protein folding. derived from a biological source. The organic matter that makes up living bioluminescence The emission lumi- organisms provides a potential of light without heat (see nescence source of trapped energy that is be- ) by living organisms. The ginning to be exploited to supply the phenomenon occurs in glow-worms Ü Û ever-increasing energy demand and re ies, bacteria and fungi, and Ü around the world. An example of a in many deep-sea sh (among oth- biofuel is rapeseed oil, which can be ers); in animals it may serve as a means of protection (e.g. by disguis- used in place of diesel fuel in mod- Ü iÜed engines. The methyl ester of ing the shape of a sh) or species this oil, rapeseed methyl ester (RME), recognition or it may provide mating can be used in unmodiÜed diesel en- signals. The light is produced during the oxidation of a compound called gines and is sometimes known as luciferin (the composition of which biodiesel. Other biofuels include varies according to the species), the *biogas and *gasohol. reaction being catalysed by an en- biogas A mixture of methane and zyme, luciferase. Bioluminescence carbon dioxide resulting from the may be continuous (e.g. in bacteria) anaerobic decomposition of such or intermittent (e.g. in ÜreÛies). waste materials as domestic, indus- biomarker A normal metabolite trial, and agricultural sewage. The de- that, when present in abnormal con- composition is carried out by centrations in certain body Ûuids, methanogenic bacteria; these oblig- can indicate the presence of a partic- ate anaerobes produce methane, the ular disease or toxicological condi- main component of biogas, which tion. For example, abnormal can be collected and used as an en- concentrations of glucose in the ergy source for domestic processes, blood can be indicative of diabetes such as heating, cooking, and light- mellitus (see insulin). ing. The production of biogas is carried out in special digesters, which biomolecule Any molecule that is are widely used in China and India. involved in the maintenance and metabolic processes of living organ- As well as providing a source of fuel, metabolism these systems also enable sewage, isms (see ). Biomolecules which contains pathogenic bacteria, include carbohydrate, lipid, protein, to be digested, thereby removing the nucleic acid, and water molecules; danger to humans that could other- some biomolecules are macromolecules. wise result from untreated domestic and agricultural waste. biopolymer A polymer that occurs 70

BIOCHEMISTRY b 1833 French chemist Anselme Payen (1795–1871) discovers diastase (the first enzyme to be discovered). 1836 Theodor Schwann discovers the digestive enzyme pepsin. c.1860 Louis Pasteur demonstrates fermentation is caused by ‘ferments’ in yeasts and bacteria. 1869 German biochemist Johann Friedrich Miescher (1844–95) discovers nucleic acid. 1877 Pasteur’s ‘ferments’ are designated as enzymes. 1890 German chemist Emil Fischer (1852–1919) proposes the ‘lock-and-key’ mechanism to explain enzyme action. 1901 Japanese chemist Jokichi Takamine (1854–1922) isolates adrenaline (the first hormone to be isolated). 1903 German biologist Eduard Buchner (1860–1917) discovers the enzyme zymase (causing fermentation). 1904 British biologist Arthur Harden (1865–1940) discovers coenzymes. 1909 Russian-born US biochemist Phoebus Levene (1869–1940) identifies ribose in RNA. 1921 Canadian physiologist Frederick Banting (1891–1941) and US physiologist Charles Best (1899–1978) isolate insulin. 1922 Alexander Fleming discovers the enzyme lysozyme. 1925 Russian-born British biologist David Keilin (1887–1963) discovers cytochrome. 1926 US biochemist James Sumner (1877–1955) crystallizes urease (the first enzyme to be isolated). 1929 German chemist Hans Fischer (1881–1945) determines the structure of haem (in haemoglobin). K. Lohman isolates ATP from muscle. 1930 US biochemist John Northrop (1891–1987) isolates the stomach enzyme pepsin. 1932 Swedish biochemist Hugo Theorell (1903–82) isolates the muscle protein myoglobin. 1937 Hans Krebs discovers the Krebs cycle. 1940 German-born US biochemist Fritz Lipmann (1899–1986) proposes that ATP is the carrier of chemical energy in many cells. 1943 US biochemist Britton Chance (1913– ) discovers how enzymes work (by forming an enzyme–substrate complex). 1952 US biologist Alfred Hershey (1908–97) proves that DNA carries genetic information. 1953 Francis Crick and James Watson discover the structure of DNA. 1955 Frederick Sanger discovers the amino acid sequence of insulin. 71

1956 US biochemist Arthur Kornberg (1918– ) discovers the enzyme DNA b polymerase. US molecular biologist Paul Berg (1926– ) identifies the nucleic acid later known as transfer RNA. 1957 British biologist Alick Isaacs (1921–67) discovers interferon. 1959 Austrian-born British biochemist Max Perutz (1914–2002) determines the structure of haemoglobin. 1960 South African-born British molecular biologist Sydney Brenner (1927– ) and French biochemist François Jacob (1920– ) discover messenger RNA. 1961 British biochemist Peter Mitchell (1920–92) proposes the chemiosmotic theory. Brenner and Crick discover that the genetic code consists of a series of base triplets. 1969 US biochemist Gerald Edelman (1929– ) discovers the amino acid sequence of immunoglobulin G. 1970 US virologists Howard Temin (1934–94) and David Baltimore (1938– ) discover the enzyme reverse transcriptase. US molecular biologist Hamilton Smith (1931– ) discovers restriction enzymes. 1973 US biochemists Stanley Cohen (1935– ) and Herbert Boyer (1936– ) use restriction enzymes to produce recombinant DNA. 1977 Sanger determines the complete base sequence of DNA in bacteriophage φX174. 1984 British biochemist Alec Jeffreys (1950– ) devises DNA profiling. 1985 US biochemist Kary Mullis (1944– ) invents the polymerase chain reaction for amplifying DNA. 1986 US pharmacologists Robert Furchgott (1916– ) and Louis Ignarro (1941– ) demonstrate the importance of nitric oxide as a signal molecule in the blood vascular system. 1988 US biochemist Peter Agre (1949– ) identifies a water-channel protein (aquaporin) in the plasma membrane of cells. 1994 Beginnings of DNA chip technology. 1998 US biochemist Roderick MacKinnon (1956– ) reveals detailed three- dimensional structure of potassium-ion channel in brain cells. 2001 US molecular biologist Harry Noller and colleagues produce first detailed X-ray crystallographic image of a complete ribosome. 2002 First synthetic virus created by Eckard Wimmer and associates, based on human poliovirus. 2004 A team led by David L. Spector produces the first real-time imaging of gene transcription in a living cell, using different fluorescent markers to tag nucleic acids and proteins. bioreactor 72

naturally, such as a *polysaccharide, otics, such as synthetic organic com- *protein, or *nucleic acid. pounds. b bioreactor (industrial fermenter) A biosynthesis The production of large stainless steel tank used to molecules by a living cell, which is grow producer microorganisms in the essential feature of *anabolism. the industrial production of enzymes biotechnology The development and other chemicals. After the tank of techniques for the application of is steam-sterilized, an inoculum of biological processes to the produc- the producer cells is introduced into tion of materials of use in medicine a medium that is maintained by and industry. For example, the pro- probes at optimum conditions of duction of antibiotics, cheese, and temperature, pressure, pH, and oxy- wine rely on the activity of various gen levels for enzyme production. An fungi and bacteria. Genetic engineer- agitator (stirrer) mixes the medium, ing can modify bacterial cells to syn- which is constantly aerated. It is es- thesize completely new substances, sential that the culture medium is e.g. hormones, vaccines, monoclonal sterile and contains the appropriate antibodies, etc., or introduce novel nutritional requirements for the traits into plants or animals. microorganism. When the nutrients have been utilized the product is sep- biotin A vitamin in the *vitamin B arated; if the product is an extracellu- complex. It is the *coenzyme for var- lar compound the medium can be ious enzymes that catalyse the incor- removed during the growth phase of poration of carbon dioxide into various compounds. Adequate the microorganisms, but an intracel- amounts are normally produced by lular product must be harvested the intestinal bacteria; other sources when the batch culture growth stops. include cereals, vegetables, milk, and Some bioreactors are designed for liver. continuous culture. biotite An important rock-forming biosensor A device that uses an im- silicate mineral, a member of the mobilized agent to detect or measure *mica group of minerals, in common a chemical compound. The agents in- with which it has a sheetlike crystal clude enzymes, antibiotics, or- structure. It is usually black, dark ganelles, or whole cells. A reaction brown, or green in colour. between the immobilized agent and the molecule being analysed is trans- bipy See dipyridyl. duced into an electronic signal. This bipyramid See complex. signal may be produced in response dipyridyl to the presence of a reaction product, bipyridyl See . the movement of electrons, or the biradical (diradical) A radical that appearance of some other factor (e.g. has two unpaired electrons at differ- light). Biosensors are being used in- ent points in the molecule, so that creasingly in diagnostic tests: these the radical centres are independent allow quick, sensitive, and speciÜc of each other. analysis of a wide range of biological birefringence double refrac- products, including antibiotics, vita- See tion. mins, and other important biomolecules (such as glucose), as well as the Birge–Sponer extrapolation A determination of certain *xenobi- method used to calculate the heat of 73 bistability dissociation of a molecule by extrap- high Hall effect when placed in mag- olation from observed band spectra. netic Üelds. It is used to make low- The dissociation energy D0 is equal to melting-point casting alloys with tin the sum of the vibrational quanta ∆G, and cadmium. These alloys expand b where ∆G represents the energy be- on solidiÜcation to give clear replica- tween two successive vibrational tion of intricate features. It is also states. This means that D0 is approxi- used to make thermally activated mately equal to the area under the safety devices for Üre-detection and curve of ∆G plotted against the vibra- sprinkler systems. More recent appli- tional quantum number v. If the Ürst cations include its use as a catalyst few vibrational quanta are observed, for making acrylic Übres, as a constituent of malleable iron, as a car- an approximate value of D0 can be obtained by a linear extrapolation of rier of uranium–235 fuel in nuclear this curve. If a sufÜcient number of reactors, and as a specialized thermo- vibrational quanta have been ob- couple material. Bismuth compounds served, a considerable improvement (when lead-free) are used for cosmetics and medical preparations. It is at- in the value of D0 can be obtained by taking the curvature of the ∆G curve tacked by oxidizing acids, steam (at into account by quadratic (and/or) high temperatures), and by moist higher terms. The technique was put halogens. It burns in air with a blue Û forward by R. T. Birge and H. Sponer ame to produce yellow oxide fumes. Ü in 1926. C. G. Junine rst demonstrated that it was different from lead in 1753. Birkeland–Eyde process A A Ü process for the xation of nitrogen • Information from the WebElements site by passing air through an electric arc to produce nitrogen oxides. It bisphosphonates (diphospho- was introduced in 1903 by the Nor- nates) A class of medical drugs used wegian chemists Kristian Birkeland in the treatment of osteoporosis and (1867–1913) and Samuel Eyde other conditions that involve fragile (1866–1940). The process is economic bones. The bisphosphonates attack only if cheap hydroelectricity is avail- osteoclasts (i.e. the bone cells that able. break down bone tissue). The general formula is O P–C(R1R3)–PO . The side conformation 3 3 bisecting See . chain R1 is a simple group (–H, –OH, 2 bismuth Symbol Bi. A white crys- –Cl). R is usually a longer chain (e.g. talline metal with a pinkish tinge be- –S–C6H4–Cl or –(CH2)5–NH2. longing to *group 15 (formerly VB) of bistability The ability of a system the periodic table; a.n. 83; r.a.m. to exist in two steady states. Bistabil- 208.98; r.d. 9.78; m.p. 271.3°C; b.p. ity is a necessary condition for oscil- 1560°C. The most important ores are lations to occur in chemical bismuthinite (Bi2S3) and bismite reactions. The two steady states of a (Bi2O3). Peru, Japan, Mexico, Bolivia, bistable system are not states of ther- and Canada are major producers. The modynamic equilibrium, as they are metal is extracted by carbon reduc- associated with conditions far from tion of its oxide. Bismuth is the most equilibrium. Chemical reactions in- diamagnetic of all metals and its volving bistability jump suddenly thermal conductivity is lower than from one state to the other state any metal except mercury. The metal when a certain concentration of one has a high electrical resistance and a of the participants in the reaction is bisulphate 74

reached. The effect is analogous to blast furnace A furnace for smelt- the phenomenon of supercooling, i.e. ing iron ores, such as haematite the cooling of a liquid below its (Fe2O3) or magnetite (Fe3O4), to make b freezing point without freezing. See *pig iron. The furnace is a tall refrac- also oscillating reaction. tory-lined cylindrical structure that is bisulphate See hydrogensulphate. charged at the top with the dressed ore (see beneficiation), coke, and a hydrogensulphite bisulphite See ; Ûux, usually limestone. The conver- aldehydes . sion of the iron oxides to metallic bite angle See chelate. iron is a reduction process in which carbon monoxide and hydrogen are bittern The solution of salts re- the reducing agents. The overall reac- maining when sodium chloride is tion can be summarized thus: crystallized from sea water. → Fe3O4 + 2CO + 2H2 3Fe + 2CO2 + bitumen See petroleum. 2H2O bituminous coal See coal. The CO is obtained within the fur- bituminous sand See oil sand. nace by blasting the coke with hot air from a ring of tuyeres about two- biuret test A biochemical test to thirds of the way down the furnace. detect proteins in solution, named The reaction producing the CO is: after the substance biuret (H2NCON- → 2C + O2 2CO HCONH2), which is formed when urea is heated. Sodium hydroxide is In most blast furnaces hydrocarbons mixed with the test solution and (oil, gas, tar, etc.) are added to the drops of 1% copper(II) sulphate solu- blast to provide a source of hydrogen. tion are then added slowly. A positive In the modern direct-reduction result is indicated by a violet ring, process the CO and H2 may be pro- caused by the reaction of *peptide duced separately so that the reduc- bonds in the proteins or peptides. tion process can proceed at a lower Such a result will not occur in the temperature. The pig iron produced presence of free amino acids. by a blast furnace contains about 4% carbon and further reÜning is usually bivalent (divalent) Having a va- required to produce steel or cast iron. lency of two. blasting gelatin Black, Joseph (1728–99) British A high explosive chemist and physician, born in made from nitroglycerine and gun France. He studied at Glasgow and cotton (cellulose nitrate). Edinburgh, where his thesis (1754) bleaching powder A white solid contained the Ürst accurate descrip- regarded as a mixture of calcium tion of the chemistry of carbon diox- chlorate(I), calcium chloride, and cal- ide. In 1757 he discovered latent cium hydroxide. It is prepared on a Ü heat, and was the rst to distinguish large scale by passing chlorine gas between heat and temperature. through a solution of calcium hy- blackdamp (choke damp) Air left droxide. Bleaching powder is sold on depleted in oxygen following the ex- the basis of available chlorine, which plosion of Üredamp in a mine. is liberated when it is treated with a dilute acid. It is used for bleaching black lead See carbon. paper pulps and fabrics and for steril- blanc Üxe See barium sulphate. izing water. 75 Bohr theory blende A naturally occurring metal German scientist J. Böhm. See alu- sulphide, e.g. zinc blende ZnS. minium hydroxide. Bloch’s theorem A theorem relat- Bohr, Niels Henrik David (1885– b ing to the *quantum mechanics of 1962) Danish physicist. In 1913 he crystals stating that the wave func- published his explanation of how tion ψ for an electron in a periodic atoms, with electrons orbiting a potential has the form ψ(r) = central nucleus, achieve stability by exp(ik•r)U(r), where k is the wave assuming that their angular momen- vector, r is a position vector, and U(r) tum is quantized. Movement of elec- is a periodic function that satisÜes trons from one orbit to another is U(r + R) = U(r), for all vectors R of the accompanied by the absorption or Bravais lattice of the crystal. Block’s emission of energy in the form of theorem is interpreted to mean that light, thus accounting for the series the wave function for an electron in of lines in the emission *spectrum of a periodic potential is a plane wave hydrogen. For this work Bohr was modulated by a periodic function. awarded the 1922 Nobel Prize for This explains why a free-electron physics. See bohr theory. model has some success in describing bohrium Symbol Bh. A radioactive the properties of certain metals al- *transactinide element; a.n. 107. It though it is inadequate to give a was Ürst made in 1981 by Peter Arm- quantitative description of the prop- bruster and a team in Darmstadt, erties of most metals. Block’s theo- Germany, by bombarding bismuth- rem was formulated by the 209 nuclei with chromium-54 nuclei. German-born US physicist Felix Only a few atoms of bohrium have en- Bloch (1905–83) in 1928. See also ever been detected. ergy band. A block See periodic table. • Information from the WebElements site block copolymer See polymer. Bohr magneton See magneton. blood pigment Any one of a Bohr theory The theory published group of metal-containing coloured in 1913 by Niels *Bohr to explain the protein compounds whose function line spectrum of hydrogen. He as- is to increase the oxygen-carrying ca- sumed that a single electron of mass pacity of blood. m travelled in a circular orbit of radius r, at a velocity v, around a posi- blue vitriol See copper(ii) tively charged nucleus. The angular sulphate. momentum of the electron would boat See ring conformations. then be mvr. Bohr proposed that electrons could only occupy orbits in boat conformation See confor- which this angular momentum had mation. certain Üxed values, h/2π, 2h/2π, BOD See biochemical oxygen de- 3h/2π,…nh/2π, where h is the Planck mand. constant. This means that the angular momentum is quantized, i.e. can body-centred cubic (b.c.c.) See only have certain values, each of cubic crystal. which is a multiple of n. Each permit- boehmite A mineral form of a ted value of n is associated with an mixed aluminium oxide and hydrox- orbit of different radius and Bohr as- ide, AlO.OH. It is named after the sumed that when the atom emitted boiling point 76

or absorbed radiation of frequency ν, boiling-point curve. The two curves the electron jumped from one orbit would coincide for an ideal mixture, to another; the energy emitted or ab- but generally they are different be- b sorbed by each jump is equal to hν. cause of deviations from *Raoult’s This theory gave good results in pre- law. In some cases, they may show a dicting the lines observed in the maximum or minimum and coincide spectrum of hydrogen and simple at some intermediate composition, ions such as He+, Li2+, etc. The idea of explaining the formation of quantized values of angular momen- *azeotropes. tum was later explained by the wave boiling-point elevation See nature of the electron. Each orbit has elevation of boiling point. to have a whole number of wavelengths around it; i.e. nλ = 2πr, where Boltzmann, Ludwig Eduard λ is the wavelength and n a whole (1844–1906) Austrian physicist. He number. The wavelength of a particle held professorships in Graz, Vienna, is given by h/mv, so nh/mv = 2πr, Munich, and Leipzig, where he which leads to mvr = nh/2π. Modern worked on the kinetic theory of atomic theory does not allow sub- gases (see maxwell–boltzmann dis- atomic particles to be treated in the tribution) and on thermodynamics same way as large objects, and Bohr’s (see boltzmann equation). He suf- reasoning is somewhat discredited. fered from depression and commit- However, the idea of quantized angu- ted suicide. lar momentum has been retained. Boltzmann constant Symbol k. boiling point (b.p.) The tempera- The ratio of the universal gas con- ture at which the saturated vapour stant (R) to the Avogadro constant pressure of a liquid equals the exter- (NA). It may be thought of therefore nal atmospheric pressure. As a conse- as the gas constant per molecule: quence, bubbles form in the liquid × –23 –1 k = R/NA = 1.380 658(12) 10 JK and the temperature remains con- It is named after Ludwig *Boltzmann. stant until all the liquid has evaporated. As the boiling point of a liquid Boltzmann equation An equation depends on the external atmospheric used in the study of a collection of pressure, boiling points are usually particles in *nonequilibrium statisti- quoted for standard atmospheric cal mechanics, particularly their pressure (760 mmHg = 101 325 Pa). transport properties. The Boltzmann equation describes a quantity called boiling-point–composition dia- the distribution function, f, which gram A graph showing how the gives a mathematical description of boiling point and vapour composi- the state and how it is changing. The tion of a mixture of two liquids de- distribution function depends on a pends on the composition of the position vector r, a velocity vector v, mixture. The abscissa shows the and the time t; it thus provides a sta- range of compositions from 100% A tistical statement about the positions at one end to 100% B at the other. and velocities of the particles at any The diagram has two curves: the time. In the case of one species of lower one gives the boiling points (at particle being present, Boltzmann’s a Üxed pressure) for the different equation can be written compositions. The upper one is plot- ∂ ∂ a ∂ ∂v v ∂ ∂r ∂ ∂ ted by taking the composition of f/ t + .( f/ ) + .( f/ ) = ( f/ t)coll, vapour at each temperature on the where a is the acceleration of bodies 77 borane

∂ ∂ between collisions and ( f/ t)coll is the calculated in this way are called aver- rate of change of f(r,v,t) due to colli- age bond energies or bond–energy sions. The Boltzmann equation can terms. They depend to some extent be used to calculate *transport on the molecule chosen; the C–H b coefÜcients, such as conductivity. The bond energy in methane will differ equation was proposed by Ludwig slightly from that in ethane. The *Boltzmann in 1872. bond dissociation energy is a different measurement, being the energy Boltzmann formula An equation required to break a particular bond; concerning the entropy S of a system e.g. the energy for the process: that derives from statistical mechan- → • • ics. It states that entropy is related to CH4(g) CH3 (g) + H (g) the number W of distinguishable bond enthalpy See bond energy. ways in which the equation S = k lnW, where k is the Boltzmann con- bonding orbital See orbital. stant, can describe the system. It ex- bond order A value indicating the presses in quantitative terms the degree of bonding between two concept that entropy is a measure of atoms in a molecule relative to a sin- the disorder of a system. It was dis- gle bond. Bond orders are theoretical covered in the late 19th century by values depending on the way the cal- Ludwig *Boltzmann while he was culation is done. For example, in studying statistical mechanics. ethane the bond order of the carbon- bomb calorimeter An apparatus carbon bond is 1. In ethene, the bond used for measuring heats of combus- order is 2. In benzene the bond order tion (e.g. caloriÜc values of fuels and as calculated by molecular orbital foods). It consists of a strong con- theory is 1.67. tainer in which the sample is sealed bone black See charcoal. with excess oxygen and ignited electrically. The heat of combustion at borane (boron hydride) Any of a constant volume can be calculated group of compounds of boron and from the resulting rise in tempera- hydrogen, many of which can be pre- ture. pared by the action of acid on magnesium boride (MgB2). Others are bond See chemical bond. made by pyrolysis of the products of bond dissociation energy See this reaction in the presence of hy- bond energy. drogen and other reagents. They are all volatile, reactive, and oxidize bond energy An amount of energy readily in air, some explosively so. associated with a bond in a chemical The boranes are a remarkable group compound. It is obtained from the of compounds in that their structures heat of atomization. For instance, in cannot be described using the con- methane the bond energy of the C–H ventional two-electron covalent bond bond is one quarter of the enthalpy model (see electron-deficient com- of the process pound). The simplest example is di- CH (g) → C(g) + 4H(g) 4 H H H Bond energies (or bond enthalpies) can be calculated from the standard BB enthalpy of formation of the com- H H pound and from the enthalpies of at- H omization of the elements. Energies Borane borate 78

borane (B2H6): see formula. Other bo- hedral BO3(OH) group. ‘Hydrated’ boranes include B4H10, B5H9, B5H11, rates are ones containing –OH B6H10, and B10H4. The larger borane groups; many examples occur natu- b molecules have open or closed poly- rally. Anhydrous borates, which con- hedra of boron atoms. In addition, tain BO3 groups, can be made by there is a wide range of borane deriv- melting together boric acid and atives containing atoms of other el- metal oxides. ements, such as carbon and borax (disodium tetraborate-10- phosphorus. Borohydride ions of the water) A colourless monoclinic solid, type B H 2– also exist. Boranes and 6 6 Na B O .10H O, soluble in water and borohydride ions are classiÜed ac- 2 4 7 2 very slightly soluble in ethanol; mon- cording to their structure. Those oclinic; r.d. 1.73; loses 8H O at 75°C; with a complete polyhedron are said 2 loses 10H O at 320°C. The formula to have a closo-structure. Those in 2 gives a misleading impression of the which the polyhedron is incomplete structure. The compound contains by loss of one vertex have a nido- the ion [B O (OH) ]2– (see borate). structure (from the Greek for ‘nest’). 4 5 4 Attempts to recrystallize this com- Those with open structures by re- pound above 60.8°C yield the penta- moval of two or more vertices have hydrate. The main sources are the an arachno structure (from the Greek borate minerals kernite (Na B O . for ‘spider’). See also wade’s rules. 2 4 7 4H2O) and tincal (Na2B4O7.10H2O). borate Any of a wide range of ionic The ores are puriÜed by carefully compounds that have negative ions controlled dissolution and recrystal- containing boron and oxygen (see lization. On treatment with mineral formulae). Lithium borate, for exam- acids borax gives boric acid. ple, contains the simple anion Borax is a very important sub- – B(OH)4 . Most borates, however, are stance in the glass and ceramics in- inorganic polymers with rings, dustries as a raw material for making chains, or other networks based on borosilicates. It is also important as a the planar BO3 group or the tetra- metallurgical Ûux because of the abil-

O– OO– BB

OO B

O– OH B O3– as in Na B O 3 6 3 3 6 B– O O O– HO B O B OH B O– O O O B O– B– O B OH

n– 2– (BO2)n as in CaB2O4 [B4O5(OH)4] as in borax Na2B4O7.10H2O

Borate 79 boride ity of molten borates to dissolve In the solid there is considerable metal oxides. In solution it partially hydrogen bonding between H3BO3 hydrolyses to boric acid and can thus molecules resulting in a layer struc- act as a buffer. For this reason it is ture, which accounts for the easy b used as a laundry pre-soak. It is used cleavage of the crystals. H3BO3 mol- medicinally as a mild alkaline anti- ecules also exist in dilute solutions septic and astringent for the skin and but in more concentrated solutions mucous membranes. polymeric acids and ions are formed Disodium tetraborate is the source (e.g. H4B2O7; pyroboric acid or of many industrially important boron tetrahydroxomonoxodiboric(III) acid). compounds, such as barium borate The compound is a very weak acid (fungicidal paints), zinc borate (Üre- but also acts as a Lewis *acid in ac- retardant additive in plastics), and cepting hydroxide ions: boron phosphate (heterogeneous acid ˆ – + B(OH)3 + H2O B(OH)4 + H catalyst in the petrochemicals indus- If solid boric acid is heated it loses try). water and transforms to another acid borax-bead test A simple labora- at 300°C. This is given the formula tory test for certain metal ions in HBO2 but is in fact a polymer salts. A small amount of the salt is (HBO2)n. It is called metaboric acid or, mixed with borax and a molten bead technically, polydioxoboric(III) acid. formed on the end of a piece of plat- Boric acid is used in the manufac- inum wire. Certain metals can be ture of glass (borosilicate glass), identiÜed by the colour of the bead glazes and enamels, leather, paper, produced in the oxidizing and reduc- adhesives, and explosives. It is widely ing parts of a Bunsen Ûame. For ex- used (particularly in the USA) in de- ample, iron gives a bead that is red tergents, and because of the ability of when hot and yellow when cold in fused boric acid to dissolve other the oxidizing Ûame and a green bead metal oxides it is used as a Ûux in in the reducing Ûame. brazing and welding. Because of its mild antiseptic properties it is used borazon See boron nitride. in the pharmaceutical industry and Bordeaux mixture A mixture of as a food preservative. copper(II) sulphate and calcium hy- boric anhydride See boron(iii) droxide in water, used as a fungicide. oxide. boric acid Any of a number of boric oxide See boron(iii) oxide. acids containing boron and oxygen. Used without qualiÜcation the term boride A compound of boron with applies to the compound H3BO3 a metal. Most metals form at least (which is also called orthoboric acid one boride of the type MB, MB2, MB4, or, technically, trioxoboric(III) acid). MB6, or MB12. The compounds have a This is a white or colourless solid variety of structures; in particular, that is soluble in water and ethanol; the hexaborides contain clusters of ° triclinic; r.d. 1.435; m.p. 169 C. It oc- B6 atoms. The borides are all hard curs naturally in the condensate high-melting materials with metal- from volcanic steam vents (sufÜoni). like conductivity. They can be made Commercially, it is made by treating by direct combination of the el- borate minerals (e.g. kernite, ements at high temperatures (over ° Na2B4O7.4H2O) with sulphuric acid 2000 C) or, more usually, by high- followed by recrystallization. temperature reduction of a mixture Born, Max 80

→ + ∆ of the metal oxide and boron oxide M(g) M (g) + e H3 using carbon or aluminium. Chemi- This is obtained from the ionization b cally, they are stable to nonoxidizing potential. acids but are attacked by strong oxi- (4) Ionization of the nonmetal: dizing agents and by strong alkalis. → – ∆ X(g) + e X (g) H4 Magnesium boride (MgB2) is unusual in that it can be hydrolysed to bo- This is the electron afÜnity. ranes. Industrially, metal borides are (5) Formation of the ionic solids: + – → + – ∆ used as refractory materials. The M (g) + X (g) M X (s) HL most important are CrB, CrB2, TiB2, Equating the enthalpies gives: and ZnB . Generally, they are fabri- 2 ∆H = ∆H + ∆H + ∆H + ∆H + ∆H cated using high-temperature pow- f 1 2 3 4 L ∆ der metallurgy, in which the article from which HL can be found. It is is produced in a graphite die at over named after the German physicist 2000°C and at very high pressure. Max Born (1882–1970) and Fritz Items are pressed as near to Ünal *Haber. shape as possible as machining re- bornite An important ore of copper quires diamond cutters and is ex- composed of a mixed copper–iron tremely expensive. sulphide, Cu5FeS4. Freshly exposed Born, Max (1882–1970) German- surfaces of the mineral are a metallic born British physicist who was reddish-brown but a purplish irides- awarded the 1954 Nobel Prize for cent tarnish soon develops – hence it physics (with W. Bothe) for his work is popularly known as peacock ore. on statistical mechanics. With Bornite is mined in Chile, Peru, Bo- *Heisenberg he also developed ma- livia, Mexico, and the USA. trix mechanics. Born–Oppenheimer approxima- Born–Haber cycle A cycle of reaction An *adiabatic approximation tions used for calculating the lattice used in molecular and solid-state energies of ionic crystalline solids. physics in which the motion of For a compound MX, the lattice en- atomic nuclei is taken to be so much ergy is the enthalpy of the reaction slower than the motion of electrons M+(g) + X–(g) → M+X–(s) ∆H that, when calculating the motions L of electrons, the nuclei can be taken The standard enthalpy of formation to be in Üxed positions. This approxi- of the ionic solid is the enthalpy of mation was justiÜed using perturba- the reaction tion theory by Max Born and the US ½ → + – ∆ M(s) + X2(g) M X (s) Hf physicist Julius Robert Oppenheimer The cycle involves equating this en- (1904–67) in 1927. thalpy (which can be measured) to borohydride ions See borane. the sum of the enthalpies of a number of steps proceeding from the el- boron Symbol B. An element of ements to the ionic solid. The steps *group 13 (formerly IIIB) of the peri- are: odic table; a.n. 5; r.a.m. 10.81; r.d. (1) Atomization of the metal: 2.34–2.37 (amorphous); m.p. 2300°C; → ∆ b.p. 2550°C. It forms two allotropes; M(s) M(g) H1 amorphous boron is a brown powder (2) Atomization of the nonmetal: but metallic boron is black. The ½ → ∆ X2(g) X(g) H2 metallic form is very hard (9.3 on (3) Ionization of the metal: Mohs’ scale) and is a poor electrical 81 borosilicate conductor at room temperature. At posed by hot water; r.d. 2.25 (hexago- least three crystalline forms are pos- nal); sublimes above 3000°C. Boron sible; two are rhombohedral and the nitride is manufactured by heating other tetragonal. The element is boron oxide to 800°C on an acid-solu- b never found free in nature. It occurs ble carrier, such as calcium phos- as orthoboric acid in volcanic springs phate, in the presence of nitrogen or in Tuscany, as borates in kernite ammonia. It is isoelectronic with car- (Na2B4O7.4H2O), and as colemanite bon and, like carbon, it has a very (Ca2B6O11.5H2O) in California. Sam- hard cubic form (borazon) and a ples usually contain isotopes in the softer hexagonal form; unlike ratio of 19.78% boron–10 to 80.22% graphite this is a nonconductor. It is boron–11. Extraction is achieved by used in the electrical industries vapour-phase reduction of boron where its high thermal conductivity trichloride with hydrogen on electri- and high resistance are of especial cally heated Ülaments. Amorphous value. boron can be obtained by reducing boron(III) oxide (boric anhydride the trioxide with magnesium pow- ; der. Boron when heated reacts with boric oxide; diboron trioxide) A oxygen, halogens, oxidizing acids, glassy solid, B2O3, that gradually ab- and hot alkalis. It is used in semicon- sorbs water to form boric acid. It has ductors and in Ülaments for special- some amphoteric characteristics and ized aerospace applications. forms various salts. Amorphous boron is used in Ûares, boron trichloride A colourless giving a green coloration. The iso- fuming liquid, BCl3, which reacts tope boron–10 is used in nuclear re- with water to give hydrogen chloride actor control rods and shields. The and boric acid; r.d. 1.349; m.p. element was discovered in 1808 by –107°C; b.p. 12.5°C. Boron trichloride Sir Humphry *Davy and by J. L. *Gay- is prepared industrially by the Lussac and L. J. Thenard. exothermic chlorination of boron A carbide at above 700°C, followed by • Information from the WebElements site fractional distillation. An alternative, but more expensive, laboratory boron carbide A black solid, B4C, method is the reaction of dry chlo- soluble only in fused alkali; it is ex- ½ rine with boron at high temperature. tremely hard, over 9 on Mohs’ Boron trichloride is a Lewis *acid, scale; rhombohedral; r.d. 2.52; m.p. ° > ° forming stable addition compounds 2350 C; b.p. 3500 C. Boron carbide with such donors as ammonia and is manufactured by the reduction of the amines and is used in the labora- boric oxide with petroleum coke in tory to promote reactions that liber- an electric furnace. It is used largely ate these donors. The compound is as an abrasive, but objects can also important industrially as a source of be fabricated using high-temperature pure boron (reduction with hydro- powder metallurgy. Boron nitride is gen) for the electronics industry. It is also used as a neutron absorber be- also used for the preparation of bo- cause of its high proportion of ranes by reaction with metal hy- boron–10. drides. boron hydride See borane. borosilicate Any of a large number boron nitride A solid, BN, insolu- of substances in which BO3 and SiO4 ble in cold water and slowly decom- units are linked to form networks Bosch process 82

with a wide range of structures. A Borosilicate glasses are particularly • Information on colour coding of gas important; the addition of boron to containers b the silicate network enables the glass to be fused at lower temperatures bound state A system in which than pure silica and also extends the two (or more) parts are bound to- plastic range of the glass. Thus such gether in such a way that energy is glasses as Pyrex have a wider range required to split them. An example of applications than soda glasses (nar- of a bound state is a *molecule row plastic range, higher thermal ex- formed from two (or more) *atoms. pansion) or silica (much higher bowsprit See ring conformations. melting point). Borosilicates are also used in glazes and enamels and in Boyle, Robert (1627–91) English the production of glass wools. chemist and physicist, born in Ire- land. After moving to Oxford in 1654 Bosch process See haber process. he worked on gases, using an air Bose–Einstein statistics See pump made by Robert Hooke. In quantum statistics. 1662 he discovered *Boyle’s law. In chemistry he worked on *Ûame tests boson A particle or system that and acid-base *indicators. Boyle is obeys Bose–Einstein statistics (see quantum statistics). Combining generally regarded as the person who quantum mechanics with special rel- established chemistry as a modern subject, distinct from alchemy. He ativity theory gives a result that a Ü Ü boson has to have an integer spin. A was the rst to give a de nition of a *photon is an example of a boson. chemical element. bottled gas Gas supplied under Boyle’s law The volume (V) of a pressure in metal cylinders. The term given mass of gas at a constant tem- includes pressurized gas (e.g. oxygen perature is inversely proportional to and nitrogen cylinders) and gases its pressure (p), i.e. pV = constant. liqueÜed under pressure (e.g. liquid This is true only for an *ideal gas. butane for use as a fuel). Colour con- This law was discovered in 1662 by ventions are used to identify the type Robert *Boyle. On the continent of of gas or, in some cases, the speciÜc Europe it is known as Mariotte’s law gas. The colour indicating the con- after E. Mariotte (1620–84), who dis- tents is that of the shoulder of the covered it independently in 1676. See cylinder at the top. The convention is also gas laws. not international, and practice differs Brackett series See hydrogen in different countries. In the UK, the spectrum. convention is: Yellow for toxic or corrosive gases Bragg, Sir William Henry Red for Ûammable gases (1862–1942) British physicist, who Light blue for oxidizing gases with his son Sir William Lawrence Maroon for acetylene Bragg (1890–1971), was awarded the Dark green for argon 1915 Nobel Prize for physics for their Grey for carbon dioxide pioneering work on X-ray crystallog- Brown for helium raphy. He also constructed an X-ray Blue for nitrous oxide spectrometer for measuring the Black for nitrogen wavelengths of X-rays. In the 1920s, White for oxygen while director of the Royal Institu- 83 brighteners tion in London, he initiated X-ray dif- range of wavelengths down to a min- fraction studies of organic molecules. imum value, which depends on the energy of the incident particles. Bragg peak A peak in the scatter- Bremsstrahlung are produced by a b ing pattern in X-ray diffraction of a metal target when it is bombarded crystal. The intensity of Bragg peaks is proportional to the square of the by electrons. number of the scatterers. If X-ray brewing The process by which scattering from a solid produces beer is made. Fermentation of sugars Bragg peaks this indicates that the from barley grain by the yeasts Sac- solid has long-range order. Bragg charomyces cerevisiae and S. uvarum (or peaks are named after Sir William S. carlsbergenesis) produces alcohol. In Lawrence *Bragg, who discovered the Ürst stage the barley grain is them in 1912. The scattering of X- soaked in water, a process known as rays from a set of planes in a crystal malting. The grain is then allowed to that gives rise to Bragg peaks is germinate and the natural enzymes called Bragg scattering. of the grain (the amylases and the Bragg’s law When a beam of X- maltases) convert the starch to mal- rays (wavelength λ) strikes a crystal tose and then to glucose. The next surface in which the layers of atoms stage is kilning or roasting, in which or ions are separated by a distance d, the grains are dried and crushed. The the maximum intensity of the colour of a beer depends on the tem- reÛected ray occurs when sinθ = perature used for this process: the nλ/2d, where θ (known as the Bragg higher the temperature, the darker angle) is the complement of the the beer. In the next stage, mashing, angle of incidence and n is an inte- the crushed grain is added to water Ü ger. The law enables the structure of at a speci c temperature and any re- many crystals to be determined. It maining starch is converted to sugar; was discovered in 1912 by Sir the resultant liquid is the raw ma- William Lawrence Bragg. terial of brewing, called wort. The yeast is then added to the wort to chain branched chain See . convert the sugar to alcohol, fol- brass A group of alloys consisting lowed by hops, which give beer its of copper and zinc. A typical yellow characteristic Ûavour. Hops are the brass might contain about 67% cop- female Ûowers of the vine Humulus per and 33% zinc. lupulus; they contain resins (humu- Ü lones, cohumulones, and adhumu- Bravais lattice An in nite array of lones) that give beer its distinctive lattice points. A Bravais lattice can bitter taste. have only fourteen space groups divided into seven crystal systems. It is bridge An atom or group joining named after Auguste Bravais two other atoms in a molecule. See (1811–63). aluminium chloride; borane. Bremsstrahlung (German: braking brighteners Substances added to radiation) The *X-rays emitted when detergents or used to treat textiles or a charged particle, especially a fast paper in order to brighten the electron, is rapidly slowed down, as colours or, particularly, to enhance when it passes through the electric whiteness. Blueing agents are used in Üeld around an atomic nucleus. The laundries to give a slight blue cast to X-rays cover a whole continuous white material in order to counteract Brillouin zone 84

yellowing. Fluorescent brighteners bromate A salt or ester of a bromic are compounds that absorb visible or acid. ultraviolet radiation and Ûuoresce in bromic(I) acid (hypobromous acid) b the blue region of the optical spec- A yellow liquid, HBrO. It is a weak trum. acid but a strong oxidizing agent. Brillouin zone A cell in a recipro- bromic(V) acid A colourless liquid, cal lattice. The Ürst Brillouin zone is HBrO , made by adding sulphuric the cell of smallest volume enclosed 3 acid to barium bromate. It is a strong by those planes that are perpendicu- acid. lar bisectors of reciprocal lattice vectors. Higher Brillouin zones also bromide See halide. exist. Brillouin zones are used in the bromination A chemical reaction theory of energy levels in a periodic in which a bromine atom is intro- potential, as in a crystal. They are duced into a molecule. See also halo- named after the French physicist genation. Léon Brillouin (1889–1969), who introduced them into the theory of bromine Symbol Br. A *halogen el- crystals in 1930. ement; a.n. 35; r.a.m. 79.909; r.d. 3.13; m.p. –7.2°C; b.p. 58.78°C. It is a Brinell hardness A scale for meas- red volatile liquid at room tempera- uring the hardness of metals intro- ture, having a red-brown vapour. duced around 1900 by the Swedish Bromine is obtained from brines in metallurgist Johann Brinell (1849– 1925). A small chromium-steel ball is the USA (displacement with chlo- pressed into the surface of the metal rine); a small amount is obtained by a load of known weight. The ratio from sea water in Anglesey. Large of the mass of the load in kilograms quantities are used to make 1,2-di- to the area of the depression formed bromoethane as a petrol additive. It in square millimetres is the Brinell is also used in the manufacture of number. many other compounds. Chemically, it is intermediate in reactivity be- Brin process A process formerly tween chlorine and iodine. It forms used for making oxygen by heating compounds in which it has oxidation barium oxide in air to form the per- states of 1, 3, 5, or 7. The liquid is oxide and then heating the peroxide harmful to human tissue and the at higher temperature (>800°C) to vapour irritates the eyes and throat. produce oxygen The element was discovered in 1826 → by Antoine Balard. 2BaO2 2BaO + O2 A Britannia metal A silvery alloy • Information from the WebElements site consisting of 80–90% tin, 5–15% antimony, and sometimes small percent- bromoethane (ethyl bromide) A Û ages of copper, lead, and zinc. It is colourless ammable liquid, C2H5Br; used in bearings and some domestic r.d. 1.46; m.p. –119°C; b.p. 38.4°C. It articles. is a typical *haloalkane, which can be prepared from ethene and hydro- British thermal unit (Btu) The Im- gen bromide. Bromoethane is used as perial unit of heat, being originally a refrigerant. the heat required to raise the temperature of 1lb of water by 1°F. 1 Btu bromoform See is now deÜned as 1055.06 joules. tribromomethane; haloforms. 85 brusselator bromomethane (methyl bromide) copper and metals other than tin are A colourless volatile nonÛammable called bronzes – aluminium bronze is ° liquid, CH3Br; r.d. 1.68; m.p. –93 C; a mixture of copper and aluminium. b.p. 3.56°C. It is a typical *halo- Other special bronzes include *bell b alkane. metal, *gun metal, and *beryllium bronze. N-bromosuccinimide (NBS) A crystalline solid, C4O2NBr, used ex- Brownian movement The contin- tensively as a reagent for elec- uous random movement of micro- trophilic addition of bromine. It acts scopic solid particles (of about 1 by producing a small constant supply micrometre in diameter) when sus- of bromine in solution pended in a Ûuid medium. First ob- + – → C4O2NBr + H + Br C4O2NH + served by the British botanist Robert Br2. Brown (1773–1858) in 1827 when studying pollen particles, it was origi- Br nally thought to be the manifestation of some vital force. It was later recog- O N O nized to be a consequence of bombardment of the particles by the continually moving molecules of the N-bromosuccinimide liquid. The smaller the particles the more extensive is the motion. The effect is also visible in particles of smoke suspended in a still gas. bromothymol blue An acid–base *indicator that is yellow in acid solu- A tions and blue in alkaline solutions. • Robert Brown’s original paper It changes colour over the pH range • Jean Perrin’s paper 6–8. brown-ring test A test for ionic ni- Brønsted, Johannes Nicolaus trates. The sample is dissolved and (1879–1947) Danish physical chemist. iron(II) sulphate solution added in a He worked on thermochemistry and test tube. Concentrated sulphuric electrochemistry and is best known acid is then added slowly so that it for the Lowry–Brønsted theory of forms a separate layer. A brown ring *acids and bases, which he proposed (of Fe(NO)SO4) at the junction of the (independently of Lowry) in 1923. liquids indicates a positive result. Brønsted acid See acid. brucite A mineral form of *magnesium hydroxide, Mg(OH) . Brønsted base See acid. 2 brusselator A type of chemical re- bronze Any of a group of alloys of action mechanism that leads to an copper and tin, sometimes with lead *oscillating reaction. It involves the and zinc present. The amount of tin conversion of reactants A and B into varies from 1% to 30%. The alloy is products C and B by a series of four hard and easily cast and extensively steps: used in bearings, valves, and other → machine parts. Various improved A X bronzes are produced by adding 2X + Y → 3Y other elements; for instance, phos- B + X → Y + C phor bronzes contain up to 1% phosphorus. In addition certain alloys of X → D Buchner funnel 86

Autocatalysis occurs as in the by trapping metal ions within the C60 *Lotka–Volterra mechanism and the cage. Some of these have semicon- *oregonator. If the concentrations of ducting properties. The electric-arc b A and B are maintained constant, the method of producing C60 also leads concentrations of X and Y oscillate to a smaller number of fullerenes with time. A graph of the concentra- such as C70, which have less symmet- tion of X against that of Y is a closed rical molecular structures. It is also loop (the limit cycle of the reaction). possible to produce forms of carbon The reaction settles down to this in which the atoms are linked in a limit cycle whatever the initial con- cylindrical, rather than spherical, centrations of X and Y, i.e. the limit framework with a diameter of a few cycle is an *attractor for the system. nanometres. They are known as The reaction mechanism is named buckytubes (or nanotubes). after the city of Brussels, where the A research group that discovered it is • Information about IUPAC nomenclature based. and representation of fullerenes and Buchner funnel A type of funnel related compounds with an internal perforated tray on which a Ûat circular Ülter paper can be placed, used for Ültering by suc- tion. It is named after the German chemist Eduard Buchner (1860– 1917). buckminsterfullerene A form of carbon composed of clusters of 60 carbon atoms bonded together in a polyhedral structure composed of pentagons and hexagons (see illustration). Originally it was identiÜed in 1985 in products obtained by Üring a high-power laser at a graphite target. It can be made by an electric arc struck between graphite electrodes Buckminsterfullerene in an inert atmosphere. The molecule, C60, was named after the US buckyball See buckminster- architect Richard Buckminster Fuller fullerene. (1895–1983) because of the resemblance of the structure to the geo- buckytube See buckminster- desic dome, which Fuller invented. fullerene. The molecules are informally called buffer A solution that resists buckyballs; more formally, the substance itself is also called fullerene. change in pH when small amounts of The substance is a yellow crystalline an acid or alkali are added over a cer- solid (fullerite), soluble in benzene. tain range or when the solution is di- Various fullerene derivatives are luted. Acidic buffers consist of a known in which organic groups are weak acid with a salt of the acid. The attached to carbon atoms on the salt provides the negative ion A–, sphere. In addition, it is possible to which is the conjugate base of the produce novel enclosure compounds acid HA. An example is carbonic acid 87 Bunsen cell and sodium hydrogencarbonate. buna rubber A type of synthetic Basic buffers have a weak base and rubber based in polymerization of a salt of the base (to provide the butadiene (buta-1,3-diene). The name conjugate acid). An example is am- comes from Bu (for butadiene) and b monia solution with ammonium Na (for sodium, which was used as a chloride. catalyst in the original polymeriza- In an acidic buffer, for example, tion reaction). An improved form, molecules HA and ions A– are pre- known as Buna-S was developed by sent. When acid is added most of the copolymerizing butadiene with extra protons are removed by the styrene. In 1934, Buna-N was in- base: vented, in which the styrene was re- A– + H+ → HA placed by acrylonitrile, giving a product with better oil resistance (see When base is added, most of the nitrile rubber). extra hydroxide ions are removed by reaction with undissociated acid: Bunsen, Robert Wilhelm OH– + HA → A– + H O (1811–99) German chemist, who held 2 professorships at Kassel, Marburg, Thus, the addition of acid or base and Heidelberg. His early researches changes the pH very little. The hy- on arsenic-containing compounds drogen-ion concentration in a buffer cost him an eye in an explosion. He is given by the expression then turned to gas analysis and spec- + – Ka = [H ] = [A ]/[HA] troscopy, enabling him and *Kirch- i.e. it depends on the ratio of conju- hoff to discover the elements gate base to acid. As this is not al- *caesium (1860) and *rubidium tered by dilution, the hydrogen-ion (1861). He also popularized the use of concentration for a buffer does not the *Bunsen burner and developed change much during dilution. the *Bunsen cell. In the laboratory, buffers are used Bunsen burner A laboratory gas to prepare solutions of known stable burner having a vertical metal tube pH. Natural buffers occur in living or- into which the gas is led, with a hole ganisms, where the biochemical re- in the side of the base of the tube to actions are very sensitive to change admit air. The amount of air can be in pH. The main natural buffers are regulated by a sleeve on the tube. – – 2– H2CO3/HCO3 and H2PO4 /HPO4 . When no air is admitted the Ûame is Buffer solutions are also used in luminous and smoky. With air, it has medicine (e.g. in intravenous injec- a faintly visible hot outer part (the tions), in agriculture, and in many oxidizing part) and an inner blue industrial processes (e.g. dyeing, cone where combustion is incom- fermentation processes, and the plete (the cooler reducing part of the food industry). Ûame). The device is named after bumping Violent boiling of a liquid Robert *Bunsen, who used a similar caused by superheating so that bub- device (without a regulating sleeve) bles form at a pressure above atmos- in 1855. pheric pressure. It can be prevented Bunsen cell A *primary cell de- by putting pieces of porous pot in vised by Robert *Bunsen consisting the liquid to enable bubbles of of a zinc cathode immersed in dilute vapour to form at the normal boil- sulphuric acid and a carbon anode ing point. immersed in concentrated nitric acid. burette 88

The electrolytes are separated by a occurs in living organisms as an in- porous pot. The cell gives an e.m.f. of termediate in metabolism, especially b about 1.9 volts. in the *Krebs cycle. burette A graduated glass tube with a tap at one end leading to a OH H Üne outlet tube, used for delivering 2 O C known volumes of a liquid (e.g. in C O titration). H2 buta-1,3-diene (butadiene) A OH colourless gaseous hydrocarbon, Butanedioic acid ° CH2:CHCH:CH2; m.p. –109 C; b.p. ° –4.5 C. It is made by catalytic dehy- butanoic acid (butyric acid) A drogenation of butane (from petro- colourless liquid water-soluble acid, leum or natural gas) and polymerized ° C3H7COOH; r.d. 0.96; b.p. 163 C. It is in the production of synthetic rub- × –5 –3 a weak acid (Ka = 1.5 10 mol dm bers. The compound is a conjugated at 25°C) with a rancid odour. Its es- *diene in which the electrons in the ters are present in butter and in pi orbitals are partially delocalized human perspiration. The acid is used over the whole molecule. It can have to make esters for Ûavourings and trans and cis forms, the latter taking perfumery. part in *Diels–Alder reactions. butanol Either of two aliphatic butanal (butyraldehyde) A colour- alcohols with the formula C4H9OH. less Ûammable liquid aldehyde, Butan-1-ol, CH (CH ) OH, is a ° 3 2 3 C3H7CHO; r.d. 0.8; m.p. –99 C; b.p. primary alcohol; r.d. 0.81; m.p. 75.7°C. –89.5°C; b.p. 117.3°C. Butan-2-ol, butane A gaseous hydrocarbon, CH3CH(OH)C2H5, is a secondary al- –3 ° cohol; r.d. 0.81; m.p. –114.7°C; b.p. C4H10; d. 0.58 g cm ; m.p. –138 C; ° b.p. 0°C. Butane is obtained from 100 C. Both are colourless volatile petroleum (from reÜnery gas or liquids obtained from butane and are by cracking higher hydrocarbons). used as solvents. The fourth member of the *alkane butanone (methyl ethyl ketone) A series, it has a straight chain of colourless Ûammable water-soluble carbon atoms and is isomeric with liquid, CH3COC2H5; r.d. 0.8; m.p. 2-methylpropane (CH3CH(CH3)CH3, –86.4°C; b.p. 79.6°C. It can be made formerly called isobutane). It can eas- by the catalytic oxidation of butane ily be liqueÜed under pressure and is and is used as a solvent. supplied in cylinders for use as a fuel butene (butylene) Either of two gas. It is also a raw material for mak- isomers with the formula C H : ing buta-1,3-diene (for synthetic rub- 4 8 1-butene (CH CH CH:CH ), which is ber). 3 2 2 made by passing 1-butanol vapour butanedioic acid (succinic acid) A over heated alumina, and but-2-ene colourless crystalline fatty acid, (CH3CH:CHCH3), which is made by ° (CH2)2(COOH)2; r.d. 1.6; m.p. 185 C; heating 2-butanol with sulphuric b.p. 235°C. A weak carboxylic acid, it acid. They are unpleasant-smelling is produced by fermentation of sugar gases used in the manufacture of or ammonium tartrate and used as a polymers. The isomer 2-methyl- sequestrant and in making dyes. It propane ((CH3)2C:CH2) was formerly 89 B–Z reaction called isobutene or isobutylene. See je = jce = jae, alkenes. where jce is the equilibrium cathode butenedioic acid Either of current and jae is the equilibrium b two isomers with the formula anode current. F is the Faraday con- HCOOHC:CHCOOH. Both com- stant, η is the overpotential, R is the pounds can be regarded as deriva- gas constant, T is the thermodynamic tives of ethene in which a hydrogen temperature, and α is a quantity atom on each carbon has been re- called the *transfer coefÜcient. placed by a –COOH group. The com- butterÛy effect See chaos. pounds show cis–trans isomerism. The trans form is fumaric acid (r.d. butylene See butene. ° 1.64; sublimes at 165 C) and the cis butyl group The organic group form is maleic acid (r.d. 1.59; m.p. CH (CH ) –. 139–140°C). Both are colourless crys- 3 2 3 talline compounds used in making butyl rubber A type of synthetic synthetic resins. The cis form is rubber obtained by copolymerizing rather less stable than the trans form 2-methylpropene (CH2:C(CH3)CH3; and converts to the trans form at isobutylene) and methylbuta-1,3- ° 120 C. Unlike the trans form it can diene (CH2:C(CH3)CH:CH2, isoprene). eliminate water on heating to form a Only small amounts of isoprene cyclic anhydride containing a (about 2 mole %) are used. The rub- –CO.O.CO– group (maleic anhydride). ber can be vulcanized. Large Fumaric acid is an intermediate in amounts were once used for tyre the *Krebs cycle. inner tubes. butyraldehyde See butanal. butyric acid See butanoic acid. by-product A compound formed during a chemical reaction at the same time as the main product. Com- cis-butenedioic acid trans-butenedioic acid mercially useful by-products are ob- (maleic acid) (fumaric acid) tained from a number of industrial Butenedioic acid processes. For example, calcium chloride is a by-product of the *Solvay process for making sodium carbon- Butler–Volmer equation An ate. Propanone is a by-product in the equation for the rate of an electro- manufacture of *phenol. chemical reaction; it describes the benzoylecgonine current density at an electrode in BZ See . terms of the overpotential. The But- B–Z reaction (Belousov–Zhabotin- ler–Volmer equation is given by: skii reaction) A chemical reaction α η that shows a periodic colour change j = ja – jc = je[exp(1 – )F /RT – exp(– αF/RT)], between magenta and blue with a period of about one minute. It occurs where ja and jc are the individual with a mixture of sulphuric acid, cathode and anode currents respec- potassium bromate(V), cerium sul- tively, and je is the equilibrium cur- phate, and propanedioic acid. The rent, called the exchange current colour change is caused by alternat- density. By deÜnition ing oxidation–reductions in which B–Z reaction 90

cerium changes its oxidation state which there is a regular periodic (Ce3+ gives a magenta solution while change in the concentration of one Ce4+ gives a blue solution). The B–Z or more reactants. The mechanism is b reaction is an example of a chemical highly complicated, involving a large *oscillating reaction – a reaction in number of steps. See brusselator. C

cacodyl An oily liquid, greenockite and is used as a pigment Û (CH3)2AsAs(CH3)2. It has a characteris- and in semiconductors and uores- tic odour of garlick and is poisonous. cent materials. Cacodyl was one of the Ürst organo- caesium Symbol Cs. A soft silvery- metallic compounds to be synthe- white metallic element belonging to sized (by heating arsenic with *group 1 (formerly IA) of the periodic potassium acetate). The group table; a.n. 55; r.a.m. 132.905; r.d. (CH ) As– is the cacodyl group. 3 2 1.88; m.p. 28.4°C; b.p. 678°C. It oc- cadmium Symbol Cd. A soft bluish curs in small amounts in a number metal belonging to *group 12 (for- of minerals, the main source being merly IIB) of the periodic table; a.n. carnallite (KCl.MgCl2.6H2O). It is ob- 48; r.a.m. 112.41; r.d. 8.65; m.p. tained by electrolysis of molten cae- 320.9°C; b.p. 765°C. The element’s sium cyanide. The natural isotope is name is derived from the ancient caesium–133. There are 15 other name for calamine, zinc carbonate radioactive isotopes. Caesium–137 ZnCO3, and it is usually found associ- (half-life 33 years) is used as a gamma ated with zinc ores, such as spha- source. As the heaviest alkali metal, lerite (ZnS), but does occur as the caesium has the lowest ionization po- mineral greenockite (CdS). Cadmium tential of all elements, hence its use is usually produced as an associate in photoelectric cells, etc. product when zinc, copper, and lead A ores are reduced. Cadmium is used in • Information from the WebElements site low-melting-point alloys to make solders, in Ni–Cd batteries, in bearing caesium chloride structure A alloys, and in electroplating (over type of ionic crystal structure in 50%). Cadmium compounds are used which the anions are at the eight cor- as phosphorescent coatings in TV ners of a cubic unit cell with one tubes. Cadmium and its compounds cation at the centre of the cell. It can are extremely toxic at low concentra- equivalently by described as cations tions; great care is essential where at the corners of the cell with an solders are used or where fumes are anion at the centre. Each type of ion emitted. It has similar chemical prop- has a coordination number of 8. Ex- erties to zinc but shows a greater ten- amples of compounds with this dency towards complex formation. structure are CsCl, CsBr, CsI, CsCN, The element was discovered in 1817 CuZn, and NH4Cl. by F. Stromeyer. caesium clock An *atomic clock A that depends on the energy differ- • Information from the WebElements site ence between two states of the caesium–133 nucleus when it is in a cadmium cell See weston cell. magnetic Üeld. In one type, atoms of cadmium sulphide A water- caesium–133 are irradiated with insoluble compound, CdS; r.d. 4.82. radio-frequency radiation, whose fre- It occurs naturally as the mineral quency is chosen to correspond to caffeine 92

the energy difference between the calcite One of the most common two states. Some caesium nuclei ab- and widespread minerals, consisting sorb this radiation and are excited to of crystalline calcium carbonate, the higher state. These atoms are CaCO3. Calcite crystallizes in the c deÛected by a further magnetic Üeld, rhombohedral system; it is usually which causes them to hit a detector. colourless or white and has a hard- A signal from this detector is fed ness of 3 on the Mohs’ scale. It has back to the radio-frequency oscillator the property of double refraction, to prevent it drifting from the reso- which is apparent in Iceland spar – nant frequency of 9 192 631 770 the transparent variety of calcite. It is hertz. In this way the device is locked an important rock-forming mineral to this frequency with an accuracy and is a major constituent in lime- better than 1 part in 1013. The cae- stones, marbles, and carbonatites. sium clock is used in the *SI unit calcium deÜnition of the second. Symbol Ca. A soft grey metallic element belonging to caffeine (1,3,7-trimethylxanthine) *group 2 (formerly IIA) of the peri- ° An alkaloid, C8H10N4O2; m.p. 235 C; odic table; a.n. 20; r.a.m. 40.08; r.d. sublimes at 176°C. It is a stimulant 1.54; m.p. 839°C; b.p. 1484°C. Cal- and diuretic and is present in cof- cium compounds are common in the fee, tea, and some soft drinks. See earth’s crust; e.g. limestone and mar- methylxanthines . ble (CaCO3), gypsum (CaSO4.2H2O), Û cage compound See clathrate. and uorite (CaF2). The element is extracted by electrolysis of fused cal- cage effect An effect occurring in cium chloride and is used as a getter certain condensed-phase reactions in in vacuum systems and a deoxidizer which fragments are formed and in producing nonferrous alloys. It is their diffusion is hindered by a sur- also used as a reducing agent in the rounding ‘cage’ of molecules. The ini- extraction of such metals as thorium, tial fragments are consequently more zirconium, and uranium. likely to recombine or to react to- Calcium is an essential element for gether to form new products. living organisms, being required for Cahn–Ingold–Prelog system See normal growth and development. In cip system. animals it is an important constituent of bones and teeth and is calamine A mineral form of zinc present in the blood, being required carbonate, ZnCO3 (smithsonite), al- for muscle contraction and other though in the USA the same name is metabolic processes. In plants it is a given to a hydrated zinc silicate constituent (in the form of calcium (hemimorphite). The calamine used pectate) of the middle lamella. medicinally in lotions for treating A sunburn and other skin conditions is basic zinc carbonate coloured pink • Information from the WebElements site with a trace of iron(III) oxide. calcium acetylide See calcium dicarbide calcination The formation of a cal- . cium carbonate deposit from hard calcium bicarbonate See calcium water. See hardness of water. hydrogencarbonate. calcinite A mineral form of *potas- calcium carbide See calcium di- sium hydrogencarbonate, KHCO3. carbide. 93 calcium hydrogencarbonate calcium carbonate A white solid, The reaction has been used as a Ü CaCO3, which is only sparingly solu- method of xing nitrogen in coun- ble in water. Calcium carbonate tries in which cheap electricity is decomposes on heating to give *cal- available to make the calcium dicar- cium oxide (quicklime) and carbon bide (the cyanamide process). Cal- c dioxide. It occurs naturally as the cium cyanamide can be used as a minerals *calcite (rhombohedral; r.d. fertilizer because it reacts with water 2.71) and *aragonite (rhombic; r.d. to give ammonia and calcium car- 2.93). Rocks containing calcium carbonate: bonate dissolve slowly in acidiÜed → CaCN2(s) + 3H2O(l) CaCO3(s) + rainwater (containing dissolved CO2) 2NH (g) to cause temporary hardness. In the 3 laboratory, calcium carbonate is pre- It is also used in the production of cipitated from *limewater by carbon melamine, urea, and certain cyanide dioxide. Calcium carbonate is used in salts. making lime (calcium oxide) and is calcium dicarbide (calcium the main raw material for the acetylide; calcium carbide; carbide) A *Solvay process. colourless solid compound, CaC2; ° calcium chloride A white deli- tetragonal; r.d. 2.22; m.p. 450 C; b.p. 2300°C. In countries in which elec- quescent compound, CaCl2, which is soluble in water; r.d. 2.15; m.p. tricity is cheap it is manufactured by 782°C; b.p. >1600°C. There are a heating calcium oxide with either number of hydrated forms, including coke or ethyne at temperatures ° the monohydrate, CaCl .H O, the di- above 2000 C in an electric arc fur- 2 2 2+ hydrate, CaCl .2H O (r.d. 0.84), and nace. The crystals consist of Ca and 2 2 – C2 ions arranged in a similar way to the hexahydrate, CaCl2.6H2O (trigonal; r.d. 1.71; the hexahydrate loses the ions in sodium chloride. When ° water is added to calcium dicarbide, 4H2O at 30 C and the remaining ° the important organic raw material 2H2O at 200 C). Large quantities of it are formed as a byproduct of the ethyne (acetylene) is produced: → *Solvay process and it can be pre- CaC2(s) + 2H2O(l) Ca(OH)2(s) + pared by dissolving calcium carbon- C2H2(g) ate or calcium oxide in hydrochloric Û acid. Crystals of the anhydrous salt calcium uoride A white crys- can only be obtained if the hydrated talline solid, CaF2; r.d. 3.2; m.p. ° ° salt is heated in a stream of hydrogen 1360 C; b.p. 2500 C. It occurs natu- Û chloride. Solid calcium chloride is rally as the mineral * uorite (or Û used in mines and on roads to reduce uorspar) and is the main source of dust problems, whilst the molten salt Ûuorine. See also fluorite structure. is the electrolyte in the extraction of calcium hydrogencarbonate (cal- calcium. An aqueous solution of calcium bicarbonate) A compound, cium chloride is used in refrigeration Ca(HCO3)2, that is stable only in solu- plants. tion and is formed when water con- calcium cyanamide A colourless taining carbon dioxide dissolves solid, CaCN2, which sublimes at calcium carbonate: ° → 1300 C. It is prepared by heating cal- CaCO3(s) + H2O(l) + CO2(g) ° cium dicarbide at 800 C in a stream Ca(HCO3)2(aq) of nitrogen: It is the cause of temporary *hard- → CaC2(s) + N2(g) CaCN2(s) + C(s) ness in water, because the calcium calcium hydroxide 94

ions react with soap to give scum. limestone is heated in a tall tower Calcium hydrogencarbonate is unsta- (lime kiln) to a temperature above ble when heated and decomposes to 550°C: give solid calcium carbonate. This ex- ˆ CaCO3(s) CaO(s) + CO2(g) plains why temporary hardness is re- Although the reaction is reversible, c moved by boiling and the formation the carbon dioxide is carried away by of ‘scale’ in kettles and boilers. the upward current through the kiln calcium hydroxide (slaked lime) A and all the limestone decomposes. white solid, Ca(OH)2, which dissolves Calcium oxide is used to make cal- sparingly in water (see limewater); cium hydroxide, as a cheap alkali for hexagonal; r.d. 2.24. It is manufac- treating acid soil, and in extractive tured by adding water to calcium metallurgy to produce a slag with the oxide, a process that evolves much impurities (especially sand) present heat and is known as slaking. It is in metal ores. used as a cheap alkali to neutralize calcium phosphate(V) A white the acidity in certain soils and in the insoluble powder, Ca (PO ) ; r.d. 3.14. manufacture of mortar, whitewash, 3 4 2 It is found naturally in the mineral bleaching powder, and glass. *apatite, Ca5(PO4)3(OH,F,Cl), and as calcium nitrate A white deliques- rock phosphate. It is also the main cent compound, Ca(NO3)2, that is constituent of animal bones. Calcium very soluble in water; cubic; r.d. 2.50; phosphate can be prepared by mix- m.p. 561°C. It can be prepared by ing solutions containing calcium ions neutralizing nitric acid with calcium and hydrogenphosphate ions in the carbonate and crystallizing it from presence of an alkali: solution as the tetrahydrate 2– – → 3– HPO4 + OH PO4 + H2O Ca(NO ) .4H O, which exists in two 3 2 2 2+ 3– → monoclinic crystalline forms (α, r.d. 3Ca + 2PO4 Ca3(PO4)2 1.9; β, r.d. 1.82). There is also a trihy- It is used extensively as a fertilizer. drate, Ca(NO3)2.3H2O. The anhydrous The compound was formerly called salt can be obtained from the hy- calcium orthophosphate (see phos- drate by heating but it decomposes phates). on strong heating to give the oxide, calcium stearate See calcium oc- nitrogen dioxide, and oxygen. Cal- tadecanoate. cium nitrate is sometimes used as a nitrogenous fertilizer. calcium sulphate A white solid compound, CaSO ; r.d. 2.96; 1450°C. calcium octadecanoate (calcium 4 It occurs naturally as the mineral stearate) An insoluble white salt, *anhydrite, which has a rhombic Ca(CH (CH ) COO) , which is formed 3 2 16 2 structure, transforming to a mono- when soap is mixed with water con- clinic form at 200°C. More com- taining calcium ions and is the scum monly, it is found as the dihydrate, produced in hard-water regions. *gypsum, CaSO4.2H2O (monoclinic; calcium oxide (quicklime) A white r.d. 2.32). When heated, gypsum solid compound, CaO, formed by loses water at 128°C to give the heating calcium in oxygen or by the hemihydrate, 2CaSO4.H2O, better thermal decomposition of calcium known as *plaster of Paris. Calcium carbonate; cubic; r.d. 3.35; m.p. sulphate is sparingly soluble in water 2580°C; b.p. 2850°C. On a large scale, and is a cause of permanent *hard- calcium carbonate in the form of ness of water. It is used in the manu- 95 calorific value facture of certain paints, ceramics, of calixarene molecules to mimic en- and paper. The naturally occurring zyme action. forms are used in the manufacture of calmodulin A protein, consisting sulphuric acid. of 148 amino-acid residues, that acts Calgon Tradename for a water- as a receptor for calcium ions in c softening agent. See hardness of many calcium-regulated processes in water. both animal and plant cells. Calmo- dulin mediates reactions catalysed by caliche A mixture of salts found in many enzymes. deposits between gravel beds in the Atacama and Tarapaca regions of calomel See mercury(i) chloride. Chile. They vary from 4 m to 15 cm calomel half cell (calomel elec- thick and were formed by periodic trode) A type of half cell in which leaching of soluble salts during wet the electrode is mercury coated with geological epochs, followed by drying calomel (HgCl) and the electrolyte is out of inland seas in dry periods. a solution of potassium chloride and They are economically important as a saturated calomel. The standard elec- source of nitrates. A typical composi- trode potential is –0.2415 volt (25°C). tion is NaNO3 17.6%, NaCl 16.1%, In the calomel half cell the reactions Na2SO4 6.5%, CaSO4 5.5%, MgSO4 are 3.0%, KNO 1.3%, Na B O 0.94%, 3 2 4 7 HgCl(s) ˆ Hg+(aq) + Cl–(aq) KClO3 0.23%, NaIO3 0.11%, sand and gravel to 100%. Hg+(aq) + e ˆ Hg(s) californium Symbol Cf. A radio- The overall reaction is active metallic transuranic element HgCl(s) + e ˆ Hg(s) + Cl–(aq) belonging to the *actinoids; a.n. 98; – This is equivalent to a Cl2(g)|Cl (aq) mass number of the most stable iso- half cell. tope 251 (half-life about 700 years). Nine isotopes are known; cali- calorie The quantity of heat re- fornium–252 is an intense neutron quired to raise the temperature of 1 source, which makes it useful in neu- gram of water by 1°C (1 K). The calo- tron *activation analysis and poten- rie, a c.g.s. unit, is now largely re- tially useful as a radiation source in placed by the *joule, an *SI unit. 1 medicine. The element was Ürst pro- calorie = 4.186 8 joules. duced by Glenn Seaborg (1912–99) Calorie (kilogram calorie; kilocalo- and associates in 1950. rie) 1000 calories. This unit is still in A limited use in estimating the energy • Information from the WebElements site value of foods, but is obsolescent. calixarenes Compounds that have caloriÜc value The heat per unit molecules with a cuplike structure mass produced by complete combus- (the name comes from the Greek tion of a given substance. CaloriÜc calix, cup). The simplest, has four values are used to express the energy phenol molecules joined by four values of fuels; usually these are ex- –CH2– groups into a ring (forming pressed in megajoules per kilogram the base of the ‘cup’). The four phe- (MJ kg–1). They are also used to meas- nol hexagons point in the same di- ure the energy content of foodstuffs; rection to form a cavity that can bind i.e. the energy produced when the substrate molecules. Interest has food is oxidized in the body. The been shown in the potential ability units here are kilojoules per gram calorimeter 96

(kJ g–1), although Calories (kilocalo- mosan camphor tree, but can now be ries) are often still used in nontechni- synthesized. The compound has a cal contexts. CaloriÜc values are characteristic odour associated with measured using a *bomb calorime- its use in mothballs. It is a plasticizer c ter. in celluloid. CH calorimeter Any of various devices 3 used to measure thermal properties, C Ü such as calori c values or heats of H C CO bomb 2 chemical reactions. See also H C C CH calorimeter 3 3 . H C CH 2 2 Calvin, Melvin (1911–97) US bio- C H chemist. After World War II, at the Lawrence Radiation Laboratory, Camphor Berkeley, he investigated the light-independent reactions of *photosyn- Canada balsam A yellow-tinted thesis. Using radioactive carbon-14 to resin used for mounting specimens label carbon dioxide he discovered in optical microscopy. It has similar the *Calvin cycle, for which he was optical properties to glass. awarded the 1961 Nobel Prize for candela Symbol Cd. The *SI unit of chemistry. luminous intensity equal to the lumi- Calvin cycle The metabolic path- nous intensity in a given direction of way of the light-independent stage of a source that emits monochromatic × 12 *photosynthesis, which occurs in the radiation of frequency 540 10 Hz stroma of the chloroplasts. The path- and has a radiant intensity in that di- way was elucidated by Melvin rection of 1/683 watt per steradian. *Calvin and his co-workers and in- cane sugar See sucrose. volves the Üxation of carbon dioxide and its subsequent reduction to car- cannabinoids A large group of bohydrate. During the cycle, carbon structurally related phenolic com- dioxide combines with *ribulose bis- pounds found in the plant Cannabis phosphate, through the mediation of sativa. The main one is *tetrahydro- the enzyme ribulose bisphosphate cannabinol (THC), which is the com- carboxylase, to form an unstable pound responsible for the affects of six-carbon compound that breaks cannabis. It affects cannabinoid re- down to form two molecules of the ceptors found in the brain (and also three-carbon compound glycerate in the spleen). The name ‘cannabi- 3-phosphate. This is converted to noid’ is also applied to structurally glyceraldehyde 3-phosphate, which is unrelated compounds found natu- used to regenerate ribulose bisphos- rally in animal tissue and having an affect on the cannabinoid receptors. phate and to produce glucose and These endocannabinoids (endog- fructose. enous cannabinoids) are believed to calx A metal oxide formed by heat- act as ‘messengers’ between cells. ing an ore in air. cannabis An illegal drug produced camphor A white crystalline cyclic from the plant Cannabis sativa. The Û ketone, C10H16O; r.d. 0.99; m.p. dried in orescences of the plant are 179°C; b.p. 204°C. It was formerly ob- known as marijuana and the thick tained from the wood of the For- resin produced from the plant is 97 caprolactam known as hashish. The normal in a reservoir containing an elec- method of using cannabis is to trolyte (e.g. a buffer solution). The smoke it, although it can also be source reservoir contains a positive taken in certain foods. Cannabis is a electrode and the destination reser- class C controlled substance in the voir contains a negative electrode. A c UK. It contains a large number of re- high potential difference is main- lated compounds known as herbal tained between the electrodes. Com- *cannabinoids. The main active components of the sample Ûow through ponent is *tetrahydrocannabinol the buffer solution in the capillary (THC). under the inÛuence of the electric Üeld. Their mobility depends on their Cannizzaro, Stanislao charge, size, and shape, and the com- (1826–1910) Italian chemist. He dis- ponents separate as they move covered the *Cannizzaro reaction in through the tube. They are detected 1853. His main contribution to chem- close to the end of the capillary, usu- istry was his recognition of the valid- ally by ultraviolet absorption. Capil- ity of *Avogadro’s law and his liary gel electrophoresis (CGE) is used deduction that common gases such for separating large charged species, as H2 and O2 are diatomic molecules. such as DNA fragments. The capil- This led to the establishment of a re- liary is Ülled with a gel and separa- liable system of atomic and molecu- tion depends on the size of the lar weights. species. In CE, a graph of detector Cannizzaro reaction A reaction of output against time is known as an aldehydes to give carboxylic acids electropherogram. Individual compo- and alcohols. It occurs in the pres- nents can be identiÜed by their reten- ence of strong bases with aldehydes tion times in the capillary. The that do not have alpha hydrogen technique can be highly sensitive atoms. For example, benzenecar- and is widely used in forensic labora- baldehyde gives benzenecarboxylic tories. acid and benzyl alcohol: capillary gel electrophoresis → capillary electro- 2C6H5CHO C6H5COOH + (CGE) See phoresis C6H5CH2OH . Aldehydes that have alpha hydrogen capillary zone electrophoresis atoms undergo the *aldol reaction in- (CZE) See capillary electro- stead. The Cannizzaro reaction is an phoresis. example of a *disproportionation. It decanoic acid was discovered by Stanislao *Canniz- capric acid See . zaro. caproic acid See hexanoic acid. canonical form One of the possi- caprolactam (6-hexanelactam) A ble structures of a molecule that to- white crystalline substance, gether form a *resonance hybrid. capillary A tube of small diameter. capillary electrophoresis (CE) A technique for investigating mixtures of charged species. In capilliary zone electrophoresis (CZE), the sample is introduced into a Üne capillary tube, with each end of the capillary placed Caprolactam caprylic acid 98

° C6H11NO; r.d. 1.02; m.p. 69–71 C; b.p. types of organic reaction (e.g. the 139°C. It is a *lactam containing the *aldol reaction). Ü –NH.CO– group with ve CH2 groups carbazole A white crystalline com- making up the rest of the seven- pound found with anthracene, membered ring. Caprolactam is used ° ° c C12H9N; m.p. 238 C; b.p. 335 C. It is in making *nylon. used in the manufacture of dyestuffs. caprylic acid See octanoic acid. 9 1 9a 9b capture Any of various processes in 8 2 which a system of particles absorbs an extra particle. There are several 7 5 3 5a examples in atomic and nuclear N 4a 6 4 physics. For instance, a positive ion H may capture an electron to give a Carbazole neutral atom or molecule. Similarly, a neutral atom or molecule capturing carbene A species of the type R2C:, an electron becomes a negative ion. in which the carbon atom has two An atomic nucleus may capture a electrons that do not form bonds. neutron to produce a different (often Methylene, :CH2, is the simplest ex- unstable) nucleus. Another type of ample. Carbenes are highly reactive nuclear capture is the process in and exist only as transient intermedi- which the nucleus of an atom ab- ates in certain organic reactions. sorbs an electron from the innermost They attack double bonds to give cy- orbit (the K shell) to transform into a clopropane derivatives. They also different nucleus. In this process cause insertion reactions, in which (called K capture) the atom is left in the carbene group is inserted bean excited state and generally decays tween the carbon and hydrogen by emission of an X-ray photon. atoms of a C–H bond: Radiative capture is any such C–H + :CR → C–CR –H process in which the capture results 2 2 in an excited state that decays by carbenium ion See carbocation. emission of photons. A common example is neutron capture to yield an carbide Any of various compounds excited nucleus, which decays by of carbon with metals or other more electropositive elements. True car- emission of a gamma ray. 4– bides contain the ion C as in Al4C3. carat 1. A measure of Üneness (pu- These are saltlike compounds giving rity) of gold. Pure gold is described as methane on hydrolysis, and were for- 24-carat gold. 14-carat gold contains merly called methanides. Com- 2– 14 parts in 24 of gold, the remainder pounds containing the ion C2 are usually being copper. 2. A unit of also saltlike and are known as dicar- mass equal to 0.200 gram, used to bides. They yield ethyne (acetylene) measure the masses of diamonds and on hydrolysis and were formerly other gemstones. called acetylides. The above types of compound are ionic but have par- carbamide See urea. tially covalent bond character, but carbanion An organic ion with a boron and silicon form true covalent negative charge on a carbon atom; carbides, with giant molecular struc- – i.e. an ion of the type R3C . Carban- tures. In addition, the transition met- ions are intermediates in certain als form a range of interstitial 99 carbon carbides in which the carbon atoms hydrates of much greater molecular occupy interstitial positions in the weight and complexity; examples are metal lattice. These substances are starch, glycogen, and cellulose. Car- generally hard materials with metal- bohydrates perform many vital roles lic conductivity. Some transition met- in living organisms. Sugars, notably c als (e.g. Cr, Mn, Fe, Co, and Ni) have glucose, and their derivatives are es- atomic radii that are too small to sential intermediates in the conver- allow individual carbon atoms in the sion of food to energy. Starch and interstitial holes. These form car- other polysaccharides serve as energy bides in which the metal lattice is stores in plants. Cellulose, lignin, and distorted and chains of carbon atoms others form the supporting cell walls and woody tissue of plants. Chitin is exist (e.g. Cr3C2, Fe3C). Such compounds are intermediate in character a structural polysaccharide found in between interstitial carbides and the body shells of many invertebrate ionic carbides. They give mixtures of animals. hydrocarbons on hydrolysis with A water or acids. • Information about IUPAC nomenclature carbocation An ion with a positive carbolic acid See phenol. charge that is mostly localized on a carbon Symbol C. A nonmetallic el- carbon atom. There are two types: ement belonging to *group 14 (for- Carbonium ions Ü have ve bonds to merly IVB) of the periodic table; a.n. the carbon atom and a complete 6; r.a.m. 12.011; m.p. ∼3550°C; b.p. outer shell of eight electrons. A sim- ∼ ° + 4827 C. Carbon has three main al- ple example is the ion CH5 , which lotropic forms (see allotropy). has a trigonal bipyramidal shape. *Diamond (r.d. 3.52) occurs natu- Ions of this type are transient rally and can be produced syntheti- species. They can be produced by cally. It is extremely hard and has electron impact and detected by highly refractive crystals. The hard- mass spectroscopy. ness of diamond results from the co- Carbenium ions have three bonds valent crystal structure, in which to the carbon atom and are planar, each carbon atom is linked by cova- with six outer electrons and a vacant lent bonds to four others situated at p-orbital. Ions of this type are inter- the corners of a tetrahedron. The mediates in a number of organic re- C–C bond length is 0.154 nm and the actions (for example, in the SN1 bond angle is 109.5°. mechanism of *nucleophilic substitu- Graphite (r.d. 2.25) is a soft black tion). Certain carbenium ions are sta- slippery substance (sometimes called bilized by delocalization of the black lead or plumbago). It occurs charge. An example is the orange-red naturally and can also be made by + - salt (C6H5)3C Cl . Carbenium ions can the *Acheson process. In graphite be produced by *superacids. the carbon atoms are arranged in lay- carbocyclic See cyclic. ers, in which each carbon atom is surrounded by three others to which carbohydrate One of a group of it is bound by single or double bonds. organic compounds based on the The layers are held together by much general formula Cx(H2O)y. The sim- weaker van der Waals’ forces. The plest carbohydrates are the *sugars carbon–carbon bond length in the (saccharides), including glucose and layers is 0.142 nm and the layers are sucrose. *Polysaccharides are carbo- 0.34 nm apart. Graphite is a good carbon assimilation 100

conductor of heat and electricity. It most important carbonate minerals has a variety of uses including electri- are *calcite, *dolomite, and *magne- cal contacts, high-temperature equip- site. See also aragonite. ment, and as a solid lubricant. carbonation The solution of car- c Graphite mixed with clay is the ‘lead’ bon dioxide in a liquid under pres- in pencils (hence its alternative sure. name). The third crystalline allotrope is fullerite (see buckminster- carbon bisulphide See carbon fullerene). There are also several disulphide. amorphous forms of carbon, such as carbon black A Üne carbon powder *carbon black and *charcoal. made by burning hydrocarbons in in- There are two stable isotopes of sufÜcient air. It is used as a pigment carbon (proton numbers 12 and 13) and a Üller (e.g. for rubber). and four radioactive ones (10, 11, 14, 15). Carbon–14 is used in *carbon carbon cycle 1. One of the major dating. cycles of chemical elements in the Carbon forms a large number of environment. Carbon (as carbon compounds because of its unique dioxide) is taken up from the atmos- ability to form stable covalent bonds phere and incorporated into the tis- with other carbon atoms and also sues of plants in *photosynthesis. It with hydrogen, oxygen, nitrogen, may then pass into the bodies of ani- and sulphur atoms, resulting in the mals as the plants are eaten. During formation of a variety of compounds the respiration of plants, animals, containing chains and rings of car- and organisms that bring about de- bon atoms. composition, carbon dioxide is re- A turned to the atmosphere. The combustion of fossil fuels (e.g. coal • Information from the WebElements site and peat) also releases carbon diox- carbon assimilation The incorpo- ide into the atmosphere. See illustra- ration of carbon from atmospheric tion. 2. (in physics) A series of carbon dioxide into organic mol- nuclear reactions in which four hy- ecules. This process occurs during drogen nuclei combine to form a he- *photosynthesis. See carbon cycle. lium nucleus with the liberation of energy, two positrons, and two neu- carbonate A salt of carbonic acid trinos. The process is believed to be 2– containing the carbonate ion, CO3 . the source of energy in many stars The free ion has a plane triangular and to take place in six stages. In this structure. Metal carbonates may be series carbon–12 acts as if it were a ionic or may contain covalent catalyst, being reformed at the end of metal–carbonate bonds (complex car- the series: bonates) via one or two oxygen 12C + 1 H → 13 N + γ atoms. The carbonates of the alkali 6 1 7 13 → 13 + ν metals are all soluble but other car- 7 N 6 C + e + e bonates are insoluble; they all react 13 C + 1 H → 14 N + γ with mineral acids to release carbon 6 1 7 14 1 → 15 γ dioxide. 7 N + 1 H 8 O + 15 → 15 + ν carbonate minerals A group of 8 O 7 N + e + e common rock-forming minerals con- 15 N + 1 H → 12C + 4 He. 2– 7 1 6 2 taining the anion CO3 as the fundamental unit in their structure. The carbon dating (radiocarbon dating) 101 carbon dioxide

carbon dioxide in the atmosphere

respiration in combustion respiration decomposers c

carbon organic death carbon compounds in fossil respiration photosynthesis compounds in dead organic fuels in animals matter

fossilization feeding death

organic compounds in green plants

Carbon cycle

A method of estimating the ages of developed by Willard F. Libby archaeological specimens of biologi- (1908–80) and his coworkers in cal origin. As a result of cosmic radia- 1946–47. tion a small number of atmospheric nitrogen nuclei are continuously carbon dioxide A colourless being transformed by neutron bom- odourless gas, CO2, soluble in water, –3 bardment into radioactive nuclei of ethanol, and acetone; d. 1.977 g dm ° ° ° carbon–14: (0 C); m.p. –56.6 C; b.p. –78.5 C. It occurs in the atmosphere (0.04% by vol- 14 N + n → 14 C + p 7 6 ume) but has a short residence time Some of these radiocarbon atoms Ü in this phase as it is both consumed nd their way into living trees and by plants during *photosynthesis and other plants in the form of carbon produced by respiration and by com- dioxide, as a result of photosynthesis. bustion. It is readily prepared in the When the tree is cut down photosyn- laboratory by the action of dilute thesis stops and the ratio of radiocarbon atoms to stable carbon atoms acids on metal carbonates or of heat begins to fall as the radiocarbon de- on heavy-metal carbonates. Carbon cays. The ratio 14C/12C in the speci- dioxide is a by-product from the men can be measured and enables manufacture of lime and from fer- the time that has elapsed since the mentation processes. tree was cut down to be calculated. Carbon dioxide has a small liquid The method has been shown to give range and liquid carbon dioxide is consistent results for specimens up produced only at high pressures. The to some 40 000 years old, though its molecule CO2 is linear with each oxy- accuracy depends upon assumptions gen making a double bond to the car- concerning the past intensity of the bon. Chemically, it is unreactive and cosmic radiation. The technique was will not support combustion. It dis- carbon disulphide 102

solves in water to give *carbonic are used in strong composite ma- acid. terials for use at high temperatures. Large quantities of solid carbon carbonic acid A dibasic acid, dioxide (dry ice) are used in processes H2CO3, formed in solution when car- c requiring large-scale refrigeration. It bon dioxide is dissolved in water: is also used in Üre extinguishers as a ˆ desirable alternative to water for CO2(aq) + H2O(l) H2CO3(aq) most Üres, and as a constituent of The acid is in equilibrium with dis- medical gases as it promotes exhala- solved carbon dioxide, and also disso- tion. It is also used in carbonated ciates as follows: ˆ + – drinks. H2CO3 H + HCO3 The level of carbon dioxide in the K = 4.5 × 10–7 mol dm–3 atmosphere has increased by some a – ˆ 2– + 12% in the last 100 years, mainly be- HCO3 CO3 + H × –11 –3 cause of extensive burning of fossil Ka = 4.8 10 mol dm fuels and the destruction of large The pure acid cannot be isolated, areas of rain forest. This has been although it can be produced in ether postulated as the main cause of the solution at –30°C. Carbonic acid gives average increase of 0.5°C in global rise to two series of salts: the *car- temperatures over the same period, bonates and the *hydrogencarbon- through the *greenhouse effect. ates. Steps are now being taken to prevent carbonium ion See carbocation. further increases in atmospheric CO2 concentration and subsequent global carbonize (carburize) To change an warming. organic compound into carbon by heating, or to coat something with carbon disulphide (carbon bisul- carbon in this way. phide) A colourless highly refractive carbon monoxide A colourless liquid, CS2, slightly soluble in water and soluble in ethanol and ether; r.d. odourless gas, CO, sparingly soluble 1.261; m.p. –110°C; b.p. 46.3°C. Pure in water and soluble in ethanol and benzene; d. 1.25 g dm–3 (0°C); m.p. carbon disulphide has an ethereal ° ° Û odour but the commercial product is –199 C; b.p. –191.5 C. It is ammable contaminated with a variety of other and highly toxic. In the laboratory it sulphur compounds and has a most can be made by the dehydration of unpleasant smell. It was previously methanoic acid (formic acid) using manufactured by heating a mixture concentrated sulphuric acid. Industri- of wood, sulphur, and charcoal; mod- ally it is produced by the oxidation of ern processes use natural gas and sul- natural gas (methane) or (formerly) phur. Carbon disulphide is an by the water-gas reaction. It is excellent solvent for oils, waxes, rub- formed by the incomplete combus- ber, sulphur, and phosphorus, but its tion of carbon and is present in car- exhaust gases. use is decreasing because of its high It is a neutral oxide, which burns toxicity and its Ûammability. It is in air to give carbon dioxide, and is a used for the preparation of xanthates good reducing agent, used in a num- in the manufacture of viscose yarns. ber of metallurgical processes. It has carbon Übres Fibres of carbon in the interesting chemical property which the carbon has an oriented of forming a range of transition Ü crystal structure. Carbon bres are metal carbonyls, e.g. Ni(CO)4. Car- made by heating textile Übres and bon monoxide is able to use vacant 103 carbyne p-orbitals in bonding with metals; globin combines with carbon monox- the stabilization of low oxidation ide. Carbon monoxide competes with states, including the zero state, is a oxygen for haemoglobin, with which consequence of this. This also ac- it binds strongly: the afÜnity of counts for its toxicity, which is due haemoglobin for carbon monoxide is c to the binding of the CO to the iron 250 times greater than that for oxy- in haemoglobin, thereby blocking gen. This reduces the availability of the uptake of oxygen. haemoglobin for combination with _ (and transport of) oxygen and ac- CO_ carbon monoxide _ _ counts for the toxic effects of carbon OC_ _ O carbon dioxide monoxide on the respiratory system. _ OC_ _ C _ C _ O tricarbon dioxide (carbon suboxide) carboxylate An anion formed from a *carboxylic acid. For example, Carbon monoxide ethanoic acid gives rise to the – ethanoate ion, CH3COO . carbon suboxide See tricarbon carboxyl group The organic group dioxide. –COOH, present in *carboxylic acids. carbon tetrachloride See tetrachloromethane. carboxylic acids Organic compounds containing the group –CO.OH carbonyl chloride (phosgene) A (the carboxyl group; i.e. a carbonyl colourless gas, COCl2, with an odour group attached to a hydroxyl group). of freshly cut hay. It is used in or- In systematic chemical nomenclature ganic chemistry as a chlorinating carboxylic-acid names end in the agent, and was formerly used as a sufÜx -oic, e.g. ethanoic acid, war gas. CH3COOH. They are generally weak carbonyl compound A compound acids. Many long-chain carboxylic containing the carbonyl group >C=O. acids occur naturally as esters in fats Aldehydes, ketones, and carboxylic and oils and are therefore also acids are examples of organic car- known as *fatty acids. See also glyc- bonyl compounds. Inorganic car- erides. bonyls are complexes in which A carbon monoxide has coordinated to • Information about IUPAC nomenclature a metal atom or ion, as in *nickel ligand carbonyl, Ni(CO)4. See also . O carbonyl group The group >C=O, R C carboxyl group found in aldehydes, ketones, carboxylic acids, amides, etc., and in in- OH organic carbonyl complexes (see carbonyl compound). Carboxylic acid carboranes Compounds similar to carbonize the *boranes, but with one or more carburize See . boron atoms replaced by carbon carbylamine reaction See iso- atoms. cyanide test. silicon carbide carborundum See . carbyne A transient species of the carboxyhaemoglobin The highly type R–C≡, with three nonbonding stable product formed when *haemo- electrons on the carbon atom. For- carcinogen 104

merly, carbynes were called methyli- b. Adiabatic expansion with a fall of dynes. temperature to T2. c. Isothermal compression at temper- carcinogen Any agent that pro- ature T2 with heat q2 given out. duces cancer, e.g. tobacco smoke, d. Adiabatic compression with a rise c certain industrial chemicals, and of temperature back to T1. ionizing radiation (such as X-rays According to the Carnot principle, the and ultraviolet rays). efÜciency of any reversible heat en- Carius method A method of deter- gine depends only on the tempera- mining the amount of sulphur and ture range through which it works, halogens in an organic compound, by rather than the properties of the heating the compound in a sealed working substances. In any reversible Ü η tube with silver nitrate in concen- engine, the ef ciency ( ) is the ratio of the work done (W) to the heat trated nitric acid. The compound is η decomposed and silver sulphide and input (q1), i.e. = W/q1. As, according Ü * halides are precipitated, separated, to the rst law of thermodynamics, W = q – q , it follows that η = and weighed. 1 2 (q1 – q2)/q1. For the Kelvin tempera- η carnallite A mineral consisting of a ture scale, q1/q2 = T1/T2 and = Ü hydrated mixed chloride of potas- (T1 – T2)/T1. For maximum ef ciency sium and magnesium, KCl.MgCl2. T1 should be as high as possible and 6H2O. T2 as low as possible. carnauba wax A natural wax obtained from the leaves of the copaiba P T q isothermal palm of South America. It is ex- 1 1 expansion tremely hard and brittle and is used adiabatic in varnishes and to add hardness and expansion adiabatic T2q2 lustre to other waxes in polishes. compression isothermal Carnot, Nicolas Léonard Sadi compression (1796–1832) French physicist, who Ürst worked as a military engineer. He then turned to scientiÜc research and in 1824 published his analysis of V Ü the ef ciency of heat engines. The Carnot cycle key to this analysis is the thermodynamic *Carnot cycle and the even- tual introduction of the idea of carnotite A radioactive mineral *entropy in thermodynamics. He consisting of hydrated uranium died at an early age from cholera. potassium vanadate, K (UO ) (VO ) .nH O. It varies in col- Carnot cycle The most efÜcient 2 2 2 4 2 2 our from bright yellow to lemon- or cycle of operations for a reversible greenish-yellow. It is a source of ura- *heat engine. Published in 1824 by nium, radium, and vanadium. The Nicolas *Carnot, it consists of four chief occurrences are in the Colorado operations on the working substance Plateau, USA; Radium Hill, Australia; in the engine (see illustration): and Katanga, Zaïre. a. Isothermal expansion at thermody- carnot namic temperature T1 with heat q1 Carnot principle See taken in. cycle. 105 cascade process

Caro’s acid See peroxo- thus allowing electrons to Ûow sulphuric(vi) acid. through the system. 2. A lipid- soluble molecule that can bind to carotene A member of a class of lipid-insoluble molecules and trans- *carotenoid pigments. Examples are port them across membranes. Carrier β-carotene and lycopene, which col- c molecules have speciÜc sites that in- our carrot roots and ripe tomato teract with the molecules they trans- fruits respectively. α- and β-carotene port. Several different molecules may yield vitamin A when they are bro- compete for transport by the same ken down during animal digestion. carrier. carotenoid Any of a group of yel- Carr–Purcell–Meiboom–Gill se- low, orange, red, or brown plant pig- quence See cpmg sequence. ments chemically related to terpenes. Carotenoids are responsible for the CARS (coherent anti-Stokes Raman characteristic colour of many plant spectroscopy) A form of Raman spec- organs, such as ripe tomatoes, car- troscopy (see raman effect) enabling rots, and autumn leaves. They also the intensity of Raman transitions to absorb light energy and pass this on be increased. In this technique two to chlorophyll molecules in the light- laser beams with different frequen- dependent reactions of photosynthe- cies pass through a sample producing sis. electromagnetic radiation of several frequencies. It is possible to adjust Carothers, Wallace Hume the frequency of the lasers so that (1896–1937) US industrial chemist, one of the frequencies corresponds who joined the Du Pont company to that of a Stokes line from the sam- where he worked on polymers. In ple and the coherent emission has 1931 he produced *neoprene, a syn- the frequency of the anti-Stokes line thetic rubber. His greatest success with a high intensity. CARS enables came in 1935 with the discovery of Raman spectra to be obtained even the polyamide that came to be in the presence of other radiation. known as *nylon. Carothers, who One application of CARS is to obtain suffered from depression, committed Raman spectra from bodies in Ûames. suicide. Using this technique temperatures in carrageenan A naturally occurring different parts of the Ûame can be es- polysaccharide isolated from red timated from the intensity of the algae. The polymer is composed of d- transitions. galactose units, many of which are cascade liqueÜer An apparatus sulphated. K-carrageenan is a gelling for liquefying a gas of low *critical agent with uses similar to those of temperature. Another gas, already *agar. below its critical temperature, is carrier gas The gas that carries the liquiÜed and evaporated at a reduced sample in *gas chromatography. pressure in order to cool the Ürst gas to below its critical temperature. In carrier molecule 1. A molecule practice a series of steps is often that plays a role in transporting elec- used, each step enabling the critical trons through the *electron trans- temperature of the next gas to be port chain. Carrier molecules are reached. usually proteins bound to a nonprotein group; they can undergo oxida- cascade process Any process that tion and reduction relatively easily, takes place in a number of steps, usu- case hardening 106

ally because the single step is too ture since 1957 and around 50 000 inefÜcient to produce the desired re- new numbers are added each week. sult. For example, in various ura- A nium-enrichment processes the • Further information from the CAS site c separation of the desired isotope is • A free service for Ünding CAS numbers only poorly achieved in a single from CambridgeSoft stage; to achieve better separation the process has to be repeated a cassiterite A yellow, brown, or number of times, in a series, with black form of tin(IV) oxide, SnO2, the enriched fraction of one stage that forms tetragonal, often twinned, being fed to the succeeding stage for crystals; the principal ore of tin. It oc- further enrichment. Another exam- curs in hydrothermal veins and meta- ple of cascade process is that operat- somatic deposits associated with acid ing in a *cascade liqueÜer. igneous rocks and in alluvial (placer) deposits. The chief producers are case hardening The hardening of Malaysia, Indonesia, Democratic Re- the surface layer of steel, used for public of Congo, and Nigeria. tools and certain mechanical components. The commonest method is to cast iron A group of iron alloys carburize the surface layer by heat- containing 1.8 to 4.5% of carbon. It is Ü ing the metal in a hydrocarbon or by usually cast into speci c shapes ready dipping the red hot metal into for machining, heat treatment, or as- molten sodium cyanide. Diffusion of sembly. It is sometimes produced dinitrogen into the surface layer to rect from the *blast furnace or it form nitrides is also used. may be made from remelted *pig iron. casein One of a group of phosphate-containing proteins (phospho- Castner–Kellner cell An elec- proteins) found in milk. Caseins are trolytic cell used industrially for the production of *sodium hydroxide. easily digested by the enzymes of Ü young mammals and represent a The usually iron cell is lled with major source of phosphorus. brine (sodium chloride solution) and employs liquid mercury as the cath- CAS registry A database of chemi- ode. The sodium liberated there cal compounds, certain mixtures, forms an amalgam with the mercury, and biological sequences maintained which is run off and reacted with by the Chemical Abstracts Service of water to give sodium hydroxide (and the American Chemical Society. In hydrogen); the mercury is then re- the registry every entry has a unique used. Chlorine gas produced at the CAS registry number (CASRN). This anode is another valuable by-product. has three parts: up to six digits, fol- castor oil lowed by two digits, followed by one A pale-coloured oil ex- digit. For example, the CAS number tracted from the castor-oil plant. It contains a mixture of glycerides of water is 7732-18-5. The Ünal digit of fatty acids, the predominant is a check number. CAS registry acid being ricinoleic acid, numbers are used for searching C H (OH)COOH. It is used as a chemical databases. The size of the 17 32 *drying oil in paints and varnishes registry is immense, with over 32 and medically as a laxative. million substances and 59 million sequences. It identiÜes every chemical catabolism The metabolic break- that has been described in the litera- down of large molecules in living or- 107 catenation ganisms to smaller ones, with the re- by incomplete combustion of hydro- lease of energy. Respiration is an ex- carbons, and nitrogen oxides pro- ample of a catabolic series of duced by nitrogen in the air reacting reactions. See metabolism. Compare with oxygen at high engine tempera- anabolism. tures. Hydrocarbons and carbon c monoxide can be controlled by a catalysis The process of changing higher combustion temperature and the rate of a chemical reaction by use a weaker mixture. However, the of a *catalyst. higher temperature and greater avail- catalyst A substance that increases ability of oxygen arising from these the rate of a chemical reaction with- measures encourage formation of out itself undergoing any permanent nitrogen oxides. The use of three- chemical change. Catalysts that have way catalytic converters solves this the same phase as the reactants are problem by using platinum and homogeneous catalysts (e.g. *en- palladium catalysts to oxidize the zymes in biochemical reactions or hydrocarbons and the CO and transition-metal complexes used in rhodium catalysts to reduce the ni- the liquid phase for catalysing or- trogen oxides back to nitrogen. These ganic reactions). Those that have a three-way catalysts require that the different phase are hetereogeneous air–fuel ratio is strictly stochiometric. catalysts (e.g. metals or oxides used Some catalytic converters promote in many industrial gas reactions). The oxidation reactions only, leaving the catalyst provides an alternative path- nitrogen oxides unchanged. Three- way by which the reaction can pro- way converters can reduce hydrocar- ceed, in which the activation energy bons and CO emissions by some 85%, at the same time reducing nitrogen is lower. It thus increases the rate at oxides by 62%. which the reaction comes to equilibrium, although it does not alter the catalytic cracking See cracking. position of the equilibrium. The cata- catalytic rich gas process See crg lyst itself takes part in the reaction process. and consequently may undergo physical change (e.g. conversion into pow- cataphoresis See electrophoresis. der). In certain circumstances, very catechol See 1,2-dihydroxy- small quantities of catalyst can speed benzene. up reactions. Most catalysts are also highly speciÜc in the type of reaction catecholamine Any of a class of they catalyse, particularly enzymes amines that possess a catechol in biochemical reactions. Generally, (C6H4(OH)2) ring. Including the term is used for a substance that *dopamine, *adrenaline, and *nora- increases reaction rate (a positive cat- drenaline, they function as neuro- alyst). Some reactions can be slowed transmitters and/or hormones. down by negative catalysts (see inhi- catenane A type of compound con- bition). sisting of two or more large rings catalytic converter A device used that are interlocked like the links of in the exhaust systems of motor vehi- a chain. In a catenane, there is no cles to reduce atmospheric pollution. chemical bonding between the rings; the rings are held together by *me- The three main pollutants produced chanical bonding. by petrol engines are: unburnt hydrocarbons, carbon monoxide produced catenation 1. The formation of cathetometer 108

chains of atoms in chemical com- cationic detergent See pounds. 2. The formation of a *cate- detergent. nane compound by mechanical cationic dye See dyes. bonding. cationic resin See ion exchange. c cathetometer A telescope or microscope Ütted with crosswires in caustic Describing a substance that the eyepiece and mounted so that it is strongly alkaline (e.g. caustic soda). can slide along a graduated scale. caustic potash See potassium hy- Cathetometers are used for accurate droxide. measurement of lengths without mechanical contact. The microscope caustic soda See sodium hydrox- type is often called a travelling micro- ide. scope. Cavendish, Henry (1731–1810) cathine (β-hydroxyamphetamine) British chemist and physicist, born in France. Although untrained, his in- An alkaloid, C9H13NO. It may contribute to the stimulant activity of heritance from his grandfather, the *khat. Duke of Devonshire, enabled him to live as a recluse and study science. In cathinone (β-ketoamphetamine) his experiments with gases (1766), he An alkaloid, C9H11NO. It is the main correctly distinguished between hy- active ingredient in fresh *khat. drogen and carbon dioxide. In 1784 cathode A negative electrode. In he showed that sparking mixtures of *electrolysis cations are attracted to hydrogen and oxygen produced pure the cathode. In vacuum electronic de- water, showing that water is not an vices electrons are emitted by the element. Also in sparking experi- cathode and Ûow to the *anode. It is ments with air, Cavendish recog- nised a small percentage unchanged therefore from the cathode that elec- Ü trons Ûow into these devices. How- (later identi ed as noble gases). Cavendish also did signiÜcant work ever, in a primary or secondary cell in physics. the cathode is the electrode that spontaneously becomes negative dur- c.c.p. Cubic close packing. See close ing discharge, and from which there- packing. fore electrons emerge. CD See circular dichroism. cathodic protection See sacrificial protection CD spectrum (circular dichroism . spectrum) The spectrum obtained by cation A positively charged ion, i.e. plotting the variable IR – IL against an ion that is attracted to the cath- frequency of the incident electro- ode in *electrolysis. Compare anion. magnetic radiation, where IR and IL

Cathine 109 cell are the absorption intensities for the spontaneous reaction is one in right- and left-circularly polarized which copper ions in solution take light, respectively. One application of electrons from the electrode and are CD spectroscopy is to determine the deposited on the electrode as copper conÜgurations of complexes of transi- atoms. In this case, the copper ac- c tion metals. If the CD spectra of simi- quires a positive charge. lar transition metal complexes The two half cells can be con- (including similarity of geometry) are nected by using a porous pot for the taken, the features of their CD spec- liquid junction (as in the *Daniell tra are also similar. cell) or by using a salt bridge. The re- CE See capillary electrophoresis. sulting cell can then supply current if the electrodes are connected celestine A mineral form of stron- through an external circuit. The cell tium sulphate, SrSO4. is written cell 1. A system in which two elec- Zn(s)|Zn2+(aq)|Cu2+(aq)|Cu trodes are in contact with an elec- E = 1.10 V trolyte. The electrodes are metal or Here, E is the e.m.f. of the cell equal carbon plates or rods or, in some to the potential of the right-hand cases, liquid metals (e.g. mercury). In electrode minus that of the left-hand an *electrolytic cell a current from electrode for zero current. Note that an outside source is passed through ‘right’ and ‘left’ refer to the cell as the electrolyte to produce chemical written. Thus, the cell could be writ- electrolysis change (see ). In a ten *voltaic cell, spontaneous reactions Cu(s)|Cu2+(aq)|Zn2+(aq)|Zn(s) between the electrodes and electrolyte(s) produce a potential differ- E = –1.10 V ence between the two electrodes. The overall reaction for the cell is Voltaic cells can be regarded as Zn(s) + Cu2+(aq) → Cu(s) + made up of two *half cells, each Zn2+(aq) composed of an electrode in contact This is the direction in which the cell with an electrolyte. For instance, a reaction occurs for a positive e.m.f. zinc rod dipped in zinc sulphate solu- The cell above is a simple example 2+ tion is a Zn|Zn half cell. In such a of a chemical cell; i.e. one in which system zinc atoms dissolve as zinc the e.m.f. is produced by a chemical ions, leaving a negative charge on difference. Concentration cells are the electrode cells in which the e.m.f. is caused by Zn(s) → Zn2+(aq) + 2e a difference of concentration. This The solution of zinc continues until may be a difference in concentration the charge build-up is sufÜcient to of the electrolyte in the two half prevent further ionization. There is cells. Alternatively, it may be an elec- then a potential difference between trode concentration difference (e.g. the zinc rod and its solution. This different concentrations of metal in cannot be measured directly, since an amalgam, or different pressures of measurement would involve making gas in two gas electrodes). Cells are contact with the electrolyte, thereby also classiÜed into cells without introducing another half cell (see transport (having a single electrolyte) electrode potential). A rod of cop- and with transport (having a liquid per in copper sulphate solution com- junction across which ions are trans- prises another half cell. In this case ferred). Various types of voltaic cell cellophane 110

exist, used as sources of current, stituent of the cell walls of all plants, standards of potential, and experi- many algae, and some fungi and is mental set-ups for studying electro- responsible for providing the rigidity chemical reactions. See also dry cell; of the cell wall. It is an important c primary cell; secondary cell; constituent of dietary Übre. The lithium battery. Übrous nature of extracted cellulose 2. See kerr effect (for Kerr cell). has led to its use in the textile industry for the production of cotton, cellophane Reconstituted cellulose artiÜcial silk, etc. in the form of a thin transparent sheet, made by extruding a viscous cellulose acetate See cellulose cellulose xanthate solution through a ethanoate. Ü ne slit into a bath of acid (see cellulose ethanoate (cellulose ac- rayon ). It is commonly used as a etate) A compound prepared by wrapping material, especially for treating cellulose (cotton linters or foodstuffs, but is being replaced by wood pulp) with a mixture of Û polypropene because of its amma- ethanoic anhydride, ethanoic acid, bility. and concentrated sulphuric acid. Cel- cellular plastic See expanded plas- lulose in the cotton is ethanoylated tic. and when the resulting solution is treated with water, cellulose cellular plastics Solid synthetic ethanoate forms as a Ûocculent white materials having an open structure. mass. It is used in lacquers, nonshat- A common example is rigid *poly- terable glass, varnishes, and as a Übre styrene foam used in insulation and (see also rayon). packaging. cellulose nitrate A highly Ûamma- celluloid A transparent highly ble material made by treating cellu- Ûammable substance made from cel- lose (wood pulp) with concentrated lulose nitrate with a camphor plasti- nitric acid. Despite the alternative cizer. It was formerly widely used as name nitrocellulose, the compound is a thermoplastic material, especially in fact an ester (containing CONO2 for Ülm (a use now discontinued groups), not a nitro compound Û owing to the ammability of cellu- (which would contain C–NO2). It is loid). used in explosives (as guncotton) and celluloid. cellulose A polysaccharide that consists of a long unbranched chain Celsius scale A *temperature scale of glucose units. It is the main con- in which the Üxed points are the

CH2OH CH2OH CH2OH O O O H H H H H H O 14 1 O 4 O OH H O OH H OH H H H H

H OH H OH H OH

14 b-glycosidic bonds

Cellulose 111 cerussite temperatures at standard pressure of ties are separated by rotating them in ice in equilibrium with water (0°C) a tube in a horizontal circle. The and water in equilibrium with steam denser particles tend to move along (100°C). The scale, between these two the length of the tube to a greater ra- temperatures, is divided in 100 de- dius of rotation, displacing the c grees. The degree Celsius (°C) is equal lighter particles to the other end. in magnitude to the *kelvin. This scale was formerly known as the ceramics Inorganic materials, such centigrade scale; the name was ofÜ- as pottery, enamels, and refractories. cially changed in 1948 to avoid con- Ceramics are metal silicates, oxides, fusion with a hundredth part of a nitrides, etc. grade. It is named after the Swedish cerebroside Any one of a class of astronomer Anders Celsius (1701–44), *glycolipids in which a single sugar who devised the inverted form of unit is bound to a sphingolipid (see this scale (ice point 100°, steam point phospholipid). The most common 0°) in 1742. cerebrosides are galactocerebrosides, cement Any of various substances containing the sugar group galactose; used for bonding or setting to a hard they are found in the plasma mem- material. Portland cement is a mix- branes of neural tissue and are abun- ture of calcium silicates and alumi- dant in the myelin sheaths of nates made by heating limestone neurones. (CaCO3) with clay (containing alumi- cerium Symbol Ce. A silvery metal- nosilicates) in a kiln. The product is lic element belonging to the *lan- Ü ground to a ne powder. When thanoids; a.n. 58; r.a.m. 140.12; r.d. mixed with water it sets in a few 6.77 (20°C); m.p. 799°C; b.p. 3426°C. hours and then hardens over a It occurs in allanite, bastnasite, longer period of time due to the for- cerite, and monazite. Four isotopes mation of hydrated aluminates and occur naturally: cerium–136, –138, silicates. –140, and –142; Üfteen radioisotopes cementation Any metallurgical have been identiÜed. Cerium is used process in which the surface of a in mischmetal, a rare-earth metal metal is impregnated by some other containing 25% cerium, for use in substance, especially an obsolete lighter Ûints. The oxide is used in process for making steel by heating the glass industry. It was discovered bars of wrought iron to red heat for by Martin Klaproth (1743–1817) in several days in a bed of charcoal. See 1803. also case hardening. A cementite See steel. • Information from the WebElements site centi- Symbol c. A preÜx used in cermet A composite material con- the metric system to denote one hun- sisting of a ceramic in combination dredth. For example, 0.01 metre = 1 with a sintered metal, used when a centimetre (cm). high resistance to temperature, corrosion, and abrasion is needed. centigrade scale See celsius scale. cerussite An ore of lead consisting of lead carbonate, PbCO . It is usually centrifugal pump See pump. 3 of secondary origin, formed by the centrifuge A device in which solid weathering of *galena. Pure cerussite or liquid particles of different densi- is white but the mineral may be grey cetane 112

due to the presence of impurities. It German-born British biochemist, forms well-shaped orthorhombic who began his research career at crystals. It occurs in the USA, Spain, Cambridge University in 1933. Two and SW Africa. years later he joined *Florey at Ox- c cetane See hexadecane. ford, where they isolated and puriÜed *penicillin. They also devel- cetane number A number that oped a method of producing the drug provides a measure of the ignition in large quantities and carried out its characteristics of a Diesel fuel when Ürst clinical trials. The two men it is burnt in a standard Diesel en- shared the 1945 Nobel Prize for phys- gine. It is the percentage of cetane iology or medicine with penicillin’s (hexadecane) in a mixture of cetane discoverer, Alexander *Fleming. and 1-methylnaphthalene that has the same ignition characteristics as chain reaction A reaction that is the fuel being tested. Compare oc- self-sustaining as a result of the prod- tane number. ucts of one step initiating a subsequent step. Chemical chain reactions CFC See chlorofluorocarbon. usually involve free radicals as inter- CGE Capillary gel electrophoresis. mediates. An example is the reaction See capillary electrophoresis. of chlorine with hydrogen initiated by ultraviolet radiation. A chlorine c.g.s. units A system of *units molecule is Ürst split into atoms: based on the centimetre, gram, and → • • second. Derived from the metric sys- Cl2 Cl + Cl tem, it was badly adapted to use with These react with hydrogen as follows • → • thermal quantities (based on the in- Cl + H2 HCl + H Ü consistently de ned *calorie) and • → • with electrical quantities (in which H + Cl2 HCl + Cl etc. two systems, based respectively on Combustion and explosion reactions unit permittivity and unit permeabil- involve similar free-radical chain re- ity of free space, were used). For sci- actions. Ü enti c purposes c.g.s. units have now chair See ring conformations. been replaced by *SI units. chair conformation See confor- chain A line of atoms of the same mation. type in a molecule. In a straight chain the atoms are attached only to single chalcedony A mineral consisting atoms, not to groups. Propane, for in- of a microcrystalline variety of stance, is a straight-chain alkane, *quartz. It occurs in several forms, CH3CH2CH3, with a chain of three including a large number of semi- carbon atoms. A branched chain is precious gemstones; for example, one in which there are side groups sard, carnelian, jasper, onyx, chryso- attached to the chain. Thus, 3-ethyl- prase, agate, and tiger’s-eye. octane, CH3CH2CH(C2H5)C5H11, is a chalcogens See group 16 elements. branched-chain alkane in which there is a side chain (C2H5) attached chalconides Binary compounds to the third carbon atom. A closed formed between metals and group 16 chain is a *ring of atoms in a mol- elements; i.e. oxides, sulphides, se- ecule; otherwise the molecule has an lenides, and tellurides. open chain. chalcopyrite (copper pyrites) A Chain, Sir Ernst Boris (1906–79) brassy yellow mineral consisting of a 113 charcoal mixed copper–iron sulphide, CuFeS2, because two pieces of initial data dif- crystallizing in the tetragonal sys- fering by even a very small amount tem; the principal ore of copper. It is will give widely different results at a similar in appearance to pyrite and later time. Originally, the theory was gold. It crystallizes in igneous rocks introduced to describe unpredictabil- c and hydrothermal veins associated ity in meteorology, as exempliÜed by with the upper parts of acid igneous the butterÛy effect. It has been sug- intrusions. Chalcopyrite is the most gested that the dynamical equations widespread of the copper ores, occur- governing the weather are so sensi- ring, for example, in Cornwall (UK), tive to the initial data that whether Sudbury (Canada), Chile, Tasmania or not a butterÛy Ûaps its wings in (Australia), and Rio Tinto (Spain). one part of the world may make the difference between a tornado occur- chalk A very Üne-grained white ring or not occurring in some other rock composed of the skeletal re- part of the world. In chemistry, mains of microscopic sea creatures, chaos theory has been used to ex- such as plankton, and consisting plain oscillatory (clock) reactions, largely of *calcium carbonate such as the *B–Z reaction, and (CaCO ). It is used in toothpaste and 3 chaotic reactions. cosmetics. It should not be confused with blackboard ‘chalk’, which is chaotic dynamics See chaos. made from calcium sulphate. chaotic reaction A type of chemi- change of phase (change of state) cal reaction in which the concentra- A change of matter in one physical tions of reactants show chaotic *phase (solid, liquid, or gas) into an- behaviour. This may occur when the other. The change is invariably ac- reaction involves a large number of companied by the evolution or complex interlinked steps. Under absorption of energy. such conditions, it is possible for the reaction to display unpredictable chaos Unpredictable and appar- changes with time. See also bistabil- ently random behaviour arising in a ity; oscillating reaction. system that is governed by determin- istic laws (i.e. laws that, given the charcoal A porous form of carbon state of the system at a given time, produced by the destructive distilla- uniquely determine the state at any tion of organic material. Charcoal other time). Chaotic dynamics are from wood is used as a fuel. All forms widespread in nature: examples are of charcoal are porous and are used the turbulent Ûow of Ûuids, the dy- for adsorbing gases and purifying namics of planetary moons, oscilla- and clarifying liquids. There are sev- tions in electrical circuits, and the eral types depending on the source. long-term unpredictability of the Charcoal from coconut shells is a par- weather. In such situations, chaos ticularly good gas adsorbent. Animal arises because the mathematical charcoal (or bone black) is made by equations expressing the (determinis- heating bones and dissolving out the tic) laws in question are nonlinear calcium phosphates and other min- and extremely sensitive to the initial eral salts with acid. It is used in sugar conditions. It is impossible to specify reÜning. Activated charcoal is char- data at a given time to a sufÜcient de- coal that has been activated for ad- gree of precision to be able to predict sorption by steaming or by heating the future behaviour of the system in a vacuum. Chargaff’s rule 114

Chargaff’s rule The principle that electrolytes they are positive and in any sample of DNA the amount of negative ions. adenine equals the amount of charge density 1. The electric thymine and the amount of guanine charge per unit volume of a medium equals the amount of cytosine. It is a c or body (volume charge density). consequence of *base pairing. The 2. The electric charge per unit sur- rule was published in 1950 by the face area of a body (surface charge Austrian–American biochemist Erwin density). Chargaff (1905–2002). charge-transfer complex A charge A property of some *ele- chemical compound in which there mentary particles that gives rise to is weak coordination involving the an interaction between them and transfer of charge between two mol- consequently to the host of material ecules. An example is phenoquinone, phenomena described as electrical. in which the phenol and quinone Charge occurs in nature in two molecules are not held together by forms, conventionally described as formal chemical bonds but are asso- positive and negative in order to dis- ciated by transfer of charge between tinguish between the two kinds of in- the compounds’ aromatic ring sys- teraction between particles. Two tems. particles that have similar charges (both negative or both positive) inter- Charles, Jacques Alexandre act by repelling each other; two par- César (1746–1823) French chemist ticles that have dissimilar charges and physicist, who became professor (one positive, one negative) interact of physics at the Paris Conservatoire by attracting each other. des Arts et Métiers. He is best re- The natural unit of negative charge membered for discovering *Charles’ is the charge on an *electron, which law (1787), relating to the volume is equal but opposite in effect to the and temperature of a gas. In 1783 he positive charge on the proton. Large- became the Ürst person to make an scale matter that consists of equal ascent in a hydrogen balloon. numbers of electrons and protons is Charles’ law The volume of a Üxed electrically neutral. If there is an ex- mass of gas at constant pressure excess of electrons the body is nega- pands by a constant fraction of its tively charged; an excess of protons volume at 0°C for each Celsius de- results in a positive charge. A Ûow of gree or kelvin its temperature is charged particles, especially a Ûow raised. For any *ideal gas the fraction of electrons, constitutes an electric is approximately 1/273. This can be current. Charge is measured in expressed by the equation V = V (1 + coulombs, the charge on an electron 0 t/273), where V is the volume at 0°C being 1.602 × 10–19 coulombs. 0 and V is its volume at t°C. This is charge carrier The entity that equivalent to the statement that the transports electric charge in an elec- volume of a Üxed mass of gas at con- tric current. The nature of the carrier stant pressure is proportional to its depends on the type of conductor: in thermodynamic temperature, V = kT, metals, the charge carriers are elec- where k is a constant. The law re- trons; in *semiconductors the carri- sulted from experiments begun ers are electrons (n-type) or positive around 1787 by Jacques *Charles but *holes (p-type); in gases the carriers was properly established only by the are positive ions and electrons; in more accurate results published in 115 chemical bond

2+ → 1802 by Joseph *Gay-Lussac. Thus the [Cu(OH2)6] + en [Cu(en)(OH2)4 + law is also known as Gay-Lussac’s 2H2O law. An equation similar to that results in a net increase in the num- given above applies to pressures for ber of molecules (from 2 to 3). The ideal gases: p = p0(1 + t/273), a rela- reaction c tionship known as Charles’ law of 2+ → pressures. See also gas laws. [Cu(OH2)6] + 2NH3 Cu(NH3)2(OH2)4 + 2H2O cheddite Any of a group of high ex- involves no net increase in the num- plosives made from nitro compounds ber of molecules. As a result, the mixed with sodium or potassium chelate reaction has a larger reaction chlorate. entropy and is more favourable. chelate An inorganic complex in cheletropic reaction A type of ad- which a *ligand is coordinated to a dition reaction in which a conjugated metal ion at two (or more) points, so molecule forms two single bonds that there is a ring of atoms including the metal. The process is known from terminal atoms of the conju- as chelation. Chelating agents are gated system to a single atom on an- classiÜed according to the number of other molecule to give a cyclic points at which they can coordinate. adduct. Cheletropic reactions are A ligand such as diaminoethane types of *pericyclic reaction. (H2N(CH2)2NH2), which can coordi- ChemDraw A widely used chemi- nate at two points, is said to be cal drawing and modelling program bidentate. The angle made between produced by CambridgeSoft. two bonds coordinating to a metal A atom is the bite angle of the ligand. • The CambridgeSoft website, giving See also sequestration. details about ChemDraw and associated

H2C software NH2 H C chemical bond A strong force of 2 NH Cu2+ 2 attraction holding atoms together in NH a molecule or crystal. Typically chem- 2 CH 2 ical bonds have energies of about NH 2 1000 kJ mol–1 and are distinguished CH2 from the much weaker forces be- Chelate tween molecules (see van der waals’ force). There are various types. chelate effect The effect in which Ionic (or electrovalent) bonds can be a chelate complex is generally more formed by transfer of electrons. For stable than the analogous complex instance, the calcium atom has an formed with monodentate ligands. electron conÜguration of [Ar]4s2, i.e. For example, the complex ion [Cu(en) it has two electrons in its outer shell. 2+ 2 5 (OH2)4] is more stable than the com- The chlorine atom is [Ne]3s 3p , with 2+ plex ion [Cu(NH3)2 (OH2)4] . Here, en seven outer electrons. If the calcium denotes the bidentate ethylene di- atom transfers two electrons, one to amine (1,2-diaminoethane) ligand. each chlorine atom, it becomes a The main cause of the chelate effect Ca2+ ion with the stable conÜgura- is the effect of reaction entropy tion of an inert gas [Ar]. At the same when the complex is formed. Thus, time each chlorine, having gained the reaction one electron, becomes a Cl– ion, also chemical cell 116

with an inert-gas conÜguration [Ar]. ionic and covalent contributions. The bonding in calcium chloride is Thus, in hydrogen chloride, H–Cl, the electrostatic attraction between the bonding is predominantly cova- the ions. lent with one pair of electrons shared Covalent bonds are formed by shar- between the two atoms. However, c ing of valence electrons rather than the chlorine atom is more elec- by transfer. For instance, hydrogen tronegative than the hydrogen and atoms have one outer electron (1s1). has more control over the electron In the hydrogen molecule, H2, each pair; i.e. the molecule is polarized atom contributes 1 electron to the with a positive charge on the hydro- bond. Consequently, each hydrogen gen and a negative charge on the atom has control of 2 electrons – one chlorine, forming a *dipole. See also of its own and the second from the banana bond; hydrogen bond; other atom – giving it the electron metallic bond; multicentre bond; conÜguration of an inert gas [He]. In multiple bond. the water molecule, H O, the oxygen 2 chemical cell See cell. atom, with six outer electrons, gains control of an extra two electrons sup- chemical combination The com- plied by the two hydrogen atoms. bination of elements to give com- This gives it the conÜguration [Ne]. pounds. There are three laws of Similarly, each hydrogen atom gains chemical combination. control of an extra electron from the (1) The law of constant composition oxygen, and has the [He] electron states that the proportions of the el- conÜguration. ements in a compound are always A particular type of covalent bond the same, no matter how the com- is one in which one of the atoms sup- pound is made. It is also called the plies both the electrons. These are law of constant proportions or known as coordinate (semipolar or deÜnite proportions. dative) bonds, and written A→B, (2) The law of multiple proportions where the direction of the arrow de- states that when two elements A and notes the direction in which elec- B combine to form more than one trons are donated. compound, then the masses of B that Covalent or coordinate bonds in combine with a Üxed mass of A are which one pair of electrons is shared in simple ratio to one another. For are electron-pair bonds and are example, carbon forms two oxides. known as single bonds. Atoms can In one, 12 grams of carbon is com- also share two pairs of electrons to bined with 16 grams of oxygen (CO); form double bonds or three pairs in in the other 12 g of carbon is com- orbital triple bonds. See . bined with 32 grams of oxygen (CO2). In a compound such as sodium The oxygen masses combining with a chloride, Na+Cl–, there is probably Üxed mass of carbon are in the ratio complete transfer of electrons in 16:32, i.e. 1:2. forming the ionic bond (the bond is (3) The law of equivalent proportions said to be heteropolar). Alternatively, states that if two elements A and B in the hydrogen molecule H–H, the each form a compound with a third pair of electrons is equally shared be- element C, then a compound of A tween the two atoms (the bond is ho- and B will contain A and B in the rel- mopolar). Between these two ative proportions in which they react extremes, there is a whole range of with C. For example, sulphur and intermediate bonds, which have both carbon both form compounds with 117 chemically induced dynamic nuclear polarization hydrogen. In methane 12 g of carbon relative numbers of molecules react- react with 4 g of hydrogen. In hydro- ing, are called the stoichiometric gen sulphide, 32 g of sulphur react coefÜcients. The sum of the coefÜ- with 2 g of hydrogen (i.e. 64 g of S for cients of the reactants minus the 4 g of hydrogen). Sulphur and carbon sum of the coefÜcients of the prod- c form a compound in which the C:S ucts (x + y – z – w in the example) is ratio is 12:64 (i.e. CS2). The law is the stoichiometric sum. If this is zero sometimes called the law of recipro- the equation is balanced. Sometimes cal proportions. a generalized chemical equation is considered chemical dating An absolute *dat- ν ν → ν ing technique that depends on meas- 1A1 + 2A2 + … … nAn + ν uring the chemical composition of a n+1An+1 … specimen. Chemical dating can be In this case the reaction can be writ- Σν used when the specimen is known to ten iAi = 0, where the convention is undergo slow chemical change at a that stoichiometric coefÜcients are known rate. For instance, phosphate positive for reactants and negative in buried bones is slowly replaced by for products. The stoichiometric sum Û Σν uoride ions from the ground water. is i. Measurement of the proportion of chemical equilibrium A re- Ûuorine present gives a rough esti- versible chemical reaction in which mate of the time that the bones have the concentrations of reactants and been in the ground. Another, more products are not changing with time accurate, method depends on the because the system is in thermody- fact that amino acids in living organ- namic equilibrium. For example, the isms are l-optical isomers. After reversible reaction death, these racemize and the age of ˆ bones can be estimated by measuring 3H2 + N2 2NH3 the relative amounts of d- and l- is in chemical equilibrium when the amino acids present. See amino acid rate of the forward reaction racemization. → 3H2 + N2 2NH3 chemical engineering The study is equal to the rate of the back reac- of the design, manufacture, and oper- tion ation of plant and machinery in in- → 2NH3 3H2 + N2 dustrial chemical processes. See also equilibrium constant. chemical equation A way of de- chemical equivalent See equiva- noting a chemical reaction using the lent weight. symbols for the participating particles (atoms, molecules, ions, etc.); for chemical fossil Any of various or- example, ganic compounds found in ancient xA + yB → zC + wD geological strata that appear to be biological in origin and are assumed The single arrow is used for an irre- to indicate that life existed when the versible reaction; double arrows (ˆ) rocks were formed. The presence of are used for reversible reactions. chemical fossils in Archaean strata When reactions involve different indicates that life existed over 3000 phases it is usual to put the phase in million years ago. brackets after the symbol (s = solid; l = liquid; g = gas; aq = aqueous). The chemically induced dynamic nu- numbers x, y, z, and w, showing the clear polarization See cidnp. chemical markup language 118

chemical markup language chemical shift A change in the (CML) A markup language used to normal wavelength of absorption or express chemical information. CML is emission of electromagnetic wave- an extension of XML (extensible length in a process in which there is markup language), which is widely a nuclear energy change (as in the c used to store and transfer text. A par- *Mössbauer effect and *nuclear mag- ticular feature of XML is the use of netic resonance) or a change in elec- tags to identify different types of in- tron energy levels in the inner shells formation. These are commonly in of an atom (as in X-ray *photoelec- angle brackets. In the text version of tron spectroscopy). this dictionary, for example, head- chemical warfare The use of toxic words are surrounded by the tags chemical substances in warfare or …, cross references are military operations. A large number identiÜed by …, etc. In of chemicals have been designed or CML, there are tags for many differ- used for warfare, including pul- ent types of chemical information. monary agents (chlorine, *carbonyl For example, structures can be repre- chloride, *diphosgene), blister sented by tagged data giving the agents, (*lewisite, *sulphur mustard, atoms and their coordinates in two *nitrogen mustard), and the *nerve or three dimensions. Reactions can agents. Chemical warfare agents are also be indicated, as well as data for classiÜed as weapons of mass destruc- spectra and other properties. tion by the United Nations. See also A chemical weapons convention. • An FAQ produced by the Chemistry Department, Cambridge University Chemical Weapons Convention An agreement dating from 1993 on chemical potential Symbol: µ. For the production, stockpiling, and use a given component in a mixture, the of chemical weapons (see chemical coefÜcient ∂G/∂n, where G is the warfare). It came into force in 1997 Gibbs free energy and n the amount and is administered by the Organiza- of substance of the component. The tion for the Prohibition of Chemical chemical potential is the change in Weapons (OPCW). Gibbs free energy with respect to A change in amount of the component, • The OPCW website, giving details of the with pressure, temperature, and convention amounts of other components being constant. Components are in equilib- chemiluminescence See lumines- rium if their chemical potentials are cence. equal. chemiosmotic theory A theory chemical reaction A change in postulated by the British biochemist which one or more chemical el- Peter Mitchell (1920–92) to explain ements or compounds (the reactants) the formation of ATP in the mito- form new compounds (the products). chondrial *electron transport chain. All reactions are to some extent re- As electrons are transferred along versible; i.e. the products can also the electron carrier system in the react to give the original reactants. inner mitochondrial membrane, hy- However, in many cases the extent of drogen ions (protons) are actively this back reaction is negligibly small, transported (via *hydrogen carriers) and the reaction is regarded as irre- into the space between the inner and versible. outer mitochondrial membranes, 119 Chinese white

2H+ 2H+ 2H+ inner mitochondrial membrane

NADH cytochrome cytochrome dehydrogenase b-c complex oxidase 1 c mitochondrial matrix 3ADP + P ATPase

6H+ 3 ATP

Chemiosmotic theory which thus contains a higher concen- features including calculation of mo- tration of protons than the matrix. lecular weight, calculation of per- This creates an electrochemical gra- centages of elements present, IUPAC dient across the inner membrane, name generation, and viewing in Ras- down which protons move back into Mol. the matrix. This movement occurs A through special channels associated • Details and download from Advanced with ATP synthetase, the enzyme Chemistry Development website that catalyses the conversion of ADP flint to ATP, and is coupled with the phos- chert See . phorylation of ADP. A similar gradi- Chile saltpetre A commercial min- ent is created across the thylakoid eral largely composed of *sodium ni- membranes of chloroplasts during trate from the caliche deposits in the light-dependent reactions of Chile. Before the ammonia-oxidation *photosynthesis. process for nitrates most imported chemisorption See adsorption. Chilean saltpetre was used by the chemical industry; its principal use chemistry The study of the el- today is as an agricultural source of ements and the compounds they nitrogen. form. Chemistry is mainly concerned with effects that depend on the outer Chime A plug-in that allows chemi- electrons in atoms. See biochemistry; cal structures to be displayed within geochemistry; inorganic chem- a web page. The structures, which Ü istry; organic chemistry; physical are embedded as mol les, can be chemistry. viewed interactively; i.e. they can be rotated, expanded, exported, and chemoinformatics The branch of rendered in *RasMol. The program is chemistry concerned with methods the copyright of MDL Information of representing molecules and reac- Systems, Inc., and is available free tions and with the design and use of (subject to a license agreement). databases for storing chemical infor- A mation. • A downloadable version of Chime at ChemSketch A commonly used the MDL website (free registration chemical drawing program for 2D required) and 3D structures, copyright of Ad- china clay See kaolin. vanced Chemistry Development, Inc. The program has certain additional Chinese white See zinc oxide. chirality 120

chirality The property of existing chloral hydrate See 2,2,2- in left- and right-handed structural trichloroethanediol. forms. See optical activity. chlorates Salts of the chloric acids; chirality element The part of a i.e. salts containing the ions ClO– – c molecule that makes it exist in left- (chlorate(I) or hypochlorite), ClO2 – and right-handed forms. In most (chlorate(III) or chlorite), ClO3 (chlo- – cases this is a chirality centre (i.e. an rate(V)), or ClO4 (chlorate(VII) or per- asymmetric atom). In certain cases chlorate). When used without the element is a chirality axis. For ex- speciÜcation of an oxidation state the ample, in allenenes of the type term ‘chlorate’ refers to a chlorate(V) R1R2C=C=CR3R4 the C=C=C chain is a salt. chirality axis. Certain ring compounds may display chirality as a re- chloric acid Any of the oxoacids of sult of a chirality plane in the chlorine: *chloric(I) acid, *chloric(III) molecule. acid, *chloric(V) acid, and *chloric(VII) acid. The term is com- chirooptical spectroscopy Spec- monly used without speciÜcation of troscopy making use of the proper- the oxidation state of chlorine to ties of chiral substances when they mean chloric(V) acid, HClO3. interact with polarized light of various wavelengths. Optical rotatory chloric(I) acid (hypochlorous acid) dispersion (the change of optical rota- A liquid acid that is stable only in so- tion with wavelength) and *circular lution, HOCl. It may be prepared by dichroism are old examples of chi- the reaction of chlorine with an agi- rooptical spectroscopy. More recent tated suspension of mercury(II) oxide. types of chirooptical spectroscopy are Because the disproportionation of – vibrational optical rotatory disper- the ion ClO is slow at low tempera- sion, vibrational circular dichroism. tures chloric(I) acid may be produced, (VCD), and Raman optical activity along with chloride ions by the reac- (ROA), which are all the result of tion of chlorine with water at 0°C. At the interaction between chiral sub- higher temperatures disproportiona- – stances and polarized infrared tion to the chlorate(V) ion, ClO3 , electromagnetic radiation; these takes place. Chloric(I) acid is a very techniques are known as vibrational weak acid but is a mild oxidizing optical activity (VOA), as they are as- agent and is widely used as a bleach- sociated with transitions in the vibra- ing agent. tional energy levels in the electronic chloric(III) acid (chlorous acid) A ground state of a molecule. Chiro- pale-yellow acid known only in solu- optical spectroscopy is used in the tion, HClO2. It is formed by the reac- analysis of the structure of mol- tion of chlorine dioxide and water ecules. and is a weak acid and an oxidizing chitin A *polysaccharide compris- agent. d ing chains of N-acetyl- -glucosamine, chloric(V) acid (chloric acid) A a derivative of glucose. Chitin is colourless unstable liquid, HClO3; r.d. structurally very similar to cellulose 1.2; m.p. <–20°C; decomposes at and serves to strengthen the support- 40°C. It is best prepared by the reac- ing structures of various inverte- tion of barium chlorate with sul- brates. It also occurs in fungi. phuric acid although chloric(V) acid chloral See trichloroethanal. is also formed by the disproportiona- 121 chlorite tion of chloric(I) acid in hot solutions. bleaching, and the manufacture of a It is both a strong acid and a power- large number of organic chemicals. ful oxidizing agent; hot solutions of It reacts directly with many el- the acid or its salts have been known ements and compounds and is a to detonate in contact with readily strong oxidizing agent. Chlorine c oxidized organic material. compounds contain the element in the 1, 3, 5, and 7 oxidation states. It chloric(VII) acid (perchloric acid) was discovered by Karl Scheele in An unstable liquid acid, HClO ; r.d. 4 1774 and Humphry Davy conÜrmed 1.76; m.p. –112°C; b.p. 39°C (50 it as an element in 1810. mmHg); explodes at about 90°C at atmospheric pressure. There is also a A monohydrate (r.d. 1.88 (solid), 1.77 • Information from the WebElements site ° (liquid); m.p. 48 C; explodes at about chlorine dioxide A yellowish-red ° 110 C) and a dihydrate (r.d. 1.65; m.p. explosive gas, ClO ; d. 3.09 g dm–3; ° ° 2 –17.8 C; b.p. 200 C). Commercial m.p. –59.5°C; b.p. 9.9°C. It is soluble chloric(VII) acid is a water azeotrope, in cold water but decomposed by hot which is 72.5% HClO4, boiling at water to give chloric(VII) acid, chlo- ° 203 C. The anhydrous acid may be rine, and oxygen. Because of its high prepared by vacuum distillation of reactivity, chlorine dioxide is best the concentrated acid in the presence prepared by the reaction of sodium of magnesium perchlorate as a dehy- chlorate and moist oxalic acid at drating agent. Chloric(VII) acid is 90°–100°C, as the product is then di- both a strong acid and a strong oxi- luted by liberated carbon dioxide. dizing agent. It is widely used to de- Commercially the gas is produced by compose organic materials prior to the reaction of sulphuric acid con- analysis, e.g. samples of animal or taining chloride ions with sulphur vegetable matter requiring heavy- dioxide. Chlorine dioxide is widely metal analysis. used as a bleach in Ûour milling and chloride See halide. in wood pulping and also Ünds application in water puriÜcation. chlorination 1. A chemical reaction in which a chlorine atom is in- chlorine monoxide See dichlo- troduced into a compound. See rine oxide. halogenation . 2. The treatment of chlorite 1. See chlorates. 2. A water with chlorine to disinfect it. group of layered silicate minerals, chlorine Symbol Cl. A *halogen el- usually green or white in colour, that ement; a.n. 17; r.a.m. 35.453; d. 3.214 are similar to the micas in structure gdm–3; m.p. –100.98°C; b.p. –34.6°C. and crystallize in the monoclinic It is a poisonous greenish-yellow system. Chlorites are composed of gas and occurs widely in nature as complex silicates of aluminium, sodium chloride in seawater and as magnesium, and iron in combina- halite (NaCl), carnallite (KCl.MgCl2. tion with water, with the formula 6H2O), and sylvite (KCl). It is manu- (Mg,Al,Fe)12(Si,Al)8O20(OH)16. They are factured by the electrolysis of brine most common in low-grade meta- and also obtained in the *Downs morphic rocks and also occur as sec- process for making sodium. It has ondary minerals in igneous rocks as many applications, including the alteration products of pyroxenes, am- chlorination of drinking water, phiboles, and micas. The term is de- chloroacetic acids 122

× –2 rived from chloros, the Greek word CHCl2COOH 5.0 10 × –1 for green. CCl3COOH 2.3 10 chloroacetic acids See chloro- chloroethene (vinyl chloride) A ethanoic acids . gaseous compound, CH2:CHCl; r.d. ° ° c chlorobenzene A colourless highly 0.911; m.p. –153.8 C; b.p. –13.37 C. It Û is made by chlorinating ethene to ammable liquid, C6H5Cl; r.d. 1.106; m.p. –45.43°C; b.p. 131.85°C. It is pre- give dichloroethane, then removing pared by the direct chlorination of HCl: → → benzene using a halogen carrier (see C2H4 + Cl2 CH2ClCH2Cl friedel–crafts reaction), or manu- CH2CHCl factured by the *Raschig process. It The compound is used in making is used mainly as an industrial sol- PVC. vent. chloroÛuorocarbon (CFC) A type 2-chlorobuta-1,3-diene (chloro- of compound in which some or all of prene) A colourless liquid chlori- the hydrogen atoms of a hydrocar- nated diene, CH2:CClCH:CH2; r.d. bon (usually an alkane) have been re- 0.96; b.p. 59°C. It is polymerized to placed by chlorine and Ûuorine make synthetic rubbers (e.g. neo- atoms. Most chloroÛuorocarbons are prene). chemically unreactive and are stable chlorocruorin A greenish iron-con- at high temperatures. They are used taining respiratory pigment that oc- as aerosol propellants, refrigerants, curs in the blood of polychaete and solvents, and in the manufacture worms. It closely resembles *haemo- of rigid packaging foam. A com- globin. monly encountered commercial name for these compounds is freon, chloroethane (ethyl chloride) A e.g. freon 12 is dichlorodiÛuoro- Û colourless ammable gas, C2H5Cl; methane (CCl F ). ChloroÛuoro- ° ° 2 2 m.p. –136.4 C; b.p. 12.3 C. It is made carbons, because of their chemical by reaction of ethene and hydrogen inertness, can diffuse unchanged into chloride and used in making lead the upper atmosphere. Here, photo- tetraethyl for petrol. chemical reactions cause them to chloroethanoic acids (chloroacetic break down and react with ozone (see acids) Three acids in which hydro- ozone layer). For this reason, their gen atoms in the methyl group of use has been discouraged. ethanoic acid have been replaced by chloroform See trichloro- chlorine atoms. They are: mono- methane. chloroethanoic acid (CH2ClCOOH); chloromethane (methyl chloride) dichloroethanoic acid (CHCl2COOH); A colourless Ûammable gas, CH Cl; trichloroethanoic acid (CCl3COOH). 3 The presence of chlorine atoms in r.d. 0.916; m.p. –97.1°C; b.p. –24.2°C. the methyl group causes electron It is a *haloalkane, made by direct withdrawal from the COOH group chlorination of methane and used as and makes the chloroethanoic acids a local anaesthetic and refrigerant. stronger acids than ethanoic acid it- chlorophenol Any of the various –3 self. The Ka values (in moles dm at compounds produced by chlorinating ° 25 C) are a *phenol. Chlorophenols are fairly × –5 CH3COOH 1.7 10 acidic and have many uses, including × –3 CH2ClCOOH 1.3 10 antiseptics, disinfectants, herbicides, 123 chromatogram insecticides, and wood preservatives, uid crystal in which molecules lie in and in making dyes. They form con- sheets at angles that change by a densation polymers of the bakelite small amount between each sheet. type with methanal. The pitch of the helix so formed is similar to the wavelength of visible chlorophyll One of a number of c pigments, including (chlorophyll a light. Cholesteric liquid crystals and chlorophyll b), that are responsi- therefore diffract light and have ble for the green colour of most colours that depend on the tempera- plants. Chlorophyll molecules are the ture. (The word cholesteric is derived principal sites of light absorption in from the Greek for bile solid.) the light-dependent reactions of cholesterol A *sterol occurring *photosynthesis. They are magne- widely in animal tissues and also in sium-containing *porphyrins, chemi- some higher plants and algae. It can cally related to *cytochrome and exist as a free sterol or esteriÜed with *haemoglobin. a long-chain fatty acid. Cholesterol is CHCH CH absorbed through the intestine or 2 3 manufactured in the liver. It serves CH principally as a constituent of blood CH C H plasma lipoproteins and of the 3 2 5 N N lipid–protein complexes that form cell membranes. It is also important CH Mg CH as a precursor of various steroids, es- H N N pecially the bile acids, sex hormones, CH CH and adrenocorticoid hormones. The 3 3 derivative 7-dehydrocholesterol is C H converted to vitamin D3 by the action CH CH CO of sunlight on skin. Increased levels 2 of dietary and blood cholesterol have CH COOCH 2 3 been associated with atherosclerosis, a condition in which lipids accumu- CCO late on the inner walls of arteries and C H O hydrocarbon eventually obstruct blood Ûow. 20 39 (phytol) chain choline An amino alcohol, Chlorophyll CH2OHCH2N(CH3)3OH. It occurs widely in living organisms as a chloroplatinic acid A reddish crys- constituent of certain types of talline compound, H2PtCl6, made by phospholipids – the lecithins and dissolving platinum in aqua regia. sphingomyelins – and in the neurotransmitter *acetylcholine. It is chloroprene See 2-chlorobuta-1,3- Ü diene sometimes classi ed as a member . of the vitamin B complex. chlorosulphanes disulphur See chromate A salt containing the ion dichloride. 2– CrO4 . chlorous acid chloric(iii) acid See . chromatogram A record obtained choke damp See blackdamp. by *chromatography. The term is applied to the developed records of cholecalciferol See vitamin d. *paper chromatography and *thin- cholesteric crystal A type of *liq- layer chromatography and also to the chromatography 124

graphical record produced in *gas chromatographic technique in which chromatography. the size of the sample molecules is important. chromatography A technique for See also affinity chromatog- analysing or separating mixtures of raphy; gas chromatography; high- c gases, liquids, or dissolved sub- performance liquid chromatogra- stances. The original technique (in- phy; paper chromatography; rf vented by the Russian botanist value; thin-layer chromatography. Mikhail Tsvet in 1906) is a good example of column chromatography. A chrome alum See potassium vertical glass tube is packed with an chromium sulphate. adsorbing material, such as alumina. chrome iron ore A mixed The sample is poured into the col- iron–chromium oxide, FeO.Cr2O3, umn and continuously washed used to make ferrochromium for through with a solvent (a process chromium steels. known as elution). Different components of the sample are adsorbed to chrome red A basic lead chromate, different extents and move down the PbO.PbCrO4, used as a red pigment. column at different rates. In Tsvet’s chrome yellow Lead chromate, original application, plant pigments PbCrO4, used as a pigment. were used and these separated into chromic acid coloured bands in passing down the A hypothetical acid, H CrO , known only in chromate column (hence the name chromatog- 2 4 salts. raphy). The usual method is to collect the liquid (the eluate) as it passes out chromic anhydride See chro- from the column in fractions. mium(vi) oxide. In general, all types of chromatog- chromic compounds Compounds raphy involve two distinct phases – containing chromium in a higher (+3 the stationary phase (the adsorbent or +6) oxidation state; e.g. chromic material in the column in the exam- oxide is chromium(VI) oxide (CrO ). ple above) and the moving phase (the 3 solution in the example). The separa- chromite A spinel mineral, tion depends on competition for mol- FeCr2O4; the principal ore of chro- ecules of sample between the mium. It is black with a metallic moving phase and the stationary lustre and usually occurs in massive phase. The form of column chro- form. It is a common constituent of matography above is an example of peridotites and serpentines. The adsorption chromatography, in chief producing countries are which the sample molecules are ad- Turkey, South Africa, Russia, the sorbed on the alumina. In partition Philippines, and Zimbabwe. chromatography, a liquid (e.g. water) chromium Symbol Cr. A hard sil- is Ürst absorbed by the stationary very *transition element; a.n. 24; phase and the moving phase is an r.a.m. 52.00; r.d. 7.19; m.p. 1857°C; immiscible liquid. The separation is b.p. 2672°C. The main ore is then by *partition between the two chromite (FeCr2O4). The metal has a liquids. In ion-exchange chromatog- body-centred-cubic structure. It is ex- raphy (see ion exchange), the process tracted by heating chromite with involves competition between differ- sodium chromate, from which ent ions for ionic sites on the station- chromium can be obtained by elec- ary phase. *Gel Ültration is another trolysis. Alternatively, chromite can 125 chromous compounds be heated with carbon in an electric oxide at 420–450°C and 200–300 at- furnace to give ferrochrome, which mospheres. The compound is unsta- is used in making alloy steels. The ble. metal is also used as a shiny decora- chromium(VI) oxide (chromium tive electroplated coating and in the c manufacture of certain chromium trioxide; chromic anhydride) A red compound, CrO3; rhombic; r.d. 2.70; compounds. ° At normal temperatures the metal m.p. 196 C. It can be made by careful is corrosion-resistant. It reacts with addition of concentrated sulphuric dilute hydrochloric and sulphuric acid to an ice-cooled concentrated acids to give chromium(II) salts. aqueous solution of sodium dichro- These readily oxidize to the more sta- mate with stirring. The mixture is Ü ble chromium(III) salts. Chromium then ltered through sintered glass, also forms compounds with the +6 washed with nitric acid, then dried at ° oxidation state, as in chromates, 120 C in a desiccator. 2– Chromium(VI) oxide is an ex- which contain the CrO4 ion. The element was discovered in 1797 by tremely powerful oxidizing agent, Vauquelin. especially to organic matter; it im- A mediately inÛames ethanol. It is an acidic oxide and dissolves in water to • Information from the WebElements site form ‘chromic acid’, a powerful oxi- chromium dioxide See chro- dizing agent and cleansing Ûuid for mium(iv) oxide. glassware. At 400°C, chromium(VI) chromium(II) oxide A black insol- oxide loses oxygen to give uble powder, CrO. Chromium(II) chromium(III) oxide. oxide is prepared by oxidizing chromium potassium sulphate chromium amalgam with air. At high A red crystalline solid, temperatures hydrogen reduces it to K2SO4.Cr2(SO4)3.24H2O; r.d. 1.91. It is the metal. used as a mordant See also alums. chromium(III) oxide (chromium chromium sesquioxide See sesquioxide) A green crystalline chromium(iii) oxide. water-insoluble salt, Cr2O3; r.d. 5.21; m.p. 2435°C; b.p. 4000°C. It is ob- chromium steel Any of a group of tained by heating chromium in a *stainless steels containing 8–25% of stream of oxygen or by heating am- chromium. A typical chromium steel monium dichromate. The industrial might contain 18% of chromium, 8% preparation is by reduction of of nickel, and 0.15% of carbon. sodium dichromate with carbon. Chromium steels are highly resistant Chromium(III) oxide is amphoteric, to corrosion and are used for cutlery, dissolving in acids to give chro- chemical plant, ball bearings, etc. mium(III) ions and in concentrated chromium trioxide See solutions of alkalis to give chromites. chromium(vi) oxide. It is used as a green pigment in glass, porcelain, and oil paint. chromophore A group causing coloration in a *dye. Chromophores are chromium(IV) oxide (chromium generally groups of atoms having de- dioxide) A black insoluble powder, ° localized electrons. CrO2; m.p. 300 C. It is prepared by the action of oxygen on chromous compounds Com- chromium(VI) oxide or chromium(III) pounds containing chromium in its chromyl chloride 126

lower (+2) oxidation state; e.g. chro- cine-substitution A type of substi- mous chloride is chromium(II) chlo- tution reaction in which the entering ride (CrCl2). group takes a position on an atom adjacent to the atom to which the chromyl chloride (chromium oxy- leaving group is attached. See also c chloride) A dark red liquid, CrO Cl ; 2 2 tele-substitution. r.d. 1.911; m.p. –96.5°C; b.p. 117°C. It is evolved as a dark-red vapour on ad- cinnabar A bright red mineral dition of concentrated sulphuric acid form of mercury(II) sulphide, HgS, to a mixture of solid potassium crystallizing in the hexagonal sys- dichromate and sodium chloride; it tem; the principal ore of mercury. It condenses to a dark-red covalent liq- is deposited in veins and impregna- uid, which is immediately hydrolysed tions near recent volcanic rocks and by solutions of alkalis to give the yel- hot springs. The chief sources in- low chromate. Since bromides and clude Spain, Italy, and Yugoslavia. iodides do not give analogous com- cinnamic acid (3-phenylpropenoic Ü pounds this is a speci c test for acid) A white crystalline aromatic chloride ions. The compound is a *carboxylic acid, C6H5CH:CHCOOH; powerful oxidizing agent, exploding r.d. 1.248 (trans isomer); m.p. on contact with phosphorus and in- 135–136°C; b.p. 300°C. Esters of cin- Û aming sulphur and many organic namic acid occur in some essential compounds. oils. serpentine chrysotile See . CIP system (Cahn–Ingold–Prelog CI See colour index. system) A system for the unambiguous description of stereoisomers used CIDNP (chemically induced dynamic in the R–S convention (see absolute nuclear polarization) A mechanism configuration) and in the *E–Z con- Û enabling nuclear spin to in uence vention. The system involves a se- the direction of a chemical reaction. quence rule for determining a This can occur in certain cases, even conventional order of ligands. The though the gap between energy lev- rule is that the atom bonded directly els of nuclear spin states in a mag- to the chiral centre or double bond is netic Üeld is very much smaller than considered and the ligand in which the dissociation energies of chemical this atom has the highest proton bonds. Two radicals, the electrons of number takes precedence. So, for ex- which have parallel spins, can only ample, I takes precedence over Cl. If combine if the conversion of a *trip- two ligands have bonding atoms with let to a *singlet can take place. In a the same proton number, then sub- magnetic Üeld, a triplet has three stituents are taken into account (with nondegenerate states called T0, T+, the substituent of highest proton and T–. For a triplet-to-singlet conver- number taking precedence). Thus, sion to take place, one electron must –C2H5 has a higher precedence than precess faster than the other for a –CH3. If a double (or triple) bond oc- sufÜcient time to enable a 180° phase curs to a substituent, then the sub- difference to develop. This difference stituent is counted twice (or three in precession can arise when the nu- times). An isotope of high nucleon clear spin interacts with the electron number takes precedence over one on the radical, by means of hyperÜne of lower nucleon number. Hydrogen coupling. always has lowest priority in this sys- 127 Clapeyron–Clausius equation tem. For example, the sequence for A some common ligands is I, Br, Cl, • Information about IUPAC nomenclature SO3H, OCOCH3, OCH3, OH, NO2, NH2, cisplatin A platinum complex, cis- COOCH3, CONH2, COCH3, CHO, [PtCl (NH ) ], used in cancer treat- CH2OH, C6H5, C2H5, CH3, H. The sys- 2 3 2 c tem was jointly developed by the ment to inhibit the growth of British chemists Robert Sidney Cahn tumours. It acts by binding between (1899–1981) and Sir Christopher Kelk strands of DNA. Ingold (1893–1970) and the Bosnian– citrate A salt or ester of citric acid. Swiss chemist Vladimir Prelog (1906– 98). citric acid A white crystalline hydroxycarboxylic acid, circular birefringence A phenom- HOOCCH2C(OH)(COOH)CH2COOH; enon in which there is a difference r.d. 1.54; m.p. 153°C. It is present in between the refractive indices of the citrus fruits and is an intermediate in molecules of a substance for right- the *Krebs cycle in plant and animal and left-circularly polarized light. Cir- cells. cular birefringence depends on the way in which the electromagnetic citric-acid cycle See krebs cycle. Ü eld interacts with the molecule, and CL20 See hniw. is affected by the handedness of the molecule, and hence its polarizabil- Claisen condensation A reaction ity. If a molecule has the shape of a of esters in which two molecules of helix, the polarizability is dependent the ester react to give a keto ester, on whether or not the electric Üeld e.g. Ü → of the electromagnetic eld rotates in 2CH3COOR CH3COCH2COOR the same sense as the helix, thus giv- + ROH ing rise to circular birefringence. The reaction is catalysed by sodium circular dichroism (CD) The pro- ethoxide, the mechanism being simi- duction of an elliptically polarized lar to that of the *aldol reaction. It is wave when a linearly polarized light named after Ludwig Claisen wave passes through a substance that (1851–1930). has differences in the extinction Clapeyron–Clausius equation Ü A coef cients for left- and right-handed differential equation that describes polarized light. The size of this effect φ π λ η η φ the relationship between variables is given by = / ( l – r), where is when there is a change in the state the ellipticity of the beam that of a system. In a system that has two emerges (in radians), λ is the wave- η η *phases of the same substance, for length of the light, and l and r are example solid and liquid, heat is the absorption indices of the left- and added or taken away very slowly so right-handed circularly polarized that one phase can change reversibly light, respectively. Circular dichro- into the other while the system re- ism is a property of optically active mains at equilibrium. If the two molecules and is used to obtain infor- phases are denoted A and B, the mation about proteins. See also cd Clapeyron–Clausius equation is: spectrum. dp/dT = L/T(VB–VA), circular polarization See polariza- where p is the pressure, T is the ther- tion of light. modynamic temperature, L is the cis See isomerism; torsion angle. heat absorbed per mole in the Clark cell 128

change from A to B, and VB and VA The process was perfected in 1902 by are the volumes of B and A respec- the French scientist Georges Claude tively. In the case of a transition from (1870–1960). liquid to vapour, the volume of the claudetite A mineral form of *ar- liquid can be ignored. Taking the c senic(III) oxide, As O . vapour to be an *ideal gas, the 4 6 Clapeyron–Clausius equation can be Clausius, Rudolf Julius Em- written: manuel (1822–88) German physi- 2 cist, who held teaching posts in dlogep/dT = L/RT Berlin and Zurich, before going to The Clapeyron–Clausius equation is Würzburg in 1869. He is best known named after the French engineer for formulating the second law of Benoit-Pierre-Émile Clapeyron *thermodynamics in 1850, indepen- (1799–1864) and Rudolf *Clausius. dently of William Thomson (Lord Clark cell A type of *voltaic cell *Kelvin). In 1865 he introduced the consisting of an anode made of zinc concept of *entropy, and later con- amalgam and a cathode of mercury tributed to electrochemistry and elec- both immersed in a saturated solu- trodynamics (see clausius–mossotti tion of zinc sulphate. The Clark cell equation). was formerly used as a standard of Clausius inequality An inequality e.m.f.; the e.m.f. at 15°C is 1.4345 that relates the change in entropy dS volts. It is named after the British sci- in an irreversible process to the heat entist Hosiah Clark (d. 1898). supplied to the system dQ and the Clark process See hardness of thermodynamic temperature T, i.e. water. dS ≥ dQ/T. In the case of an irreversible adiabatic change, where dQ = clathrate A solid mixture in which 0, the Clausius inequality has the small molecules of one compound or form dS > 0, which means that for element are trapped in holes in the this type of change the entropy of crystal lattice of another substance. the system must increase. The in- Clathrates are sometimes called equality is named after Rudolf Julius enclosure compounds or cage com- Emmanuel *Clausius. The Clausius pounds, but they are not true com- inequality can be used to demon- pounds (the molecules are not held strate that entropy increases in such by chemical bonds). Quinol and ice processes as the free expansion of a both form clathrates with such sub- gas and the cooling of an initially hot stances as sulphur dioxide and xenon. substance. Claude process A process for liq- Clausius–Mossotti equation A uefying air on a commercial basis. relation between the *polarizability Air under pressure is used as the α of a molecule and the dielectric working substance in a piston en- constant ε of a dielectric substance gine, where it does external work made up of molecules with this po- and cools adiabatically. This cool air larizability. The Clausius–Mossotti is fed to a counter-current heat ex- equation can be written in the form changer, where it reduces the tem- α π ε ε perature of the next intake of = (3/4 N)/[( – 1)/( – 2)], high-pressure air. The same air is re- where N is the number of molecules compressed and used again, and after per unit volume. The equation pro- several cycles eventually liqueÜes. vides a link between a microscopic 129 close packing quantity (the polarizability) and a tic properties of clay; the particles macroscopic quantity (the dielectric have the property of being able to constant); it was derived using hold water. The chief groups of clay macroscopic electrostatics by the Ital- minerals are: ian physicist Ottaviano Fabrizio kaolinite, Al4Si4O10(OH)8, the chief c Mossotti (1791–1863) in 1850 and in- constituent of *kaolin; dependently by Rudolf *Clausius in halloysite, Al4Si4(OH)8O10.4H2O; 1879. It works best for gases and is illite, KAl4(Si,Al)8O18.2H2O; only approximately true for liquids montmorillonite, (Na,Ca)0.33(Al,Mg)2- Û or solids, particularly if the dielectric Si4O10(OH)2.nH2O, formed chie y constant is large. Compare lorentz– through alteration of volcanic ash; lorenz equation. and Claus process A process for obtain- vermiculite, (Mg,Fe,Al)3(Al,Si)4- ing sulphur from hydrogen sulphide O10(OH)2.4H2O, used as an insulat- (from natural gas or crude oil). It in- ing material and potting soil. volves two stages. First, part of the cleavage The splitting of a crystal hydrogen sulphide is oxidized to sul- along planes of atoms in the lattice. phur dioxide: Clemmensen reduction A 2H S + 3O → 2SO + 2H O 2 2 2 2 method of reducing a *carbonyl Subsequently, the sulphur dioxide re- group (C=O) to CH2, using zinc amal- acts with hydrogen sulphide to pro- gam and concentrated hydrochloric duce sulphur: acid. It is used as a method of ascend- → SO2 + 2H2S 3S + 2H2O ing a homologous series. The reac- The second stage occurs at 300°C and tion is named after Erik Clemmensen needs an iron or aluminium oxide (1876–1941). catalyst. clinal See torsion angle. Ü clay A ne-grained deposit consist- clock reaction See b–z reaction; Û ing chie y of *clay minerals. It is oscillating reaction. characteristically plastic and virtually impermeable when wet and cracks closed chain See chain; ring. when it dries out. In geology the size close packing The packing of of the constituent particles is usually spheres so as to occupy the mini- taken to be less than 1/256 mm. In mum amount of space. In a single soil science clay is regarded as a soil plane, each sphere is surrounded by with particles less than 0.002 mm in six close neighbours in a hexagonal size. arrangement. The spheres in the sec- clay minerals Very small particles, ond plane Üt into depressions in the chieÛy hydrous silicates of alu- Ürst layer, and so on. Each sphere has minium, sometimes with magne- 12 other touching spheres. There are sium and/or iron substituting for all two types of close packing. In hexag- or part of the aluminium, that are onal close packing the spheres in the the major constituents of clay ma- third layer are directly over those in terials. The particles are essentially the Ürst, etc., and the arrangement of crystalline (either platy or Übrous) planes is ABAB…. In cubic close pack- with a layered structure, but may be ing the spheres in the third layer oc- amorphous or metalloidal. The clay cupy a different set of depressions minerals are responsible for the plas- than those in the Ürst. The arrange- closo-structure 130

ment is ABCABC…. See also cubic coated with a lipid membrane and crystal. contain enzymes that are capable of A converting such substances as glu- • An interactive version of cubic close- cose into more complex molecules, c packing such as starch. Coacervate droplets • An interactive version of hexagonal arise spontaneously under appropri- close packing ate conditions and may have been the prebiological systems from borane closo-structure See . which living organisms originated. club drug A drug typically used by coagulation The process in which young people at clubs, parties, etc. colloidal particles come together irre- Examples are ecstasy, gammahydrox- versibly to form larger masses. Coag- ybutyric acid (GHB), and metham- ulation can be brought about by phetamines. adding ions to change the ionic Clusius column A device for sepa- strength of the solution and thus rating isotopes by thermal diffusion. destabilize the colloid (see floccula- One form consists of a vertical col- tion). Ions with a high charge are umn some 30 metres high with a particularly effective (e.g. alum, con- heated electric wire running along its taining Al3+, is used in styptics to co- axis. The lighter isotopes in a gaseous agulate blood). Another example of mixture of isotopes diffuse faster ionic coagulation is in the formation than the heavier isotopes. Heated by of river deltas, which occurs when the axial wire, and assisted by nat- colloidal silt particles in rivers are co- ural convection, the lighter atoms agulated by ions in sea water. Alum are carried to the top of the column, and iron (III) sulphate are also used where a fraction rich in lighter iso- for coagulation in sewage treatment. topes can be removed for further en- Heating is another way of coagulat- richment. ing certain colloids (e.g. boiling an egg coagulates the albumin). cluster compound A compound in which groups of metal atoms are coal A brown or black carbonaceous joined together by metal–metal deposit derived from the accumula- bonds. The formation of such com- tion and alteration of ancient vegeta- pounds is a feature of the chemistry tion, which originated largely in of certain elements, particularly swamps or other moist environ- molybdenum and tungsten, but also ments. As the vegetation decom- vanadium, tantalum, niobium, and posed it formed layers of peat, which uranium. Isopoly compounds are were subsequently buried (for exam- ones in which the cluster contains ple, by marine sediments following a atoms of the same element; het- rise in sea level or subsidence of the eropoly compounds contain a mix- land). Under the increased pressure ture of different elements. and resulting higher temperatures the peat was transformed into coal. CML See chemical markup lan- Two types of coal are recognized: guage. humic (or woody) coals, derived from coacervate An aggregate of macro- plant remains; and sapropelic coals, molecules, such as proteins, lipids, which are derived from algae, spores, and nucleic acids, that form a stable and Ünely divided plant material. *colloid unit with properties that re- As the processes of coaliÜcation (i.e. semble living matter. Many are the transformation resulting from 131 cobalt the high temperatures and pressures) structive distillation of coal. For- continue, there is a progressive trans- merly, coal tar was obtained as a by- formation of the deposit: the propor- product in manufacturing *coal gas. tion of carbon relative to oxygen Now it is produced in making coke rises and volatile substances and for steel making. The crude tar con- c water are driven out. The various tains a large number of organic stages in this process are referred to compounds, such as benzene, naph- as the ranks of the coal. In ascending thalene, methylbenzene, phenols, order, the main ranks of coal are: lig- etc., which can be obtained by distil- nite (or brown coal), which is soft, lation. The residue is pitch. At one brown, and has a high moisture con- time coal tar was the major source of tent; subbituminous coal, which is organic chemicals, most of which are used chieÛy by generating stations; now derived from petroleum and bituminous coal, which is the most natural gas. abundant rank of coal; semibitumi- cobalamin (vitamin B ) See vita- nous coal; semianthracite coal, which 12 min b complex. has a Üxed carbon content of between 86% and 92%; and anthracite cobalt Symbol Co. A light-grey coal, which is hard and black with a *transition element; a.n. 27; r.a.m. Üxed carbon content of between 92% 58.933; r.d. 8.9; m.p. 1495°C; b.p. and 98%. 2870°C. Cobalt is ferromagnetic Most deposits of coal were formed below its Curie point of 1150°C. during the Carboniferous and Per- Small amounts of metallic cobalt are mian periods. More recent periods of present in meteorites but it is usually coal formation occurred during the extracted from ore deposits worked early Jurassic and Tertiary periods. in Canada, Morocco, and Zaïre. It is Coal deposits occur in all the major present in the minerals cobaltite, continents; the leading producers in- smaltite, and erythrite but also asso- clude the USA, China, Ukraine, ciated with copper and nickel as sul- Poland, UK, South Africa, India, Aus- phides and arsenides. Cobalt ores are tralia, and Germany. Coal is used as a usually roasted to the oxide and then fuel and in the chemical industry; by- reduced with carbon or water gas. products include coke and coal tar. Cobalt is usually alloyed for use. Al- nico is a well-known magnetic alloy coal gas A fuel gas produced by the and cobalt is also used to make stain- destructive distillation of coal. In the less steels and in high-strength alloys late-19th and early-20th centuries that are resistant to oxidation at high coal gas was a major source of energy temperatures (for turbine blades and and was made by heating coal in the cutting tools). absence of air in local gas works. Typ- The metal is oxidized by hot air ically, it contained hydrogen (50%), and also reacts with carbon, phos- methane (35%), and carbon monoxide phorus, sulphur, and dilute mineral (8%). By-products of the process were acids. Cobalt salts, usual oxidation *coal tar and coke. The use of this states II and III, are used to give a type of gas declined with the increas- brilliant blue colour in glass, tiles, ing availability of natural gas, al- and pottery. Anhydrous cobalt(II) though since the early 1970s interest chloride paper is used as a qualitative has developed in using coal in mak- test for water and as a heat-sensitive ing *SNG. ink. Small amounts of cobalt salts are coal tar A tar obtained from the de- essential in a balanced diet for mam- cobalt(II) oxide 132

mals (see essential element). ArtiÜ- roxylon coca). It stimulates the central cially produced cobalt–60 is an im- nervous system and has effects simi- portant radioactive tracer and lar to the amphetamines. It was origi- cancer-treatment agent. The element nally used as a local anaesthetic. The c was discovered by Georg Brandt illegal drug is usually the soluble hy- (1694–1768) in 1737. drochloride. This can be converted A into the free-base form (known as • Information from the WebElements site crack cocaine) by dissolving in water and heating with sodium bicarbon- cobalt(II) oxide A pink solid, CoO; ate. Cocaine is a class A drug in the cubic; r.d. 6.45; m.p. 1935°C. The ad- UK. It can be detected by *Scott’s dition of potassium hydroxide to a test. solution of cobalt(II) nitrate gives a bluish-violet precipitate, which on O

boiling is converted to pink impure CH3 cobalt(II) hydroxide. On heating this O in the absence of air, cobalt(II) oxide N

is formed. The compound is readily CH3 oxidized in air to form tricobalt O tetroxide, Co3O4, and is readily reduced by hydrogen to the metal. O cobalt(III) oxide (cobalt sesquioxide) A black grey insoluble solid, Cocaine Co2O3; hexagonal or rhombic; r.d. Û Û 5.18; decomposes at 895°C. It is pro- cocurrent ow Flow of two uids duced by the ignition of cobalt ni- in the same direction with transfer of matter or heat between them. Com- trate; the product however never has countercurrent flow the composition corresponding ex- pare .

actly to cobalt(III) oxide. On heating codeine An alkaloid C18H21NO3 it readily forms Co3O4, which con- found in opium. It is structurally sim- tains both Co(II) and Co(III), and is ilar to morphine, from which it is easily reduced to the metal by hydro- produced, and is used in the form of gen. Cobalt(III) oxide dissolves in the sulphate or phosphate as a strong acid to give unstable brown painkiller and cough medicine. solutions of trivalent cobalt salts. Codeine is converted into morphine With dilute acids cobalt(II) salts are in the liver. It is used to some extent formed. as a recreational drug. In the UK it is a class B drug but can be obtained in cobalt steel Any of a group of composite over-the-counter prepara- alloy *steels containing 5–12% of tions in which it has a low concentra- cobalt, 14–20% of tungsten, usually tion and is combined with paraceta- with 4% of chromium and 1–2% of mol or ibuprofen. See also opioids. vanadium. They are very hard but somewhat brittle. Their main use is coenzyme An organic nonprotein in high-speed tools. molecule that associates with an enzyme molecule in catalysing bio- cobalt thiocyanate test See chemical reactions. Coenzymes usu- scott’s test. ally participate in the substrate– cocaine A powerful drug present in enzyme interaction by donating or the leaver of the coca plant (Eryth- accepting certain chemical groups. 133 collagen

Many vitamins are precursors of Thus, they are much more difÜcult to coenzymes. See also cofactor. oxidize and are more resistant to corrosion. In addition, the fact that they coenzyme A (CoA) A complex or- have d-electrons makes them show ganic compound that acts in conjunc- variable valency (CuI, CuII, and CuIII; tion with enzymes involved in c AgI and AgII; AuI and AuIII) and form a various biochemical reactions, no- wide range of coordination com- tably the oxidation of pyruvate via pounds. They are generally classiÜed the *Krebs cycle and fatty-acid oxida- with the *transition elements. tion and synthesis. It comprises principally the B vitamin pantothenic coke A form of carbon made by the acid, the nucleotide adenine, and a destructive distillation of coal. Coke ribose-phosphate group. is used for blast-furnaces and other metallurgical and chemical processes coenzyme Q See ubiquinone. requiring a source of carbon. Lower- cofactor A nonprotein component grade cokes, made by heating the essential for the normal catalytic ac- coal to a lower temperature, are used tivity of an enzyme. Cofactors may as smokeless fuels for domestic heat- be organic molecules (*coenzymes) ing. or inorganic ions. They may activate colchicine An *alkaloid derived the enzyme by altering its shape or from the autumn crocus, Colchicum they may actually participate in the autumnale. It inhibits cell division. chemical reaction. Colchicine is used in genetics, cytol- coherent anti-Stokes Raman ogy, and plant breeding research and spectroscopy See cars. also in cancer therapy to inhibit cell division. coherent units A system of *units of measurement in which derived OMe units are obtained by multiplying or dividing base units without the use MeO OMe of numerical factors. *SI units form a coherent system; for example the MeO unit of force is the newton, which is equal to 1 kilogram metre per second squared (kg m s–2), the kilogram, metre, and second all being base O units of the system. cohesion The force of attraction NH between like molecules. O coinage metals A group of three Me malleable ductile transition metals Colchicine forming group 11 (formerly IB) of the *periodic table: copper (Cu), silver Ü (Ag), and gold (Au). Their outer elec- collagen An insoluble brous pro- tronic conÜgurations have the form tein found extensively in the connec- nd10(n+1)s1. Although this is similar to tive tissue of skin, tendons, and that of alkali metals, the coinage bone. The polypeptide chains of col- metals all have much higher ioniza- lagen (containing the amino acids tion energies and higher (and posi- glycine and proline predominantly) tive) standard electrode potentials. form triple-stranded helical coils that collective oscillation 134

are bound together to form Übrils, mixture of a solid and liquid (or two which have great strength and lim- liquids) is passed between two discs a ited elasticity. Collagen accounts for small distance apart, which rotate over 30% of the total body protein of very rapidly relative to each other. c mammals. Other types of colloid mills are the valve and oriÜce types, in which the collective oscillation An oscilla- Ü tion in a many-body system in which mixture is forced through an ori ce all the particles in the system take at a very high speed and then strikes part in a cooperative. Plasma oscilla- a breaker ring. Applications of colloid tions provide an example of collec- mills occur in food processing, in tive oscillations. In systems described paint manufacture, and in the phar- by quantum mechanics, collective os- maceutical industry. cillations are quantized to give collec- colloids Colloids were originally tive excitations. deÜned by Thomas *Grahamin 1861 colligation The combination of as substances, such as starch or two free radicals to form a covalent gelatin, which will not diffuse • • → bond, as in H3C + Cl CH3Cl. It is through a membrane. He distin- the reverse of *homolytic Üssion. guished them from crystalloids (e.g. inorganic salts), which would pass colligative properties Properties through membranes. Later it was rec- that depend on the concentration of ognized that colloids were distin- particles (molecules, ions, etc.) pre- guished from true solutions by the sent in a solution, and not on the na- presence of particles that were too ture of the particles. Examples of small to be observed with a normal colligative properties are osmotic microscope yet were much larger pressure (see osmosis), *lowering of than normal molecules. Colloids are vapour pressure, *depression of now regarded as systems in which freezing point, and *elevation of boil- there are two or more phases, with ing point. one (the dispersed phase) distributed collision density The number of in the other (the continuous phase). collisions that occur in unit volume Moreover, at least one of the phases in unit time when a given particle has small dimensions (in the range Ûux passes through matter. 10–9–10–6 m). Colloids are classiÜed in collision quenching See external various ways. conversion. Sols are dispersions of small solid particles in a liquid. The particles Ü collodion A thin lm of cellulose may be macromolecules or may be nitrate made by dissolving the cellu- clusters of small molecules. Lyopho- lose nitrate in ethanol or ethoxy- bic sols are those in which there is ethane, coating the surface, and no afÜnity between the dispersed evaporating the solvent. phase and the liquid. An example is colloid mills Machines used to silver chloride dispersed in water. In grind aggregates into very Üne parti- such colloids the solid particles have cles or to apply very high shearing a surface charge, which tends to stop forces within a Ûuid to produce col- them coming together. Lyophobic loid suspensions or emulsions in sols are inherently unstable and in which the particle sizes are less than time the particles aggregate and 1 micrometer. One type of colloid form a precipitate. Lyophilic sols, on mill is called a disc mill, in which a the other hand, are more like true so- 135 combinatorial chemistry lutions in which the solute molecules tron has energy levels similar to are large and have an afÜnity for the those of a particle in a box. F-centres solvent. Starch in water is an exam- can be produced by chemical activity ple of such a system. Association col- or by irradiation. loids are systems in which the colour index (CI) A list, regarded as c dispersed phase consists of clusters deÜnitive, of dyes and pigments, of molecules that have lyophobic and which includes information on their lyophilic parts. Soap in water is an as- commercial names, method of appli- sociation colloid (see micelle). cation, colour fastness, etc. Emulsions are colloidal systems in which the dispersed and continuous colour photography Any of vari- phases are both liquids, e.g. oil-in- ous methods of forming coloured water or water-in-oil. Such systems images on Ülm or paper by photo- require an emulsifying agent to stabi- graphic means. One common process lize the dispersed particles. is a subtractive reversal system that Gels are colloids in which both dis- utilizes a Ülm with three layers of persed and continuous phases have a light-sensitive emulsion, one re- three-dimensional network through- sponding to each of the three pri- out the material, so that it forms a mary colours. On development a jelly-like mass. Gelatin is a common black image is formed where the example. One component may some- scene is blue. The white areas are times be removed (e.g. by heating) to dyed yellow, the complementary col- leave a rigid gel (e.g. silica gel). our of blue, and the blackened areas Other types of colloid include are bleached clean. A yellow Ülter aerosols (dispersions of liquid or between this emulsion layer and solid particles in a gas, as in a mist the next keeps blue light from the or smoke) and foams (dispersions of second emulsion, which is green- gases in liquids or solids). sensitive. This is dyed magenta where no green light has fallen. The colophony See rosin Ünal emulsion is red-sensitive and is colorimetric analysis Quantita- given a cyan (blue-green) image on tive analysis of solutions by estimat- the negative after dying. When white ing their colour, e.g. by comparing it light shines through the three dye with the colours of standard solu- layers the cyan dye subtracts red tions. where it does not occur in the scene, the magenta subtracts green, and the colour centre A defect in a crystal yellow subtracts blue. The light pro- that changes the way in which it ab- jected by the negative therefore re- sorbs light or other electromagnetic constructs the original scene either radiation. Impurities in the crystal af- as a transparency or for use with fect the bind structure and allow transitions in different regions of the printing paper. spectrums. Impurity colour centres columbium A former name for the are responsible for the characteristic element *niobium. colours of many gemstones. A par- column chromatography See ticular type of colour centre is an F- chromatography. centre. This is a missing negative ion in an ionic crystal, where the overall combinatorial chemistry A tech- charge neutrality occurs by trapping nique in which very large numbers an electron in the vacancy. The elec- of related compounds are formed in combustion 136

octahedral square-planar

tetrahedral trigonal-bipyramid

Complexes

small quantities in cells on a plate, of the basis vectors of the sublattice. and properties are investigated by The phase transition between a com- some automated technique. It is par- mensurate lattice and an incommen- ticularly used in the development of surate lattice can be analysed using new drugs. the Frenkel–Kontorowa model. An ion that is common to two or more combustion A chemical reaction components in a mixture. In a solu- in which a substance reacts rapidly tion of XCl, for example, the addition with oxygen with the production of of another chloride YCl, may precipi- heat and light. Such reactions are tate XCl because the solubility prod- often free-radical chain reactions, uct is exceeded as a result of the which can usually be summarized as _ extra concentration of Cl ions. This the oxidation of carbon to form its is an example of a common-ion oxides and the oxidation of hydrogen effect. to form water. See also flame. common salt See sodium commensurate lattice A lattice chloride. that can be divided into two or more sublattices, with the basis vectors of competitive inhibition See inhi- the lattice being a rational multiple bition. 137 Compton effect complementarity The concept there are two components. See also that a single model may not be ade- phase rule. quate to explain all the observations compound A substance formed by made of atomic or subatomic sys- Ü tems in different experiments. For the combination of elements in xed c example, *electron diffraction is best proportions. The formation of a com- explained by assuming that the elec- pound involves a chemical reaction; Ü tron is a wave (see de broglie wave- i.e. there is a change in the con gura- length), whereas the *photoelectric tion of the valence electrons of the effect is described by assuming that atoms. Compounds, unlike mixtures, it is a particle. The idea of two differ- cannot be separated by physical ent but complementary concepts to means. See also molecule. Ü treat quantum phenomena was rst comproportionation A reaction put forward by the Danish physicist in which an element in a higher oxi- Niels Bohr (1855–1962) in 1927. dation state reacts with the same el- complex A compound in which ement in a lower oxidation state to molecules or ions form coordinate give the element in an intermediate bonds to a metal atom or ion (see il- oxidation state. For example lustration). Often complexes occur as 2+ → + 2+ Ag (aq) + Ag(s) 2Ag (aq) complex ions, such as [Cu(H2O)6] or 3– It is the reverse of *disproportiona- Fe[(CN)6] . A complex may also be a tion. neutral molecule (e.g. PtCl2(NH3)2). The formation of such coordination Compton, Arthur Holly complexes is typical behaviour of (1892–1962) US physicist, who be- transition metals. The complexes came professor of physics at the Uni- formed are often coloured and have versity of Chicago in 1923. He is best unpaired electrons (i.e. are paramag- known for his discovery (1923) of the netic). See also ligand; chelate. *Compton effect, for which he complex ion See complex. shared the 1927 Nobel Prize for complexometric analysis A type physics with C. T. R. Wilson. of volumetric analysis in which the Compton effect The reduction in reaction involves the formation of an the energy of high-energy (X-ray or inorganic *complex. gamma-ray) photons when they are component A distinct chemical scattered by free electrons, which species in a mixture. If there are no thereby gain energy. The phenome- reactions taking place, the number of non, Ürst observed in 1923 by A. H. components is the number of sepa- *Compton, occurs when the photon rate chemical species. A mixture of collides with an electron; some of the water and ethanol, for instance, has photon’s energy is transferred to the two components (but is a single electron and consequently the pho- ν ν phase). A mixture of ice and water ton loses energy h( 1 – 2), where h is ν ν has two phases but one component the *Planck constant and 1 and 2 (H2O). If an equilibrium reaction oc- are the frequencies before and after ν >ν curs, the number of components is collision. As 1 2, the wavelength taken to be the number of chemical of the radiation increases after the species minus the number of reac- collision. This type of inelastic scat- tions. Thus, in tering is known as Compton scatter- ˆ H2 + I2 2HI ing and is similar to the *Raman computational chemistry 138

effect. See also inverse compton ef- tively lower density of particles. By fect. random motion, particles will move from the area of high concentration computational chemistry The towards the area of low concentra- use of computers in chemical re- tion, by the process of *diffusion, c search. With the increase in process- until the particles are evenly distrib- ing power of computers, calculations uted in the solution or gas. on individual molecules and on chemical systems have become im- concerted reaction A type of reac- portant tools for research and indus- tion in which there is only one stage trial development. With simple rather than a series of steps. The SN2 molecules, predictions can be made mechanism in *nucleophilic substitu- about electronic structure and prop- tions is an example. See also peri- erties using *ab-initio calculations. cyclic reactions. For more complex molecules *semi- condensation The change of a empirical calculations are used. vapour or gas into a liquid. The The Üeld has been particularly exchange of phase is accompanied by panded by the *density-functional the evolution of heat (see latent method of treating large molecules heat). and by the availability of software for analysing molecular behaviour and condensation polymerization structure. See also molecular model- See polymer. ling. condensation pump See diffu- concentrated Describing a solu- sion pump. tion that has a relatively high con- condensation reaction A chemi- centration of solute. cal reaction in which two molecules concentration The quantity of dis- combine to form a larger molecule solved substance per unit quantity of with elimination of a small mol- aldehydes ke- a solution. Concentration is meas- ecule (e.g. H2O). See ; ured in various ways. The amount of tones. substance dissolved per unit volume condenser A device used to cool a of the solution (symbol c) has units of vapour to cause it to condense to a mol dm–3 or mol l–1. It is now called liquid. See liebig condenser. amount concentration (formerly molarity). The mass concentration (sym- conducting polymer An organic bol ρ) is the mass of solute per unit polymer that conducts electricity. volume of solution. It has units of Conducting polymers have a crys- kg dm–3, g cm–3, etc. The molality is talline structure in which chains of the amount of substance per unit conjugated unsaturated carbon– mass of solvent, commonly given in carbon bonds are aligned. Examples units of mol kg–1. See also mole frac- are polyacetylene and polypyrrole. tion. There has been considerable interest in the development of such materials concentration cell See cell. because they would be cheaper and concentration gradient (diffusion lighter than metallic conductors. gradient) The difference in concen- They do, however, tend to be chemi- tration between a region of a solu- cally unstable and, so far, no com- tion or gas that has a high density of mercial conducting polymers have particles and a region that has a rela- been developed. 139 conformation conductiometric titration A type that has n degrees of freedom, where of titration in which the electrical the values q describe the degrees of conductivity of the reaction mixture freedom. For example, in a gas of N is continuously monitored as one atoms each atom has three positional reactant is added. The equivalence coordinates, so the conÜguration c point is the point at which this space is 3N-dimensional. If the parti- undergoes a sudden change. The cles also have internal degrees of method is used for titrating coloured freedom, such as those caused by vi- solutions, which cannot be used with bration and rotation in a molecule, normal indicators. then these must be included in the conÜguration space, which is conse- conduction band See energy quently of a higher dimension. See bands. also statistical mechanics. conductivity water See distilled conformation One of the very water. large number of possible spatial Condy’s Ûuid A mixture of cal- arrangements of atoms that can be cium and potassium permanganates interconverted by rotation about a (manganate(VII)) used as an antisep- single bond in a molecule. In the case tic. of ethane, H3C–CH3, one methyl conÜguration 1. The arrangement group can rotate relative to the of atoms or groups in a molecule. other. There are two extreme cases. 2. The arrangement of electrons In one, the C–H bonds on one group about the nucleus of an *atom. align with the C–H bonds on the other (as viewed along the C–C conÜguration space The n-dimen- bond). This is an eclipsed conforma- sional space with coordinates tion (or eclipsing conformation) and (q1,q2,…,qn) associated with a system corresponds to a maximum in a

eclipsed conformation anti conformation gauche conformation

= methyl group

Conformations of butane (sawhorse projection)

R H R O

R R RR O H bisecting conformation eclipsed conformation

Conformations of R3CHO (Newman projection) conformational analysis 140

graph of potential energy against ro- conformational analysis The de- tation angle. In the other the C–H termination or estimation of the bonds on one group bisect the angle relative energies of possible confor- between two C–H bonds on the mations of a molecule and the effect c other. This is a staggered conforma- of these on the molecule’s proper- tion (or bisecting conformation) and ties. corresponds to a minimum in the potential-energy curve. In the case of conformational isomer (con- ethane, the energy difference be- former) One of a set of stereoisomers tween the two conformations is that differ from each other by tor- small (2.8 kcal/mole) and effective sion angle or angles (where only free rotation occurs under normal structures that are minima of poten- conditions. tial energy are considered). In the case of butane, rotation can congeners Elements that belong occur about the bond between the to the same group in the periodic second and third carbon atoms, table. MeH2C–CH2Me. The highest maximum occurs when the methyl conjugate acids and bases See acid groups are eclipsed (they are at their . closest). Maxima of lower energies conjugated Describing double or occur when the methyl groups on triple bonds in a molecule that are one carbon atom eclipse the hydro- separated by one single bond. For ex- gen atoms on the other. The lowest ample, the organic compound buta- potential energy occurs when a C–Me 1,3-diene, H2C=CH–CH=CH2, has bond bisects the angle between the conjugated double bonds. In such C–H bonds (i.e. the two methyl molecules, there is some delocaliza- groups are furthest apart). This is tion of electrons in the pi orbitals be- called the anti conformation. A lesser tween the carbon atoms linked by minimum occurs when the methyl the single bond. group bisects the angle between a C–H bond and a C–Me bond. This conjugate solutions Solutions arrangement is called the gauche formed between two liquids that conformation. are partially miscible with one an- Similar rotational arrangements other; such solutions are in equilib- occur in other types of molecule. For rium at a particular temperature. instance, in a compound such as Examples of conjugate solutions R3C–CHO, with a C=O double bond, are phenol in water and water in one conformation has the C=O bond phenol. bisecting the angle between C–R conjugation Delocalization of pi bonds. This is called the bisecting electrons as occurs in *conjugated conformation (note that the C–H systems. Conjugation can also in- bond eclipses one of the C–R bonds volve d orbitals and lone pairs of in this case). The other has the C=O electrons. bond eclipsing a C–R bond. This is the eclipsed or eclipsing conforma- connection table A way of repre- tion. (In this case the C–H bond bi- senting a molecule as a table show- sects the angle between the C–R ing the atoms, their coordinates, and bonds.) the links between them. The MDL See also torsion angle; ring con- *molÜle format uses a connection formations. table in its representation of struc- 141 contact process ture. Connection tables are a useful added to it). According to Einstein’s way of storing molecular data, both mass–energy relationship, every from the point of view of graphics quantity of energy (E) has a mass (m), programs and also for database which is given by E/c2, where c is the searches, in which it is possible to speed of light. Therefore if mass is c use the table to look for substruc- conserved, the law of conservation of tures. energy must be of equally wide application. conservation The sensible use of the earth’s natural resources in order consolute temperature The tem- to avoid excessive degradation and perature at which two partially mis- impoverishment of the environment. cible liquids become fully miscible as It should include the search for alter- the temperature is increased. native food and fuel supplies when constantan An alloy having an these are endangered (as by defor- Ü electrical resistance that varies only estation and over shing); an aware- very slightly with temperature (over ness of the dangers of *pollution; a limited range around normal room and the maintenance and preserva- temperatures). It consists of copper tion of natural habitats and the cre- (50–60%) and nickel (40–50%) and is ation of new ones, e.g. nature used in resistance wire, thermocou- reserves, national parks, and sites of ples, etc. special scientiÜc intrest (SSSIs). constant-boiling mixture See conservation law A law stating azeotrope. that the total magnitude of a certain physical property of a system, such constant proportions See chemical combination as its mass, energy, or charge, re- . main unchanged even though there constitutional isomerism See iso- may be exchanges of that property merism. between components of the system. For example, imagine a table with a contact insecticide Any insecti- pesticide bottle of salt solution (NaCl), a bottle cide (see ) that kills its target insect by being absorbed through the of silver nitrate solution (AgNO3), and a beaker standing on it. The mass of cuticle or by blocking the spiracles, this table and its contents will not rather than by being ingested. change even when some of the con- contact process A process for tents of the bottles are poured into making sulphuric acid from sulphur the beaker. As a result of the reaction dioxide (SO2), which is made by burn- between the chemicals two new sub- ing sulphur or by roasting sulphide stances (silver chloride and sodium ores. A mixture of sulphur dioxide nitrate) will appear in the beaker: and air is passed over a hot catalyst → → NaCl + AgNO3 AgCl + NaNO3, 2SO2 + O2 2SO3 but the total mass of the table and its The reaction is exothermic and the contents will not change. This con- conditions are controlled to keep the servation of mass is a law of wide temperature at an optimum 450°C. and general applicability, which is Formerly, platinum catalysts were true for the universe as a whole, pro- used but vanadium–vanadium oxide vided that the universe can be con- catalysts are now mainly employed sidered a closed system (nothing (although less efÜcient, they are less escaping from it, nothing being susceptible to poisoning). The sul- continuous phase 142

phur trioxide is dissolved in sul- from the cylinder can move down phuric acid into the boiler to be heated. In some → modern systems, where small-bore H2SO4 + SO3 H2S2O7 pipes are used or it is inconvenient to and the oleum is then diluted. c place the cylinder above the boiler, continuous phase See colloids. the circulation between boiler and continuous spectrum See spec- hot-water cylinder relies upon a trum. pump. This is an example of forced convection, where hot Ûuid is trans- controlled substance In the UK, ferred from one region to another by an illegal or restricted drug. There a pump or fan. are three classes with different legal penalties for possession and for deal- converter The reaction vessel in ing. the *Bessemer process or some simi- Class A drugs (the most harmful) in- lar steel-making process. clude cocaine, ecstasy, heroin, LSD, coomassie blue A biological dye and magic mushrooms. Ampheta- used for staining proteins. mines prepared for injection are also coordinate bond See chemical class A drugs. bond. Class B drugs include amphetamines, methylphenidate, and pholcodine. coordination compound A com- Class C drugs include cannabis, pound in which coordinate bonds are gammahydroxybutyric acid (GHB), formed (see chemical bond). The ketamine, and some painkillers. term is used especially for inorganic Drug use is governed by the Misuse *complexes. of Drugs Act, 1971 and 2005. Some A substances fall under the Medicines • Information about IUPAC nomenclature Act and are classiÜed as prescription- only, pharmacist without prescrip- coordination number The num- tion, or generally available. ber of groups, molecules, atoms, or A ions surrounding a given atom or ion in a complex or crystal. For instance, • Information about the Misuse of Drugs in a square-planar complex the cen- Act from the Home OfÜce website tral ion has a coordination number of convection A process by which four. In a close-packed crystal (see heat is transferred from one part of a close packing) the coordination Ûuid to another by movement of the number is twelve. Ûuid itself. In natural convection the Cope rearrangement A type of movement occurs as a result of grav- rearrangement of dienes in which ity; the hot part of the Ûuid expands, the position of a substituent changes. becomes less dense, and is displaced For instance the substituted diene by the colder denser part of the Ûuid as this drops below it. This is the CH2=CH–CH(R)– CH2–CH=CH2, process that occurs in most domestic when heated, can convert to hot-water systems between the boiler RCH=CH–CH2–CH2–CH=CH2. and the hot-water cylinder. A natural It involves the formation of a cyclic convection current is set up transfer- transition state, and is an example of ring the hot water from the boiler up a *sigmatropic reaction. to the cylinder (always placed above the boiler) so that the cold water copolymer See polymer. 143 copper glance

– copper Symbol Cu. A red-brown per(I) is present as the [CuCl2] com- *transition element; a.n. 29; r.a.m. plex ion. On pouring the solution 63.546; r.d. 8.92; m.p. 1083.4°C; b.p. into air-free distilled water copper(I) 2567°C. Copper has been extracted chloride precipitates. It must be kept for thousands of years; it was known free of air and moisture since it oxi- c to the Romans as cuprum, a name dizes to copper(II) chloride under linked with the island of Cyprus. The those conditions. metal is malleable and ductile and an Copper(I) chloride is essentially co- excellent conductor of heat and elec- valent and its structure is similar to tricity. Copper-containing minerals that of diamond; i.e. each copper include cuprite (Cu2O) as well as atom is surrounded tetrahedrally by azurite (2CuCO3.Cu(OH)2), chalco- four chlorine atoms and vice versa. pyrite (CuFeS2), and malachite In the vapour phase, dimeric and (CuCO3.Cu(OH)2). Native copper ap- trimeric species are present. pears in isolated pockets in some Copper(I) chloride is used in conjunc- parts of the world. The large mines tion with ammonium chloride as a in the USA, Chile, Canada, Zambia, catalyst in the dimerization of Democratic Republic of Congo, and ethyne to but-1-ene-3-yne (vinyl Peru extract ores containing sul- acetylene), which is used in the pro- phides, oxides, and carbonates. They duction of synthetic rubber. In the are usually worked by smelting, laboratory a mixture of copper(I) leaching, and electrolysis. Copper chloride and hydrochloric acid is metal is used to make electric cables used for converting benzene diazo- and wires. Its alloys, brass (copper– nium chloride to chlorobenzene – zinc) and bronze (copper–tin), are the Sandmeyer reaction. used extensively. Water does not attack copper but copper(II) chloride A brown- yellow powder, CuCl2; r.d. 3.386; in moist atmospheres it slowly forms ° a characteristic green surface layer m.p. 620 C. It exists as a blue-green dihydrate (rhombic; r.d. 2.54; loses (patina). The metal will not react ° with dilute sulphuric or hydrochloric H2O at 100 C). The anhydrous solid is acids, but with nitric acid oxides of obtained by passing chlorine over nitrogen are formed. Copper com- heated copper. It is predominantly pounds contain the element in the covalent and adopts a layer structure +1 and +2 oxidation states. Copper(I) in which each copper atom is sur- compounds are mostly white (the rounded by four chlorine atoms at a oxide is red). Copper(II) salts are blue distance of 0.23 and two more at a in solution. The metal also forms a distance of 0.295. A concentrated large number of coordination com- aqueous solution is dark brown in colour due to the presence of complexes. 2– A plex ions such as [CuCl4] . On dilution the colour changes to green and • Information from the WebElements site then blue because of successive re- copperas See iron(ii) sulphate. placement of chloride ions by water molecules, the Ünal colour being that copper(I) chloride A white solid of the [Cu(H O) ]2+ ion. The dihydrate compound, CuCl; cubic; r.d. 4.14; 2 6 ° ° can be obtained by crystallizing the m.p. 430 C; b.p. 1490 C. It is ob- solution. tained by boiling a solution containing copper(II) chloride, excess copper copper glance A mineral form of turnings, and hydrochloric acid. Cop- copper(I) sulphide, Cu2S. copper(II) nitrate 144

copper(II) nitrate A blue deliques- it is also readily reduced to copper on cent solid, Cu(NO3)2.3H2O; r.d. 2.32; heating in a stream of hydrogen. m.p. 114.5°C. It may be obtained by Copper(II) oxide is soluble in dilute reacting either copper(II) oxide or acids forming blue solutions of c copper(II) carbonate with dilute nitric cupric salts. acid and crystallizing the resulting copper pyrites See chalcopyrite. solution. Other hydrates containing 6 or 9 molecules of water are known. copper(II) sulphate A blue crys- On heating it readily decomposes to talline solid, CuSO4.5H2O; triclinic; give copper(II) oxide, nitrogen diox- r.d. 2.284. The pentahydrate loses ° Ü ide, and oxygen. The anhydrous form 4H2O at 110 C and the fth H2O at can be obtained by reacting copper 150°C to form the white anhydrous with a solution of nitrogen dioxide in compound (rhombic; r.d. 3.6; decom- ethyl ethanoate. It sublimes on heat- poses above 200°C). The pentahy- ing suggesting that it is appreciably drate is prepared either by reacting covalent. copper(II) oxide or copper(II) carbonate with dilute sulphuric acid; the so- copper(I) oxide A red insoluble lution is heated to saturation and the solid, Cu O; r.d. 6.0; m.p. 1235°C. It is 2 blue pentahydrate crystallizes out on obtained by reduction of an alkaline cooling (a few drops of dilute sul- solution of copper(II) sulphate. Since phuric acid are generally added to the addition of alkalis to a solution of prevent hydrolysis). It is obtained on copper(II) salt results in the precipita- an industrial scale by forcing air tion of copper(II) hydroxide the cop- through a hot mixture of copper and per(II) ions are complexed with dilute sulphuric acid. In the pentahy- tartrate ions; under such conditions drate each copper(II) ion is sur- the concentration of copper(II) ions is rounded by four water molecules at so low that the solubility product of the corner of a square, the Üfth and copper(II) hydroxide is not exceeded. sixth octahedral positions are occu- When copper(I) oxide reacts with pied by oxygen atoms from the sul- dilute sulphuric acid a solution of phate anions, and the Üfth water copper(II) sulphate and a deposit of molecule is held in place by hydro- copper results, i.e. disproportionation gen bonding. Copper(II) sulphate has occurs. many industrial uses, including the + → 2+ Cu2O + 2H Cu + Cu + H2O preparation of the Bordeaux mixture When dissolved in concentrated (a fungicide) and the preparation of – hydrochloric acid the [CuCl2] com- other copper compounds. It is also plex ion is formed. Copper(I) oxide is used in electroplating and textile used in the manufacture of rectiÜers dying and as a timber preservative. and the production of red glass. The anhydrous form is used in the detection of traces of moisture. copper(II) oxide A black insoluble Copper(II) sulphate pentahydrate is solid, CuO; monoclinic; r.d. 6.3; m.p. also known as blue vitriol. 1326°C. It is obtained by heating either copper(II) carbonate or copper(II) coprecipitation The removal of a nitrate. It decomposes on heating substance from solution by its associ- above 800°C to copper(I) oxide and ation with a precipitate of some oxygen. Copper(II) oxide reacts read- other substance. For example, if A ily with mineral acids on warming, and B are present in solution and a with the formation of copper(II) salts; reagent is added such that A forms 145 correlation spectroscopy an insoluble precipitate, then B may gram, the types of molecular orbital be carried down with the precipitate possible for the molecules and their of A, even though it is soluble under order as a function of increasing en- the conditions. This can occur by oc- ergy can be seen as a function of inclusion or absorption. teratomic distance. Lines are drawn c linking each united-atom orbital to a cordite An explosive mixture of cel- separated atom orbital. An important lulose nitrate and nitroglycerin, with rule for determining which energy added plasticizers and stabilizers, levels correlate is the noncrossing used as a propellant for guns. rule. This states that two curves of core orbital An atomic orbital that energy plotted against interatomic is part of the inner closed shell of an distance never cross if the invariant atom. Unlike the valence orbitals, properties (e.g. parity, spin, etc.) are which are the atomic orbitals of the the same. valence shell of an atom, core or- correlation functions Quantities bitals on one atom have a small over- used in condensed-matter physics lap with core orbitals on another that are *ensemble averages of prod- atom. They are therefore usually ig- ucts of such quantities as *density at nored in simple calculations of chem- different points in space. Each parti- ical bonding. cle affects the behaviour of its neigh- Corey–Pauling rules A set of bouring particles, so creating rules, formulated by Robert Corey correlations, the range of which is at and Linus *Pauling, that govern the least as long as the range of the inter- secondary nature of proteins. The molecular forces. Thus, correlation Corey–Pauling rules are concerned functions contain a great deal of in- with the stability of structures pro- formation about systems in con- vided by *hydrogen bonds associated densed-matter physics. Correlation with the –CO–NH– peptide link. The functions can be measured in consid- Corey–Pauling rules state that: erable detail, frequently by experi- (1) All the atoms in the peptide link ments involving scattering of such lie in the same plane. particles as neutrons or of *electro- (2) The N, H, and O atoms in a hydro- magnetic radiation (e.g. light or X- gen bond are approximately on a rays) and can be calculated straight line. theoretically using *statistical me- (3) All the CO and NH groups are in- chanics. volved in bonding. correlation spectroscopy (COSY) Two important structures in which A type of spectroscopy used in *nu- the Corey–Pauling rules are obeyed clear magnetic resonance (NMR) in are the alpha helix and the *beta which the pulse sequence used is: sheet. 90°x – t1 – 90°x – acquire (t2). The CORN rule See absolute configu- delay t1 is variable and a series of ac- quisitions is made. Fourier trans- ration. forms are then performed on both correlation diagram A diagram the delay t1 and the real time t2. The that relates the energy levels of sepa- information thus gained can be rate atoms to the energy levels of di- shown on a diagram that plots con- atomic molecules and united atoms, tours of signal intensity. This repre- the two limiting states being corre- sentation of the information enables lated with each other. Using this dia- NMR spectra to be interpreted more corrosion 146

readily than in one-dimensional length; this is called the negative NMR, particularly for complex spec- Cotton effect. These effects occur for tra. coloured substances and for colourless substances with bands in the ul- corrosion Chemical or electro- traviolet. It is named after the French c chemical attack on the surface of a physicist Aimé Cotton (1859–1951). metal. See also electrolytic corrosion; rusting. coulomb Symbol C. The *SI unit of electric charge. It is equal to the corundum A mineral form of alu- charge transferred by a current of minium oxide, Al O . It crystallizes 2 3 one ampere in one second. The unit in the trigonal system and occurs as is named after Charles de Coulomb. well-developed hexagonal crystals. It is colourless and transparent when Coulomb explosion The sudden pure but the presence of other el- disruption of a molecule from which ements gives rise to a variety of the electrons have been stripped to colours. *Ruby is a red variety con- leave only the nuclei, which repel taining chromium; *sapphire is a each other because of their electric blue variety containing iron and charge. The technique of Coulomb titanium. Corundum occurs as a explosion imaging uses this effect to rock-forming mineral in both meta- investigate the shape of molecules. A morphic and igneous rocks. It is beam of high-energy neutral mol- chemically resistant to weathering ecules is produced by Ürst adding processes and so also occurs in allu- electrons, accelerating the ions in an vial (placer) deposits. The second electric Üeld, and then removing the hardest mineral after diamond (it has electrons. The beam collides with a a hardness of 9 on the Mohs’ scale), it thin metal foil having a thickness of is used as an abrasive. about 30 atoms. As the molecules pass through this foil their electrons COSY See correlation spec- are scattered and only the nuclei of troscopy. the molecules emerge. The process COT See cyclo-octatetraene. occurs within a very short period of time, shorter than the time required Cotton effect The wavelength defor a complete molecular vibration, pendence of the optical rotary disper- and consequently the nuclei retain sion curve or the *circular dichroism the molecular shape until they are curve in the neighbourhood of an ab- suddenly repulsed by the like sorption band, both having charac- charges. The nuclei then impinge on teristic shapes. If the wavelength is a detector that records their velocity decreased, the rotation angle in- and direction, thus enabling the spa- creases until it reaches a maximum tial arrangement of the original mol- and then decreases, passing through ecule to be derived. zero at the wavelength at which the maximum of absorption occurs. As coumarin (1,2-benzopyrone) A the wavelength is decreased further pleasant-smelling colourless crys- ° the angle becomes negative, until it talline compound, C9H6O2, m.p. 70 C. reaches a minimum after which it It occurs naturally in tonka (or ton- rises again. This pattern is called the quin) beans, and is synthesized from positive Cotton effect. A mirror salicylaldehyde. It forms coumarinic image of this pattern can also occur acid on hydrolysis with sodium hy- about the λ-axis, where λ is the wave- droxide. Coumarin is used in making 147 covalent radius

H CH C H

CO CO H 2 O OH c

coumarin coumarinic acid

Coumarin perfumes, to scent tobacco, and as an cases.) 2. A type of chemical reaction anticoagulant in medicine; *warfarin in which two molecules join to- is derived from it. gether; for example, the formation of coumarone See benzfuran. an *azo compound by coupling of a diazonium ion with a benzene ring. countercurrent Ûow Flow of two chemical Ûuids in opposite directions with covalent bond See bond transfer of heat or matter between . them. Compare cocurrent flow. covalent carbide See carbide. counter ion An ion of opposite covalent crystal A crystal in charge to a given ion. For example, which the atoms are held together in a crystal of sodium chloride, the by covalent bonds. Covalent crystals chloride ions can be regarded as are sometimes called macromolecular counter ions to the sodium ions. In or giant-molecular crystals. They are certain colloids, the charge on the hard high-melting substances. Exam- surface of colloidal particles is neu- ples are diamond and boron nitride. tralized by oppositely charged counter ions in the surrounding solu- covalent radius An effective ration. dius assigned to an atom in a covalent compound. In the case of a coupling 1. An interaction be- simple diatomic molecule, the cova- tween two different parts of a system lent radius is half the distance be- or between two or more systems. tween the nuclei. Thus, in Cl the Examples of coupling in the *spectra 2 of atoms and nuclei are *Russell– internuclear distance is 0.198 nm so Saunders coupling, *j-j coupling, and the covalent radius is taken to be spin–orbit coupling. In the spectra of 0.099 nm. Covalent radii can also be molecules there are Üve idealized calculated for multiple bonds; for in- ways (called the Hund coupling cases) stance, in the case of carbon the val- in which the different types of angu- ues are 0.077 nm for single bonds, lar momentum in a molecule (the 0.0665 nm for double bonds, and electron orbital angular momentum 0.0605 nm for triple bonds. The val- L, the electron spin angular momen- ues of different covalent radii can tum S, and the angular momentum sometimes be added to give internu- of nuclear rotation N) couple to form clear distances. For example, the a resultant angular momentum J. (In length of the bond in interhalogens practice, the coupling for many mol- (e.g. ClBr) is nearly equal to the sum ecules is intermediate between of the covalent radii of the halogens Hund’s cases due to interactions, involved. This, however, is not al- which are ignored in the idealized ways true because of other effects CPMG sequence 148

(e.g. ionic contributions to the bond- and other hydrocarbon sources. It in- ing). volves a nickel-based catalyst, pressures of up to 70 bar, and CPMG sequence (Carr–Purcell– temperatures between 250°C and Meiboom–Gill sequence) A sequence ° c of pulses used for spin-echo experi- 650 C depending on the feedstock. ments in nuclear magnetic resonance The reactions are: → (NMR) in which the initial pulse is CnH2n+2 + nH2O nCO + (2n +1)H2 ° ° 90 followed by a series of 180 CO + 3H → CH + H O pulses. A CPMG sequence is designed 2 4 2 → so that the spin echoes die away ex- CO + H2O CO2 + H2 ponentially with time. Spin–spin re- The result is a mixture of methane, laxation occurs characterized by a carbon monoxide, carbon dioxide, time constant T2, which can be deter- and trace amounts of ethane and mined from the decay signal. other hydrocarbons. With partial car- crack cocaine See cocaine. bon dioxide removal it is possible to produce town gas with medium cracking The process of breaking caloriÜc value containing about 30% down chemical compounds by heat. CH4, 30% H2, and 2% CO. The process The term is applied particularly to can be used to produce SNG. In this the cracking of hydrocarbons in the case there are multiple methanation kerosine fraction obtained from *pe- stages and complete removal of CO2 troleum reÜning to give smaller hy- to give a product containing about drocarbon molecules and alkenes. It 98.5% CH4, 0.9% H2, and 0.1% CO. is an important process, both as a source of branched-chain hydrocar- cristobalite A mineral form of *sil- bons suitable for gasoline (for motor icon(IV) oxide, SiO2. fuel) and as a source of ethene and critical pressure The pressure of a other alkenes. Catalytic cracking is a Ûuid in its *critical state; i.e. when it similar process in which a catalyst is is at its critical temperature and criti- used to lower the temperature re- cal volume. quired and to modify the products obtained. critical state The state of a Ûuid in which the liquid and gas phases both potassium hy- cream of tartar See have the same density. The Ûuid is drogentartrate . then at its critical temperature, criti- creosote 1. (wood creosote) An al- cal pressure, and critical volume. most colourless liquid mixture of critical temperature 1. The tem- phenols obtained by distilling tar ob- perature above which a gas cannot tained by the destructive distillation be liqueÜed by an increase of pres- of wood. It is used medically as an sure. See also critical state. 2. See antiseptic and expectorant. 2. (coal- transition point. tar creosote) A dark liquid mixture of phenols and cresols obtained by dis- critical volume The volume of a tilling coal tar. It is used for preserv- Üxed mass of a Ûuid in its *critical ing timber. state; i.e. when it is at its critical temperature and critical pressure. The cresols See methylphenols. critical speciÜc volume is its volume CRG process (catalytic rich gas per unit mass in this state: in the process) An industrial process for past this has often been called the producing fuel gas from naphtha critical volume. 149 crown ethers

Crookes, Sir William (1832–1919) atoms linking two longer chains in a British chemist and physicist, who in polymeric material. 1861 used *spectroscopy to discover crown See ring conformations. *thallium and in 1875 invented the radiometer. He also developed an im- crown ethers Organic compounds c proved vacuum tube (Crookes’ tube) with molecules containing large for studying gas discharges. Crookes rings of carbon and oxygen atoms. was also involved in industrial chem- The crown ethers are macrocyclic Ü istry and realised the importance of polyethers. The rst to be synthe- nitrogen Üxation for fertilizers. sized was the compound 18-crown-6, which consists of a ring of six crossed-beam reaction A chemi- –CH2–CH2–O– units (i.e. C12H24O6). cal reaction in which two molecular The general method of naming beams are crossed; one beam is re- crown ethers is to use the form n- garded as the incident beam of gas crown-m, where n is the number of and the other as the target gas. This atoms in the ring and m is the num- technique enables a great deal of in- ber of oxygen atoms. Substituted formation to be gained about the crown ethers can also be made. The chemical reaction since the states of crown ethers are able to form both the target and projectile mol- strongly bound complexes with ecules can be controlled. The inci- metal ions by coordination through dent beam is characterized by its the oxygen atoms. The stability of incident beam Ûux, I, which is the these complexes depends on the size number of particles per unit area per of the ion relative to the cavity avail- unit time. The scattered molecules able in the ring of the particular can be detected Ürst by ionizing crown ether. Crown ethers also form them and then detecting the ions complexes with ammonium ions electrically or using spectroscopy if (NH4+) and alkyl ammonium ions changes in the vibrational or rota- (RNH3+). They can be used for increasing the solubility of ionic salts tional states of molecules in a reac- in nonpolar solvents. For example, tion are of interest. dicyclohexyl-18-crown-6 complexes cross linkage A short side chain of with the potassium ion of potassium

oxygen

CH2 group metal ion

18-crown-6 dicyclohexyl-18-crown-6 complex

Crown ethers crucible 150

permanganate and allows it to dis- ice and some other substance (e.g. an solve in benzene, giving a purple ionic salt) obtained by freezing a so- neutral solution that can oxidize lution. many organic compounds. They also cryolite A rare mineral form of act as catalysts in certain reactions Û c sodium alumino uoride, Na3AlF6, involving organic salts by complex- which crystallizes in the monoclinic ing with the positive metal cation system. It is usually white but may and thereby increasing its separation also be colourless. The only impor- from the organic anion, which shows tant occurrence of the mineral is in a consequent increase in activity. Greenland. It is used chieÛy to lower Some of the uses of crown ethers de- Ü the melting point of alumina in the pend on their selectivity for speci c production of aluminium. sizes of anions. Thus they can be used to extract speciÜc ions from cryoscopic constant See depres- mixtures and enrich isotope mix- sion of freezing point. tures. Their selectivity also makes cryoscopy The use of *depression them useful analytical reagents. See of freezing point to determine rela- cryptands also . tive molecular masses. crucible A dish or other vessel in cryostat A vessel enabling a sample which substances can be heated to a to be maintained at a very low tem- high temperature. See also gooch perature. The *Dewar Ûask is the crucible. most satisfactory vessel for control- crude oil See petroleum. ling heat leaking in by radiation, conduction, or convection. Cryostats Crum Brown’s rule A rule that usually consist of two or more Dewar predicts how substituents will enter Ûasks nesting in each other. For ex- the benzene ring. If C6H5X is a com- ample, a liquid nitrogen bath is often pound with one substituent in the used to cool a Dewar Ûask containing benzene ring, it will produce the 1,3 a liquid helium bath. (meta) disubstituted derivative if the substance HX can be directly oxi- cryptands Compounds with large dized to HOX. If not, a mixture of 1,2 three-dimensional molecular struc- (ortho) and 1,4 (para) compounds will tures containing ether chains linked be formed. The rule was proposed in by three-coordinate nitrogen atoms. 1892 by the British chemist Alexan- Thus cryptands are macropolycyclic der Crum Brown (1838–1922). polyaza-polyethers. For example, the compound (2,2,2)-cryptand has three cryogenic pump A *vacuum chains of the form pump in which pressure is reduced –CH CH OCH CH OCH CH –. by condensing gases on surfaces 2 2 2 2 2 2 maintained at about 20 K by means These chains are linked at each end by a nitrogen atom. Cryptands, like of liquid hydrogen or at 4 K by the *crown ethers, can form coordi- means of liquid helium. Pressures nation complexes with ions that can down to 10–8 mmHg (10–6 Pa) can be Üt into the cavity formed by the open maintained; if they are used in con- three-dimensional structure, i.e. they junction with a *diffusion pump, can ‘cryptate’ the ion. Various types pressures as low as 10–15 mmHg of cryptand have been produced hav- (10–13 Pa) can be reached. ing both spherical and cylindrical cryohydrate A eutectic mixture of cavities. The cryptands have the 151 crystalline

N OO OO

N N NNO c OO OO N OO

Cryptands same kind of properties as the crown the corners of a square. The presence ethers and the same uses. In general, of these ions affects the energies of they form much more strongly the d-orbitals, causing a splitting of bound complexes and can be used to energy levels. The theory can be used stabilize unusual ionic species. For to explain the spectra of complexes example, it is possible to produce the and their magnetic properties. Lig- negative Na– ion in the compound and-Üeld theory is a development of [(2,2,2)-cryptand-Na]+Na–, which is a crystal-Üeld theory in which the over- gold-coloured crystalline substance lap of orbitals is taken into account. stable at room temperature. Cluster Crystal-Üeld theory was initiated in 2– ions, such as Pb5 , can be similarly 1929 by the German-born US physi- stabilized. cist Hans Albrecht Bethe (1906–2005) crystal A solid with a regular poly- and extensively developed in the hedral shape. All crystals of the same 1930s. substance grow so that they have the crystal habit See crystal. same angles between their faces. However, they may not have the crystal lattice The regular pattern same external appearance because of atoms, ions, or molecules in a crys- different faces can grow at different talline substance. A crystal lattice can rates, depending on the conditions. be regarded as produced by repeated The external form of the crystal is re- translations of a unit cell of the lat- ferred to as the crystal habit. The tice. See also crystal system. atoms, ions, or molecules forming A the crystal have a regular arrange- • An interactive version of the structures ment and this is the crystal structure. of a range of crystals from a US Navy crystal defect An imperfection in website the regular lattice of a crystal. See • Crystal models from the University of Feature (pp 152–153). Calgary crystal-Üeld theory A theory of crystalline Having the regular in- the electronic structures of inorganic ternal arrangement of atoms, ions, or *complexes, in which the complex is molecules characteristic of crystals. assumed to consist of a central metal Crystalline materials need not neces- atom or ion surrounded by ligands sarily exist as crystals; all metals, for that are ions. For example, the com- example, are crystalline although 2– 2+ plex [PtCl4] is thought of as a Pt they are not usually seen as regular ion surrounded by four Cl– ions at geometric crystals. 152

CRYSTAL DEFECTS

A crystal *lattice is formed by a repeated arrangement of atoms, ions, or molecules. Within one cubic centimetre of material one can expect to find up to 1022 atoms and it is extremely unlikely that all of these will be arranged in c perfect order. Some atoms will not be exactly in the right place with the result that the lattice will contain *defects. The presence of defects within the crystal structure has profound consequences for certain bulk properties of the solid, such as the electrical resistance and the mechanical strength. Point defects Local crystal defects called point defects, appear as either impurity atoms or gaps in the lattice. Impurity atoms can occur in the lattice either at interstitial sites (between atoms in a non-lattice site) or at substitutional sites (replacing an atom in the host lattice). Lattice gaps are called vacancies and arise when an atom is missing from its site in the lattice. Vacancies are sometimes called Schottky defects. A vacancy in which the missing atom has moved to an interstitial position is known as a Frenkel defect. Colour centres In ionic crystals, the ions and vacancies always arrange themselves so that there is no build-up of one type of charge in any small volume of the crystal. If ions or charges are introduced into or removed from the lattice, there will, in general, be an accompanying rearrangement of the ions and their outer valence electrons. This rearrangement is called charge compensation and is most dramatically observed in colour centres. If certain crystals are irradiated with X-rays, gamma rays, neutrons, or electrons a colour change is observed. For example, diamond may be coloured blue by electron bombardment and quartz may be coloured brown by irradiation with neutrons. The high-energy radiation produces defects in the lattice and, in an attempt to maintain charge neutrality, the crystal undergoes some measure of charge compensation. Just as electrons around an atom have a series of discrete permitted energy levels, so charges residing at point defects exhibit sets of discrete levels, which are separated from one another by energies corresponding to wavelengths in the visible region of the spectrum. Thus light of certain wavelengths can be absorbed at the defect sites, and the material appears to be coloured. Heating the irradiated crystal can, in many cases, repair the irradiation damage and the crystal loses its coloration. Dislocations Non-local defects may involve entire planes of atoms. The most important of

Formation of a Schottky defect Formation of a Frenkel defect

Point defects in a two-dimensional crystal 153

these is called a dislocation. Dislocations are essentially line-defects; that is, there is an incomplete plane of atoms in the crystal lattice. In 1934, Taylor, Orowan, and Polanyi independently proposed the concept of the dislocation to account for the mechanical strength of metal crystals. Their microscopic studies revealed that when a metal crystal is plastically deformed, the c deformation does not occur by a separation of individual atoms but rather by a slip of one plane of atoms over another plane. Dislocations provide a mechanism for this slipping of planes that does not require the bulk movement of crystal material. The passage of a dislocation in a crystal is similar to the movement of a ruck in a carpet. A relatively large force is required to slide the carpet as a whole. However, moving a ruck over the carpet can inch it forward without needing such large forces. This movement of dislocations is called plastic flow. Strength of materials In practice most metal samples are polycrystalline; that is they consist of many small crystals or grains at different angles to each other. The boundary between two such grains is called a grain boundary. The plastic flow of dislocations may be hindered by the presence of grain boundaries, impurity atoms, and other dislocations. Pure metals produced commercially are generally too weak to be of much mechanical use. The weakness of these samples can be attributed to the ease with which the dislocations are able to move within the sample. Slip, and therefore deformation, can then occur under relatively low stresses. Impurity atoms, other dislocations, and grain boundaries can all act as obstructions to the slip of atomic planes. Traditionally, methods of making metals stronger involved introducing defects that provide regions of disorder in the material. For example, in an alloy, such as steel, impurity atoms (e.g. carbon) are introduced into the lattice during the forging process. The perfection of the iron lattice structure is disturbed and the impurities oppose the dislocation motion. This makes for greater strength and stiffness. The complete elimination of dislocations may seem an obvious way to strengthen materials. However, this has only proved possible for hair-like single crystal specimens called whiskers. These whiskers are only a few micrometers thick and are seldom more than a few millimetres long; nevertheless their strength approaches the theoretical value.

AE H A

G G B D B D

F F C C

Dislocation in a two-dimensional crystal. The extra plane of atoms AB causes strain at bond CD. On breaking, the bond flips across to form CB. This incremental movement shifts the dislocation across so that the overall effect is to slide the two planes BDG and CF over each other. crystallite 154

crystallite A small crystal, e.g. one H H of the small crystals forming part of a microcrystalline substance. C C crystallization The process of H H c forming crystals from a liquid or gas. C C crystallography The study of crystal form and structure. See also x-ray crystallography. C C H H crystalloids See colloids.

crystal meth See amphetamines. C C

crystal structure See crystal. H H crystal system A method of classi- Cubane fying crystalline substances on the basis of their unit cell. There are Ürst synthesized in 1964 by Philip seven crystal systems. If the cell is a Eaton. The C–C–C bond angle of 90° parallelopiped with sides a, b, and c is highly strained and cubane and its and if α is the angle between b and c, derivatives have been investigated as β the angle between a and c, and γ high-energy fuels and explosives. In the angle between a and b, the sys- particular, octanitrocubane, in which tems are: the hydrogen atoms are replaced by α β γ ° (1) cubic a=b=c and = = =90 –NO2 groups is possibly the most (2) tetragonal a=b≠c and α=β=γ=90° powerful chemical explosive known, (3) rhombic (or orthorhombic) a≠b≠c although, so far, only small amounts and α=β=γ=90° have been synthesized. It decom- (4) hexagonal a=b≠c and α=β=γ=90° poses to carbon dioxide and nitro- (5) trigonal a=b≠c and α=β=γ≠90° gen: ≠ ≠ α γ °≠β (6) monoclinic a b c and = =90 C (NO ) → 8CO + 4N (7) triclinic a=b=c and α≠β≠γ 8 2 8 2 2 cubic close packing See close crystal test A type of *presump- packing tive test in which a substance is . identiÜed by the formation of charac- cubic crystal A crystal in which teristic crystals when a certain the unit cell is a cube (see crystal reagent is added. Usually, such tests system). There are three possible are conducted using a microscope packings for cubic crystals: simple (microcrystal test). An example is the cubic, face-centred cubic, and body- *acetone–chlor–haemin test for centred cubic. See illustration oppo- blood. site. CS gas The vapour from a white A solid, C6H4(Cl)CH:C(CN)2, causing • An interactive version of a simple cubic tears and choking, used in ‘crowd crystal control’. • An interactive version of a body-centred cubic crystal cubane A crystalline hydrocarbon, • An interactive version of a face-centred C H ; r.d. 1.29; m.p. 131°C. It has a 8 8 cubic crystal novel structure with eight carbon atoms at the corners if a cube, each cubic zircona (CZ) A crystal form attached to a hydrogen. Cubane was or zircon(IV) oxide (zircon dioxide, 155 Curie, Marie

ZrO2) made by fusing ZrO2 and allow- oxidation state; e.g. cupric chloride is ing it to cool under controlled condi- copper(II) chloride (CuCl2). tions. It is used as an inexpensive cuprite A red mineral cubic form of diamond substitute in jewellery. copper(I) oxide, Cu2O; an important Often it is erroneously called *zircon. ore of copper. It occurs where de- c cumene process An industrial posits of copper have been subjected process for making phenol from ben- to oxidation. The mineral has been zene. A mixture of benzene vapour mined as a copper ore in Chile, and propene is passed over a phos- Democratic Republic of Congo, Bo- phoric acid catalyst at 250°C and livia, Australia, Russia, and the USA. high pressure cupronickel A type of corrosion-re- → sistant alloy of copper and nickel C6H6 + CH3CH:CH2 containing up to 45% nickel. C6H5CH(CH3)2 The product is called cumene, and it cuprous compounds Compounds can be oxidized in air to a peroxide, containing copper in its lower (+1) oxidation state; e.g. cuprous chloride C6H5C(CH3)2O2H. This reacts with di- is copper(I) chloride (CuCl). lute acid to give phenol (C6H5OH) and propanone (acetone, CH3OCH3), curare A resin obtained from the which is a valuable by-product. bark of South American trees of the cupellation A method of separat- genera Strychnos and Chondrodendron ing noble metals (e.g. gold or silver) that causes paralysis of voluntary muscle. It acts by blocking the action from base metals (e.g. lead) by melt- of the neurotransmitter *acetyl- ing the mixture with a blast of hot choline at neuromuscular junctions. air in a shallow porous dish (the Curare is used as an arrow poison by cupel). The base metals are oxidized, South American Indians and was for- the oxide being carried away by the merly used as a muscle relaxant in blast of air or absorbed by the porous surgery. container. curie The former unit of *activity cuprammonium ion The tetraam- (see radiation units). It is named 2+ minecopper(II) ion [Cu(NH3)4] . See after Marie *Curie. ammine. Curie, Marie (Marya Sklodowska; cupric compounds Compounds 1867–1934) Polish-born French containing copper in its higher (+2) chemist, who went to Paris in 1891.

body-centred simple cubic face-centred Cubic crystal Curie point 156

She married the physicist Pierre Curie A (1859–1906) in 1895 and soon began • Information from the WebElements site work on seeking radioactive elements other than uranium in cutting agent (adulterant) A sub- pitchblende (to account for its un- stance used to dilute illegal drugs c such as heroine and cocaine. Exam- expectedly high radioactivity). By Û 1898 she had discovered *radium ples include our, starch, sugar, and and *polonium, although it took her caffeine. four years to purify them. In 1903 cyanamide 1. An inorganic salt 2– calcium the Curies shared the Nobel Prize for containing the ion CN2 . See physics with Henri *Becquerel, who cyanamide. 2. A colourless crys- had discovered radioactivity. In 1911 talline solid, H2NCN, made by the ac- Marie Curie was awarded the Nobel tion of carbon dioxide on hot Prize for chemistry. sodamide. It is a weakly acidic compound (the parent acid of cyanamide Curie point (Curie temperature) salts) that is soluble in water and The temperature at which a ferro- ethanol. It is hydrolysed to urea in magnetic substance loses its ferro- acidic solutions. magnetism and becomes only calcium paramagnetic. For iron the Curie cyanamide process See cyanamide point is 760°C and for nickel 356°C. . It is named after Pierre *Curie. cyanate See cyanic acid. Curie’s law The susceptibility (χ) of cyanic acid An unstable explosive a paramagnetic substance is propor- acid, HOCN. The compound has the tional to the thermodynamic temper- structure H–O–C≡N, and is also called ature (T), i.e. χ = C/T, where C is the fulminic acid. Its salts and esters are Curie constant. A modiÜcation of this cyanates (or fulminates). The com- law, the Curie–Weiss law, is more pound is a volatile liquid, which generally applicable. It states that readily polymerizes. In water it hy- χ = C/(T – θ), where θ is the Weiss drolyses to ammonia and carbon constant, a characteristic of the ma- dioxide. It is isomeric with another terial. The law was Ürst proposed by acid, H–N=C=O, which is known as Pierre *Curie and modiÜed by an- isocyanic acid. Its salts and esters are other French physicist, Pierre-Ernest isocyanates. Weiss (1865–1940). cyanide 1. An inorganic salt con- curium Symbol Cm. A radioactive taining the cyanide ion CN–. Cya- metallic transuranic element belong- nides are extremely poisonous ing to the *actinoids; a.n. 96; mass because of the ability of the CN– ion number of the most stable isotope to coordinate with the iron in 247 (half-life 1.64 × 107 years); r.d. haemoglobin, thereby blocking the (calculated) 13.51; m.p. 1340±40°C. uptake of oxygen by the blood. 2. A There are nine known isotopes. The metal coordination complex formed element was Ürst identiÜed by Glenn with cyanide ions. Seaborg (1912–99) and associates in cyanide process A method of ex- 1944 and Ürst produced by L. B. tracting gold by dissolving it in potas- Werner and I. Perlman in 1947 by sium cyanide (to form the complex – bombarding americium–241 with ion [Au(CN)2] ). The ion can be re- neutrons. duced back to gold with zinc. 157 cyclic

O N O OH N OH

N N N N c H H

O OH

Cyanuric acid cyanine dyes A class of dyes that formed by the addition of hydrogen contain a –CH= group linking two ni- cyanide to aldehydes or ketones (in trogen-containing heterocyclic rings. the presence of a base). The Ürst step They are used as sensitizers in pho- is attack by a CN– ion on the car- tography. bonyl carbon atom. The Ünal product is a compound in which a –CN and cyanoacrylate A compound –OH group are attached to the same formed by the condensation of an carbon atom. For example, ethanal alkyl cyanoethanoate and methanal. reacts as follows Cyanoacrylates have the general for- → CH3CHO + HCN CH3CH(OH)(CN) mula CH2:C(CN)COOR. The methyl and ethyl esters are the basis of vari- The product is 2-hydroxypropanoni- ous ‘superglues’, which rapidly poly- trile. Cyanohydrins of this type can merize in air (under the action of be oxidized to α-hydroxy carboxylic moisture) to form strong adhesives. acids. cyanocobalamin See vitamin b cyanuric acid A white crystalline complex. water-soluble trimer of cyanic acid, (HNCO)3. It is a cyclic compound hav- cyanogen A colourless gas, (CN)2, ing a six-membered ring made of al- with a pungent odour; soluble in ternating imide (NH) and carbonyl water, ethanol, and ether; d. 2.335 (CO) groups (i.e. three –NH–C(O)– gdm–3; m.p. –27.9°C; b.p. –20.7°C. units). It can also exist in a phenolic The compound is very toxic. It may form (three –N=C(OH)– units). be prepared in the laboratory by heating mercury(II) cyanide; industri- cyclamates Salts of the acid, ally it is made by gas-phase oxidation C6H11.NH.SO3H, where C6H11– is a cy- of hydrogen cyanide using air over a clohexyl group. Sodium and calcium silver catalyst, chlorine over acti- cyclamates were formerly used as vated silicon(IV) oxide, or nitrogen sweetening agents in soft drinks, etc, dioxide over a copper(II) salt. until their use was banned when Cyanogen is an important intermedi- they were suspected of causing can- ate in the preparation of various fer- cer. tilizers and is also used as a stabilizer cyclic Describing a compound that in making nitrocellulose. It is an ex- has a ring of atoms in its molecules. ample of a *pseudohalogen. In homocyclic compounds all the atoms in the ring are the same type, cyano group The group –CN in a e.g. benzene (C H ) and cyclohexane chemical compound. See nitriles. 6 6 (C6H12). These two examples are also cyanohydrins Organic compounds examples of carbocyclic compounds; cyclic AMP 158

i.e. the rings are of carbon atoms. If cyclic diene hydrocarbon, C4H4, with different atoms occur in the ring, as its four carbon atoms in a square in pyridine (C5H5N), the compound is ring. It is made by degradation of its said to be heterocyclic. metal complexes, and has a short lifetime of a few seconds. It under- cyclic AMP A derivative of *ATP c goes addition reactions with alkynes. that is widespread in animal cells as a second messenger in many bio- cyclohexadiene-1,4-dione chemical reactions induced by hor- (benzoquinone; quinone) A yellow ° mones. Upon reaching their target solid, C6H4O2; r.d. 1.3; m.p. 116 C. It cells, the hormones activate adeny- has a six-membered ring of carbon late cyclase, the enzyme that cataly- atoms with two opposite carbon ses cyclic AMP production. Cyclic atoms linked to oxygen atoms (C=O) AMP ultimately activates the en- and the other two pairs of carbon zymes of the reaction induced by the atoms linked by double bonds hormone concerned. Cyclic AMP is (HC=CH). The compound is used in also involved in controlling gene ex- making dyes. See also quinhydrone pression and cell division, in im- electrode. mune responses, and in nervous cyclohexane A colourless liquid transmission. *cycloalkane, C6H12; r.d. 0.78; m.p. cyclization The formation of a 6.5°C; b.p. 81°C. It occurs in petro- cyclic compound from an open-chain leum and is made by passing ben- compound. See ring. zene and hydrogen under pressure Ü over a heated Raney nickel catalyst at cyclo- Pre x designating a cyclic 150°C, or by the reduction of cyclo- compound, e.g. a cycloalkane or a hexanone. It is used as a solvent and cyclosilicate. paint remover and can be oxidized cycloaddition A reaction in which using hot concentrated nitric acid to two or more unsaturated compounds hexanedioic acid (adipic acid). The cy- form a cyclic adduct or in which a clohexane ring is not planar and can cyclic compound is formed by addi- adopt boat and chair *conformation between unsaturated parts of tions; in formulae it is represented the same molecule. In cycloaddition, by a single hexagon. there is no net reduction in bond cyclonite A highly explosive nitro multiplicity. The *Diels–Alder reac- compound, (CH2N.NO2)3. It has a tion is an example. Cycloadditions cyclic structure with a six-membered may be stepwise reactions or may be ring of alternating CH2 groups and *pericyclic reactions. nitrogen atoms, with each nitrogen cycloalkanes Cyclic saturated hy- being attached to a NO2 group. It is drocarbons containing a ring of car- made by nitrating hexamine, bon atoms joined by single bonds. NO They have the general formula 2 CnH2n, for example cyclohexane, C6H12, etc. In general they behave N like the *alkanes but are rather less 2CH CH2 reactive. A N N C NO O2N 2 • Information about IUPAC nomenclature H2 cyclobutadiene A short-lived Cyclonite 159 cystine

C6H12N4, which is obtained from ammonia and methanal. Cyclonite is a very powerful explosive used mainly for military purposes. It is also called RDX. The abbreviation is for ‘Re- c search Department composition X’, used at the Chemical Research and Development Department, Wool- wich. cyclo-octatetraene (COT) A yellow liquid cyclic compound, C8H8, with eight carbon atoms and four double bonds in its ring; b.p. 142°C. It is Cyclophane made by polymerizing ethyne (acetylene) in the presence of nickel salts. cyclophane A compound consist- It forms addition compounds and cer- ing of one or more aromatic rings tain metal complexes, such as ura- forming part of a larger ring system nocene, (C8H8)2U. in which aliphatic chains of the CH2 cyclopentadiene A colourless liq- groups link the aromatic rings. Com- Ü uid cyclic *alkene, C5H6; r.d. 0.8021; pounds of this type have the suf x m.p. –97.2°C; b.p. 40.0°C. It is pre- phane in their names. Depending on pared as a by-product during the frac- the sizes of the (CH2)n chains, the aro- tional distillation of crude benzene matic rings may not be planar. from coal tar. It undergoes condensa- A tion reactions with ketones to give • Information about IUPAC nomenclature highly coloured compounds (ful- venes) and readily undergoes poly- cyclopropane A colourless gas, ° merization at room temperature to C3H6, b.p. –34.5 C, whose molecules give the dimer, dicyclopentadiene. contain a triangular ring of carbon Cyclopentadiene itself is not an aro- atoms. It is made by treating 1,3-di- matic compound because it does not bromopropane with zinc metal, and have the required number of pi elec- is used as a general anaesthetic. trons. However, removal of a hydro- cyclosarin A highly toxic colourless gen atom produces the stable – liquid, C7H14FO2P; r.d. 1.13; m.p. cyclopentadienyl ion, C5H5 , which –30°C; b.p. 239°C. it is a Ûuorinated does have aromatic properties. In organophosphorus compound, particular, the ring can coordinate to (Ûuoromethylphosphoryl)oxycyclo- positive ions in such compounds as hexane. Cyclosarin was discovered in *ferrocene. 1949 and belongs to the G-series of H H *nerve agents (GF). C C cyclotetramethylenetetran- CH CH itramine See hmx. C cylinder gas See bottled gas. H2 amino acid Cyclopentadiene cysteine See . cystine A molecule resulting from cyclopentadienyl ion See cy- the oxidation reaction between the clopentadiene. sulphydryl (–SH) groups of two cys- cytidine 160

teine molecules (see amino acid). cytochrome Any of a group of This often occurs between adjacent proteins, each with an iron-contain- cysteine residues in polypeptides. ing *haem group, that form part of The resultant disulphide bridges the *electron transport chain in mi- (–S–S–) are important in stabilizing tochondria and chloroplasts. Elec- c the structure of protein molecules. trons are transferred by reversible changes in the iron atom between cytidine A nucleoside comprising the reduced Fe(II) and oxidized Fe(III) one cytosine molecule linked to a d- states. ribose sugar molecule. The derived nucleotides, cytidine mono-, di-, and cytosine A *pyrimidine derivative. triphosphate (CMP, CDP, and CTP re- It is one of the principal component spectively) participate in various bio- bases of *nucleotides and the nucleic chemical reactions, notably in acids *DNA and *RNA. phospholipid synthesis.

Cytosine

CZ See cubic zircona. CZE Capillary zone electrophoresis. See capillary elec- Cytidine trophoresis. D

2,4-D 2,4-dichlorophenoxyacetic sures of its components, i.e. the sum acid (2,4-dichlorophenoxyethanoic of the pressures that each compo- acid): a synthetic auxin frequently nent would exert if it were present used as a weedkiller of broad-leaved alone and occupied the same volume weeds. See also pesticide. as the mixture of gases. Strictly Dakin reaction See baeyer– speaking, the principle is true only villiger reaction. for ideal gases. The law was discovered by John Dalton. dalton See atomic mass unit. Daniell cell A type of primary Dalton, John (1766–1844) British *voltaic cell with a copper positive chemist and physicist. In 1801 he for- electrode and a negative electrode of mulated his law of partial pressures a zinc amalgam. The zinc-amalgam dalton’s law (see ), but he is best re- electrode is placed in an electrolyte membered for *Dalton’s atomic of dilute sulphuric acid or zinc sul- theory, which he announced in phate solution in a porous pot, 1803. Dalton also studied colour which stands in a solution of copper blindness (a condition, once called sulphate in which the copper elec- Daltonism, that he shared with his trode is immersed. While the reac- brother). tion takes place ions move through Dalton’s atomic theory A theory the porous pot, but when it is not in of *chemical combination, Ürst use the cell should be dismantled to stated by John *Dalton in 1803. It in- prevent the diffusion of one elec- volves the following postulates: trolyte into the other. The e.m.f. of (1) Elements consist of indivisible the cell is 1.08 volts with sulphuric small particles (atoms). acid and 1.10 volts with zinc sul- (2) All atoms of the same element are phate. It was invented in 1836 by the identical; different elements have dif- British chemist John Daniell (1790– ferent types of atom. 1845). (3) Atoms can neither be created nor photosynthesis destroyed. dark reaction See . (4) ‘Compound elements’ (i.e. com- darmstadtium Symbol Ds. A radio- pounds) are formed when atoms of active transactinide; a.n. 110. It has different elements join in simple ra- several isotopes; the most stable tios to form ‘compound atoms’ (i.e. being 281Ds, with a half-life of about molecules). 1.6 minutes. It can be produced by Dalton also proposed symbols for bombarding a plutonium target with atoms of different elements (later re- sulphur nuclei or by bombarding a placed by the present notation using lead target with nickel nuclei. Its letters). chemical properties probably resem- Dalton’s law The total pressure of ble those of platinum. Darmstadtium a mixture of gases or vapours is was named after the German city of equal to the sum of the partial pres- Darmstadt, the location of the Insti- date-rape drugs 162

tute for Heavy Ion Research where it Institution, London, in 1801 and Üve was Ürst produced. years later isolated potassium and A sodium by electrolysis. He also pre- • Information from the WebElements site pared barium, boron, calcium, and strontium as well as proving that date-rape drugs Drugs that are chlorine and iodine are elements. In used to render the victim unable to 1816 he invented the *Davy lamp. d resist a sexual assault. Particular drugs used for this purpose are Ûuni- Davy lamp An oil-burning miner’s trazepam and gammahydroxybutyric safety lamp invented by Sir Humphry acid (GHB). Davy in 1816 when investigating Üredamp (methane) explosions in dating techniques Methods of es- coal mines. The lamp has a metal timating the age of rocks, palaeonto- gauze surrounding the Ûame, which logical specimens, archaeological cools the hot gases by conduction sites, etc. Relative dating techniques and prevents ignition of gas outside date specimens in relation to one an- the gauze. If Üredamp is present it other; for example, stratigraphy is burns within the gauze cage, and used to establish the succession of lamps of this type are still used for fossils. Absolute (or chronometric) testing for gas. techniques give an absolute estimate of the age and fall into two main d-block elements The block of el- groups. The Ürst depends on the exis- ements in the *periodic table consist- tence of something that develops at a ing of scandium, yttrium, and seasonally varying rate, as in den- lanthanum together with the three drochronology and varve dating. The periods of transition elements: tita- other uses some measurable change nium to zinc, zirconium to cadmium, that occurs at a known rate, as in and hafnium to mercury. These el- *chemical dating, radioactive (or ra- ements all have two outer s-electrons diometric) dating (see carbon dating; and have d-electrons in their penulti- fission-track dating; potassium– mate shell; i.e. an outer electron argon dating; rubidium–strontium conÜguration of the form (n–1)dxns2, dating; uranium–lead dating), where x is 1 to 10. See also transition *amino acid racemization, and *ther- elements. moluminescence. DDT Dichlorodiphenyltrichloro- dative bond See chemical bond. ethane; a colourless organic crystalline compound, (ClC6H4)2CH(CCl3), daughter 1. A nuclide produced by made by the reaction of trichloro- radioactive *decay of some other nu- methanal with chlorobenzene. DDT clide (the parent). 2. An ion or free is the best known of a number of radical produced by dissociation or chlorine-containing *pesticides used reaction of a parent ion or radical. extensively in agriculture in the Davisson–Germer experiment 1940s and 1950s. The compound is See electron diffraction. stable, accumulates in the soil, and concentrates in fatty tissue, reaching Davy, Sir Humphry (1778–1829) dangerous levels in carnivores high British chemist, who studied gases at in the food chain. Restrictions are the Pneumatic Institute in Bristol, now placed on the use of DDT and where he discovered the anaesthetic similar pesticides. properties of *dinitrogen oxide (nitrous oxide). He moved to the Royal deacetylation The removal of an 163 Debye–Hückel–Onsager theory acetyl group (–COCH3) from a mol- fraction substantiated this argument ecule. Deacetylation is an important and the de Broglie wave became the reaction in several biochemical path- basis of *wave mechanics. ways, including the *Krebs cycle. debye A unit of electric dipole mo- Deacon process A former process ment in the electrostatic system, for making chlorine by oxidizing hy- used to express dipole moments of drogen chloride in air at 450°C using molecules. It is the dipole moment d a copper chloride catalyst. It was produced by two charges of opposite patented in 1870 by Henry Deacon sign, each of 1 statcoulomb and (1822–76). placed 10–18 cm apart, and has the × –30 deactivation A partial or complete value 3.335 64 10 coulomb reduction in the reactivity of a sub- metre. It is named after Peter Debye stance, as in the poisoning of a cata- (1884–1966). lyst. Debye, Peter Joseph William deamination The removal of an (1884–1966) Dutch-born physical chemist who worked on a number of amino group (–NH2) from a compound. Enzymatic deamination oc- topics. He introduced the idea of elec- curs in the liver and is important in tric dipole moments in molecules amino-acid metabolism, especially in and, in 1923, working with Erich their degradation and subsequent ox- Hückel, he published the *Debye- idation. The amino group is removed Hückel theory of electrolytes. Debye as ammonia and excreted, either un- was awarded the 1936 Nobel Prize changed or as urea or uric acid. for chemistry. de Broglie, Louis-Victor Pierre Debye–Hückel–Onsager theory Raymond (1892–1987) French A theory providing quantitative re- physicist, who taught at the Sor- sults for the conductivity of ions in bonne in Paris for 34 years. He is best dilute solutions of strong elec- known for his 1924 theory of trolytes, which enables the wave–particle duality (see de broglie *Kohlrausch equation to be derived. wavelength), which reconciled the This theory can be stated as: K = A + Λ0 Λ0 corpuscular and wave theories of B m, where m is the limiting molar light and proved important in quan- conductivity. tum theory. For this work he was A = z2eF2/3πη(2/εRT)½, awarded the 1929 Nobel Prize. B = qz3eF/24πεRT(2/εRT)½, de Broglie wavelength The where z is the charge of an ion, e is wavelength of the wave associated the charge of an electron, F is Fara- with a moving particle. The wave- day’s constant, η is the viscosity of length (λ) is given by λ = h/mv, where the liquid, R is the gas constant, T is h is the Planck constant, m is the the thermodynamic temperature, mass of the particle, and v its veloc- and q = 0.586 in the case of a 1,1 elec- ity. The de Broglie wave was Ürst trolyte. The Debye–Hückel–Onsager suggested by Louis de Broglie in 1924 theory uses the same assumptions on the grounds that electromagnetic and approximations as the waves can be treated as particles *Debye–Hückel theory and is also (photons) and one could therefore limited to very dilute solutions (usu- expect particles to behave in some ally less than 10–3 M) for which there circumstances like waves. The subse- is good agreement between theory quent observation of *electron dif- and experiment. The modiÜcations Debye–Hückel theory 164

were made by the Norwegian-born which assumes that each atom has US chemist Lars Onsager (1903–76). the same vibrational frequency, Debye–Hückel theory A theory to Debye postulated that there is a continuous range of frequencies that explain the nonideal behaviour of ν electrolytes, published in 1923 by cuts off at a maximum frequency D, Peter *Debye and Erich Hückel which is characteristic of a particular d (1896–1980). It assumes that elec- solid. The theory leads to the conclusion that the speciÜc heat capacity of trolytes in solution are fully dissoci- 3 ated and that nonideal behaviour solids is proportional to T , where T arises because of electrostatic interac- is the thermodynamic temperature. tions between the ions. The theory This result is in very good agreement shows how to calculate the extra free with experiment at low tempera- energy per ion resulting from such tures. interactions, and consequently the A key quantity in this theory is the θ Ü θ activity coefÜcient. It gives a good de- Debye temperature, D, de ned by D = ν scription of nonideal electrolyte be- h Dk, where h is the Planck constant haviour for very dilute solutions, but and k is the Boltzmann constant. The cannot be used for more concen- Debye temperature is characteristic trated electrolytes. of a particular solid. For example, the Debye temperature of sodium is 150 Debye–Scherrer method A tech- K and the Debye temperature of cop- nique used in *X-ray diffraction in per is 315 K. which a crystal in powder form is Üxed to a thin Übre or thin silica Debye–Waller factor A quantity tube, which is then rotated in the that characterizes the effect of lattice path of a beam of monochromatic X- vibrations on the scattering intensity rays. A circular diffraction ring, in X-ray diffraction by crystals. The called the Debye–Scherrer ring, con- existence of the Debye–Waller factor centric with the undeÛected beam was pointed out and calculated by is formed. The diffraction pattern is Peter *Debye in 1913–1914 and Ivar recorded on a cylindrical Ülm, which Waller in 1923–1925. Because the has its axis parallel to the axis of ro- amplitude of lattice vibrations in- tation of the material. The Debye– creases with temperature, it was Scherrer method is used to obtain in- thought in the very early days of X- formation about the material. The ray diffraction studies that the dif- grains of the powdered crystal must fraction pattern would disappear at be much larger than the atomic di- high temperatures. The work of mensions in order for them to dif- Debye and Waller showed that the fract X-rays. lattice vibrations at higher temperatures reduce the intensities of the dif- Debye temperature See debye fracted radiation but do not destroy theory of specific heat. the diffraction pattern altogether. Debye theory of speciÜc heat A deca- Symbol da. A preÜx used in theory of the speciÜc heat capacity of the metric system to denote ten solids put forward by Peter *Debye in times. For example, 10 coulombs = 1 1912, in which it was assumed that decacoulomb (daC). the speciÜc heat is a consequence of the vibrations of the atoms of the lat- decahydrate A crystalline hydrate tice of the solid. In contrast to the containing ten moles of water per *Einstein theory of speciÜc heat, mole of compound. 165 degassing decalin (decahydronaphthalene) A the decay constant or the disintegra- liquid bicyclic hydrocarbon, C10H18, tion constant. The reciprocal of the used as a solvent. There are two decay constant is the mean life. The stereoisomers, cis (b.p. 198°C) and time required for half the original ° ½ trans (b.p. 185 C), made by the cat- nuclides to decay (i.e. N = N0) is alytic hydrogenation of naphthalene called the half-life of the nuclide. The at high temperatures and pressures. same terms are applied to elementary particles that spontaneously d H H H transform into other particles. For H H example, a free neutron decays into a proton and an electron. 2. The rever- sion of excited states of atoms or H H molecules to the ground state. H H H deci- Symbol d. A preÜx used in the Decalin metric system to denote one tenth. For example, 0.1 coulomb = 1 deci- decanedioic acid (sebacic acid) A coulomb (dC); 0.1 metre = 1 decime- white crystalline dicarboxylic acid, tre (dm). HOOC(CH2)8COOH; r.d. 1.12; m.p. 131–134.5°C; b.p. 294.4°C (100 decoction A solution made by boiling material (e.g. plant substances) in mmHG). Obtained from castor oil, it Ü is used in plasticizers, lubricants, and water, followed by ltration. cosmetics and in the production of decomposition 1. The chemical other organic chemicals. breakdown of organic matter into its decanoic acid (capric acid) A white constituents by the action of bacteria crystalline straight-chain saturated and other organisms. 2. A chemical reaction in which a compound *carboxylic acid, CH3(CH2)8COOH; m.p. 31.5°C. Its esters are used in per- breaks down into simpler com- fumes and Ûavourings. pounds or into elements. decantation The process of sepa- decrepitation A crackling noise rating a liquid from a settled solid produced when certain crystals are suspension or from a heavier immis- heated, caused by changes in struc- cible liquid by carefully pouring it ture resulting from loss of water of into a different container. crystallization. crystal defect decarboxylation The removal of defect See . carbon dioxide from a molecule. De- defect state A quantum mechani- carboxylation is an important reac- cal state that exists due to the pres- tion in many biochemical processes, ence of a *crystal defect. such as the *Krebs cycle and the syn- Ü chemi- thesis of fatty acids. de nite proportions See cal combination. decay 1. The spontaneous transfor- Û mation of one radioactive nuclide de agration A type of explosion into a daughter nuclide, which may in which the shock wave arrives be- be radioactive or may not, with the fore the reaction is complete (be- emission of one or more particles or cause the reaction front moves more slowly than the speed of sound in the photons. The decay of N0 nuclides to give N nuclides after time t is given medium). γ γ by N = N0exp(– t), where is called degassing The removal of dis- degeneracy 166

solved, absorbed, or adsorbed gases grees of freedom, corresponding to from a liquid or solid. Degassing is the three coordinates in space re- important in vacuum systems, where quired to specify its position. The gas absorbed in the walls of the vac- mean energy per atom for each uum vessel starts to desorb as the degree of freedom is the same, ac- pressure is lowered. cording to the principle of the *equi- d degeneracy The state of being *de- partition of energy, and is equal to generate. kT/2 for each degree of freedom (where k is the *Boltzmann constant degenerate Having quantum and T is the thermodynamic temper- states with the same energy. For ex- ature). Thus for a monatomic gas the Ü ample, the ve d-orbitals in an iso- total molar energy is 3LkT/2, where L lated transition-metal atom have the is the Avogadro constant (the num- same energy (although they have dif- ber of atoms per mole). As k = R/L, ferent spatial arrangements) and are where R is the molar gas constant, thus degenerate. The application of a the total molar energy is 3RT/2. Ü magnetic or electric eld may cause In a diatomic gas the two atoms re- the quantum states to have different quire six coordinates between them, crystal-field theory energies (see ). giving six degrees of freedom. Com- In this case, the degeneracy is said to monly these are interpreted as six in- be ‘lifted’. dependent ways of storing energy: on degenerate rearrangement A this basis the molecule has three de- rearrangement of a molecule in grees of freedom for different direc- which the product is chemically in- tions of translational motion, and in distinguishable from the reactant. addition there are two degrees of Degenerate rearrangements can be freedom for rotation of the molecu- detected by using isotopic labelling. lar axis and one vibrational degree of degradation A type of organic freedom along the bond between the chemical reaction in which a com- atoms. The rotational degrees of free- pound is converted into a simpler dom each contribute their share, compound. An example is the *Bar- kT/2, to the total energy; similarly the bier–Wieland degradation. vibrational degree of freedom has an equal share of kinetic energy and degree A division on a *tempera- must on average have as much po- ture scale. tential energy. The total energy per degrees absolute See absolute. molecule for a diatomic gas is therefore 3kT/2 (for translational energy of degrees of freedom 1. The num- the whole molecule) plus 2kT/2 (for ber of independent parameters re- Ü rotational energy) plus 2kT/2 (for vi- quired to specify the con guration of brational energy), i.e. a total of 7kT/2. a system. This concept is applied in 2. The least number of independent * the kinetic theory to specify the variables required to deÜne the state number of independent ways in of a system in the *phase rule. In this which an atom or molecule can take sense a gas has two degrees of free- up energy. There are however vari- dom (e.g. temperature and pressure). ous sets of parameters that may be chosen, and the details of the conse- dehydration 1. Removal of water quent theory vary with the choice. from a substance. 2. A chemical re- For example, in a monatomic gas action in which a compound loses each atom may be allotted three de- hydrogen and oxygen in the ratio 167 delta bonding

2:1. For instance, ethanol passed over groscopic solid to such an extent that hot pumice undergoes dehydration a concentrated solution of the solid to ethene: eventually forms. → C2H5OH – H2O CH2:CH2 delocalization The spreading of Substances such as concentrated sul- valence electrons over two or more phuric acid, which can remove H2O bonds in a chemical compound. In in this way, are known as dehydrat- certain compounds, the valence elec- d ing agents. For example, with sul- trons cannot be regarded as re- phuric acid, methanoic acid gives stricted to deÜnite bonds between carbon monoxide: the atoms but move over several HCOOH – H O → CO atoms in the molecule. Such elec- 2 trons are said to be delocalized. Delo- dehydrogenase Any enzyme that calization occurs particularly when catalyses the removal of hydrogen the compound contains alternating atoms (*dehydrogenation) in biologi- (conjugated) double or triple bonds, cal reactions. Dehydrogenases occur the delocalized electrons being those in many biochemical pathways but in the pi *orbitals. The molecule is are particularly important in driving then more stable than it would be if the *electron-transport-chain reac- the electrons were localized, an ef- tions of cell respiration. They work fect accounting for the properties of in conjunction with the hydrogen- benzene and other aromatic com- accepting coenzymes *NAD and pounds. The energy difference be- *FAD. tween the actual delocalized state and a localized state is the delocaliza- dehydrogenation A chemical re- tion energy. Another example is in action in which hydrogen is removed the ions of carboxylic acids, contain- from a compound. Dehydrogenation ing the carboxylate group –COO–. In of organic compounds converts sin- terms of a simple model of chemical gle carbon–carbon bonds into double bonding, this group would have the bonds. It is usually effected by means carbon joined to one oxygen by a of a metal catalyst or – in biological double bond (i.e. C=O) and the other systems – by *dehydrogenases. joined to O– by a single bond (C–O–). dehydrohalogenation A type of In fact, the two C–O bonds are identi- chemical reaction in which a hydro- cal because the extra electron on the gen halide is removed from a mol- O– and the electrons in the pi bond ecule with formation of a double of C=O are delocalized over the three bond. A simple example is the forma- atoms. Delocalization of electrons is tion of ethene from chloroethane a feature of metallic bonding. The de- using alcoholic potassium hydroxide: localization energy of molecules can → be calculated approximately using CH3CH2Cl + KOH CH2 = CH2 + the *Hückel approximation, as was KCl +H2O. done originally by Hückel. However, deionized water Water from modern computing power enables which ionic salts have been removed delocalization energy to be calculated by ion-exchange. It is used for many using *ab-initio calculations, even for purposes as an alternative to distilled large molecules. See also localization. water. delta bonding Chemical bonding deliquescence The absorption of involving delta (δ) orbitals. A δ orbital water from the atmosphere by a hy- is so called because it resembles a delta-brass 168

d-orbital when viewed along the axis Crystals that grow in this way (den- of a molecule and has two units of dritic growth) have a branching tree- orbital angular momentum around like appearance. the internuclear axis. The formation dendrochronology An absolute of δ bonds originates in the overlap *dating technique using the growth of d-orbitals on different atoms. Delta rings of trees. It depends on the fact orbitals contribute to the bonding of that trees in the same locality show a d cluster compounds of transition metals. characteristic pattern of growth rings resulting from climatic conditions. delta-brass A strong hard type of Thus it is possible to assign a deÜnite *brass that contains, in addition to date for each growth ring in living copper and zinc, a small percentage trees, and to use the ring patterns to of iron. It is mainly used for making date fossil trees or specimens of cartridge cases. wood (e.g. used for buildings or ob- deltahedron A polyhedron that jects on archaeological sites) with has triangular faces. See wade’s lifespans that overlap those of living rules. trees. The bristlecone pine (Pinus aris- iron tata), which lives for up to 5000 delta-iron See . years, has been used to date speci- delta orbital See delta bonding. mens over 8000 years old. Fossil specimens accurately dated by delta value A quantity that meas- dendrochronology have been used ures the shift in *nuclear magnetic * resonance (NMR). to make corrections to the carbon- dating technique. Dendrochronology denature 1. To add a poisonous or is also helpful in studying past cli- unpleasant substance to ethanol to matic conditions. Analysis of trace el- make it unsuitable for human con- ements in sections of rings can also sumption (see methylated spirits). provide information on past atmos- 2. To produce a structural change in pheric pollution. a protein or nucleic acid that results Ü in the reduction or loss of its biologi- denitri cation A chemical process cal properties. Denaturation is caused in which nitrates in the soil are re- by heat, chemicals, and extremes of duced to molecular nitrogen, which pH. The differences between raw and is released into the atmosphere. This boiled eggs are largely a result of de- process is effected by the bacterium naturation. 3. To add another iso- Pseudomonas denitriÜcans, which uses tope to a Üssile material to make it nitrates as a source of energy for unsuitable for use in a nuclear other chemical reactions in a manner weapon. similar to respiration in other organisms. Compare nitrification. See ni- dendrimer (dendritic polymer) A trogen cycle. type of macromolecule in which a number of chains radiate out from a de novo pathway Any metabolic central atom or cluster of atoms. pathway in which a *biomolecule is Dendritic polymers have a number synthesized from simple precursor of possible applications. See also molecules. Nucleotide synthesis is an supramolecular chemistry. example. dendrite A crystal that has density The mass of a substance branched in growth into two parts. per unit of volume. In *SI units it is 169 designer drug measured in kg m–3. See also relative in tannins and other natural prod- density; vapour density. ucts. density-function theory A derivative A compound that is de- method for the theoretical treatment rived from some other compound of molecules, in which the electron and usually maintains its general density is considered rather than the structure, e.g. trichloromethane interactions of individual electrons. It (chloroform) is a derivative of d is successful for large molecules. methane. deoxyribonucleic acid See dna. derived unit See base unit. depolarization The prevention of desalination The removal of salt *polarization in a *primary cell. For from sea water for irrigation of the example, maganese(IV) oxide (the de- land or to provide drinking water. polarizer) is placed around the posi- The process is normally only eco- tive electrode of a *Leclanché cell to nomic if a cheap source of energy, oxidize the hydrogen released at this such as the waste heat from a nu- electrode. clear power station, can be used. De- salination using solar energy has the depression of freezing point greatest economic potential since The reduction in the freezing point shortage of fresh water is most acute of a pure liquid when another sub- in hot regions. The methods em- stance is dissolved in it. It is a *col- ployed include evaporation, often ligative property – i.e. the lowering under reduced pressure (Ûash evapo- of the freezing point is proportional ration); freezing (pure ice forms from to the number of dissolved particles freezing brine); *reverse osmosis; (molecules or ions), and does not de- *electrodialysis; and *ion exchange. pend on their nature. It is given by ∆t = K C , where C is the molar con- dessicant A drying agent. There f m m are many types, including anhydrous centration of dissolved solute and Kf is a constant (the cryoscopic constant) calcium chloride, anhydrous calcium for the solvent used. Measurements sulphate, concentrated sulphuric of freezing-point depression (using a acid, phosphorus (V) oxide, solid * Beckmann thermometer) can be used sodium hydroxide, lime, and silica for Ünding relative molecular masses gel. of unknown substances. desiccation A method of preserv- A ing organic material by the removal • Raoult’s original paper of its water content. Cells and tissues can be preserved by desiccation after depsides A class of compounds lowering the samples to freezing formed by condensation of a pheno- temperatures; thereafter they can be lic carboxylic acid (such as gallic acid) stored at room temperature. with a similar compound, the reaction being between the carboxylic desiccator A container for drying acid group on one molecule and a substances or for keeping them free phenolic OH group on the other. from moisture. Simple laboratory Depsides are similar to esters, except desiccators are glass vessels contain- that the OH group is linked directly ing a drying agent, such as silica gel. to an aromatic ring. In such com- They can be evacuated through a tap in the lid. pounds, the –O–CO– group is called a depside linkage. Depsides are found designer drug A synthetic drug desorption 170

with a chemical structure that is sim- solvent for many compounds, it will ilar to that of an existing drug. De- not dissolve grease and natural oils. signer drugs are produced either to Detergents are compounds that cause increase potency or to avoid detec- such nonpolar substances to go into tion. An example is ecstasy (MDMA), solution in water. *Soap is the origi- which is a variant of MDA. nal example, owing its action to the presence of ions formed from desorption The removal of ad- d long-chain fatty acids (e.g. the sorbed atoms, molecules, or ions octadecanoate (stearate) ion, from a surface. – CH3(CH2)16COO ). These have two destructive distillation The parts: a nonpolar part (the hydro- process of heating complex organic carbon chain), which attaches to the substances in the absence of air so grease; and a polar part (the –COO– that they break down into a mixture group), which is attracted to the of volatile products, which are con- water. A disadvantage of soap is that densed and collected. At one time it forms a scum with hard water (see the destructive distillation of coal (to hardness of water) and is relatively give coke, coal tar, and coal gas) was expensive to make. Various synthetic the principal source of industrial or- (‘soapless’) detergents have been de- ganic chemicals. veloped from petrochemicals. The desulphuration The removal of commonest, used in washing pow- sulphur from a compound. Desulphu- ders, is sodium dodecylbenzene- sulphonate, which contains ration has been implicated in the – toxicity of a number of sulphur- CH3(CH2)11C6H4SO2O ions. This, like containing organic compounds: it is soap, is an example of an anionic de- postulated that the atomic sulphur tergent, i.e. one in which the active released into a cell by desulphura- part is a negative ion. Cationic deter- tion, which is highly electrophilic, gents have a long hydrocarbon can bind to proteins and thereby chain connected to a positive ion. Usually they are amine salts, as in alter their function. + – CH3(CH2)15N(CH3)3 Br , in which the + detailed balance The cancellation polar part is the –N(CH3)3 group. of the effect of one process by an- Nonionic detergents have non- other process that operates at the ionic polar groups of the type same time with the opposite effect. –C2H4–O–C2H4–OH, which form An example of detailed balance is hydrogen bonds with the water. provided by a chemical reaction be- Synthetic detergents are also used as tween two molecular species A and wetting agents, emulsiÜers, and sta- B, which results in the formation of bilizers for foam. the molecular species C and D. De- detonating gas electrolytic tailed balance for this chemical reac- See gas. tion occurs if the rate at which the reaction A + B → C + D occurs is deuterated compound A com- equal to the rate at which the reac- pound in which some or all of the tion C + D → A + B occurs. The equi- hydrogen–1 atoms have been re- librium state in thermodynamics is placed by deuterium atoms. characterized by detailed balance. deuterium (heavy hydrogen) Sym- detergent A substance added to bol D. The isotope of hydrogen that water to improve its cleaning proper- has a mass number 2 (r.a.m. 2.0144). ties. Although water is a powerful Its nucleus contains one proton and 171 dextrorotatory one neutron. The abundance of deu- molecule. It has a high bond strain terium in natural hydrogen is about and will spontaneously slowly con- 0.015%. It is present in water as the vert to benzene. The unsubstituted oxide HDO (see also heavy water), compound was Ürst synthesized in from which it is usually obtained by 1963. electrolysis or fractional distillation. Dewar Ûask A vessel for storing Its chemical behaviour is almost hot or cold liquids so that they main- d identical to hydrogen although deu- tain their temperature independently terium compounds tend to react of the surroundings. Heat transfer to rather more slowly than the corre- the surroundings is reduced to a min- sponding hydrogen compounds. Its imum: the walls of the vessel consist physical properties are slightly differ- of two thin layers of glass (or, in ent from those of hydrogen, e.g. b.p. large vessels, steel) separated by a 23.6 K (hydrogen 20.4 K). vacuum to reduce conduction and deuterium oxide See heavy convection; the inner surface of a water. glass vessel is silvered to reduce radiation; and the vessel is stoppered to deuteron A nucleus of a deuterium prevent evaporation. It was devised atom, consisting of a proton and a around 1872 by Sir James *Dewar neutron bound together; the ion D+ and is also known by its Ürst trade formed by ionization of a deuterium name Thermos Ûask. See also cryo- atom. See also hydron. stat. Devarda’s alloy An alloy of copper Dewar structure A structure of (50%), aluminium (45%) and zinc (5%), *benzene proposed by Sir James used in chemical tests for the nitrate *Dewar, having a hexagonal ring of ion (in alkaline solutions it reduces a six carbon atoms with two opposite nitrate to ammonia). atoms joined by a long single bond devitriÜcation Loss of the amor- across the ring and with two double phous nature of glass as a result of C–C bonds, one on each side of the crystallization. hexagon. Dewar structures con- Dewar, Sir James (1842–1923) tribute to the resonance hybrid of British chemist and physicist, born in benzene. Scotland. In 1875 he became a pro- dextran A glutinous glucose poly- fessor at Cambridge University, while mer produced by certain bacteria. It carrying out much of his experimen- can be made by fermenting sucrose tal work at the Royal Institution in (cane sugar) and is used as a thicken- London. He began studying gases at ing agent, as a stabilizer in ice cream, low temperatures and in 1872 in- and as a substitute for plasma in vented the *Dewar Ûask. In 1891, to- blood transfusions. Esters with sul- gether with Frederick Abel (1827– phuric acid yield sodium salts that 1902), he developed the smokeless are employed as anticoagulant drugs. propellant explosive *cordite, and in dextrin An intermediate polysac- 1898 was the Ürst to liquefy hydro- charide compound resulting from gen. the hydrolysis of starch to maltose by Dewar benzene An isomer of ben- amylase enzymes. zene, C H . Dewar benzene has the 6 6 dextro- form See optical activity. systematic name bicyclo [2,2,0] hexa- 2,5-diene, and is a nonplanar bicyclic dextrorotatory Denoting a chemi- dextrose 172

cal compound that rotates the plane readily incorporated along with of polarization of plane-polarized other oxides into a wide range of light to the right (clockwise as ob- glassy materials. served by someone facing the on- The reason for this relationship is a coming radiation). See optical combination of the trends to increase activity. size down a group and to decrease size along a period, and a similar, but dextrose See glucose. d reversed, effect in electronegativity, diagonal relationship A relation- i.e. decrease down a group and in- ship within the periodic table by crease along a period. which certain elements in the second dialysis A method by which large period have a close chemical similar- molecules (such as starch or protein) ity to their diagonal neighbours in and small molecules (such as glucose the next group of the third period. or amino acids) in solution may be This is particularly noticeable with separated by selective diffusion the following pairs. through a semipermeable mem- Lithium and magnesium: brane. For example, if a mixed solu- (1) both form chlorides and bromides tion of starch and glucose is placed in that hydrolyse slowly and are soluble a closed container made of a semi- in ethanol; permeable substance (such as Cello- (2) both form colourless or slightly phane), which is then immersed in a coloured crystalline nitrides by direct beaker of water, the smaller glucose reaction with nitrogen at high tem- molecules will pass through the peratures; membrane into the water while the (3) both burn in air to give the nor- starch molecules remain behind. The mal oxide only; cell membranes of living organisms (4) both form carbonates that decom- are semipermeable, and dialysis pose on heating. takes place naturally in the kidneys Beryllium and aluminium: for the excretion of nitrogenous (1) both form highly refractory ox- waste. An artiÜcial kidney (dialyser) ides with polymorphs; utilizes the principle of dialysis by (2) both form crystalline nitrides that taking over the functions of diseased are hydrolysed in water; kidneys. (3) addition of hydroxide ion to solu- magnetism tions of the salts gives an amphoteric diamagnetism See . hydroxide, which is soluble in excess diaminobenzene (phenylenedi- hydroxide giving beryllate or alumi- amine) Any one of three isomeric 2– – nate ions [Be(OH)4] and [Al(OH)4] ; aromatic compounds, C6H4(NH2)2, (4) both form covalent halides and co- with strong basic properties. 1,2- valent alkyl compounds that display Diaminobenzene is a yellow-brown bridging structures; crystalline solid, m.p. 104°C, used as (5) both metals dissolve in alkalis. a photographic developer and in Boron and silicon: making dyes. 1,3-Diaminobenzene is (1) both display semiconductor prop- a colourless crystalline solid, m.p. erties; 63°C, which turns brown on standing (2) both form hydrides that are unsta- in air. It is made by reducing 1,3- ble in air and chlorides that hydrol- dinitrobenzene with iron and hy- yse in moist air; drochloric acid, and is used for mak- (3) both form acidic oxides with cova- ing dyes. 1,4-Diaminobenzene is a lent crystal structures, which are white crystalline solid, m.p. 147°C, 173 diastereoisomers which rapidly darkens on standing in South Africa but others are found in air. It is made by the reduction of 1,4- Tanzania, the USA, Russia, and Aus- nitrophenylamine and is used as a tralia. Diamonds also occur in river photographic develop and hair dye. deposits that have been derived from weathered kimberlite, notably in 1,2-diaminoethane (ethylenedi- Brazil, Zaïre, Sierra Leone, and India. amine) A colourless fuming liquid Industrial diamonds are increasingly that smells of ammonia, d ° being produced synthetically. H2NCH2CH2NH2; b.p. 116 C. It is made from ammonia and 1,2- dichloroethane, which are heated under pressure with a copper(I) chloride catalyst. In air it absorbs water and carbon dioxide to form diaminoethane carbamate. With fatty acids diaminoethane produces soaps, employed as emulsifying agents and detergents. It is also used as a solvent Diamond (for resins) and in making coatings, paper and textiles. Crystals of its tar- diamond-anvil cell A device for trate derivative are employed in producing very high pressures. A piezoelectric devices. It is a bidentate sample of material to be subject to ligand, given the symbol en in formu- the high pressure is placed in a cav- lae. ity between two high-quality dia- 1,6-diaminohexane (hexamethyl- monds. The diamond-anvil cell enediamine) A solid colourless operates like a nutcracker, with pres- ° sures up to 1 megabar (1011 Pa) being amine, H2N(CH2)6NH2; m.p. 41 C; b.p. 204°C. It is made by oxidizing cyclo- exerted by turning a screw. The pres- hexane to hexanedioic acid, reacting sure exerted can be determined by this with ammonia to give the am- spectroscopy for small samples of monium salt, and dehydrating the ruby in the material being com- salt to give hexanedionitrile pressed, while the sample itself is observed optically. A use of the (NC(CH2)6CN). This is reduced with hydrogen to the diamine. The com- diamond-anvil cell is to study the in- pound is used, with hexanedioic acid, sulator–metal transition in such sub- for producing *nylon 6,6. stances as iodine as the pressure is increased. This type of study is the diamond The hardest known min- nearest laboratory approach to the eral (with a hardness of 10 on Mohs’ structure of matter in the conditions scale). It is an allotropic form of pure obtaining in the interior of the earth. *carbon that has crystallized in the cubic system, usually as octahedra or diaspore A mineral form of a cubes, under great pressure. Dia- mixed aluminium oxide and hydrox- mond crystals may be colourless and ide, AlO.OH. See aluminium hydrox- transparent or yellow, brown, or ide. black. They are highly prized as gem- diastase See amylase. stones but also have extensive uses in industry, mainly for cutting and diastereoisomers Stereoisomers grinding tools. Diamonds occur in an- that are not identical and yet not cient volcanic pipes of kimberlite; mirror images. For instance, the d- the most important deposits are in form of tartaric acid and the meso diatomaceous earth 174

form constitute a pair of diastereoiso- low temperature (below 5°C). The ni- mers. See optical activity. trous acid is produced in the reaction diatomaceous earth See kiesel- mixture from sodium nitrite and hy- guhr. drochloric acid: → + ArNH2 + NaNO2 + HCl ArN N + diatomic molecule A molecule – + – Cl + Na + OH + H2O formed from two atoms (e.g. H2 or d HCl). dibasic acid An *acid that has two diatomite See kieselguhr. acidic hydrogen atoms in its molecules. Sulphuric (H2SO4) and car- diazepam A *benzodiazepine used bonic (H2CO3) acids are common medically to treat anxiety, convul- examples. sions, insomnia, and alcohol with- boron(iii) drawal. It is widely used, and often diboron trioxide See known under its tradename Valium. oxide.

CH 1,2-dibromoethane A colourless 3 O liquid *haloalkane, BrCH CH Br; r.d. N 2 2 2.2; m.p. 9.79°C; b.p. 131.36°C. It is made by addition of bromine to N ethene and used as an additive in Cl petrol to remove lead during combustion as the volatile lead bromide. dicarbide See carbide.

Diazepam dicarboxylic acid A *carboxylic acid having two carboxyl groups in diazine See azine. its molecules. In systematic chemical nomenclature, dicarboxylic acids are diazo compounds Organic com- denoted by the sufÜx -dioic; e.g. hexa- pounds containing two linked nitro- nedioic acid, HOOC(CH ) COOH. gen compounds. The term includes 2 4 *azo compounds, diazonium com- dichlorine oxide (chlorine monox- pounds, and also such compounds as ide) A strongly oxidizing orange gas, diazomethane, CH2N2. Cl2O, made by oxidation of chlorine A using mercury(II) oxide. It is the acid • Information about IUPAC nomenclature anhydride of chloric(I) acid. diazonium salts Unstable salts dichlorobenzene Any one of three + isomeric liquid aromatic compounds, containing the ion C6H5N2 (the diazonium ion: see formula). They are C6H4Cl2. 1,2-Dichlorobenzene (b.p. ° formed by *diazotization reactions. 179 C) and 1,4-dichlorobenzene (b.p. 174°C) are made by chlorinating benzene with an iron catalyst and separating the mixed isomers by fractional distillation; 1,3-dichlorobenzene (b.p. 172°C) is made from Diazonium salt one of the other two by catalytic iso- merization. The 1,2 isomer is used as diazotization The formation of a an insecticide and in making dyes; *diazonium salt by reaction of an the 1,4 compound is employed as a aromatic amine with nitrous acid at deodorant and moth repellent. 175 Dieterici equation

H2 CH COOH C 2 H COOH CH CH CH C + CH CH CH C H COOH CH2 COOH C H 2 d Diels-Alder reaction dichloroethanoic acid See (1902–58). Diels and Alder shared the chloroethanoic acids. 1950 Nobel Prize for chemistry. dichloromethane (methylene chlo- Diels–Alder reaction A type of ride) A colourless, slightly toxic liq- chemical reaction in which a com- ° uid, CH2Cl2, b.p. 41 C. It has a pound containing two double bonds characteristic odour similar to that of separated by a single bond (i.e. a con- trichloromethane (chloroform), from jugated *diene) adds to a suitable which it is made by heating with compound containing one double zinc and hydrochloric acid. It is used bond (known as the dienophile) to as a refrigerant and solvent (for paint give a ring compound. In the stripping and degreasing). dienophile, the double bond must 2,4-dichlorophenoxyacetic acid have a carbonyl group on each side. See 2,4-d. It is named after the German dichroism The property of some chemists Otto *Diels and Kurt Alder. crystals, such as tourmaline, of selec- diene An *alkene that has two dou- tively absorbing light vibrations in ble bonds in its molecule. If the two one plane while allowing light vibra- bonds are separated by one single tions at right angles to this plane to bond, as in buta-1,3-diene pass through. Polaroid is a synthetic CH :CHCH:CH , the compound is a dichroic material. See polarization. 2 2 conjugated diene. dichromate(VI) A salt containing diels alder reac- the ion Cr O –. Solutions containing dienophile See – 2 7 tion dichromate(VI) ions are strongly oxi- . dizing. Dieterici equation An *equation 1,2-didehydrobenzene See ben- of state for a gas of the form zyne. P(V – b)[exp (a/VRT)] = RT, dielectric constant See permittiv- where P is the pressure, V is the vol- ity. ume, T is the thermodynamic tem- Diels, Otto Paul Hermann perature, R is the gas constant, and a (1876–1954) German organic chemist and b are constants characteristic of who worked mostly at the University the gas. The Dieterici equation is a Ü of Kiel. In 1906 he discovered tricar- modi cation of van der Waals’ equation, which takes account of the pres- bon dioxide (C3O2). Diels also did important work on steroids but is re- sure gradient at the boundary of the membered for his discovery in 1928 gas. At low pressures the Dieterici of the *Diels–Alder reaction, which equation becomes identical to van he made with his assistant Kurt Alder der Waals’ equation. diethanolamine 176

diethanolamine See ethanol- down to 10–7 Pa can be reached by so- amine. phisticated forms of the diffusion pump. diethyl ether See ethoxyethane. Û fick’s law differential scanning calorime- diffusive ux See . try (DSC) See thermal analysis. dihedral (dihedron) An angle differential thermal analysis formed by the intersection of two d planes (e.g. two faces of a polyhe- (DTA) See thermal analysis. dron). The dihedral angle is the angle diffusion 1. The process by which formed by taking a point on the line different substances mix as a result of intersection and drawing two lines of the random motions of their com- from this point, one in each plane, ponent atoms, molecules, and ions. perpendicular to the line of intersec- In gases, all the components are per- tion. fectly miscible with each other and dihydrate A crystalline hydrate mixing ultimately becomes nearly containing two moles of water per uniform, though slightly affected by mole of compound. gravity (see also graham’s law). The diffusion of a solute through a sol- dihydric alcohol See diol. vent to produce a solution of uni- dihydrogen The normal form of form concentration is slower, but molecular hydrogen, H2, used to dis- otherwise very similar to the process tinguish it from hydrogen atoms. of gaseous diffusion. In solids, diffusion occurs very slowly at normal 1,2-dihydroxybenzene (catechol) temperatures. See also fick’s law. A colourless crystalline phenol, ° 2. The passage of elementary parti- C6H4(OH)2; r.d. 1.15; m.p. 105 C; b.p. ° cles through matter when there is a 245 C. It is used as a photographic high probability of scattering and a developer. low probability of capture. 1,3-dihydroxybenzene (resorcinol) diffusion constant See fick’s law. A colourless crystalline aromatic ° compound, C6H4(OH)2; m.p. 111 C. It diffusion gradient See concen- is made by the fusion of benzene- tration gradient. disulphonic acid with sodium hy- diffusion limited aggregation droxide and used mainly in the (DLA) A process of aggregation domi- dyestuffs industry. On fusing with Û nated by particles diffusing and hav- phthalic anhydride it forms uores- ing a nonzero probability of sticking cein dyes. It is also used to make together irreversibly when they cold-setting adhesives (with touch. The clusters formed by DLA methanal), plasticizers, and resins. are *fractal in type. 2,3-dihydroxybutanedioic acid tartaric acid diffusion pump (condensation See . pump) A vacuum pump in which oil diketones Organic compounds that or mercury vapour is diffused have two carbonyl (>C=O) groups (see through a jet, which entrains the gas ketones). There are three kinds, de- molecules from the container in pending on the location of the car- which the pressure is to be reduced. bonyl groups. 1,2-Diketones (also The diffused vapour and entrained called α-diketones), R.CO.CO.R′, have gas molecules are condensed on the the carbonyl groups on adjacent cooled walls of the pump. Pressures carbon atoms. The aliphatic 1,2- 177 p-dimethylaminobenzaldehyde diketones are pungent-smelling yel- cleus in which it is statistically un- low oils, whereas the aromatic com- likely that there will be more than pounds are crystalline solids. 1,3- one such nucleus in a molecule (un- β Ü- Diketones (or -diketones), R.CO.CH2. less the substance has been arti CO.R′, are more acidic and exist in cially enriched with isotopes having both keto and enol forms (see keto– that nucleus). An example of a dilute enol tautomerism); they form stable spin species is 13C since it has a nat- compounds with metals. 1,4-Dike- ural abundance of 1.1%. This means d γ tones (or -diketones), R.CO.CH2.CH2. that for dilute spin systems it is usu- CO.R′, also exist and readily re- ally not necessary to consider spin– arrange to form cyclic compounds. spin interactions between two nuclei of the dilute spin species, e.g. 13C–13C dilatancy See newtonian fluid. in the same molecule. The opposite dilation (dilatation) An increase in of a dilute spin species is an abun- volume. dant spin species, an example being dilead(II) lead(IV) oxide A red the proton. powder, Pb3O4; r.d. 9.1; decomposes dilution The volume of solvent in at 500°C to lead(II) oxide. It is pre- which a given amount of solute is pared by heating lead(II) oxide to dissolved. 400°C and has the unusual property dilution law See ostwald’s dilu- of being black when hot and red- tion law. orange when cold. The compound is nonstoichiometric, generally contain- dimer An association of two identi- ing less oxygen than implied by the cal molecules linked together. The formula. It is largely covalent and has molecules may react to form a larger Pb(IV)O6 octahedral groups linked to- molecule, as in the formation of dini- gether by Pb(II) atoms, each joined to trogen tetroxide (N2O4) from nitro- three oxygen atoms. It is used in gen dioxide (NO2), or the formation glass making but its use in the paint of an *aluminium chloride dimer industry has largely been discontin- (Al2Cl6) in the vapour. Alternatively, ued because of the toxicity of lead. they may be held by hydrogen bonds. Dilead(II) lead(IV) oxide is commonly For example, carboxylic acids form called red lead or, more accurately, dimers in organic solvents, in which red lead oxide. hydrogen bonds exist between the O of the C=O group and the H of the Dillie–Koppanyi test A presump- –O–H group. tive test for barbituates. The Dillie– Koppanyi test reagent has two solu- p-dimethylaminobenzaldehyde tions: a 1% solution of colbalt acetate in methanol, follow by a 5% solution O H of isopropylamine (CH3CH(CH3)NH2) in methanol. Barbiturates give a reddish-violet colour. diluent A substance added to dilute a solution or mixture (e.g. a *Üller). dilute Describing a solution that has a relatively low concentration of N solute. 3CH CH3 dilute spin species A type of nu- p-dimethylaminobenzaldehyde dimethylbenzenes 178

(p-DMAB) A substituted aldehyde dinitrogen oxide (nitrous oxide) A –3 used in a presumptive test for LSD colourless gas, N2O, d. 1.97 g dm ; and for psilocybin. A 1% solution of p- m.p. –90.8°C; b.p. –88.5°C. It is solu- BMAB is used in 10% mineral acid (ei- ble in water, ethanol, and sulphuric ther sulphuric acid or hydrochloric acid. It may be prepared by the con- acid). This reagent is called Ehrlick’s trolled heating of ammonium nitrate d reagent or Van Urk’s reagent. LSD is (chloride free) to 250°C and passing indicated by a purple colouration; the gas produced through solutions psilocybin by a red-brown colour. of iron(II) sulphate to remove impuri- dimethylbenzenes (xylenes) ties of nitrogen monoxide. It is rela- Three compounds with the formula tively unreactive, being inert to (CH ) C H , each having two methyl halogens, alkali metals, and ozone at 3 2 6 4 normal temperatures. It is decom- groups substituted on the benzene ° ring. 1,2-dimethylbenzene is ortho- posed on heating above 520 C to ni- xylene, etc. A mixture of the isomers trogen and oxygen and will support (b.p. 135–145°C) is obtained from pe- the combustion of many compounds. troleum and is used as a clearing Dinitrogen oxide is used as an anaes- agent in preparing specimens for op- thetic gas (‘laughing gas’) and as an tical microscopy. aerosol propellant. dimethylformamide (DMF) A dinitrogen tetroxide A colourless colourless liquid compound, to pale yellow liquid or a brown gas, ° (CH ) NCHO; r.d. 0.944; m.p. –61°C; N2O4; r.d. 1.45 (liquid); m.p. –11.2 C; 3 2 ° b.p. 153°C. The systematic name is b.p. 21.2 C. It dissolves in water with N,N-dimethylmethanamide. It can be reaction to give a mixture of nitric made from methanoic acid (formic acid and nitrous acid. It may be read- acid) and dimethylamine, and is ily prepared in the laboratory by the widely used as a solvent. reaction of copper with concentrated nitric acid; mixed nitrogen oxides theo- 1,3-dimethylxanthine See containing dinitrogen oxide may also phylline . be produced by heating metal ni- 3,7-dimethylxanthine See theo- trates. The solid compound is wholly bromine. N2O4 and the liquid is about 99% N2O4 at the boiling point; N2O4 is dia- dimethylglyoxime (DMG) A magnetic. In the gas phase it dissoci- colourless solid, (CH3CNOH)2, m.p. nitrogen dioxide ° ° ates to give 234 C. It sublimes at 215 C and ˆ slowly polymerizes if left to stand. It N2O4 2NO2 is used in chemical tests for nickel, Because of the unpaired electron this with which it forms a dark-red com- is paramagnetic and brown. Liquid plex. N2O4 has been widely studied as a nonaqueous solvent system (self- dimethyl sulphoxide (DMSO) A + – ° ionizes to NO and NO3 ). Dinitrogen colourless solid, (CH3)2SO; m.p. 18 C; ° tetroxide, along with other nitrogen b.p. 189 C. It is used as a solvent and oxides, is a product of combustion as a reagent in organic synthesis. engines and is thought to be involved dimorphism See polymorphism. in the depletion of stratospheric ozone. dinitrogen The normal form of molecular nitrogen, N2 used to distin- dinucleotide A compound consist- guish it from nitrogen atoms. ing of two *nucleotides. 179 dipole diol (dihydric alcohol) An *alcohol and slightly basic (forming salts with containing two hydroxyl groups per mineral acids). Its derivatives are em- molecule. ployed as stabilizers for synthetic rubber and rocket fuels. dioxan A colourless toxic liquid, ° C4H8O2; r.d. 1.03; m.p. 11 C; b.p. diphenylamine test A presump- 101.5°C. The molecule has a six- tive test for nitrates. The reagent is a membered ring containing four CH2 solution of diphenylamine d groups and two oxygen atoms at op- ((C6H5)2NH) in sulphuric acid. A posi- posite corners. It can be made from tive result is indicated by a blue col- ethane-1,2-diol and is used as a sol- our. It was once used in testing for vent. gunshot residue, but is not particu- dioxin (2,4,7,8-tetrachlorodibenzo- larly reliable. p-dioxin) A toxic solid, formed in the diphenylmethanone (benzo- manufacture of the herbicide *2,4,5- phenone) A colourless solid, ° T and present as an impurity in C6H5COC6H5, m.p. 49 C. It has a char- Agent Orange. It can cause skin dis- acteristic smell and is used in making Ügurement (chloracne) and severe perfumes. It is made from benzene fetal defects. and benzoyl chloride using the dioxonitric(III) acid See nitrous *Friedel–Crafts reaction with alu- acid. minium chloride as catalyst. dioxygen The normal form of mo- diphosgene A colourless liquid, ClCO.O.CCl , originally used in 1916 lecular oxygen, O2, used to distin- 3 guish it from oxygen atoms or from by Germany in World War I as a chemical warfare agent. It is now ozone (O3). used as a reagent in organic synthe- dioxygenyl compounds Com- sis. See also carbonyl chloride. pounds containing the positive ion + Û O2 , as in dioxygenyl hexa uoro- diphosphane (diphosphine) A yel- platinate O2PtF6 – an orange solid low liquid, P2H4, which is sponta- that sublimes in vacuum at 100°C. neously Ûammable in air. It is Other ionic compounds of the type obtained by hydrolysis of calcium + – O2 [MF6] can be prepared, where M phosphide. Many of the references to is P, As, or Sb. the spontaneous Ûammability of phosphine (PH ) are in fact due to dipeptide A compound consisting 3 traces of P H as impurities. of two amino acid units joined at the 2 4 diphosphane amino (–NH2) end of one and the car- diphosphine See . boxyl (–COOH) end of the other. This bisphospho- peptide diphosphonates See peptide bond (see ) is formed nates. by a condensation reaction that involves the removal of one molecule dipolar bond See chemical bond. of water. dipole A pair of separated opposite diphenylamine A colourless crys- electric charges. The dipole moment talline aromatic compound, (symbol µ) is the product of the posi- ° (C6H5)2NH; m.p. 54 C. It is made by tive charge and the distance between heating phenylamine (aniline) with the charges. Dipole moments are phenylamine hydrochloride. It is a often stated in *debyes; the SI unit is secondary amine and is both slightly the coulomb metre. In a diatomic acidic (forming an N-potassium salt) molecule, such as HCl, the dipole dipole–dipole interaction 180

moment is a measure of the polar na- nary compounds that are used in her- ture of the bond (see polar mol- bicides such as Paraquat (44′) and Di- ecule); i.e. the extent to which the quat (22′). average electron charge is displaced towards one atom (in the case of HCl, the electrons are attracted towards the more electronegative chlorine d atom). In a polyatomic molecule, the N N dipole moment is the vector sum of the dipole moments of the individual bonds. In a symmetrical molecule, N N such as tetrachloromethane (CCl4), there is no overall dipole moment, although the individual C–Cl bonds are Dipyridyl polar. dipole–dipole interaction The Diquat Tradename for a herbicide. interaction of two systems, such as See dipyridyl. atoms or molecules, by their *dipole Dirac, Paul Adrien Maurice moments. The energy of dipole– (1902–84) British physicist, who dipole interaction depends on the shared the 1933 Nobel Prize for relative orientation and the strength physics with Erwin *Schrödinger of the dipoles and how far apart they for developing Schrödinger’s non- are. A water molecule has a perma- relativistic wave equations to take nent dipole moment, thus causing a account of relativity. Dirac also in- dipole–dipole interaction if two vented, independently of Enrico water molecules are near each other. Fermi, the form of *quantum statis- Although isolated atoms do not have tics known as Fermi–Dirac statistics. permanent dipole moments, a dipole moment can be induced by the pres- Dirac constant See planck con- ence of another atom near it, thus stant. leading to induced dipole–dipole in- Dirac equation A version of the teractions. Dipole–dipole interactions nonrelativistic *Schrödinger equa- are responsible for *van der Waals’ tion taking special relativity theory forces and *surface tension in liq- into account. The Dirac equation is uids. needed to discuss the quantum me- dipole radiation See forbidden chanics of electrons in heavy atoms transitions. and, more generally, to discuss Üne- structure features of atomic spectra, dipyridyl (bipyridyl) A compound such as *spin–orbit coupling. The formed by linking two pyridine rings equation was put forward by Paul by a single C–C bond, (C H N) . Vari- 5 4 2 * ous isomers are possible depending Dirac in 1928. It can be solved ex- on the relative positions of the nitro- actly in the case of the hydrogen gen atoms. A mixture of isomers can atom but can only be solved using be made by reacting pyridine with approximation techniques for more sodium metal and oxidizing the re- complicated atoms. sulting sodium salt. The 22′-isomer is diradical See biradical. a powerful chelating agent denoted direct dye See dyes. bipy in chemical formulae. Both the 22′- and 44′-isomers can form quater- disaccharide A sugar consisting of 181 dissipative system two linked *monosaccharide mol- disordered solid A material that ecules. For example, sucrose com- neither has the structure of a perfect prises one glucose molecule and one *crystal lattice nor of a crystal lattice fructose molecule bonded together. with isolated *crystal defects. In a discharge 1. The conversion of the random alloy, one type of disordered chemical energy stored in a *sec- solid, the order of the different types ondary cell into electrical energy. of atom occurs at random. Another d 2. The release of electric charge from type of disordered solid is formed by a capacitor in an external circuit. introducing a high concentration of 3. The passage of charge carriers defects, with the defects distributed through a gas at low pressure in a randomly throughout the solid. In an discharge tube. A potential difference *amorphous solid, such as glass, applied between cathode and anode there is a random network of atoms creates an electric Üeld that acceler- with no lattice. ates any free electrons and ions to disperse dye See dyes. their appropriate electrodes. Colli- colloids sions between electrons and gas mol- disperse phase See . ecules create more ions. Collisions dispersion forces See van der also produce excited ions and mol- waals’ force. ecules (see excitation), which decay displacement reaction See substi- with emission of light in certain tution reaction parts of the tube. . disconnection See retrosynthetic disproportionation A type of analysis. chemical reaction in which the same compound is simultaneously reduced disilane See silane. and oxidized. For example, copper(I) dislocation See crystal defect. chloride disproportionates thus: 2CuCl → Cu + CuCl disodium hydrogenphos- 2 phate(V) (disodium orthophos- The reaction involves oxidation of phate) A colourless crystalline solid, one molecule I → II Na2HPO4, soluble in water and insol- Cu Cu + e uble in ethanol. It is known as the di- and reduction of the other hydrate (r.d. 2.066), heptahydrate CuI + e → Cu (r.d. 1.68), and dodecahydrate (r.d. 1.52). It may be prepared by titrating The reaction of halogens with hy- phosphoric acid with sodium hydrox- droxide ions is another example of a ide to an alkaline end point (phe- disproportionation reaction, for ex- nolphthalein) and is used in treating ample – ˆ – boiler feed water and in the textile Cl2(g) + 2OH (aq) Cl (aq) + – industry. ClO (aq) + H2O(l) disodium orthophosphate See The reverse process is *compropor- disodium hydrogenphosphate(v). tionation. disodium tetraborate-10-water dissipative system A system that See borax. involves *irreversible processes. All real systems are dissipative (in con- d-isomer optical activity See . trast to such idealized systems as the d-isomer See absolute configura- frictionless pendulum, which is in- tion. variant under time reversal). In a dis- dissociation 182

sipative system the system is moving when calcium carbonate is main- towards a state of equilibrium, which tained at a constant high tempera- can be regarded as moving toward a ture in a closed container, the point attractor in phase space; this is dissociation pressure at that tempera- equivalent to moving towards the ture is the pressure of carbon dioxide minimum of the free energy, F. from the equilibrium ˆ d dissociation The breakdown of a CaCO3(s) CaO(s) + CO2(g) molecule, ion, etc., into smaller mol- distillation The process of boiling ecules, ions, etc. An example of disso- a liquid and condensing and collect- ciation is the reversible reaction of ing the vapour. The liquid collected hydrogen iodide at high tempera- is the distillate. It is used to purify tures liquids and to separate liquid mix- ˆ 2HI(g) H2(g) + I2(g) tures (see fractional distillation; The *equilibrium constant of a re- steam distillation). See also de- versible dissociation is called the structive distillation; extractive dissociation constant. The term ‘dis- distillation. sociation’ is also applied to ionization distilled water Water puriÜed by reactions of *acids and *bases in distillation so as to free it from dis- water; for example solved salts and other compounds. ˆ + – HCN + H2O H3O + CN Distilled water in equilibrium with which is often regarded as a straight- the carbon dioxide in the air has a forward dissociation into ions conductivity of about 0.8 × 10–6 –1 HCN ˆ H+ + CN– siemens cm . Repeated distillation in a vacuum can bring the conductivity The equilibrium constant of such a down to 0.043 × 10–6 siemens cm–1 at dissociation is called the acid dissoci- 18°C (sometimes called conductivity ation constant or acidity constant, water). The limiting conductivity is given by due to self ionization: H O ˆ H+ + + – 2 Ka = [H ][A ]/[HA] OH–. See also deionized water. for an acid HA (the concentration of disulphur dichloride (sulphur water [H O] can be taken as con- 2 monochloride) An orange–red liquid, stant). K is a measure of the strength a S Cl , which is readily hydrolysed by of the acid. Similarly, for a nitroge- 2 2 water and is soluble in benzene and nous base B, the equilibrium ether; r.d. 1.678; m.p. –80°C; b.p. ˆ + – B + H2O BH + OH 136°C. It may be prepared by passing is also a dissociation; with the base chlorine over molten sulphur; in the dissociation constant, or basicity con- presence of iodine or metal chlorides stant, given by sulphur dichloride, SCl2, is also + – formed. In the vapour phase S Cl Kb = [BH ][OH ]/[B] 2 2 molecules have Cl–S–S–Cl chains. For a hydroxide MOH, The compound is used as a solvent + – Kb = [M ][OH ]/[MOH] for sulphur and can form higher chlorosulphanes of the type dissociation pressure When a Cl–(S) –Cl (n < 100), which are of solid compound dissociates to give n great value in *vulcanization one or more gaseous products, the processes. dissociation pressure is the pressure of gas in equilibrium with the solid disulphuric(VI) acid (pyrosulphuric at a given temperature. For example, acid) A colourless hygroscopic crys- 183 DNA talline solid, H2S2O7; r.d. 1.9; m.p. tential can be calculated as a func- 35°C. It is commonly encountered tion of distance, with colloid stability mixed with sulphuric acid as it is being attained when the two forces formed by dissolving sulphur triox- balance each other. The DLVO theory ide in concentrated sulphuric acid. is the basis for understanding colloid The resulting fuming liquid, called stability and has a considerable oleum or Nordhausen sulphuric acid, amount of experimental support. is produced during the *contact However, it is inadequate for the d process and is also widely used in the properties of colloids in the aggre- *sulphonation of organic com- gated state, which depend on short- pounds. See also sulphuric acid. range interactions taking into account the speciÜc properties of dithionate A salt of dithionic acid, ions, rather than regarding them as containing the ion S O 2–, usually 2 6 point particles. formed by the oxidation of a sulphite using manganese(IV) oxide. The ion p-DMAB See p-dimethylaminoben- has neither pronounced oxidizing zadehyde. nor reducing properties. DMF See dimethylformamide. dithionic acid An acid, H2S2O6, DMG See dimethylglyoxime. known in the form of its salts (dithionates). DMSO See dimethyl sulphoxide. dithionite See sulphinate. DNA (deoxyribonucleic acid) The genetic material of most living organ- dithionous acid See sulphinic acid isms, which is a major constituent of . the chromosomes within the cell nu- divalent (bivalent) Having a va- cleus and plays a central role in the lency of two. determination of hereditary characteristics by controlling protein syn- DLA See diffusion limited aggre- thesis in cells. DNA is a nucleic acid gation. composed of two chains of *nu- D-lines Two close lines in the yel- cleotides in which the sugar is de- low region of the visible spectrum of oxyribose and the bases are sodium, having wavelengths 589.0 *adenine, *cytosine, *guanine, and and 589.6 nm. As they are prominent *thymine (compare rna). The two and easily recognized they are used chains are wound round each other as a standard in spectroscopy. and linked together by hydrogen bonds between speciÜc complemen- dl-isomer See optical activity; tary bases to form a spiral ladder- racemic mixture. shaped molecule (double helix: see DLVO theory A theory of colloid illustration). stability proposed in the 1940s by the When the cell divides, its DNA also Soviet scientists Boris Derjaguin and replicates in such a way that each of Lev Landau and independently by the the two daughter molecules is identi- Dutch scientists Evert Verwey and cal to the parent molecule. The Theo Overbeek. The DLVO theory hydrogen bonds between the com- takes account of two types of force in plementary bases on the two strands a stable colloid: the van der Waals’ of the parent molecule break and the force, which is attractive and binds strands unwind. Using as building particles together, and electrostatic bricks nucleotides present in the repulsion. The total interaction po- nucleus, each strand directs the Döbereiner’s triads 184

– 2 AT O O deoxyribose P OOCH– bases O OH CH 2 O hydrogen P bond H OO– 2 d O O CH O TA P OO H nucleotide – 2 GC O O CH O O P CG O – O OH H sugar– phosphate Detail of molecular structure of sugar–phosphate backbone backbone. Each deoxyribose unit is attached to a phosphate group and a base, forming a nucleotide

TA hydrogen bond thymine adenine GC (T) H (A) CH AT 3.4 nm 3 O HN N

base N NH N N N H O sugar– NH O N AT phosphate backbone NHN N CG N N OHN H Double helical structure of DNA cytosine guanine (C) (G) The four bases of DNA, showing the hydrogen bonding between base pairs

DNA

synthesis of a new one complemen- triads are now recognized as consecu- tary to itself. Replication is initiated, tive members of the groups of the controlled, and stopped by means of periodic table. Examples are: lithium, polymerase enzymes. sodium, and potassium; calcium, Döbereiner’s triads A set of triads strontium, and barium; and chlorine, of chemically similar elements noted bromine, and iodine. by Johann Döbereiner (1780–1849) in A 1817. Even with the inaccurate • Döbereiner’s original paper atomic mass data of the day it was dodecanoic acid (lauric acid) A observed that when each triad was white crystalline *fatty acid, arranged in order of increasing CH (CH ) COOH; r.d. 0.8; m.p. 44°C; atomic mass, then the mass of the 3 2 10 b.p. 225°C. Glycerides of the acid are central member was approximately present in natural fats and oils (e.g. the average of the values for the coconut and palm-kernel oil). other two. The chemical and physical properties were similarly related. The dodecene A straight-chain alkene, 185 dose

CH3(CH2)9CH:CH2, obtained from pe- donor An atom, ion, or molecule troleum and used in making *dode- that provides a pair of electrons in cylbenzene. forming a coordinate bond. dodecylbenzene A hydrocarbon, dopa (dihydroxyphenylalanine) A CH3(CH2)11C6H5, manufactured by a derivative of the amino acid tyrosine. Friedel–Crafts reaction between do- It is found in particularly high levels decene (CH3(CH2)9CH:CH2) and ben- in the adrenal glands and is a precur- d zene. It can be sulphonated, and the sor in the synthesis of *dopamine, sodium salt of the sulphonic acid is *noradrenaline, and *adrenaline. the basis of common *detergents. The laevorotatory form, L-dopa, is administered in the treatment of dolomite A carbonate mineral con- Parkinson’s disease, in which brain sisting of a mixed calcium–magne- levels of dopamine are reduced. sium carbonate, CaCO3.MgCO3, crystallizing in the rhombohedral H system. It is usually white or colour- 2 less. The term is also used to denote OH C CH a rock with a high ratio of magne- 2 sium to calcium carbonate. See lime- NH2 stone. OH domain A functional unit of the Dopa tertiary structure of a *protein. It consists of chains of amino acids dopamine A *catecholamine that folded into alpha helices and *beta is a precursor in the synthesis of *no- sheets to form a globular structure. radrenaline and *adrenaline. It also Different domains are linked to- functions as a neurotransmitter in gether by relatively straight sections the brain. of polypeptide chain to form the protein molecule. Domains allow a de- H H2 2 gree of movement in the protein OH C C structure. OH Donnan equilibrium The equilib- NH2 rium set up when two solutions are OH separated by a membrane permeable Dopamine to some but not all of the ions in the solutions. In practice, the membrane d-orbital See orbital. is often permeable to the solvent and small ions but not to charged entities dose A measure of the extent to of colloidal size or such polyelec- which matter has been exposed to trolytes as proteins. An electrical po- *ionizing radiation. The absorbed tential develops between the two dose is the energy per unit mass ab- sides of the membrane with the two sorbed by matter as a result of such solutions having varying osmotic exposure. The SI unit is the gray, al- pressure. Donnan equilibrium is though it is often measured in rads (1 named after the British chemist Fred- rad = 0.01 gray; see radiation units). erick George Donnan (1870–1956), The maximum permissible dose is the who developed the theory of mem- recommended upper limit of ab- brane equilibrium. Donnan equilib- sorbed dose that a person or organ rium is important in biology. should receive in a speciÜed period dosimeter 186

according to the International Com- in certain spectra, e.g. the two lines mission on Radiological Protection. that make up the sodium D-lines. dosimeter Any device used to Downs process A process for ex- measure absorbed *dose of ionizing tracting sodium by the electrolysis of radiation. Methods used include the molten sodium chloride. The Downs ionization chamber, photographic cell has a central graphite anode sur- d Ülm, or the rate at which certain rounded by a cylindrical steel cath- chemical reactions occur in the pres- ode. Chlorine released is led away ence of ionizing radiation. through a hood over the anode. Molten sodium is formed at the cath- double bond See chemical bond. ode and collected through another double decomposition See hood around the top of the cathode metathesis. cylinder (it is less dense than the double layer See electrical dou- sodium chloride). The two hoods and ble layer. electrodes are separated by a coaxial cylindrical steel gauze. A small double refraction The property, amount of calcium chloride is added possessed by certain crystals (notably to the sodium chloride to lower its calcite), of forming two refracted rays melting point. The sodium chloride from a single incident ray. The ordi- is melted electrically and kept nary ray obeys the normal laws of re- molten by the current through the fraction. The other refracted ray, cell. More sodium chloride is added called the extraordinary ray, follows as the electrolysis proceeds. different laws. The light in the ordinary ray is polarized at right angles Dow process A method of extract- to the light in the extraordinary ray. ing magnesium from sea water by Along an optic axis the ordinary and adding calcium hydroxide to precipi- extraordinary rays travel with the tate magnesium hydroxide. same speed. Some crystals, such as Dragendorff test A *presumptive calcite, quartz, and tourmaline, have test for alkaloids. The Dragendorff only one optic axis; they are uniaxial reagent has two solutions. One is bis- crystals. Others, such as mica and se- muth nitrate in acetic acid. This is lenite, have two optic axes; they are followed by a sodium nitrate solu- biaxial crystals. The phenomenon is tion. Alkaloids are indicated by a red- also known as birefringence and the dish-brown deposit. double-refracting crystal as a birefrin- dropping-mercury electrode See gent crystal. See also polarization. polarography. double salt A crystalline salt in dry cell A primary or secondary cell which there are two different anions in which the electrolytes are re- and/or cations. An example is the strained from Ûowing in some way. mineral dolomite, CaCO .MgCO , 3 3 Many torch, radio, and calculator bat- which contains a regular arrange- teries are *Leclanché cells in which ment of Ca2+ and Mg2+ ions in its the electrolyte is an ammonium chlo- crystal lattice. *Alums are double sul- ride paste and the container is the phates. Double salts only exist in the negative zinc electrode (with an solid; when dissolved they act as a outer plastic wrapping). Various mixture of the two separate salts. modiÜcations of the Leclanché cell Double oxides are similar. are used in dry cells. In the zinc chlo- doublet A pair of associated lines ride cell, the electrolyte is a paste of 187 Dumas’ method zinc chloride rather than ammonium conÜrmed in 1970 at Dubna and at chloride. The electrical characteris- Berkeley, California. It can be made tics are similar to those of the by bombarding californium–249 nu- Leclanché cell but the cell works bet- clei with nitrogen–15 nuclei. Only a ter at low temperatures and has few atoms have ever been made. more efÜcient depolarization charac- A teristics. A number of alkaline sec- • Information from the WebElements site d ondary cells can be designed for use as dry cells. In these, the electrolyte Dulong and Petit’s law For a is a liquid (sodium or potassium hy- solid element the product of the rela- Ü droxide) held in a porous material or tive atomic mass and the speci c in a gel. Alkaline dry cells typically heat capacity is a constant equal to –1 –1 have zinc–manganese dioxide, silver about 25 J mol K . Formulated in oxide–zinc, nickel–cadmium, or these terms in 1819 by the French nickel–iron electrode systems (see scientists Pierre Dulong (1785–1838) nickel–iron accumulator). For spe- and Alexis Petit (1791–1820), the law cialized purposes, dry cells and bat- in modern terms states: the molar teries have been produced with solid heat capacity of a solid element is ap- electrolytes. These may contain a proximately equal to 3R, where R is solid crystalline salt, such as silver io- the *gas constant. The law is only ap- dide, an ion-exchange membrane, or proximate but applies with fair accu- an organic wax with a small amount racy at normal temperatures to of dissolved ionic material. Such cells elements with a simple crystal struc- deliver low currents. They are used ture. in miniature cells for use in elec- A tronic equipment. • Original paper dry ice Solid carbon dioxide used as Dumas, Jean Baptiste André a refrigerant. It is convenient because (1800–84) French chemist, who be- it sublimes at –78°C (195 K) at stan- came an apothecary in Geneva, dard pressure rather than melting. where in 1818 he investigated the use of iodine to treat goitre. He then drying oil A natural oil, such as lin- took up chemistry and moved to seed oil, that hardens on exposure to Paris. In 1826 he devised a method of the air. Drying oils contain unsatu- measuring *vapour density. He went rated fatty acids, such as linoleic and on to discover various organic com- linolenic acids, which polymerize on pounds, including anthracene (1832), oxidation. They are used in paints, urethane (1833), and methanol varnishes, etc. (1834), which led him in 1840 to pro- DSC Differential scanning calorime- pose the theory of types (functional try. See thermal analysis. groups). D-series See absolute configura- Dumas’ method 1. A method of tion. Ünding the amount of nitrogen in an organic compound. The sample is DTA Differential thermal analysis. weighed, mixed with copper(II) See thermal analysis. oxide, and heated in a tube. Any ni- dubnium Symbol Db. A radioactive trogen present in the compound is *transactinide element; a.n. 105. It converted into oxides of nitrogen, was Ürst reported in 1967 by a group which are led over hot copper to re- at Dubna near Moscow and was duce them to nitrogen gas. This is duplet 188

collected and the volume measured, G in methanol). The dye is excited by from which the mass of nitrogen in a an external source. The solvent known mass of sample can be found. broadens the states into bands and 2. A method of Ünding the relative consequently laser action can be ob- molecular masses of volatile liquids tained over a range of wavelengths. by weighing. A thin-glass bulb with a This allows one to select a speciÜc d long narrow neck is used. This is wavelength (using a grating) and to weighed full of air at known temper- change the wavelength of the laser. ature, then a small amount of sample Such a device is called a tuneable is introduced and the bulb heated (in laser. Dye lasers are also used in pro- a bath) so that the liquid is vaporized ducing very short pulses of radiation. and the air is driven out. The tip of The technique is to use a dye that the neck is sealed and the bulb stops absorbing radiation when a cooled and weighed at known (room) high proportion of its molecules be- temperature. The volume of the bulb come excited. The cavity then be- Ü is found by lling it with water and comes resonant and a pulse of weighing again. If the density of air radiation is produced. This technique is known, the mass of vapour in a can give pulses of about 10 nanosec- known volume can be calculated. onds duration and is used in *femto- The techniques are named after chemistry. Jean Baptiste André *Dumas. dyes Substances used to impart col- duplet A pair of electrons in a cova- our to textiles, leather, paper, etc. lent chemical bond. Compounds used for dyeing Duquenois–Levine test A widely (dyestuffs) are generally organic com- used presumptive test for tetrahydro- pounds containing conjugated dou- cannabinol and other cannabinoids. ble bonds. The group producing the The Duquenois–Levine reagent has a colour is the *chromophore; other solution containing 2% vanillin and noncoloured groups that inÛuence or 1% ethanal. Concentrated hydrochlo- intensify the colour are called *aux- ric acid is added and then chloro- ochromes. Dyes can be classiÜed ac- form. A purple colour in the cording to the chemical structure of chloroform layer indicates a positive the dye molecule. For example, azo result. dyes contain the –N=N– group (see azo compounds Duralumin Tradename for a class ). In practice, they Ü of strong lightweight aluminium al- are classi ed according to the way in loys containing copper, magnesium, which the dye is applied or is held on manganese, and sometimes silicon. the substrate. Duralumin alloys combine strength Acid dyes are compounds in with lightness and are extensively which the chromophore is part of used in aircraft, racing cars, etc. a negative ion (usually an organic – sulphonate RSO2O ). They can be Dutch metal An alloy of copper used for protein Übres (e.g. wool and and zinc, which can be produced in silk) and for polyamide and acrylic very thin sheets and used as imita- Übres. Originally, they were applied tion gold leaf. It spontaneously in- Û from an acidic bath. Metallized dyes ames in chlorine. are forms of acid dyes in which the dye laser A type of laser in which negative ion contains a chelated the active material is a dye dissolved metal atom. Basic dyes have chro- in a suitable solvent (e.g. Rhodanine mophores that are part of a positive 189 dystectic mixture ion (usually an amine salt or ionized The evolution of dynamical systems imino group). They are used for can be very complex, even for sys- acrylic Übres and also for wool and tems with only a few degrees of free- silk, although they have only moder- dom, sometimes involving such ate fastness with these materials. considerations as ergodicity, which Direct dyes are dyes that have a originated in *statistical mechanics. high afÜnity for cotton, rayon, and The evolution of dynamical systems d other cellulose Übres. They are ap- can be studied using the phase space plied directly from a neutral bath for the system. *Chaos is an example containing sodium chloride or of the complex behaviour that can sodium sulphate. Like acid dyes, they occur in a dynamical system. are usually sulphonic acid salts but dynamic equilibrium See equilib- are distinguished by their greater rium. substantivity (afÜnity for the substrate), hence the alternative name dynamite Any of a class of high ex- substantive dyes. plosives based on nitroglycerin. The Vat dyes are insoluble substances original form, invented in 1867 by used for cotton dyeing. They usually Alfred Nobel, consisted of nitroglyc- contain keto groups, C=O, which are erin absorbed in kieselguhr. Modern reduced to C–OH groups, rendering dynamites, which are used for blast- the dye soluble (the leuco form of the ing, contain sodium or ammonium dye). The dye is applied in this form, nitrate sensitized with nitroglycerin then oxidized by air or oxidizing and use other absorbers (e.g. wood agents to precipitate the pigment in pulp). the Übres. Indigo and anthroquinone dysprosium Symbol Dy. A soft sil- dyes are examples of vat dyes. Sul- phur dyes are dyes applied by this very metallic element belonging to * technique using sodium sulphide so- the lanthanoids; a.n. 66; r.a.m. 162.50; r.d. 8.551 (20°C); m.p. 1412°C; lution to reduce and dissolve the dye. ° Sulphur dyes are used for cellulose b.p. 2562 C. It occurs in apatite, Übres. gadolinite, and xenotime, from Disperse dyes are insoluble dyes which it is extracted by an ion- applied in the form of a Üne disper- exchange process. There are seven natural isotopes and twelve artiÜcial sion in water. They are used for cellu- Ü Ü lose acetate and other synthetic isotopes have been identi ed. It nds Übres. limited use in some alloys as a neu- Reactive dyes are compounds that tron absorber, particularly in nuclear contain groups capable of reacting technology. It was discovered by Paul with the substrate to form covalent Lecoq de Boisbaudran (1838–1912) in bonds. They have high substantivity 1886. and are used particularly for cellu- A lose Übres. • Information from the WebElements site dynamical system A system gov- dystectic mixture A mixture of erned by dynamics (either classical substances that has a constant maxi- mechanics or quantum mechanics). mum melting point. E

EAC Emergency action code. See and xenon 0.00001%. In addition to hazchem code. water vapour, air in some localities contains sulphur compounds, hydro- Earnshaw’s theorem The princi- gen peroxide, hydrocarbons, and ple that a system of particles interact- dust particles. ing via an inverse square law, such as Coulomb’s law of electrostatics, can- ebonite See vulcanite. not exist in a state of static equilib- ebullioscopic constant See eleva- rium. The Reverend Samuel tion of boiling point. Earnshaw (1805–88) proved this result in 1842. The theorem is of funda- ebullioscopy The use of *elevation mental importance in chemistry of boiling point to determine relative since it shows that it is not possible molecular masses. to construct a correct model of atoms echelon A form of interferometer and molecules if the electrons are consisting of a stack of glass plates taken to be stationary. arranged stepwise with a constant earth The planet that orbits the sun offset. It gives a high resolution and between the planets Venus and Mars. is used in spectroscopy to study Ü The earth consists of three layers: the hyper ne line structure. gaseous atmosphere (see earth’s at- eclipsed See conformation. mosphere), the liquid hydrosphere, eclipsed conformation See con- and the solid lithosphere. The solid formation part of the earth also consists of . three layers: the crust with a mean eclipsing See conformation. thickness of about 32 km under the ecstasy (methylenedioxymetham- land and 10 km under the seas; the phetamine; MDMA) A designer drug mantle , which extends some based on methamphetamine (see am- 2900 km below the crust; and the phetamines). Originally intended as core, part of which is believed to be an appetite suppressant, it produces liquid. The composition of the crust a feeling of euphoria and is widely is: oxygen 47%, silicon 28%, alu- used as a club drug. It is a class A minium 8%, iron 4.5%, calcium 3.5%, drug in the UK. sodium and potassium 2.5% each, and magnesium 2.2%. Hydrogen, H carbon, phosphorus, and sulphur are 2 H C N O all present to an extent of less than CH 1%. 3 CH earth’s atmosphere The gas that O 3 surrounds the earth. The composition of dry air at sea level is: nitrogen Ecstasy 78.08%, oxygen 20.95%, argon 0.93%, carbon dioxide 0.03%, neon 0.0018%, Edison cell See nickel–iron accu- helium 0.0005%, krypton 0.0001%, mulator. 191 eigenvalues

EDTA Ethylenediaminetetraacetic which a crystalline hydrate loses acid, water, forming a powdery deposit on the crystals. (HOOCCH2)2N(CH2)2N(CH2COOH)2, a compound that acts as a chelating effusion The Ûow of a gas through agent, reversibly binding with iron, a small aperture. The relative rates at magnesium, and other metal ions. It which gases effuse, under the same is used in certain culture media conditions, is approximately in- bound with iron, which it slowly re- versely proportional to the square leases into the medium, and also in roots of their densities. e some forms of quantitative analysis. Ehrenfest classiÜcation A EELS (electron energy loss spec- classiÜcation of phase transitions in troscopy) A technique for studying terms of their thermodynamic prop- adsorbates and their dissociation. A erties put forward by the Dutch beam of electrons is reÛected from a physicist Paul Ehrenfest (1880–1933). surface and the energy loss they suf- A Ürst-order phase transition is a fer upon reÛection is measured. This phase transition in which the Ürst de- loss can be used to interpret the vi- rivative of the chemical potential is brational spectrum of the adsorbate. discontinuous. In a Ürst-order phase Very small amounts of adsorbate can transition there is a nonzero change be detected using EELS (as few as 50 in the value of the enthalpy, entropy, atoms in a sample); this is particu- and volume at the transition temper- larly useful for light elements that ature. Melting and boiling are exam- are not readily detected using other ples of Ürst-order phase transitions. techniques. In a second-order phase transition, the Ürst derivative of the chemical effervescence The formation of potential is continuous but its second gas bubbles in a liquid by chemical derivative is not continuous. In a sec- reaction. ond-order phase transition there is efÜciency A measure of the perfor- no jump in the value of the enthalpy, mance of a machine, engine, etc., entropy, and volume at the transition being the ratio of the energy or temperature. Examples of second- power it delivers to the energy or order phase transitions include the power fed to it. In general, the efÜ- transition to ferromagnetism and ciency of a machine varies with the order–disorder transitions in alloys. conditions under which it operates Ehrlich’s reagent See p-dimethyl- and there is usually a load at which it aminobenzadehyde. operates with the highest efÜciency. The thermal efÜciency of a heat en- eigenfunction In general, a solu- gine is the ratio of the work done by tion of an *eigenvalue equation. In the engine to the heat supplied by *quantum mechanics eigenfunctions the fuel. For a reversible heat engine occur as the allowed wave functions Ü this ef ciency equals (T1 – T2)/T1, of a system and so satisfy the where T1 is the thermodynamic tem- *Schrödinger equation. perature at which all the heat is eigenvalues The allowed set of val- taken up and T is the thermody- 2 ues of the constants in an eigenvalue namic temperature at which it is equation. In an eigenvalue equation given out (see carnot cycle). For real the left-hand side consists of an oper- engines it is always less than this. ator and an *eigenfunction, upon efÛorescence The process in which the operator operates; the Einstein, Albert 192

right-hand side of the equation con- also given by Bρ, with the coefÜcient sists of the product of the same B of induced emission being equal to eigenfunction and a constant. Eigen- the coefÜcient of absorption. The rate value equations occur in quantum of spontaneous emission is given by mechanics, with the eigenvalues A, where A is the Einstein A coefÜ- being the values of the quantized cient of spontaneous emission. The A quantities. In particular, the and B coefÜcients are related by A = Schrödinger equation is an eigen- 8πhν3B/c3, where h is the Planck con- e value equation with the allowed stant, ν is the frequency of electro- quantized energy levels being the magnetic radiation, and c is the speed eigenvalues of this equation. of light. The coefÜcients were put for- Einstein, Albert (1879–1955) Ger- ward by Albert *Einstein in 1916–17 man-born US physicist, who took in his analysis of the quantum theory Swiss nationality in 1901. A year of radiation. later he went to work in the Bern Einstein equation 1. The mass– Ü patent of ce. In 1905 he published energy relationship announced by Al- Ü Û ve enormously in uential papers, bert *Einstein in 1905 in the form E one on *Brownian movement, one = mc2, where E is a quantity of enon the *photoelectric effect, one on ergy, m its mass, and c is the speed of the special theory of relativity, and light. It presents the concept that en- one on energy and inertia (which in- ergy possesses mass. 2. The relation- cluded the famous expression E = ship E = hf – W, where E is the mc2). In 1915 he published the gen- max max maximum kinetic energy of elec- eral theory of relativity, concerned trons emitted in the photoemissive mainly with gravitation. In 1921 he effect, h is the Planck constant, f the was awarded the Nobel Prize for frequency of the incident radiation, physics. In 1933, as a Jew, Einstein * decided to remain in the USA (where and W the work function of the he was lecturing), as Hitler had come emitter. This is also written Emax = hf – φe, where e is the electronic charge to power. For the remainder of his φ life he sought a uniÜed Üeld theory. and a potential difference, also In 1939 he informed President Roo- called the work function. (Sometimes φ sevelt that an atom bomb was feasi- W and are distinguished as work ble and that Germany might be able function energy and work function to make one. potential.) The equation can also be applied to photoemission from gases, Ü Ü Einstein coef cients Coef cients when it has the form: E = hf – I, used in the quantum theory of radia- where I is the ionization potential of tion, related to the probability of a the gas. transition occurring between the ground state and an excited state (or einsteinium Symbol Es. A radio- vice versa) in the processes of *inactive metallic transuranic element duced emission and *spontaneous belonging to the *actinoids; a.n. 99; emission. For an atom exposed to mass number of the most stable iso- electromagnetic radiation, the rate of tope 254 (half-life 270 days). Eleven ρ absorption Ra is given by Ra = B , isotopes are known. The element was where ρ is the density of electromag- Ürst identiÜed by A. Ghiorso and as- netic radiation and B is the Einstein B sociates in debris from the Ürst hy- coefÜcient associated with absorp- drogen bomb explosion in 1952. tion. The rate of induced emission is Microgram quantities of the element 193 electrical double layer did not become available until 1961. E-isomer See e–z convention. It is named after Albert *Einstein. elastic collision A collision in A which the total kinetic energy of the • Information from the WebElements site colliding bodies after collision is Einstein–Smoluchowski equa- equal to their total kinetic energy be- tion A relation between the diffu- fore collision. Elastic collisions occur sion coefÜcient D and the distance only if there is no conversion of ki- λ that a particle can jump when dif- netic energy into other forms, as in the collision of atoms. In the case of e fusing in a time τ. The Einstein– macroscopic bodies this will not be Smoluchowski equation, which is the case as some of the energy will D = λ2/2τ, gives a connection between become heat. In a collision between the microscopic details of particle dif- polyatomic molecules, some kinetic fusion and the macroscopic quanti- energy may be converted into vibra- ties associated with the diffusion, tional and rotational energy of the such as the viscosity. The equation is molecules. derived by assuming that the particles undergo a random walk. The elastin A Übrous protein that is the quantities in the equation can be re- major constituent of the yellow elas- lated to quantities in the *kinetic tic Übres of connective tissue. It is theory of gases, with λ/τ taken to be rich in glycine, alanine, proline, and the mean speed of the particles and other nonpolar amino acids that are λ their mean free path. The Einstein– cross-linked, making the protein rela- Ü Smoluchowski equation was derived tively insoluble. Elastic bres can by Albert Einstein and the Polish stretch to several times their length physicist Marian Ritter von Smolan- and then return to their original size. Smoluchowski. Elastin is particularly abundant in elastic cartilage, blood-vessel walls, Einstein theory of speciÜc heat ligaments, and the heart. A theory of the speciÜc heat capacity elastomer A natural or synthetic of solids put forward by Albert *Ein- rubber or rubberoid material, which stein in 1906, in which it was as- has the ability to undergo deforma- sumed that the speciÜc heat capacity tion under the inÛuence of a force is a consequence of the vibrations of and regain its original shape once the the atoms of the lattice of the solid. force has been removed. Einstein assumed that each atom has the same frequency ν. The theory electret A permanently electriÜed leads to the correct conclusion that substance or body that has opposite the speciÜc heat of solids tends to charges at its extremities. Electrets zero as the temperature goes to ab- resemble permanent magnets in solute zero, but does not give a cor- many ways. An electret can be made rect quantitative description of the by cooling certain waxes in a strong Ü low-temperature behaviour of the electric eld. speciÜc heat capacity. In the *Debye electrical double layer A model theory of speciÜc heat, and in other of the interface between an electrode analyses of this problem, Einstein’s and the solution close to it. In this simplifying approximation was im- model a sheet of one type of elec- proved on by taking account of the trical charge surrounds the surface fact that the frequencies of lattice vi- of the electrode and a sheet of brations can have a range of values. the opposite charge surrounds the electric-arc furnace 194

Ürst sheet in the solution. In the electrochromatography See elec- Helmholtz model the double layer is trophoresis. regarded as consisting of two planes electrocyclic reaction A type of of charge, with the inner plane of cyclic rearrangement in which a ions from the solution being caused sigma bond is formed between two by the charge on the electrode and terminal carbon atoms of a conju- the outer plane being caused by op- gated molecule, resulting in a de- positely charged ions in the solution crease of one in the number of pi Ü e responding to the rst layer of ions. bonds present. In the *Gouy–Chapman model (diffuse double layer) thermal motion of ions is taken into account. Neither model is completely successful since the Helmholtz model exaggerates the rigidity of the structure of the Electrocyclic reaction charges and the Gouy–Chapman model underestimates the rigidity of electrode 1. A conductor that the structure. The Stern model improves on both models by assuming emits or collects electrons in a cell, that the ions next to the electrode thermionic valve, semiconductor de- have a rigid structure, while taking vice, etc. The anode is the positive electrode and the cathode is the neg- the second layer to be as described half cell by the Gouy–Chapman model. ative electrode. 2. See . electric-arc furnace A furnace electrodeposition The process of used in melting metals to make al- depositing one metal on another by loys, especially in steel manufacture, electrolysis, as in *electroforming in which the heat source is an elec- and *electroplating. tric arc. In the direct-arc furnace, electrode potential The potential such as the Héroult furnace, an arc is difference produced between the formed between the metal and an electrode and the solution in a *half electrode. In the indirect-arc furnace, cell. It is not possible to measure this such as the Stassano furnace, the arc directly since any measurement in- is formed between two electrodes volves completing the circuit with and the heat is radiated onto the the electrolyte, thereby introducing metal. another half cell. Standard electrode Š Ü electrochemical cell See cell. potentials E are de ned by measuring the potential relative to a stan- electrochemical equivalent dard *hydrogen half cell using 1.0 Symbol z. The mass of a given el- molar solution at 25°C. The conven- ement liberated from a solution of its tion is to designate the cell so that ions in electrolysis by one coulomb the oxidized form is written Ürst. For faraday’s laws of charge. See . example, electrochemical series See elec- Pt(s)|H (g)H+(aq)|Zn2+(aq)|Zn(s) tromotive series 2 . The e.m.f. of this cell is –0.76 volt (i.e. electrochemistry The study of the zinc electrode is negative). Thus chemical properties and reactions in- the standard electrode potential of volving ions in solution, including the Zn2+|Zn half cell is –0.76 V. Elec- electrolysis and electric cells. trode potentials are also called reduc- 195 electrolytic refining tion potentials. See also electromo- form neutral species. Alternatively, tive series. atoms of the electrode can lose elec- electrodialysis A method of ob- trons and go into solution as positive taining pure water from water con- ions. In either case the reaction is an taining a salt, as in *desalination. oxidation. At the cathode, positive The water to be puriÜed is fed into a ions in solution can gain electrons to cell containing two electrodes. Be- form neutral species. Thus cathode tween the electrodes is placed an reactions are reductions. array of *semipermeable membranes electrolyte A liquid that conducts e alternately semipermeable to posi- electricity as a result of the presence tive ions and negative ions. The ions of positive or negative ions. Elec- tend to segregate between alternate trolytes are molten ionic compounds pairs of membranes, leaving pure or solutions containing ions, i.e. solu- water in the other gaps between tions of ionic salts or of compounds membranes. In this way, the feed that ionize in solution. Liquid metals, water is separated into two streams: in which the conduction is by free one of pure water and the other of electrons, are not usually regarded as more concentrated solution. electrolytes. Solid conductors of ions, electroendosmosis See electro- as in the sodium–sulphur cell, are osmosis. also known as electrolytes. electroforming A method of form- electrolytic cell A cell in which ing intricate metal articles or parts electrolysis occurs; i.e. one in which by *electrodeposition of the metal on current is passed through the elec- a removable conductive mould. trolyte from an external source. electrokinetic potential (zeta po- electrolytic corrosion Corrosion tential) Symbol ζ. The electric poten- that occurs through an electrochemi- tial associated with an *electrical cal reaction. See rusting. double layer around a colloid at the electrolytic gas (detonating gas) radius of shear, relative to the value The highly explosive gas formed by of the potential in the bulk of the so- the electrolysis of water. It consists of lution far from the colloid, where the two parts hydrogen and one part oxy- radius of shear is the radius of the gen by volume. entity made up of the colloid and the rigid layer of ions at the surface of electrolytic reÜning The puriÜca- the colloid. tion of metals by electrolysis. It is commonly applied to copper. A large lumines- electroluminescence See piece of impure copper is used as the cence . anode with a thin strip of pure cop- electrolysis The production of a per as the cathode. Copper(II) sul- chemical reaction by passing an elec- phate solution is the electrolyte. tric current through an electrolyte. Copper dissolves at the anode: Cu → In electrolysis, positive ions migrate Cu2+ + 2e, and is deposited at the to the cathode and negative ions to cathode. The net result is transfer of the anode. The reactions occurring pure copper from anode to cathode. depend on electron transfer at the Gold and silver in the impure copper electrodes and are therefore redox re- form a so-called anode sludge at the actions. At the anode, negative ions bottom of the cell, which is recov- in solution may lose electrons to ered. electrolytic separation 196

electrolytic separation A method electromeric effect See elec- of separating isotopes by exploiting tronic effects. the different rates at which they are electrometallurgy The uses of released in electrolysis. It was for- electrical processes in the separation merly used for separating deuterium of metals from their ores, the reÜn- and hydrogen. On electrolysis of ing of metals, or the forming or plat- water, hydrogen is formed at the ing of metals. cathode more readily than deu- e terium, thus the water becomes en- electromotive force (e.m.f.) The riched with deuterium oxide. greatest potential difference that can be generated by a particular source electromagnetic radiation En- of electric current. In practice this ergy resulting from the acceleration may be observable only when the of electric charge and the associated source is not supplying current, be- electric Üelds and magnetic Üelds. cause of its internal resistance. The energy can be regarded as waves propagated through space (requiring electromotive series (electro- no supporting medium) involving os- chemical series) A series of chemical cillating electric and magnetic Üelds elements arranged in order of their at right angles to each other and to *electrode potentials. The hydrogen + →½ the direction of propagation. In a vac- electrode (H + e H2) is taken as uum the waves travel with a con- having zero electrode potential. El- stant speed (the speed of light) of ements that have a greater tendency 2.9979 × 108 metres per second; if than hydrogen to lose electrons to material is present they are slower. their solution are taken as elec- Alternatively, the energy can be re- tropositive; those that gain electrons garded as a stream of *photons trav- from their solution are below hydro- elling at the speed of light, each gen in the series and are called elec- photon having an energy hc/λ, where tronegative. The series shows the h is the Planck constant, c is the order in which metals replace one speed of light, and λ is the wave- another from their salts; electroposi- length of the associated wave. A fu- tive metals will replace hydrogen sion of these apparently conÛicting from acids. The chief metals and hy- concepts is possible using the meth- drogen, placed in order in the series, ods of *quantum mechanics. The are: potassium, calcium, sodium, characteristics of the radiation de- magnesium, aluminium, zinc, pend on its wavelength. See electro- cadmium, iron, nickel, tin, lead, magnetic spectrum. hydrogen, copper, mercury, silver, platinum, gold. This type of series is electromagnetic spectrum The sometimes referred to as an activity range of wavelengths over which series. electromagnetic radiation extends. The longest waves (105–10–3 metres) electron An elementary particle are radio waves, the next longest with a rest mass of 9.109 3897(54) × (10–3–10–6 m) are infrared waves, 10–31 kg and a negative charge of then comes the narrow band (4–7 × 1.602 177 33(49) × 10–19 coulomb. 10–7 m) of visible radiation, followed Electrons are present in all atoms in by ultraviolet waves (10–7–10–9 m) groupings called shells around the and *X-rays and gamma radiation nucleus; when they are detached (10–9–10–14 m). from the atom they are called free 197 electron diffraction electrons. The antiparticle of the elec- the electron (nonrelativistic). Thus, tron is the positron. the momentum (p) of the electron is Ü √(2eVm). As the de Broglie wave- electron af nity Symbol A. The λ energy change occurring when an length ( ) of an electron is given by h/p, where h is the Planck constant, atom or molecule gains an electron λ √ to form a negative ion. For an atom then = h/ (2eVm). For an accelerat- or molecule X, it is the energy re- ing voltage of 3600 V, the wave- leased for the electron-attachment re- length of the electron beam is 0.02 × 4 action nanometre, some 3 10 times e shorter than visible radiation. X(g) + e → X–(g) Electrons then, like X-rays, show Often this is measured in electron- diffraction effects with molecules volts. Alternatively, the molar en- and crystals in which the interatomic ∆ thalpy change, H, can be used. spacing is comparable to the wave- electron capture 1. The formation length of the beam. They have the of a negative ion by an atom or mol- advantage that their wavelength can ecule when it acquires an extra free be set by adjusting the voltage. Un- electron. 2. A radioactive transfor- like X-rays they have very low pene- mation in which a nucleus acquires trating power. The Ürst observation an electron from an inner orbit of of electron diffraction was by George the atom, thereby transforming, ini- Paget *Thomson in 1927, in an ex- tially, into a nucleus with the same periment in which he passed a beam mass number but an atomic number of electrons in a vacuum through a one less than that of the original nu- very thin gold foil onto a photo- cleus (capture of the electron trans- graphic plate. Concentric circles forms a proton into a neutron). This were produced by diffraction of elec- type of capture is accompanied by trons by the lattice. The same year emission of an X-ray photon or Auger Clinton J. Davisson (1881–1958) and electron as the vacancy in the inner Lester Germer (1896–1971) per- orbit is Ülled by an outer electron. formed a classic experiment in electron conÜguration See con- which they obtained diffraction pat- figuration. terns by glancing an electron beam off the surface of a nickel crystal. electron-deÜcient compound A Both experiments were important compound in which there are fewer veriÜcations of de Broglie’s theory electrons forming the chemical and the new quantum theory. bonds than required in normal elec- Electron diffraction, because of the tron-pair bonds. Such compounds use low penetration, cannot easily be borane *multicentre bonds. See . used to investigate crystal structure. electron diffraction Diffraction of It is, however, employed to measure a beam of electrons by atoms or mol- bond lengths and angles of molecules ecules. The fact that electrons can be in gases. Moreover, it is extensively diffracted in a similar way to light used in the study of solid surfaces and X-rays shows that particles can and absorption. The main techniques act as waves (see de broglie wave- are low-energy electron diffraction length). An electron (mass m, charge (LEED) in which the electron beam is e) accelerated through a potential dif- reÛected onto a Ûuorescent screen, ference V acquires a kinetic energy and high-energy electron diffraction mv2/2 = eV, where v is the velocity of (HEED) used either with reÛection or electronegative 198

transmission in investigating thin positive ions to be taken into ac- Ülms. count. electronegative Describing el- electronic effects Effects by ements that tend to gain electrons which the reactivity at one part of a and form negative ions. The halogens molecule is affected by electron at- are typical electronegative elements. traction or repulsion originating in For example, in hydrogen chloride, another part of a molecule. Often the chlorine atom is more elec- this is called an *inductive effect (or e tronegative than the hydrogen and resonance effect), although some- the molecule is polar, with negative times the term ‘inductive effect’ is re- charge on the chlorine atom. There served for an inÛuence transmitted are various ways of assigning values through chemical bonds and is dis- for the electronegativity of an el- tinguished from a Üeld effect, which ement. Mulliken electronegativities is transmitted through space. An in- are calculated from E = (I + A)/2, ductive effect through chemical where I is ionization potential and A bonds was formerly called a me- is electron afÜnity. More commonly, someric effect (or mesomerism) or an Pauling electronegativities are used. electromeric effect. It is common to These are based on bond dissociation refer to all effects (through bonds or energies using a scale in which through space) as resonance effects. Ûuorine, the most electronegative el- electronic spectra of molecules ement, has a value 4. Some other val- The spectra associated with transi- ues on this scale are B 2, C 2.5, N 3.0, tions between the electronic states of O 3.5, Si 1.8, P 2.1, S 2.5, Cl 3.0, Br molecules. These transitions corre- 2.8. spond to the visible or ultraviolet electron energy loss spectros- regions of the electromagnetic spec- copy See eels. trum. There are changes in vibrational and rotational energy when electron Ûow The transfer of elec- electronic transitions occur. Conse- trons along a series of carrier mol- quently there are spectral bands asso- ecules in the *electron transport ciated with changes in vibrational chain. motion, with these bands having Üne electron gas A model of the elec- structure due to changes in rota- trons in a metal or a plasma in which tional motion. Because electronic they are regarded as forming a gas transitions are associated with that interacts with a uniformly dis- changes in vibrational motion the tributed background of positive corresponding spectra are sometimes charge to ensure that the system is called vibrational spectra. The elec- electrically neutral. The electron gas tronic spectra of molecules are used is analysed theoretically using either to obtain information about energy classical or quantum statistical me- levels in molecules, interatomic dis- chanics and the kinetic theory of tances, dissociation energies of mol- gases. The electron-gas model ac- ecules, and force constants of counts for many properties of metals chemical bonds. and plasmas in a qualitative and ap- electron microscope A form of proximately quantitative way but microscope that uses a beam of elec- cannot give an accurate quantitative trons instead of a beam of light (as in account of these systems, as this the optical microscope) to form a would require the motions of the large image of a very small object. In 199 electron-transfer reaction optical microscopes the resolution is tical probability, at normal tempera- limited by the wavelength of the tures, that there will be slightly more light. High-energy electrons, how- in the lower state than in the higher. ever, can be associated with a consid- By applying microwave radiation to erably shorter wavelength than light; the sample a transition to the higher for example, electrons accelerated to state can be achieved. The precise en- an energy of 105 electronvolts have a ergy difference between the two wavelength of 0.004 nanometre (see states of an electron depends on the de broglie wavelength) enabling a surrounding electrons in the atom or e resolution of 0.2–0.5 nm to be molecule. In this way the position of achieved. The transmission electron unpaired electrons can be investi- microscope has an electron beam, gated. The technique is used particu- sharply focused by electron lenses, larly in studying free radicals and passing through a very thin metal- paramagnetic substances such as in- lized specimen (less than 50 nanome- organic complexes. It is also called tres thick) onto a Ûuorescent screen, electron-spin resonance (ESR). See also where a visual image is formed. This nuclear magnetic resonance; image can be photographed. The endor. scanning electron microscope can be electron probe microanalysis used with thicker specimens and (EPM) A method of analysing a very forms a perspective image, although small quantity of a substance (as lit- Ü the resolution and magni cation are tle as 10–13 gram). The method con- lower. In this type of instrument a sists of directing a very Ünely focused beam of primary electrons scans beam of electrons on to the sample the specimen and those that are to produce the characteristic X-ray reÛected, together with any sec- spectrum of the elements present. It ondary electrons emitted, are col- can be used quantitatively for el- lected. This current is used to ements with atomic numbers in ex- modulate a separate electron beam cess of 11. in a TV monitor, which scans the electron-spin resonance See elec- screen at the same frequency, conse- tron paramagnetic resonance quently building up a picture of the . specimen. The resolution is limited electron-transfer reaction A to about 10–20 nm. chemical reaction that involves the electron-nuclear double reso- transfer, addition, or removal of elec- nance See endor. trons. Electron-transfer reactions often involve complexes of transition electron paramagnetic reso- metals. In such complexes one gen- nance (EPR) A spectroscopic eral mechanism for electron transfer method of locating electrons within is the inner-sphere mechanism, in the molecules of a paramagnetic sub- which two complexes form an inter- stance (see magnetism) in order to mediate, with ligand bridges en- provide information regarding its abling electrons to be transferred bonds and structure. The spin of an from one complex to another com- unpaired electron is associated with plex. The other main mechanism is a magnetic moment that is able to the outer-sphere mechanism, in align itself in one of two ways with which two complexes retain all their an applied external magnetic Üeld. ligands, with electrons passing from These two alignments correspond to one complex to the other. different energy levels, with a statis- The rates of electron-transfer reac- electron transport chain 200

tions vary enormously. These rates along the chain. The ATP is then car- can be explained in terms of the way ried across the mitochondrial mem- in which molecules of the solvent brane in exchange for ADP. An solvating the reactants rearrange so electron transport chain also occurs as to solvate the products in the case in *photosynthesis. of the outer-sphere mechanism. In electronvolt Symbol eV. A unit of the case of the inner-sphere (ligand- energy equal to the work done on an bridged) reactions the rate of the re- electron in moving it through a po- e action depends on the intermediate tential difference of one volt. It is and the way in which the electron is used as a measure of particle ener- transferred. gies although it is not an *SI unit. 1 electron transport chain (elec- eV = 1.602 × 10–19 joule. tron transport system) A sequence of electroorganic reaction An or- biochemical reduction–oxidation re- ganic reaction produced in an elec- actions that effects the transfer of trolytic cell. Electroorganic reactions electrons through a series of carriers. are used to synthesize compounds An electron transport chain, also that are difÜcult to produce by con- known as the respiratory chain, ventional techniques. An example of Ü forms the nal stage of aerobic respi- an electroorganic reaction is *Kolbe’s ration. It results in the transfer of method of synthesizing alkanes. electrons or hydrogen atoms derived from the *Krebs cycle to molecular electroosmosis The movement of oxygen, with the formation of water. a polar liquid through a membrane Û At the same time it conserves energy under the in uence of an applied Ü from food or light in the form of electric eld. The linear velocity of Û Ü *ATP. The chain comprises a series of ow divided by the eld strength is carrier molecules that undergo re- the electroosmotic mobility. Elelec- versible reduction–oxidation reac- troosmosis was formerly called elec- tions, accepting electrons and then troendosmosis. donating them to the next carrier in electroosmotic mobility See elec- the chain – a process known as electroosmosis. tron Ûow. In the mitochondria, electropherogram See capillary NADH and FADH2, generated by the electrophoresis Krebs cycle, transfer their electrons . to a chain including coenzyme Q (see electrophile An ion or molecule ubiquinone) and a series of *cy- that is electron deÜcient and can ac- tochromes. This process is coupled to cept electrons. Electrophiles are the formation of ATP at three sites often reducing agents and Lewis

inner mitochondrial membrane H+ H+ H+ 2e– 2e– 2e– NADH cytochrome cytochrome dehydrogenase ubiquinone b-c complex oxidase 1

NADH H O +H+ 2 NAD+ 2H+ + ½O 2 Electron transport chain 201 electrospray ionization

*acids. They are either positive ions nents can be identiÜed by the rate + (e.g. NO2 ) or molecules that have a at which they move. In gel elec- positive charge on a particular atom trophoresis the medium is a gel, typi- (e.g. SO3, which has an electron- cally made of polyacrylamide (see deÜcient sulphur atom). In organic page), agarose, or starch. reactions they tend to attack nega- Electrophoresis, which has also tively charged parts of a molecule. been called electrochromatography, Compare nucleophile. is used extensively in studying mixtures of proteins, nucleic acids, car- electrophilic addition An *addi- e bohydrates, enzymes, etc. In clinical tion reaction in which the Ürst step is medicine it is used for determining attack by an electrophile (e.g. a posi- the protein content of body Ûuids. tive ion) on an electron-rich part of the molecule. An example is addition electrophoretic deposition A to the double bonds in alkenes. technique for coating a material electrophilic substitution A making it an electrode in a bath con- *substitution reaction in which the taining a colloidal suspension of Ürst step is attack by an electrophile. charged particles. Under suitable con- Electrophilic substitution is a feature ditions, the particles are attracted to, of reactions of benzene (and its com- and deposited on, the electrode. Elec- pounds) in which a positive ion ap- trophoretic deposition is used exten- proaches the delocalized pi electrons sively in industry; for example, in on the benzene ring. applying paint to metal components. electrophoresis (cataphoresis) A electrophoretic effect The effect in which the mobility of ions in solu- technique for the analysis and sepa- Û ration of colloids, based on the move- tion moving under the in uence of an applied electric Üeld is affected by ment of charged colloidal particles in Û an electric Üeld. There are various ex- the ow of ions of opposite charge in perimental methods. In one the sam- the opposite direction. ple is placed in a U-tube and a buffer electroplating A method of plat- solution added to each arm, so that ing one metal with another by *elec- there are sharp boundaries between trodeposition. The articles to be buffer and sample. An electrode is plated are made the cathode of an placed in each arm, a voltage ap- electrolytic cell and a rod or bar of plied, and the motion of the bound- the plating metal is made the anode. aries under the inÛuence of the Üeld Electroplating is used for covering is observed. The rate of migration of metal with a decorative, more expen- the particles depends on the Üeld, sive, or corrosion-resistant layer of the charge on the particles, and on another metal. other factors, such as the size and electropositive Describing el- shape of the particles. More simply, ements that tend to lose electrons electrophoresis can be carried out and form positive ions. The alkali using an adsorbent, such as a strip of metals are typical electropositive el- Ülter paper, soaked in a buffer with ements. two electrodes making contact. The sample is placed between the elec- electrospray ionization (ESI) A trodes and a voltage applied. Differ- technique for producing ions for ent components of the mixture mass spectrometry, used especially migrate at different rates, so the sam- for obtaining ions from large mol- ple separates into zones. The compo- ecules, which would be likely to pro- electrovalent bond 202

duce fragment ions in electron- lioscopic constant of the solvent and impact impact ionization sources. if this is known, the molecular The sample is dissolved in a volatile weight of the solute can be calcu- solvent, which may also contain lated from the measured value of ∆t. volatile acids or bases so that the The elevation is measured by a Beck- sample exists in an ionic form. The mann thermometer. See also colliga- solution is forced through a charged tive properties. metal capillary tube and forms an elimination reaction A reaction e aerosol. Evaporation of the solvent in which one molecule decomposes results in single ions of the sample, into two, one much smaller than the which are analyzed by the mass spec- other. trometer. Electrospray ionization is the ionization technique often used Elinvar Trade name for a nickel– in chromatography–mass spectrome- chromium steel containing about try. 36% nickel, 12% chromium, and smaller proportions of tungsten and chemical electrovalent bond See manganese. Its elasticity does not bond . vary with temperature and it is electrum 1. An alloy of gold and therefore used to make hairsprings silver containing 55–88% of gold. for watches. 2. A *German silver alloy containing Ellingham diagram A diagram 52% copper, 26% nickel, and 22% used to show the conditions under zinc. which a metal oxide can be reduced element A substance that cannot to a metal. The standard Gibbs free be decomposed into simpler sub- energy of formation of the oxide is stances. In an element, all the atoms considered, for example, ½ → have the same number of protons or M + O2 MO electrons, although the number of This value, ∆GŠ, is plotted against neutrons may vary. There are 92 temperature. In general, the result is naturally occurring elements. See a straight line. In some cases, there is also periodic table; transuranic el- an abrupt change in the line’s slope ements; transactinide elements. at a point because of a phase change. ∆ Š elementary particle One of the The value of G for the reducing fundamental particles of which mat- agent is also plotted. For example, if the reducing agent is carbon forming ter is composed, such as the electron, ∆ Š proton, or neutron. carbon dioxide, it is G for the reaction elementary reaction A reaction C + O → CO with no intermediates; i.e. one that 2 2 takes place in a single step with a single transition state. 2M + O2 = 2MO free CO + O2 = 2CO2 elevation of boiling point An in- energy crease in the boiling point of a liquid when a solid is dissolved in it. The el- C + O2 = CO2 evation is proportional to the number of particles dissolved (molecules ∆ 2C + O2 = 2CO or ions) and is given by t = kBC, where C is the molal concentration temperature of solute. The constant kB is the ebul- Ellingham diagram 203 enamine

Reduction can occur in the range of Africa, and Kaligunan in India. Emer- temperatures in which the carbon alds can also be successfully synthe- curve is lower than the metal curve. sized. The diagram was devised by the emergency action code See physical chemist H. J. T. Ellingham. hazchem code. elliptical polarization See polar- emery A rock composed of corun- ization of light. dum (natural aluminium oxide, elongation (in protein synthesis) Al2O3) with magnetite, haematite, or e The phase in which amino acids are spinel. It occurs on the island of linked together by sequentially Naxos (Greece) and in Turkey. Emery formed peptide bonds to form a is used as an abrasive and polishing polypeptide chain. Elongation factors material and in the manufacture of are proteins that – by binding to a certain concrete Ûoors. tRNA–amino-acid complex – enable electromotive force the correct positioning of this com- e.m.f. See . plex on the ribosome, so that transla- emission spectrum See spectrum. tion can proceed. empirical Denoting a result that is eluate See chromatography; elu- obtained by experiment or observation . tion rather than from theory. chromatography elu- eluent See ; empirical formula See formula. tion. emulsiÜcation (in digestion) The elution The process of removing an breakdown of fat globules in the duo- adsorbed material (adsorbate) from denum into tiny droplets, which pro- an adsorbent by washing it in a liq- vides a larger surface area on which uid (eluent). The solution consisting the enzyme pancreatic *lipase can of the adsorbate dissolved in the elu- act to digest the fats into fatty acids ent is the eluate. Elution is the and glycerol. EmulsiÜcation is as- process used to wash components of sisted by the action of the bile salts a mixture through a *chromatogra- in bile. phy column. emulsion A *colloid in which small elutriation The process of suspend- particles of one liquid are dispersed ing Ünely divided particles in an up- in another liquid. Usually emulsions ward Ûowing stream of air or water involve a dispersion of water in an to wash and separate them into sized oil or a dispersion of oil in water, and fractions. are stabilized by an emulsiÜer. Com- emanation The former name for monly emulsiÜers are substances, the gas radon, of which there are such as *detergents, that have three isotopes: Rn–222 (radium ema- lyophobic and lyophilic parts in their nation), Rn–220 (thoron emanation), molecules. and Rn–219 (actinium emanation). en The symbol for ethylene diamine emerald The green gem variety of (1,2-diaminoethane) functioning as a *beryl: one of the most highly prized bidentate ligand, used in formulae. gemstones. The Ünest specimens occur in the Muzo mines, Colombia. enamine A type of compound with Other occurrences include the Ural the general formula Mountains, the Transvaal in South R1R2C = C(R3)–NR4R5, enantiomeric pair 204

where R is a hydrocarbon group or the signal strength. The technique, hydrogen. Enamines can be produced which is usually done at low temper- by condensation of an aldehyde or atures, is used to investigate para- ketone with a secondary amine. magnetic molecules. See also electron paramagnetic resonance enantiomeric pair A pair of mol- . ecules consisting of one chiral mol- endothermic Denoting a chemical ecule and the mirror image of this reaction that takes heat from its sur- molecule. The molecules making up roundings. Compare exothermic. e an enantiomeric pair rotate the plane end point The point in a titration of polarized light in equal, but oppo- at which reaction is complete as site, directions. shown by the *indicator. enantiomers See optical activity. energy A measure of a system’s enantiomorph See optical activ- ability to do work. Like work itself, it ity. is measured in joules. Energy is con- Ü enantiomorphism See optical ac- veniently classi ed into two forms: tivity. potential energy is the energy stored in a body or system as a consequence enantiotopic Denoting a ligand a, of its position, shape, or state (this in- attached to a *prochiral centre, in cludes gravitational energy, electrical which the replacement of this ligand energy, nuclear energy, and chemical with a ligand d (which is different energy); kinetic energy is energy of from the ligands a, b, and c attached motion and is usually deÜned as the to the prochiral centre) gives rise to a work that will be done by the body pair of enantiomers Cabcd. possessing the energy when it is enantiotropy See allotropy. brought to rest. For a body of mass m having a speed v, the kinetic energy endo- PreÜx used to designate a 2 2 is mv /2 (classical) or (m – m0)c (rela- bridged ring molecule with a sub- tivistic). The rotational kinetic energy stituent on the ring that is on the of a body having an angular velocity same side as the bridge. If the sub- ω is Iω2/2, where I is its moment of stituent is on the opposite side the inertia. compound is designated exo-. The *internal energy of a body is endocannabinoids See cannabi- the sum of the potential energy and noids. the kinetic energy of its component atoms and molecules. ENDOR Electron-nuclear double resonance. A magnetic resonance tech- energy bands A range of energies nique involving exitation of both that electrons can have in a solid. In electron spins and nuclear spins. Two a single atom, electrons exist in dis- sources of radiation are used. One is crete *energy levels. In a crystal, in a Üxed source at microwave fre- which large numbers of atoms are quency, which partially saturates the held closely together in a lattice, electron spins. The other is a variable electrons are inÛuenced by a number radiofrequency source, which excites of adjacent nuclei and the sharply the atomic nuclear spins. Excitation deÜned levels of the atoms become of the nuclear spins affects the elec- bands of allowed energy; this ap- tron spins by hyperÜre coupling, in- proach to energy levels in solids is creasing the relaxation time of the often known as the band theory. excited electron spins and increasing Each band represents a large number 205 energy level

EEE

conduction band

forbidden band conduction conduction band band electron distribution valence band e valence valence band band

Insulator Conductor Semiconductor

Energy bands of allowed quantum states. Between the top of the valence band can move the bands are forbidden bands. The by thermal agitation into the conduc- outermost electrons of the atoms (i.e. tion band (at absolute zero, a semi- the ones responsible for chemical conductor would act as an insulator). bonding) form the valence band of Doped semiconductors have extra the solid. This is the band, of those bands in the forbidden gap. See illus- occupied, that has the highest en- tration. ergy. energy level Ü Ü The band structure of solids ac- A de nite xed en- counts for their electrical properties. ergy that a molecule, atom, electron, or nucleus can have. In an atom, for In order to move through the solid, Ü the electrons have to change from example, the atom has a xed energy one quantum state to another. This corresponding to the *orbitals in can only occur if there are empty which its electrons move around the quantum states with the same en- nucleus. The atom can accept a quan- ergy. In general, if the valence band tum of energy to become an excited excitation is full, electrons cannot change to atom (see ) if that extra en- new quantum states in the same ergy will raise an electron to a per- band. For conduction to occur, the mitted orbital. Between the ground electrons have to be in an unÜlled state, which is the lowest possible band – the conduction band. Metals energy level for a particular system, are good conductors either because and the Ürst excited state there are the valence band and the conduction no permissible energy levels. Accord- band are only half-Ülled or because ing to the *quantum theory, only the conduction band overlaps with certain energy levels are possible. An the valence band; in either case va- atom passes from one energy level to cant states are available. In insulators the next without passing through the conduction band and valence fractions of that energy transition. band are separated by a wide forbid- These levels are usually described by den band and electrons do not have the energies associated with the indi- enough energy to ‘jump’ from one to vidual electrons in the atoms, which the other. In intrinsic semiconduc- are always lower than an arbitrary tors the forbidden gap is narrow and, level for a free electron. The energy at normal temperatures, electrons at levels of molecules also involve quan- Engel’s salt 206

tized vibrational and rotational mo- remains constant and the enthalpy of tion. reaction, ∆H, is equal to ∆U + p∆V. For an exothermic reaction ∆H is Engel’s salt See potassium carbon- taken to be negative. ate. entropy Symbol S. A measure of enolate ion A negative ion ob- the unavailability of a system’s en- tained from an enol, by removal of a ergy to do work; in a closed system, hydrogen atom. Enolate ions can an increase in entropy is accompa- have two forms: one with a single e nied by a decrease in energy avail- C–C bond and the negative charge on ability. When a system undergoes a the beta carbon atom and the other reversible change the entropy (S) with a double C–C bond and the neg- changes by an amount equal to the ative charge on the oxygen atom. energy (Q) transferred to the system enols Compounds containing the by heat divided by the thermody- group –CH=C(OH)– in their mol- namic temperature (T) at which this ecules. See also keto–enol tau- occurs, i.e. ∆S = ∆Q/T. However, all tomerism. real processes are to a certain extent enrichment The process of increas- irreversible changes and in any ing the abundance of a speciÜed iso- closed system an irreversible change tope in a mixture of isotopes. It is is always accompanied by an increase usually applied to an increase in the in entropy. proportion of U–235, or the addition In a wider sense entropy can be in- of Pu–239 to natural uranium for use terpreted as a measure of disorder; in a nuclear reactor or weapon. the higher the entropy the greater the disorder (see boltzmann ensemble A set of systems of parti- formula). As any real change to a cles used in *statistical mechanics to closed system tends towards higher describe a single system. The concept entropy, and therefore higher disor- of an ensemble was put forward by der, it follows that the entropy of the the US scientist Josiah Willard Gibbs universe (if it can be considered a (1839–1903) in 1902 as a way of cal- closed system) is increasing and its culating the time average of the sin- available energy is decreasing. This gle system, by averaging over the increase in the entropy of the uni- systems in the ensemble at a Üxed verse is one way of stating the sec- time. An ensemble of systems is con- ond law of *thermodynamics. structed from knowledge of the sin- envelope ring conformations gle system and can be represented as See . a set of points in phase space with enyl complex A type of complex each system of the ensemble repre- in which there is a link between the sented by a point. Ensembles can be metal atom or ion and the pi elec- constructed both for isolated systems trons of a double bond. *Zeise’s salt and for open systems. was the Ürst known example. enthalpy Symbol H. A thermody- enzyme A protein that acts as a namic property of a system deÜned *catalyst in biochemical reactions. by H = U + pV, where H is the en- Each enzyme is speciÜc to a particu- thalpy, U is the internal energy of the lar reaction or group of similar reac- system, p its pressure, and V its vol- tions. Many require the association of ume. In a chemical reaction carried certain nonprotein *cofactors in out in the atmosphere the pressure order to function. The molecule un- 207 ephedrine dergoing reaction (the substrate) or disappearance of the substrate. As binds to a speciÜc *active site on the the concentration of the substrate is enzyme molecule to form a short- increased the rate of reaction in- lived intermediate (see enzyme– creases proportionally up to a certain substrate complex): this greatly in- point, after which any further increases (by a factor of up to 1020) the crease in substrate concentration no rate at which the reaction proceeds longer increases the reaction rate (see to form the product. Enzyme activity michaelis–menten curve). At this is inÛuenced by substrate concentra- point, all active sites of the enzyme e tion and by temperature and pH, are saturated with substrate; any fur- which must lie within a certain ther increase in the rate of reaction range. Other molecules may compete will occur only if more enzyme is for the active site, causing *inhibi- added. Reaction rates are also af- tion of the enzyme or even irre- fected by the presence of inhibitors versible destruction of its catalytic (see inhibition), temperature, and pH properties. (see enzyme). Enzyme production is governed by enzyme–substrate complex The a cell’s genes. Enzyme activity is fur- intermediate formed when a sub- ther controlled by pH changes, alter- strate molecule interacts with the ations in the concentrations of *active site of an enzyme. Following essential cofactors, feedback inhibi- the formation of an enzyme– tion by the products of the reaction, substrate complex, the substrate and activation by another enzyme, molecule undergoes a chemical reac- either from a less active form or an tion and is converted into a new inactive precursor (zymogen). Such product. Various mechanisms for the changes may themselves be under formation of enzyme–substrate com- the control of hormones or the plexes have been suggested, includ- enzyme ki- nervous system. See also ing the *induced-Üt model and the netics . *lock-and-key mechanism. Enzymes are classiÜed into six major groups, according to the type ephedrine An alkaloid, of reaction they catalyse: (1) oxidore- C6H5CH(OH)CH(CH3)NHCH3 found in ductases; (2) transferases; (3) hydro- plants of the genus Ephedra, once lases; (4) lyases; (5) isomerases; (6) used as a bronchodilator in the treat- ligases. The names of most individual ment of asthma. It is also used as a enzymes also end in -ase, which is stimulant and appetite suppressant. added to the names of the substrates Structurally, it is a phenylethylamine on which they act. Thus lactase is the and is similar to amphetamines, al- enzyme that acts to break down lac- though less active. It is, however, tose; it is classiÜed as a hydrolase. widely used in the illegal synthesis of A methamphetamine. The molecule has two chiral centres. If the stereo- • Information about IUPAC nomenclature enzyme inhibition See inhibition. OH H enzyme kinetics The study of the N rates of enzyme-catalysed reactions. CH Rates of reaction are usually meas- 3 Ü ured by using the puri ed enzyme in CH3 vitro with the substrate and then observing the formation of the product Ephedrine epimerism 208

chemical conformations are opposite viscosity of water (e.g. in Üre Üght- (i.e. 1R,2S or 1S,1R) the name ing). ephedrine is used. If the conforma- epoxy resins Synthetic resins pro- tions are the same (1R,2R or 1S,2S) duced by copolymerizing epoxide then the compound is called pseu- compounds with phenols. They con- doephedrine. tain –O– linkages and epoxide groups epimerism A type of optical iso- and are usually viscous liquids. They merism in which a molecule has two can be hardened by addition of e chiral centres; two optical isomers agents, such as polyamines, that (epimers) differ in the arrangement form cross-linkages. Alternatively, about one of these centres. See also catalysts may be used to induce fur- optical activity. ther polymerization of the resin. epinephrine See adrenaline. Epoxy resins are used in electrical equipment and in the chemical in- epitaxy (epitaxial growth) Growth dustry (because of resistance to of a layer of one substance on a sin- chemical attack). They are also used gle crystal of another, such that the as adhesives. crystal structure in the layer is the EPR See electron paramagnetic same as that in the substrate. It is resonance used in making semiconductor de- . vices. epsomite A mineral form of *mag- EPM See electron probe micro- nesium sulphate heptahydrate, analysis. MgSO4.7H2O. Epsom salt See magnesium sul- epoietin (EPO) A hormone that reg- phate ulates the production of red blood . cells in the body. The drug is used equation of state An equation medically to treat anemia, and is also that relates the pressure p, volume V, used illegally by athletes participat- and thermodynamic temperature T ing in endurance events. of an amount of substance n. The epoxides Compounds that contain simplest is the ideal *gas law: oxygen atoms in their molecules as pV = nRT, part of a three-membered ring (see where R is the universal gas constant. formula). Epoxides are thus cyclic Applying only to ideal gases, this ethers. equation takes no account of the volume occupied by the gas molecules CH CH 3 3 (according to this law if the pressure is inÜnitely great the volume be- 3CH CH3 O comes zero), nor does it take into account any forces between molecules. Epoxides A more accurate equation of state would therefore be epoxyethane (ethylene oxide) A Û (p + k)(V – nb) = nRT, colourless ammable gas, C2H4O; m.p. –111°C; b.p. 13.5°C. It is a cyclic where k is a factor that reÛects the ether (see epoxides), made by the cat- decreased pressure on the walls of alytic oxidation of ethene. It can be the container as a result of the attrac- hydrolysed to ethane-1,2-diol and tive forces between particles, and nb also polymerizes to …-OC2H4-O-C2H4- is the volume occupied by the parti- …, which is used for lowering the cles themselves when the pressure is 209 equilibrium constant inÜnitely high. In van der Waals’s expression for equilibrium constant. equation, proposed by the Dutch This form of the equilibrium con- physicist J. D. van der Waals (1837– stant was originally introduced in 1923), 1863 by C. M. Guldberg and P. Waage k = n2a/V2, using the law of *mass action. They derived the expression by taking the where a is a constant. This equation more accurately reÛects the behav- rate of the forward reaction x y iour of real gases; several others have kf[A] [B] done better but are more complicated. and that of the back reaction e z w equatorial 1. See ring conforma- kb[C] [D] tions. 2. See apical. Since the two rates are equal at equi- equilibrium A state in which a sys- librium, the equilibrium constant Kc tem has its energy distributed in the is the ratio of the rate constants kf/kb. statistically most probable manner; a The principle that the expression is a state of a system in which forces, constant is known as the equilibrium inÛuences, reactions, etc., balance law or law of chemical equilibrium. each other out so that there is no net The equilibrium constant shows change. A body is said to be in ther- the position of equilibrium. A low mal equilibrium if no net heat ex- value of Kc indicates that [C] and [D] change is taking place within it or are small compared to [A] and [B]; i.e. between it and its surroundings. A that the back reaction predominates. system is in *chemical equilibrium It also indicates how the equilibrium when a reaction and its reverse are shifts if concentration changes. For proceeding at equal rates (see also example, if [A] is increased (by equilibrium constant). These are adding A) the equilibrium shifts to- examples of dynamic equilibrium, in wards the right so that [C] and [D] in- which activity in one sense or direc- crease, and Kc remains constant. tion is in aggregate balanced by com- For gas reactions, partial pressures parable reverse activity. are used rather than concentrations. The symbol Kp is then used. Thus, in equilibrium constant For a re- the example above versible reaction of the type K = p zp w/p xp y xA + yB ˆ zC + wD p C D A B It can be shown that, for a given re- chemical equilibrium occurs when ∆ν ∆ν action Kp = Kc(RT) , where is the the rate of the forward reaction difference in stoichiometric coefÜ- equals the rate of the back reaction, cients for the reaction (i.e. z + w – x – so that the concentrations of prod- y). Note that the units of Kp and Kc de- ucts and reactants reach steady-state pend on the numbers of molecules values. It can be shown that at equi- appearing in the stoichiometric equa- librium the ratio of concentrations tion. The value of the equilibrium [C]z[D]w/[A]x[B]y constant depends on the tempera- is a constant for a given reaction and ture. If the forward reaction is Üxed temperature, called the equilib- exothermic, the equilibrium constant rium constant Kc (where the c indi- decreases as the temperature rises; if cates concentrations have been used). endothermic it increases (see also Note that, by convention, the prod- van’t hoff’s isochore). ucts on the right-hand side of the re- The expression for the equilibrium action are used on the top line of the constant can also be obtained by equilibrium law 210

thermodynamics; it can be shown *lanthanoids; a.n. 68; r.a.m. 167.26; that the standard equilibrium con- r.d. 9.006 (20°C); m.p. 1529°C; b.p. stant KŠ is given by exp(–∆GŠ/RT), 2863°C. It occurs in apatite, gadolin- where ∆GŠ is the standard Gibbs ite, and xenotine from certain free energy change for the complete sources. There are six natural iso- reaction. Strictly, the expressions topes, which are stable, and twelve above for equilibrium constants are artiÜcial isotopes are known. It has true only for ideal gases (pressure) or been used in alloys for nuclear tech- e inÜnite dilution (concentration). For nology as it is a neutron absorber; it accurate work *activities are used. is being investigated for other poten- equilibrium law See equilibrium tial uses. It was discovered by Carl constant. MosanderA (1797–1858) in 1843. equipartition of energy The • Information from the WebElements site theory, proposed by Ludwig *Boltz- mann and given some theoretical ergocalciferol See vitamin d. support by James Clerk *Maxwell, ergodic hypothesis A hypothesis that the energy of gas molecules in a in *statistical mechanics concerning large sample under thermal *equilib- phase space. If a system of N atoms rium is equally divided among their or molecules is enclosed in a Üxed available *degrees of freedom, the volume, the state of this system is average energy for each degree of given by a point in 6N-dimensional freedom being kT/2, where k is the phase space with q representing co- *Boltzmann constant and T is the i ordinates and pi representing mo- thermodynamic temperature. The menta. Taking the energy E to be proposition is not generally true if constant, a representative point in quantum considerations are impor- phase space describes an orbit on the tant, but is frequently a good approx- surface E(qi,pi) = c, where c is a con- imation. stant. The ergodic hypothesis states equivalence point The point in a that the orbit of the representative titration at which reaction is com- point in phase space eventually goes plete. See indicator. through all points on the surface. The quasi-ergodic hypothesis states equivalent proportions See chemical combination that the orbit of the representative . point in phase space eventually equivalent weight The mass of comes close to all points on the sur- an element or compound that could face. In general, it is very difÜcult to combine with or displace one gram prove the ergodic or quasi-ergodic of hydrogen (or eight grams of oxy- hypotheses for a given system. See gen or 35.5 grams of chlorine) in a also ergodicity. chemical reaction. The equivalent ergodicity A property of a system weight represents the ‘combining that obeys the *ergodic hypothesis. power’ of the substance. For an el- The ergodicity of systems has been ement it is the relative atomic mass discussed extensively in the founda- divided by the valency. For a com- tions of *statistical mechanics, al- pound it depends on the reaction though it is now thought by many considered. physicists to be irrelevant to the erbium Symbol Er. A soft silvery problem. Considerations of ergodic- metallic element belonging to the ity occur in dynamics, since the be- 211 esterification haviour can be very complex even ber of elements required by living or- for simple *dynamical systems (see ganisms to ensure normal growth, attractor). Systems, such as *spin development, and maintenance. glasses, in which ergodicity is Apart from the elements found in or- thought not to hold are described as ganic compounds (i.e. carbon, hydro- having broken ergodicity. gen, oxygen, and nitrogen), plants, animals, and microorganisms all re- ergosterol A *sterol occurring in quire a range of elements in inor- fungi, bacteria, algae, and plants. It is ganic forms in varying amounts, converted into vitamin D by the ac- e 2 depending on the type of organism. tion of ultraviolet light. The major elements, present in tis- ESCA See photoelectron spec- sues in relatively large amounts troscopy. (greater than 0.005%), are calcium, eserine (physostigmine) An alka- phosphorus, potassium, sodium, loid, derived from the calabar bean chlorine, sulphur, and magnesium. plant, that inhibits cholinesterase by The trace elements occur at much covalently binding with it (see inhibi- lower concentrations and thus re- tion). Eserine is used to treat the eye quirements are much less. The most condition glaucoma. important are iron, manganese, zinc, copper, iodine, cobalt, selenium,

CH3 molybdenum, chromium, and sili- CH 3 con. Each element may fulÜl one or O NH more of a variety of metabolic roles. N essential fatty acids * 3CH O Fatty acids N that must normally be present in the diet of certain animals, including CH 3 man. Essential fatty acids, which in- Eserine clude *linoleic and *linolenic acids, all possess double bonds at the same ESI See electrospray ionization. two positions along their hydrocarbon chain and so can act as precur- electron paramagnetic ESR See sors of *prostaglandins. DeÜciency of resonance . essential fatty acids can cause der- essential amino acid An *amino matosis, weight loss, irregular acid that an organism is unable to oestrus, etc. An adult human re- synthesize in sufÜcient quantities. It quires 2–10 g linoleic acid or its must therefore be present in the diet. equivalent per day. In man the essential amino acids are essential oil A natural oil with a arginine, histidine, lysine, threonine, distinctive scent secreted by the methionine, isoleucine, leucine, va- glands of certain aromatic plants. line, phenylalanine, and tryptophan. *Terpenes are the main constituents. These are required for protein syn- Essential oils are extracted from thesis and deÜciency leads to re- plants by steam distillation, extrac- tarded growth and other symptoms. tion with cold neutral fats or solvents Most of the amino acids required by (e.g. alcohol), or pressing and used in man are also essential for all other perfumes, Ûavourings, and medicine. multicellular animals and for most Examples are citrus oils, Ûower oils protozoans. (e.g. rose, jasmine), and oil of cloves. essential element Any of a num- esteriÜcation A reaction of an al- esters 212

cohol with an acid to produce an methyl group attached to each car- ester and water; e.g. bon atom. It is used as a drug for in- ˆ ducing sleep. Addition of dilute acid CH3OH + C6H5COOH below 0°C gives ethanal tetramer (or CH3OOCC6H5 + H2O metaldehyde), which has a similar The reaction is an equilibrium and is structure to the trimer but with an slow under normal conditions, but eight-membered ring. It is used as a can be speeded up by addition of a solid fuel in portable stoves and in strong acid catalyst. The ester can slug pellets. e often be distilled off so that the reaction can proceed to completion. The ethanamide (acetamide) A colour- reverse reaction is ester hydrolysis or less solid crystallizing in the form of *saponiÜcation. See also labelling. long white crystals with a characteristic smell of mice, CH CONH ; r.d. esters Organic compounds formed 3 2 1.159; m.p. 82.3°C; b.p. 221.25°C. It is by reaction between alcohols and made by the dehydration of ammo- acids (see illustration). Esters formed nium ethanoate or by the action of from carboxylic acids have the gen- ammonia on ethanoyl chloride, eral formula RCOOR′. Examples are ethanoic anhydride, or ethyl ethyl ethanoate, CH COOC H , and 3 2 5 ethanoate. methyl propanoate, C2H5COOCH3. Es- ters containing simple hydrocarbon ethane A colourless Ûammable groups are volatile fragrant sub- gaseous hydrocarbon, C2H6; m.p. stances used as Ûavourings in the –183°C; b.p. –89°C. It is the second food industry. Triesters, molecules member of the *alkane series of hy- containing three ester groups, occur drocarbons and occurs in natural gas. dep- in nature as oils and fats. See also ethanedial See glyoxal. sides; glycerides. A ethanedioic acid See oxalic acid. • Information about IUPAC nomenclature ethane-1,2-diol (ethylene glycol; glycol) A colourless viscous hygro- ethanal (acetaldehyde) A colourless scopic liquid, CH OHCH OH; m.p. highly Ûammable liquid aldehyde, 2 2 –11.5°C; b.p. 198°C. It is made by hy- CH CHO; r.d. 0.78; m.p. –121°C; b.p. 3 drolysis of epoxyethane (from 20.8°C. It is made from ethene by the ethene) and used as an antifreeze *Wacker process and used as a start- and a raw material for making *poly- ing material for making many or- esters (e.g. Terylene). ganic compounds. The compound polymerizes if dilute acid is added to ethanenitrile (acetonitrile, methyl give ethanal trimer (or paraldehyde), cyanide) A poisonous liquid, CH3CN; which contains a six-membered ring b.p. 82°C. It is made by dehydrating of alternating carbon and oxygen ethanamide or from ammonia and atoms with a hydrogen atom and a ethyne. It is a good polar solvent and

C C2H5 O CH OHHO 3 C C2H5 +H2O CH3 O methanol propanoic acid methyl propanoate water Esters 213 ethanoyl chloride is employed for dissolving ionic com- duce ethanol solutions containing pounds when water cannot be used. more than 15% ethanol by volume. Distillation can produce a constant- ethanoate (acetate) A salt or ester boiling mixture containing 95.6% of ethanoic acid (acetic acid). ethanol and 4.4% water. Pure ethanol ethanoic acid (acetic acid) A clear (absolute alcohol) is made by remov- viscous liquid or glassy solid *car- ing this water by means of drying boxylic acid, CH3COOH, with a char- agents. acteristically sharp odour of vinegar; The main industrial use of ethanol e r.d. 1.049; m.p. 16.6°C; b.p. 117.9°C. is as a solvent although at one time it The pure compound is called glacial was a major starting point for mak- ethanoic acid. It is manufactured by ing other chemicals. For this it was the oxidation of ethanol or by the ox- produced by fermentation of mo- idation of butane in the presence of lasses. Now ethene has replaced dissolved manganese(II) or cobalt(II) ethanol as a raw material and indus- ethanoates at 200°C, and is used in trial ethanol is made by hydrolysis of making ethanoic anhydride for pro- ethene. ducing cellulose ethanoates. It is also ethanolamine Any of three low- used in making ethenyl ethanoate melting hygroscopic colourless (for polyvinylacetate). The compound solids. They are strong bases, smell of is formed by the fermentation of al- ammonia, and absorb water readily cohol and is present in vinegar, to form viscous liquids. Mono- which is made by fermenting beer or ethanolamine, HOCH CH NH , is a wine. ‘Vinegar’ made from ethanoic 2 2 2 primary *amine, m.p. 10.5°C; di- acid with added colouring matter is ethanolamine, (HOCH CH ) NH, is a called ‘nonbrewed condiment’. In liv- 2 2 2 secondary amine, m.p. 28°C; and tri- ing organisms it combines with *co- ethanolamine, (HOCH CH ) N, is a enzyme A to form acetyl coenzyme 2 2 3 tertiary amine, m.p. 21°C. All are A, which plays a crucial role in en- made by heating ethylene oxide with ergy metabolism. concentrated aqueous ammonia ethanoic anhydride (acetic anhy- under pressure and separating the dride) A pungent-smelling colourless products by fractional distillation. ° liquid, (CH3CO)2O, b.p. 139.5 C. It is With fatty acids they form neutral used in organic synthesis as an soaps, used as emulsifying agents *ethanoylating agent (attacking an and detergents, and in bactericides –OH or –NH group) and in the manu- and cosmetics. facture of aspirin and cellulose plas- ethanoylating agent (acetylating tics. It hydrolyses in water to give agent) A chemical reagent used to ethanoic acid. introduce an ethanoyl group ethanol (ethyl alcohol) A colourless (–COCH3) instead of hydrogen in an water-soluble *alcohol, C2H5OH; r.d. organic compound. Examples include 0.789 (0°C); m.p. –117.3°C; b.p. *ethanoic anhydride and ethanoyl ° –78.3 C. It is the active principle in chloride (acetyl chloride, CH3COCl). intoxicating drinks, in which it is ethanoyl chloride (acetyl chloride) produced by fermentation of sugar A colourless liquid acyl chloride (see using yeast acyl halides), CH COCl, with a pun- → 3 C6H12O6 2C2H5OH + 2CO2 gent smell; r.d. 1.105; m.p. –112.15°C; The ethanol produced kills the yeast b.p. 50.9°C. It is made by reacting and fermentation alone cannot pro- ethanoic acid with a halogenating ethanoyl group 214

agent such as phosphorus(III) chlo- is an anaesthetic and useful organic ride, phosphorus(V) chloride, or sul- solvent. See ethers. phur dichloride oxide and is used to ethyl 3-oxobutanoate (ethyl introduce ethanoyl groups into or- acetoacetonate) A colourless liquid ganic compounds containing –OH, acylation ester with a pleasant odour, –NH2, and –SH groups. See . CH3COCH2COOC2H5; r.d. 1.03; m.p. ethanoyl group (acetyl group) The <–80°C; b.p. 180.4°C. It can be pre- organic group CH3CO–. pared by reacting ethyl ethanoate e (CH COOC H ) with sodium or ethene (ethylene) A colourless 3 2 5 sodium ethoxide. The compound Ûammable gaseous hydrocarbon, shows keto–enol *tautomerism and C H ; m.p. –169°C; b.p. –103.7°C. It is 2 4 contains about 7% of the enol form, the Ürst member of the *alkene se- CH C(OH):CHCOOC H , under nor- ries of hydrocarbons. It is made by 3 2 5 mal conditions. Sometimes known as cracking hydrocarbons from petro- acetoacetic ester, it is used in organic leum and is now a major raw ma- synthesis. terial for making other organic chemicals (e.g. ethanal, ethanol, ethyl acetate See ethyl ethanoate. ethane-1,2-diol). It can be polymer- ethyl acetoacetonate See ethyl 3- ized to *polyethene. It occurs natu- oxobutanoate. rally in plants, in which it acts as a growth substance promoting the ethyl alcohol See ethanol. ripening of fruits. ethylamine A colourless Ûamma- ethenone See ketene. ble volatile liquid, C2H5NH2; r.d. 0.69; m.p. –81°C; b.p. 16.6°C. It is a pri- ethenyl ethanoate (vinyl acetate) mary amine made by reacting An unsaturated organic ester, CH : 2 chloroethane with ammonia and CHOOCCH ; r.d. 0.9; m.p. –100°C; 3 used in making dyes. b.p. 73°C. It is made by catalytic reaction of ethanoic acid and ethene and ethylbenzene A colourless Ûam- used to make polyvinylacetate. mable liquid, C6H5C2H5; r.d. 0.867; m.p. –95°C; b.p. 136°C. It is made ether ethoxyethane ethers See ; . from ethene and ethybenzene by a ethers Organic compounds contain- *Friedel–Crafts reaction and is used ing the group –O– in their molecules. in making phenylethene (for poly- Examples are dimethyl ether, styrene). CH3OCH3, and diethyl ether, bromoethane ethoxyethane ethyl bromide See . C2H5OC2H5 (see ). They are volatile highly Ûammable com- ethylene See ethene. pounds made by dehydrating alco- ethylenediamine See 1,2-diamino- holsA using sulphuric acid. ethane. ethylene glycol See ethane- • Information about IUPAC nomenclature 1,2-diol. ethoxyethane (diethyl ether; ethylene oxide See epoxyethane. ether) A colourless Ûammable volatile ether, C2H5OC2H5; r.d. 0.71; ethyl ethanoate (ethyl acetate) A m.p. –116°C; b.p. 34.5°C. It can be colourless Ûammable liquid ester, ° made by *Williamson’s synthesis. It C2H5OOCCH3; r.d. 0.9; m.p. –83.6 C; 215 EXAFS b.p. 77.06°C. It is used as a solvent the oxide in phosphors for television and in Ûavourings and perfumery. screens. It was discovered by Sir ethyl group The organic group AWilliam Crookes in 1889. CH3CH2–. • Information from the WebElements site ethyl iodide See iodoethane. eutectic mixture A solid solution ethyne (acetylene) A colourless un- consisting of two or more substances stable gas, C H , with a characteristic 2 2 and having the lowest freezing point sweet odour; r.d. 0.618; m.p. –80.8°C; of any possible mixture of these com- e b.p. –84.0°C. It is the simplest mem- ponents. The minimum freezing ber of the *alkyne series of unsatu- point for a set of components is rated hydrocarbons, and is prepared called the eutectic point. Low- by the action of water on calcium di- melting-point alloys are usually carbide or by adding alcoholic potassium hydroxide to 1,2-dibromoethane. eutectic mixtures. It can be manufactured by heating Evans balance See gouy balance. methane to 1500°C in the presence evaporation The change of state of a catalyst. It is used in oxyacety- of a liquid into a vapour at a temper- lene welding and in the manufacture ature below the boiling point of the of ethanal and ethanoic acid. Ethyne can be polymerized easily at high liquid. Evaporation occurs at the sur- temperatures to give a range of prod- face of a liquid, some of those mol- ucts. The inorganic saltlike dicar- ecules with the highest kinetic 2– energies escaping into the gas phase. bides contain the ion C2 , although ethyne itself is a neutral compound The result is a fall in the average ki- (i.e. not a protonic acid). netic energy of the molecules of the liquid and consequently a fall in its eudiometer An apparatus for temperature. measuring changes in volume of Ü gases during chemical reactions. A exa- Symbol E. A pre x used in the 18 simple example is a graduated glass metric system to denote 10 times. 18 tube sealed at one end and inverted For example, 10 metres = 1 exame- in mercury. Wires passing into the tre (Em). tube allow the gas mixture to be EXAFS (extended X-ray absorption sparked to initiate the reaction be- Üne structure) Oscillations of the X- tween gases in the tube. ray absorption coefÜcient beyond an europium Symbol Eu. A soft silvery absorption edge. The physical cause metallic element belonging to the of EXAFS is the modiÜcation of the *lanthanoids; a.n. 63; r.a.m. 151.96; Ünal state of a photoelectron *wave r.d. 5.245 (20°C); m.p. 822°C; b.p. function caused by back-scattering 1597°C. It occurs in small quantities from atoms surrounding the excited in bastanite and monazite. Two sta- atom. EXAFS is used to determine ble isotopes occur naturally: eu- structure in chemical, solid state, or ropium–151 and europium–153, both biological systems; it is especially of which are neutron absorbers. Ex- useful in those systems in which it is perimental europium alloys have not possible to use diffraction tech- been tried for nuclear-reactor parts niques. EXAFS experiments are usu- but until recently the metal has not ally performed using synchrotron been available in sufÜcient quanti- radiation. It is possible to interpret ties. It is widely used in the form of EXAFS experiments using single- excimer 216

scattering theory for short-range meson. An association of an electron order. and a positron is also regarded as an exotic atom (known as positronium). excimer See exciplex. This has a mean life of about 10–75, exciplex A combination of two dif- decaying to give three photons. ferent atoms that exists only in an expanded plastic (cellular plastic) excited state. When an exciplex A plastic in the form of a rigid solid emits a photon of electromagnetic ra- foam. Polystyrene, used as a packing diation, it immediately dissociates material, is a common example. e into the atoms, rather than reverting to the ground state. A similar tran- explosive A compound or mixture sient excited association of two that, when ignited or detonated, un- atoms of the same kind is an dergoes a rapid violent chemical re- excimer. An example of an exciplex action that produces large amounts is the species XeCl* (the asterisk indi- of gas and heat, accompanied by cates an excited state), which can be light, sound and a high-pressure formed by an electric discharge in shock wave. Low explosives burn xenon and chlorine. This is used in comparatively slowly when ignited, the exciplex laser, in which a popula- and are employed as propellants in tion inversion is produced by an elec- Ürearms and guns; they are also used trical discharge. in blasting. Examples include *gunpowder and various smokeless pro- excitation A process in which a nu- pellants, such as *cordite. High cleus, electron, atom, ion, or mol- explosives decompose very rapidly ecule acquires energy that raises it to to produce an uncontrollable blast. a quantum state (excited state) Examples of this type include *dyna- higher than that of its *ground state. mite, *nitroglycerine, and *trinitro- The difference between the energy in toluene (TNT); they are exploded the ground state and that in the ex- using a detonator. Other high-power cited state is called the excitation en- explosives include pentaerythritol ergy. See energy level. tetranitrate (PETN) and ammonium exclusion principle See pauli ex- nitride/fuel oil mixture (ANFO). Cy- clusion principle. clonite (RDX) is a military high explo- exo- See endo-. sive; mixed with oils and waxes, it forms a plastic explosive (such as exothermic Denoting a chemical Semtex). See also Chronology. reaction that releases heat into its extended X-ray absorption Üne surroundings. Compare endothermic. structure See exafs. exotic atom A species in which extender some other charged particle replaces An inert substance added the electron or nucleon. An example to a product (paint, rubber, washing is an atom in which an electron has powder, etc.) to dilute it (for economy) or to modify its physical prop- been replaced by another negatively erties. charged particle, such as a muon or meson. In this case the negative par- extensive variable A quantity in ticle eventually collides with the nu- a *macroscopic system that is pro- cleus with the emission of X-ray portional to the size of the system. photons. Another system is one in Examples of extensive variables in- which the nucleus of an atom has clude the volume, mass, and total en- been replaced by a positively charged ergy. If an extensive variable is 217 external conversion divided by an arbitrary extensive external conversion A process in variable, such as the volume, an *in- which molecules in electronically extensive variable results. A macro- cited states pass to a lower electronic scopic system can be described by state (which is frequently the ground one extensive variable and a set of in- state) by colliding with other mol- tensive variables. ecules. In this process the electronic

EXPLOSIVES e

900–1000 Gunpowder developed in China. 1242 English monk Roger Bacon (1220–92) describes the preparation of gunpowder. c.1250 German alchemist Berthold Schwarz claims to have reinvented gunpowder. 1771 French chemist Pierre Woulfe discovers picric acid (originally used as a yellow dye). 1807 Scottish cleric Alexander Forsyth (1767–1843) discovers mercury fulminate. 1833 French chemist Henri Braconnot (1781–1855) nitrates starch, making a highly flammable compound (crude nitrocellulose). 1838 French chemist Théophile Pelouze (1807–67) nitrates paper, making crude nitrocellulose. 1845 German chemist Christian Schönbein (1799–1868) nitrates cotton, making nitrocellulose. 1846 Italian chemist Ascania Sobrero (1812–88) discovers nitroglycerine. 1863 Swedish chemist J. Wilbrand discovers trinitrotoluene (TNT). Swedish chemist Alfred Nobel (1833–96) invents a detonating cap based on mercury fulminate. 1867 Alfred Nobel invents dynamite by mixing nitroglycerine and kieselguhr. 1871 German chemist Hermann Sprengel shows that picric acid can be used as an explosive. 1875 Alfred Nobel invents blasting gelatin (nitroglycerine mixed with nitrocellulose). 1885 French chemist Eugène Turpin discovers ammonium picrate (Mélinite). 1888 Alfred Nobel invents a propellant from nitroglycerine and nitrocellulose (Ballistite). 1889 British scientists Frederick Abel (1826–1902) and James Dewar invent a propellant (Cordite) similar to Ballistite. 1891 German chemist Bernhard Tollens (1841–1918) discovers pentaerythritol tetranitrate (PETN). 1899 Henning discovers cyclotrimethylenetrinitramine (RDX or cyclonite). 1905 US army officer B. W. Dunn (1860–1936) invents ammonium picrate explosive (Dunnite). 1915 British scientists invent amatol (TNT + ammonium nitrate). 1955 US scientists develop ammonium nitrate–fuel oil mixtures (ANFO) as industrial explosives. extraction 218

energy is eventually converted into vated state. A similar equation (with- heat. Since this process involves colli- out the K) has been used to describe sions, the rate at which it occurs de- transport processes, such as diffu- pends on how frequently collisons sion, thermal conductivity, and vis- occur. As a result, this process occurs cosity in dense gases and liquids. In much faster in liquids than in gases. these cases it is assumed that the It is sometimes called collision main kinetic process is the motion of quenching. a molecule to a vacant site near it. e extraction The separation of a The equation is derived by assuming component from its mixture by selec- that the reactants are in equilibrium tive solubility. See partition. with the excited state. This assumption of equilibrium is not necessarily extractive distillation A distilla- correct for small activation energies. tion technique in which a solvent is The Eyring equation is named after added to the mixture in order to sep- Henry *Eyring, who derived it and arate two closely boiling compo- applied it widely in the theory of nents. The added solvent is usually chemical reactions and transport nonvolatile and is selected for its processes. ability to have different effects on the volatilities of the components. E–Z convention A convention for the description of a molecule show- extraordinary ray See double re- ing cis-trans isomerism (see iso- fraction. merism). In a molecule ABC=CDE, extrinction coefÜcient See ab- where A,B,D, and E are substituent sorption coefficient. groups, the sequence rule (see cip system extrusion reaction See insertion ) is applied to the pair A and B to Ü reaction. nd which has priority and similarly to the pair C and D. If the two groups Eyring, Henry (1901–81) US of highest priority are on the same chemist who worked at Princeton side of the bond then the isomer is and Utah. His main work was on designated Z (from German chemical kinetics and he is noted for zusammen, together). If they are on the *Eyring equation for absolute re- opposite sides the isomer is desig- action rates. nated E (German entgegen, opposite). Eyring equation An equation used The letters are used in the names extensively to describe chemical re- of compounds; for example (E)- actions. For a rate constant κ, it is butenedioic acid (fumaric acid) and given by (Z)-butenedioic acid (maleic acid). In κ = K(kT/h)exp(–∆G‡/kT), compounds containing two (or more) where k is the *Boltzmann constant, double bonds numbers are used to T is the thermodynamic temperature, designate the bonds (e.g. (2E, 4Z)-2,4- h is the *Planck constant, ∆G‡ is the hexadienoic acid). The system is less free energy of activation, and K is a ambiguous than the cis/trans system constant called the transmission of describing isomers. coefÜcient, which is the probability A that a chemical reaction takes place • Information about the convention from once the system has reached the acti- IUPAC F

FAB mass spectroscopy See fast- convert to the *Celsius scale the for- atom bombardment mass spec- mula is C =5(F – 32)/9. troscopy . Fajan’s method See adsorption face-centred cubic (f.c.c.) See indicator. cubic crystal . Fajans’ rules Rules indicating the fac-isomer See isomerism. extent to which an ionic bond has covalent character caused by polariza- Û FAD ( avin adenine dinucleotide) A tion of the ions. Covalent character is *coenzyme important in various bio- more likely if: chemical reactions. It comprises a (1) the charge of the ions is high; phosphorylated vitamin B2 (ri- (2) the positive ion is small or the Û bo avin) molecule linked to the nu- negative ion is large; cleotide adenine monophosphate (3) the positive ion has an outer elec- (AMP). FAD is usually tightly bound tron conÜguration that is not a noble- to the enzyme forming a Ûavopro- gas conÜguration. tein. It functions as a hydrogen ac- The rules were introduced by the ceptor in dehydrogenation reactions, Polish-born US chemist Kasimir Fa- being reduced to FADH2. This in turn jans (1887–1975). is oxidized to FAD by the *electron fall-out 1. (radioactive fall-out) transport chain, thereby generating Radioactive particles deposited from ATP (two molecules of ATP per mol- the atmosphere either from a nu- ecule of FADH2). clear explosion or from a nuclear ac- Fahrenheit, Gabriel Daniel cident. Local fall-out, within 250 km (1686–1736) German physicist, who of an explosion, falls within a few became an instrument maker in Am- hours of the explosion. Tropospheric sterdam. In 1714 he developed the fall-out consists of Üne particles de- mercury-in-glass thermometer, and posited all round the earth in the ap- devised a temperature scale to go proximate latitude of the explosion with it (see fahrenheit scale). within about one week. Stratospheric fall-out may fall anywhere on earth Fahrenheit scale A temperature over a period of years. The most dan- Ü scale in which (by modern de nition) gerous radioactive isotopes in fall-out the temperature of boiling water is are the Üssion fragments iodine–131 taken as 212 degrees and the temper- and strontium–90. Both can be taken ature of melting ice as 32 degrees. It up by grazing animals and passed on was invented in 1714 by G. D. to human populations in milk, milk *Fahrenheit, who set the zero at the products, and meat. Iodine–131 accu- lowest temperature he knew how to mulates in the thyroid gland and obtain in the laboratory (by mixing strontium–90 accumulates in bones. ice and common salt) and took his 2. (chemical fall-out) Hazardous own body temperature as 96°F. The chemicals discharged into and subse- scale is no longer in scientiÜc use. To quently released from the atmos- farad 220

phere, especially by factory chim- stant, z the charge of the ion, and M neys. the relative ionic mass. These are the modern forms of the farad Symbol F. The SI unit of ca- laws. Originally, they were stated by pacitance, being the capacitance of a Michael *Faraday in a different form: capacitor that, if charged with one (1) The amount of chemical change coulomb, has a potential difference produced is proportional to the quan- of one volt between its plates. 1 F = 1 tity of electricity passed. CV–1. The farad itself is too large for (2) The amount of chemical change most applications; the practical unit –6 produced in different substances by a is the microfarad (10 F). The unit is Üxed quantity of electricity is propor- f named after Michael *Faraday. tional to the electrochemical equiva- Faraday, Michael (1791–1867) lent of the substance. British chemist and physicist, who re- fast-atom bombardment mass ceived little formal education. He spectroscopy (FAB mass spec- started to experiment on electricity troscopy) A technique in *mass spec- and in 1812 attended lectures by Sir troscopy in which ions are produced Humphry *Davy at the Royal Institu- by bombardment with high-energy tion; a year later he became Davy’s neutral atoms or molecules. It is used assistant. He remained at the Institu- for samples that are nonvolatile or tion until 1861. Faraday’s chemical are thermally unstable. discoveries include the liquefaction Û of chlorine (1823) and benzene (1825) fat A mixture of lipids, chie y as well as the laws of electrolysis (see *triglycerides, that is solid at normal faraday’s laws). He is also remem- body temperatures. Fats occur widely bered for his work in physics: in in plants and animals as a means of 1821 he demonstrated electromag- storing food energy, having twice the Ü netic rotation (the principle of the calori c value of carbohydrates. In electric motor) and in 1832 discov- mammals, fat is deposited in a layer ered electromagnetic induction (the beneath the skin (subcutaneous fat) principle of the dynamo). and deep within the body as a specialized adipose tissue. Faraday constant Symbol F. The Fats derived from plants and Üsh electric charge carried by one mole generally have a greater proportion of electrons or singly ionized ions, of unsaturated *fatty acids than i.e. the product of the *Avogadro those from mammals. Their melting constant and the charge on an elec- points thus tend to be lower, causing tron (disregarding sign). It has the a softer consistency at room tempera- value 9.648 5309(29) × 104 coulombs tures. Highly unsaturated fats are liq- per mole. This number of coulombs uid at room temperatures and are is sometimes treated as a unit of elec- therefore more properly called *oils. tric charge called the faraday. fatty acid An organic compound Faraday’s laws Two laws describ- consisting of a hydrocarbon chain ing electrolysis: and a terminal carboxyl group (see (1) The amount of chemical change carboxylic acids). Chain length during electrolysis is proportional to ranges from one hydrogen atom the charge passed. (methanoic, or formic, acid, HCOOH) (2) The charge required to deposit or to nearly 30 carbon atoms. Ethanoic liberate a mass m is given by Q = (acetic), propanoic (propionic), and Fmz/M, where F is the Faraday con- butanoic (butyric) acids are impor- 221 feldspars tant in metabolism. Long-chain fatty until all the hydrocarbon chain has acids (more than 8–10 carbon atoms) been used up. most commonly occur as con- f-block elements The block of el- stituents of certain lipids, notably ements in the *periodic table consist- glycerides, phospholipids, sterols, ing of the lanthanoid series (from and waxes, in which they are cerium to lutetium) and the actinoid esteriÜed with alcohols. These long- series (from thorium to lawrencium). chain fatty acids generally have an They are characterized by having two even number of carbon atoms; un- s-electrons in their outer shell (n) and branched chains predominate over f-electrons in their inner (n–1) shell. branched chains. They may be satu- f f.c.c. Face-centred cubic. See cubic rated (e.g. *palmitic (hexadecanoic) crystal. acid and *stearic (octadecanoic) acid) or unsaturated, with one double F-centre See colour centre. bond (e.g. *oleic (cis-octodec-9-enoic) Fehling’s test A chemical test to acid) or two or more double bonds, detect reducing sugars and aldehydes in which case they are called polyun- in solution, devised by the German saturated fatty acids (e.g. *linoleic chemist H. C. von Fehling (1812–85). acid and *linolenic acid). See also es- Fehling’s solution consists of Fehlings sential fatty acids. A (copper(II) sulphate solution) and The physical properties of fatty Fehling’s B (alkaline 2,3-dihydroxybu- acids are determined by chain tanedioate (sodium tartrate) solu- length, degree of unsaturation, and tion), equal amounts of which are chain branching. Short-chain acids added to the test solution. After boil- are pungent liquids, soluble in water. ing, a positive result is indicated by As chain length increases, melting the formation of a brick-red precipi- points are raised and water-solubility tate of copper(I) oxide. Methanal, decreases. Unsaturation and chain being a strong reducing agent, also branching tend to lower melting produces copper metal; ketones do points. not react. The test is now little used, having been replaced by *Benedict’s fatty-acid oxidation (β-oxidation) test. The metabolic pathway in which fats are metabolized to release energy. feldspars A group of silicate minerals, the most abundant minerals in Fatty-acid oxidation occurs continu- the earth’s crust. They have a struc- ally but does not become a major ture in which (Si,Al)O tetrahedra are source of energy until the animal’s 4 linked together with potassium, carbohydrate resources are ex- sodium, and calcium and very occa- hausted, for example during starva- sionally barium ions occupying the tion. Fatty-acid oxidation occurs large spaces in the framework. The Û chie y in the mitochondria. A series chemical composition of feldspars of reactions cleave off two carbon may be expressed as combinations of atoms at a time from the hydrocar- the four components: anorthite (An), bon chain of the fatty acid. These CaAl2Si2O8; albite (Ab), NaAlSi3O8; or- two-carbon fragments are combined thoclase (Or), KAlSi3O8; celsian (Ce), with *coenzyme A to form acetyl BaAl2Si2O8. The feldspars are subdi- coenzyme A (acetyl CoA), which then vided into two groups: the alkali enters the *Krebs cycle. The forma- feldspars (including microcline, or- tion of acetyl CoA occurs repeatedly thoclase, and sanidine), in which feldspathoids 222

potassium is dominant with a complexes, which only exist for smaller proportion of sodium and about a picosecond (10–12 s). In a fem- negligible calcium; and the plagio- tochemical experiment a femtosec- clase feldspars, which vary in compo- ond pulse causes dissociation of a sition in a series that ranges from molecule. A series of femtosecond pure sodium feldspar (albite) through pulses is then released, the frequency to pure calcium feldspar (anorthite) of the pulses being that of an absorp- with negligible potassium. Feldspars tion of one of the products of the dis- form colourless, white, or pink crys- sociation. The absorption can be used tals with a hardness of 6 on the as a measure of the abundance of the f Mohs’ scale. product of the dissociation. This type of study enables the course of the feldspathoids A group of alkali mechanism of a chemical reaction to aluminosilicate minerals that are be studied in detail. similar in chemical composition to the *feldspars but are relatively deÜ- Fenton’s reagent A mixture of hy- cient in silica and richer in alkalis. drogen peroxide and iron(II) sulphate The structure consists of a frame- used to produce free radicals by reac- work of (Si,Al)O4 tetrahedra with alu- tions of the type 2+ → 3+ . - minium and silicon atoms at their Fe + H2O2 Fe + OH + OH centres. The feldspathoids occur . Fe3+ + H O → Fe2+ + OOH + H+ chieÛy with feldspars but do not co- 2 2 It is used in water treatment and as a exist with free quartz (SiO2) as they react with silica to yield feldspars. reagent in organic synthesis to intro- The chief varieties of feldspathoids duce a OH group into an aromatic drug. are: nepheline, KNa3(AlSiO4)4; leucite, KAlSi2O6; analcime, NaAlSi2O6.H2O; fermentation A form of anaerobic cancrinite, Na8(AlSiO4)6(HCO3)2; and respiration occurring in certain mi- the sodalite subgroup comprising: croorganisms, e.g. yeasts. Alcoholic sodalite, 3(NaAlSiO4).NaCl; nosean, fermentation comprises a series of 3(NaAlSiO4).Na2SO4; haüyne, biochemical reactions by which pyru- 3(NaAlSiO4).CaSO4; lazurite vate (the end product of *glycolysis) (Na,Ca)8(Al,Si)12O24(S,SO4) (see is converted to ethanol and carbon lapis lazuli). dioxide. Fermentation is the basis of FEM See field-emission the baking, wine, and beer indus- microscope. tries. femto- Symbol f. A preÜx used in fermi A unit of length formerly –15 used in nuclear physics. It is equal to the metric system to denote 10 . –15 For example, 10–15 second = 1 femto- 10 metre. In SI units this is equal second (fs). to 1 femtometre (fm). It was named after Enrico *Fermi. femtochemistry The investigation Fermi–Dirac statistics See quan- of chemical processes that occur on tum statistics the timescale of a femtosecond (10–15 . s). Femtochemistry has become possi- Fermi level The energy in a solid ble as a result of the development of at which the average number of par- *lasers capable of being pulsed in ticles per quantum state is ½; i.e. one femtoseconds. This has enabled ob- half of the quantum states are occu- servations to be made on very short- pied. The Fermi level in conductors lived species, such as activated lies in the conduction band (see en- 223 ferrocene ergy bands), in insulators it lies in ferricyanide A compound contain- 3– the valence band, and in semicon- ing the complex ion [Fe(CN)6] , i.e. ductors it falls in the gap between the hexacyanoferrate(III) ion. the conduction band and the valence ferrimagnetism See magnetism. band. At absolute zero all the electrons would occupy energy levels up ferrite 1. A member of a class of to the Fermi level and no higher lev- mixed oxides MO.Fe2O3, where M is a els would be occupied. It is named metal such as cobalt, manganese, after the Italian-born US physicist En- nickel, or zinc. The ferrites are ce- rico Fermi (1901–54). ramic materials that show either ferrimagnetism or ferromagnetism, but f fermion An *elementary particle are not electrical conductors. For (or bound state of an elementary par- this reason they are used in high- ticle, e.g. an atomic nucleus or an frequency circuits as magnetic cores. atom) with half-integral spin; i.e. 2. See steel. a particle that conforms to Fermi– ferroalloys Alloys of iron with Dirac statistics (see quantum statis- other elements made by smelting tics). Compare boson. mixtures of iron ore and the metal fermium Symbol Fm. A radioactive ore; e.g. ferrochromium, ferrovana- metallic transuranic element belong- dium, ferromanganese, ferrosilicon, ing to the *actinoids; a.n. 100; mass etc. They are used in making alloy number of the most stable isotope *steels. 257 (half-life 10 days). Ten isotopes ferrocene An orange-red crys- Ü are known. The element was rst talline solid, Fe(C H ) ; m.p. 173°C. It Ü 5 5 2 identi ed by A. Ghiorso and associ- can be made by adding the ionic ates in debris from the Ürst hydro- + – compound Na C5H5 (cyclopentadi- gen-bomb explosion in 1952. It is enyl sodium, made from sodium and named after Enrico Fermi. cyclopentadiene) to iron(III) chloride. A In ferrocene, the two rings are paral- • Information from the WebElements site lel, with the iron ion sandwiched between them (hence the name ferrate An iron-containing anion, sandwich compound 2– : see formula). FeO4 . It exists only in strong alka- The bonding is between pi orbitals line solutions, in which it forms pur- on the rings and d-orbitals on the ple solutions. Fe2+ ion. The compound can undergo ferric alum One of the *alums, electrophilic substitution on the C5H5 rings (they have some aromatic char- K2SO4.Fe2(SO4)3.24H2O, in which the aluminium ion Al3+ is replaced by acter). It can also be oxidized to the + the iron(III) (ferric) ion Fe3+. blue ion (C5H5)2Fe . Ferrocene is the ferric chloride test A *presumptive test for morphine. The reagent is a 10% solution of ferric chloride (iron(III) chloride, Fe Cl3) in water. With morphine a blue-green col- Fe oration occurs, changing to green. ferric compounds Compounds of iron in its +3 oxidation state; e.g. ferric chloride is iron(III) chloride, FeCl3. Ferrocene ferrocyanide 224

Ürst of a class of similar complexes crease the process of eutrophication. called *sandwich compounds. Its sys- Bacteria that can Üx nitrogen are tematic name is di-π-cyclopentadi- sometimes added to the soil to in- enyl iron(II). crease its fertility; for example, in ferrocyanide A compound con- tropical countries the cyanobac- 4– terium Anabaena is added to rice pad- taining the complex ion [Fe(CN)6] , i.e. the hexacyanoferrate(II) ion. dies to increase soil fertility. Ü protein ferroelectric materials Ceramic brous protein See . dielectrics, such as Rochelle salt and Fick’s law A law describing the dif- f barium titanate, that have a domain fusion that occurs when solutions of structure making them analogous to different concentrations come into ferromagnetic materials. They ex- contact, with molecules moving from hibit hysteresis and usually the regions of higher concentration to re- piezoelectric effect. gions of lower concentration. Fick’s ferromagnetism See magnetism. law states that the rate of diffusion dn/dt, called the diffusive flux and de- ferrosoferric oxide See triiron tetroxide. noted J, across an area A is given by: dn/dt = J = –DA∂c/∂x, where D is a con- ferrous compounds Compounds stant called the diffusion constant, of iron in its +2 oxidation state; e.g. ∂c/∂x is the concentration gradient of ferrous chloride is iron(II) chloride, the solute, and dn/dt is the amount of FeCl2. solute crossing the area A per unit fertilizer Any substance that is time. D is constant for a speciÜc added to soil in order to increase its solute and solvent at a speciÜc tem- productivity. Fertilizers can be of nat- perature. Fick’s law was formulated ural origin, such as composts, or they by the German physiologist Adolf can be made up of synthetic chemi- Eugen Fick (1829–1901) in 1855. cals, particularly nitrates and phos- Üeld effect See electronic phates. Synthetic fertilizers can effects. increase crop yields dramatically, but when leached from the soil by rain, Üeld-emission microscope (FEM) which runs into lakes, they also in- A type of electron microscope in

metal liquid tip helium

high negative voltage electrons

fluorescent screen

to vacuum pump

Field-emission microscope 225 fine structure which a high negative voltage is ap- veloped to indicate the extent that plied to a metal tip placed in an evac- the wearer has been exposed to uated vessel some distance from a harmful radiation. glass screen with a Ûuorescent coat- Ülter A device for separating solid ing. The tip produces electrons by particles from a liquid or gas. The Üeld emission, i.e. the emission of simplest laboratory Ülter for liquids electrons from an unheated sharp is a funnel in which a cone of paper metal part as a result of a high elec- Ü tric Üeld. The emitted electrons form ( lter paper) is placed. Special con- an enlarged pattern on the Ûuores- tainers with a porous base of sintered glass are also used. See also gooch cent screen, related to the individual crucible f exposed planes of atoms. As the reso- . lution of the instrument is limited by Ülter pump A simple laboratory the vibrations of the metal atoms, it vacuum pump in which air is re- is helpful to cool the tip in liquid he- moved from a system by a jet of lium. Although the individual atoms water forced through a narrow noz- forming the point are not displayed, zle. The lowest pressure possible is individual adsorbed atoms of other the vapour pressure of water. substances can be, and their activity Ü is observable. ltrate The clear liquid obtained by Ültration. Üeld-ionization microscope Ü (Üeld-ion microscope; FIM) A type of ltration The process of separating Ü electron microscope that is similar in solid particles using a lter. In vac- Ü principle to the *Üeld-emission uum ltration, the liquid is drawn Ü microscope, except that a high posi- through the lter by a vacuum Ü Ü tive voltage is applied to the metal pump. Ultra ltration is ltration tip, which is surrounded by low- under pressure. pressure gas (usually helium) rather FIM See field-ionization micro- than a vacuum. The image is formed scope. in this case by Üeld ionization: ion- Ü ization at the surface of an unheated ne chemicals Chemicals pro- solid as a result of a strong electric duced industrially in relatively small Üeld creating positive ions by elec- quantities and with a high purity; tron transfer from surrounding e.g. dyes and drugs. atoms or molecules. The image is Üneness of gold A measure of the Û formed by ions striking the uores- purity of a gold alloy, deÜned as the cent screen. Individual atoms on the parts of gold in 1000 parts of the surface of the tip can be resolved alloy by mass. Gold with a Üneness of and, in certain cases, adsorbed atoms 750 contains 75% gold, i.e. 18 *carat atom-probe may be detected. See also gold. field-ion microscopy. Üne structure Ü Closely spaced spec- ller A solid inert material added to tral lines arising from transitions be- a synthetic resin or rubber, either to tween energy levels that are split by change its physical properties or sim- the vibrational or rotational motion ply to dilute it for economy. of a molecule or by electron spin. Ülm badge A lapel badge contain- They are visible only at high resolu- ing masked photographic Ülm worn tion. HyperÜne structure, visible only by personnel who could be exposed at very high resolution, results from to ionizing radiation. The Ülm is de- the inÛuence of the atomic nucleus finger domain 226

on the allowed energy levels of the with horizontal bonds representing atom. bonds coming out of the page and Ünger domain A Ünger-shaped vertical bonds representing bonds in structure produced in a protein when the plane of the page or behind the page. It is named after Emil Fischer. a series of the constituent amino absolute configuration acids combines with a metal atom. See . Üredamp Methane formed in coal Fischer–Tropsch process An in- mines. dustrial method of making hydrocarbon fuels from carbon monoxide and Ü re extinguisher A substance that hydrogen. The process was invented Û f smothers ames or prevents their in 1933 and used by Germany in spreading. Many substances can be World War II to produce motor fuel. used. Liquids include water, sodium Hydrogen and carbon monoxide are hydrogencarbonate solution, chlori- mixed in the ratio 2:1 (water gas was nated organic compounds such as used with added hydrogen) and tetrachloromethane and carbon passed at 200°C over a nickel or dioxide-containing foams. Solid cobalt catalyst. The resulting hydro- extinguishers, such as sodium or carbon mixture can be separated into potassium hydrogencarbonate, pro- a higher-boiling fraction for Diesel duce carbon dioxide gas when engines and a lower-boiling gasoline strongly heated; solid carbon dioxide fraction. The gasoline fraction con- (dry ice) may also be used. tains a high proportion of straight- Ürst-order reaction See order. chain hydrocarbons and has to be Fischer, Emil Hermann (1852– reformed for use in motor fuel. Alco- 1919) German organic chemist who hols, aldehydes, and ketones are also studied under *Kekulé in Bonn and present. The process is also used in later under *Baeyer in Strasbourg. the manufacture of SNG from coal. It He moved to Munich in 1875 and is named after the German chemist Würzburg in 1885. Fischer is noted Franz Fischer (1852–1932) and the for his extensive pioneering work Czech Hans Tropsch (1839–1935). on natural products and especially Üssion-track dating A method of his work on sugar chemistry. In 1899 estimating the age of glass and other he began work on synthesizing pep- mineral objects by observing the tides and proteins. Fischer was tracks made in them by the Üssion awarded the 1902 Nobel Prize for fragments of the uranium nuclei that chemistry. they contain. By irradiating the ob- Fischer, Hans (1881–1945) German jects with neutrons to induce Üssion organic chemist who worked mainly and comparing the density and num- in Munich. He worked on por- ber of the tracks before and after ir- phyrins, synthesizing haemin in radiation it is possible to estimate the 1927. Fischer worked also on chloro- time that has elapsed since the ob- phyll, showing that it was a por- ject solidiÜed. phyrin containing magnesium. In Fittig reaction See wurtz reac- 1944 he synthesized bilirubin. He tion. was awarded the 1930 Nobel Prize for chemistry. Üxation See nitrogen fixation. Fischer projection A type of *pro- Üxed point A temperature that jection in which a molecule is drawn can be accurately reproduced to en- 227 flip-flop able it to be used as the basis of a Ûavin adenine dinucleotide See *temperature scale. fad. Ûagpole See ring conformations. Ûavones A group of flavanoid compounds found in many plants. Ûame A hot luminous mixture of gases undergoing combustion. The chemical reactions in a Ûame are mainly free-radical chain reactions O and the light comes from Ûuores- cence of excited molecules or ions or from incandescence of small solid f particles (e.g. carbon). Ûame test A simple test for met- O als, in which a small amount of the sample (usually moistened with hy- Flavone structure drochloric acid) is placed on the end Û of a platinum wire and held in a Bun- avonoids A group of naturally oc- sen Ûame. Certain metals can be de- curring phenolic compounds many tected by the colour produced: of which are plant pigments. They in- Û barium (green), calcium (brick red), clude the anthocyanins, avonols, Û lithium (crimson), potassium (lilac), and avones. sodium (yellow), strontium (red). Ûavoprotein See fad. Ûash photolysis A technique for Fleming, Sir Alexander studying free-radical reactions in (1881–1955) British bacteriologist, gases. The apparatus used typically born in Scotland. He studied medi- consists of a long glass or quartz tube cine at St Mary’s Hospital, London, holding the gas, with a lamp outside where he remained all his life. In the tube suitable for producing an in- 1922 he identiÜed lysozyme, an en- tense Ûash of light. This dissociates zyme that destroys bacteria, and in molecules in the sample creating free 1928 discovered the antibiotic *peni- radicals, which can be detected spec- cillin. He shared the 1945 Nobel Prize troscopically by a beam of light for physiology or medicine with *Flo- passed down the axis of the tube. It is rey and *Chain, who Ürst isolated the possible to focus the spectrometer on drug. an absorption line for a particular Ûint (chert) Very hard dense nod- product and measure its change in ules of microcrystalline quartz and intensity with time using an oscillo- chalcedony found in chalk and lime- scope. In this way the kinetics of very stone. fast free-radical gas reactions can be Û Û studied. ip- op The movement (transverse diffusion) of a lipid molecule from Ûash point The temperature at one surface of a *lipid bilayer mem- which the vapour above a volatile liq- brane to the other, which occurs at a uid forms a combustible mixture very slow rate. This contrasts with with air. At the Ûash point the appli- the much faster rate at which lipid cation of a naked Ûame gives a mo- molecules exchange places with mentary Ûash rather than sustained neighbouring molecules on the same combustion, for which the tempera- surface of the membrane (lateral dif- ture is too low. fusion). flocculation 228

Ûocculation The process in which working in Cambridge and ShefÜeld, particles in a colloid aggregate into he returned to Oxford in 1935. There larger clumps. Often, the term is he teamed up with Ernst *Chain and used for a reversible aggregation of by 1939 they succeeded in isolating particles in which the forces holding and purifying *penicillin. They also the particles together are weak and developed a method of producing the the colloid can be redispersed by agi- drug in large quantities and carried tation. The stability of a lyophobic out its Ürst clinical trials. The two colloidal dispersion depends on the men shared the 1945 Nobel Prize for existence of a layer of electric charge physiology or medicine with peni- f on the surface of the particles. cillin’s discoverer, Alexander *Flem- Around this are attracted electrolyte ing. ions of opposite charge, which form Flory temperature (theta tempera- a mobile ionic ‘atmosphere’. The re- ture) Symbol θ. The unique tempera- sult is an electrical double layer on ture at which the attractions and the particle, consisting of an inner repulsions of a polymer in a solution Ü shell of xed charges with an outer cancel each other. It is analogous to mobile atmosphere. The potential en- the Boyle temperature of a nonideal ergy between two particles depends gas. A polymer solution at the Flory on a repulsive interaction between temperature is called a theta (θ) solu- double layers on adjacent particles tion. At the Flory temperature the and an attractive interaction due to virial coefÜcient B, asociated with the *van der Waals’ forces between the excluded volume of the polymer, is particles. zero, which results in the polymer At large separations, the repulsive chain behaving almost ideally. This forces dominate, and this accounts enables the theory of polymer solu- for the overall stability of the colloid. tions at the Flory temperature to pro- As the particles become closer to- vide a more accurate description of gether, the potential energy in- events than for polymer solutions at creases to a maximum and then falls other temperatures, even if the poly- sharply at very close separations, mer solution is concentrated. It is not where the van der Waals’ forces always possible to attain the Flory dominate. This potential-energy min- temperature experimentally. The imum corresponds to *coagulation Flory temperature is named after the and is irreversible. If the *ionic US physicist Paul Flory (1910–85). strength of the solution is high, the Ûotation froth flotation ionic atmosphere around the parti- See . cles is dense and the potential-energy Ûuctuation–dissipation theorem curve shows a shallow minimum at A theory relating quantities in equi- larger separation of particles. This librium and *nonequilibrium statisti- corresponds to Ûocculation of the cal mechanics and microscopic and particles. Ions with a high charge are macroscopic quantities. The Ûuctua- particularly effective for causing Ûoc- tion–dissipation theorem was Ürst de- culation and coagulation. rived for electrical circuits with noise in 1928 by H. Nyquist; a general theo- Ûocculent Aggregated in woolly rem in statistical mechanics was de- masses; used to describe precipitates. rived by H. B. Callen and T. A. Florey, Howard Walter, Baron Welton in 1951. The underlying prin- (1898–1968) Australian pathologist, ciple of the Ûuctuation–dissipation who moved to Oxford in 1922. After theorem is that a nonequilibrium 229 fluorine state may have been reached either rious as a so-called ‘date rape’ drug. It as a result of a random Ûuctuation or is quickly eliminated from the body, an external force (such as an electric difÜcult to detect, and causes amne- or magnetic Üeld) and that the evolu- sia, so that victims cannot remember tion towards equilibrium is the same events that occur when under the in both cases (for a sufÜciently inÛuence of the drug. small Ûuctuation). The Ûuctuation– 3CH dissipation theorem enables *trans- O Ü port coef cients to be calculated in N terms of response to external Üelds. Ûuidization A technique used in f some industrial processes in which N solid particles suspended in a stream 2NH of gas are treated as if they were in the liquid state. Fluidization is useful for transporting powders, such as F coal dust. Fluidized beds, in which solid particles are suspended in an upward stream, are extensively used in the chemical industry, particularly Flunitrazepam in catalytic reactions where the powdered catalyst has a high surface Ûuorescein A yellowish-red dye area. They are also used in furnaces, that produces yellow solutions with a being formed by burning coal in a green Ûuorescence. It is used in trac- hot turbulent bed of sand or ash ing water Ûow and as an *adsorption through which air is passed. The bed indicator. behaves like a Ûuid, enabling the Û luminescence combustion temperature to be re- uorescence See . duced so that the production of Ûuoridation The process of adding polluting oxides of nitrogen is dimin- very small amounts of Ûuorine salts ished. By adding limestone to the (e.g. sodium Ûuoride, NaF) to drink- bed with the fuel, the emission of ing water to prevent tooth decay. The sulphur dioxide is reduced. Ûuoride becomes incorporated into High-pressure Ûuidized beds are the Ûuoroapatite (see apatite) of the also used in power-station furnaces growing teeth and reduces the inci- in a combined cycle in which the dence of dental caries. products of combustion from the Ûuoride See halide. Ûuidized bed are used to drive a gas turbine, while a steam-tube boiler in Ûuorination A chemical reaction the Ûuid bed raises steam to drive a in which a Ûuorine atom is intro- steam turbine. This system both induced into a molecule. See halogena- creases the efÜciency of the combustion. tion process and reduces pollution. Ûuorine Symbol F. A poisonous Ûunitrazepam A *benzodiazepine pale yellow gaseous element belong- used medically in some countries as ing to group 17 (formerly VIIB) of the a powerful hypnotic, sedative, and periodic table (the *halogens); a.n. 9; muscle relaxant. It was marketed in r.a.m. 18.9984; d. 1.7 g dm–3; m.p. the US under the tradename Rohyp- –219.62°C; b.p. –188.1°C. The main Û nol. Flunitrazepam has become noto- mineral sources are * uorite (CaF2) fluorite 230

Û and *cryolite (Na3AlF). The element is atoms of hydrocarbons by uorine obtained by electrolysis of a molten atoms. Their inertness and high sta- mixture of potassium Ûuoride and bility to temperature make them hydrogen Ûuoride. It is used in the suitable for a variety of uses as oils, synthesis of organic Ûuorine com- polymers, etc. See also chlorofluoro- pounds. Chemically, it is the most re- carbon; halon. active and electronegative of all 5-Ûuorouracil (5-FU) A Ûuorine de- elements. It is a highly dangerous el- rivative of the pyrimidine uracil. It is ement, causing severe chemical used in chemotherapy where it in- burns on contact with the skin. The hibits the cell’s ability to synthesize element was identiÜed by Scheele in f DNA. It is often used in a treatment 1771 and Ürst isolated by Moissan in regime along with cisplatin. 1886. A Ûux 1. A substance applied to the • Information from the WebElements site surfaces of metals to be soldered to inhibit oxidation. 2. A substance Ûuorite (Ûuorspar) A mineral form used in the smelting of metals to as- Û of calcium uoride, CaF2, crystalliz- sist in the removal of impurities as ing in the cubic system. It is variable slag. in colour; the most common Ûuorites Û are green and purple (blue john), but uxional molecule A molecule other forms are white, yellow, or that undergoes alternate very rapid brown. Fluorite is used chieÛy as a rearrangements of its atoms and thus Ü Ûux material in the smelting of iron only has a speci c structure for a and steel; it is also used as a source of very short period of time. For exam- Ûuorine and hydroÛuoric acid and in ple, the molecule ClF3 has a T-shape ° the ceramic and optical-glass indus- at low temperatures (–60 C); at room Û tries. temperature the uorine atoms change position very rapidly and ap- Û uorite structure A type of ionic pear to have identical positions. crystal structure in which the cations have an expanded face-centred cubic foam A dispersion of bubbles in a arrangement with the anions occupy- liquid. Foams can be stabilized by ing both types of tetrahedral hole. *surfactants. Solid foams (e.g. ex- The cations have a coordination panded polystyrene or foam rubber) number of 8 and the anions have a are made by foaming the liquid and coordination number of 4. Examples allowing it to set. See also colloids. of compounds with this structure are foaming agent (blowing agent) A CaF2, BaCl2, and PbO2. substance used to produce a liquid or The antiÛuorite structure is the op- solid foam (e.g. an expanded plastic). posite arrangement, with anions in Physical agents are compressed the fcc array with coordination num- gases; chemical foaming agents are ber 8 and cations in the tetrahedral substances that release gas under cer- holes with coordination number 4. tain conditions (e.g. sodium hydro- Û Examples of the anti urite structure gencarbonate). are K2O, Li2O, Na2O, K2S, and Na2S. A Fokker–Planck equation An equation in *nonequilibrium statisti- • An interactive version of the structure cal mechanics that describes a super- Ûuorocarbons Compounds ob- position of a dynamic friction tained by replacing the hydrogen (slowing-down) process and a diffu- 231 formalin sion process for the evolution of vari- than allowed transitions. There are ables in a system. The Fokker–Planck three reasons why forbidden transi- equation, which can be used to tions may occur: analyse such problems as *Brownian (1) the selection rule that is violated movement, is soluble using statistical is only an approximate rule. An ex- methods and the theory of probabil- ample is provided by those selection ity. It is named after the Dutch physi- rules that are only exact in the ab- cist Adriaan Fokker (1887–1968) and sence of *spin–orbit coupling. When Max *Planck. spin–orbit coupling is taken into account, the forbidden transitions be- folacin See folic acid. come allowed – their strength f folic acid (folacin) A vitamin of the increasing with the size of the *vitamin B complex. In its active spin–orbit coupling; form, tetrahydrofolic acid, it is a (2) the selection rule is valid for di- *coenzyme in various reactions in- pole radiation, i.e. in the interaction volved in the metabolism of amino between a quantum-mechanical sys- acids, purines, and pyrimidines. It is tem, such as an atom, and an electro- synthesized by intestinal bacteria magnetic Üeld, only the (variable) and is widespread in food, especially electric dipole moment is considered. green leafy vegetables. DeÜciency Actual transitions may involve mag- causes poor growth and nutritional netic dipole radiation or quadrupole anaemia. radiation; food additive A substance added (3) the selection rule only applies for to a food during its manufacture or an atom, molecule, etc., in isolation and does not necessarily apply if ex- processing in order to improve its Ü keeping qualities, texture, appear- ternal elds, collisions, etc., are taken ance, or stability or to enhance its into account. taste or colour. Additives are usually force constant A constant charac- present in minute quantities; they in- terizing the strength of the bond in a clude colouring materials, sweeten- diatomic molecule. Near the equilib- ers, preservatives, *antioxidants, rium position, Re, of the potential en- emulsiÜers, and stabilizers. In most ergy curve of a diatomic molecule, countries the additives used must be the potential energy V is accurately selected from an approved list of represented by a parabola of the 2 such compounds, which have been form V = k/2(R – Re) , where R is the tested for safety, and they must be internuclear distance and k is the listed on the food labels of individual force constant. The greater the force products. constant, the stronger is the bond be- fool’s gold See pyrite. tween the atoms, since the walls of the potential curve become steeper. forbidden band See energy Regarding the molecular vibrations bands. as simple harmonic motion, the forbidden transitions Transitions force constant occurs in the analysis between energy levels in a quantum- of the vibrational energy levels. mechanical system that are not al- forced convection See convec- lowed to take place because of tion. *selection rules. In practice, forbid- formaldehyde See methanal. den transitions can occur, but they do so with much lower probability formalin A colourless solution of formate 232

methanal (formaldehyde) in water four-circle diffractometer An in- with methanol as a stabilizer; r.d. strument used in X-ray crystallogra- 1.075–1.085. When kept at tempera- phy to automatically determine the tures below 25°C a white polymer of shape and symmetry of the unit cell methanal separates out. It is used as of a crystal. The crystal is placed in a disinfectant and preservative for the goniometer head with an arbi- biological specimens. trary orientation. If the dimensions formate See methanoate. of the unit cell have been determined, they can be used to calculate formic acid See methanoic acid. the settings of the four angles of the formula A way of representing a diffractometer needed to observe a f Ü Û chemical compound using symbols speci c (h k l) re ection, where (h k l) for the atoms present. Subscripts are are Miller indices. A computer con- used for the numbers of atoms. The trols the settings so that each (h k l) molecular formula simply gives the is examined in turn and the diffrac- types and numbers of atoms present. tion intensity measured. This infor- For example, the molecular formula mation enables the electron density of ethanoic acid is C H O . The em- to be calculated, as the intensity of 2 4 2 Û pirical formula gives the atoms in the re ection for a set of (h k l) their simplest ratio; for ethanoic acid planes is proportional to the square of the modulus of a function called it is CH2O. The structural formula gives an indication of the way the the structure factor Fhkl of the set, atoms are arranged. Commonly, this which is related to the electron den- is done by dividing the formula into sity. groups; ethanoic acid can be written Fourier analysis The representa- CH3.CO.OH (or more usually simply tion of a function f(x), which is peri- CH3COOH). Structural formulae can odic in x, as an inÜnite series of sine also show the arrangement of atoms and cosine functions: or groups in space. ∞ f(x) = a /2 + ∑ (a cosnx + b sinnx) 0 n=1 n n formula weight The relative mo- A series of this type is called a lecular mass of a compound as calcu- *Fourier series. If the function is pe- lated from its molecular formula. riodic with a period 2π, the coefÜ- formylation A chemical reaction cients a0, an, bn are: that introduces a formyl group ∫+π a0 = –π f(x)dx, (methanoyl, –CHO) into an organic +π ∫ molecule. an = –π f(x) cosnxdx, (n = 1,2,3,…), +π ∫ π formyl group The group HCO–. bn = –π 1/ f(x)sinnxdx, (n = 1,2,3,…). fossil fuel Coal, oil, and natural Fourier analysis and Fourier series gas, the fuels used by man as a are named after the French mathe- source of energy. They are formed matician and engineer Joseph Fourier from the remains of living organisms (1768–1830). Fourier series have and all have a high carbon or hydro- many important applications in gen content. Their value as fuels re- mathematics, science, and engineering, having been invented by Fourier lies on the exothermic oxidation of Ü carbon to form carbon dioxide (C + in the rst quarter of the 19th cen- → tury in his analysis of the problem of O2 CO2) and the oxidation of hy- ½ → heat conduction. drogen to form water (H2 + O2 H2O). Fourier series An expansion of a 233 fractional crystallization periodic function as a series of tion Ünishes in an initially unoccu- trigonometric functions. Thus, pied state F, having started in a state I, which is not the ground state. As f(x) = a0 + (a1cosx + b1sinx) + the state F is initially unoccupied, (a2cos2x + b2sin2x)+…, any population in I constitutes popu- where a , a , b , b , etc., are con- 0 1 1 2 lation inversion. Thus laser action is stants, called Fourier coefÜcients. The Ü Ü possible if I is suf ciently metastable. series was rst formulated by Joseph If transitions from F to the ground * Fourier and is used in Fourier analy- state G are rapid, population inver- sis. sion is maintained since this lowers Fourier transform An integral the population in F caused by the f transform of the type: transition in the laser action. ∞ ∫ –xy F(y) = –∞ f(x)e dy. fractal A curve or surface generated The inverse is: by a process involving successive sub- ∞ Û f(x) = (1/2π) ∫ F(y)eixydy. division. For example, a snow ake –∞ curve can be produced by starting Fourier transform techniques are with an equilateral triangle and di- used in obtaining information from viding each side into three segments. spectra, especially in NHR and in- The middle segments are then re- fourier- frared spectroscopy (see placed by two equal segments, which transform infrared ). would form the sides of a smaller Fourier-transform infrared (FT-IR) equilateral triangle. This gives a 12- Infrared spectroscopy in which com- sided star-shaped Ügure. The next puters are part of the spectroscopic stage is to subdivide each of the sides apparatus and use *Fourier trans- of this Ügure in the same way, and so forms to enable the curve of inten- on. The result is a developing Ügure sity against wave number to be that resembles a snowÛake. In the plotted with very high sensitivity. limit, this Ügure has ‘fractional di- This has allowed spectra to be ob- mension’ – i.e. a dimension between tained in the far infrared region; pre- that of a line (1) and a surface (2); the Û viously it was difÜcult to attain dimension of the snow ake curve is spectra in this region as the resolu- 1.26. The study of this type of ‘self- Ü tion was obscured by the signal-to- similar’ gure is used in certain noise ratio being too high to resolve branches of chemistry – for example, the vibrational and/or rotational crystal growth. Fractals are also im- spectra of small molecules in their portant in *chaos theory and in com- gas phase. FT-IR has been used in re- puter graphics. search on the atmosphere. Another fraction See fractional distilla- application of this technique is the tion. detection of impurities in samples of fractional crystallization A condensed matter. method of separating a mixture of four-level laser A laser in which soluble solids by dissolving them in a four energy levels are involved. The suitable hot solvent and then lower- disadvantage of a three-level laser is ing the temperature slowly. The least that it is difÜcult to attain population soluble component will crystallize inversion because many molecules out Ürst, leaving the other compo- have to be raised from their ground nents in solution. By controlling the state to an excited state by pumping. temperature, it is sometimes possible In a four-level laser, the laser transi- to remove each component in turn. fractional distillation 234

fractional distillation (fractiona- trons (see born–oppenheimer approx- tion) The separation of a mixture of imation), an electronic transition oc- liquids by distillation. Effective sepa- curs much more rapidly than the ration can be achieved by using a time required for the nuclei to re- long vertical column (fractionating spond to it. Therefore, in a diagram column) attached to the distillation showing the electronic states of the vessel and Ülled with glass beads. molecule as a function of internu- Vapour from the liquid rises up the clear distance, the most intense elec- column until it condenses and runs tronic transition is represented by a back into the vessel. The rising vertical line. For this reason a transi- f vapour in the column Ûows over the tion obeying the Franck–Condon descending liquid, and eventually a principle is called a vertical steady state is reached in which transition; when it occurs the relative there is a decreasing temperature positions of the nuclei remain un- gradient up the column. The vapour changed. The Franck–Condon princi- in the column has more volatile com- ple is named after James Franck ponents towards the top and less (1882–1964), who stated it in 1925, volatile components at the bottom. and Edward Condon, who formu- Various fractions of the mixture can lated it mathematically in terms of be drawn off at points on the col- quantum mechanics in 1928. umn. Industrially, fractional distilla- Frankland, Sir Edward tion is performed in large towers (1825–99) British organic chemist who was the containing many perforated trays. It Ü is used extensively in petroleum rst to produce organometallic com- reÜning. pounds (zinc dialkyls). He is remembered as the originator of the theory fractionating column See frac- of valency and introduced a method tional distillation. of writing structural formulas. fractionation See fractional dis- Frasch process A method of ob- tillation. taining sulphur from underground francium Symbol Fr. A radioactive deposits using a tube consisting of element belonging to *group 1 (for- three concentric pipes. Superheated merly IA) of the periodic table; a.n. steam is passed down the outer pipe 87; r.d. 2.4; m.p. 27±1°C; b.p. to melt the sulphur, which is forced 677±1°C. The element is found in up through the middle pipe by com- uranium and thorium ores. All 22 pressed air fed through the inner known isotopes are radioactive, the tube. The steam in the outer casing most stable being francium–223. The keeps the sulphur molten in the existence of francium was conÜrmed pipe. It was named after the German- by Marguerite Perey in 1939. born US chemist Hermann Frasch A (1851–1914). • Information from the WebElements site Fraunhofer, Josef von (1787–1826) German physicist, who Franck–Condon principle A prin- trained as an optician. In 1814 he ob- ciple governing the intensity of tran- served dark lines in the spectrum of sitions in the vibrational structure the sun (see fraunhofer lines). He during an electronic transition in a also studied diffraction. molecule. The principle states that since nuclei are much heavier and Fraunhofer lines Dark lines in the move much more slowly than elec- solar spectrum, discovered by Josef 235 Freundlich isotherm von Fraunhofer, that result from the process, ∆F represents the useful absorption by elements in the solar work available. chromosphere of some of the wave- free radical An atom or group of lengths of the visible radiation emit- atoms with an unpaired valence elec- ted by the hot interior of the sun. tron. Free radicals can be produced free electron See electron. by photolysis or pyrolysis in which a free-electron approximation bond is broken without forming ions homolytic fission The approximation resulting from (see ). Because of the assumption that electrons in their unpaired valence electron, most *metals can be analysed using the free radicals are extremely reactive. chain reaction *kinetic theory of gases, without tak- See also . f ing the periodic potential of the freeze drying A process used in metal into account. This approxima- dehydrating food, blood plasma, and tion gives a good qualitative account other heat-sensitive substances. The of some properties of metals, such as product is deep-frozen and the ice their electrical conductivity. At very trapped in it is removed by reducing low temperatures it is necessary to the pressure and causing it to sub- use quantum statistical mechanics lime. The water vapour is then re- rather than classical statistical me- moved, leaving an undamaged dry chanics. The free-electron approxi- product. mation does not, however, give an adequate quantitative description of freezing mixture A mixture of the properties of metals. It can be im- components that produces a low proved by the nearly free electron ap- temperature. For example, a mixture proximation, in which the periodic of ice and sodium chloride gives a potential is treated as a perturbation temperature of –20°C. on the free electrons. freezing-point depression See free energy A measure of a sys- depression of freezing point. tem’s ability to do work. The Gibbs Frenkel defect See crystal defect. free energy (or Gibbs function), G, is deÜned by G = H – TS, where G is the Frenkel–Kontorowa model A energy liberated or absorbed in a re- one-dimensional model of atoms, versible process at constant pressure such as xenon, adsorbed on a peri- and constant temperature (T), H is odic substrate, such as graphite. This the *enthalpy and S the *entropy of model, which can be used to investi- the system. Changes in Gibbs free en- gate the nature of the lattice formed ergy, ∆G, are useful in indicating the by the adsorbed gas, was invented in conditions under which a chemical 1938 by Y. I. Frenkel and T. Kon- reaction will occur. If ∆G is positive torowa and independently in 1949 by the reaction will only occur if energy F. C. Frank and J. H. Van der Merwe. is supplied to force it away from the The Frenkel–Kontorowa model can equilibrium position (i.e. when ∆G = be used to investigate the phase tran- 0). If ∆G is negative the reaction will sition between a *commensurate lat- proceed spontaneously to equilib- tice and an *incommensurate lattice. rium. freon See chlorofluorocarbon. The Helmholtz free energy (or Helmholtz function), F, is deÜned by F Freundlich isotherm An isotherm = U – TS, where U is the *internal en- for adsorption with the form θ = 1/c2 ergy. For a reversible isothermal c1P , where the fractional coverage Friedel–Crafts reaction 236

θ is the ratio of adsorption sites occu- (for acyl substitution). The reactions pied to the number of adsorption occur at high temperature (about sites available, c1, and c2 are con- 100°C) with an aluminium chloride stants, and P is the pressure of gas. catalyst. The catalyst acts as an elec- The Freundlich isotherm is fre- tron acceptor for a lone pair on the quently used with adsorption from halide atom. This polarizes the liquid solutions, when it has the haloalkane or acyl halide, producing × 1/c2 form: w = c1 c , where w is the a positive charge on the alkyl or acyl mass of solute adsorbed per unit group. The mechanism is then elec- mass of adsorbent (a quantity called trophilic substitution. Alcohols and f the mass fraction) and c is the con- alkenes can also undergo Friedel– centration of the solution. Like other Crafts reactions. The reaction is isotherms the Freundlich isotherm named after the French chemist agrees with experiment for limited Charles Friedel (1832–99) and the US ranges of parameter. If the limits of chemist James M. Craft (1839–1917). applicability have been established these isotherms, such as the Fre- Froehde’s test A *presumptive undlich isotherm, are useful in con- test for opioids. Froehde’s reagent sidering heterogeneous catalysis. consists of 0.5 gram of sodium molybdate (Na MoO ) dissolved in Friedel–Crafts reaction 2 4 A type of 100 ml of concentrated sulphuric reaction in which an alkyl group acid. LSD gives a blue-green colour, (from a haloalkane) or an acyl group heroin gives purple to olive green, (from an acyl halide) is substituted on and mescaline gives a greenish col- a benzene ring (see illustration). The our. product is an alkylbenzene (for alkyl substitution) or an alkyl aryl ketone frontier orbital One of two or-

CH3

+ CH3CI

benzene chloromethane methylbenzene (toluene)

FriedelCrafts methylation

C CH3

+ CH3COCI

benzene ethanoyl chloride phenyl methyl ketone FriedelCrafts acetylation

Friedel-Crafts reactions 237 fuel cell bitals in a molecule: the *highest oc- unwanted gangue. The mixture is cupied molecular orbital (HOMO) and ground to a powder and water and a the *lowest unoccupied molecular frothing agent added. Air is blown orbital (LUMO). These two molecular through the water. With a suitable orbitals are usually the most impor- frothing agent, the bubbles adhere tant ones in determining chemical only to particles of ore and carry and spectroscopic properties of the them to the surface, leaving the molecule. gangue particles at the bottom. frontier-orbital theory A theory fructose (fruit sugar; laevulose) A of the reactions of molecules that simple sugar, C6H12O6, stereoiso- emphasizes the energies and symme- meric with glucose (see monosaccha- f tries of *frontier orbitals. Frontier or- ride). (Although natural fructose is bital theory was developed by the the d-form, it is in fact laevorotatory.) Japanese chemist Kenichi Fukui Fructose occurs in green plants, (1919–98) in the 1950s and is an al- fruits, and honey and tastes sweeter ternative approach to the *Wood- than sucrose (cane sugar), of which it ward–Hoffmann rules. It has been is a constituent. Derivatives of fruc- very successful in explaining such re- tose are important in the energy me- actions as the *Diels–Alder reaction. tabolism of living organisms. Some polysaccharide derivatives (fructans) Frost diagram A graph showing are carbohydrate energy stores in how standard electrode potentials certain plants. vary with oxidation state for different oxidation states of an element. fructose 1,6-bisphosphate An in- Frost diagrams can be constructed termediate formed in the initial stage from *Latimer diagrams. For an el- of *glycolysis by the phosphorylation ement M, the standard electrode po- of glucose using ATP. Š tential E is calculated for the fruit sugar See fructose. reaction. FT-IR See fourier-transform in- M(N) + Ne– → M(0) frared. where M(N) indicates the species in 5-fluorouracil oxidation state N and M(0) indicates 5-FU See . the zero oxidation state. The Frost di- fuel A substance that is oxidized or agram is then obtained by plotting otherwise changed in a furnace or NEŠ against N for the different heat engine to release useful heat or species. NEŠ is proportional to the energy. For this purpose wood, veg- standard Gibbs free energy of the etable oil, and animal products have particular half reaction. largely been replaced by *fossil fuels In a Frost diagram, the lowest since the 18th century. point corresponds to the most stable The limited supply of fossil fuels oxidation state of the element. Also, and the expense of extracting them the slope of a line between two from the earth has encouraged the points is the standard potential of development of nuclear fuels to pro- the couple represented by the points. duce electricity. Like Latimer diagrams, Frost dia- fuel cell A cell in which the chemi- grams depend on pH. cal energy of a fuel is converted di- froth Ûotation A method of sepa- rectly into electrical energy. The rating mixtures of solids, used indus- simplest fuel cell is one in which hy- trially for separating ores from the drogen is oxidized to form water fugacity 238

over porous sintered nickel elec- gacity is 1. The activity then equals trodes. A supply of gaseous hydrogen the fugacity. is fed to a compartment containing fullerene See buckminster- the porous anode and a supply of fullerene. oxygen is fed to a compartment con- buckminster- taining the porous cathode; the elec- fullerite See fullerene trodes are separated by a third . compartment containing a hot alka- fuller’s earth A naturally occur- line electrolyte, such as potassium ring clay material (chieÛy montmoril- hydroxide. The electrodes are porous lonite) that has the property of f to enable the gases to react with the decolorizing oil and grease. In the electrolyte, with the nickel in the past raw wool was cleaned of grease electrodes acting as a catalyst. At the and whitened by kneading it in anode the hydrogen reacts with the water with fuller’s earth; a process hydroxide ions in the electrolyte to known as fulling. Fuller’s earth is form water, with the release of two now widely used to decolorize fats electrons per hydrogen molecule: and oils and also as an insecticide H + 2OH– → 2H O + 2e– carrier and drilling mud. The largest 2 2 deposits occur in the USA, UK, and At the cathode, the oxygen reacts Japan. with the water, taking up electrons, cyanic acid to form hydroxide ions: fulminate See . ½ – → – cyanic acid O2 + H2O + 2e 2OH fulminic acid See . The electrons Ûow from the anode to fumaric acid See butenedioic the cathode through an external cir- acid. cuit as an electric current. The device functional group The group of is a more efÜcient converter of elec- atoms responsible for the characteris- tric energy than a heat engine, but it tic reactions of a compound. The is bulky and requires a continuous functional group is –OH for alcohols, supply of gaseous fuels. Their use to –CHO for aldehydes, –COOH for car- power electric vehicles is being ac- boxylic acids, etc. tively explored. fundamental See harmonic. fugacity Symbol f. A thermodynamic function used in place of par- fundamental constants (universal tial pressure in reactions involving constants) Those parameters that do real gases and mixtures. For a compo- not change throughout the universe. nent of a mixture, it is deÜned by dµ The charge on an electron, the speed = RTd(lnf), where µ is the chemical of light in free space, the Planck constant, the gravitational constant, the potential. It has the same units as electric constant, and the magnetic pressure and the fugacity of a gas is constant are all thought to be exam- equal to the pressure if the gas is ples. ideal. The fugacity of a liquid or solid A is the fugacity of the vapour with which it is in equilibrium. The ratio • fundamental physical constants from of the fugacity to the fugacity in NIST some standard state is the *activity. fundamental units A set of inde- For a gas, the standard state is cho- pendently deÜned *units of measure- sen to be the state at which the fu- ment that forms the basis of a 239 fusion system of units. Such a set requires on standing in air. It is the aldehyde three mechanical units (usually of derivative of *furan and occurs in length, mass, and time) and one elec- various essential oils and in *fusel trical unit; it has also been found oil. It is used as a solvent for extract- convenient to treat certain other ing mineral oils and natural resins quantities as fundamental, even and itself forms resins with some though they are not strictly inde- aromatic compounds. pendent. In the metric system the ring centimetre–gram–second (c.g.s.) fused ring See . system was replaced by the metre– A kilogram–second (m.k.s.) system; the • Information about IUPAC nomenclature f latter has now been adapted to provide the basis for *SI units. In British fusel oil A mixture of high- Imperial units the foot–pound– molecular weight *alcohols contain- second (f.p.s.) system was formerly ing also esters and fatty acids, some- used. times formed as a toxic impurity in the distillation products of alcoholic pesticide fungicide See . fermentation. It is used as a source of furan A colourless liquid com- higher alcohols and in making paints ° pound, C4H4O; r.d. 0.94; m.p. –86 C; and plastics. b.p. 31.4°C. It has a Üve-membered fusible alloys Alloys that melt at ring consisting of four CH groups 2 low temperature (around 100°C). and one oxygen atom. They have a number of uses, including constant-temperature baths, pipe bending, and automatic sprinklers to O provide a spray of water to prevent Üres from spreading. Fusible alloys Furan are usually *eutectic mixtures of bis- furanose A *sugar having a Üve- muth, lead, tin, and cadmium. membered ring containing four car- *Wood’s metal, *Rose’s metal, and bon atoms and one oxygen atom. Lipowitz’s alloy are examples of alloys that melt in the range 70–100°C. furfural A colourless liquid, ° C5H4O2, b.p. 162 C, which darkens fusion Melting. G

GABA See gamma-aminobutyric frequently in association with silver, acid. arsenic, copper, zinc, and antimony. Important deposits occur in Australia Gabriel reaction A method of (at Broken Hill), Germany, the USA making a primary *amine (free from (especially in Missouri, Kansas, and any secondary or tertiary amine im- Oklahoma), and the UK. purities) from a haloalkane (alkyl halide) using potassium phthalimide. gallic acid (3,4,5-trihydroxybenzoic It is named after Siegmund Gabriel acid) A colourless crystalline aro- (1851–1924). matic compound, C6H2(OH)3COOH; m.p. 253°C. It occurs in wood, oak gadolinium Symbol Gd. A soft sil- galls, and tea, and is a component of very metallic element belonging to tannins. It can be made from tannin the *lanthanoids; a.n. 64; r.a.m. by acid hydrolysis or fermentation. ° ° 157.25; r.d. 7.901 (20 C); m.p. 1313 C; Gallic acid is used to make ink and ° b.p. 3266 C. It occurs in gadolinite, various dyes. On heating it yields xenotime, monazite, and residues *pyrogallol. from uranium ores. There are seven stable natural isotopes and eleven OH artiÜcial isotopes are known. Two of the natural isotopes, gadolinium–155 OH OH and gadolinium–157, are the best neutron absorbers of all the elements. The metal has found limited applications in nuclear technology and in ferromagnetic alloys (with cobalt, copper, iron, and cerium). OH O Gadolinium compounds are used in Gallic acid electronic components. The element was discovered by Jean de Marignac gallium Symbol Ga. A soft silvery (1817–94) in 1880. metallic element belonging to group A 13 (formerly IIIB) of the periodic • Information from the WebElements site table; a.n. 31; r.a.m. 69.72; r.d. 5.90 (20°C); m.p. 29.78°C; b.p. 2403°C. It galactose A simple sugar, C6H12O6, stereoisomeric with glucose, that oc- occurs in zinc blende, bauxite, and curs naturally as one of the products kaolin, from which it can be extracted by fractional electrolysis. It of the enzymic digestion of milk also occurs in gallite, CuGaS , to an sugar (lactose) and as a constituent of 2 extent of 1%; although bauxite only gum arabic. contains 0.01% this is the only com- galena A mineral form of lead(II) mercial source. The two stable iso- sulphide, PbS, crystallizing in the topes are gallium–69 and gallium–71; cubic system; the chief ore of lead. It there are eight radioactive isotopes, usually occurs as grey metallic cubes, all with short half-lives. The metal 241 gas has only a few minor uses (e.g. as an synthesized by *decarboxylation of activator in luminous paints), but gal- the amino acid glutamate. lium arsenide is extensively used as a gammahydroxybutyric acid See semiconductor in many applications. 4-hydroxybutanoic acid. Gallium corrodes most other metals because it rapidly diffuses into their gamma-iron See iron. lattices. Most gallium(I) and some gal- gamma radiation Electromag- lium(II) compounds are unstable. The Ü Ü netic radiation emitted by excited element was rst identi ed by Paul atomic nuclei during the process of Lecoq de Boisbaudran (1838–1912) in passing to a lower excitation state. 1875. Gamma radiation ranges in energy A from about 10–15 to 10–10 joule (10 • Information from the WebElements site keV to 10 MeV) corresponding to a g –10 GALP See glyceraldehyde 3-phos- wavelength range of about 10 to –14 phate. 10 metre. A common source of gamma radiation is cobalt–60, the galvanic cell See voltaic cell. decay process of which is: 60 β 60 γ 60 galvanized iron Iron or steel that 27Co → 28Ni → 28Ni has been coated with a layer of zinc The de-excitation of nickel–60 is ac- to protect it from corrosion in a companied by the emission of process invented by Luigi Galvani. gamma-ray photons having energies Corrugated mild-steel sheets for 1.17 MeV and 1.33 MeV. rooÜng and mild-steel sheets for dust- bins, etc., are usually galvanized by gangue Rock and other waste ma- dipping them in molten zinc. The terial present in an ore. formation of a brittle zinc–iron alloy garnet Any of a group of silicate is prevented by the addition of small minerals that conform to the general quantities of aluminium or magne- formula A3B2(SiO4)3. The elements sium. Wire is often galvanized by a representing A may include magne- cold electrolytic process as no alloy sium, calcium, manganese, and forms in this process. Galvanizing is iron(II); those representing B may an effective method of protecting include aluminium, iron(III), steel because even if the surface is chromium, or titanium. Six varieties scratched, the zinc still protects the of garnet are generally recognized: underlying metal. See sacrificial pyrope, Mg3Al2Si3O12; protection. 2+ almandine, Fe3 Al2Si3O12; gamma-aminobutyric acid spessartite, Mn3Al2Si3O12; (GABA) An inhibitory neurotransmit- grossularite, Ca3Al2Si3O12; 3+ ter in the central nervous system andradite, Ca3(Fe ,Ti)2Si3O12; (principally the brain) that is capable uvarovite, Ca3Cr2Si3O12. of increasing the permeability of Varieties of garnet are used as gem- postsynaptic membranes. GABA is stones and abrasives. gas A state of matter in which the O matter concerned occupies the whole H2 of its container irrespective of its 2NH C quantity. In an *ideal gas, which C C OH obeys the *gas laws exactly, the mol- H H 2 2 ecules themselves would have a neg- Gamma-aminobutyric acid ligible volume and negligible forces gas chromatography 242

between them, and collisions be- in molecules are characteristic in in- tween molecules would be perfectly frared spectra this produces informa- elastic. In practice, however, the be- tion to supplement that obtained by haviour of real gases deviates from mass spectrometry. It is easier to per- the gas laws because their molecules form FT-IR for gas chromatography occupy a Ünite volume, there are than for liquid chromatography be- small forces between molecules, and cause carrier solvents in liquid in polyatomic gases collisions are to a chromatography absorb infrared certain extent inelastic (see equation radiation. of state). gas constant (universal molar gas gas chromatography A technique constant) Symbol R. The constant g for separating or analysing mixtures that appears in the universal gas of gases by *chromatography. The equation (see gas laws). It has the apparatus consists of a very long tube value 8.314 510(70) J K–1 mol–1. containing the stationary phase. This gas equation See gas laws. may be a solid, such as kieselguhr (gas–solid chromatography, or GSC), gasiÜcation The conversion of or a nonvolatile liquid, such as a hy- solid or liquid hydrocarbons to fuel drocarbon oil coated on a solid sup- gas. Solid fuels such as coal or coke port (gas–liquid chromatography, or are converted into producer gas (car- GLC). The sample is often a volatile bon monoxide) or water gas (carbon liquid mixture, which is vaporized monoxide and hydrogen) by the ac- and swept through the column by a tion of air (or oxygen) and steam. carrier gas (e.g. hydrogen). The com- Solid fuels may also be hydrogenated ponents of the mixture pass through to produce methane. Liquid fuels, the column at different rates because from petroleum, are gasiÜed to pro- they adsorb to different extents on duce synthesis gas (carbon monoxide the stationary phase. They are de- and hydrogen) or town gas (mostly tected as they leave, either by meas- hydrogen and methane), usually by uring the thermal conductivity of the *cracking or *hydrogenation. Û gas or by a ame detector. gas laws Laws relating the temper- Gas chromatography is usually ature, pressure, and volume of an used for analysis; components can be Ü *ideal gas. *Boyle’s law states that identi ed by the time they take to the pressure (p) of a specimen is in- pass through the column. It is some- versely proportional to the volume times also used for separating mix- (V) at constant temperature (pV = con- tures. stant). The modern equivalent of Gas chromatography is often used *Charles’ law states that the volume to separate a mixture into its compo- is directly proportional to the ther- nents, which are then directly in- modynamic temperature (T) at con- jected into a mass spectrometer. This stant pressure (V/T = constant); technique is known as gas chro- originally this law stated the con- matography–mass spectroscopy or stant expansivity of a gas kept at con- GCMS. stant pressure. The pressure law gas chromatography infrared states that the pressure is directly (GC-IR) A form of *Fourier-transform proportional to the thermodynamic infrared (FT-IR) used to identify small temperature for a specimen kept at amounts of gas obtained by gas chro- constant volume. The three laws can matography. Since functional groups be combined in the universal gas 243 Gattermann reaction equation, pV = nRT, where n is the ally by means of a mercury manome- amount of gas in the specimen and R ter and a barometer. * is the gas constant. The gas laws Gattermann–Koch reaction A re- were Ürst established experimentally → action of the type C6H6 C6H5CHO, for real gases, although they are used to introduce aldehyde groups obeyed by real gases to only a limited onto benzene rings. It is used in the extent; they are obeyed best at high industrial manufacture of benzalde- temperatures and low pressures. See hyde. A mixture of hydrogen chlo- equation of state also . ride and carbon monoxide is passed gasohol A mixture of petrol (gaso- through benzene using a Lewis acid line) and alcohol (i.e. typically such as aluminium chloride as a cata- ethanol at 10%, or methanol at 3%), lyst. An intermediate H–CϵOC+ is g used as an alternative fuel for cars formed and electrophilic substitution and other vehicles in many coun- takes place on the benzene ring. tries. The ethanol is obtained as a There is a variation of the reaction, *biofuel by fermentation of agricul- sometimes simply called the Gatter- tural crops or crop residues, for ex- mann reaction, in which the –CHO ample sugar cane waste. Many cars group is substituted onto the ben- can also use a mixture of 85% ethanol zene ring of a phenol. In this reac- and 15% petrol, called E85. Ethanol- tion hydrogen cyanide is used rather based gasohol has a higher octane than carbon monoxide. A mixture of rating and burns more completely zinc cyanide and hydrochloric acid than conventional petrol, thus lower- produces the hydrogen cyanide along ing some emissions. However, the with zinc chloride to act as the cata- + ethanol can damage certain engine lyst. The electrophile is HCNH components, such as rubber seals. which produces an imine intermedi- Methanol-based gasohol is more toxic ate → and corrosive, and its emissions in- C6H5OH HOC6H4CH=NH clude formaldehyde, a known car- This then hydrolyses to the aldehyde cinogen. HOC6H4CHO. If an organic cyanide gas oil A high-density petroleum (nitrite) RCN is used, a ketone is pro- fraction (between kerosene and lubri- duced. The reaction was discovered cating oil), whose molecules have up by Ludwig Gattermann and J.C. Koch to 25 carbon atoms. It is used as a do- in 1897. The use of hydrogen cyanide mestic and industrial heating fuel. was reported by Gattermann in 1907. Gattermann reaction gasoline See petroleum. A variation of the *Sandmeyer reaction for gas-phase electrophoresis See preparing chloro- or bromoarenes by ion-mobility spectrometry. reaction of the diazonium com- gas thermometer A device for pound. In the Gattermann reaction measuring temperature in which the the aromatic amine is added to Û sodium nitrite and the halogen acid working uid is a gas. It provides the ° most accurate method of measuring (10 C), then fresh copper powder temperatures in the range 2.5 to (e.g. from Zn + CuSO4) is added and the solution warmed. The diazonium 1337 K. Using a Üxed mass of gas a salt then forms the haloarene, e.g. constant-volume thermometer meas- + – → ures the pressure of a Üxed volume C6H5N2 Cl C6H5Cl + N2 of gas at relevant temperatures, usu- The copper acts as a catalyst. The re- gauche 244

action is easier to perform than the ate it towards its appropriate elec- Sandmeyer reaction and takes place trode, causing an avalanche of fur- at lower temperature, but generally ther ionizations by collision. The gives lower yields. It was discovered consequent current pulses can be in 1890 by the German chemist Lud- counted in electronic circuits or sim- wig Gattermann (1860–1920). See also ply ampliÜed to work a small loud- gattermann–koch reaction. speaker in the instrument. It was Ü gauche See conformation; tor- rst devised in 1908 by the German sion angle. physicist Hans Geiger (1882–1945). Geiger and W. Müller produced an Gay-Lussac, Joseph (1778–1850) improved design in 1928. French chemist and physicist whose g discovery of the laws of chemical gel A lyophilic *colloid that has co- combination in gases helped to estab- agulated to a rigid or jelly-like solid. lish the atomic theory. It also led to In a gel, the disperse medium has *Avogadro’s law. See also charles’ formed a loosely-held network of law. linked molecules through the dispersion medium. Examples of gels are Gay Lussac’s law 1. When gases silica gel and gelatin. combine chemically the volumes of the reactants and the volume of the gelatin(e) A colourless or pale yel- product, if it is gaseous, bear simple low water-soluble protein obtained relationships to each other when by boiling collagen with water and measured under the same conditions evaporating the solution. It swells of temperature and pressure. The law when water is added and dissolves in was Ürst stated in 1808 by J. L. Gay- hot water to form a solution that sets Lussac and led to *Avogadro’s law. to a gel on cooling. It is used in pho- 2. See charles’ law. tographic emulsions and adhesives, gaylussite A mineral consisting and in jellies and other foodstuffs. of a hydrated mixed carbonate of gel electrophoresis See elec- sodium and calcium, Na2CO3. trophoresis. CaCO .5H O. 3 2 gel Ültration A type of column GC-IR See gas chromatography in- *chromatography in which a mix- frared. ture of liquids is passed down a col- GCMS See gas chromatography. umn containing a gel. Small molecules in the mixture can enter Geiger counter (Geiger–Müller pores in the gel and move slowly counter) A device used to detect and down the column; large molecules, measure ionizing radiation. It con- which cannot enter the pores, move sists of a tube containing a low- more quickly. Thus, mixtures of mol- pressure gas (usually argon or neon ecules can be separated on the basis with methane) and a cylindrical hol- of their size. The technique is used low cathode through the centre of Ü particularly for separating proteins which runs a ne-wire anode. A po- but it can also be applied to other tential difference of about 1000 volts polymers and to cell nuclei, viruses, is maintained between the elec- etc. trodes. An ionizing particle or photon passing through a window into gelignite A high explosive made the tube will cause an ion to be pro- from nitroglycerin, cellulose nitrate, duced and the high p.d. will acceler- sodium nitrate, and wood pulp. 245 Gibbs, Josiah Willard gem Designating molecules in 5.36; m.p. 937°C; b.p. 2830°C. It is which two functional groups are found in zinc sulphide and in certain attached to the same atom in a other sulphide ores, and is mainly molecule. For example, 1,1- obtained as a by-product of zinc dichloroethane (CH3CHCl2) is a gem smelting. It is also present in some dihalide and can be named gem- coal (up to 1.6%). Small amounts are dichloroethane. Compare vicinal. used in specialized alloys but the main use depends on its semiconduc- geminate pair A pair of molecules, ions, etc., in close proximity tor properties. Chemically, it forms surrounded by a solvent cage (see compounds in the +2 and +4 oxida- cage effect). tion states, the germanium(IV) compounds being the more stable. The geochemistry The scientiÜc study element also forms a large number g of the chemical composition of the of organometallic compounds. Pre- earth. It includes the study of the dicted in 1871 by *Mendeleev (eka- abundance of the earth’s elements silicon), it was discovered by Winkler and their isotopes and the distribu- in 1886. tion of the elements in environments A of the earth (lithosphere, atmos- • Information from the WebElements site phere, biosphere, and hydrosphere). German silver (nickel silver) An geometrical isomerism See iso- alloy of copper, zinc, and nickel, merism. often in the proportions 5:2:2. It re- gerade Symbol g. Describing a mo- sembles silver in appearance and is lecular orbital of a homonuclear di- used in cheap jewellery and cutlery atomic molecule with even parity and as a base for silver-plated wire. (gerade is the German word for even). See also electrum. This means that during the process getter A substance used to remove of inversion, the sign of the orbital is small amounts of other substances unchanged upon going from any from a system by chemical combina- point in the molecule through the tion. For example, a metal such as centre of inversion to the corre- magnesium may be used to remove sponding point on the other side. the last traces of air when achieving The symbol g is written as a sub- a high vacuum. Various getters are script. The opposite of gerade is also employed to remove impurities ungerade, symbol u. The symbols g from semiconductors. and u are used to determine selection rules for diatomic molecules. These GHB Gammahydroxybutyric acid. symbols are only applicable to See 4-hydroxybutanoic acid. homonuclear diatomic molecules, as gibberellic acid (GA3) A plant heteronuclear diatomic molecules, growth substance, abundant in such as CO, do not have a centre of young actively growing areas of the inversion. plant, that is involved in stem elon- geraniol An alcohol, C9H15CH2OH, gation. A *terpene, it was discovered present in a number of essential oils. in 1954. Gibberellic acid and related growth substances are called gib- germanium Symbol Ge. A lustrous berellins. hard metalloid element belonging to group 14 (formerly IVB) of the peri- Gibbs, Josiah Willard (1839–1903) odic table; a.n. 32; r.a.m. 72.59; r.d. US mathematician and physicist, Gibbs–Duhem equation 246

who spent his entire academic career giga- Symbol G. A preÜx used in the at Yale University. During the 1870s metric system to denote one thou- he developed the theory of chemical sand million times. For example, 109 thermodynamics, devising functions joules = 1 gigajoule (GJ). such as Gibbs *free energy; he also gilbert derived the *phase rule. In mathe- Symbol Gb. The c.g.s. unit matics he introduced vector notation. of magnetomotive force equal to 10/4π (= 0.795 77) ampere-turn. It is Gibbs–Duhem equation An equa- named after the English physician tion describing the relation between and physicist William Gilbert the chemical potentials of species in (1544–1603), who studied magnet- a mixture. If ni is the amount of ism. µ species i and i is the chemical poten- g tial of species i, the Gibbs–Duhem glacial ethanoic acid See ethanoic acid equation states that . ∑ µ nid i = 0 glass Any noncrystalline solid; i.e. a i This equation implies that the chemi- solid in which the atoms are random cal potentials for the species in a and have no long-range ordered pat- mixture do not change indepen- tern. Glasses are often regarded as su- dently. Thus, in the case of a binary percooled liquids. Characteristically mixture, if the chemical potential of they have no deÜnite melting point, one species increases, the chemical but soften over a range of tempera- potential of the other species must tures. decrease. The equation was derived The common glass used in win- independently by J. W. *Gibbs and dows, bottles, etc., is soda glass, the French physicist P. Duhem which is made by heating a mixture (1861–1916). of lime (calcium oxide), soda (sodium carbonate), and sand (silicon(IV) Gibbs free energy (Gibbs function) oxide). It is a form of calcium silicate. See free energy. Borosilicate glasses (e.g. Pyrex) are Gibbs–Helmholtz equation An made by incorporating some boron equation used in thermodynamics to oxide, so that silicon atoms are re- show the temperature dependence of placed by boron atoms. They are the *Gibbs free energy. It has the tougher than soda glass and more re- form: sistant to temperature changes, ∂ ∂ ( G/ T)p = (G – H)/T, hence their use in cooking utensils where G is the Gibbs free energy, H is and laboratory apparatus. Glasses for the enthalpy, T is the thermody- special purposes (e.g. optical glass) namic temperature, and p is the pres- have other elements added (e.g. bar- sure (which is held constant). The ium, lead). See also spin glass. Gibbs–Helmholtz equation can be de- glass electrode A type of *half ∂ ∂ rived from ( G/ T)p = – S and S = (H – cell having a glass bulb containing an G)T using the rules of differentiation. acidic solution of Üxed pH, into The equation was derived by J. W. which dips a platinum wire. The *Gibbs and H. L. F. von *Helmholtz. glass bulb is thin enough for hydro- gibbsite A mineral form of hy- gen ions to diffuse through. If the drated *aluminium hydroxide bulb is placed in a solution contain- (Al(OH)3). It is named after the US ing hydrogen ions, the electrode po- mineralogist George Gibbs (d. 1833). tential depends on the hydrogen-ion 247 glutaric acid concentration. Glass electrodes are duced by the decay of excited atoms used in pH measurement. and molecules. glass Übres Melted glass drawn gluconic acid An optically into thin Übres some 0.005 mm–0.01 active hydroxycarboxylic acid, Ü mm in diameter. The bres may be CH2(OH)(CHOH)4COOH. It is the car- spun into threads and woven into boxylic acid corresponding to the al- fabrics, which are then impregnated dose sugar glucose, and can be made with resins to give a material that is by the action of certain moulds. both strong and corrosion resistant. glucosan Any one of a class of It is used in car bodies, boat building, *polysaccharide compounds that can and similar applications. be converted to glucose by hydroly- glauberite A mineral consisting of sis. Glucosans include dextrin, starch, g a mixed sulphate of sodium and cal- and cellulose. cium, Na2SO4.CaSO4. glucose (dextrose; grape sugar) A Glauber’s salt *Sodium sulphate white crystalline sugar, C6H12O6, oc- decahydrate, Na2SO4.10H2O, used as curring widely in nature. Like other a laxative. It is named after Johann *monosaccharides, glucose is opti- Glauber (1604–68). cally active: most naturally occurring glucose is dextrorotatory. Glucose GLC (gas–liquid chromatography) and its derivatives are crucially im- See gas chromatography. portant in the energy metabolism of global warming See greenhouse living organisms. Glucose is also a effect. constituent of many polysaccharides, globin See haemoglobin. most notably starch and cellulose. These yield glucose when broken globular protein See protein. down, for example by enzymes dur- globulin Any of a group of globular ing digestion. proteins that are generally insoluble glucuronic acid A compound, in water and present in blood, eggs, OC6H9O6, derived from the oxidation milk, and as a reserve protein in of glucose. It is an important con- seeds. Blood serum globulins com- stituent of gums and mucilages. Glu- α α β prise four types: 1-, 2-, and - curonic acid can combine with globulins, which serve as carrier pro- hydroxyl (–OH), carboxyl (–COOH), or γ teins; and -globulins, which include amino (–NH2) groups to form a glu- the immunoglobulins responsible for curonide. The addition of a glu- immune responses. curonide group to a molecule glove box A metal box that has (glucuronidation) generally increases gloves Ütted to ports in its walls. It is the solubility of a compound; hence used to manipulate mildly radio- glucuronidation plays an important active materials and in laboratory role in the excretion of foreign sub- techniques in which an inert, sterile, stances. dry, or dust-free atmosphere has to glucuronide See glucuronic acid. be maintained. glutamic acid See amino acid. glow discharge An electrical dis- glutamine See amino acid. charge that passes through a gas at low pressure and causes the gas to glutaric acid See pentanedioic become luminous. The glow is pro- acid. glyceraldehyde 3-phosphate 248

glyceraldehyde 3-phosphate store of carbohydrate energy in ani- (GALP) A triose phosphate, mal cells and is present as granular CHOCH(OH)CH2OPO3H2, that is an in- clusters of minute particles. termediate in the *Calvin cycle (see photosynthesis glycogenesis The conversion of also ) and glycolysis. glucose to glycogen, which is stimu- glycerate 3-phosphate A phos- lated by insulin from the pancreas. phorylated three-carbon monosac- Glycogenesis occurs in skeletal mus- charide that is an intermediate in the cles and to a lesser extent in the *Calvin cycle of photosynthesis and liver. Glucose that is taken up by also in *glycolysis. It was formerly cells is phosphorylated to glucose 6- known as 3-phosphoglycerate or phos- phosphate; this is converted succes- phoglyceric acid (PGA). sively to glucose 1-phosphate, uridine g diphosphate glucose, and Ünally to glycerides (acylglycerols) Fatty-acid glycogen. Compare glycogenolysis. esters of glycerol. EsteriÜcation can occur at one, two, or all three hy- glycogenolysis The conversion of droxyl groups of the glycerol mol- glycogen to glucose, which occurs in ecule producing mono-, di-, and the liver and is stimulated by triglycerides respectively. *Triglyc- glucagon from the pancreas and erides are the major constituent of adrenaline from the adrenal medulla. fats and oils found in living organ- These hormones activate an enzyme isms. Alternatively, one of the hy- that phosphorylates glucose mol- droxyl groups may be esteriÜed with ecules in the glycogen chain to form a phosphate group forming a phos- glucose 1-phosphate, which is con- phoglyceride (see phospholipid) or to verted to glucose 6-phosphate. This is a sugar forming a *glycolipid. then converted to glucose by a phos- phatase enzyme. In skeletal muscle glycerine See glycerol. glycogen is degraded to glucose 6- glycerol (glycerine; propane- phosphate, which is then converted 1,2,3,-triol) A trihydric alcohol, into pyruvate and used in ATP production during glycolysis and the HOCH2CH(OH)CH2OH. Glycerol is a colourless sweet-tasting viscous liq- Krebs cycle. However, pyruvate can uid, miscible with water but insolu- also be converted, in the liver, to glu- ble in ether. It is widely distributed cose; thus muscle glycogen is indirectly a source of blood glucose. in all living organisms as a con- glycogenesis stituent of the *glycerides, which Compare . yield glycerol when hydrolysed. glycol See ethane-1,2-diol. glycerophospholipids See phos- glycolic acid (hydroxyethanoic pholipids. acid) A colourless crystalline com- ° glycine See amino acid. pound, CH2(OH)COOH; m.p. 80 C. It occurs in sugar cane and sugar beet, glycobiology The study of carbo- and is made by the electrolytic re- hydrates and carbohydrate com- duction of oxalic acid or by boiling a plexes, especially *glycoproteins. solution of sodium monochloro- glycogen (animal starch) A *poly- ethanoate. Glycolic acid is used in saccharide consisting of a highly making textiles and leather and for branched polymer of glucose occur- cleaning metals. ring in animal tissues, especially in glycolipid Any of a group of sugar- liver and muscle cells. It is the major containing lipids, in which the lipid 249 glycosidic bond portion of the molecule is usually the receptors of leucocytes. Viral gly- based on glycerol (see glyceride) or coproteins can also act as target mol- sphingosine and the sugar is typically ecules and help viruses to detect galactose, glucose, or inositol. Glycol- certain types of host cell; for exam- ipids are components of biological ple, a glycoprotein on the surface of membranes. In animal plasma mem- HIV (the AIDS virus) enables the virus branes they are found in the outer to Ünd and infect white blood cells. layer of the lipid bilayer; the simplest glycosaminoglycan Any one of a animal glycolipids are the *cerebro- group of polysaccharides that contain sides. Plant glycolipids are glycerides amino sugars (such as glucosamine). in which the sugar group is most Formerly known as mucopolysaccha- commonly galactose. They are the rides, they include *hyaluronic acid principal lipid constituents of chloro- and chondroitin, which provide lu- g plasts. brication in joints and form part of glycolysis (Embden–Meyerhof path- the matrix of cartilage. The three- way) The series of biochemical reac- dimensional structure of these mol- tions in which glucose is broken ecules enables them to trap water, down to pyruvate with the release of which forms a gel and gives gly- usable energy in the form of *ATP. cosaminoglycans their elastic proper- One molecule of glucose undergoes ties. two phosphorylation reactions glycoside Any one of a group of and is then split to form two triose- compounds consisting of a pyranose phosphate molecules. Each of these sugar residue, such as glucose, linked is converted to pyruvate. The net en- to a noncarbohydrate residue (R) by a ergy yield is two ATP molecules per *glycosidic bond: the hydroxyl group glucose molecule. In aerobic respira- (–OH) on carbon-1 of the sugar is re- tion pyruvate then enters the *Krebs placed by –OR. Glycosides are widely cycle. Alternatively, when oxygen is distributed in plants; examples are in short supply or absent, the pyru- the *anthocyanin pigments and the vate is converted to various products cardiac glycosides, such as digoxin by anaerobic respiration. Other sim- and ouabain, which are used medici- ple sugars, e.g. fructose and galac- nally for their stimulant effects on tose, and glycerol (from fats) enter the heart. the glycolysis pathway at intermediate stages. glycosidic bond (glycosidic link) The type of chemical linkage be- glycoprotein A carbohydrate tween the monosaccharide units of linked covalently to a protein. disaccharides, oligosaccharides, and Formed in the Golgi apparatus in the polysaccharides, which is formed by process of *glycosylation, glycopro- the removal of a molecule of water teins are important components of (i.e. a *condensation reaction). The cell membranes. They are also con- bond is normally formed between stituents of body Ûuids, such as the carbon-1 on one sugar and the mucus, that are involved in lubrica- carbon-4 on the other. An α-glyco- tion. Many of the hormone receptors sidic bond is formed when the –OH on the surfaces of cells have been group on carbon-1 is below the plane identiÜed as glycoproteins. Glycopro- of the glucose ring and a β-glycosidic teins produced by viruses attach bond is formed when it is above the themselves to the surface of the host plane. Cellulose is formed of glucose cell, where they act as markers for molecules linked by 1-4 β-glycosidic glycosidic link 250

CH2OH CH2OH CH2OH CH2OH

O O condensation O H H O

C1 C4 C1 C4 + H O OH OH OH OH 2 O OH OH OH OH OH OH 1–4 α-glycosidic OH OH OH bond OH

glucose glucose maltose water Glycosidic bond

bonds, whereas starch is composed be measured from the pressure rise g of 1-4 α-glycosidic bonds. in the tube. glycosidic link See glycoside. gold Symbol Au. A soft yellow mal- glycosylation The process in leable metallic *transition element; which a carbohydrate is joined to an- a.n. 79; r.a.m. 196.967; r.d. 19.32; ° ± ° other molecule, such as a protein to m.p. 1064.43 C; b.p. 2807 2 C. Gold form a *glycoprotein or to a lipid to has a face-centred-cubic crystal struc- form a glycolipid (see glyceride). Gly- ture. It is found as the free metal in cosylation occurs in the rough endo- gravel or in quartz veins, and is also plasmic reticulum and the Golgi present in some lead and copper sul- apparatus of cells. phide ores. It also occurs combined with silver in the telluride sylvanite, glyoxal (ethanedial) A low melting- (Ag,Au)Te . It is used in jewellery, point yellow crystalline solid, CHO- 2 dentistry, and electronic devices. CHO, m.p. 15°C. It is a dialdehyde. It gives off a green vapour when heated Chemically, it is unreactive, being unaffected by oxygen. It reacts with and burns with a violet-coloured ° Ûame. Glyoxal is made by the cat- chlorine at 200 C to form gold(III) alytic oxidation of ethanediol and is chloride. It forms a number of com- used to harden the gelatin employed plexes with gold in the +1 and +3 ox- for photographic emulsions. idationA states. goethite A yellow-brown mineral, FeO.OH, crystallizing in the or- • Information from the WebElements site thorhombic system. It is formed as a Goldschmidt process A method result of the oxidation and hydration of extracting metals by reducing the of iron minerals or as a direct precip- oxide with aluminium powder, e.g. itate from marine or fresh water (e.g. Cr O + 2Al → 2Cr + Al O in swamps and bogs). Most *limonite 2 3 2 3 is composed largely of cryptocrys- The reaction can also be used to pro- talline goethite. Goethite is mined as duce molten iron (see thermite). It an ore of iron. was discovered by the German chemist Hans Goldschmidt Golay cell A transparent cell con- (1861–1923). taining gas, used to detect *infrared radiation. Incident radiation is ab- Gooch crucible A porcelain dish sorbed within the cell, causing a rise with a perforated base over which a in the gas temperature and pressure. layer of asbestos is placed, used for The amount of incident radiation can Ültration in gravimetric analysis. It is 251 greenhouse effect named after the US chemist Frank Graham, Thomas (1805–69) Scot- Gooch (1852–1929). tish chemist, who became professor Gouy balance A method of meas- of chemistry at Glasgow University in 1830, moving to University College, uring magnetic susceptibility. The London, in 1837. His 1829 paper on sample is suspended from a balance, gaseous diffusion introduced *Gra- with the bottom part of the sample ham’s law. He went on to study diffu- between the poles of an electromag- sion in liquids, leading in 1861 to the net. When the magnetic Üeld is deÜnition of *colloids. switched on, the sample experiences a Üeld gradient which causes an ap- Graham’s law The rates at which parent change in weight. In particu- gases diffuse is inversely propor- lar, paramagnetic substances show tional to the square roots of their g an increase in weight, which, after densities. This principle is made use correction for a smaller diamagnetic of in the diffusion method of separat- contribution, can be used to calculate ing isotopes. The law was formulated the paramagnetic part of the suscep- in 1829 by Thomas *Graham. tibility. This can be used to calculate gram Symbol g. One thousandth of the number of unimpaired electrons a kilogram. The gram is the funda- in the sample. Magnetic measure- mental unit of mass in *c.g.s. units ments of this type are widely used to and was formerly used in such units investigate the electronic structures as the gram-atom, gram-molecule, of metal complexes. The Evans bal- and gram-equivalent, which have ance is a portable version of the now been replaced by the *mole. Gouy balance using permanent magnets and giving a direct readout. grape sugar See glucose. Other methods of measuring mag- graphite See carbon. netic susceptibility include magnetic resonance techniques and the use of graphitic compounds Substances a SQID (superconducting quantum in which atoms or molecules are interference device). trapped between the layers in graphite. See lamellar solids. Gouy–Chapman model A model of the *electrical double layer in gravimetric analysis A type of which thermal motion causing disor- quantitative analysis that depends on dering of the layer is taken into ac- weighing. For instance, the amount count. This is very similar to the of silver in a solution of silver salts *Debye–Hückel theory of the ionic could be measured by adding excess hydrochloric acid to precipitate silver atmosphere around an ion, except Ü that the concept of a central single chloride, ltering the precipitate, ion is replaced by that of an inÜnite washing, drying, and weighing. plane electrode. The Gouy–Chapman gray Symbol Gy. The derived SI unit model understates the structure in a of absorbed dose of ionizing radia- double layer, but can be improved by tion (see radiation units). It is the Stern model, in which the ions named after the British radiobiologist closest to the electrode are ordered L. H. Gray (1905–65). and the ions are described by the greenhouse effect An effect oc- Gouy–Chapman model outside the curring in the atmosphere because of Ürst layer. the presence of certain gases (green- graft copolymer See polymer. house gases) that absorb infrared ra- greenhouse gas 252

diation. Light and ultraviolet radia- C2H5MgBr, etc.). They actually have tion from the sun are able to pen- the structure R2Mg.MgCl2, and can be etrate the atmosphere and warm the made by reacting a haloalkane with earth’s surface. This energy is re-radi- magnesium in ether; they are rarely ated as infrared radiation, which, be- isolated but are extensively used in cause of its longer wavelength, is organic synthesis, when they are absorbed by such substances as car- made in one reaction mixture. Grig- bon dioxide. Emissions of carbon nard reagents have a number of reac- dioxide from human activities have tions that make them useful in increased markedly in the last 150 organic synthesis. With methanal years or so. The overall effect is that they give a primary alcohol the average temperature of the earth CH MgCl + HCHO → CH CH OH g and its atmosphere is increasing (so- 3 3 2 called global warming). The effect is Other aldehydes give a secondary al- similar to that occurring in a green- cohol → house, where light and long-wave- CH3CHO + CH3MgCl length ultraviolet radiation can pass (CH3)2CHOH through the glass into the green- With alcohols, hydrocarbons are house but the infrared radiation is formed absorbed by the glass and part of it is CH MgCl + C H OH → C H CH re-radiated into the greenhouse. 3 2 5 2 5 3 The greenhouse effect is seen as a Water also gives a hydrocarbon → major environmental hazard. Aver- CH3MgCl + H2O CH4 age increases in temperature are The compounds are named after likely to change weather patterns their discoverer, the French chemist and agricultural output. It is already Victor Grignard (1871–1935). causing the polar ice caps to melt, with a corresponding rise in sea Grotrian diagram A diagram that level. Carbon dioxide, from fossil-fuel summarizes the energy levels and power stations and car exhausts, is the *allowed transitions between the main greenhouse gas. Other con- these energy levels in an atom. The tributory pollutants are nitrogen energy is plotted vertically with a oxides, ozone, methane, and horizontal line for each energy level. chloroÛuorocarbons. The intensity of the transition can be represented on a Grotrian diagram greenhouse greenhouse gas See by allowing the thickness of the line effect . to represent the transition propor- greenockite A mineral form of tional to the intensity. Grotrian dia- cadmium sulphide, CdS. grams are named after the German spectroscopist W. Grotrian, who in- green vitriol See iron(ii) sulphate. vented them in 1928. Griess test A test for nitrates or ni- Grottius–Draper law A law in trites, once widely used to detect the photochemistry stating that only the possible existence of gunshot light absorbed by a substance or sub- residue. stances is effective in bringing about Grignard reagents A class of chemical change. Not all the light organometallic compounds of mag- falling on the substances will neces- nesium, with the general formula sarily bring about chemical change, RMgX, where R is an organic group since some of it can be re-emitted in and X a halogen atom (e.g. CH3MgCl, the form of heat or light. The light 253 group 2 elements does not need to be absorbed directly tinuous. An example of a continuous by the reacting substances; it is possi- group is the set of rotations about a ble, in photosensitization for exam- Üxed axis. The rotation group thus ple, that light can be absorbed by an formed underlies the quantum inert substance, which subsequently theory of angular momentum, which transfers the absorbed energy (as has many applications to atoms and thermal energy) to the reactants. nuclei. ground state The lowest stable en- group 0 elements See noble ergy state of a system, such as a mol- gases. ecule, atom, or nucleus. See energy group 1 elements A group of el- level. ements in the *periodic table: group 1. See periodic table. 2. A lithium (Li), sodium (Na), potassium g mathematical structure consisting of (K), rubidium (Rb), caesium (Cs), and a set of elements A, B, C, etc., for Francium (Fr). They are known as the which there exists a law of composi- *alkali metals. Formerly, they were tion, referred to as ‘multiplication’. classiÜed in group I, which consisted Any two elements can be combined of two subgroups: group IA (the main to give a ‘product’ AB. group) and group IB. Group IB con- (1) Every product of two elements is sisted of the *coinage metals, copper, an element of the set. silver, and gold, which comprise (2) The operation is associative, i.e. group 11 and are usually considered A(BC) = (AB)C. with the *transition elements. (3) The set has an element I, called the identity element, such that IA = group 2 elements A group of el- AI = A for all A in the set. ements in the *periodic table: beryl- (4) Each element of the set has an in- lium (Be), magnesium (Hg), calcium verse A–1 belonging to the set such (Ca), strontium (Sr), barium (Ba), and that AA–1 = A–1A = I. radium (Ra). They are known as the Although the law of combination is *alkaline-earth metals. Formerly, Ü called ‘multiplication’ this does not they were classi ed in group II, necessarily have its usual meaning. which consisted of two subgroups: alka- For example, the set of integers group IIA (the main group, see line-earth metals forms a group if the law of composi- ) and group IIB. tion is addition. Group IIB consisted of the three met- Two elements A, B of a group com- als zinc (Zn), cadmium (Cd), and mer- mute if AB = BA. If all the elements of cury (Hg), which have two s-electrons a group commute with each other outside Ülled d-subshells. Moreover, the group is said to be Abelian. If this none of their compounds have is not the case the group is said to be unÜlled d-levels, and the metals are non-Abelian. regarded as nontransition elements. The interest of group theory in They now form group 12 and are physics and chemistry is in analysing sometimes called the zinc group. Zinc symmetry. Discrete groups have a and cadmium are relatively elec- Ünite number of elements, such as tropositive metals, forming com- the symmetries involved in rotations pounds containing divalent ions Zn2+ and reÛections of molecules, which or Cd2+. Mercury is more unreactive give rise to point groups. Continuous and also unusual in forming mer- groups have an inÜnite number of el- cury(I) compounds, which contain 2+ ements where the elements are con- the ion Hg2 . groups 3–12 254

groups 3–12 See transition el- elements of group 13 have a vacant ements. p-orbital they display many electron- acceptor properties. For example, group 13 elements A group of el- many boron compounds form ements in the *periodic table: boron adducts with donors such as ammo- (B), aluminium (Al), gallium (Ga), in- nia and organic amines (acting as dium (In), and thallium (Tl), which all Lewis acids). A large number of com- have outer electronic conÜgurations plexes of the type [BF ]–, [AlCl ]–, ns2np1 with no partly Ülled inner lev- 4 4 [InCl ]–, [TlI ]– are known and the els. They are the Ürst members of the 4 4 heavier members can expand their p-block. The group differs from the coordination numbers to six as in alkali metals and alkaline-earth met- [AlF ]3– and [TlCl ]3–. This acceptor als in displaying a considerable varia- 6 6 g property is also seen in bridged tion in properties as the group is dimers of the type Al Cl . Another descended. Formerly, they were 2 6 feature of group 13 is the increasing classiÜed in group III, which con- stability of the monovalent state sisted of two subgroups: group IIIB down the group. The electron (the main group) and group IIIA. conÜguration ns2np1 suggests that Group IIIA consisted of scandium only one electron could be lost or (Sc), yttrium (Yt), and lanthanum (La), shared in forming compounds. In which are generally considered with fact, for the lighter members of the the *lanthanoids, and actinium (Ac), group the energy required to pro- which is classiÜed with the *acti- mote an electron from the s-subshell noids. Scandium and yttrium now be- to a vacant p-subshell is small. It is long to group 3 (along with lutetium more than compensated for by the and lawrencium). resulting energy gain in forming Boron has a small atomic radius three bonds rather than one. This en- and a relatively high ionization energy gain is less important for the ergy. In consequence its chemistry is heavier members of the group. Thus, largely covalent and it is generally aluminium forms compounds of the classed as a metalloid. It forms a type AlCl in the gas phase at high large number of volatile hydrides, temperatures. Gallium similarly some of which have the uncommon forms such compounds and bonding characteristic of *electron- gallium(I) oxide (Ga O) can be iso- deÜcient compounds. It also forms a 2 lated. Indium has a number of weakly acidic oxide. In some ways, known indium(I) compounds (e.g. boron resembles silicon (see diago- InCl, In O, In I[InIIICl ]). Thallium has nal relationship). 2 3 6 stable monovalent compounds. In As the group is descended, atomic aqueous solution, thallium(I) com- radii increase and ionization energies pounds are more stable than the cor- are all lower than for boron. There is responding thallium(III) compounds. an increase in polar interactions and See inert-pair effect. the formation of distinct M3+ ions. This increase in metallic character is group 14 elements A group of el- clearly illustrated by the increasing ements in the *periodic table: carbon basic character of the hydroxides: (C), silicon (Si), germanium (Ge), tin boron hydroxide is acidic, aluminium (Sn), and lead (Pb), which all have and gallium hydroxides are ampho- outer electronic conÜgurations ns2np2 teric, indium hydroxide is basic, and with no partly Ülled inner levels. For- thallium forms only the oxide. As the merly, they were classiÜed in group 255 group 16 elements

IV, which consisted of two sub- methane (CH4) to the unstable groups: IVB (the main group) and plumbane (PbH4). group IVA. Group IVA consisted of ti- group 15 elements A group of el- tanium (Ti), zirconium (Zr), and ements in the *periodic table: nitro- hafnium (Hf), which now form group gen (N), phosphorus (P), arsenic (As), 4 and are generally considered with antimony (Sb), and bismuth (Bi), the *transition elements. which all have outer electronic The main valency of the elements conÜgurations ns2np3 with no partly is 4, and the members of the group Ülled inner levels. Formerly, they show a variation from nonmetallic to were classiÜed in group V, which metallic behaviour in moving down consisted of two subgroups: group VB the group. Thus, carbon is a non- (the main group) and group VA. metal and forms an acidic oxide g Group VA consisted of vanadium (V), (CO ) and a neutral oxide. Carbon 2 niobium (Nb), and tantalum (Ta), compounds are mostly covalent. One which are generally considered with allotrope (diamond) is an insulator, the *transition elements: although graphite is a fairly good The lighter elements (N and P) are conductor. Silicon and germanium nonmetals; the heavier elements are are metalloids, having semiconduc- metalloids. The lighter elements are tor properties. Tin is a metal, but electronegative in character and have does have a nonmetallic allotrope fairly large ionization energies. Nitro- (grey tin). Lead is deÜnitely a metal. gen has a valency of 3 and tends to Another feature of the group is the form covalent compounds. The other tendency to form divalent com- elements have available d-sublevels pounds as the size of the atom in- and can promote an s-electron into creases. Thus carbon has only the one of these to form compounds highly reactive carbenes. Silicon with the V oxidation state. Thus, forms analogous silylenes. Germa- they have two oxides P O , P O , nium has an unstable hydroxide 2 3 2 5 Sb O , Sb O , etc. In the case of bis- (Ge(OH) ), a sulphide (GeS), and 2 3 2 5 2 muth, the pentoxide Bi O is difÜcult halides. The sulphide and halides dis- 2 5 to prepare and unstable – an exam- proportionate to germanium and the ple of the increasing stability of the germanium(IV) compound. Tin has a III oxidation state in going from number of tin(II) compounds, which phosphorus to bismuth. The oxides are moderately reducing, being oxi- also show how there is increasing dized to the tin(IV) compound. Lead metallic (electropositive) character has a stable lead(II) state. See inert- down the group. Nitrogen and phos- pair effect. phorus have oxides that are either In general, the reactivity of the el- neutral (N O, NO) or acidic. Bismuth ements increases down the group 2 trioxide (Bi O ) is basic. Bismuth is from carbon to lead. All react with 2 3 the only member of the group that oxygen on heating. The Ürst four forms a well-characterized positive form the dioxide; lead forms the ion Bi3+. monoxide (i.e. lead(II) oxide, PbO). Similarly, all will react with chlorine group 16 elements A group of el- to form the tetrachloride (in the case ements in the *periodic table: oxy- of the Ürst four) or the dichloride (for gen (O), sulphur (S), selenium (Se), lead). Carbon is the only one capable tellurium (Te), and polonium (Po), of reacting directly with hydrogen. which all have outer electronic The hydrides all exist from the stable conÜgurations ns2np4 with no partly group 17 elements 256

Ülled inner levels. They are also observed in groups 13, 14, and 15. called the chalcogens. Formerly, they The chlorides are limited to M2Cl2 Ü were classi ed in group VI, which and MCl4; the bromides are similar consisted of two subgroups: group except that sulphur only forms S2Br2. VIB (the main group) and group VIA. All metallic elements form oxides Group VIA consisted of chromium and sulphides and many form se- (Cr), molybdenum (Mo), and tungsten lenides. (W), which now form group 6 are Ü group 17 elements A group of el- generally classi ed with the *transi- ements in the *periodic table: tion elements. Ûuorine (F), chlorine (Cl), bromine The conÜgurations are just two Ü (Br), iodine (I), and astatine (At). They electrons short of the con guration are known as the *halogens. For- g of a noble gas and the elements are merly, they were classiÜed in group characteristically electronegative and VII, which consisted of two sub- almost entirely nonmetallic. Ioniza- groups: group VIIB (the main group) tion energies are high, (O 1314 to Po and group VIIA. Group VIIA consisted –1 813 kJ mol ) and monatomic cations of the elements manganese (Mn), are not known. Polyatomic cations technetium (Te), and rhenium (Re), + 2+ 2+ 2+ do exist, e.g. O2 , S8 , Se8 , Te4 . which now form group 7 and are Electronegativity decreases down the usually considered with the transi- group but the nearest approach to tion elements. metallic character is the occurrence of ‘metallic’ allotropes of selenium, group 18 elements A group of el- tellurium, and polonium along with ements in the *periodic table: he- some metalloid properties, in partic- lium (He), neon (Ne), argon (Ar), ular, marked photoconductivity. The krypton (Kr), xenon (Xe), and radon Ü elements of group 16 combine with a (Rn). Formerly classi ed as group 0 wide range of other elements and the elements, they are usually referred to bonding is largely covalent. The elas the *noble gases. ements all form hydrides of the type group representation A group of XH2. Apart from water, these ma- mathematical objects that is homo- terials are all toxic foul-smelling morphic to (i.e. has the same mathe- gases; they show decreasing thermal matical structure as) the original stability with increasing relative group. In particular, group represen- atomic mass of X. The hydrides dis- tations made up of square matrices solve in water to give very weak are of interest. The dimension of the acids (acidity increases down the representation is the number of rows group). Oxygen forms the additional or group representation columns of hydride H2O2 (hydrogen peroxide), the matrix. The *irreducible repre- but sulphur forms a range of sul- sentations of a group, i.e. the repre- phanes, such as H2S2, H2S4, H2S6. sentations that cannot be expressed Û Oxygen forms the uorides O2F2 in terms of lower-dimensional repre- Û and OF2, both powerful uorinating sentations, are of great importance agents; sulphur forms analogous in quantum mechanics since the en- Ûuorides along with some higher ergy levels of a quantum mechanical Û uorides, S2F2, SF2, SF4, SF6, S2F10. Se- system are labelled by the irreducible lenium and tellurium form only the representations of the symmetry Û higher uorides MF4 and MF6; this is group of the system. This enables in contrast to the formation of lower *selection rules for the system to be valence states by heavier elements derived. 257 gyromagnetic ratio

GSC (gas–solid chromatography) See containing 88–90% copper, 8–10% tin, gas chromatography. and 2–4% zinc. Formerly used for can- nons, it is still used for bearings and G-series See nerve agents. other parts that require high resis- guanidine A crystalline basic com- tance to wear and corrosion. pound HN:C(NH2)2, related to urea. gunpowder An explosive consist- guanine A *purine derivative. It is ing of a mixture of potassium nitrate, one of the major component bases of sulphur, and charcoal. *nucleotides and the nucleic acids gypsum A monoclinic mineral *DNA and *RNA. form of hydrated *calcium sulphate, Ü gum Any of a variety of substances CaSO4.2H2O. It occurs in ve vari- obtained from plants. Typically they eties: rock gypsum, which is often g are insoluble in organic solvents but red stained and granular; gypsite, an form gelatinous or sticky solutions impure earthy form occurring as a with water. Gum resins are mixtures surface deposit; alabaster, a pure Ü of gums and natural resins. Gums are ne-grained translucent form; satin Ü produced by the young xylem vessels spar, which is brous and silky; and of some plants (mainly trees) in re- selenite, which occurs as transparent sponse to wounding or pruning. The crystals in muds and clays. It is used exudate hardens when it reaches the in the building industry and in the plant surface and thus provides a manufacture of cement, rubber, temporary protective seal while the paper, and plaster of Paris. cells below divide to form a perma- gyromagnetic ratio Symbol γ. nent repair. Excessive gum formation The ratio of the angular momentum is a symptom of some plant diseases. of an atomic system to its magnetic moment. The inverse of the gyro- guncotton See cellulose nitrate. magnetic ratio is called the magneto- gun metal A type of bronze usually mechanical ratio. H

Haber, Fritz (1868–1934) German containing potassium and alu- chemist who worked at the Karl- minium oxide promoters. The higher sruhe Technical Institute, where he the pressure the greater the yield, al- perfected the *Haber process for though there are technical difÜcul- making ammonia in 1908. As a Jew, ties in using very high pressures. A he left Germany in 1933 to go into pressure of about 250 atmospheres is exile in Britain, working in Cam- commonly employed. bridge at the Cavendish Laboratory. The process is of immense impor- For his Haber process, he was tance for the Üxation of nitrogen for awarded the 1918 Nobel Prize for fertilizers. It was developed in 1908 chemistry. by Fritz *Haber and was developed for industrial use by Carl Bosch Haber process An industrial (1874–1940), hence the alternative process for producing ammonia by name Haber–Bosch process. The ni- reaction of nitrogen with hydrogen: trogen is obtained from liquid air. ˆ N2 + 3H2 2NH3 Formerly, the hydrogen was from The reaction is reversible and *water gas and the water–gas shift exothermic, so that a high yield of reaction (the Bosch process) but now ammonia is favoured by low temper- the raw material (called synthesis ature (see le chatelier’s principle). gas) is obtained by steam *reforming However, the rate of reaction would natural gas. be too slow for equilibrium to be habit See crystal. reached at normal temperatures, so an optimum temperature of about haem (heme) An iron-containing 450°C is used, with a catalyst of iron molecule that binds with proteins as

CH2 CH3 H C 3CH

CH2 N N

CH Fe CH

N N

CH 3 CH3 C H

HOOC COOH Haem 259 hafnium a *cofactor or *prosthetic group to globular proteins occurring widely in form the haemoproteins. These are animals as oxygen carriers in blood. *haemoglobin, *myoglobin, and the Vertebrate haemoglobin comprises *cytochromes. Essentially, haem two pairs of polypeptide chains, comprises a *porphyrin with its four known as α-chains and β-chains nitrogen atoms holding the iron(II) (forming the globin protein), with atom as a chelate. This iron can re- each chain folded to provide a bind- versibly bind oxygen (as in haemo- ing site for a *haem group. Each of globin and myoglobin) or (as in the the four haem groups binds one cytochromes) conduct electrons by oxygen molecule to form oxy- conversion between the iron(II) and haemoglobin. Dissociation occurs in iron(III) series. oxygen-depleted tissues: oxygen is re- haematin test (Teichmann test) A leased and haemoglobin is reformed. test for blood using the presence of The haem groups also bind other in- characteristic haematin crystals. It organic molecules, including carbon h was introduced in 1853 by Ludwig monoxide (to form carboxyhaemo- Teichmann. globin). In vertebrates, haemoglobin haematite A mineral form of is contained in the red blood cells (erythrocytes). iron(III) oxide, Fe2O3. It is the most important ore of iron and usually oc- haemoglobinic acid A very weak curs in two main forms: as a massive acid formed inside red blood cells red kidney-shaped ore (kidney ore) when hydrogen ions combine with and as grey to black metallic crystals haemoglobin. The presence of the known as specular iron ore. hydrogen ions, which are produced Haematite is the major red colour- by the dissociation of carbonic acid, ing agent in rocks; the largest de- encourages oxyhaemoglobin to disso- posits are of sedimentary origin. In ciate into haemoglobin and oxygen. industry haematite is also used as a The oxygen diffuses into the tissue polishing agent (jeweller’s rouge) and cells and the haemoglobin acts as a in paints. *buffer for the excess hydrogen ions, haemochromogen test which it takes up to form haemoglo- (Takayama test) A test used to binic acid. conÜrm the presence of blood. It hafnium Symbol Hf. A silvery lus- is a microcrystal test exploiting trous metallic *transition element; the characteristic appearance of a.n. 72; r.a.m. 178.49; r.d. 13.3; m.p. haemochromogen crystals observed ± ° ° under a microscope. The rest was in- 2227 20 C; b.p. 4602 C. The element troduced in Japan in 1912 by Masao is found with zirconium and is ex- Takayama. tracted by formation of the chloride and reduction by the Kroll process. It haemoerythrin A red iron-contain- is used in tungsten alloys in Ülaments ing respiratory pigment that occurs and electrodes and as a neutron ab- in the blood of annelids and some sorber. The metal forms a passive other invertebrates. Its structure is oxide layer in air. Most of its com- essentially the same as that of pounds are hafnium(IV) complexes; *haemoglobin except the prosthetic less stable hafnium(III) complexes group has a different chemical com- also exist. The element was Ürst re- position. ported by Urbain in 1911, and its ex- haemoglobin One of a group of istence was Ünally established by Hahn, Otto 260

Dirk Coster (1889–1950) and George half-width Half the width of a de Hevesey (1885–1966) in 1923. spectrum line (or in some cases the A full width) measured at half its height. • Information from the WebElements site halide A compound of a halogen Hahn, Otto (1879–1968) German with another element or group. The chemist, who studied in London halides of typical metals are ionic Û + – (with William *Ramsay) and Canada (e.g. sodium uoride, Na F ). Metals (with Ernest *Rutherford) before re- can also form halides in which the turning to Germany in 1907. In 1917, bonding is largely covalent (e.g. aluminium chloride, AlCl ). Organic com- together with Lise *Meitner, he dis- 3 pounds are also sometimes referred covered protactinium. In the late to as halides; e.g. the alkyl halides 1930s he collaborated with Fritz (see haloalkanes) and the *acyl Strassmann (1902–80) and in 1938 halides. Halides are named Ûuorides, bombarded uranium with slow neu- h chlorides, bromides, or iodides. trons. Among the products was barium, but it was Meitner (now in halite (rock salt) Naturally occur- Sweden) who the next year inter- ring *sodium chloride (common salt, preted the process as nuclear Üssion. NaCl), crystallizing in the cubic sys- In 1944 Hahn received the Nobel tem. It is chieÛy colourless or white Prize for chemistry. (sometimes blue) when pure but the presence of impurities may colour it transactinide el- hahnium See grey, pink, red, or brown. Halite ements . often occurs in association with an- half cell An electrode in contact hydrite and gypsum. with a solution of ions, forming part Hall–Heroult cell An electrolytic of a *cell. Various types of half cell cell used industrially for the extrac- exist, the simplest consisting of a tion of aluminium from bauxite. The metal electrode immersed in a solu- bauxite is Ürst puriÜed by dissolving tion of metal ions. Gas half cells have it in sodium hydroxide and Ültering a gold or platinum plate in a solution off insoluble constituents. Alu- with gas bubbled over the metal minium hydroxide is then precipi- plate. The commonest is the *hydro- tated (by adding CO2) and this is gen half cell. Half cells can also be decomposed by heating to obtain formed by a metal in contact with an pure Al2O3. In the Hall–Heroult cell, insoluble salt or oxide and a solution. the oxide is mixed with cryolite (to The *calomel half cell is an example lower its melting point) and the of this. Half cells are commonly re- molten mixture electrolysed using ferred to as electrodes. graphite anodes. The cathode is the lining of the cell, also of graphite. half chair See ring conforma- The electrolyte is kept in a molten tions. state (about 850°C) by the current. half-life See decay. Molten aluminium collects at the half sandwich See sandwich com- bottom of the cell and can be tapped off. Oxygen forms at the anode, and pound. gradually oxidizes it away. The cell is half-thickness The thickness of a named after the US chemist Charles speciÜed material that reduces the in- Martin Hall (1863–1914), who discov- tensity of a beam of radiation to half ered the process in 1886, and the its original value. French chemist Paul Heroult (1863– 261 halogenation

1914), who discovered it indepen- haloform reaction A reaction for dently in the same year. producing *haloforms from methyl ketones. An example is the produc- hallucinogen A drug or chemical tion of chloroform from propanone that causes alterations in perception using sodium chlorate(I) (or bleach- (usually visual), mood, and thought. ing powder): Common hallucinogenic drugs in- → clude *lysergic acid diethylamide CH3COCH3 + 3NaOCl CH3COCl3 (LSD) and mescaline. There is no + 3NaOH common mechanism of action for The substituted ketone then reacts to this class of compounds although give chloroform (trichloromethane): many hallucinogens are structurally → CH3COCCl3 + NaOH NaOCOCH3 similar to neurotransmitters in the + CHCl3 central nervous system, such as sero- The reaction can also be used for tonin and the catecholamines. making carboxylic acids, since h halo A broad ring appearing in the RCOCH3 gives the product NaOCOR. electron diffraction, neutron diffrac- It is particularly useful for aromatic tion, or X-ray diffraction patterns of acids as the starting ketone can be materials that are not crystals. Haloes made by a Friedel–Crafts acylation. of this type occur in gases and liquids The reaction of methyl ketones as well as in noncrystalline solids. with sodium iodate(I) gives iodoform (triiodomethane), which is a yellow haloalkanes (alkyl halides) Organic solid with a characteristic smell. This compounds in which one or more reaction is used in the iodoform test hydrogen atoms of an alkane have to identify methyl ketones. It also been substituted by halogen atoms. gives a positive result with a sec- Examples are chloromethane, CH3Cl, ondary alcohol of the formula dibromoethane, CH2BrCH2Br, etc. Ü RCH(OH)CH3 (which is rst oxidized Haloalkanes can be formed by direct to a methylketone) or with ethanol reaction between alkanes and halo- (oxidized to ethanal, which also un- gens using ultraviolet radiation. They dergoes the reaction). are usually made by reaction of an alcohol with a halogen carrier. haloforms The four compounds with formula CHX3, where X is a halocarbons Compounds that con- halogen atom. They are chloroform tain carbon and halogen atoms and Û (CHCl3), and, by analogy, uoroform (sometimes) hydrogen. The simplest (CHF3), bromoform (CHBr3), and iod- are compounds such as tetra- oform (CHI3). The systematic names chloromethane (CCl4), tetrabro- are trichloromethane, triÛuoro- momethane (CBr4), etc. The methane, etc. *haloforms are also simple halocar- halo- * Û halogenating agent See bons. The chloro uorocarbons genation (CFCs) contain carbon, chlorine, and . Ûuorine. Similar to these are hy- halogenation A chemical reaction drochloroÛuorocarbons (HCFCs), in which a halogen atom is intro- which contain carbon, chlorine, duced into a compound. Halogena- Ûuorine, and hydrogen, and the hy- tions are described as chlorination, droÛuorocarbons (HFCs), which con- Ûuorination, bromination, etc., ac- tain carbon, Ûuorine, and hydrogen. cording to the halogen involved. The *halons are a class of halocar- Halogenation reactions may take bons that contain bromine. place by direct reaction with the halogens 262

halogen. This occurs with alkanes, uid, and iodine and astatine are where the reaction involves free radi- solids. All exist as diatomic mol- cals and requires high temperature, ecules. ultraviolet radiation, or a chemical The name ‘halogen’ comes from initiator; e.g. the Greek ‘salt-producer’, and the el- → ements react with metals to form C2H6 + Br2 C2H5Br + HBr The halogenation of aromatic com- ionic halide salts. They also combine with nonmetals, the activity decreas- pounds can be effected by elec- Û trophilic substitution using an ing down the group: uorine reacts aluminium chloride catalyst: with all nonmetals except nitrogen → and the noble gases helium, neon, C6H6 + Cl2 C6H5Cl + HCl and argon; iodine does not react with Halogenation can also be carried out any noble gas, nor with carbon, ni- using compounds, such as phospho- trogen, oxygen, or sulphur. The el- h rus halides (e.g. PCl3) or sulphur di- ements Ûuorine to iodine all react halide oxides (e.g. SOCl2), which react with hydrogen to give the acid, with with –OH groups. Such compounds the activity being greatest for are called halogenating agents. Addi- Ûuorine, which reacts explosively. tion reactions are also referred to as Chlorine and hydrogen react slowly halogenations; e.g. at room temperature in the dark → C2H4 + Br2 CH2BrCH2Br (sunlight causes a free-radical chain reaction). Bromine and hydrogen halogens (group 17 elements) A react if heated in the presence of a group of elements in the *periodic catalyst. Iodine and hydrogen react Û table (formerly group VIIB): uorine only slowly and the reaction is not (F), chlorine (Cl), bromine (Br), iodine complete. There is a decrease in oxi- (I), and astatine (At). All have a char- dizing ability down the group from Ü acteristic electron con guration of Ûuorine to iodine. As a consequence, 2 5 noble gases but with outer ns np each halogen will displace any halo- electrons. The outer shell is thus one gen below it from a solution of its Ü electron short of a noble-gas con gu- salt, for example: ration. Consequently, the halogens – → – are typical nonmetals; they have Cl2 + 2Br Br2 + 2Cl high electronegativities – high elec- The halogens also form a wide vari- tron afÜnities and high ionization en- ety of organic compounds in which ergies. They form compounds by the halogen atom is linked to carbon. gaining an electron to complete the In general, the aryl compounds are stable conÜguration; i.e. they are more stable than the alkyl com- good oxidizing agents. Alternatively, pounds and there is decreasing resis- they share their outer electrons to tance to chemical attack down the form covalent compounds, with sin- group from the Ûuoride to the iodide. gle bonds. Fluorine has only a valency of 1, al- All are reactive elements with the though the other halogens can have reactivity decreasing down the higher oxidation states using their group. The electron afÜnity decreases vacant d-electron levels. There is also down the group and other properties evidence for increasing metallic be- also show a change from Ûuorine to haviour down the group. Chlorine astatine. Thus, the melting and boil- and bromine form compounds with ing points increase; at 20°C, Ûuorine oxygen in which the halogen atom is and chlorine are gases, bromine a liq- assigned a positive oxidation state. 263 Hantz–Widman system

Only iodine, however, forms positive ity of side-chain derivatives of aro- + – ions, as in I NO3 . matic compounds. It arises from a halon A compound obtained by re- comparison between rate constants placing the hydrogen atoms of a hy- for various reactions with the rate of drocarbon by bromine along with hydrolysis of benzyl chloride on the other halogen atoms (see halocar- one hand and a comparison between bons), for instance halon 1211 is bro- equilibrium constants (such as the Û dissociation constant of benzoic acid) mochlorodi uoromethane (CF2BrCl) and halon 1301 is bromotriÛuoro- on the other hand. The Hammett equation can be written in the form methane (CF3Br). Halons are very sta- ρ ble and unreactive and are widely log(k/k0) = log(K/K0), where log(K/K0) used in Üre extinguishers. There is refers to comparing dissociation constants to the dissociation constant, concern that they are being broken ° down in the atmosphere to bromine, K0, of benzoic acid in water at 25 C, which reacts with ozone, leading to and log(k/k0) refers to comparing h depletion of *ozone layer, and their rates of reaction to the rate, k0, of hy- use is being curtailed. Although more drolysis of benzyl chloride. The term σ *chloroÛuorocarbons are present in log(K/K0) = is called the substituent the atmosphere, halons are between constant, since the nature of the sub- three and ten times more destructive stituent affects the strength of the σ of ozone. benzoic acid. If is positive, the substituent is electron attracting, while halothane (1-chloro-1-bromo-2,2,2,- if σ is negative the substituent is elec- Û tri uoroethane) A colourless, non- tron donating. ρ is a reaction con- Û ammable oily liquid, CHBrClCF3; stant, which is determined for a ° b.p. 51 C. It smells like trichloro- given reaction by the slope of a methane and has a sickly burning σ graph of log(k/k0) against . The nu- taste. Halothane is widely used as a merical value of ρ depends on tem- general anaesthetic, often adminis- perature and the type of solvent. tered also with oxygen and dinitro- The Hammett equation applies to gen oxide. meta- and para- substituents (pro- Hamiltonian Symbol H. A function vided that resonance interaction used to express the energy of a sys- from the substituents does not occur) tem in terms of its momentum and but not to ortho-substituents. positional coordinates. In simple Hantz–Widman system A system cases this is the sum of its kinetic for naming heterocyclic compounds, and potential energies. In Hamilton- independently introduced for nam- ian equations, the usual equations ing 5- and 6- membered nitrogen used in mechanics (based on forces) monocyclic compounds by A. are replaced by equations expressed Hantzsch (1887) and O. Widman in terms of momenta. This method (1888). Subsequently it was extended of formulating mechanics (Hamilton- Ü to other ring sizes and other hetero- ian mechanics) was rst introduced atoms. It is based on preÜxes denot- by Sir William Rowan Hamilton ing the hetero atoms(s) (e.g. aza- for (1805–65). The Hamiltonian operator nitrogen, thia- for sulphur, oxa- for is used in quantum mechanics in the oxygen), and items denoting ring * Schrödinger equation. size and saturation. For example, 4- Hammett equation An equation membered rings have ete for unsatu- relating the structure to the reactiv- rated rings and etane for saturated; hapticity 264

5- membered rings have ole for un- etc., and in certain industrial saturated and olane for saturated processes. Various methods of water compounds. Rules are given for nam- softening are used. In public sup- ing compounds with two or more plies, the temporary hardness can be different heteroatoms and for num- removed by adding lime (calcium hy- bering the atoms. The system is droxide), which precipitates calcium widely used both for monocyclic carbonate compounds and for heterocyclic com- → Ca(OH)2(aq) + Ca(HCO3)2(aq) ponents of polycyclic compounds. 2CaCO3(s) + 2H2O(l) A This is known as the Clark process (or • Information about IUPAC nomenclature as ‘clarking’). It does not remove per- hapticity Symbol η. The number of manent hardness. Both temporary electrons in a ligand that are directly and permanent hardness can be coordinated to a metal. treated by precipitating calcium car- h bonate by added sodium carbonate – hsab principle hard acid See . hence its use as a washing soda and hard base See hsab principle. in bath salts. Calcium (and other) ions can also be removed from water hardening of oils The process of by ion-exchange using zeolites (e.g. converting unsaturated esters of Permutit). This method is used in *fatty acids into (more solid) satu- small domestic water-softeners. An- rated esters by hydrogenation using a other technique is not to remove the nickel catalyst. It is used in the man- Ca2+ ions but to complex them and ufacture of margarine from vegetable prevent them reacting further. For oils. domestic use polyphosphates (con- 6– hardness of water The presence taining the ion P6O18 , e.g. Calgon) in water of dissolved calcium or mag- are added. Other sequestering agents nesium ions, which form a scum are also used for industrial water. See with soap and prevent the formation also sequestration. of a lather. The main cause of hard Hargreaves process See potas- water is dissolved calcium hydrogen- sium sulphate. carbonate (Ca(HCO3)2), which is formed in limestone or chalk regions harmonic An oscillation having a by the action of dissolved carbon frequency that is a simple multiple dioxide on calcium carbonate. This of a fundamental sinusoidal oscilla- type is known as temporary hardness tion. The fundamental frequency of a because it is removed by boiling: sinusoidal oscillation is usually called → the Ürst harmonic. The second har- Ca(HCO3)2(aq) CaCO3(s) + H2O(l) monic has a frequency twice that of + CO2(g) the fundamental, and so on. The precipitated calcium carbonate is the ‘fur’ (or ‘scale’) formed in kettles, harmonic oscillator A system that boilers, pipes, etc. In some areas, oscillates with simple harmonic mo- hardness also results from dissolved tion. The harmonic oscillator is ex- calcium sulphate (CaSO4), which can- actly soluble in both classical not be removed by boiling (perma- mechanics and quantum mechanics. nent hardness). Many systems exist for which har- Hard water is a considerable prob- monic oscillators provide very good lem in washing, reducing the efÜ- approximations. Atoms vibrating ciency of boilers, heating systems, about their mean positions in mol- 265 HCFC ecules or crystal lattices at low tem- and can be used quantitatively to peratures can be regarded as good ap- show chemical periodicity. proximations to harmonic oscillators hashish See cannabis. in quantum mechanics. Even if a system is not exactly a harmonic oscilla- hassium Symbol Hs. A radioactive tor the solution of the harmonic *transactinide element; a.n. 108. It oscillator is frequently a useful start- was Ürst made in 1984 by Peter Arm- ing point for treating such systems bruster and a team in Darmstadt, using *perturbation theory. Compare Germany. It can be produced by bom- anharmonic oscillator. barding lead-208 nuclei with iron-58 harpoon mechanism A mecha- nuclei. Only a few atoms have ever nism suggested to explain the reac- been produced. The name comes tion between such metal atoms as from the Latinized form of Hesse, the Ü sodium or potassium and such halo- German state where it was rst syn- h gen molecules as bromine, e.g. K + thesized. → A Br2 KBr + Br. As the alkali atom approaches the bromine molecule its • Information from the WebElements site valence electron moves to the hazchem code (emergency action bromine molecule (thus providing a code; EAC) A code designed to be dis- ‘harpoon’). There are then two ions played when hazardous chemicals with a Coulomb attraction between are transported or stored in bulk. It is them. As a result the ions move to- used to help the emergency services gether and the reaction takes place. to take action quickly in any acci- This mechanism, which has been worked out quantitatively, explains dent. The code consists of a number why the reaction occurs far more followed by one or two letters. The readily than might be expected tak- number indicates the type of sub- ing into account only mechanical col- stance to be used in treating the acci- Ü lisions between the alkali-metal dent (e.g. stream of water, ne spray, Ü atoms and halogen molecules. foam, dry agent). The rst letter indicates the type of protective clothing Hartree–Fock procedure A self- needed along with information about Ü consistent eld (SCF) procedure used the possibility of violent reaction on Ü to nd approximate *wave functions whether the substance should be and energy levels in many-electron contained or diluted. The second let- atoms. This procedure was intro- ter, where it exists, is letter E, indi- duced by the English mathematician cating that people have to be and physicist Douglas Hartree in evacuated from the neighbourhood 1928 and improved by the Soviet of the incident. In the UK, the code is physicist Vladimir Fock in 1930 (by usually displayed as part of a panel, taking into account the Pauli exclu- which includes an international UN sion principle). The initial wave func- number for the substance, a tele- tions can be taken to be hydrogenic phone number for specialist advice, atomic orbitals. The resulting equa- the company name, and a symbol in- tions can be solved numerically using dicating the danger (e.g., a skull and a computer. The results of the crossbones for toxic substances). Hartree–Fock theory are sufÜciently accurate to show that electron den- HCFC (hydrochloroÛuorocarbon) See sity occurs in shells around atoms halocarbons. h.c.p. 266

h.c.p. Hexagonal close packing. See given substance crystallizes from a close packing. saturated solution of the same substance. headspace The space above a sample held in a sealed container. Head- heat of dissociation The energy space analysis is used in forensic absorbed when one mole of a given science to investigate the volatile substance is dissociated into its con- constituents of a sample. stituent elements. heat capacity (thermal capacity) heat of formation The energy lib- The ratio of the heat supplied to an erated or absorbed when one mole of object or specimen to its consequent a compound is formed in their *stan- rise in temperature. The speciÜc heat dard states from its constituent el- capacity is the ratio of the heat sup- ements. plied to unit mass of a substance to heat of neutralization The en- h its consequent rise in temperature. ergy liberated in neutralizing one The molar heat capacity is the ratio mole of an acid or base. of the heat supplied to unit amount of a substance to its consequent rise heat of reaction The energy liber- in temperature. In practice, heat ca- ated or absorbed as a result of the pacity (C) is measured in joules per complete chemical reaction of molar kelvin, speciÜc heat capacity (c) in amounts of the reactants. JK–1 kg–1, and molar heat capacity heat of solution The energy liber- –1 –1 (Cm) in J K mol . For a gas, the val- ated or absorbed when one mole of a ues of c and Cm are commonly given given substance is completely dis- either at constant volume, when only solved in a large volume of solvent its *internal energy is increased, or (strictly, to inÜnite dilution). at constant pressure, which requires deuterium a greater input of heat as the gas is heavy hydrogen See . allowed to expand and do work heavy metal A metal with a high against the surroundings. The sym- relative atomic mass. The term is bols for the speciÜc and molar heat usually applied to common transition capacities at constant volume are cv metals, such as copper, lead, and and Cv, respectively; those for the zinc. These metals are a cause of en- speciÜc and molar heat capacities at vironmental *pollution (heavy-metal constant pressure are cp and Cp. pollution) from a number of sources, including lead in petrol, industrial heat engine A device for convert- efÛuents, and leaching of metal ions ing heat into work. Engines usually from the soil into lakes and rivers by work on cycles of operation, the acid rain. most efÜcient of which would be the *Carnot cycle. heavy spar A mineral form of *barium sulphate, BaSO . heat of atomization The energy 4 required to dissociate one mole of a heavy water (deuterium oxide) given substance into atoms. Water in which hydrogen atoms, 1H, are replaced by the heavier isotope heat of combustion The energy deuterium, 2H (symbol D). It is a liberated when one mole of a given colourless liquid, which forms hexag- substance is completely oxidized. onal crystals on freezing. Its physical heat of crystallization The en- properties differ from those of ‘nor- ergy liberated when one mole of a mal’ water; r.d. 1.105; m.p. 3.8°C; b.p. 267 Helmholtz free energy

° 101.4 C. Deuterium oxide, D2O, oc- small amount of helium–3. There are curs to a small extent (about 0.003% also two short-lived radioactive iso- by weight) in natural water, from topes: helium–5 and –6. It occurs in which it can be separated by frac- ores of uranium and thorium and in tional distillation or by electrolysis. It some natural-gas deposits. It has a va- is useful in the nuclear industry be- riety of uses, including the provision cause of its ability to reduce the ener- of inert atmospheres for welding and gies of fast neutrons to thermal semiconductor manufacture, as a re- energies and because its absorption frigerant for superconductors, and as cross-section is lower than that of hy- a diluent in breathing apparatus. It is drogen and consequently it does not also used in Ülling balloons. Chemi- Û appreciably reduce the neutron ux. cally it is totally inert and has no In the laboratory it is used for *la- known compounds. It was discovered belling other molecules for studies of in the solar spectrum in 1868 by reaction mechanisms. Water also Joseph Lockyer (1836–1920). h contains the compound HDO. A Ü hecto- Symbol h. A pre x used in • Information from the WebElements site the metric system to denote 100 times. For example, 100 coulombs = helium–neon laser A laser in 1 hectocoulomb (hC). which the medium is a mixture of helium and neon in the mole ratio Heisenberg, Werner Karl 1:5 respectively. An electric discharge (1901–76) German physicist, who became a professor at the University of is used to excite a He atom to the 1 1 Ü Leipzig and, after World War II, at metastable 1s 2s con guration. the Kaiser Wilhelm Institute in Göt- Since the excitation energy coincides tingen. In 1923 he was awarded the with an excitation energy of neon, Nobel Prize for his work on matrix transfer of energy between helium mechanics. But he is best known for and neon atoms can readily occur his 1927 discovery of the *uncer- during collisions. These collisions tainty principle. lead to highly excited neon atoms with unoccupied lower energy states. Heisenberg uncertainty princi- Thus, population inversion occurs ple See uncertainty principle. giving rise to laser action with a helicate A type of inorganic mol- wavelength of 633 nm. (Many other ecule containing a double helix of spectral lines are also produced in bipyridyl-derived molecules formed the process.) around a chain of up to Üve copper(I) ions. See also supramolecular chem- Helmholtz, Hermann Ludwig istry. Ferdinand von (1821–94) German physiologist and physicist. In 1850 he helium Symbol He. A colourless measured the speed of a nerve im- odourless gaseous nonmetallic el- pulse and in 1851 invented the oph- ement belonging to group 18 of the thalmoscope. Helmholtz discovered noble gases periodic table (see ); a.n. the conservation of energy (1847), 2; r.a.m. 4.0026; d. 0.178 g dm–3; m.p. ° ° giving many examples of its applica- –272.2 C (at 20 atm.); b.p. –268.93 C. tion, and also introduced the concept The element has the lowest boiling of *free energy. point of all substances and can be so- lidiÜed only under pressure. Natural Helmholtz free energy See free helium is mostly helium–4, with a energy. Helmholtz model 268

electrical ° Helmholtz model See atoms, C16H33COOH; m.p. 59–61 C; double layer. b.p. 227°C (100 mm Hg). It is present in certain natural fats. heme See haem. heptahydrate A crystalline hy- hemiacetals See acetals. drate that has seven moles of water hemicellulose A *polysaccharide per mole of compound. found in the cell walls of plants. The heptane A liquid straight-chain branched chains of this molecule alkane obtained from petroleum, bind to cellulose microÜbrils, form- ° Ü C7H16; r.d. 0.684; m.p. –90.6 C; b.p. ing a network of cross-linked bres. 98.4°C. In standardizing *octane hemihedral form The form of a numbers, heptane is given a value crystal in which only half the num- zero. ber of faces required for the symme- heptaoxodiphosphoric(V) acid holohedral h try are present. Compare See phosphoric(v) acid. form. heptavalent (septivalent) Having a hemihydrate A crystalline hydrate valency of seven. containing two molecules of com- cannabi- pound per molecule of water (e.g. herbal cannabinoids See noids. 2CaSO4.H2O). pesticide hemiketals See ketals. herbicide See . henry Symbol H. The *SI unit of in- Hermann–Mauguin system (inductance equal to the inductance of a ternational system) A notation closed circuit in which an e.m.f. of used to describe the symmetry of one volt is produced when the elec- point groups. In contrast to the Û tric current in the circuit varies uni- *Schoen ies system, which is used formly at a rate of one ampere per for isolated molecules (e.g. in spec- second. It is named after the U.S. troscopy), the Hermann–Mauguin physicist Joseph Henry (1797–1878). system is used in *crystallography. Some of the categories are the same Henry’s law At a constant tempera- as the SchoenÛies system. n is the ture the mass of gas dissolved in a same group as Cn. nmm is the same liquid at equilibrium is proportional group as Cnv. There are two ms be- to the partial pressure of the gas. The cause of two distinct types of mirror law, discovered in 1801 by the British plane containing the n-fold axis. n22 chemist and physician William is the same group as Dn. The other Henry (1775–1836), is a special case categories do not coincide with the of the partition law. It applies only to SchoenÛies system. is a group with gases that do not react with the sol- an n-fold rotation–inversion_ _ axis_ and vent. includes_ C3h as 6, S4 as 4, S6 as 3, and S as 1. n/m is the same group as C heparin A glycosaminoglycan (mu- 2 _ nh except that C is regarded as 6. n2m copolysaccharide) with anticoagulant 3h is the same group as D , except that properties, occurring in vertebrate nd D is regarded as 62m. n/m 2/m 2/m, tissues, especially the lungs and 3h abbreviated to n/mmm, is the same blood vessels. group as Dnh, _except that D3h is heptadecanoic acid (margaric regarded as 62m. (Unlike the acid) A white crystalline carboxylic SchoenÛies system, the Hermann– acid with a linear chain of carbon Mauguin system regards the three- 269 hexachlorobenzene

∆ fold axis as a special case.) As regards (the heat of formation of ethane, Hf) the cubic_ groups, Oh is denoted m3m and (or 4/m 3 2/m), O is denoted 432, T is ½ → _ h C2H6(g) + 3 O2 2CO2(g) + 3H2O(l) denoted m3 (or 2/m 3), T is denoted _ d (the heat of combustion of ethane, 43m, and T is denoted 23. In the Her- ∆H ). As ∆H can be measured and as mann–Mauguin system all the cubic E E ∆ ∆ ∆ ∆ groups have 3 as the second number Hf + HE = 2 Hc + 3 HH ∆ because of the three-fold axis that oc- Hf can be found. Another example curs in all cubic groups. is the use of the *Born–Haber cycle to obtain lattice energies. The law heroin (diacetylmorphine) A highly was Ürst put forward in 1840 by the addictive drug produced by acetylat- Swiss-born Russian chemist Germain ing *morphine. It is usually used as Henri Hess (1802–50). It is sometimes the hydrochloride. In the UK it is a called the law of constant heat sum- class A drug but it can be prescribed mation and is a consequence of the h as a painkiller under the name dia- law of conservation of energy. morphine. The Marquis and Froedhe tests are used to give an initial indica- hetero atom An odd atom in the tion of heroin. ring of a heterocyclic compound. For instance, nitrogen is the hetero atom hertz Symbol Hz. The *SI unit of in pyridine. frequency equal to one cycle per second. It is named after Heinrich heterocyclic See cyclic. *Hertz. heterogeneous Relating to two or Hess’s law If reactants can be con- more phases, e.g. a heterogeneous homogeneous verted into products by a series of re- *catalyst. Compare . actions, the sum of the heats of these heterolytic Üssion The breaking reactions (with due regard to their of a bond in a compound in which sign) is equal to the heat of reaction the two fragments are oppositely for direct conversion from reactants charged ions. For example, HCl → H+ to products. More generally, the over- + Cl–. Compare homolytic fission. all energy change in going from reactants to products does not depend on heteronuclear Denoting a mol- the route taken. The law can be used ecule in which the atoms are of dif- to obtain thermodynamic data that ferent elements. cannot be measured directly. For ex- heteropolar bond See chemical ample, the heat of formation of bond. ethane can be found by considering heteropoly compound See clus- the reactions: ter compound. 2C(s) + 3H (g) + 3½O (g) → 2CO (g) 2 2 2 heteropolymer See polymer. + 3H2O(l) ∆ The heat of this reaction is 2 HC + Heusler alloys Ferromagnetic al- ∆ ∆ ∆ 3 HH, where HC and HH are the loys containing no ferromagnetic el- heats of combustion of carbon and ements. The original alloys contained hydrogen respectively, which can be copper, manganese, and tin and were measured. By Hess’s law, this is equal Ürst made by Conrad Heusler (19th- to the sum of the energies for two century mining engineer). stages: hexachlorobenzene A colourless → 2C(s) + 3H2(g) C2H6(g) crystalline compound, C6Cl6; m.p. hexacyanoferrate(II) 270

227°C. It is made by the chlorination tiseptic for treating urinary infec- of benzene with an iron(III) chloride tions in medicine. It is used in the catalyst or by treating hexachlorocy- production of *cyclonite. clohexane with chlorine in hexa- chloroethane. It is used to preserve N wood and dress seeds, and in the manufacture of hexaÛuorobenzene. N N N hexacyanoferrate(II) (ferrocyanide) A complex iron-containing Hexamine 4– anion, [Fe(CN6)] , used as a solution of its potassium salt as a test for fer- hexanedioate (adipate) A salt or ric iron (iron(III)), with which it ester of hexanedioic acid. forms a dark blue precipitate of h Prussian blue. The sodium salt is hexanedioic acid (adipic acid) A used as an anticaking agent in com- carboxylic acid, (CH2)4(COOH)2; r.d. mon salt. 1.36; m.p. 153°C; b.p. 265°C (100 mmHg). It is used in the manufacture hexacyanoferrate(III) (ferri- of *nylon 6,6. See also polymeriza- cyanide) A complex iron-containing tion 3– . anion, [Fe(CN)6)] , used as a solution of its potassium salt as a test for fer- 6-hexanelactam See caprolactam. rous iron (iron(II)), with which it hexanitrohexaazaisowurtzitane forms a dark blue precipitate of See hniw. Prussian blue. hexanoate (caproate) A salt or hexadecane (cetane) A colourless ester of hexanoic acid. liquid straight-chain alkane hydrocarbon, C16H34, used in standardizing hexanoic acid (caproic acid) A liq- *cetane numbers of Diesel fuel. uid fatty acid, CH3(CH2)4COOH; r.d. ° ° hexadecanoate See palmitate. 0.93; m.p. –3.4 C; b.p. 205 C. Glyc- erides of the acid occur naturally in hexadecanoic acid See palmitic cow and goat milk and in some veg- acid. etable oils. hexagonal close packing See hexose A *monosaccharide that close packing. has six carbon atoms in its mol- hexagonal crystal See crystal sys- ecules. tem . hexyl group (hexyl radical) The hexahydrate A crystalline com- organic group CH3CH2CH2CH2- pound that has six moles of water CH2CH2–, derived from hexane. per mole of compound. HFC (hydroÛuorocarbon) See halo- hexamethylenetetramine See carbons. hexamine . highest occupied molecular or- hexamine (hexamethylenete- bital (HOMO) The orbital in a mol- tramine) A white crystalline com- ecule that has the highest energy pound, C6H12N4, made by the level occupied at the temperature of condensation of methanal with am- absolute zero. The highest occupied monia. It has been used as a solid molecular orbital and the lowest un- fuel for camping stoves and as an an- occupied molecular orbital (LUMO) 271 HMX are the two *frontier orbitals of the Hildebrand rule If the density of molecule. the vapour phase above a liquid is high frequency (HF) A radio fre- constant, the molar entropy of vaporization is constant. This law does not quency in the range 3–30 megahertz; hold if molecular association takes i.e. having a wavelength in the range place in the liquid or if the liquid is 10–100 metres. subject to quantum-mechanical high-performance liquid chro- effects, e.g. as in superÛuidity. The matography (HPLC) A sensitive rule is named after the US chemist technique for separating or analysing Joel Henry Hildebrand (1881–1983). mixtures, in which the sample is Hill reaction The release of oxygen forced through the chromatography from isolated illuminated chloro- column under pressure. plasts when suitable electron accep- high-resolution electron-loss tors (e.g. potassium ferricyanide) are spectroscopy (HRELS) A technique added to the surrounding water. The h for obtaining information about mol- reaction was discovered by Robert ecules absorbed on a surface by in- Hill (1899–1991) in 1939; the electron elastic scattering of electrons. acceptors substitute for NADP+, the high-speed steel A steel that will natural acceptor for the light-depend- remain hard at dull red heat and can ent reactions of *photosynthesis. therefore be used in cutting tools for histidine See amino acid. high-speed lathes. It usually con- histochemistry The study of the tains 12–22% tungsten, up to 5% distribution of the chemical con- chromium, and 0.4–0.7% carbon. It stituents of tissues by means of their may also contain small amounts of chemical reactions. It utilizes such vanadium, molybdenum, and other techniques as staining, light and elec- metals. tron microscopy, autoradiography, Hilbert space A linear vector space and chromatography. that can have an inÜnite number of HMX (octogen; cyclotetramethyl- dimensions. The concept is of inter- enetetranitramine) A colourless crys- est in physics because the state of a talline compound, C H N O ; r.d. 1.9; system in *quantum mechanics is 4 8 8 8 m.p. 276–286°C. It has a structure represented by a vector in Hilbert similar to that of *cyclonite (RDX), space. The dimension of the Hilbert but with an 8-membered ring rather space has nothing to do with the than a six-membered ring. An ex- physical dimension of the system. tremely powerful explosive, it is used The Hilbert space formulation of mainly for military purposes and as a quantum mechanics was put forward by the Hungarian-born US mathe- NO2 matician John von Neumann (1903–57) in 1927. Other formula- N CH CH tions of quantum mechanics, such as 2 2 * * matrix mechanics and wave me- O N N N NO chanics, can be deduced from the 2 2

Hilbert space formulation. It is 2CH CH2 named after the German mathemati- N cian David Hilbert (1862–1943), who NO invented the concept early in the 2 20th century. HMX HNIW 272

rocket propellant. The name comes cules (the Hofmann degradation). from ‘High Molecular Weight RDX’. The reaction is named after the Ger- HNIW (hexanitrohexaazaisowurtzi- man chemist August Wilhelm von Hofmann (1818–92). tane) A powerful explosive, C6N12O12. It has a three-dimensional bridged hole 1. A vacant electron position structure containing six N–NO2 in the lattice structure of a solid that groups. Also known as CL20, it is behaves like a mobile positive extremely sensitive and so far has *charge carrier. 2. A vacant electron not been produced in bulk. position in one of the inner orbitals of an atom. O2N NO2 NN holmium Symbol Ho. A soft silvery NO metallic element belonging to the O N N N 2 2 *lanthanoids; a.n. 67; r.a.m. 164.93; h r.d. 8.795 (20°C); m.p. 1474°C; b.p. ° N N 2695 C. It occurs in apatite, xeno- O N NO2 time, and some other rare-earth min- 2 erals. There is one natural isotope, HNIW holmium–165; eighteen artiÜcial isotopes have been produced. There are Hoff, Jacobus Henrikus van’t no uses for the element, which was (1852–1911) Dutch physical chemist discovered by Per Cleve (1840–1905) who Ürst recognized that a molecule and J. L. Soret in 1879. could exist in two mirror-image A forms. He proposed that these forms • Information from the WebElements site rotated the plane of polarization in opposite senses, and is generally re- holohedral form The form of a garded as the founder of stereochem- crystal in which the full number of istry. Van’t Hoff also did important faces required for the symmetry are work on other branches of physical present. Compare holohedral form. chemistry, especially chemical ther- HOMO See highest occupied mo- modynamics. He was awarded the lecular orbital. 1901 Nobel Prize for chemistry. homocyclic See cyclic. Hofmann’s reaction (Hofmann rearrangement) A reaction for making homogeneous Relating to only primary *amines from *amines one phase, e.g. a homogeneous mixture, a homogeneous *catalyst. Com- using bromine or chlorine and hetereogeneous sodium hydroxide: pare . → homoleptic compound A chemi- RCONH2 RNH2 cal complex with only one type of The halogen replaces a hydrogen ligand, as in nickel carbonyl or lead atom from the amido group to form tetraethyl. a halo-amide. This then reacts with the alkali to produce an isocyanate, homologous series A series of re- which decomposes into the amine lated chemical compounds that have and carbon dioxide. The amine has the same functional group(s) but dif- one carbon atom fewer than the fer in formula by a Üxed group of amide from which it is produced. atoms. For instance, the simple car- This technique is used to reduce the boxylic acids: methanoic (HCOOH), length of carbon chains in moles- ethanoic (CH3COOH), propanoic 273 humectant

(C2H5COOH), etc., form a homolo- ity of compounds with certain lig- gous series in which each member ands. Hard acids tend to complex differs from the next by CH2. Succes- with halide ions in the order sive members of such a series are F– > Cl– > Br– > I– called homologues. Soft acids complex in the opposite homolytic Üssion The breaking of order. Compounds that complex a bond in a compound in which the with hard acids are hard bases; ones fragments are uncharged free radi- that more readily form complexes → . . cals. For example, Cl2 Cl + Cl . with soft acids are called soft bases. Compare heterolytic fission. In general, soft acids and bases are more easily polarized than hard acids homonuclear Denoting a molecule and bases and consequently have in which the atoms are of the same more covalent character in the bond. element. The idea is an extension of the *type homopolar bond See chemical A and B metals concept to com- h bond. pounds other than metal complexes. homopolymer See polymer. Hückel, Erich (1896–1980) German physicist and theoretical chemist hormone A substance that is man- who worked with Peter *Debye at ufactured and secreted in very small Zürich on the theory of electrolytes. quantities into the bloodstream by Later he moved to Copenhagen to an endocrine gland or a specialized work with Niels *Bohr and here he nerve cell and regulates the growth produced his work on bonding in Ü or functioning of a speci c tissue or aromatic molecules. See hückel ap- organ in a distant part of the body. proximation; aromatic compound. For example, the hormone insulin controls the rate and manner in Hückel approximation A set of which glucose is used by the body. approximations used to simplify the molecular orbital analysis of conju- hornblende Any of a group of gated molecules, suggested by Erich common rock-forming minerals Hückel in 1931. In the Hückel theory, of the amphibole group with the σ orbitals are treated separately the generalized formula from the π orbitals, the shape of the (Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2. molecule being determined by the σ Hornblendes consist mainly of cal- orbitals. The Hückel theory makes cium, iron, and magnesium silicate. the approximation that interactions host–guest chemistry See between non-neighbouring atoms supramolecular chemistry. are taken to be zero. It enables calculations to be made for conjugated HPLC See high-performance liquid molecules. In particular, the *delocal- chromatography. ization energy of such molecules can HRELS See high-resolution elec- be estimated. Hückel explained the tron-loss spectroscopy. stability of benzene associated with aromaticity in this way. The theory HSAB principle A method of classi- can also be used to analyse delocal- fying Lewis acids and bases (See acid) ized bonding in solids. developed by Ralph Pearson in the Hückel rule See aromatic com- 1960s. The acronym stands for ‘hard pound and soft acids and bases’. It is based . in empirical measurements of stabil- humectant A substance used to Humphreys series 274

maintain moisture levels. Humec- hybrid orbital See orbital. tants are generally *hygroscopic. For hydracid See binary acid. example, glycerol is employed as a humectant in confectionery, food- hydrate A substance formed by stuffs, and tobacco. Other polyhydric combination of a compound with alcohols, such as mannitol and sor- water. See water of crystallization. bitol, are also used as humectant ad- hydrated alumina See aluminium ditives in the foodstuffs industry. hydroxide. Humphreys series A series of hydrated aluminium hydroxide lines in the *hydrogen spectrum See aluminium hydroxide. with the form hydration See solvation. 1/λ = R(1/62 – 1/n2), n = 7,8,9…, where λ is the wavelength associated hydrazine A colourless liquid or white crystalline solid, N2H4; r.d. 1.01 h with the lines and R is the Rydberg ° ° constant. The Humphreys series was (liquid); m.p. 1.4 C; b.p. 113.5 C. It is discovered by C. J. Humphreys in very soluble in water and soluble in 1953 and lies in the far infrared. ethanol. Hydrazine is prepared by the Raschig synthesis in which am- Hund coupling cases See cou- monia reacts with sodium(I) chlorate pling . (sodium hypochlorite) to give NH2Cl, Hund’s rules Empirical rules for in- which then undergoes further reac- terpreting atomic *spectra used to tion with ammonia to give N2H4. In- determine the lowest energy level for dustrial production must be carefully a conÜguration of two equivalent controlled to avoid a side reaction electrons (i.e. electrons with the leading to NH4Cl. The compound is a weak base giving rise to two series of same n and l quantum numbers) in a + many-electron *atom. (1) The lowest salts, those based on N2H5 , which are stable in water (sometimes writ- energy state has the maximum *mul- ten in the form N H .HCl rather than tiplicity consistent with the *Pauli 2 4 N H +Cl–), and a less stable and ex- exclusion principle. (2) The lowest 2 5 tensively hydrolysed series based on energy state has the maximum total N H 2+. Hydrazine is a powerful re- electron orbital angular momentum 2 6 ducing agent and reacts violently quantum number, consistent with with many oxidizing agents, hence rule (1). These rules were put forward its use as a rocket propellant. It re- by the German physicist Friedrich acts with aldehydes and ketones to Hund (1896–1993) in 1925. Hund’s give *hydrazones. rules are explained by quantum theory involving the repulsion be- hydrazoic acid See hydrogen tween two electrons. azide. hyaluronic acid A *glycosamino- hydrazones Organic compounds glycan (mucopolysaccharide) that is containing the group =C:NNH2, part of the matrix of connective tis- formed by condensation of substi- sue. Hyaluronic acid binds cells to- tuted hydrazines with with aldehy- gether and helps to lubricate joints. des and ketones (see illustration). It may play a role in the migration of Phenylhydrazones contain the group cells at wounds; this activity ceases =C:NNHC6H5. when hyaluronidase breaks down A hyaluronic acid. • Information about IUPAC nomenclature 275 hydrogen hydride A chemical compound of dynamic radius includes all the water hydrogen and another element or el- molecules attracted to the ion. As a ements. Non-metallic hydrides (e.g. result, it is possible for a small ion to ammonia, methane, water) are cova- have a larger hydrodynamic radius lently bonded. The alkali metals and than a large ion – if it is surrounded alkaline earths (*s-block elements) by more solvent molecules. Experi- form salt-like hydrides containing ments involving *nuclear magnetic the hydride ion H–, which produce resonance (NMR) and isotope tracers hydrogen on reacting with water. Hy- indicate that there is considerable dride-forming *transition elements movement between solvent mol- form interstitial hydrides, with the ecules within the hydrodynamic ra- hydrogen atoms ‘trapped’ within the dius and the rest of the solution. gaps in the lattice of metal atoms. hydroÛuoric acid See hydrogen Complex hydrides, such as *lithium fluoride. tetrahydroaluminate(III), have hy- h dride ions as *ligands; many are pow- hydroÛuorocarbon (HFC) See erful reducing agents. halocarbons. hydriodic acid See hydrogen io- hydrogen Symbol H. A colourless dide. odourless gaseous chemical element; hydrogen a.n. 1; r.a.m. 1.008; d. 0.0899 g dm–3; hydrobromic acid See ° ° bromide. m.p. –259.14 C; b.p. –252.87 C. It is the lightest element and the most hydrocarbons Chemical com- abundant in the universe. It is pre- pounds that contain only carbon and sent in water and in all organic com- hydrogen. A vast number of different pounds. There are three isotopes: hydrocarbon compounds exist, the naturally occurring hydrogen con- main types being the *alkanes, sists of the two stable isotopes hydro- *alkenes, *alkynes, and *arenes. gen–1 (99.985%) and *deuterium. The Ü- hydrochloric acid See hydrogen radioactive *tritium is made arti chloride. cially. The gas is diatomic and has two forms: orthohydrogen, in which amine salts hydrochloride See . the nuclear spins are parallel, and hydrochloroÛuorocarbon (HCFC) parahydrogen, in which they are an- See halocarbons. tiparallel. At normal temperatures the gas is 25% parahydrogen. In the hydrocyanic acid See hydrogen cyanide liquid it is 99.8% parahydrogen. The . main source of hydrogen is steam hydrodynamic radius The effec- *reforming of natural gas. It can also tive radius of an ion in a solution be made by the Bosch process (see measured by assuming that it is a haber process) and by electrolysis of body moving through the solution water. The main use is in the Haber and resisted by the solution’s viscos- process for making ammonia. Hydro- ity. If the solvent is water, the hydrogen is also used in various other

R″ O H R″ N N – H2O RC + NN R C H R′ H H R′

ketone hydrazine hydrazone Hydrazones hydrogen acceptor 276

industrial processes, such as the hydrogen atoms bound to electro- reduction of oxide ores, the reÜning negative atoms (F, N, O). It can be of petroleum, the production of regarded as a strong dipole–dipole hydrocarbons from coal, and the hy- attraction caused by the electron- drogenation of vegetable oils. Consid- withdrawing properties of the elec- erable interest has also been shown tronegative atom. Thus, in the water in its potential use in a ‘hydrogen molecule the oxygen atom attracts fuel economy’ in which primary en- the electrons in the O–H bonds. The ergy sources not based on fossil fuels hydrogen atom has no inner shells of (e.g. nuclear, solar, or geothermal en- electrons to shield the nucleus, and ergy) are used to produce electricity, there is an electrostatic interaction which is employed in electrolysing between the hydrogen proton and a water. The hydrogen formed is lone pair of electrons on an oxygen stored as liquid hydrogen or as metal atom in a neighbouring molecule. h hydrides. Chemically, hydrogen re- Each oxygen atom has two lone pairs acts with most elements. It was dis- and can make hydrogen bonds to covered by Henry *Cavendish in two different hydrogen atoms. The 1766. strengths of hydrogen bonds are A about one tenth of the strengths of • Information from the WebElements site normal covalent bonds. Hydrogen bonding does, however, have signiÜ- hydrogen acceptor See hydrogen cant effects on physical properties. carrier. Thus it accounts for the unusual hydrogenation 1. A chemical re- properties of *water and for the rela- action with hydrogen; in particular, tively high boiling points of H2O, HF, an addition reaction in which hydro- and NH3 (compared with H2S, HCl, gen adds to an unsaturated com- and PH3). It is also of great impor- pound. Nickel is a good catalyst for tance in living organisms. Hydrogen such reactions. 2. The process of bonding occurs between bases in the converting coal to oil by making the chains of DNA. It also occurs be- carbon in the coal combine with hy- tween the C=O and N–H groups in drogen to form hydrocarbons. See fis- proteins, and is responsible for main- cher–tropsch process; bergius taining the secondary structure. process. hydrogen azide (hydrazoic acid; azoimide) A colourless liquid, HN3; r.d. 1.09; m.p. –80°C; b.p. 37°C. It is highly toxic and a powerful reductant, which explodes in the presence of oxygen and other oxidizing agents. It may be prepared by the reaction of sodium amide and sodium 0.177nm nitrate at 175°C followed by distillation of a mixture of the resulting sodium azide and a dilute acid. See also azides.

hydrogen bond A type of electro- oxygen hydrogen static interaction between molecules occurring in molecules that have Hydrogen bond 277 hydrogen iodide

Hydrogen bonds are not purely elec- and methane with air and is used in trostatic and can be shown to have producing acrylate plastics. Hydrogen × –9 some covalent character. cyanide is a weak acid (Ka = 2.1 10 –3 hydrogen bromide A colourless mol dm ). With organic carbonyl gas, HBr; m.p. –88.5°C; b.p. –67°C. It compounds it forms *cyanohydrins. can be made by direct combination hydrogen electrode See hydro- of the elements using a platinum cat- gen half cell. alyst. It is a strong acid dissociating hydrogen Ûuoride A colourless extensively in solution (hydrobromic liquid, HF; r.d. 0.99; m.p. –83°C; b.p. acid). 19.5°C. It can be made by the action hydrogencarbonate (bicarbonate) of sulphuric acid on calcium Ûuoride. A salt of *carbonic acid in which one The compound is an extremely corro- hydrogen atom has been replaced; it sive Ûuorinating agent, which attacks thus contains the hydrogencarbonate glass. It is unlike the other hydrogen h – ion HCO3 . halides in being a liquid (a result of hydrogen carrier (hydrogen accep- *hydrogen bond formation). It is also tor) A molecule that accepts hydro- a weaker acid than the others be- Û gen atoms or ions, becoming reduced cause the small size of the uorine in the process (see oxidation– atom means that the H–F bond is reduction). The *electron transport shorter and stronger. Solutions of hy- chain, whose function is to generate drogen Ûuoride in water are known energy in the form of ATP during res- as hydroÛuoric acid. piration, involves a series of hydro- hydrogen half cell (hydrogen elec- gen carriers, including *NAD and trode) A type of *half cell in which a *FAD, which pass on the hydrogen metal foil is immersed in a solution (derived from the breakdown of glu- of hydrogen ions and hydrogen gas is cose) to the next carrier in the chain. bubbled over the foil. The standard hydrogen chloride A colourless hydrogen electrode, used in measur- fuming gas, HCl; m.p. –114.8°C; b.p. ing standard *electrode potentials, –85°C. It can be prepared in the labo- uses a platinum foil with a 1.0 M so- ratory by heating sodium chloride lution of hydrogen ions, the gas at 1 with concentrated sulphuric acid atmosphere pressure, and a tempera- ° (hence the former name spirits of ture of 25 C. It is written Pt(s)|H2(g), salt). Industrially it is made directly H+(aq), the effective reaction being → + from the elements at high tempera- H2 2H + 2e. ture and used in the manufacture of PVC and other chloro compounds. It hydrogenic Describing an atom or is a strong acid and dissociates fully ion that has only one electron; for ex- in solution (hydrochloric acid). ample, H, He+, Li2+, C5+. Hydrogenic atoms (or ions) do not involve elec- hydrogen cyanide (hydrocyanic tron–electron interactions and are acid; prussic acid) A colourless liquid easier to treat theoretically. or gas, HCN, with a characteristic odour of almonds; r.d. 0.699 (liquid at hydrogen iodide A colourless gas, 22°C); m.p. –14°C; b.p. 26°C. It is an HI; m.p. –51°C; b.p. –35.38°C. It can extremely poisonous substance be made by direct combination of formed by the action of acids on the elements using a platinum cata- metal cyanides. Industrially, it is made lyst. It is a strong acid dissociating ex- by catalytic oxidation of ammonia tensively in solution (hydroiodic acid hydrogen ion 278

or hydriodic acid). It is also a reduc- been used as an oxidant in rocket ing agent. fuels. hydrogen ion See acids. hydrogen spectrum The atomic spectrum of hydrogen is character- hydrogen molecule ion The sim- ized by lines corresponding to radia- plest type of molecule (H +), consist- 2 tion quanta of sharply deÜned ing of two hydrogen nuclei and one energy. A graph of the frequencies at electron. In the *Born–Oppenheimer which these lines occur against the approximation, in which the nuclei ordinal number that characterizes are regarded as being Üxed, the their position in the series of lines, *Schrödinger equation for the hy- produces a smooth curve indicating drogen molecule ion can be solved that they obey a formal law. In 1885 exactly. This enables ideas and ap- J. J. Balmer (1825–98) discovered the proximation techniques concerned law having the form: with chemical bonding to be tested h λ 2 2 quantitatively. 1/ = R(1/n1 + 1/n2 ) This law gives the so-called Balmer hydrogen peroxide A colourless series of lines in the visible spectrum or pale blue viscous unstable liquid, in which n = 2 and n = 3,4,5…, λ is H O ; r.d. 1.44; m.p. –0.41°C; b.p. 1 2 2 2 the wavelength associated with the 150.2°C. As with water, there is con- lines, and R is the *Rydberg constant. siderable hydrogen bonding in the In the Lyman series, discovered by liquid, which has a high dielectric Theodore Lyman (1874–1954), n1 = 1 constant. It can be made in the labo- and the lines fall in the ultraviolet. ratory by adding dilute acid to bar- ° The Lyman series is the strongest fea- ium peroxide at 0 C. Large quantities ture of the solar spectrum as ob- are made commercially by electroly- served by rockets and satellites above sis of KHSO4.H2SO4 solutions. An- the earth’s atmosphere. In the other industrial process involves Paschen series, discovered by F. catalytic oxidation (using nickel, pal- Paschen (1865–1947), n1 = 3 and the ladium, or platinum with an an- lines occur in the far infrared. The thraquinone) of hydrogen and water Brackett series (n = 4), Pfund series in the presence of oxygen. Hydrogen 1 (n1 = 5), and Humphreys series (n1 = 6) peroxide readily decomposes in light also occur in the far infrared. or in the presence of metal ions to A give water and oxygen. It is usually supplied in solutions designated by • Balmer’s paper volume strength. For example, 20- hydrogensulphate (bisulphate) A – volume hydrogen peroxide would salt containing the ion HSO4 or an yield 20 volumes of oxygen per vol- ester of the type RHSO4, where R is ume of solution. Although the *per- an organic group. It was formerly oxides are formally salts of H2O2, the called hydrosulphate. compound is essentially neutral. hydrogen sulphide (sulphuretted Thus, the acidity constant of the ion- hydrogen) A gas, H S, with an odour ization 2 of rotten eggs; r.d. 1.54 (liquid); m.p. ˆ + – H2O2 + H2O H3O + HO2 –85.5°C; b.p. –60.7°C. It is soluble in is 1.5 × 10–12 mol dm–3. It is a strong water and ethanol and may be pre- oxidizing agent, hence its use as a pared by the action of mineral acids mild antiseptic and as a bleaching on metal sulphides, typically hy- agent for cloth, hair, etc. It has also drochloric acid on iron(II) sulphide 279 4-hydroxybutanoic acid

(see kipp’s apparatus). Solutions in hydrophobic Lacking afÜnity for water (known as hydrosulphuric acid) water. See lyophobic. contain the anions HS– and minute benzene-1,4- 2– hydroquinone See traces of S and are weakly acidic. diol. Acid salts (those containing the HS– ion) are known as hydrogensulphides hydrosol A sol in which the contin- colloids (formerly hydrosulphides). In acid so- uous phase is water. See . lution hydrogen sulphide is a mild hydrosulphate See hydrogensul- reducing agent. Hydrogen sulphide phate. has an important role in traditional hydrosulphide See hydrogen sul- qualitative chemical analysis, where phide. it precipitates metals with insoluble sulphides. Hydrogen sulphide is ex- hydrosulphuric acid See hydro- ceedingly poisonous (more toxic than gen sulphide. claus h hydrogen cyanide). See also hydroxide A metallic compound process. containing the ion OH– (hydroxide hydrogensulphite (bisulphite) A ion) or containing the group –OH (hy- salt containing the ion –HSO or an droxyl group) bound to a metal atom. 3 Hydroxides of typical metals are ester of the type RHSO3, where R is an organic group. basic; those of *metalloids are amphoteric. hydroiodic acid See hydrogen io- hydroxoacid A type of acid in dide. which the acidic hydrogen is on a hy- hydrolysis A chemical reaction of a droxyl group attached to an atom compound with water. For instance, that is not attached to an oxo (=O) salts of weak acids or bases hydrolyse group. An example is → – + in aqueous solution, as in Si(OH4) + H2O Si(OH)3(O) + H3O +– – + – Na CH3COO + H2O ˆ Na + OH + Compare oxoacid. CH COOH 3 hydroxonium ion See oxonium The reverse reaction of *esteriÜca- ion. tion is another example. See also solvolysis. 4-hydroxybutanoic acid (gammahydroxybutyric acid; GHB) A hydromagnesite A mineral form naturally occurring carboxylic acid, of basic *magnesium carbonate, HO(CH2)3COOH, found in small 3MgCO3.Mg(OH)2.3H2O. amounts in most living things. It is used as a therapeutic drug to treat in- hydron The positive ion H+. The somnia, depression and alcoholism. name is used when it is not relevant GHB, as it is commonly known, is to specify the isotope, as is usually also used as an illegal club drug and the case in compounds in which the as a date-rape drug. hydrogen is in natural abundance. When the isotope is relevant then O 1 + 2 + proton ( H ), deuteron ( H ), or triton H 3 + 2 ( H ) should be used. OH C hydronium ion See oxonium ion. C C OH H H hydrophilic Having an afÜnity for 2 2 water. See lyophilic. 4-hydroxybutanoic acid 4-hydroxybutanoic acid lactone 280

4-hydroxybutanoic acid lactone molecule, as in the interaction of a (γ-butyrolactone) A colourless liquid methyl group with the benzene ring ° *lactone, C4H6O2; b.p. 206 C. It is in toluene. It is postulated to account used as a solvent and in the produc- for the stability of some carbonium tion of certain synthetic resins. ions. Hyperconjugation is often hydroxycerussite See lead(ii) car- thought of as a contribution of reso- bonate hydroxide. nance structures in which a sigma bond is broken to give ions; e.g. gly- – + hydroxyethanoic acid See C6H5CH2 H in toluene. It has been colic acid. called no-bond resonance. hydroxylamine A colourless solid, hyperÜne structure See fine ° NH2OH, m.p. 33 C. It explodes on structure. heating and may be employed as an oxidizing agent or reducing agent. It hypertonic solution A solution h is made by the reduction of nitrates that has a higher osmotic pressure or nitrites, and is used in making than some other solution. Compare hypotonic solution nylon. With aldehydes and ketones it . formsA *oximes. hypochlorite See chlorates. hypochlorous acid See chloric(i) • Information about IUPAC nomenclature acid. hydroxylation The introduction of hypophosphorus acid See phos- a hydroxyl group (–OH) into an or- phinic acid. ganic compound. For example, alkenes can be hydroxylated using hyposulphite See sulphinate. potassium permanganate or lead hyposulphurous acid See sul- ethanoate to give alcohols. In bio- phinic acid. chemistry, various enzymes can bring about hydroxylation. hypothesis See laws, theories, and hypotheses. hydroxyl group The group –OH in a chemical compound. hypotonic solution A solution that has a lower osmotic pressure 2-hydroxypropanoic acid See lactic acid than some other solution. Compare . hypertonic solution. hygroscopic Describing a sub- hypsochromic shift A shift of a stance that can take up water from spectral band to shorter wavelengths the atmosphere. See also deliquescence as a result of substitution in a mol- . ecule or as a result of a change in hyperconjugation The interaction the physical conditions. Compare of sigma bonds with pi bonds in a bathochromic shift. I

ice See water. have no means of storing energy except as translational kinetic energy. ice point The temperature at which there is equilibrium between ideal solution See raoult’s law. ice and water at standard atmos- IE Ionization energy. See ionization pheric pressure (i.e. the freezing or potential. melting point under standard conditions). It was used as a Üxed point (0°) ignition temperature The tem- on the Celsius scale, but the kelvin perature to which a substance must and the International Practical Tem- be heated before it will burn in air. perature Scale are based on the Ilkovic equation A relation used *triple point of water. in polarography relating the diffu- icosahedron A polyhedron having sion current ia and the concentration 20 triangular faces with Üve edges c. The Ilkovic equation has the form meeting at each vertex. The symme- ia = kc, where k is a constant. try of an icosahedron (known as Ü imides Organic compounds con- icosahedral symmetry) has vefold ro- taining the group –CO.NH.CO.– (the tation axes. It is impossible in *crys- imido group). tallography to have a periodic crystal A with the point group symmetry of an icosahedron (icosahedral packing). • Information about IUPAC nomenclature However, it is possible for short- imido group See imides. range order to occur with icosahedral symmetry in certain liquids and imines Compounds containing the glasses because of the dense packing group –NH– in which the nitrogen of icosahedra. Icosahedral symmetry atom is part of a ring structure, or also occurs in certain *quasicrystals, the group =NH, in which the nitro- such as alloys of aluminium and gen atom is linked to a carbon atom manganese. by a double bond. In either case, the group is referred to as an imino ideal crystal A single crystal with a group. perfectly regular lattice that contains A no impurities, imperfections, or • Information about IUPAC nomenclature other defects. imino group See imines. ideal gas (perfect gas) A hypothetical gas that obeys the *gas laws ex- Imperial units The British system actly. An ideal gas would consist of of units based on the pound and the molecules that occupy negligible yard. The former f.p.s. system was space and have negligible forces be- used in engineering and was loosely tween them. All collisions made be- based on Imperial units; for all tween molecules and the walls of the scientiÜc purposes *SI units are now container or between molecules and used. Imperial units are also being re- other molecules would be perfectly placed for general purposes by met- elastic, because the molecules would ric units. implosion 282

implosion An inward collapse of a tice can be analysed using the vessel, especially as a result of evacu- *Frenkel–Kontorowa model. ation. indene A colourless Ûammable hy- IMS See ion-mobility spectrometry. drocarbon, C9H8; r.d. 0.996; m.p. –1.8°C; b.p. 182.6°C. Indene is an incandescence The emission of aromatic hydrocarbon with a Üve- light by a substance as a result of membered ring fused to a benzene raising it to a high temperature. ring. It is present in coal tar and is InChI A type of *line notation intro- used as a solvent and raw material duced by IUPAC. The acronym stands for making other organic com- for International Chemical IdentiÜer. pounds. Originally, it was IChI (IUPAC chemi- independent-particle model A cal identiÜer). An InChI can give a model for electrons in a many- large amount of chemical informa- electron system in which the cor- tion displayed in layers and sublayers relation between electrons is either i separated by forward slashes. For ex- ignored or taken into account by re- ample, the InChI for naphthalene is garding an electron as moving in an InChI=1/C10H8/c-1-2-6-10-8-4-3-7- averaged-out potential that repre- 9(10)5-1/h1-8H sents the interactions between the electron and all the other particles in Here, InChI=1 indicates the version the system. Although the indepen- of InChI used. The remainder of the dent-particle model cannot describe string is the main layer. It consists of all aspects of many-electron systems, three sublayers. These are (1) the mo- it had some notable successes, such lecular formula (C10H8); (2) the atom- as explaining the shell structure of connection information (starting electrons in atoms. with c), and (3) the hydrogens pre- indeterminacy See uncertainty sent (starting with h). Other layers principle. can be added for such information as charge, stereochemistry, and isotopic indicator A substance used to show composition. the presence of a chemical substance A or ion by its colour. Acid–base indicators are compounds, such as phe- • An FAQ produced by the Chemistry nolphthalein and methyl orange, Department, Cambridge University that change colour reversibly, de- • Information and downloads from the pending on whether the solution is IUPAC site acidic or basic. They are usually weak incommensurate lattice A lattice acids in which the un-ionized form with long-range periodic order that HA has a different colour from the has two or more periodicities in negative ion A–. In solution the indi- which an irrational number gives the cator dissociates slightly ratio between the periodicities. An HA ˆ H+ + A– example of an incommensurate lat- In acid solution the concentration of tice occurs in certain magnetic sys- H+ is high, and the indicator is tems in which the ratio of the largely undissociated HA; in alkaline magnetic period to the atomic lattice solutions the equilibrium is displaced is an irrational number. The phase to the right and A– is formed. Useful transition between a commensurate acid–base indicators show a sharp lattice and an incommensurate lat- colour change over a range of about 283 induced-fit model

2 pH units. In titration, the point at ble indium(I), indium(II), and in- which the reaction is complete is the dium(III) compounds. The element equivalence point (i.e. the point at was discovered in 1863 by Ferdinand which equivalent quantities of acid Reich (1799–1882) and Hieronymus and base are added). The end point is Richter (1824–90). the point at which the indicator just A changes colour. For accuracy, the two • Information from the WebElements site must be the same. During a titration the pH changes sharply close to the indole A yellow solid, C8H7N, m.p. ° equivalence point, and the indicator 52 C. Its molecules consist of a ben- used must change colour over the zene ring fused to a nitrogen-contain- Ü same range. ing ve-membered ring. It occurs in Other types of indicator can be some plants and in coal tar, and is used for other reactions. Starch, for produced in faeces by bacterial ac- example, is used in iodine titrations tion. It is used in making perfumes. because of the deep blue complex it Indole has the nitrogen atom posi- forms. Oxidation–reduction indica- tioned next to the fused benzene i tors are substances that show a re- ring. An isomer with the nitrogen versible colour change between two atoms away from the fused ring isoindole oxidized and reduced forms. See also is called . adsorption indicator. indigo A blue vat dye, C16H10N2O2. It occurs as the glucoside indican in the leaves of plants of the genus In- digofera, from which it was formerly extracted. It is now made syntheti- Indole cally. indium Symbol In. A soft silvery el- induced emission (stimulated ement belonging to group 13 (for- emission) The emission of a photon merly IIIB) of the periodic table; a.n. by an excited atom or molecule in- 49; r.a.m. 114.82; r.d. 7.31 (20°C); duced by an incident photon of suit- m.p. 156.6°C; b.p. 2080±2°C. It occurs able energy. The relation between in zinc blende and some iron ores induced emission and *spontaneous Û and is obtained from zinc ue dust in emission is given by the *Einstein co- total quantities of about 40 tonnes efÜcients. The process of induced per annum. Naturally occurring in- emission is essential for the opera- dium consists of 4.23% indium–113 tion of lasers and masers. (stable) and 95.77% indium–115 (half- life 6 × 1014 years). There are a fur- induced-Üt model A proposed ther Üve short-lived radioisotopes. mechanism of interaction between The uses of the metal are small – an enzyme and a substrate. It postu- some special-purpose electroplates lates that exposure of an enzyme to a and some special fusible alloys. Sev- substrate causes the *active site of eral semiconductor compounds are the enzyme to change shape in order used, such as InAs, InP, and InSb. to allow the enzyme and substrate With only three electrons in its va- to bind (see enzyme–substrate com- lency shell, indium is an electron ac- plex). This hypothesis is generally ceptor and is used to dope pure preferred to the *lock-and-key mech- germanium and silicon; it forms sta- anism. inductive effect 284

inductive effect The effect of a less important and the lower valence group or atom of a compound in states become favoured. pulling electrons towards itself or in infrared chemiluminescence A pushing them away. Inductive effects technique for studying chemical re- can be used to explain some aspects action mechanisms by measuring of organic reactions. For instance, and analysing weak infrared emis- electron-withdrawing groups, such as sions from product molecules formed –NO2, –CN, –CHO, –COOH, and the in certain chemical reactions. If prod- halogens substituted on a benzene ucts are formed with excess energy, ring, reduce the electron density on this appears as excited vibrational the ring and decrease its susceptibil- states of the molecules, which decay ity to further (electrophilic) substitu- with emission of infrared radiation. tion. Electron-releasing groups, such Spectroscopic investigation of this ra- as –OH, –NH2, –OCH3, and –CH3, diation gives information about the elec- have the opposite effect. See also states in which the product mol- tronic effects . ecules were formed. i bioreac- industrial fermenter See infrared radiation (IR) Electro- tor . magnetic radiation with wavelengths inelastic neutron scattering A longer than that of red light but technique for investigating the mo- shorter than radiowaves, i.e. radiation of molecules by scattering neu- tion in the wavelength range 0.7 mi- trons. The neutrons pick up or lose crometre to 1 millimetre. It was energy as they move through a sam- discovered in 1800 by William Her- ple of a liquid. The analysis of neu- schel (1738–1822) in the sun’s spec- tron scattering experiments enables trum. The natural vibrational information to be obtained about the frequencies of atoms and molecules liquid. and the rotational frequencies of some gaseous molecules fall in the inert gases See noble gases. infrared region of the electromag- inert-pair effect An effect seen es- netic spectrum. The infrared absorp- pecially in groups 13 and 14 of the tion spectrum of a molecule is highly periodic table, in which the heavier characteristic of it and the spectrum elements in the group tend to form can therefore be used for molecular Ü compounds with a valency two lower identi cation. Glass is opaque to in- than the expected group valency. It is frared radiation of wavelength used to account for the existence of greater than 2 micrometres and thallium(I) compounds in group 13 other materials, such as germanium, and lead(II) in group 14. In forming quartz, and polyethylene, have to be compounds, elements in these used to make lenses and prisms. Pho- tographic Ülm can be made sensitive groups promote an electron from a µ Ülled s-level state to an empty p-level. to infrared up to about 1.2 m. The energy required for this is more infrared spectroscopy (IR spec- than compensated for by the extra troscopy) A technique for chemical energy gain in forming two more analysis and the determination of bonds. For the heavier elements, the structure. It is based on the princi- bond strengths or lattice energies ples that molecular vibrations occur in the compounds are lower than in the infrared region of the electro- those of the lighter elements. Conse- magnetic spectrum and functional quently the energy compensation is groups have characteristic absorption 285 insulin frequencies. The frequencies of most zyme–substrate complex other than interest range from 2.5 to 16 µm; the active site, known as an allosteric however, in IR spectroscopy it is site. This deforms the active site so common to use the reciprocal of the that the enzyme cannot catalyse the wavelength, and thus this range be- reaction. Noncompetitive inhibition comes 4000–625 cm–1. Examples of cannot be reversed by increasing the typical vibrations are centred on concentration of the substrate. The 2900 cm–1 for C–H stretching in alka- toxic effects of many substances are nes, 1600 cm–1 for N–H stretching in produced in this way. Inhibition by amino groups, and 2200 cm–1 for reaction products (feedback inhibi- C≡C stretching in alkynes. In an IR tion) is important in the control of spectrometer there is a source of IR enzyme activity. See also allosteric light, covering the whole frequency enzyme. range of the instrument, which is inner Describing a chemical com- split into two beams of equal inten- pound formed by reaction of one sity. One beam is passed through the part of a molecule with another part sample and the other is used as a ref- i of the same molecule. Thus, a lactam erence against which the Ürst is then is an inner amide; a lactone is an compared. The spectrum is usually inner ester. obtained as a chart showing absorption peaks, plotted against wave- inner-sphere mechanism See length or frequency. The sample can electron-transfer reaction. be a gas, liquid, or solid. See also inner transition series See tran- fourier-transform infrared. sition elements. Ingold, Sir Christopher Kelk inorganic chemistry The branch (1893–1970) British organic chemist of chemistry concerned with com- who worked chieÛy in London. His pounds of elements other than main work was on reaction mecha- carbon. Certain simple carbon nisms and, particularly, on *elec- compounds, such as CO, CO , CS , tronic effects in physical organic 2 2 and carbonates and cyanides, are chemistry. usually treated in inorganic chem- inhibition A reduction in the rate istry. of a catalysed reaction by substances insecticide See pesticide. called inhibitors. Inhibitors may work by poisoning catalysts for the reac- insertion reaction A type of tion or by removing free radicals in a chemical reaction in which an atom chain reaction. Enzyme inhibition af- or group is inserted between two fects biochemical reactions, in which atoms in a molecule. A common ex- the catalysts are *enzymes. Competi- ample is the insertion of carbene: tive inhibition occurs when the in- → R3C–X + CH2: R3CCH2X hibitor molecules resemble the The opposite of an insertion reaction substrate molecules and bind to the is an extrusion reaction. *active site of the enzyme, so preventing normal enzymatic activ- insulin A protein hormone, se- ity. Competitive inhibition can creted by the β cells of the islets of be reversed by increasing the con- Langerhans in the pancreas, that pro- centration of the substrate. In non- motes the uptake of glucose by body competitive inhibition the inhibitor cells, particularly in the liver and binds to a part of the enzyme or *en- muscles, and thereby controls its intensive variable 286

concentration in the blood. Insulin serves as a conductor of ions. Conse- was the Ürst protein whose amino- quently, such cells can be recharged acid sequence was fully determined many more times than, say, a lead- (in 1955). Underproduction of insulin acid accumulator, which eventually results in the accumulation of large suffers from degeneration of the elec- amounts of glucose in the blood and trodes. Lithium cells, based on this its subsequent excretion in the urine. principle, have been used in portable This condition, known as diabetes electronic equipment, such as cam- mellitus, can be treated successfully corders. They have also been consid- by insulin injections. ered for use in electric vehicles. intensive variable A quantity in a intercalation compound A type macroscopic system that has a well of compound in which atoms, ions, deÜned value at every point inside or molecules are trapped between the system and that remains (nearly) layers in a crystal lattice. There is no constant when the size of the system formal chemical bonding between is increased. Examples of intensive the host crystal and the trapped mol- i variables are the pressure, tempera- ecules (see also clathrate). Such com- ture, density, speciÜc heat capacity at pounds are formed by *lamellar constant volume, and viscosity. An solids and are often nonstoichiomet- intensive variable results when any ric; examples are graphitic oxide *extensive variable is divided by an (graphite–oxygen) and the mineral arbitrary extensive variable such as *muscovite. the volume. A macroscopic system interhalogen A chemical com- can be described by one extensive pound formed between two *halo- variable and a set of intensive vari- gens. Interhalogens are highly ables. reactive and volatile, made by direct intercalation cell A type of sec- combination of the elements. They ondary cell in which layered elec- include compounds with two atoms trodes, usually made of metal oxides (C1F, IBr, etc.), four atoms (C1F3, IF3, or graphite, store positive ions be- etc.), six atoms (BrF5, IF5, etc.) and IF7 tween the crystal layers of an elec- with eight atoms. trode. In one type, lithium ions form intermediate bond See chemical an intercalation compound with a bond. graphite electrode when the cell is j-j charged. During discharge, the ions intermediate coupling See coupling move through an electrolyte to the . other electrode, made of manganese intermetallic compound A com- oxide, where they are more tightly pound consisting of two or more bound. When the cell is being metallic elements present in deÜnite charged, the ions move back to their proportions in an alloy. positions in the graphite. This backwards and forwards motion of the intermolecular forces Weak ions has led to the name rocking- forces occurring between molecules. See van der waals’ force; hydrogen chair cell for this type of system. Such bond cells have the advantage that only . minor physical changes occur to the internal conversion A process in electrodes during the charging and which an excited atomic nucleus de- discharging processes and the elec- cays to the *ground state and the en- trolyte is not decomposed but simply ergy released is transferred by 287 inverse Compton effect electromagnetic coupling to one of interstellar molecules See astro- the bound electrons of that atom chemistry. rather than being released as a pho- interstitial See crystal defect. ton. The coupling is usually with an electron in the K-, L-, or M-shell of interstitial compound A com- the atom, and this conversion elec- pound in which ions or atoms of a tron is ejected from the atom with a nonmetal occupy interstitial posi- kinetic energy equal to the difference tions in a metal lattice. Such com- between the nuclear transition en- pounds often have metallic ergy and the binding energy of the properties. Examples are found in electron. The resulting ion is itself in the *carbides, *borides, and *sili- an excited state and usually subse- cides. quently emits an Auger electron or intersystem crossing A process in an X-ray photon. which a singlet excited electronic state makes a transition to a triplet internal energy Symbol U. The excited state at the point where the total of the kinetic energies of the potential energy curves for the ex- i atoms and molecules of which a sys- cited singlet and triplet states cross. tem consists and the potential ener- This transition is forbidden in the ab- gies associated with their mutual sence of *spin–orbit coupling but oc- interactions. It does not include the curs in the presence of spin–orbit kinetic and potential energies of the coupling. A triplet formed in this system as a whole nor their nuclear way is frequently in an excited vibra- energies or other intra-atomic ener- tional state. This excited triplet state gies. The value of the absolute inter- can reach its lowest vibrational state nal energy of a system in any by collisions with other molecules. particular state cannot be measured; The transition from this state to the the signiÜcant quantity is the change singlet state is forbidden in the ab- in internal energy, ∆U. For a closed sence of spin–orbit coupling but al- system (i.e. one that is not being re- lowed when there is spin–orbit plenished from outside its bound- coupling. This gives rise to the slow aries) the change in internal energy emision of electromagnetic radiation is equal to the heat absorbed by the known as *phosphorescence. system (Q) from its surroundings, less intrinsic factor See vitamin b com- the work done (W) by the system on plex. its surroundings, i.e. ∆U = Q – W. See also energy; thermodynamics. Invar A tradename for an alloy of iron (63.8%), nickel (36%), and carbon internal resistance The resistance (0.2%) that has a very low expansivity within a source of electric current, over a a restricted temperature such as a cell or generator. It can be range. It is used in watches and other calculated as the difference between instruments to reduce their sensitiv- the e.m.f. (E) and the potential differ- ity to changes in temperature. ence (V) between the terminals divided by the current being supplied inverse Compton effect The gain in energy of low-energy photons (I), i.e. r = (E – V)/I, where r is the in- when they are scattered by free elec- ternal resistance. trons of much higher energy. As a interstellar molecules See astro- consequence, the electrons lose en- chemistry. ergy. See also compton effect. inversion 288

inversion A chemical reaction in- (displacement by chlorine). There is volving a change from one optically one stable isotope, iodine–127, and active conÜguration to the opposite fourteen radioactive isotopes. It is conÜguration. The Walden inversion used in medicine as a mild antiseptic is an example. See nucleophilic sub- (dissolved in ethanol as tincture of io- stitution. dine), and in the manufacture of io- iodic acid Any of various oxoacids dine compounds. Chemically, it is of iodine, such as iodic(V) acid and less reactive than the other halogens iodic(VII) acid. When used without an and the most electropositive (metal- oxidation state speciÜed, the term lic) halogen. In solution it can be usually refers to iodic(V) acid (HIO ). determined by titration using thio- 3 sulphate solution: iodic(V) acid A colourless or very 2– → – 2– I2 + 252O3 2I + SuO6 . pale yellow solid, HIO3; r.d. 4.63; decomposes at 110°C. It is soluble in The molecule forms an intense blue water but insoluble in pure ethanol complex with starch, which is conse- i and other organic solvents. The com- quently used as an indicator. It was pound is obtained by oxidizing io- discovered in 1812 by Courtois. dine with concentrated nitric acid, A hydrogen peroxide, or ozone. It is a • Information from the WebElements site strong acid and a powerful oxidizing iodine(V) oxide (iodine pentoxide) agent. A white solid, I2O5; r.d. 4.799; decom- iodic(VII) acid (periodic acid) A hy- poses at 300–350°C. It dissolves in groscopic white solid, H5IO6, which water to give iodic(V) acid and also decomposes at 140°C and is very acts as an oxidizing agent. soluble in water, ethanol, and iodine value A measure of the ethoxyethane. Iodic(VII) acid may be amount of unsaturation in a fat or prepared by electrolytic oxidation of vegetable oil (i.e. the number of dou- concentrated solutions of iodic(V) ble bonds). It is obtained by Ünding acid at low temperatures. It is a weak the percentage of iodine by weight acid but a strong oxidizing agent. absorbed by the sample in a given iodide See halide. time under standard conditions. iodine Symbol I. A dark violet iodoethane (ethyl iodide) A colour- nonmetallic element belonging to less liquid *haloalkane, C2H5I; r.d. group 17 of the periodic table (see 1.9; m.p. –108°C; b.p. 72°C. It is made halogens); a.n. 53; r.a.m. 126.9045; by reacting ethanol with a mixture of r.d. 4.94; m.p. 113.5°C; b.p. 184.35°C. iodine and red phosphorus. The element is insoluble in water but iodoform triiodomethane soluble in ethanol and other organic See . solvents. When heated it gives a vio- iodoform test See haloform reac- let vapour that sublimes. Iodine is tion. required as a trace element (see es- iodomethane (methyl iodide) A sential element) by living organ- colourless liquid haloalkane, CH I; isms; in animals it is concentrated in 3 r.d. 2.28; m.p. –66.45°C; b.p. 42.4°C. It the thyroid gland as a constituent of can be made by reacting methanol thyroid hormones. The element is with a mixture of iodine and red present in sea water and was for- phosphorus. merly extracted from seaweed. It is now obtained from oil-well brines ion An atom or group of atoms that 289 ionic product has either lost one or more electrons, which anions X– are attached. The ex- making it positively charged (a change reaction is one in which dif- cation), or gained one or more elec- ferent anions in the solution displace trons, making it negatively charged the X– from the solid. Similarly, (an anion). See also ionization. cation exchange occurs with resins that have ionized acidic side groups ion association The electrostatic – – such as –COO or –SO2O , with posi- attraction between ions in solutions + of electrolytes that causes them to as- tive ions M attached. sociate in pairs. As a result, complete Ion exchange also occurs with inor- dissociation does not take place and ganic polymers such as *zeolites, in experimentally found electrical con- which positive ions are held at sites in the silicate lattice. These are used ductivities are lower than those pre- 2+ dicted by the *Debye–Hückel theory. for water-softening, in which Ca + In 1926, Neils Bjerrum (1879–1958) ions in solution displace Na ions in proposed improving the Debye– the zeolite. The zeolite can be regen- Hückel theory by taking ion associa- erated with sodium chloride solution. tion into account. He found that the Ion-exchange membranes are used as i signiÜcance of ion association in- separators in electrolytic cells to re- creases as the relative permittivity move salts from sea water and in of the solvent decreases. producing deionized water. Ion- exchange resins are also used as the ion exchange The exchange of stationary phase in ion-exchange ions of the same charge between a chromatography. solution (usually aqueous) and a solid in contact with it. The process occurs ion-exchange chromatography ion exchange widely in nature, especially in the See . absorption and retention of water- ionic bond See chemical bond. soluble fertilizers by soil. For exam- crystal ple, if a potassium salt is dissolved in ionic crystal See . water and applied to soil, potassium ionic product The product of the ions are absorbed by the soil and concentrations of ions present in a sodium and calcium ions are released given solution taking the stoichiome- from it. try into account. For a sodium chlo- The soil, in this case, is acting as ride solution the ionic product is an ion exchanger. Synthetic ion- [Na+][Cl–]; for a calcium chloride solu- exchange resins consist of various tion it is [Ca2+][Cl–]2. In pure water, copolymers having a cross-linked there is an equilibrium with a small three-dimensional structure to which amount of self-ionization: ionic groups have been attached. An H O ˆ H+ + OH– anionic resin has negative ions built 2 into its structure and therefore ex- The equilibrium constant of this dis- changes positive ions. A cationic resin sociation is given by + – has positive ions built in and ex- KW = [H ][OH ] changes negative ions. Ion-exchange since the concentration [H2O] can Ü resins, which are used in sugar re n- be taken as constant. KW is referred ing to remove salts, are synthetic or- to as the ionic product of water. It ganic polymers containing side has the value 10–14 mol2 dm–6 at groups that can be ionized. In anion 25°C. In pure water (i.e. no added exchange, the side groups are ion- acid or added alkali) [H+] = [OH–] = + –7 –3 ized basic groups, such as –NH3 to 10 mol dm . In this type of self- ionic radius 290

ionization of a solvent the ionic prod- in a collision with another particle or uct is also called the autoprotolysis a quantum of radiation (see pho- constant. See also solubility product; toionization). This may occur as a ph scale. result of the impact of *ionizing radiation or of thermal ionization and ionic radius A value assigned to the reaction takes the form the radius of an ion in a crystalline solid, based on the assumption that A → A+ + e the ions are spherical with a deÜnite Alternatively, ions can be formed by size. X-ray diffraction can be used to electron capture, i.e. measure the internuclear distance in A + e → A– crystalline solids. For example, in NaF the Na – F distance is 0.231 nm, ionization energy (IE) See ioniza- and this is assumed to be the sum of tion potential. + – the Na and F radii. By making cer- ionization gauge A vacuum gauge tain assumptions about the shielding consisting of a three-electrode system i effect that the inner electrons have inserted into the container in which on the outer electrons, it is possible the pressure is to be measured. Elec- to assign individual values to the + – trons from the cathode are attracted ionic radii – Na 0.096 nm; F 0.135 to the grid, which is positively bi- nm. In general, negative ions have ased. Some pass through the grid but larger ionic radii than positive ions. do not reach the anode, as it is main- The larger the negative charge, the tained at a negative potential. Some larger the ion; the larger the positive of these electrons do, however, col- charge, the smaller the ion. See also hydrodynamic radius lide with gas molecules, ionizing . them and converting them to posi- ionic strength Symbol I. A func- tive ions. These ions are attracted to tion expressing the effect of the the anode; the resulting anode cur- charge of the ions in a solution, rent can be used as a measure of the equal to the sum of the molality of number of gas molecules present. each type of ion present multiplied Pressure as low as 10–6 pascal can be Σ 2 by the square of its charge. I = mizi . measured in this way. ionization The process of produc- ionization potential (IP) Symbol I. ing *ions. Certain molecules (see The minimum energy required to re- electrolyte) ionize in solution; for move an electron from a speciÜed example, *acids ionize when dis- atom or molecule to such a distance solved in water (see also solvation): that there is no electrostatic interac- HCl → H+ + Cl– tion between ion and electron. Origi- nally deÜned as the minimum Electron transfer also causes ioniza- potential through which an electron tion in certain reactions; for exam- would have to fall to ionize an atom, ple, sodium and chlorine react by the the ionization potential was meas- transfer of a valence electron from ured in volts. It is now, however, the sodium atom to the chlorine deÜned as the energy to effect an ion- atom to form the ions that constitute ization and is conveniently measured a sodium chloride crystal: in electronvolts (although this is not → + – Na + Cl Na Cl an SI unit) or joules per mole. The Ions may also be formed when an synonymous term ionization energy atom or molecule loses one or more (IE) is often used. electrons as a result of energy gained The energy to remove the least 291 ion-mobility spectrometry strongly bound electron is the Ürst where the dot before a radical indi- ionization potential. Second, third, cates an unpaired electron and an * and higher ionization potentials can denotes an excited species. also be measured, although there is ion-microprobe analysis A tech- some ambiguity in terminology. nique for analysing the surface com- Thus, in chemistry the second ionization potential is often taken to be the position of solids. The sample is bombarded with a narrow beam (as minimum energy required to remove µ an electron from the singly charged small as 2 m diameter) of high- ion; the second IP of lithium would energy ions. Ions ejected from the be the energy for the process surface by sputtering are detected by mass spectrometry. The technique al- Li+ → Li2+ + e lows quantitative analysis of both In physics, the second ionization po- chemical and isotopic composition tential is the energy required to re- for concentrations as low as a few move an electron from the next to parts per million. highest energy level in the neutral atom or molecule; e.g. ion-mobility spectrometry (IMS) i → + A technique for detecting low con- Li Li + e, Ü + centrations of speci c compounds, where Li is an excited singly based on the rate at which their ions charged ion produced by removing migrate through an electric Üeld. The an electron from the K-shell. instrument operates in the gas phase A at atmospheric pressure. The sample • A table of values from NIST vapour enters an ionizing region, ionizing radiation Radiation of where ions can be produced by a va- sufÜciently high energy to cause riety of methods. In compact instru- *ionization in the medium through ments the source is usually a small which it passes. It may consist of a amount of radioactive material. The stream of high-energy particles (e.g. ions are allowed in pulses into a drift electrons, protons, alpha-particles) or tube, where they move to a detector short-wavelength electromagnetic ra- under the inÛuence of a homoge- diation (ultraviolet, X-rays, gamma- neous electric Üeld. The rate of move- rays). This type of radiation can cause ment depends on the way the ions extensive damage to the molecular interact with neutral molecules in structure of a substance either as a the tube and this depends on the result of the direct transfer of energy ion’s size and shape. The spectrum is to its atoms or molecules or as a re- a plot of detector signal against time, sult of the secondary electrons re- and is characteristic of the sample leased by ionization. In biological being ionized. Ion-mobility spectrom- tissue the effect of ionizing radiation eters are compact, sensitive, and fast- can be very serious, usually as a con- acting. They are widely used in sequence of the ejection of an elec- screening for drugs and explosives at tron from a water molecule and the airports, border crossings, etc. often oxidizing or reducing effects of the the technique is to wipe a swab over resulting highly reactive species: luggage and place it in the instru- → – 2H2O e + H2O + H2O* ment. More sophisticated instruments combine IMS with gas H O* → .OH + .H 2 chromatography or mass spectrome- + → + H2O + H2O .OH + H3O try. The technique is sometimes ionophore 292

referred to as gas-phase electro- inum metals); a.n. 77; r.a.m. 192.20; phoresis. r.d. 22.42; m.p. 2410°C; b.p. 4130°C. It occurs with platinum and is ionophore A relatively small hy- mainly used in alloys with platinum drophobic molecule that facilitates and osmium. The element forms a the transport of ions across lipid range of iridium(III) and iridium(IV) membranes. Most ionophores are complexes. It was discovered in 1804 produced by microorganisms. There by Smithson Tennant (1761–1815). are two types of ionophore: channel formers, which combine to form a A channel in the membrane through • Information from the WebElements site which ions can Ûow; and mobile ion iron Symbol Fe. A silvery malleable carriers, which transport ions across and ductile metallic *transition el- a membrane by forming a complex ement; a.n. 26; r.a.m. 55.847; r.d. with the ion. 7.87; m.p. 1535°C; b.p. 2750°C. The ion pair A pair of oppositely main sources are the ores *haematite i charged ions produced as a result of (Fe2O3), *magnetite (Fe3O4), limonite a single ionization; e.g. (FeO(OH)nH2O), ilmenite (FeTiO3), siderite (FeCO ), and pyrite (FeS ). HCl → H+ + Cl–. 3 2 The metal is smelted in a *blast fur- Sometimes a positive ion and an elec- nace to give impure *pig iron, which tron are referred to as an ion pair, as is further processed to give *cast in iron, *wrought iron, and various A → A+ + e–. types of *steel. The pure element has three crystal forms: alpha-iron, stable ion pump A type of *vacuum below 906°C with a body-centred- pump that can reduce the pressure cubic structure; gamma-iron, stable in a container to about 1 nanopascal between 906°C and 1403°C with a by passing a beam of electrons nonmagnetic face-centred-cubic through the residual gas. The gas is structure; and delta-iron, which is ionized and the positive ions formed the body-centred-cubic form above are attracted to a cathode within 1403°C. Alpha-iron is ferromagnetic the container where they remain up to its Curie point (768°C). The el- trapped. The pump is only useful at ement has nine isotopes (mass num- very low pressures, i.e. below about bers 52–60), and is the fourth most 1 micropascal. The pump has a lim- abundant in the earth’s crust. It is re- ited capacity because the absorbed quired as a trace element (see essen- ions eventually saturate the surface tial element) by living organisms. of the cathode. A more effective Iron is quite reactive, being oxidized pump can be made by simultane- by moist air, displacing hydrogen ously producing a Ülm of metal by from dilute acids, and combining ion impact (sputtering), so that with nonmetallic elements. It forms fresh surface is continuously pro- ionic salts and numerous complexes duced. The device is then known as with the metal in the +2 or +3 oxida- a sputter-ion pump. tion states. Iron(VI) also exists in the 2– IP See ionization potential. ferrate ion FeO4 , and the element also forms complexes in which its ox- IR See infrared radiation. idation number is zero (e.g. Fe(CO)5). iridium Symbol Ir. A silvery metal- A lic *transition element (see also plat- • Information from the WebElements site 293 iron(III) sulphate iron(II) chloride A green-yellow oxide. The compound has the sodium deliquescent compound, FeCl2; chloride structure, indicating its hexagonal; r.d. 3.16; m.p. 670°C. It ionic nature, but the crystal lattice is also exists in hydrated forms: deÜcient in iron(II) ions and it is non- FeCl2.2H2O (green monoclinic; r.d. stoichiometric. Iron(II) oxide dis- 2.36) and FeCl2.4H2O (blue-green solves readily in dilute acids. monoclinic deliquescent; r.d. 1.93). iron(III) oxide A red-brown to Anhydrous iron(II) chloride can be black insoluble solid, Fe O ; trigonal; made by passing a stream of dry hy- 2 3 r.d. 5.24; m.p. 1565°C. There is also a drogen chloride over the heated hydrated form, Fe O .xH O, which is metal; the hydrated forms can be 2 3 2 a red-brown powder; r.d. 2.44–3.60. made using dilute hydrochloric acid (See rusting.) or by recrystallizing with water. It is converted into iron(III) chloride by Iron(III) oxide occurs naturally as * the action of chlorine. haematite and can be prepared by heating iron(III) hydroxide or iron(II) iron(III) chloride A black-brown sulphate. It is readily reduced on i solid, FeCl3; hexagonal; r.d. 2.9; m.p. heating in a stream of carbon 306°C; decomposes at 315°C. It also monoxide or hydrogen. exists as the hexahydrate FeCl3.6H2O, pyrite a brown-yellow deliquescent crys- iron pyrites See . talline substance (m.p. 37°C; b.p. iron(II) sulphate An off-white ° 280–285 C). Iron(III) chloride is pre- solid, FeSO4.H2O; monoclinic; r.d. pared by passing dry chlorine over 2.970. There is also a heptahydrate, iron wire or steel wool. The reaction FeSO4.7H2O; blue-green monoclinic; proceeds with incandescence when r.d. 1.898; m.p. 64°C. The heptahy- started and iron(III) chloride sublimes drate is the best known iron(II) salt as almost black iridescent scales. The and is sometimes called green vitriol compound is rapidly hydrolysed in or copperas. It is obtained by the ac- moist air. In solution it is partly tion of dilute sulphuric acid on iron hydrolysed; hydrolysis can be sup- in a reducing atmosphere. The anhy- pressed by the addition of hydro- drous compound is very hygroscopic. chloric acid. The compound dissolves It decomposes at red heat to give in many organic solvents, forming iron(III) oxide, sulphur trioxide, and solutions of low electrical conductiv- sulphur dioxide. A solution of iron(II) ity: in ethanol, ethoxyethane, and sulphate is gradually oxidized on ex- pyridine the molecular weight cor- posure to air, a basic iron(III) sul- responds to FeCl3 but is higher in phate being deposited. other solvents corresponding to iron(III) sulphate A yellow hygro- Fe2Cl6. The vapour is also dimerized. In many ways the compound resem- scopic compound, Fe2(SO4)3; rhom- bles aluminium chloride, which it bic; r.d. 3.097; decomposes above may replace in Friedel–Crafts reac- 480°C. It is obtained by heating an tions. aqueous acidiÜed solution of iron(II) sulphate with hydrogen peroxide: iron(II) oxide A black solid, FeO; → cubic; r.d. 5.7; m.p. 1420°C. It can be 2FeSO4 + H2SO4 + H2O2 obtained by heating iron(II) oxalate; Fe2(SO4)3 + 2H2O the carbon monoxide formed pro- On crystallizing, the hydrate duces a reducing atmosphere thus Fe2(SO4)3.9H2O is formed. The acid preventing oxidation to iron(III) sulphate Fe2(SO4)3.H2SO4.8H2O is de- irreducible representation 294

posited from solutions containing a changes in a closed system occur in sufÜcient excess of sulphuric acid. the direction of increasing entropy. irreducible representation A irreversible process See irre- representation of a symmetry opera- versibility; reversible process. tion of a group, which cannot be ex- irreversible reaction See chemi- pressed in terms of a representation cal reaction. of lower dimension. When the representation of the group is in matrix IR spectroscopy See infrared form (i.e. a set of matrices that multi- spectroscopy. ply in the same way as the elements isentropic process Any process of the group), the matrix represen- that takes place without a change of tation cannot be put into block- *entropy. The quantity of heat trans- diagonal form by constructing a lin- ferred, δQ, in a reversible process is ear combination of the basis func- proportional to the change in en- tions. The importance of irreducible tropy, δS, i.e. δQ = TδS, where T is the i representations in *quantum me- thermodynamic temperature. There- chanics is that the energy levels of fore, a reversible *adiabatic process the system are labelled by the irre- is isentropic, i.e. when δQ equals ducible representations of the sym- zero, δS also equals zero. metry group of the system, thus enabling *selection rules to be de- Ising model A model for magnetic duced. In contrast to an irreducible systems in which atomic *spins have representation, a reducible represen- to be aligned either parallel or an- tation can be expressed in terms of a tiparallel to a given direction. The representation of lower dimension, Ising model was introduced, and with a reducible matrix representa- solved in the case of one dimension, tion that can be put into block diago- by E. Ising in 1925. Ising found that nal form by constructing a linear in one dimension, in the absence of combination of the basis functions. an external magnetic Üeld, there is no spontaneous magnetization at any irreversibility The property of a temperature above absolute zero. system that precludes a change to The study of *phase transitions in the system from being a *reversible the Ising model in dimensions process. The paradox that although greater than one has been very im- the equations describing the bodies portant to the general understanding in a system, such as Newton’s laws of of phase transitions. In two dimen- motion, Maxwell’s equation, or sions, the Ising model was Ürst solved Schrödinger’s equation are invariant exactly by L. Onsager in 1944. In under time reversal, events involving three dimensions, approximation systems made up from large num- techniques, frequently involving bers of these bodies are not re- renormalization have to be used. versible. The process of scrambling an egg is an example. The resolution ISIS/Draw A commonly used chem- of this paradox requires the concept ical drawing program for 2D and 3D of *entropy using *statistical me- structures, copyright of MDL Infor- chanics. Irreversibility occurs in the mation Systems, Inc. The program transition from an ordered arrange- has certain additional features in- ment to a disordered arrangement, cluding calculation of molecular which is a natural trend, since weight, calculation of percentages of 295 isomerism elements present, IUPAC name gen- surrounding water prevents coagula- eration, and viewing in RasMol. tion, the isoelectric point is at the A minimum of stability. The isoelectric • A limited version of ISIS/Draw at the point is characterized by the value of Elsevier MDL website (free registration the pH at that point. Above the iso- required) electric pH level the substance acts as a base and below this level it acts as Ü iso- Pre x denoting that a com- an acid. For example, at the isoelec- pound is an *isomer, e.g. isopentane tric point the pH of gelatin is 4.7. Pro- (CH3CH(CH3)C2H5, 2-methylbutane) is teins precipitate most readily at their an isomer of pentane. isoelectric points; this property can isobar 1. A curve on a graph indi- be utilized to separate mixtures of cating readings taken at constant proteins or amino acids. pressure. 2. One of two or more nu- isoelectronic Denoting different clides that have the same number of molecules that have the same num- nucleons but different *atomic number of electrons. For example N2 and i bers. Radium–88, actinium–89, and CO are isoelectronic. The energy thorium–90 are isobars as each has a level diagrams of isoelectronic mol- *nucleon number of 228. ecules are therefore similar. cyanic acid isocyanate See . isoenzyme See isozyme. cyanic acid isocyanic acid See . isoindole See indole. isonitrile isocyanide See . isoleptic complex A metal com- isocyanide test A test for primary plex in which all the ligands are the amines by reaction with an alcoholic same. solution of potassium hydroxide and isoleucine See amino acid. trichloromethane. → isomerism The existence of chemi- RNH2 + 3KOH + CHCl3 RNC + 3KCl + 3H O cal compounds (isomers) that have 2 the same molecular formulae but dif- The isocyanide RNC is recognized by ferent molecular structures or differ- its unpleasant smell. This reaction of ent arrangements of atoms in space. primary amines is called the carby- In structural (or constitutional iso- lamine reaction. merism) isomerism the molecules isoelectric point The point at have different molecular structures: which a substance (such as a colloid i.e. they may be different types of or protein) has zero net electric compound or they may simply differ charge. Usually such substances are in the position of the functional positively or negatively charged, de- group in the molecule. Structural iso- pending on whether hydrogen ions mers generally have different physi- or hydroxyl ions are predominantly cal and chemical properties. In absorbed. At the isoelectric point the stereoisomerism, the isomers have net charge on the substance is zero, the same formula and functional as positive and negative ions are ab- groups, but differ in the arrangement sorbed equally. The substance has its of groups in space. Optical isomerism minimum conductivity at its isoelec- is one form of this (see optical activ- tric point and therefore coagulates ity). Another type is cis–trans iso- best at this point. In the case of hy- merism (formerly geometrical drophilic substances, in which the isomerism), in which the isomers isomerism 296

H H H H H H

HCCCCI HCCCH

H H H H CI H

1-chloropropane 2-chloropropane structural isomers in which the functional group has different positions

H H H H

HCOCH HCCOH

H H H H

methoxymethane ethanol i structural isomers in which the functional groups are different

HCH3 CH3 CH3

C C C C

CH3 H H H

trans-but-2-ene cis-but-2-ene cis–trans isomers in which the groups are distributed on a double bond

CI I ICI

Pt Pt

I CI I CI

cis–trans isomers in a square-planar complex

O OH

CC C C

keto form enol form keto–enol tautomerism Isomerism

have different positions of groups complexes with formulae of the type

with respect to a double bond or ring MX3Y3 can display a different type of or central atom (see illustration). isomerism. If the three X ligands are Octahedral complexes can display in a plane that includes the metal cis-trans isomerism if they have for- atom and the three Y ligands are in a mulae of the type MX2Y4. Octahedral different plane at right angles, then 297 isopolymorphism

Y Y

Y X

X M X X M Y

Y Y

Y Y trans-isomer cis-isomer

X X

X Y

X M Y X M Y i Y Y

Y X fac-isomer mer-isomer

Isomerism the structure is a mer-isomer (merid- A ional). If the three X ligands are all • Information about IUPAC nomenclature on one face of the octahedron and octane octane the three Y ligands are on an oppo- iso-octane See ; number site face, then it is a fac-isomer (fa- . cial). See also ambidentate; e–z isopleth A vertical line in a liq- convention. uid–vapour phase diagram consisting isomers See isomerism. of a line of constant composition of the whole system as the pressure is isometric 1. (in crystallography) changed. The word isopleth comes Denoting a system in which the axes from the Greek for ‘equal abun- are perpendicular to each other, as in dance’. See also tie line. cubic crystals. 2. Denoting a line on isopoly compound See cluster a graph illustrating the way in which compound temperature and pressure are inter- . related at constant volume. isopolymorphism The property of isomorphism The existence of two a substance with more than one crystalline structure that is isomorphous or more substances (isomorphs) that with the crystalline structures of an- have the same crystal structure, so other substance (see polymorphism). that they are able to form *solid solu- For example, antimony(III) oxide, tions. Sb2O3, has both rhombic and octahe- isonitrile (isocyanide; carbylamine) dral structures and these are isomor- An organic compound containing the phous with the similar structures of group –NC, in which the bonding is arsenic(III) oxide, As2O3; both oxides to the nitrogen atom. therefore exhibit isopolymorphism. isoprene 298

isoprene A colourless liquid diene, the gas uranium hexaÛuoride), distil- CH2:C(CH3)CH:CH2. The systematic lation (formerly used to produce name is 2-methylbuta-1,3-diene. The heavy water), electrolysis (requiring isoprene structure is the fundamen- cheap electrical power), thermal dif- tal structural unit in *terpenes and fusion (formerly used to separate ura- natural *rubber. The compound itself nium isotopes, but now considered is used in making synthetic rubbers. uneconomic) and centrifuging. Vari- isoquinoline See quinoline. ous laser methods (involving the excitation of one isotope and its isotactic polymer See polymer. subsequent separation by electro- isotherm 1. A line on a map or magnetic means) have also been chart joining points or places of employed. equal temperature. 2. A curve on a isotopic isomers See isotopomers. graph representing readings taken at constant temperature (e.g. the rela- isotopic number (neutron excess) tionship between the pressure and The difference between the number i volume of a gas at constant tempera- of neutrons in an isotope and the ture). number of protons. isothermal process Any process isotopologues Substances that dif- that takes place at constant tempera- fer only in isotopic composition, e.g. ture. In such a process heat is, if nec- CH3OH, CH2DOH, CHD2OH, CD3OH, essary, supplied or removed from the CD3OD. system at just the right rate to main- isotopomers (isotopic isomers) tain constant temperature. Compare Molecules that have the same num- adiabatic process . bers of each type of isotopic atom isotonic Describing solutions that but in different arrangements in the have the same osmotic pressure. molecule. For example, CH2DOH and CH OD are isotopomers. isotope One of two or more atoms 3 of the same element that have the isotropic Denoting a medium same number of protons in their nu- whose physical properties are inde- cleus but different numbers of neu- pendent of direction. Compare trons. Hydrogen (1 proton, no anisotropic. neutrons), deuterium (1 proton, 1 isozyme (isoenzyme) One of several neutron), and tritium (1 proton, 2 forms of an enzyme in an individual neutrons) are isotopes of hydrogen. or population that catalyse the same Most elements in nature consist of a reaction but differ from each other mixture of isotopes. See isotope separation in such properties as substrate . afÜnity and maximum rates of en- isotope separation The separa- zyme–substrate reaction (see tion of the *isotopes of an element michaelis–menten curve). from each other on the basis of slight itaconic acid A product of the fer- differences in their physical proper- mentation of the Ülamentous fungus ties. For laboratory quantities the Aspergillus niger. Itaconic acid is used most suitable device is often the commercially in the production of mass spectrometer. On a larger scale adhesives and paints. the methods used include gaseous diffusion (widely used for separating IUPAC International Union of Pure isotopes of uranium in the form of and Applied Chemistry. An interna- 299 IUPAC tional nongovernmental body and other data. The IUPAC rules for formed in 1919 to foster worldwide systematic chemical nomenclature communications in chemical science, are widely adopted. IUPAC also intro- both academic and industrial. IUPAC duced the *InChI line notation for is the international authority deÜn- compounds. ing recommended terminology, A atomic weights, isotopic abundances, • The IUPAC home page

i J

Jablonski diagram A diagram energy levels) the actual structure of that represents the electronic energy the molecule or ion is distorted so as levels (and their relative positions) of to split the energy levels (‘raise’ the a molecule. A Jablonski diagram en- degeneracy). The effect is observed in ables radiative transitions between inorganic complexes. For example, 2+ energy levels in molecules, as well as the ion [Cu(H2O)6] is octahedral and such phenomena as internal conver- the six ligands might be expected to sion, to be shown. The vertical axis occupy equidistant positions at the measures energy, with the electronic corners of a regular octahedron. In energy levels in their lowest vibra- fact, the octahedron is distorted, tional states being short horizontal with four ligands in a square and two lines. The vibrational energy levels of opposite ligands further away. If the a particular electronic state are ‘original’ structure has a centre of drawn as shaded regions above the symmetry, the distorted structure horizontal line for that state. In a must also have a centre of symmetry. Jablonski diagram the horizontal lo- The effect was predicted theoretically cation of the electronic state is not by H. A. Jahn (1907–79) and Edward related to the internuclear distance Teller (1908–2003) in 1937. between the atoms in these states. jargoon See zircon. jacinth See zircon. jasper An impure variety of *chal- jade A hard semiprecious stone cedony. It is associated with iron ores consisting either of jadeite or and as a result contains iron oxide nephrite. Jadeite, the most valued of impurities that give the mineral its the two, is a sodium aluminium py- characteristic red or reddish-brown roxene, NaAlSi2O6. It is prized for its colour. Jasper is used as a gemstone. intense translucent green colour but white, green and white, brown, and jet A variety of *coal that can be cut orange varieties also occur. The only and polished and is used for jew- important source of jadeite is in the ellery, ornaments, etc. Mogaung region of upper Burma. jeweller’s rouge Red powdered Nephrite is one of the amphibole haematite, iron(III) oxide, Fe2O3. It is group of rock-forming minerals. It a mild abrasive used in metal clean- occurs in a variety of colours, includ- ers and polishes. ing green, yellow, white, and black. Sources include the Siberia, Turk- j-j coupling A type of *coupling oc- istan, New Zealand, Alaska, China, curring between electrons in atoms and W USA. and nucleons in nuclei, in which See jade the energies associated with the jadeite . spin–orbit interactions are much Jahn–Teller effect If a likely struc- higher than the energies associated ture of a nonlinear molecule or ion with electrostatic repulsion. *Multi- would have degenerate orbitals (i.e. plets of many-electron atoms having two molecular orbitals with the same a large atomic number are character- 301 Joule–Thomson effect ized by j-j coupling. The multiplets of William Thomson (Lord *Kelvin), he many atoms and nuclei are interme- discovered the *Joule–Thomson ef- diate between j-j coupling and *Rus- fect. sell–Saunders coupling, a state called Joule’s law The *internal energy intermediate coupling . of a given mass of gas is independent Joliot-Curie, Irène (1897–1956) of its volume and pressure, being a French physicist, daughter of Marie function of temperature alone. This and Pierre *Curie, who was educated law, which was formulated by James by her mother and her scientist asso- Prescott Joule, applies only to *ideal ciates. In 1921 she began work at the gases (for which it provides a deÜni- Radium Institute, becoming director tion of thermodynamic temperature) in 1946. In 1926 she married Frédéric as in a real gas intermolecular forces Joliot (1900–58). They shared the would cause changes in the internal 1935 Nobel Prize for chemistry for energy should a change of volume their discovery of artiÜcial radioactiv- occur. See also joule–thomson ity the previous year. effect. joliotium See transactinide el- Joule–Thomson effect (Joule– ements. Kelvin effect) The change in temper- j ature that occurs when a gas expands JMol A commonly used molecular through a porous plug into a region viewing program similar to RasMol. of lower pressure. For most real gases It can be used as an applet in a web the temperature falls under these cir- page. cumstances as the gas has to do inter- A nal work in overcoming the • Details and a download for JMol from intermolecular forces to enable the SourceForge expansion to take place. This is a de- joule Symbol J. The *SI unit of viation from *Joule’s law. There is work and energy equal to the work usually also a deviation from *Boyle’s done when the point of application law, which can cause either a rise or of a force of one newton moves, in a fall in temperature since any in- the direction of the force, a distance crease in the product of pressure and of one metre. 1 joule = 107 ergs = volume is a measure of external work done. At a given pressure, there 0.2388 calorie. It is named after is a particular temperature, called James Prescott *Joule. the inversion temperature of the gas, Joule, James Prescott (1818–89) at which the rise in temperature British physicist. In 1840 he discov- from the Boyle’s law deviation is bal- ered the relationship between elec- anced by the fall from the Joule’s law tric current passing through a wire, deviation. There is then no tempera- its resistance, and the amount of ture change. Above the inversion heat produced. In 1849 he gave an ac- temperature the gas is heated by ex- count of the kinetic theory of gases, pansion, below it, it is cooled. The ef- and a year later announced his best- fect was discovered by James Prescott known Ünding, the mechanical Joule working in collaboration with equivalent of heat. Later, with William Thomson (later Lord Kelvin). K

kainite A naturally occurring dou- stable chains. Kekulé is best remem- ble salt of magnesium sulphate and bered for his proposed structure for potassium chloride, MgSO4.KCl.3H2O. {benzene} in 1865, which he correctly interpreted as having a sym- kalinite A mineral form of *alu- metrical ring of six carbon atoms. minium potassium sulphate (Al2(SO4)3.K2SO4.24H2O). Kekulé structure A proposed structure of *benzene in which the kaolin (china clay) A soft white clay molecule has a hexagonal ring of car- that is composed chieÛy of the min- bon atoms linked by alternating dou- eral kaolinite (see clay minerals). It is ble and single bonds. Kekulé formed during the weathering and structures contribute to the reso- hydrothermal alteration of other nance hybrid of benzene. The struc- clays or feldspar. Kaolin is mined in ture was suggested in 1865 by the UK, France, the Czech Republic, Friedrich August *Kekulé. and USA. Besides its vital importance in the ceramics industry it is also kelvin Symbol K. The *SI unit of used extensively as a Üller in the thermodynamic *temperature equal manufacture of rubber, paper, paint, to the fraction 1/273.16 of the ther- and textiles and as a constituent of modynamic temperature of the medicines. *triple point of water. The magnitude of the kelvin is equal to that of Kastle Meyer test (phenolph- the degree Celsius (centigrade), but a thalein test) A presumptive test used temperature expressed in degrees to indicate blood. Phenolphthalein celsius is numerically equal to the and hydrogen peroxide are used; re- temperature in kelvins less 273.15 action with haemoglobin in the (i.e. °C = K – 273.15). The *absolute blood gives a pink colour. zero of temperature has a tempera- katharometer An instrument for ture of 0 K (–273.15°C). The former comparing the thermal conductivi- name degree kelvin (symbol °K) be- ties of two gases by comparing the came obsolete by international agree- rate of loss of heat from two heating ment in 1967. The unit is named coils surrounded by the gases. The in- after Lord Kelvin. strument can be used to detect the Kelvin, Lord (William Thomson; presence of a small amount of an im- 1824–1907) British physicist, born in purity in air and is also used as a de- Belfast, who became professor of tector in gas chromatography. natural philosophy at Glasgow Uni- Kekulé, Friedrich August von versity in 1846. He carried out im- Stradonitz (1829–96) German portant experimental work on chemist, who became professor at electromagnetism, inventing the Ghent (1858) and later at Bonn mirror galvanometer and contribut- (1867). He studied the structures of ing to the development of telegra- organic molecules and was the Ürst phy. He also worked with James to recognise that carbon atoms form *Joule on the *Joule–Thomson (or 303 ketamine

Joule–Kelvin) effect. His main theo- beam of monochromatic light, wave- retical work was in {thermodynam- length λ, such that ics}, in which he stressed the δ λ = (no – ne)x/ , importance of the conservation of where x is the length of the light energy. He also introduced the con- path in the cell. Kerr also showed cept of absolute zero and the Kelvin empirically that the ratio temperature scale based on it; the λ 2 unit of thermodynamic temperature (no – ne) = BE , is named after him. In 1896 he was where E is the Üeld strength and B is created Baron Kelvin of Largs. a constant, called the Kerr constant, which is characteristic of the sub- keratin Any of a group of Übrous stance and approximately inversely *proteins occurring in hair, feathers, proportional to the thermodynamic hooves, and horns. Keratins have temperature. coiled polypeptide chains that com- The Kerr shutter consists of a Kerr bine to form supercoils of several cell Ülled with a liquid, such as ni- polypeptides linked by disulphide trobenzene, placed between two bonds between adjacent cysteine crossed polarizers; the electric Üeld is amino acids. arranged to be perpendicular to the kerosine See petroleum. axis of the light beam and at 45° to the axis of the polarizers. In the ab- k Kerr effect The ability of certain Ü substances to refract differently light sence of a eld there is no optical path through the device. When the waves whose vibrations are in two di- Ü rections (see double refraction) eld is switched on the nitrobenzene when the substance is placed in an becomes doubly refracting and a electric Üeld. The effect, discovered path opens between the crossed po- in 1875 by John Kerr (1824–1907), is larizers. caused by the fact that certain mol- ketals Organic compounds, similar ecules have electric *dipoles, which to *acetals, formed by addition of an tend to be orientated by the applied alcohol to a ketone. If one molecule Üeld; the normal random motions of of ketone (RR′CO) reacts with one the molecules tends to destroy this molecule of alcohol R″OH, then a orientation and the balance is struck hemiketal is formed. The rings of ke- by the relative magnitudes of the tose sugars are hemiketals. Further Üeld strength, the temperature, and reaction produces a full ketal ′ ″ the magnitudes of the dipole mo- (RR C(OR )2). ments. A The Kerr effect is observed in a • Information about IUPAC nomenclature Kerr cell, which consists of a glass cell containing the liquid or gaseous ketamine A vetinary anaesthetic substance; two capacitor plates are inserted into the cell and light is CH O 3 passed through it at right angles to the electric Üeld. There are two NH principal indexes of refraction: no (the ordinary index) and ne (the extraordinary index). The difference in the velocity of propagation in the cell causes a phase difference, δ, be- Cl tween the two waves formed from a Ketamine ketene 304

that is used illegally as a club drug. It ketones Organic compounds that is a class A drug in the UK. contain the carbonyl group (>C=O) linked to two hydrocarbon groups. ketene (keten) 1. The compound The ketone group is a carbonyl group CH =C=O (ethenone). It can be pre- 2 with two single bonds to other car- pared by pyrolysis of propanone. The bon atoms. In systematic chemical compound reacts readily with com- nomenclature, ketone names end pounds containing hydroxyl or with the sufÜx -one. Examples are amino groups and is used as an acety- propanone (acetone), CH3COCH3, and lating agent. 2. Any of a class of butanone (methyl ethyl ketone), compounds of the type R1R2=C=O, CH3COC2H5. Ketones can be made by where R1 and R2 are organic groups. oxidizing secondary alcohols to con- Ketenes are reactive compounds and vert the C–OH group to C=O. Certain are often generated in a reaction ketones form addition compounds medium for organic synthesis. with sodium hydrogensulphate(IV) A (sodium hydrogensulphite). They also • Information about IUPAC nomenclature form addition compounds with hydrogen cyanide to give *cyanohy- keto–enol tautomerism A form drins and with alcohols to give of tautomerism in which a com- *ketals. They undergo condensation k pound containing a –CH2–CO– group reactions to yield *oximes, *hydra- (the keto form of the molecule) is in zones, phenylhydrazones, and *semi- equilibrium with one containing the carbazones. These are reactions that –CH=C(OH)– group (the enol). It oc- they share with aldehydes. Unlike curs by migration of a hydrogen aldehydes, they do not affect atom between a carbon atom and the Fehling’s solution or Tollens reagent oxygen on an adjacent carbon. See and do not easily oxidize. Strong oxi- isomerism. dizing agents produce a mixture of keto form See keto–enol tau- carboxylic acids; butanone, for exam- tomerism. ple, gives ethanoic and propanoic acids. ketohexose See monosaccharide. A ketol An organic compound that • Information about IUPAC nomenclature has both an alcohol (-CH OH) and a 2 ketopentose See monosaccharide. keto (=CO) group. Ketols are made by a condensation reaction between two ketose See monosaccharide. ketones or by the oxidation of dihy- khat A plant, Catha edilis, found in dric alcohols (glycols). East Africa and the Arabian peninsu- ketone body Any of three com- lar. The leaves are chewed to give a pounds, acetoacetic acid (3-oxobu- mildly stimulating and euphoric effect. Its activity comes from two alka- tanoic acid, CH3COCH2COOH), β-hydroxybutyric acid (3-hydroxybu- loids: *cathine and, in fresh leaves, the more potent *cathinone. It is a tanoic acid, CH3CH(OH)CH2COOH), and acetone or (propanone, controlled substance in many countries including the US. In the UK it is CH3COCH3), produced by the liver as a result of the metabolism of body not a controlled substance and is fat deposits. Ketone bodies are nor- used by certain Somalian and Yemeni mally used as energy sources by pe- ethnic groups. ripheral tissues. kieselguhr (diatomaceous earth; di- 305 kinetic theory atomite) A soft Üne-grained deposit the deuterated compound would be consisting of the siliceous skeletal re- slightly lower because of the lower mains of diatoms, formed in lakes vibrational frequency of the C–D and ponds. Kieselguhr is used as an bond. Such effects are used in investi- absorbent, Ültering material, Üller, gating the mechanisms of chemical and insulator. reactions. kieserite A mineral form of *mag- kinetic energy See energy. nesium sulphate monohydrate, kinetic isotope effect See kinetic MgSO .H O. 4 2 effect. kilo- Symbol k. A preÜx used in the kinetics The branch of physical metric system to denote 1000 times. chemistry concerned with measuring For example, 1000 volts = 1 kilovolt and studying the rates of chemical (kV). reactions. The main aim of chemical kilogram Symbol kg. The *SI unit kinetics is to determine the mecha- of mass deÜned as a mass equal to nism of reactions by studying the that of the international platinum– rate under different conditions (tem- iridium prototype kept by the Inter- perature, pressure, etc.). See also acti- national Bureau of Weights and vated-complex theory; arrhenius Measures at Sèvres, near Paris. equation. k kimberlite A rare igneous rock kinetic theory A theory, largely that often contains diamonds. It oc- the work of Count *Rumford, James curs as narrow pipe intrusions but is Prescott *Joule, and James Clerk often altered and fragmented. It con- *Maxwell, that explains the physical sists of olivine and phlogopite mica, properties of matter in terms of the usually with calcite, serpentine, and motions of its constituent particles. other minerals. The chief occur- In a gas, for example, the pressure is rences of kimberlite are in South due to the incessant impacts of the Africa, especially at Kimberley (after gas molecules on the walls of the which the rock is named), and in the container. If it is assumed that the Yakutia area of Siberia. molecules occupy negligible space, exert negligible forces on each other kinematic viscosity Symbol ν. except during collisions, are perfectly The ratio of the *viscosity of a liquid elastic, and make only brief collisions to its density. The SI unit is m2 s–1. with each other, it can be shown that kinetic effect A chemical effect the pressure p exerted by one mole that depends on reaction rate rather of gas containing n molecules each of than on thermodynamics. For exam- mass m in a container of volume V, ple, diamond is thermodynamically will be given by: _ less stable than graphite; its apparent p = nmc 2/3V, stability depends on the vanishingly _ where cc2 is the mean square speed slow rate at which it is converted. of the molecules. As according to the *Overpotential in electrolytic cells is *gas laws for one mole of gas: pV = another example of a kinetic effect. RT, where T is the thermodynamic Kinetic isotope effects are changes in temperature, and R is the molar *gas reaction rates produced by isotope constant, it follows that: substitution. For example, if the slow _ 2 step in a chemical reaction is the RT = nmc /3 breaking of a C–H bond, the rate for Thus, the thermodynamic tempera- Kipp’s apparatus 306

ture of a gas is proportional to the Kipp’s apparatus A laboratory ap- mean square speed of its molecules. paratus for making a gas by the reac- As the average kinetic *energy of_ tion of a solid with a liquid (e.g. the translation of the molecules is mc 2/2, reaction of hydrochloric acid with the temperature is given by: iron sulphide to give hydrogen sul- _ T = (mc 2/2)(2n/3R) phide). It consists of three intercon- nected glass globes arranged The number of molecules in one vertically, with the solid in the mid- mole of any gas is the *Avogadro dle globe. The upper and lower constant, N ; therefore in this equa- A globes are connected by a tube and tion n = N . The ratio R/N is a con- A A contain the liquid. The middle globe stant called the *Boltzmann constant has a tube with a tap for drawing off (k). The average kinetic energy of gas. When the tap is closed, pressure translation of the molecules of one of gas forces the liquid down in the mole of any gas is therefore 3kT/2. bottom reservoir and up into the top, For monatomic gases this is propor- and reaction does not occur. When tional to the *internal energy (U) of the tap is opened, the release in pres- the gas, i.e. sure allows the liquid to rise into the U = NA3kT/2 middle globe, where it reacts with the solid. It is named after Petrus k and as k = R/NA U = 3RT/2 Kipp (1808–64). For diatomic and polyatomic gases Kirchhoff, Gustav Robert the rotational and vibrational ener- (1824–87) German physicist, who in gies also have to be taken into ac- 1850 became a professor at Breslau count (see degrees of freedom). and four years later joined Robert In liquids, according to the kinetic {Bunsen} at Heidelberg. In 1845, theory, the atoms and molecules still while still a student, he formulated move around at random, the temper- {Kirchhoff’s laws} concerning elec- ature being proportional to their av- tric circuits. With Bunsen he worked erage kinetic energy. However, they on spectroscopy, a technique that led are sufÜciently close to each other them to discover the elements cae- for the attractive forces between mol- sium (1860) and rubidium (1861). ecules to be important. A molecule Kjeldahl’s method A method for that approaches the surface will ex- measuring the percentage of nitro- perience a resultant force tending to gen in an organic compound. The keep it within the liquid. It is, there- compound is boiled with concen- fore, only some of the fastest moving trated sulphuric acid and copper(II) molecules that escape; as a result sulphate catalyst to convert any ni- the average kinetic energy of those trogen to ammonium sulphate. Al- that fail to escape is reduced. In this kali is added and the mixture heated way evaporation from the surface to distil off ammonia. This is passed of a liquid causes its temperature to into a standard acid solution, and the fall. amount of ammonia can then be In a crystalline solid the atoms, found by estimating the amount of ions, and molecules are able only to unreacted acid by titration. The vibrate about the Üxed positions of a amount of nitrogen in the original *crystal lattice; the attractive forces specimen can then be calculated. The are so strong at this range that no method was developed by the Danish free movement is possible. chemist Johan Kjeldahl (1849–1900). 307 Kohlrausch equation knocking The metallic sound pro- compounds, such as alcohols). How- duced by a spark-ignition petrol en- ever, it is claimed that the presence gine under certain conditions. It is in the atmosphere of incompletely caused by rapid combustion of the burnt aromatics constitutes a cancer unburnt explosive mixture in the risk. combustion chambers ahead of the Û Û Knoevenagel reaction A reaction ame front. As the ame travels from in which an aldehyde RCHO reacts the sparking plug towards the piston with malonic acid (CH (COOH) ) it compresses and heats the unburnt 2 2 Û with subsequent loss of CO2 to give gases ahead of it. If the ame front an unsaturated carboxylic acid moves fast enough, normal combus- RCH=CHCOOH. Thus, the chain tion occurs and the explosive mix- length is increased by 2. The reaction ture is ignited progressively by the Û is base-catalysed; usually pyridine is ame. If it moves too slowly, ignition used. The reaction is named after of the last part of the unburnt gas Emil Knoevenagel (1865–1921). can occur very rapidly before the Ûame reaches it, producing a shock knotane See molecular knot. wave that travels back and forth knot theory A branch of mathe- across the combustion chamber. The matics used to classify knots and en- result is overheating, possible dam- tanglements. Knot theory has age to the plugs, an undesirable applications to the study of the prop- k noise, and loss of power (probably erties of polymers and the statistical due to preignition caused by over- mechanics of certain models of phase heated plugs). Knocking can be transitions. avoided by an engine design that in- Û molecular creases turbulence in the combustion Knudsen ow See flow chamber and thereby increases Ûame . speed. It also can be avoided by re- Kohlrausch equation An equa- ducing the compression ratio, but tion that describes the molar conduc- this involves loss of efÜciency. The tivities of strong electrolytes at low Λ Λ0 Κ ½, most effective method is to use high- concentration, i.e. m = m – c octane number Λ octane fuel (see ), where m is the molar conductivity, Λ0 which has a longer self-ignition m is the limiting molar conductivity delay than low-octane fuels. This can (the molar conductivity in the limit be achieved by the addition of an of zero concentration when the ions antiknock agent, such as lead(IV) do not interact with each other), Κ is tetraethyl, to the fuel, which re- a coefÜcient related to the stoichiom- tards the combustion chain reac- etry of the electrolyte, and c is the tions. However, lead-free petrol is concentration of the electrolyte. It is Λ0 now preferred to petrol containing possible to express m as a sum of lead tetraethyl owing to environ- the contribution of each of its ions. mental dangers arising from lead in The Kohlrausch equation was Ürst the atmosphere. In the USA the addi- stated in the 19th century by the Ger- tion of lead compounds is now for- man chemist Friedrich Kohlrausch bidden. New formulae for petrol are (1840–1910) as the result of a consid- designed to raise the octane number erable amount of experimental work. without polluting the atmosphere. With its characteristic c½ depend- These new formulae include increas- ence, the equation is explained quan- ing the content of aromatics and oxy- titatively by the existence of an ionic genates (oxygen-containing atmosphere round the ion as Kohlrausch’s law 308

analysed by the *Debye– Hückel–On- cian and economist Tjalling Charles sager theory. Koopmans (1910–85). Kohlrausch’s law If a salt is dis- Kovar A tradename for an alloy of solved in water, the conductivity of iron, cobalt, and nickel with an ex- the (dilute) solution is the sum of two pansivity similar to that of glass. It is values – one depending on the posi- therefore used in making glass-to- tive ions and the other on the nega- metal seals, especially in circum- tive ions. The law, which depends on stances in which a temperature the independent migration of ions, variation can be expected. was deduced experimentally by Kramers theorem The energy lev- Friedrich Kohlrausch. els of a system, such as an atom that ½ Kolbe’s method A method of contains an odd number of spin- making alkanes by electrolysing a so- particles (e.g. electrons), are at least lution of a carboxylic acid salt. For a double *degenerate in the absence of Ü salt Na+RCOO–, the carboxylate ions an external magnetic eld. This de- lose electrons at the cathode to give generacy, known as Kramers degen- radicals: eracy, is a consequence of time reversal invariance. Kramers theorem – → . RCOO – e RCOO was stated by the Dutch physicist k These decompose to give alkyl radi- Hendrick Anton Kramers (1894–1952) cals in 1930. Kramers degeneracy is re- . → . moved by placing the system in an RCOO R + CO2 external magnetic Üeld. The theorem Two alkyl radicals couple to give an holds even in the presence of crystal alkane Üelds (see crystal-field theory) and . . → R + R RR *spin–orbit coupling. The method can only be used for hy- Krebs, Sir Hans Adolf (1900–81) drocarbons with an even number of German-born British biochemist, carbon atoms, although mixtures of who emigrated to Britain in 1933, two salts can be electrolysed to give a working at ShefÜeld University be- mixture of three products. The fore moving to Oxford in 1954. method was discovered by the Ger- Krebs is best known for the *Krebs man chemist Herman Kolbe cycle, the basis of which he discov- (1818–84), who electrolysed pen- ered in 1937. Details were later tanoic acid (C4H9COOH) in 1849 and added by Fritz Lipmann (1899–1986), obtained a hydrocarbon, which he as- with whom Krebs shared the 1953 sumed was the substance ‘butyl’ Nobel Prize for physiology or medi- C4H9 (actually octane, C8H18). cine. Koopmans’ theorem The princi- Krebs cycle (citric acid cycle; tricar- ple that the ionization energy of a boxylic acid cycle; TCA cycle) A molecule is equal to the orbital en- cyclical series of biochemical reac- ergy of the ejected electron. It is the tions that is fundamental to the basis of the interpretation of spectra metabolism of aerobic organisms, i.e. in *photoelectron spectroscopy. animals, plants, and many micro- Koopmans’ theorem is an approxima- organisms (see illustration). The en- tion in that it ignores any reorganiza- zymes of the Krebs cycle are located tion of electrons in the ion formed. It in the mitochondria and are in close is named after the Dutch mathemati- association with the components of 309 krypton

glycolysis

pyruvate 3C

acetyl CoA 2C

+ oxaloacetate 4C NADH + H citrate 6C NAD+ isocitrate malate 4C 6C

+ fumarate 4C NAD + FADH2 NADH + H CO2 FAD succinate 4C α -ketoglutarate 5C P CO2 GTP NAD+ GDP succinyl CoA 4C NADH + H+ k Krebs cycle the *electron transport chain. The system of metabolic pathways and is two-carbon acetyl coenzyme A (acetyl involved not only in degradation and CoA) reacts with the four-carbon ox- energy production but also in the aloacetate to form the six-carbon cit- synthesis of biomolecules. It is rate. In a series of seven reactions, named after its principal discoverer, this is reconverted to oxaloacetate Sir Hans Adolf *Krebs. and produces two molecules of car- Kroll process A process for produc- bon dioxide. Most importantly, the ing certain metals by reducing the cycle generates one molecule of chloride with magnesium metal. It guanosine triphosphate (GTP – equiv- can be used for titanium, e.g. alent to 1 ATP) and reduces three TiCl + 2Mg → Ti + 2MgCl . molecules of the coenzyme *NAD to 4 2 NADH and one molecule of the coen- It is named after William Kroll, who devised the process in 1940. zyme *FAD to FADH2. NADH and FADH2 are then oxidized by the elec- krypton Symbol Kr. A colourless tron transport chain to generate gaseous element belonging to group three and two molecules of ATP re- 0 (the *noble gases) of the periodic spectively. This gives a net yield of 12 table; a.n. 36; r.a.m. 83.80; d. 3.73 molecules of ATP per molecule of gm–3; m.p. –156.6°C; b.p. –152.3°C. acetyl CoA. Krypton occurs in air (0.0001% by vol- Acetyl CoA can be derived from ume) from which it can be extracted carbohydrates (via *glycolysis), fats, by fractional distillation of liquid air. or certain amino acids. (Other amino Usually, the element is not isolated acids may enter the cycle at different but is used with other inert gases in stages.) Thus the Krebs cycle is the Ûuorescent lamps, etc. The element central ‘crossroads’ in the complex has Üve natural isotopes (mass num- Kupfer nickel 310

Û bers 78, 80, 82, 83, 84) and there are uorides, such as KrF2, have been re- Üve radioactive isotopes (76, 77, 79, ported). 81, 85). Krypton–85 (half-life 10.76 A years) is produced in Üssion reactors • Information from the WebElements site and it has been suggested that an Kupfer nickel A naturally occur- equilibrium amount will eventually ring form of nickel arsenide, NiAs; an occur in the atmosphere. The el- important ore of nickel. ement is practically inert and forms kurchatovium See transactinide very few compounds (certain elements.

k L

labelling The process of replacing a pound in which certain atoms or stable atom in a compound with a ra- groups can easily be replaced by dioisotope of the same element to other atoms or groups. The term is enable its path through a biological applied to coordination complexes in or mechanical system to be traced by which ligands can easily be replaced the radiation it emits. In some cases by other ligands in an equilibrium re- a different stable isotope is used and action. the path is detected by means of a lactams Organic compounds con- mass spectrometer. A compound containing a ring of atoms in which the taining either a radioactive or stable group –NH.CO.– forms part of the isotope is called a labelled compound ring. Lactams can be formed by reac- and the atom used is a label. If a hy- tion of an –NH2 group in one part of drogen atom in each molecule of the a molecule with a –COOH group in compound has been replaced by a tri- the other to give a cyclic amide (see tium atom, the compound is called a illustration). They can exist in an al- tritiated compound. A radioactive la- ternative tautomeric form, the lactim belled compound will behave chemi- form, in which the hydrogen atom cally and physically in the same way on the nitrogen has migrated to the as an otherwise identical stable com- oxygen of the carbonyl to give pound, and its presence can easily be –N=C(OH)–. The pyrimidine base detected using a Geiger counter. This uracil is an example of a lactam. process of radioactive tracing is A widely used in chemistry, biology, • Information about IUPAC nomenclature medicine, and engineering. For example, it can be used to follow the course of the reaction of a carboxylic acid with an alcohol to give an ester, e.g. → CH3COOH + C2H5OH C2H5COOCH3 + H2O To determine whether the noncar- bonyl oxygen in the ester comes Lactams from the acid or the alcohol, the reaction is performed with the labelled 18 compound CH3CO OH, in which the lactate A salt or ester of lactic acid oxygen in the hydroxyl group of the (i.e. a 2-hydroxypropanoate). acid has been ‘labelled’ by using the lactic acid (2-hydroxypropanoic 18O isotope. It is then found that the 18 acid) A clear odourless hygroscopic water product is H2 O; i.e. the oxy- syrupy liquid, CH3CH(OH)COOH, with gen in the ester comes from the alco- a sour taste; r.d. 1.206; m.p. 18°C; b.p. hol, not the acid. 122°C. It is prepared by the hydroly- labile Describing a chemical com- sis of ethanal cyanohydrin or the oxi- lactims 312

dation of propan-1,2-diol using dilute β-hydroxy acids, but can be synthe- nitric acid. Lactic acid is manufac- sized by other means. tured by the fermentation of lactose A (from milk) and used in the dyeing • Information about IUPAC nomenclature and tanning industries. It is an alpha hydroxy *carboxylic acid. See also op- lactose (milk sugar) A sugar com- tical activity. prising one glucose molecule linked Lactic acid is produced from pyru- to a galactose molecule. Lactose is vic acid in active muscle tissue when manufactured by the mammary oxygen is limited and subsequently gland and occurs only in milk. For removed for conversion to glucose by example, cows’ milk contains about the liver. During strenuous exercise 4.7% lactose. It is less sweet than suit may build up in the muscles, caus- crose (cane sugar). ing cramplike pains. It is also pro- Ladenburg benzene An (erro- duced by fermentation in certain neous) structure for *benzene pro- bacteria and is characteristic of sour posed by Albert Ladenburg (1842– milk. 1911), in which the six carbon atoms lactims See lactams. were arranged at the corners of a tri- A angular prism and linked by single • Information about IUPAC nomenclature bonds to each other and to the six hydrogen atoms. lactones Organic compounds con- optical activity l taining a ring of atoms in which the laevo form See . group –CO.O– forms part of the ring. laevorotatory Designating a Lactones can be formed (or regarded chemical compound that rotates the as formed) by reaction of an –OH plane of plane-polarized light to the group in one part of a molecule with left (anticlockwise for someone fac- a –COOH group in the other to ing the oncoming radiation). See opti- give a cyclic ester (see illustration). cal activity. This type of reaction occurs with fructose γ-hydroxy carboxylic acids such as the laevulose See . compound CH2(OH)CH2CH2COOH (in Lagrange multipliers (undeter- which the hydroxyl group is on the mined multipliers) Parameters, usu- third carbon from the carboxyl ally denoted λ, introduced to assist in group). The resulting γ-lactone has Ünding the maximum or minimum a Üve-membered ring. Similarly, value of a function f of several vari- δ -lactones have six-membered rings. ables x1,x2,…xn, subject to some con- β-lactones, with a four-membered straint that connects the variables. ring, are not produced directly from An important application of the method of Lagrange multipliers is its use in the derivation of the Boltz- mann distribution in statistical mechanics, in which one of the Lagrange multipliers is –1/kT, where k is the *Boltzmann constant and T is the thermodynamic temperature. La- grange multipliers were introduced by the Italian-born French mathematician Joseph-Louis Lagrange Lactones (1736–1813). 313 Landau levels

Lagrangian Symbol L. A function radiation is exactly at the absorption used to deÜne a dynamical system in peak, only molecules moving perpen- terms of functions of coordinates, ve- dicular to the line of the beam locities, and times given by: (which hence have no Doppler shift) L = T – V absorb both in the initial path and the reÛected path of the radiation. where T is the kinetic energy of the Since molecules being excited in the system and V is the potential energy of the system. The Lagrangian formu- initial path leave fewer molecules to lation of dynamics has the advantage be excited in the return path this that it does not deal with many vec- causes a less intense absorption to be tor quantities, such as forces and ac- observed. As a result a dip appears in celerations, but only with two scalar the curve, thus enabling the absorp- functions, T and V. This leads to great tion peak to be found very accu- simpliÜcations. Lagrangian dynamics rately. Lamb-dip spectroscopy is was formulated by Joseph Louis La- named after Willis Eugene Lamb grange (1736–1813). (1913–2008). LAH Lithium aluminium hydride; Lamb shift A small energy differ- 2 see lithium tetrahydro- ence between two levels ( S1/2 and 2 aluminate(iii). P1/2) in the *hydrogen spectrum. The shift results from the quantum inter- lake A pigment made by combining action between the atomic electron an organic dyestuff with an inorganic and the electromagnetic radiation. It l compound (usually an oxide, hydrox- was Ürst explained by Willis Eugene ide, or salt). Absorption of the or- Lamb. ganic compound on the inorganic substrate yields a coloured complex, lamellar solids Solid substances in as in the combination of a dyestuff which the crystal structure has dis- with a *mordant. Lakes are used in tinct layers (i.e. has a layer lattice). paints and printing inks. The *micas are an example of this type of compound. *Intercalation superfluidity lambda point See . compounds are lamellar compounds Lamb-dip spectroscopy A spec- formed by interposition of atoms, troscopic technique enabling the cen- ions, etc., between the layers of an tres of absorption lines to be existing element or compound. For determined very precisely by making example, graphite is a lamellar solid. use of the Doppler shift associated With strong oxidizing agents (e.g. a with very rapidly moving molecules. mixture of concentrated sulphuric An intense monochromatic beam of and nitric acids) it forms a nonstoi- radiofrequency electromagnetic radi- chiometric ‘graphitic oxide’, which is ation is passed through a sample of a an intercalation compound having gas with the frequency being slightly oxygen atoms between the layers of higher than that of maximum ab- carbon atoms. Substances of this type sorption. Only certain molecules are called graphitic compounds. moving at a certain speciÜc speed lamp black A Ünely divided (micro- can absorb radiation. The beam is crystalline) form of carbon made by then reÛected back through the sam- burning organic compounds in ple so that radiation is absorbed by insufÜcient oxygen. It is used as a molecules moving at exactly this black pigment and Üller. same speed, except that they are moving away from the mirror. If the Landau levels The energy levels Landé interval rule 314

found by *quantum mechanics of coefÜcient, and A(t) is a random force free electrons in a uniform magnetic describing the average effect of the Üeld. These energy levels, named Brownian motion. The Langevin after the Soviet physicist Lev Davi- equation is named after the French dovich Landau (1908–68), who physicist Paul Langevin (1872–1946). analysed the problem in 1930, have It is necessary to use statistical meth- discrete values, which are integer ods and the theory of probability to multiples of heB/m, where h is the solve the Langevin equation. Planck constant, e is the charge of Langmuir adsorption isotherm the electron, m is the mass of the Û An equation used to describe the electron, and B is the magnetic ux amount of gas adsorbed on a plane density. Each Landau level is a highly surface, as a function of the pressure *degenerate level, with each level of the gas in equilibrium with the Ü being lled by 2eB/h, where the factor surface. The Langmuir adsorption 2 is due to the spin of the electron. isotherm can be written: Landé interval rule A rule used in θ = bp/(1 + bp), interpreting atomic spectra stating where θ is the fraction of the surface that if the *spin–orbit coupling is covered by the adsorbate, p is the weak in a given multiplet, the energy pressure of the gas, and b is a con- differences between two successive J stant called the adsorption coefÜ- levels (where J is the total resultant cient, which is the equilibrium l angular momentum of the coupled constant for the process of adsorp- electrons) are proportional to the tion. The Langmuir adsorption larger of the two values of J. The rule isotherm was derived by the US was stated by the German-born US chemist Irving Langmuir (1881– physicist Alfred Landé (1888–1975) in 1957), using the *kinetic theory of 1923. It can be deduced from the gases and making the assumptions quantum theory of angular momen- that: tum. In addition to assuming *Rus- (1) the adsorption consists entirely of sell–Saunders coupling, the Landé a monolayer at the surface; interval rule assumes that the inter- (2) there is no interaction between actions between spin magnetic mo- molecules on different sites and each ments can be ignored, an assumption site can hold only one adsorbed mol- that is not correct for very light ecule; atoms, such as helium. Thus the (3) the heat of adsorption does not Landé interval rule is best obeyed by depend on the number of sites and is atoms with medium atomic num- equal for all sites. bers. The Langmuir adsorption isotherm is Langevin equation A type of ran- of limited application since for real dom equation of motion (see sto- surfaces the energy is not the same chastic process) used to study for all sites and interactions between *Brownian movement. The Langevin adsorbed molecules cannot be ignored. equation. can be written in the form v= ξv + A(t), where v is the velocity of Langmuir–Blodgett Ülm A Ülm a particle .of mass m immersed in a of molecules on a surface that can Ûuid and vis the acceleration of the contain multiple layers of Ülm. A particle; ξv is a frictional force result- Langmuir–Blodgett Ülm with multi- ing from the viscosity of the Ûuid, ple layers can be made by dipping a with ξ being a constant friction plate into a liquid so that it is cov- 315 lanthanoids ered by a monolayer and then repeat- lansfordite A mineral form of ing the process. This process, called *magnesium carbonate pentahy- the Langmuir–Blodgett technique, en- drate, MgCO3.5H2O. ables a multilayer to be built up, one lanthanide contraction See lan- monolayer at a time. Langmuir– thanoids. Blodgett Ülms have many potential practical applications, including insu- lanthanides See lanthanoids. lation for optical and semiconductor lanthanoid contraction See lan- devices and selective membranes in thanoids. biotechnology. lanthanoids (lanthanides; lan- Langmuir–Hinshelwood mecha- thanons; rare-earth elements) A se- nism A possible mechanism for a ries of elements in the *periodic bimolecular process catalyzed at a table, generally considered to range solid surface. It is assumed that two in proton number from cerium molecules are adsorbed on adjacent (58) to lutetium (71) inclusive. The sites and that a reaction takes place lanthanoids all have two outer s- via an activated complex on the sur- electrons (a 6s2 conÜguration), follow face to yield the products of the reac- lanthanum, and are classiÜed to- tion. This mechanism was suggested gether because an increasing proton by the US chemist Irving Langmuir number corresponds to increase in (1881–1957) in 1921 and developed number of 4f electrons. In fact, the 4f by the British chemist Sir Cyril Hin- and 5d levels are close in energy and l shelwood (1897–1967) in 1926. Some the Ülling is not smooth. The outer bimolecular reactions at surfaces are electron conÜgurations are as fol- in agreement with the predictions of lows: this model. See also langmuir–rideal 1 2 mechanism 57 lanthanum (La) 5d 6s . 58 cerium (Ce) 4f5d16s2 (or 4f26s2) Langmuir isotherm See langmuir 59 praseodymium (Pr) 4f36s2 adsorption isotherm. 60 neodymium (Nd) 4f46s2 61 promethium (Pm) 4f56s2 Langmuir–Rideal mechanism A 62 samarium (Sm) 4f66s2 possible mechanism for a bimolecu- 63 europium (Eu) 4f76s2 lar process catalyzed at a solid sur- 64 gadolinium (Gd) 4f75d16s2 face. It is envisaged that one of the 65 terbium (Tb) 4f96s2 molecules is adsorbed and then re- 66 dysprosium (Dy) 4f106s2 acts with a second molecule that is 67 holmium (Ho) 4f116s2 not adsorbed. This mechanism was 68 erbium (Er) 4f126s2 suggested by the US chemist Irving 69 thulium (Tm) 4f136s2 Langmuir (1881–1957) in 1921 and 70 ytterbium (Yb) 4f146s2 developed by the British chemist Sir 71 lutetium (Lu) 4f145d16s2 Eric Rideal in 1939. The Langmuir– Note that lanthanum itself does Rideal mechanism is not common not have a 4f electron but it is gener- but certain reactions involving hy- ally classiÜed with the lanthanoids drogen atoms probably occur by this because of its chemical similarities, mechanism. as are yttrium (Yt) and scandium (Sc). lanolin An emulsion of puriÜed Scandium, yttrium, and lanthanum wool fat in water, containing choles- are d-block elements; the lanthanoids terol and certain terpene alcohols and *actinoids make up the f-block. and esters. It is used in cosmetics. The lanthanoids are sometimes lanthanons 316

simply called the rare earths, al- A though strictly the ‘earths’ are their • Information from the WebElements site oxides. Nor are they particularly rare: they occur widely, usually together. lapis lazuli A blue rock that is All are silvery very reactive metals. widely used as a semiprecious stone The f-electrons do not penetrate to and for ornamental purposes. It is the outer part of the atom and there composed chieÛy of the deep blue is no f-orbital participation in bond- mineral lazurite embedded in a ma- ing (unlike the d-orbitals of the main trix of white calcite and usually also *transition elements) and the el- contains small specks of pyrite. It oc- ements form few coordination com- curs in only a few places in crys- pounds. The main compounds talline limestones as a contact contain M3+ ions. Cerium also has the metamorphic mineral. The chief highly oxidizing Ce4+ state and eu- source is Afghanistan; lapis lazuli ropium and ytterbium have a M2+ also occurs near Lake Baikal in state. Siberia and in Chile. It was formerly The 4f orbitals in the atoms are not used to make the artists’ pigment ul- very effective in shielding the outer tramarine. electrons from the nuclear charge. In Laporte selection rule A selec- going across the series the increasing tion rule in atomic spectra stating nuclear charge causes a contraction that spectral lines associated with in the radius of the M3+ ion – from electric-dipole radiation must arise l 0.1061 nm in lanthanum to from transitions between states of 0.0848 nm in lutetium. This effect, opposite parity. The Laporte selection the lanthanoid contraction (or lan- rule was discovered by O. Laporte in thanide contraction), accounts for the 1924 and was explained by the appli- similarity between the transition el- cation of group theory to the *quan- ements zirconium and hafnium. tum mechanics of atoms. In the case lanthanons See lanthanoids. of magnetic-dipole and quadrupole radiation the selection rule for spec- lanthanum Symbol La. A silvery tral lines is the opposite of the La- metallic element belonging to group porte rule, i.e. transitions are only 3 (formerly IIIA) of the periodic table allowed between states of the same and often considered to be one of the parity in these cases. *lanthanoids; a.n. 57; r.a.m. 138.91; r.d. 6.146 (20°C); m.p. 921°C; b.p. Larmor precession A precession ° of the motion of charged particles in 3457 C. Its principal ore is bastnasite, Ü Ü from which it is separated by an ion- a magnetic eld. It was rst deduced exchange process. There are two nat- in 1897 by Sir Joseph Larmor ural isotopes, lanthanum–139 (stable) (1857–1942). Applied to the orbital motion of an electron around the nu- and lanthanum–138 (half-life Ü 1010–1015 years). The metal, being py- cleus of an atom in a magnetic eld of Ûux density B, the frequency of rophoric, is used in alloys for lighter π µ Ûints and the oxide is used in some precession is given by eB/4 mv , where e and m are the electronic optical glasses. The largest use of lan- µ thanum, however, is as a catalyst in charge and mass respectively, is cracking crude oil. Its chemistry re- the permeability, and v is the velocity of the electron. This is known as the sembles that of the lanthanoids. The Larmor frequency. element was discovered by Carl Mosander (1797–1858) in 1839. laser (light ampliÜcation by stimu- 317 latent heat lated emission of radiation) A light and forth between the mirrors ampliÜer (also called an optical quickly escape through the sides of maser) usually used to produce the oscillating medium without monochromatic coherent radiation ampliÜcation. in the infrared, visible, and ultravio- Some lasers are solid, others are let regions of the *electromagnetic liquid or gas devices. Population in- spectrum. Lasers that operate in the version can be achieved by optical X-ray region of the spectrum are also pumping with Ûashlights or with being developed. other lasers. It can also be achieved Nonlaser light sources emit radia- by such methods as chemical reaction in all directions as a result of the tions and discharges in gases. spontaneous emission of photons by Lasers have found many uses since thermally excited solids (Ülament their invention in 1960. In chemistry, lamps) or electronically excited their main use has been in the study atoms, ions, or molecules (Ûuores- of photochemical reactions, in the cent lamps, etc.). The emission ac- spectroscopic investigation of mol- companies the spontaneous return of ecules, and in *femtochemistry. See the excited species to the *ground also dye laser; four-level laser; state and occurs randomly, i.e. the ra- pockels cell. diation is not coherent. In a laser, the laser spectroscopy Spectroscopy Ü atoms, ions, or molecules are rst that makes use of lasers. The beams ‘pumped’ to an excited state and of coherent monochromatic radia- l then stimulated to emit photons by tion produced by lasers have several collision of a photon of the same en- signiÜcant advantages compared ergy. This is called stimulated emis- with other spectroscopic techniques, Ü sion. In order to use it, it is rst particularly in those that employ the necessary to create a condition in the *Raman effect. amplifying medium, called population inversion, in which the majority Lassaigne’s test A method of test- of the relevant entities are excited. ing for the presence of a halogen, ni- Random emission from one entity trogen, or sulphur in an organic can then trigger coherent emission compound. A sample is heated in a from the others that it passes. In this test tube with a pellet of sodium. The way ampliÜcation is achieved. hot tube is dropped into pure water The laser ampliÜer is converted to and the fragments ground up in a an oscillator by enclosing the ampli- mortar. The presence of a halogen fying medium within a resonator. Ra- (now in the form of a sodium halide) diation then introduced along the is detected by precipitation with sil- axis of the resonator is reÛected back ver nitrate solution. Nitrogen is and forth along its path by a mirror revealed by the formation of a pre- at one end and by a partially trans- cipitate of Prussian blue on heating mitting mirror at the other end. Be- part of the solution with iron(II) sul- tween the mirrors the waves are phate solution containing hydrochlo- ampliÜed by stimulated emission. ric acid and a trace of iron(III) ions. The radiation emerges through the Lead ethanoate or sodium nitroprus- semitransparent mirror at one end side gives a precipitate with any sul- as a powerful coherent monochro- phur present. matic parallel beam of light. The latent heat Symbol L. The quantity emitted beam is uniquely parallel be- of heat absorbed or released when a cause waves that do not bounce back substance changes its physical phase latex 318

at constant temperature (e.g. from tential for nonadjacent species can solid to liquid at the melting point or be calculated, but values for common from liquid to gas at the boiling couples are often indicated by addi- point). For example, the latent heat tional arrows, Latimer diagrams de- of vaporization is the energy a sub- pend on pH and are commonly given stance absorbs from its surroundings for both acidic and alkaline condi- in order to overcome the attractive tions. See also frost diagram. forces between its molecules as it lattice The regular arrangement of changes from a liquid to a gas and in atoms, ions, or molecules in a crys- order to do work against the external talline solid. See crystal lattice. atmosphere as it expands. In thermodynamic terms the latent heat is the lattice energy A measure of the *enthalpy of evaporation (∆H), i.e. L = stability of a *crystal lattice, given by ∆H = ∆U + p∆V, where ∆U is the the energy that would be released change in the internal energy, p is per mole if atoms, ions, or molecules the pressure, and ∆V is the change in of the crystal were brought together volume. from inÜnite distances apart to form The speciÜc latent heat (symbol l) is the lattice. See born–haber cycle. the heat absorbed or released per lattice vibrations The periodic vi- unit mass of a substance in the brations of the atoms, ions, or mol- course of its isothermal change of ecules in a *crystal lattice about their phase. The molar latent heat is the mean positions. On heating, the am- l heat absorbed or released per unit plitude of the vibrations increases amount of substance during an until they are so energetic that the isothermal change of state. lattice breaks down. The temperature latex Natural rubber as it is ob- at which this happens is the melting tained from a rubber tree or any sta- point of the solid and the substance ble suspension in water of a similar becomes a liquid. On cooling, the am- synthetic polymer. Synthetic latexes plitude of the vibrations diminishes. are used to make articles from rub- At *absolute zero a residual vibration ber or plastics by such techniques as persists, associated with the *zero- dipping (rubber gloves), spreading point energy of the substance. The (waterproof cloth), and electrodeposi- increase in the electrical resistance tion (plastic-coated metal). They are of a conductor is due to increased also employed in paints and adhe- scattering of the free conduction sives. electrons by the vibrating lattice Latimer diagram A simple dia- particles. gram summarizing the standard po- laughing gas See dinitrogen tentials for an element in different oxide. oxidation states. The different lauric acid See dodecanoic acid. species are written in a horizontal line in order of decreasing oxidation Lavoisier, Antoine Laurent state, with the most highly oxidized (1743–1794) French chemist, who col- species on the left. Arrows are writ- lected taxes for the government in ten between adjacent species with Paris. In the 1770s he discovered oxy- the standard potential in volts indi- gen and nitrogen in air and demol- cated above the arrow. Often the oxi- ished the *phlogiston theory of dation number is included in the combustion by demonstrating the diagram. The standard electrode po- role of oxygen in the process. In 1783 319 laws, theories, and hypotheses he made water by burning hydrogen active metallic transuranic element in oxygen (see cavendish, henry). He belonging to the *actinoids; a.n. 103; also devised a rational nomenclature mass number of the Ürst discovered for chemical compounds. In 1794 he isotope 257 (half-life 8 seconds). A was tried by the Jacobins as an oppo- number of very short-lived isotopes nent of the Revolution (because of have now been synthesized. The el- his tax-gathering), found guilty, and ement was identiÜed by Albert guillotined. Ghiorso and associates in 1961. It law of chemical equilibrium See was named after E. O. Lawrence equilibrium constant. A(1901–58). law of conservation of energy • Information from the WebElements site See conservation law. laws of chemical combination law of conservation of mass See chemical combination conservation law. See . law of constant composition See laws, theories, and hypotheses chemical combination. In science, a law is a descriptive principle of nature that holds in all cir- law of deÜnite proportions See cumstances covered by the wording chemical combination. of the law. There are no loopholes in law of mass action See mass ac- the laws of nature and any excep- tion. tional event that did not comply with l the law would require the existing law of multiple proportions See law to be discarded or would have to chemical combination . be described as a miracle. Epony- law of octaves (Newlands’ law) mous laws are named after their dis- An attempt at classifying elements coverers (e.g. *Boyle’s law); some made by John Newlands (1837–98) in laws, however, are known by their 1863. He arranged 56 elements in subject matter (e.g. the law of conser- order of increasing atomic mass in vation of mass), while other laws use groups of eight, pointing out that both the name of the discoverer and each element resembled the element the subject matter to describe them eight places from it in the list. He (e.g. Newton’s law of gravitation). drew an analogy with the notes of a A description of nature that en- musical scale. Newlands’ octaves compasses more than one law but were groups of similar elements dis- has not achieved the uncontrovert- tinguished in this way: e.g. oxygen ible status of a law is sometimes and sulphur; nitrogen and phospho- called a theory. Theories are often rus; and Ûuorine, chlorine, bromine, both eponymous and descriptive of and iodine. In some cases it was nec- the subject matter (e.g. Einstein’s essary to put two elements in the theory of relativity and Darwin’s same position. The proposal was re- theory of evolution). jected at the time. See periodic table. A hypothesis is a theory or law that A retains the suggestion that it may not • John Newlands’ paper be universally true. However, some hypotheses about which no doubt law of reciprocal proportions still lingers have remained hypothe- See chemical combination. ses (e.g. Avogadro’s hypothesis), for lawrencium Symbol Lr. A radio- no clear reason. Clearly there is a de- layer lattice 320

gree of overlap between the three (PbCO3), and litharge (PbO). The concepts. metal is extracted by roasting the ore to give the oxide, followed by reduc- layer lattice A crystal structure in tion with carbon. Silver is also recov- which the atoms are chemically ered from the ores. Lead has a variety bonded in plane layers, with rela- of uses including building construc- tively weak forces between atoms in tion, lead-plate accumulators, bullets, adjacent layers. Graphite and micas and shot, and is a constituent of such are examples of substances having alloys as solder, pewter, bearing met- * layer lattices (i.e. they are lamellar als, type metals, and fusible alloys. solids). Chemically, it forms compounds lazurite See lapis lazuli. with the +2 and +4 oxidation states, the lead(II) state being the more sta- LCAO (linear combination of atomic ble. orbitals) A molecular *orbital A formed by the linear combination of atomic orbitals. The LCAO approxi- • Information from the WebElements site mation arises because with an elec- lead(II) acetate See lead(ii) tron that is very close to a nucleus, ethanoate. the potential energy of the electron is dominated by the interaction be- lead–acid accumulator An accu- tween the electron and the nucleus. mulator in which the electrodes are Thus, very close to the nucleus of an made of lead and the electrolyte con- l sists of dilute sulphuric acid. The atom, A, in a molecule, the *wave electrodes are usually cast from a function of the molecule is very simi- lead alloy containing 7–12% of anti- lar to the wave function of the atom mony (to give increased hardness and A. The LCAO approximation shows corrosion resistance) and a small the increase in electron density asso- amount of tin (for better casting ciated with chemical bonding. The properties). The electrodes are coated LCAO method takes account of the with a paste of lead(II) oxide (PbO) symmetry of the molecule using sym- and Ünely divided lead; after inser- metry-adapted linear combinations tion into the electrolyte a ‘forming’ (SALC). current is passed through the cell to LCP See liquid-crystal polymer. dopa lead (Pb) 2e lead(IV) oxide L-dopa See . PbO2 L–D process See basic-oxygen process. – + leaching Extraction of soluble com- Pb2+ Pb2+ ponents of a solid mixture by perco- lating a solvent through it. 2– + 2– 2H2SO4 2SO4 + 4H 2O lead Symbol Pb. A heavy dull grey soft ductile metallic element belong- 2PbSO4 ing to *group 14 (formerly IVB) of 2H2O the periodic table; a.n. 82; r.a.m. 207.19; r.d. 11.35; m.p. 327.5°C; b.p. 1740°C. The main ore is the sulphide sulphuric acid galena (PbS); other minor sources include anglesite (PbSO4), cerussite Lead-acid accumulator 321 lead(IV) hydride convert the PbO on the negative it has the disadvantage of reacting plate into a sponge of Ünely divided with hydrogen sulphide in industrial lead. On the positive plate the PbO is atmospheres and producing black converted to lead(IV) oxide (PbO2). lead sulphide. The poisonous nature The equation for the overall reaction of lead compounds has also con- during discharge is: tributed to the declining importance → PbO2 + 2H2SO4 + Pb 2PbSO4 + of this material. 2H2O lead-chamber process An obso- The reaction is reversed during lete method of making sulphuric charging. Each cell gives an e.m.f. of acid by the catalytic oxidation of sul- about 2 volts and in motor vehicles a phur dioxide with air using a potas- 12-volt battery of six cells is usually sium nitrate catalyst in water. The used. The lead–acid battery produces process was carried out in lead con- 80–120 kJ per kilogram. Compare tainers (which was expensive) and nickel iron accumulator – . only produced dilute acid. It was re- lead(II) carbonate A white solid, placed in 1876 by the *contact PbCO3, insoluble in water; rhombic; process. r.d. 6.6. It occurs as the mineral lead dioxide See lead(iv) oxide. *cerussite, which is isomorphous with aragonite and may be prepared lead(II) ethanoate (lead(II) ac- in the laboratory by the addition of etate) A white crystalline solid, cold ammonium carbonate solution Pb(CH3COO)2, soluble in water and l to a cold solution of a lead(II) salt (ac- slightly soluble in ethanol. It exists as etate or nitrate). It decomposes at the anhydrous compound (r.d. 3.25; 315°C to lead(II) oxide and carbon m.p. 280°C), as a trihydrate, dioxide. Pb(CH3COO)2.3H2O (monoclinic; r.d. ° lead(II) carbonate hydroxide 2.55; loses water at 75 C), and as a (white lead; basic lead carbonate) A decahydrate, Pb(CH3COO)2.10H2O (rhombic; r.d. 1.69). The common powder, 2PbCO3.Pb(OH)2, insoluble in water, slightly soluble in aqueous form is the trihydrate. Its chief inter- carbonate solutions; r.d. 6.14; decom- est stems from the fact that it is solu- poses at 400°C. Lead(II) carbonate hy- ble in water and it also forms a droxide occurs as the mineral variety of complexes in solution. It hydroxycerussite (of variable compo- was once known as sugar of lead be- sition). It was previously manufac- cause of its sweet taste. tured from lead in processes using lead(IV) ethanoate (lead tetra- spent tanning bark or horse manure, acetate) A colourless solid, which released carbon dioxide. It is Pb(CH3COO)4, which decomposes in currently made by electrolysis of water and is soluble in pure ethanoic mixed solutions (e.g. ammonium ni- acid; monoclinic; r.d. 2.228; m.p. trate, nitric acid, sulphuric acid, and 175°C. It may be prepared by dissolv- acetic acid) using lead anodes. For the ing dilead(II) lead(IV) oxide in warm highest grade product the lead must ethanoic acid. In solution it behaves be exceptionally pure (known in the essentially as a covalent compound trade as ‘corroding lead’) as small (no measurable conductivity) in con- amounts of metallic impurity impart trast to the lead(II) salt, which is a grey or pink discolorations. The ma- weak electrolyte. terial was used widely in paints, both for art work and for commerce, but lead(IV) hydride See plumbane. lead monoxide 322

lead monoxide See lead(ii) oxide. of lead(II) ethanoate. The material known as basic lead(II) sulphate may lead(II) oxide (lead monoxide) A be made by shaking together lead(II) solid yellow compound, PbO, which sulphate and lead(II) hydroxide in is insoluble in water; m.p. 886°C. It water. This material has been used in exists in two crystalline forms: white paint in preference to lead(II) litharge (tetrahedral; r.d. 9.53) and carbonate hydroxide, as it is not so massicot (rhombic; r.d. 8.0). It can be susceptible to discoloration through prepared by heating the nitrate, and reaction with hydrogen sulphide. is manufactured by heating molten The toxicity of lead compounds has lead in air. If the temperature used is led to a decline in the use of these lower than the melting point of the compounds. oxide, the product is massicot; above this, litharge is formed. Variations in lead(II) sulphide A black crys- the temperature and in the rate of talline solid, PbS, which is insoluble cooling give rise to crystal vacancies in water; r.d. 7.5; m.p. 1114°C. It oc- and red, orange, and brown forms of curs naturally as the metallic-looking litharge can be produced. The oxide mineral *galena (the principal ore of is amphoteric, dissolving in acids to lead). It may be prepared in the labo- give lead(II) salts and in alkalis to ratory by the reaction of hydrogen give *plumbates. sulphide with soluble lead(II) salts. Lead(II) sulphide has been used as an lead(IV) oxide (lead dioxide) A electrical rectiÜer. l dark brown or black solid with a ru- lead(iv) tile lattice, PbO2, which is insoluble lead tetra-acetate See in water and slightly soluble in con- ethanoate. centrated sulphuric and nitric acids; lead(IV) tetraethyl (tetraethyl r.d. 9.375; decomposes at 290°C. lead) A colourless liquid, Pb(C2H5)4, Lead(IV) oxide may be prepared by insoluble in water, soluble in ben- the oxidation of lead(II) oxide by zene, ethanol, ether, and petroleum; heating with alkaline chlorates or ni- r.d. 1.659; m.p. –137°C; b.p. 200°C. It trates, or by anodic oxidation of may be prepared by the reaction of lead(II) solutions. It is an oxidizing hydrogen and ethene with lead but a agent and readily reverts to the more convenient laboratory and in- lead(II) oxidation state, as illustrated dustrial method is the reaction of a by its conversion to Pb O and PbO 3 4 sodium–lead alloy with chloroethane. on heating. It reacts with hydrochlo- A more recent industrial process is ric acid to evolve chlorine. Lead(IV) the electrolysis of ethylmagnesium oxide has been used in the manufac- chloride (the Grignard reagent) using ture of safety matches and was a lead anode and slowly running ad- widely used until the mid-1970s as an ditional chloroethane onto the cath- adsorbent for sulphur dioxide in pol- ode. Lead tetraethyl is used in fuel lution monitoring. for internal-combustion engines lead(II) sulphate A white crys- (along with 1,2-dibromoethane) to in- talline solid, PbSO4, which is virtually crease the *octane number and re- insoluble in water and soluble in so- duce knocking. The use of lead(IV) lutions of ammonium salts; r.d. 6.2; tetraethyl in petrol results in the m.p. 1170°C. It occurs as the mineral emission of hazardous lead com- anglesite; it may be prepared in the pounds into the atmosphere. Pres- laboratory by adding any solution sure from environmental groups has containing sulphate ions to solutions encouraged a reduction in the use of 323 LEED lead(IV) tetraethyl and an increasing sent and thus decreases the pressure. use of lead-free petrol. The standard enthalpy change for the forward reaction is negative (i.e. Leblanc process An obsolete the reaction is exothermic). Thus, an process for manufacturing sodium increase in temperature displaces the carbonate. The raw materials were sodium chloride, sulphuric acid, equilibrium to the left since this coke, and limestone (calcium carbon- tends to reduce the temperature. The * ate), and the process involved two equilibrium constant thus falls with stages. First the sodium chloride was increasing temperature. heated with sulphuric acid to give A sodium sulphate: • Le Chatelier’s original paper → 2NaCl(s) + H2SO4(l) Na2SO4(s) + Leclanché cell A primary *voltaic 2HCl(g) cell consisting of a carbon rod (the The sodium sulphate was then anode) and a zinc rod (the cathode) heated with coke and limestone: dipping into an electrolyte of a → 10–20% solution of ammonium chlo- Na2SO4 + 2C + CaCO3 Na2CO3 + CaS + 2CO ride. *Polarization is prevented by 2 using a mixture of manganese diox- Calcium sulphide was a by-product, ide mixed with crushed carbon, held the sodium carbonate being ex- in contact with the anode by means tracted by crystallization. The of a porous bag or pot; this reacts process, invented in 1783 by the with the hydrogen produced. This l French chemist Nicolas Leblanc Ü wet form of the cell, devised in 1867 (1742–1806), was the rst for produc- by Georges Leclanché (1839–82), has ing sodium carbonate synthetically an e.m.f. of about 1.5 volts. The *dry (earlier methods were from wood ash cell based on it is widely used in and other vegetable sources). By the torches, radios, and calculators. end of the 19th century it had been largely replaced by the *Solvay lectin Any of a group of proteins, process. derived from plants, that can bind to speciÜc oligosaccharides on the sur- lechatelierite A mineral form of face of cells, causing the cells to * silicon(IV) oxide, SiO2. clump together. Lectins can be used Le Chatelier’s principle If a sys- to identify mutant cells in cell cul- tem is in equilibrium, any change tures and to determine blood groups imposed on the system tends to shift as they can cause the agglutination the equilibrium to nullify the effect of red blood cells. Lectins are found of the applied change. The principle, in seeds of legumes and in other tis- which is a consequence of the law of sues, in which they are thought to conservation of energy, was Ürst act as a toxin. stated in 1888 by Henri Le Chatelier LEED (low-energy electron diffrac- (1850–1936). It is applied to chemical tion) A technique used to study the equilibria. For example, in the gas re- structure of crystal surfaces and action processes taking place on these sur- ˆ 2SO2 + O2 2SO3 faces. The surface is bombarded with an increase in pressure on the reac- a monochromatic electron beam 10–4 tion mixture displaces the equilib- to 10–3 m in diameter, with energies rium to the right, since this reduces between 6 and 600 V. The electrons the total number of molecules pre- are diffracted by the surface atoms Lennard-Jones potential 324

and then collected on a Ûuorescent functions and thus of their overlap, screen. Both the surface structure which describes the repulsion. and changes that occur after leucine See amino acid. chemisorption and surface reactions can be investigated in this way. It is leuco form See dyes. necessary for the surface to be care- leucomalachite green test A fully cleaned and kept at ultrahigh *presumptive test for blood. The vacuum pressure. Although many reagent is the dye leucomalachite surfaces are altered by the electron green dissolved in water along with beam and therefore cannot be stud- sodium perborate (NaBO3). A blue- ied using this method, there are green colour indicates a positive re- enough surfaces and surface sult. processes that can be studied using LEED to make it a very useful tech- lever rule A rule enabling the relative amounts of two phases a and b, nique. DifÜculties in interpreting which are in equilibrium, to be LEED patterns arise as multiple- found by a construction in a phase scattering theory, rather than single- diagram. (For example, a can be gas scattering theory (as in X-ray or neu- and b can be liquid.) The distances l tron scattering), is required. See also a and l along the horizontal *tie line electron diffraction. b of the phase diagram are measured. Lennard-Jones potential l A po- The lever rule states that nala = nblb, tential used to give an approximate where na is the amount of phase a description of the potential energy and nb is the amount of phase b. The interaction, V, of molecules as a func- rule takes its name from the similar tion of intermolecular distance r. The form of the rule, mala = mblb, relating general form of the Lennard-Jones the moments of two masses ma and potential is mb about a pivot in a lever. n 6 V = Cn/r – C6/r , Lewis, Gilbert Newton (1875– Ü where Cn and C6 are coef cients that 1946) US physical chemist who spent depend on the speciÜc molecules and most of his career at Berkeley, Cali- n is greater than 6 so that at small fornia. His ideas on chemical bond- separations the repulsion term domi- ing were extremely inÛuential, and nates the interaction, the r–6 term he introduced the idea of a stable being attractive. The value n = 12 is octet of electrons and of a covalent frequently chosen. In this case the bond being a shared pair of elec- Lennard-Jones potential is given by: trons. He also introduced the concept acid 12 6 of Lewis acids and bases (see ). V = 4W[(r0/r) – (r0/r) ], acid where W is the depth of the potential Lewis acid and base See . well and r0 is the separation at which Liebermann’s reaction A method V = 0. The minimum value of the of testing for phenols. A small sam- well occurs at the separation re = ple of the test substance and a crystal 1/6 2 r0. The representation of the re- of sodium nitrite are dissolved in pulsive part of the interaction by a warm sulphuric acid. The solution is 1/r12 term is not realistic; a much then poured into excess aqueous al- more realistic term is the exponen- kali, when the formation of a blue- tial term, exp(–r/r0), as it is closer to green colour indicates the presence the exponential decay of the wave of a phenol. 325 ligase

Liebermann test A *presumptive the lone pair from ligand to metal, test sometimes used for cocaine and and a pi bond by back donation of morphine. The Liebermann reagent electrons on the metal to empty d-or- is a solution of potassium nitrite bitals on the ligand. Carbon monox- (KNO2) in sulphuric acid. With mor- ide is the most important such phine a black colour is produced; co- ligand, forming metal carbonyls (e.g. caine gives a yellow colour. Ni(CO)4). Liebig, Justus von (1803–73) Ger- The examples given above are ex- man organic chemist who worked at amples of monodentate ligands (liter- Gessen in Frankfurt. Liebig was the ally: ‘having one tooth’), in which Ürst to recognise that two different there is only one point on each lig- chemical compounds can have the and at which coordination can occur. same formula. He also developed a Some ligands are polydentate; i.e. method of analysing organic com- they have two or more possible coor- pounds by burning them and weigh- dination points. For instance, 1,2-di- ing the carbon dioxide and water aminoethane, H2NC2H4NH2, is a produced. With his students, he bidentate ligand, having two coordi- analysed many compounds and was nation points. Certain polydentate extremely inÛuential in the develop- ligands can form *chelates. ment of organic chemistry. ligand-Üeld theory An extension Ü Liebig condenser A laboratory of *crystal- eld theory describing the condenser having a straight glass properties of compounds of transi- l tube surrounded by a coaxial glass tion-metal ions or rare-earth ions in jacket through which cooling water which covalent bonding between the ligand is passed. The device is named after surrounding molecules (see ) the German organic chemist Justus and the transition-metal ions is taken von Liebig (1803–73). into account. This may involve using valence-bond theory or molecular- ligand An ion or molecule that do- orbital theory. Ligand-Üeld theory nates a pair of electrons to a metal was developed extensively in the atom or ion in forming a coordina- 1930s. As with crystal-Üeld theory, tion *complex. Molecules that func- ligand-Üeld theory indicates that en- tion as ligands are acting as Lewis acid ergy levels of the transition-metal bases (see ). For example, in the ions are split by the surrounding lig- complex hexaquocopper(II) ion 2+ ands, as determined by *group [Cu(H2O)6] six water molecules coor- theory. The theory has been very suc- dinate to a central Cu2+ ion. In the 2– cessful in explaining the optical, tetrachloroplatinate(II) ion [PtCl4] , – spectroscopic, and magnetic proper- four Cl ions are coordinated to a ties of the compounds of transition- central Pt2+ ion. A feature of such lig- metal and rare-earth ions. ands is that they have lone pairs of electrons, which they donate to ligase Any of a class of enzymes empty metal orbitals. A certain class that catalyse the formation of cova- of ligands also have empty p- or d- lent bonds using the energy released orbitals in addition to their lone pair by the cleavage of ATP. Ligases are of electrons and can produce com- important in the synthesis and repair plexes in which the metal has low of many biological molecules, includ- oxidation state. A double bond is ing DNA, and are used in genetic en- formed between the metal and the gineering to insert foreign DNA into ligand: a sigma bond by donation of cloning vectors. light-dependent reaction 326

light-dependent reaction See calcite structure; hot limewater photosynthesis. yields an aragonite structure. light-independent reaction See limit cycle See attractor. photosynthesis. limonite A generic term for a lignin A complex organic polymer group of hydrous iron oxides, that is deposited within the cellulose mostly amorphous. *Goethite and of plant cell walls during secondary *haematite are important con- thickening. LigniÜcation makes the stituents, together with colloidal walls woody and therefore rigid. silica, clays, and manganese oxides. lignite See coal. Limonite is formed by direct precipi- calcium oxide tation from marine or fresh water in lime See . shallow seas, lagoons, and bogs (thus limestone A sedimentary rock that it is often called bog iron ore) and by is composed largely of carbonate oxidation of iron-rich minerals. It is minerals, especially carbonates of used as an ore of iron and as a pig- calcium and magnesium. *Calcite ment. and *aragonite are the chief miner- Lindemann–Hinshelwood mech- als; *dolomite is also present in the anism dolomitic limestones. There are A mechanism for unimolecu- many varieties of limestones but lar chemical reactions put forward by l most are deposited in shallow water. the British physicist Frederick Linder- Organic limestones (e.g. *chalk) are mann (1886–1957) in 1921 and exam- formed from the calcareous skele- ined in more detail by the British tons of organisms; precipitated lime- chemist Sir Cyril Hinshelwood stones include oolite, which is (1897–1967) in 1927. The mechanism composed of ooliths – spherical bod- postulates that a molecule of A be- ies formed by the precipitation of comes excited by colliding with an- carbonate around a nucleus; and clas- other molecule of A, and that having tic limestones are derived from frag- been excited there is a possibility ments of pre-existing calcareous that it undergoes unimolecular rocks. decay. If the process of unimolecular decay is sufÜciently slow, the reac- limewater A saturated solution of tion has a Ürst-order rate law, in *calcium hydroxide in water. When agreement with experiment. The Lin- carbon dioxide gas is bubbled through limewater, a ‘milky’ precipi- demann–Hinshelwood mechanism tate of calcium carbonate is formed: predicts that if the concentration of → A is reduced, the reaction kinetics be- Ca(OH)2(aq) + CO2(g) CaCO3(s) + come second order. This change from H O(l) 2 Ürst to second order agrees with ex- If the carbon dioxide continues to be periment qualitatively, although it bubbled through, the calcium car- does not do so quantitatively. The bonate eventually redissolves to form mechanism fails quantitatively be- a clear solution of calcium hydrogen- cause the molecule has to be excited carbonate: in a speciÜc way for a reaction to → CaCO3(s) + CO2(g) + H2O(g) take place. The RRK and RRKM theo- Ca(HCO3)2(aq) ries improve on this deÜciency of the If cold limewater is used the original Lindemann–Hinshelwood mecha- calcium carbonate precipitated has a nism. 327 lipid bilayer linear combination of atomic lipid Any of a diverse group of or- orbitals See lcao. ganic compounds, occurring in living linear molecule A molecule in organisms, that are insoluble in which the atoms are in a straight water but soluble in organic solvents, such as chloroform, benzene, etc. line, as in carbon dioxide, O=C=O. Ü Linear molecules have only two rota- Lipids are broadly classi ed into two tional degrees of freedom. categories: complex lipids, which are esters of long-chain fatty acids and linear rotor See moment of iner- include the *glycerides (which con- tia. stitute the *fats and *oils of animals line notation A notation system and plants), *glycolipids, *phospho- for writing the structure of a chemi- lipids, and *waxes; and simple lipids, cal compound as a string of letters, which do not contain fatty acids and numbers, and symbols. Examples of include the *steroids and *terpenes. line notation are *Wiswesser line no- Lipids have a variety of functions in tation (WLN), *SMILES, *SYBYL line living organisms. Fats and oils are a notation (SLN), *ROSDAL, and convenient and concentrated means *InChI. of storing food energy in plants and animals. Phospholipids and *sterols, spectrum line spectrum See . such as cholesterol, are major com- linoleic acid A liquid polyunsatu- ponents of cell membranes (see lipid rated *fatty acid with two double bilayer). Waxes provide vital water- Ü l bonds, CH3(CH2)4CH:CHCH2- proo ng for body surfaces. Terpenes CH:CH(CH2)7COOH. Linoleic acid is include vitamins A, E, and K, and abundant in plant fats and oils, e.g. phytol (a component of chlorophyll) linseed oil, groundnut oil, and soya- and occur in essential oils, such as bean oil. It is an *essential fatty acid. menthol and camphor. Steroids include the adrenal hormones, sex hor- linolenic acid A liquid polyunsatu- mones, and bile acids. rated *fatty acid with three double Lipids can combine with proteins bonds in its structure: to form lipoproteins, e.g. in cell CH CH CH:CHCH CH:CHCH CH:CH- 3 2 2 2 membranes. In bacterial cell walls, (CH ) COOH. It occurs in certain 2 7 lipids may associate with polysaccha- plant oils, e.g. linseed and soya-bean rides to form lipopolysaccharides. oil, and in algae. It is one of the *essential fatty acids. A • Information about IUPAC nomenclature linseed oil A pale yellow oil of lipids pressed from Ûax seed. It contains a mixture of glycerides of fatty acids, lipid bilayer The arrangement of including linoleic acid and linolenic lipid molecules in biological mem- acid. It is a *drying oil, used in oil branes, which takes the form of a paints, varnishes, linoleum, etc. double sheet. Each lipid molecule comprises a hydrophilic ‘head’ (hav- Linz–Donawitz process See basic- Ü oxygen process ing a high af nity for water) and a . hydrophobic ‘tail’ (having a low lipase An enzyme secreted by the afÜnity for water). In the lipid bilayer pancreas and the glands of the small the molecules are aligned so that intestine of vertebrates that catalyses their hydrophilic heads face out- the breakdown of fats into fatty acids wards, forming the outer and inner and glycerol. surfaces of the membrane, while the lipoic acid 328

hydrophobic tails face inwards, away cially *liqueÜed petroleum gas and from the external aqueous environ- liqueÜed natural gas. ment. liqueÜed natural gas (LNG) See lipoic acid A vitamin of the *vita- liquefied petroleum gas. min B complex. It is one of the liqueÜed petroleum gas (LPG) *coenzymes involved in the decar- Various petroleum gases, principally boxylation of pyruvate by the en- propane and butane, stored as a liq- zyme pyruvate dehydrogenase. Good uid under pressure. It is used as an sources of lipoic acid include liver engine fuel and has the advantage of and yeast. causing very little cylinder-head de- lipolysis The breakdown of storage posits. Ü lipids in living organisms. Most long- Lique ed natural gas (LNG) is a sim- term energy reserves are in the form ilar product and consists mainly of of triglycerides in fats and oils. When methane. However, it cannot be Ü these are needed, e.g. during starva- lique ed simply by pressure as it has tion, lipase enzymes convert the a low critical temperature of 190 K triglycerides into glycerol and the and must therefore be cooled to below this temperature before it will component fatty acids. These are Ü then transported to tissues and oxi- liquefy. Once lique ed it has to be stored in well-insulated containers. It dized to provide energy. provides a convenient form in which l lipoprotein See lipid. to ship natural gas in bulk from oil lipowitz alloy A low-melting wells or gas-only wells to users. It is (70–74°C) alloy of bismuth (50%), lead also used as an engine fuel. (27%), tin (13%), and cadmium (10%). liquid A phase of matter between that of a crystalline solid and a *gas. liquation The separation of mix- In a liquid, the large-scale three- tures of solids by heating to a tem- dimensional atomic (or ionic or mo- perature at which lower-melting lecular) regularity of the solid is ab- components liquefy. sent but, on the other hand, so is the liquefaction of gases The conver- total disorganization of the gas. Al- sion of a gaseous substance into a liq- though liquids have been studied for uid. This is usually achieved by one many years there is still no compre- of four methods or by a combination hensive theory of the liquid state. It of two of them: is clear, however, from diffraction (1) by vapour compression, provided studies that there is a short-range that the substance is below its *criti- structural regularity extending over cal temperature; several molecular diameters. These (2) by refrigeration at constant pres- bundles of ordered atoms, molecules, sure, typically by cooling it with a or ions move about in relation to colder Ûuid in a countercurrent heat each other, enabling liquids to have Ü exchanger; almost xed volumes, which adopt (3) by making it perform work adia- the shape of their containers. batically against the atmosphere in a liquid crystal A substance that reversible cycle; Ûows like a liquid but has some order (4) by the *Joule–Thomson effect. in its arrangement of molecules. Ne- Large quantities of liqueÜed gases matic crystals have long molecules all are now used commercially, espe- aligned in the same direction, but 329 lithium carbonate otherwise randomly arranged. Cho- ing it also reacts with nitrogen and lesteric and smectic liquid crystals hydrogen. Its chemistry differs some- also have aligned molecules, which what from that of the other group 1 are arranged in distinct layers. In elements because of the small size of cholesteric crystals, the axes of the the Li+ ion. molecules are parallel to the plane of A the layers; in smectic crystals they • Information from the WebElements site are perpendicular. lithium aluminium hydride See liquid-crystal polymer A polymer lithium tetrahydroaluminate(iii). with a liquid-crystal structure, this being the most thermodynamically lithium battery A type of voltaic stable. Liquid-crystal polymers con- cell containing lithium or lithium tain long rigid chains and combine compounds. The most commonly strength with lightness. They are, used has a metallic lithium anode however, difÜcult to produce com- and a manganese dioxide (MnO2) mercially. cathode, the electrolyte being a solution of lithium salts in an organic sol- liquidus A line on a phase diagram vent. Batteries of this type have an above which a substance is liquid. output of about 3 volts. They are l-isomer See optical activity. more expensive than alkaline batter- absolute configura- ies, but last longer. Li–MnO2 batteries L-isomer See Û tion. are also produced in a at disk form l for use in digital watches and other litharge See lead(ii) oxide. small portable devices. A number of lithia See lithium oxide. other more specialized lithium primary batteries are available but are lithium Symbol Li. A soft silvery not in general use. Ü metal, the rst member of group 1 The lithium-ion battery is a (formerly IA) of the periodic table rechargeable cell. The anode is car- alkali metals (see ); a.n. 3; r.a.m. bon and the cathode is a metal oxide ° 6.939; r.d. 0.534; m.p. 180.54 C; b.p. (e.g. cobalt(IV) oxide, CoO ). The elec- ° 2 1347 C. It is a rare element found in trolyte is a lithium salt such as the spodumene (LiAlSi2O6), petalite borate (LiBO4) or chlorate (LiClO4) in (LiAlSi4O10), the mica lepidolite, and an organic solvent. The action of the certain brines. It is usually extracted cell depends on movement of Li ions by treatment with sulphuric acid to between anode and cathode with oxi- give the sulphate, which is converted dation of the cobalt ions during to the chloride. This is mixed with a charging and reduction during dis- small amount of potassium chloride, charge. Lithium-ion batteries are melted, and electrolysed. The stable light and have a low self-discharge isotopes are lithium–6 and lithium–7. rate, although the capacity does dete- Lithium–5 and lithium–8 are short- riorate with age. They are extensively lived radioisotopes. The metal is used used in mobile phones, laptops, cam- to remove oxygen in metallurgy and corders, and similar devices, as well as a constituent of some Al and Mg as electric cars. alloys. It is also used in batteries and is a potential tritium source for fu- lithium carbonate A white solid, ° sion research. Lithium salts are used Li2CO3; r.d. 2.11; m.p. 723 C; decom- in psychomedicine. The element re- poses above 1310°C. It is produced acts with oxygen and water; on heat- commercially by treating the ore lithium deuteride 330

with sulphuric acid at 250°C and acting lime with lithium salts or leaching the product to give a solu- lithium ores. Lithium hydroxide is tion of lithium sulphate. The carbon- basic but has a closer resemblance to ate is then obtained by precipitation group 2 hydroxides than to the other with sodium carbonate solution. group 1 hydroxides (an example of Lithium carbonate is used in the pre- the Ürst member of a periodic group vention and treatment of manic- having atypical properties). depressive disorders. It is also used lithium oxide (lithia) A white crys- industrially in ceramic glazes. talline compound, Li2O; cubic; r.d. lithium deuteride See lithium hy- 2.01; m.p. 1700°C. It can be obtained dride. from a number of lithium ores; the main uses are in lubricating greases, lithium hydride A white solid, ceramics, glass and refractories, and LiH; cubic; r.d. 0.82; m.p. 680°C; de- as a Ûux in brazing and welding. composes at about 850°C. It is produced by direct combination of the lithium sulphate A white or elements at temperatures above colourless crystalline material, ° 500 C. The bonding in lithium hy- Li2SO4, soluble in water and insoluble dride is believed to be largely ionic; in ethanol. It forms a monohydrate i.e. Li+H– as supported by the fact (monoclinic; r.d. 1.88) and an anhy- that hydrogen is released from the drous form, which exists in α- (mono- anode on electrolysis of the molten clinic), β- (hexagonal) and γ- (cubic) l salt. The compound reacts violently forms; r.d. 2.23. The compound is and exothermically with water to prepared by the reaction of the hy- yield hydrogen and lithium hydrox- droxide or carbonate with sulphuric ide. It is used as a reducing agent to acid. It is not isomorphous with prepare other hydrides and the 2H other group 1 sulphates and does not isotopic compound, lithium form alums. deuteride, is particularly valuable for lithium tetrahydroaluminate(III) deuterating a range of organic com- (lithium aluminium hydride; LAH) A pounds. Lithium hydride has also white or light grey powder, LiAlH ; been used as a shielding material for 4 r.d. 0.917; decomposes at 125°C. It is thermal neutrons. prepared by the reaction of excess lithium hydrogencarbonate A lithium hydride with aluminium compound, LiHCO3, formed by the chloride. The compound is soluble in reaction of carbon dioxide with aque- ethoxyethane, reacts violently with ous lithium carbonate and known water to release hydrogen, and is only in solution. It has found medici- widely used as a powerful reducing nal uses similar to those of lithium agent in organic chemistry. It should carbonate and is sometimes included always be treated as a serious Üre risk in proprietary mineral waters. in storage. lithium hydroxide A white crys- litmus A water-soluble dye ex- talline solid, LiOH, soluble in water, tracted from certain lichens. It turns slightly soluble in ethanol and insolu- red under acid conditions and blue ble in ether. It is known as the mono- under alkaline conditions, the colour hydrate (monoclinic; r.d. 1.51) and in change occurring over the pH range the anhydrous form (tetragonal, r.d. 4.5–8.3 (at 25°C). It is not suitable for 1.46; m.p. 450°C; decomposes at titrations because of the wide range 924°C). The compound is made by re- over which the colour changes, but is 331 lone pair used as a rough *indicator of acidity or decrease of one unit represents a or alkalinity, both in solution and as tenfold increase or decrease in the litmus paper (absorbent paper soaked quantity measured. Decibels and pH in litmus solution). measurements are common examples of logarithmic scales of measure- litre Symbol l. A unit of volume in ment. 2. A scale on the axis of a the metric system regarded as a spe- graph in which an increase of one cial name for the cubic decimetre. It unit represents a tenfold increase in was formerly deÜned as the volume the variable quantity. If a curve y = xn of 1 kilogram of pure water at 4°C at is plotted on graph paper with loga- standard pressure, which is equiva- rithmic scales on both axes, the re- lent to 1.000 028 dm3. sult is a straight line of slope n, i.e. lixiviation The separation of mix- logy = nlogx, which enables n to be tures by dissolving soluble con- determined. stituents in water. London formula A formula giving LNG See liquefied petroleum gas. an expression for the induced- dipole–induced-dipole interaction localization The conÜnement of between molecules (called the dis- electrons to a particular atom in a persion interaction or London inter- molecule or to a particular chemical action). The London formula for the bond. interaction energy V is given by V= 6 ⅔α′ α′ localized bond A *chemical bond –C/r , where C = 1 2I1I2/(I1 + I2). l α′ α′ in which the electrons forming the Here 1 and 2 are the polarizability bond remain between (or close to) volumes of molecule 1 and 2 respec- delocal- the linked atoms. Compare tively, I1 and I2 are the ionization ization. energies of molecules 1 and 2 respec- lock-and-key mechanism A tively, and r is the distance between mechanism proposed in 1890 by the molecules. The London formula Emil Fischer (1852–1919) to explain is named after Fritz London (1900– binding between the active site of an 54), who derived it. The interaction described by the London formula is enzyme and a substrate molecule. usually the dominant term in inter- The active site was thought to have a molecular forces (unless hydrogen Üxed structure (the lock), which ex- bonds are present). actly matched the structure of a speciÜc substrate (the key). Thus the lone pair A pair of electrons having enzyme and substrate interact to opposite spin in an orbital of an form an *enzyme–substrate complex. atom. For instance, in ammonia the The substrate is converted to prod- nitrogen atom has Üve electrons, ucts that no longer Üt the active site three of which are used in forming and are therefore released, liberating single bonds with hydrogen atoms. the enzyme. Recent observations The other two occupy a Ülled atomic made by X-ray diffraction studies orbital and constitute a lone pair (see have shown that the active site of an illustration). The orbital containing enzyme is more Ûexible than the these electrons is equivalent to a sin- lock-and-key theory would suggest. gle bond (sigma orbital) in spatial orientation, accounting for the lodestone See magnetite. pyramidal shape of the molecule. In logarithmic scale 1. A scale of the water molecule, there are two measurement in which an increase lone pairs on the oxygen atom. In long period 332

stant rate and the product is removed at a constant rate, i.e. the reaction is in a steady state (but not in chemical equilibrium). The mecha- N nism involves three steps: H H R + X → 2X H X + Y → 2Y Lone pair Y → P The Ürst two steps involve *auto- considering the shapes of molecules, catalysis: the Ürst step is catalysed by repulsions between bonds and lone the reactant X and the second by the pairs can be taken into account: reactant Y. The kinetics of such a re- lone pair–lone pair > lone action can be calculated numerically, pair–bond > bond–bond. showing that the concentrations of both X and Y increase and decrease periodic table long period See . periodically with time. This results Lorentz–Lorenz equation A rela- from the autocatalytic action. Ini- tion between the *polarizability α of tially, the concentration of X is small, a molecule and the refractive index but, as it increases, there is a rapid n of a substance made up of mol- increase in the rate of the Ürst reac- ecules with this polarizability. The tion because of the autocatalytic ac- l Lorentz–Lorenz equation can be writ- tion of X. As the concentration of X ten in the form α = (3/4πN) [(n2–1/ builds up, the rate of the second reac- (n2 + 2)], where N is the number of tion also increases. Initially, the con- molecules per unit volume. The centration of Y is low but there is a equation provides a link between a sudden surge in the rate of step 2, re- microscopic quantity (the polarizabil- sulting from the autocatalytic action ity) and a macroscopic quantity (the of Y. This lowers the concentration of refractive index). It was derived using X and slows down step 1, so the con- macroscopic electrostatics in 1880 by centration of X falls. Less X is now Hendrik Lorentz (1853–1928) and in- available for the second step and the dependently by the Danish physicist concentration of Y also starts to fall. Ludwig Valentin Lorenz also in 1880. With this fall in the amount of Y, less Compare clausius–mossotti equa- X is removed, and the Ürst reaction tion. again begins to increase. These processes are repeated, leading to re- Loschmidt’s constant (Loschmidt peated rises and falls in the concen- number) The number of particles trations of both X and Y. The cycles * per unit volume of an ideal gas at are not in phase, peaks in the con- STP. It has the value 2.686 763(23) × 25 –3 Ü centration of Y occurring later than 10 m and was rst worked out by peaks in X. Joseph Loschmidt (1821–95). In fact, known oscillating chemical Lotka–Volterra mechanism A reactions have different mechanisms simple chemical reaction mechanism to the above, but the scheme illus- proposed as a possible mechanism of trates how oscillation may occur. *oscillating reactions. The process in- This type of process is found in Üelds volves a conversion of a reactant R other than chemistry; they were in- into a product P. The reactant Ûows vestigated by the Italian mathemati- into the reaction chamber at a con- cian Vito Volterra (1860–1940) in 333 luminol test models of biological systems (e.g. the formation of a sludge, and alka- predator–prey relationships). line-earth phenates that neutralize low-energy electron diffraction acids and reduce wear. At high temperatures, solid lubri- See leed. cants, such as graphite or molybde- lowering of vapour pressure A num disulphide, are often used. reduction in the saturated vapour SemiÛuid lubricants (greases) are pressure of a pure liquid when a used to provide a seal against mois- solute is introduced. If the solute is ture and dirt and to remain attached a solid of low vapour pressure, the to vertical surfaces. They are made decrease in vapour pressure of by adding gelling agents, such as the liquid is proportional to the metallic soaps, to liquid lubricants. concentration of particles of solute; luciferase See bioluminescence. i.e. to the number of dissolved molecules or ions per unit volume. To a luciferin See bioluminescence. Ü rst approximation, it does not de- lumen Symbol lm. The SI unit of lu- pend on the nature of the particles. minous Ûux equal to the Ûux emitted See colligative properties; raoult’s law by a uniform point source of 1 can- . dela in a solid angle of 1 steradian. lowest unoccupied molecular luminescence The emission of orbital (LUMO) The orbital in a light by a substance for any reason molecule that has the lowest unoccu- other than a rise in its temperature. l pied energy level at the absolute zero In general, atoms of substances emit of temperature. The lowest unoccu- *photons of electromagnetic energy pied molecular orbital and the high- when they return to the *ground est occupied molecular orbital state after having been in an excited (HOMO) are the two *frontier or- state (see excitation). The causes of bitals of the molecule. the excitation are various. If the ex- Lowry–Brønsted theory See acid. citing cause is a photon, the process is called photoluminescence; if it is LSD See lysergic acid diethy- an electron it is called electrolumi- lamide. nescence. Chemiluminescence is lumi- L-series See absolute configura- nescence resulting from a chemical tion. reaction (such as the slow oxidation * lubrication The use of a substance of phosphorus); bioluminescence is to prevent contact between solid sur- the luminescence produced by a living organism (such as a ÜreÛy). If the faces in relative motion in order to Ü reduce friction, wear, overheating, luminescence persists signi cantly and rusting. Liquid hydrocarbons after the exciting cause is removed it phosphorescence (oils), either derived from petroleum is called ; if it does Ûuorescence or made synthetically, are the most not it is called . This distinction is arbitrary since there must widely used lubricants as they are always be some delay; in some deÜni- relatively inexpensive, are good tions a persistence of more than 10 coolants, provide the appropriate nanoseconds (10–8 s) is treated as range of viscosities, and are ther- phosphorescence. mally stable. Additives include polymeric substances that maintain the luminol test A *presumptive test desired viscosity as the temperature for blood. The reagent is a mixture of increases, antioxidants that prevent 3-aminophthalhydrazide, sodium car- LUMO 334

bonate, and sodium perborate. When Lyman series See hydrogen spec- sprayed with the reagent, traces of trum. blood (even old blood) emit a faint lyonium ion The ion formed by chemoluminescence. adding a hydron (H+) to a solvent LUMO See lowest unoccupied mo- molecule. For example, in ethanol, lecular orbital + . C2H5OH2 is the lyonium ion. lutetium Symbol Lu. A silvery lyophilic Having an afÜnity for a metallic element belonging to the solvent (‘solvent-loving’; if the sol- *lanthanoids; a.n. 71; r.a.m. 174.97; vent is water the term hydrophilic is ° ° r.d. 9.8404 (20 C); m.p. 1663 C; b.p. used). See colloids. 3402°C. Lutetium is the least abun- Ü dant of the elements and the little lyophobic Lacking any af nity for quantities that are available have a solvent (‘solvent-hating’; if the sol- been obtained by processing other vent is water the term hydrophobic is metals. There are two natural iso- used). See colloids. topes, lutetium–175 (stable) and lyotropic mesomorph An × 10 lutetium–176 (half-life 2.2 10 arrangement taken by micelles years). The element is used as a cata- Ü Ü formed from surfactant molecules in lyst. It was rst identi ed by Gerges concentrated solutions. A lyotropic UrbainA (1872–1938) in 1907. mesomorph consists of long cylin- l ders in a fairly close-packed hexago- • Information from the WebElements site nal arrangement. Lyotropic lux Symbol lx. The SI unit of illumi- mesomorphs are sometimes called nance equal to the illumination pro- liquid crystalline phases for micelles. Û duced by a luminous ux of 1 lumen lysergic acid diethylamide (LSD) distributed uniformly over an area of A chemical derivative of lysergic acid 1 square metre. that has potent hallucinogenic prop- lyate ion The ion formed by re- erties (see hallucinogen). It occurs in moving a hydron from a molecule of the cereal-fungus ergot and is a solvent. In water, for example, the classiÜed as an ergot *alkaloid. LSD hydroxide ion (OH–) is the lyate ion. was Ürst synthesized in 1943. lye See potassium hydroxide. lysine See amino acid. M

macromolecular crystal A crys- where C and n are constants. The talline solid in which the atoms are value of α can be used in calculations all linked together by covalent to determine C and n. It was Ürst cal- bonds. Carbon (in diamond), boron culated by Erwin Madelung in 1918. nitride, and silicon carbide are exam- Magic acid See superacid. ples of substances that have macromolecular crystals. In effect, the magic-angle spinning A tech- crystal is a large molecule (hence nique used in solid-state *nuclear the alternative description giant- magnetic resonance (NMR) for molecular), which accounts for the making the line widths smaller. In hardness and high melting point of magic-angle spinning, both the such materials. dipole–dipole interaction and the chemical shift anisotropy have the macromolecule A very large mol- angular dependence 1–3cos2θ, where ecule. Natural and synthetic poly- θ is the angle between the principal mers have macromolecules, as do axis of the molecule and the applied such substances as haemoglobin. See magnetic Üeld. The ‘magic angle’ is colloids also . the angle θ that satisÜes 1–3cos2θ = 0 macroscopic Designating a size and is given by θ = 54.74°. In magic- scale very much larger than that of angle spinning the material is spun atoms and molecules. Macroscopic very rapidly at the magic angle to the objects and systems are described by applied magnetic Üeld so that the di- classical physics although *quantum pole–dipole interactions and chemi- mechanics can have macroscopic cal shift anisotropies average to zero. consequences. Compare mesoscopic; It is necessary for the frequency of microscopic. spinning to be at least as large as the width of the spectrum. This tech- Madelung constant A constant nique has been extensively used, arising in calculations of the cohe- with the spinning between 4 and sion of ionic crystals. The electrosta- 5 kHz. tic interaction per ion pair, U, is given by U(r) = – αe2/r, where α is the Magnadur Tradename for a ce- Madelung constant and e2/r is the ramic material used to make perma- Coulomb interaction between the nent magnets. It consists of sintered ions, with r being the lattice con- iron oxide and barium oxide. α stant. The value of depends on the Magnalium Tradename for an alu- type of lattice. For the sodium chlo- minium-based alloy of high reÛectiv- α ride lattice, has a value of about ity for light and ultraviolet radiation 1.75. A more realistic calculation of that contains 1–2% of copper and be- cohesion is obtained if short-range tween 5% and 30% of magnesium. repulsions with an inverse power law Strong and light, these alloys also are included, i.e. sometimes contain other elements, U(r) = αe2/r – C/rn, such as tin, lead, and nickel. magnesia 336

magnesia See magnesium oxide. MgCO3.5H2O (monoclinic; r.d. 1.73), which occurs as lansfordite. Magne- magnesite A white, colourless, or grey mineral form of *magnesium sium carbonate also occurs in the mixed salt *dolomite (CaCO3.MgCO3) carbonate, MgCO3, crystallizing in the trigonal system. It is formed as a and as basic magnesium carbon- replacement mineral of magnesium- ate in the two minerals artinite rich rocks when carbon dioxide is (MgCO3.Mg(OH)2.3H2O) and hydro- available. Magnesite is mined both as magnesite (3MgCO3.Mg(OH)2.3H2O). an ore for magnesium and as a The anhydrous salt can be formed by source of magnesium carbonate. It heating magnesium oxide in a occurs in Austria, USA, Greece, Nor- stream of carbon dioxide: → way, India, Australia, and South MgO(s) + CO2(g) MgCO3(s) Africa. Above 350°C, the reverse reaction magnesium Symbol Mg. A silvery predominates and the carbonate de- metallic element belonging to group composes. Magnesium carbonate is 2 (formerly IIA) of the periodic table used in making magnesium oxide (see alkaline-earth metals); a.n. 12; and is a drying agent (e.g. in table r.a.m. 24.305; r.d. 1.74; m.p. 648.8°C; salt). It is also used as a medical b.p. 1090°C. The element is found in antacid and laxative (the basic car- a number of minerals, including bonate is used) and is a component magnesite (MgCO3), dolomite of certain inks and glasses. (MgCO .CaCO ), and carnallite 3 3 magnesium chloride A white (MgCl2.KCl.6H2O). It is also present in m sea water, and it is an *essential el- solid compound, MgCl2. The anhy- ement for living organisms. Extrac- drous salt (hexagonal; r.d. 2.32; m.p. ° ° tion is by electrolysis of the fused 714 C; b.p. 1412 C) can be prepared chloride. The element is used in a by the direct combination of dry number of light alloys (e.g. for air- chlorine with magnesium: → craft). Chemically, it is very reactive. Mg(s) + Cl2(g) MgCl2(s) In air it forms a protective oxide coat- The compound also occurs naturally ing but when ignited it burns with as a constituent of carnallite an intense white Ûame. It also reacts (KCl.MgCl2). It is a deliquescent com- with the halogens, sulphur, and ni- pound that commonly forms the trogen. Magnesium was Ürst isolated hexahydrate, MgCl2.6H2O (mono- by Bussy in 1828. clinic; r.d. 1.57). When heated, this A hydrolyses to give magnesium oxide • Information from the WebElements site and hydrogen chloride gas. The fused magnesium bicarbonate See chloride is electrolysed to produce magnesium hydrogencarbonate. magnesium and it is also used for ÜreprooÜng wood, in magnesia ce- magnesium carbonate A white ments and artiÜcial leather, and as a compound, MgCO3, existing in anhy- laxative. drous and hydrated forms. The anhydrous material (trigonal; r.d. magnesium hydrogencarbonate 2.96) is found in the mineral *mag- (magnesium bicarbonate) A com- nesite. There is also a trihydrate, pound, Mg(HCO3)2, that is stable only MgCO3.3H2O (rhombic; r.d. 1.85), in solution. It is formed by the action which occurs naturally as nesque- of carbon dioxide on a suspension of honite, and a pentahydrate, magnesium carbonate in water: 337 magnetic moment

→ MgCO3(s) + CO2(g) + H2O(l) the mineral kieserite. The common- Mg(HCO3)2(aq) est hydrate is the heptahydrate, On heating, this process is reversed. MgSO4.7H2O (rhombic; r.d. 1.68), Magnesium hydrogencarbonate is which is called Epsom salt(s), and oc- one of the compounds responsible curs naturally as the mineral ep- for temporary *hardness in water. somite. This is a white powder with a bitter saline taste, which loses 6H2O magnesium hydroxide A white ° ° at 150 C and 7H2O at 200 C. It is solid compound, Mg(OH)2; trigonal; Ü Ü ° used in sizing and reproo ng cotton r.d. 2.36; decomposes at 350 C. Mag- and silk, in tanning leather, and in nesium hydroxide occurs naturally as the manufacture of fertilizers, explo- the mineral brucite and can be pre- sives, and matches. In medicine, it is pared by reacting magnesium sul- used as a laxative. It is also used in phate or chloride with sodium veterinary medicine for treatment of hydroxide solution. It is used in the local inÛammations and infected reÜning of sugar and in the process- wounds. ing of uranium. Medicinally it is important as an antacid (milk of magnetic moment The ratio be- magnesia) and as a laxative. tween the maximum torque (Tmax) exerted on a magnet, current- magnesium oxide (magnesia) A carrying coil, or moving charge situ- white compound, MgO; cubic; r.d. ated in a magnetic Üeld and the 3.58; m.p. 2800°C. It occurs naturally strength of that Üeld. It is thus a as the mineral periclase and is prepared commercially by thermally de- measure of the strength of a magnet m composing the mineral *magnesite: or current-carrying coil. In the Som- merfeld approach this quantity (also → MgCO3(s) MgO(s) + CO2(g) called electromagnetic moment or It has a wide range of uses, including magnetic area moment) is the ratio Û re ective coatings on optical instru- Tmax/B. In the Kennelly approach the ments and aircraft windscreens and quantity (also called magnetic dipole in semiconductors. Its high melting moment) is Tmax/H. point makes it useful as a refractory In the case of a magnet placed in a lining in metal and glass furnaces. magnetic Üeld of Üeld strength H, the magnesium peroxide A white maximum torque Tmax occurs when ° the axis of the magnet is perpendicu- solid, MgO2. It decomposes at 100 C to release oxygen and also releases lar to the Üeld. In the case of a coil of oxygen on reaction with water: N turns and area A carrying a current I, the magnetic moment can be 2MgO (s) + 2H O → 2Mg(OH) + O 2 2 2 2 shown to be m = T/B = NIA or m = T/H The compound is prepared by react- = µNIA. Magnetic moments are meas- ing sodium peroxide with magne- ured in A m2. sium sulphate solution and is used as An orbital electron has an orbital a bleach for cotton and silk. magnetic moment IA, where I is the magnesium sulphate A white sol- equivalent current as the electron uble compound, MgSO4, existing as moves round its orbit. It is given by I the anhydrous compound (rhombic; = qω/2π, where q is the electronic r.d. 2.66; decomposes at 1124°C) and charge and ω is its angular velocity. in hydrated crystalline forms. The The orbital magnetic moment is ω π monohydrate MgSO4.H2O (mono- therefore IA = q A/2 , where A is the clinic; r.d. 2.45) occurs naturally as orbital area. If the electron is spin- magnetic quantum number 338

ning there is also a spin magnetic permeability slightly in excess of moment (see spin); atomic nuclei also one. Paramagnetism occurs in all have magnetic moments. atoms and molecules with unpaired electrons; e.g. free atoms, free radi- magnetic quantum number See cals, and compounds of transition atom. metals containing ions with unÜlled magnetism A group of phenomena electron shells. It also occurs in met- associated with magnetic Üelds. als as a result of the magnetic mo- Whenever an electric current Ûows a ments associated with the spins of magnetic Üeld is produced; as the or- the conducting electrons. bital motion and the *spin of atomic (c) In ferromagnetic substances, electrons are equivalent to tiny cur- within a certain temperature range, rent loops, individual atoms create there are net atomic magnetic mo- magnetic Üelds around them, when ments, which line up in such a way their orbital electrons have a net that magnetization persists after the *magnetic moment as a result of removal of the applied Üeld. Below a their angular momentum. The mag- certain temperature, called the Curie netic moment of an atom is the vec- point (or Curie temperature) an in- tor sum of the magnetic moments of creasing magnetic Üeld applied to a the orbital motions and the spins of ferromagnetic substance will cause all the electrons in the atom. The increasing magnetization to a high macroscopic magnetic properties of a value, called the saturation magneti- substance arise from the magnetic zation. This is because a ferromag- moments of its component atoms netic substance consists of small m and molecules. Different materials (1–0.1 mm across) magnetized re- have different characteristics in an gions called domains. The total mag- applied magnetic Üeld; there are four netic moment of a sample of the main types of magnetic behaviour: substance is the vector sum of the (a) In diamagnetism the magnetiza- magnetic moments of the compo- tion is in the opposite direction to nent domains. Within each domain that of the applied Üeld, i.e. the sus- the individual atomic magnetic mo- ceptibility is negative. Although all ments are spontaneously aligned by substances are diamagnetic, it is a exchange forces, related to whether weak form of magnetism and may be or not the atomic electron spins are masked by other, stronger, forms. It parallel or antiparallel. However, in results from changes induced in the an unmagnetized piece of ferromag- orbits of electrons in the atoms of a netic material the magnetic mo- substance by the applied Üeld, the di- ments of the domains themselves are rection of the change opposing the not aligned; when an external Üeld is applied Ûux. There is thus a weak applied those domains that are negative susceptibility (of the order aligned with the Üeld increase in size of –10–8 m3 mol–1) and a relative per- at the expense of the others. In a meability of slightly less than one. very strong Üeld all the domains are (b) In paramagnetism the atoms or lined up in the direction of the Üeld molecules of the substance have net and provide the high observed mag- orbital or spin magnetic moments netization. Iron, nickel, cobalt, and that are capable of being aligned in their alloys are ferromagnetic. Above the direction of the applied Üeld. the Curie point, ferromagnetic ma- They therefore have a positive (but terials become paramagnetic. small) susceptibility and a relative (d) Some metals, alloys, and transi- 339 malachite tion-element salts exhibit another measuring and investigating the form of magnetism called antiferro- magnetic properties of compounds. It magnetism. This occurs below a cer- is used particularly for studying tran- tain temperature, called the Néel sition-metal complexes, many of temperature, when an ordered array which are paramagnetic because of atomic magnetic moments sponta- they have unpaired electrons. Meas- neously forms in which alternate mo- urement of the magnetic susceptibil- ments have opposite directions. ity allows the magnetic moment of There is therefore no net resultant the metal atom to be calculated, and magnetic moment in the absence of this gives information about the an applied Üeld. In manganese bonding in the complex. Ûuoride, for example, this antiparal- magnetomechanical ratio See lel arrangement occurs below a Néel gyromagnetic ratio. temperature of 72 K. Below this temperature the spontaneous ordering magneton A unit for measuring opposes the normal tendency of the magnetic moments of nuclear, magnetic moments to align with the atomic, or molecular magnets. The µ applied Üeld. Above the Néel temper- Bohr magneton B has the value of ature the substance is paramagnetic. the classical magnetic moment of an A special form of antiferromagnet- electron, given by ism is ferrimagnetism, a type of mag- µ π × –24 2 B = eh/4 me = 9.274 10 Am , netism exhibited by the *ferrites. In where e and m are the charge and these materials the magnetic mo- e mass of the electron and h is the ments of adjacent ions are antiparal- Planck constant. The nuclear magne- m lel and of unequal strength, or the ton, µ is obtained by replacing the number of magnetic moments in one N mass of the electron by the mass of direction is greater than those in the the proton and is therefore given by opposite direction. By suitable choice µ µ × –27 2 of rare-earth ions in the ferrite lat- N = B.me/mp = 5.05 10 Am . tices it is possible to design ferri- Ü Main-Smith–Stoner rule An em- magnetic substances with speci c pirical rule in the theory of atomic magnetizations for use in electronic structure stating that for a principal components. quantum number n the number of magnetite A black mineral form of electronic quantum states that can iron oxide crystallizing in the cubic have the orbital quantum number l is system. It is a mixed iron(II)-iron(III) 2(2l + 1). This rule describes the suboxide, Fe3O4, and is one of the major shells of atoms. It was put forward ores of iron. It is strongly magnetic on the basis of chemical evidence by and some varieties, known as lode- J. D. Main-Smith and independently stone, are natural magnets; these on the basis of magnetic and spectro- were used as compasses in the an- scopic evidence by Edmund Stoner in cient world. Magnetite is widely dis- 1924. The Main-Smith–Stoner rule is tributed and occurs as an accessory a consequence of the *Pauli exclu- mineral in almost all igneous and sion principle. The rule was one of metamorphic rocks. The largest de- the key developments that led to the posits of the mineral occur in N Swe- enunciation of the Pauli exclusion den. principle in 1925. magnetochemistry The branch of malachite A secondary mineral physical chemistry concerned with form of copper carbonate–hydroxide, MALDI 340

CuCO3.Cu(OH)2. It is bright green and malonic acid See propanedioic crystallizes in the monoclinic system acid. but usually occurs as aggregates of malt The product of the hydrolysis Übres or in massive form. It is gener- of starch by β-amylase that occurs ally found with *azurite in associa- during the germination of barley in tion with the more important copper maltose ores and is itself mined as an ore of brewing. See also . copper (e.g. in Zaïre). It is also used as maltose (malt sugar) A sugar con- an ornamental stone and as a gem- sisting of two linked glucose mol- stone. ecules that results from the action of MALDI Matrix absorption laser des- the enzyme amylase on starch. Mal- orption ionization. A technique for tose occurs in barley seeds following producing ions for mass spec- germination and drying, which is the troscopy, used especially for large basis of the malting process used in biological species. The sample is ab- the manufacture of beer and malt sorbed on an inert matrix from whisky. which ions are desorbed by a laser. malt sugar See maltose. maleic acid See butenedioic acid. mancude Describing an organic maleic anhydride A colourless compound that contains the maxi- ° mum possible number of noncumu- solid, C4H2O3, m.p. 53 C, the anhydride of cis-butenedioic acid (maleic lative double bonds, as in the acid). It is a cyclic compound with a *annulenes. It is an acronym. maxi- m ring containing four carbon atoms mum non-cumulative double. and one oxygen atom, made by the Mandelin test A *presumptive catalytic oxidation of benzene or its test for amphetamines and alkaloids. derivatives at high temperatures. It is The Mandelin reagent is a 1% solu- used mainly in the manufacture of tion of ammonium vanadate alkyd and polyester resins and (NH4VO3) in concentrated sulphuric copolymers. acid. Different substances give differ- O ent colours. Mescaline, for example, O produces an orange colour, heroin a brown colour, and amphetamine a OH blue-green colour. O OH manganate(VI) A salt containing 2– the ion MnO4 . Manganate(VI) ions O O are dark green; they are produced by manganate(VII) ions in basic solution. Maleic anhydride manganate(VII) (permanganate) A – malic acid (2-hydroxybutanedioic salt containing the ion MnO4 . Man- acid) A crystalline solid, ganate(VII) ions are dark purple and l strong oxidizing agents. HOOCCH(OH)CH2COOH. -malic acid occurs in living organisms as an in- manganese Symbol Mn. A grey termediate metabolite in the *Krebs brittle metallic *transition element, cycle and also (in certain plants) in a.n. 25; r.a.m. 54.94; r.d. 7.2; m.p. photosynthesis. It is found especially 1244°C; b.p. 1962°C. The main in the juice of unripe fruits, e.g. sources are pyrolusite (MnO2) and green apples. rhodochrosite (MnCO3). The metal 341 many-body problem can be extracted by reduction of the and a carbonyl compound to produce oxide using magnesium (*Kroll an amino-carbonyl compound. It process) or aluminium (*Gold- takes place in two stages. First the schmidt process). Often the ore is amine reacts with methanol to form mixed with iron ore and reduced in a Schiff base: an electric furnace to produce ferro- → + R2N + H2CO R2N =CH2. manganese for use in alloy steels. This then reacts with the carbonyl The element is fairly electropositive; compound: it combines with oxygen, nitrogen, + 1 2 3 → and other nonmetals when heated R2N =CH2 + R R CHCOR 1 2 3 (but not with hydrogen). Salts of R2N–CH2–C(R R )COR . manganese contain the element in The reaction was Ürst reported by the +2 and +3 oxidation states. Man- Carl Mannich in 1912. ganese(II) salts are the more stable. It mannitol A polyhydric alcohol, also forms compounds in higher oxi- CH OH(CHOH) CH OH, derived from dation states, such as manganese(IV) 2 4 2 mannose or fructose. It is the main oxide and manganate(VI) and man- soluble sugar in fungi and an impor- ganate(VII) salts. The element was tant carbohydrate reserve in brown discovered in 1774 by Karl *Scheele. algae. Mannitol is used as a sweet- A ener in certain foodstuffs and as a di- • Information from the WebElements site uretic to relieve Ûuid retention. manganese(IV) oxide (manganese mannose A *monosaccharide, dioxide) A black oxide made by heat- C6H12O6, stereoisomeric with glu- m ing manganese(II) nitrate. The com- cose, that occurs naturally only in pound also occurs naturally as polymerized forms called mannans. pyrolusite. It is a strong oxidizing These are found in plants, fungi, and agent, used as a depolarizing agent in bacteria, serving as food energy voltaic cells. stores. manganic compounds Com- manometer A device for measur- pounds of manganese in its +3 oxida- ing pressure differences, usually by tion state; e.g. manganic oxide is the difference in height of two liquid manganese(III) oxide, Mn2O3. columns. The simplest type is the U- manganin A copper alloy contain- tube manometer, which consists of a ing 13–18% of manganese and 1–4% glass tube bent into the shape of a U. of nickel. It has a high electrical re- If a pressure to be measured is fed to sistance, which is relatively insensi- one side of the U-tube and the other tive to temperature changes. It is is open to the atmosphere, the differ- therefore suitable for use in resis- ence in level of the liquid in the two tance wire. limbs gives a measure of the unknown pressure. manganous compounds Com- pounds of manganese in its +2 oxida- many-body problem The prob- tion state; e.g. manganous oxide is lem that it is very difÜcult to obtain manganese(II) oxide, MnO. exact solutions to systems involving interactions between more than two mannan mannose See . bodies – using either classical me- Mannich reaction A reaction in chanics or quantum mechanics. To which a primary or secondary amine understand the physics of many-body reacts with methanal (formaldehyde) systems it is necessary to make use of marble 342

approximation techniques or model of products can be formed if the systems. For some problems, such as alkene is not symmetrical. For in- the three-body problem in classical stance, the reaction between mechanics, it is possible to obtain C2H5CH:CH2 and HCl can give qualitative information about the sys- C2H5CH2CH2Cl or C2H5CHClCH3. tem. If there are a great many bodies In general, a mixture of products interacting, such as the molecules in occurs in which one predominates a gas, the problem can be analysed over the other. In 1870, Vladimir using the techniques of *statistical Markovnikoff (1837–1904) proposed mechanics. the rule that the main product would marble A metamorphic rock com- be the one in which the hydrogen posed of recrystallized *calcite or atom adds to the carbon having the *dolomite. Pure marbles are white larger number of hydrogen atoms but such impurities as silica or clay (the latter product above). This oc- minerals result in variations of col- curs when the mechanism is *elec- our. Marble is extensively used for trophilic addition, in which the building purposes and ornamental Ürst step is addition of H+. The use; the pure white marble from Car- electron-releasing effect of the alkyl rara in Italy is especially prized by group (C2H5) distorts the electron- sculptors. The term is applied com- distribution in the double bond, mak- mercially to any limestone or ing the carbon atom furthest from dolomite that can be cut and pol- the alkyl group negative. This is the ished. atom attacked by H+ giving the carbonium ion C H C+HCH , which fur- m margaric acid See heptadecanoic 2 5 3 ther reacts with the negative ion Cl–. acid. In some circumstances anti- marijuana See cannabis. Markovnikoff behaviour occurs, in MarkofÜan process (Markov which the opposite effect is found. process) A random process (see sto- This happens when the mechanism chastic process) in which the rate involves free radicals and is common of change of a time-dependent quan- in addition of hydrogen bromide tity ∂a(t)/∂t depends on the instanta- when peroxides are present. neous value of the quantity a(t), Markush structure A generalized where t is the time, but not on its formula or description for a related previous history. If a random process set of chemical compounds, used in can be assumed to be a Markov patent applications. It is named after process, an analysis of the process is Eugene Markush (1888–1968), an Ü greatly simpli ed enabling useful American manufacturer of dyes and equations in *nonequilibrium statis- pharmaceuticals. In 1924 he was tical mechanics and disordered solids awarded a patent for “The process to be derived. Problems involving for manufacture of dyes which com- Markov processes are solved using prises coupling with a halogen- statistical methods and the theory of substituted pyralazone, a diazotized probability. Markov processes are unsulphonated material selected named after the Russian mathemati- from the group consisting of aniline, cian Andrei Andreevich Markov homologues of aniline, and halogen (1856–1922). substitution products of aniline”. Markovnikoff’s rule When an Note that the patent was for acid HA adds to an alkene, a mixture processes to produce a range of com- 343 mass action pounds, including ones that had not a similar result) by the fact that an- actually been synthesized. In 1925 timony does not dissolve in sodium The US Patent OfÜce ruled that such chlorate(I) (hypochlorite). The test patents were valid. Markush struc- was devised in 1836 by the British tures can be described for com- chemist James Marsh (1789–1846). pounds with substituents at several martensite A solid solution of car- positions, and often many thousands iron of possible compounds are deÜned in bon in alpha-iron (see ) formed this way. An important part of chem- when *steel is cooled too rapidly for ical database searching is the ability pearlite to form from austenite. It is to Ünd possible Markush structures responsible for the hardness of to rule out priority in the patent ap- quenched steel. plication. Chemical drawing pro- mascagnite A mineral form of grams can represent such structures. *ammonium sulphate, (NH4)2SO4. For example, a bond to the centre of Ü a ring indicates substitution at any maser (microwave ampli cation by position on the ring. stimulated emission of radiation) A device for amplifying or generating Marquis test A widely used *pre- microwaves by means of stimulated sumptive test that gives a variety of emission (see laser). colour changes with a range of compounds. It is particularly useful for mass A measure of a body’s inertia, detecting opiate alkaloids and for i.e. its resistance to acceleration. Ac- amphetamines and methampheta- cording to Newton’s laws of motion, mine. Marquis reagent is a mixture if two unequal masses, m1 and m2, m of methanal (formaldehyde) solution are allowed to collide, in the absence in water with sulphuric acid. Mesca- of any other forces both will experi- line gives an orange colouration. ence the same force of collision. If With morphine, a violet colour is the two bodies acquire accelerations produced. Amphetamines give an or- a1 and a2 as a result of the collision, ange-red colour and methampheta- then m1a1 = m2a2. This equation en- mine gives an orange colour. The two ables two masses to be compared. can be distinguished by the *Simon If one of the masses is regarded test. The mechanism involves attack as a standard of mass, the mass of the aldehyde an a substituted aro- of all other masses can be meas- matic ring to form a carbocation. Fur- ured in terms of this standard. ther reaction forms a coloured dimer The body used for this purpose is a of the original molecule. 1-kg cylinder of platinum–iridium marsh gas Methane formed by rot- alloy, called the international stan- ting vegetation in marshes. dard of mass. Marsh’s test A chemical test for ar- mass action The law of mass ac- senic in which hydrochloric acid and tion states that the rate at which a zinc are added to the sample, arsine chemical reaction takes place at a being produced by the nascent hy- given temperature is proportional to drogen generated. Gas from the sam- the product of the active masses of ple is led through a heated glass tube the reactants. The active mass of a re- and, if arsine is present, it decom- actant is taken to be its molar con- poses to give a brown deposit of centration. For example, for a arsenic metal. The arsenic is distin- reaction guished from antimony (which gives xA + yB → products mass concentration 344

the rate is given by ions pass along a region surrounded R = k[A]x[B]y by four parallel rods. Variable volt- ages applied to the rods produce an where k is the *rate constant. The Ü principle was introduced by C. M. oscillating electric eld. Varying the Guldberg and P. Waage in 1863. It is frequency of osillation allows different ions to pass through to a detec- strictly correct only for ideal gases. In Û real cases *activities can be used. See tor. In a time-of- ight mass also equilibrium constant. spectrometer the ions are accelerated by an electric Üeld and then enter a mass concentration See concen- drift tube through which they pass to tration . a detector. Different types of ion are massicot See lead(ii) oxide. distinguished by their time of Ûight in the drift tube. mass number See nucleon number. masurium A former name for *technetium. mass spectroscopy A technique used to determine relative atomic matrix (pl. matrices) 1. (in chem- masses and the relative abundance of istry) A continuous solid phase in isotopes, and for chemical analysis which particles (atoms, ions, etc.) are and the study of ion reactions. In a embedded. Unstable species, such as mass spectrometer a sample (usually free radicals, can be trapped in an gaseous) is ionized and the positive unreactive substrate, such as solid ions produced are accelerated into a argon, and studied by spectroscopy. m high-vacuum region containing elec- The species under investigation are tric and magnetic Üelds. These Üelds separated by the matrix, hence the deÛect and focus the ions onto a de- term matrix isolation for this tech- tector. The Üelds can be varied in a nique. 2. (in geology) The Üne- controlled way so that ions of differ- grained material of rock in which the ent types can impinge on the detec- coarser-grained material is embed- tor. A mass spectrum is thus obtained ded. 3. (in mathematics) A set of consisting of a series of peaks of vari- quantities in a rectangular array, able intensity to which mass/charge used in certain mathematical opera- (m/e) values can be assigned. The orig- tions. The array is usually enclosed in inal ions are usually produced by large parentheses or in square brack- electron impact, although ion im- ets. pact, photoionization, Üeld ionization, *electrospray ionization, and matrix mechanics A formulation *MALDI are also used. For organic of *quantum mechanics using matri- molecules, the mass spectrum con- ces (see matrix) to represent states sists of a series of peaks, one corre- and operators. Matrix mechanics was sponding to the parent ion and the the Ürst formulation of quantum me- others to fragment ions produced by chanics to be stated (by Werner the ionization process. Different mol- Heisenberg in 1925) and was devel- ecules can be identiÜed by their char- oped by Heisenberg and Max Born acteristic pattern of lines. Analysis of (1882–1970) and the German physi- mixtures can be done by gas chro- cist Pascual Jordan (1902–80). It was matography–mass spectroscopy (see shown by Erwin Schrödinger in 1926 gas chromatography). Other types to be equivalent to the *wave me- of mass spectrometer exist. In a chanics formulation of quantum me- quadrupole mass spectrometer the chanics. 345 Mayer’s test

Matura diamond See zircon. cal mechanics, although the name was suggested by the Scottish scien- Maxwell, James Clerk (1831–79) tist Sir William Thomson British physicist, born in Edinburgh, (1824–1907; subsequently Lord who held academic posts at Ab- Kelvin). erdeen, London, and Cambridge. In the 1860s he was one of the founders Maxwell’s thermodynamic of the *kinetic theory of gases, but equations Equations in thermody- his best-known work was a mathe- namics for a given mass of a homo- matical analysis of electromagnetic geneous system, relating the entropy radiation, published in 1864. (S), pressure (p), volume (V), and ther- Maxwell–Boltzmann distribu- modynamic temperature (T). The four equations are: tion A law describing the distribu- ∂ ∂ ∂ ∂ tion of speeds among the molecules ( T/ V)s = –( p/ S)V; ∂ ∂ ∂ ∂ of a gas. In a system consisting of N T/ p)s = –( V/ S)p; molecules that are independent of ∂ ∂ ∂ ∂ ( V/ T)p = –( S/ p)T; each other except that they exchange (∂S/∂V) = –(∂p/∂T) . energy on collision, it is clearly im- T V possible to say what velocity any par- Mayer f-function A quantity that ticular molecule will have. However, occurs in the calculation of virial statistical statements regarding cer- coefÜcients; it is deÜned by f = tain functions of the molecules were exp(–V2/kT) – 1, where V2 is the two- worked out by James Clerk *Max- body interaction potential energy, k well and Ludwig *Boltzmann. One is the *Boltzmann constant, and T is form of their law states that the thermodynamic temperature. It m n = Nexp(–E/RT), where n is the num- is related to the second virial coefÜ- ber of molecules with energy in ex- cient B by: cess of E, T is the thermodynamic ∫ B = (–NA/V) fdr1dr2, temperature, and R is the *gas con- where N is the Avogadro number stant. A and V is the volume of the system. Maxwell’s demon An imaginary This equation simpliÜed to: ∞ creature that is able to open and shut π ∫ 2 B = –2 NA 0 fr dr a partition dividing two volumes of a in the case of closed-shell atoms and gas in a container, when the two vol- octahedral and tetrahedral mol- umes are initially at the same tem- ecules. When particles are so far perature. The partition operated by apart that the interaction V → 0, the demon is only opened to allow then f → 0 also, but when the parti- fast molecules through. Such a cles are so close together that the in- process would make the volume of teraction V →∞, then f → –1. This gas containing the fast molecules enables strong repulsive interactions hotter than it was at the start; the between particles to be analysed in volume of gas remaining would ac- terms of f but not of V. The function cordingly become cooler. This is named after the US physicist process would be a violation of the Joseph Mayer. second law of *thermodynamics and therefore cannot occur. Maxwell’s Mayer’s test A general *presump- demon was invented by James Clerk tive test for cocaine, morphine, Maxwell in a letter written in 1867 to heroin, and other alkaloids. Mayer’s show that the second law of thermo- reagent is a solution of potassium dynamics has its origins in *statisti- mercury iodide in water. A positive McLeod gauge 346

result is indicated by a cream precipi- tance travelled between collisions by tate. the molecules in a gas, the electrons in a metallic crystal, the neutrons in McLeod gauge A vacuum pressure a moderator, etc. According to the gauge, devised by Herbert McLeod *kinetic theory the mean free path (1841–1923), in which a relatively between elastic collisions of gas mol- large volume of a low-pressure gas is ecules of diameter d (assuming they compressed to a small volume in a are rigid spheres) is 1/√2nπd2, where glass apparatus. The volume is re- n is the number of molecules per duced to an extent that causes the unit volume in the gas. As n is pro- pressure to rise sufÜciently to sup- portional to the pressure of the gas, port a column of Ûuid high enough the mean free path is inversely pro- to read. This simple device, which re- portional to the pressure. lies on *Boyle’s law, is suitable for measuring pressures in the range 103 mean free time The average time to 10–3 pascal. that elapses between the collisions of the molecules in a gas, the electrons McMillan–Mayer theory A in a crystal, the neutrons in a moder- theory of solutions of nonelectrolytes ator, etc. See mean free path. developed by the US scientists W. G. McMillan and J. E. Mayer in 1945. mechanical bonding Bonding The theory shows that there is a one- that involves a mechanical constraint to-one correspondence between the preventing two parts of a molecule equations describing a nonideal gas separating, rather than a chemical and those describing dilute solutions linkage based on transfer or sharing m of nonelectrolytes. In particular, they of electrons. It is found in *rotax- showed that there is a correspon- anes, *catenanes, and *molecular dence between the pressure of the knots. gas and the osmotic pressure of the mechanism The way in which a solution. This enables an expansion particular chemical reaction occurs, for solutions to be written, which is described in terms of the steps in- analogous to the virial expansion of volved. For example, the hydrolysis nonideal gases with analogues of the of an alkyl chloride proceeds by the virial coefÜcients. These coefÜcients S 1 mechanism (see nucleophilic can be calculated with the analogue N substitution). of potential being the potential of mean force of N solute molecules in Mecke’s test A *presumptive test the pure solvent. The McMillan– for amphetamines, methampheta- Mayer theory can also be extended to mines, and heroin. Mecke’s reagent distribution functions. consists of 1 gram of selenious acid in 100 ml of concentrated sulphuric MDA (methylenedioxyampheta- acid. Different substances give differ- mine) A hallucinogenic drug, ent results. Ecstasy, for example, C H CO , originally designed for 10 13 2 gives a light blue colour, turning to medical use but now extensively turquoise, and then dark blue. used as a club drug. Its effects are Heroin gives a yellow colour chang- similar to those of MDMA (See ec- ing to green. LSD gives an olive-green stasy). colour, changing to black. Mescaline MDMA Methylenedioxymetham- gives a brownish-orange colour. phetamine. See ecstasy. medium frequency (MF) A radio mean free path The average dis- frequency in the range 0.3–3 mega- 347 Mendeleev, Dmitri Ivanovich hertz; i.e. having a wavelength in the which are used particularly for lami- range 100–1000 metres. nated coatings. mega- Symbol M. A preÜx used in mellitic acid (benzenehexacar- the metric system to denote one mil- boxylic acid) A colourless crystalline 6 ° lion times. For example, 10 volts = 1 compound, C6(COOH)6, m.p. 288 C. megavolt (MV). Its molecules consist of a benzene ring in which all six hydrogen atoms Meitner, Lise (1878–1968) Aus- have been substituted by carboxyl trian-born Swedish physicist and ra- (–COOH) groups. It occurs naturally diochemist who worked in Berlin in some lignite beds as honeystone with Otto *Hahn. Together they dis- (the aluminium salt), and is made by covered protactinium. Meitner and oxidizing charcoal with concentrated Hahn worked together on neutron nitric acid. It decomposes on heating bombardment of uranium. In the to form pyromellitic anhydride, used 1930s, she escaped from Austria to in making epoxy resins. Condensa- Sweden to avoid Nazi persecution. In tion products of mellitic acid are em- Stockholm, along with her nephew ployed in making a wide range of Otto Frisch (1904–79), she formulated dyes. the theory of nuclear Üssion. melting point (m.p.) The tempera- meitnerium Symbol Mt. A radio- ture at which a solid changes into a active *transactinide element; a.n. Ü liquid. A pure substance under stan- 109. It was rst made in 1982 by dard conditions of pressure (usually 1 Peter Armbruster and a team in atmosphere) has a single repro- m Darmstadt, Germany, by bombarding ducible melting point. If heat is grad- bismuth-209 nuclei with iron-58 nu- ually and uniformly supplied to a clei. Only a few atoms have ever solid the consequent rise in tempera- been detected. ture stops at the melting point until A the fusion process is complete. • Information from the WebElements site Mendeleev, Dmitri Ivanovich melamine A white crystalline com- (1834–1907) Russian chemist, who pound, C3N6H6. Melamine is a cyclic became professor of chemistry at St compound having a six-membered Petersburg in 1866. His most famous ring of alternating C and N atoms, work, published in 1869, was the with three NH2 groups. It can be compilation of the *periodic table of copolymerized with methanal to give the elements, based on the periodic thermosetting melamine resins, law.

melamine melamine polymer Melamine Mendeleev’s law 348

Mendeleev’s law See periodic element is less reactive than zinc and law. cadmium and will not displace hydrogen from acids. It is also unusual mendelevium Symbol Md. A radio- in forming mercury(I) compounds active metallic transuranic element containing the Hg 2+ ion, as well as belonging to the *actinoids; a.n. 101; 2 mercury(II) compounds containing mass number of the Ürst discovered Hg2+ ions. It also forms a number of nuclide 256 (half-life 1.3 hours). Sev- complexes and organomercury com- eral short-lived isotopes have now pounds. been synthesized. The element was Ürst identiÜed by Albert Ghiorso, A Glenn Seaborg (1912–99), and associ- • Information from the WebElements site ates in 1955. mercury cell A primary *voltaic A cell consisting of a zinc anode and a • Information from the WebElements site cathode of mercury(II) oxide (HgO) Mendius reaction A reaction in mixed with graphite. The electrolyte which an organic nitrile is reduced is potassium hydroxide (KOH) satu- by nascent hydrogen (e.g. from rated with zinc oxide, the overall re- sodium in ethanol) to a primary action being: amine: Zn + HgO → ZnO + Hg → RCN + 2H2 RCH2NH2 The e.m.f. is 1.35 volts and the cell will deliver about 0.3 ampere-hour menthol A white crystalline ter- per cm3. pene alcohol, C10H19OH; r.d. 0.89; m m.p. 42°C; b.p. 103–104°C. It has a mercury(I) chloride A white salt, ° minty taste and is found in certain Hg2Cl2; r.d. 7.15; sublimes at 400 C. It essential oils (e.g. peppermint) and is made by heating mercury(II) chlo- used as a Ûavouring. ride with mercury and is used in thiols calomel cells (so called because the mercaptans See . salt was formerly called calomel) and mercapto group See thiols. as a fungicide. mercuric compounds Com- mercury(II) chloride A white salt, ° pounds of mercury in its +2 oxida- HgCl2; r.d. 5.4; m.p. 276 C; b.p. tion state; e.g. mercuric chloride is 302°C. It is made by reacting mer- mercury(II) chloride, HgCl2. cury with chlorine and used in making other mercury compounds. mercurous compounds Com- pounds of mercury in its +1 oxida- mercury(II) fulminate A grey ½ tion state; e.g. mercury(I) chloride is crystalline solid, Hg(CNO)2. H2O, mercurous chloride, HgCl. made by the action of nitric acid on mercury and treating the solution mercury Symbol Hg. A heavy sil- formed with ethanol. It is used as a very liquid metallic element belong- detonator for cartridges and can be ing to the *zinc group; a.n. 80; r.a.m. handled safely only under cold water. 200.59; r.d. 13.55; m.p. –38.87°C; b.p. 356.58°C. The main ore is the sul- mercury(II) oxide A yellow or red phide cinnabar (HgS), which can be oxide of mercury, HgO. The red form decomposed to the elements. Mer- is made by heating mercury in oxy- cury is used in thermometers, gen at 350°C; the yellow form, which barometers, and other scientiÜc ap- differs from the red in particle size, paratus, and in dental amalgams. The is precipitated when sodium hydrox- 349 metal ide solution is added to a solution of the *microscopic and the *macro- mercury(II) nitrate. Both forms de- scopic states. Mesoscopic objects and compose to the elements at high systems require quantum mechanics temperature. The black precipitate to describe them. formed when sodium hydroxide is meta- 1. PreÜx designating a ben- added to mercury(I) nitrate solution zene compound in which two sub- is sometimes referred to as stituents are in the 1,3 positions on mercury(I) oxide (Hg2O) but is probably a mixture of HgO and free mer- the benzene ring. The abbreviation cury. m- is used; for example, m-xylene is 1,3-dimethylbenzene. Compare mercury(II) sulphide A red or ortho-; para-. 2. PreÜx designating a black compound, HgS, occurring nat- lower oxo acid, e.g. metaphosphoric urally as the minerals cinnabar (red) acid. Compare ortho-. and metacinnabar (black). It can be metabo- obtained as a black precipitate by metabolic pathway See lism bubbling hydrogen sulphide through . a solution of mercury(II) nitrate. The metabolism The sum of the chem- red form is obtained by sublimation. ical reactions that occur within living The compound is also called vermil- organisms. The various compounds ion (used as a pigment). that take part in or are formed by mer isomer See isomerism. these reactions are called metabolites. In animals many metabolites mescaline A powerful hallucino- are obtained by the digestion of food, genic compound obtained from pey- whereas in plants only the basic m Û ote – the owering head of a type of starting materials (carbon dioxide, Mexican cactus. Mescaline is a class A water, and minerals) are externally drug in the UK. It can be detected by derived. The synthesis (*anabolism) the Mecke test. and breakdown (*catabolism) of most compounds occurs by a number of CH3 H2 reaction steps, the reaction sequence O C NH2 being termed a metabolic pathway. C Some pathways (e.g. *glycolysis) are H 2 linear; others (e.g. the *Krebs cycle) are cyclic. O metabolite See metabolism. CH O 3 metaboric acid See boric acid. CH3 metal Any of a class of chemical el- Mescaline ements that are typically lustrous solids that are good conductors of meso-isomer See optical activity. heat and electricity. Not all metals mesomerism (mesomeric effect) A have all these properties (e.g. mer- former name for *resonance in mol- cury is a liquid). In chemistry, metals ecules. See also electronic effects. fall into two distinct types. Those of the s- and p-blocks (e.g. sodium and mesomorph See lyotropic meso- aluminium) are generally soft silvery morph. reactive elements. They tend to form mesoscopic Designating a size positive ions and so are described as scale intermediate between those of electropositive. This is contrasted metaldehyde 350

with typical nonmetallic behaviour forming an ‘electron gas’. The bond- of forming negative ions. The *transi- ing force is electrostatic attraction tion elements (e.g. iron and copper) between the positive metal ions and are harder substances and generally the electrons. The existence of free less reactive. They form coordination electrons accounts for the good elec- complexes. All metals have oxides trical and thermal conductivities of that are basic, although some, such metals. See also energy bands. as aluminium, have *amphoteric metallic crystal A crystalline solid properties. in which the atoms are held together metaldehyde A solid compound, by *metallic bonds. Metallic crystals C4O4H4(CH3)4, formed by polymeriza- are found in some *interstitial com- tion of ethanal (acetaldehyde) in di- pounds as well as in metals and al- lute acid solutions below 0°C. The loys. compound, a tetramer of ethanal, is metallized dye See dyes. used in slug pellets and as a fuel for portable stoves. metallocene See sandwich compound. CH3 metallography The microscopic study of the structure of metals and O O their alloys. Both optical microscopes and electron microscopes are used in CH CH3 3 this work. O O m metalloid (semimetal) Any of a class of chemical elements interme-

CH3 diate in properties between metals and nonmetals. The classiÜcation is Metaldehyde not clear cut, but typical metalloids are boron, silicon, germanium, arsenic, and tellurium. They are electri- metal fatigue A cumulative effect cal semiconductors and their oxides causing a metal to fail after repeated are amphoteric. applications of stress, none of which exceeds the ultimate tensile strength. metallurgy The branch of applied The fatigue strength (or fatigue limit) science concerned with the produc- is the stress that will cause failure tion of metals from their ores, the after a speciÜed number (usually 107) puriÜcation of metals, the manufac- of cycles. The number of cycles re- ture of alloys, and the use and perfor- quired to produce failure decreases mance of metals in engineering as the level of stress or strain in- practice. Process metallurgy is con- creases. Other factors, such as corro- cerned with the extraction and pro- sion, also reduce the fatigue life. duction of metals, while physical metallurgy concerns the mechanical metallic bond A chemical bond of behaviour of metals. the type holding together the atoms in a solid metal or alloy. In such metamict state The amorphous solids, the atoms are considered to be state of a substance that has lost its ionized, with the positive ions occu- crystalline structure as a result of the pying lattice positions. The valence radioactivity of uranium or thorium. electrons are able to move freely (or Metamict minerals are minerals almost freely) through the lattice, whose structure has been disrupted 351 methanal trimer by this process. The metamictization the Nobel prize for chemistry in 2005 is caused by alpha-particles and the for work in this field. recoil nuclei from radioactive disinte- methacrylate A salt or ester of gration. methacrylic acid (2-methylpropenoic metaphosphoric acid See phos- acid). phoric(v) acid . methacrylate resins *Acrylic metaplumbate See plumbate. resins obtained by polymerizing 2- methylpropenoic acid or its esters. metastable state A condition of a system in which it has a precarious methacrylic acid See 2-methyl- stability that can easily be disturbed. propenoic acid. It is unlike a state of stable equilib- methadone A synthetic opioid, rium in that a minor disturbance will C21H27NO, used medically as an anal- cause a system in a metastable state gesic for chronic pains and also as a to fall to a lower energy level. A book substitute for heroin in the treat- lying on a table is in a state of stable ment of addiction. Methadone is it- equilibrium; a thin book standing on self addictive and considerable edge is in metastable equilibrium. quantities of ‘street’ methadone are Supercooled water is also in a meta- used in the UK. stable state. It is liquid below 0°C; a grain of dust or ice introduced into it 3CH will cause it to freeze. An excited CH CH N 3 state of an atom or nucleus that has 3 an appreciable lifetime is also CH 3CH O m metastable. C CH stannate 2 metastannate See . H2 metathesis A type of reaction in which radicals are exchanged. In inorganic chemistry, it is also called double decomposition. A simple example is Methadone → KCL + AgNO3 KNO3 + AgCl. Metathesis of alkenes is an important methamphetamine See ampheta- type of reaction in synthetic organic mines. chemistry. It involves exchange of groups. For example methanal (formaldehyde) A colourless gas, HCHO; r.d. 0.815 (at –20°C); RHC=CH + RHC=CH → RHC=CHR 2 2 m.p. –92°C; b.p. –21°C. It is the sim- + H C=CH . 2 2 plest *aldehyde, made by the cat- Reactions of this type are catalysed alytic oxidation of methanol (500°C; by metal alkylides (containing an silver catalyst) by air. It forms two M=CR2 grouping) and the intermedi- polymers: *methanal trimer and ate is a four-membered ring contain- polymethanal. See also formalin. ing the metal ion. The catalysts most often used are the Schrock catalysts methanal trimer A cyclic trimer based on molybdenum and the of methanal, C3O3H6, obtained by dis- Grubbs catalysts based on ruthenium. tillation of an acidic solution of The American chemists Richard methanal. It has a six-membered ring Schrock and Robert Grubbs shared of alternating –O– and –CH2– groups. methanation 352

methanation A method of manu- methylamine A colourless Ûamma- ° facturing methane from carbon ble gas, CH3NH2; m.p. –93.5 C; b.p. monoxide or dioxide by high- –6.3°C. It can be made by a catalytic pressure catalytic hydrogenation. reaction between methanol and am- It is often used to improve the monia and is used in the manufac- caloriÜc value of town gas. ture of other organic chemicals. methane A colourless odourless methylated spirits A mixture ° ° gas, CH4; m.p. –182.5 C; b.p. –164 C. consisting mainly of ethanol with Methane is the simplest hydrocar- added methanol (∼9.5%), pyridine bon, being the Ürst member of the (∼0.5%), and blue dye. The additives *alkane series. It is the main con- are included to make the ethanol un- ∼ stituent of natural gas ( 99%) and as drinkable so that it can be sold with- such is an important raw material for out excise duty for use as a solvent producing other organic compounds. and a fuel (for small spirit stoves). It can be converted into methanol by catalytic oxidation. methylation A chemical reaction in which a methyl group (CH –) is in- carbide 3 methanide See . troduced in a molecule. A particular methanoate (formate) A salt or example is the replacement of a hy- ester of methanoic acid. drogen atom by a methyl group, as in a *Friedel–Crafts reaction. methanoic acid (formic acid) A colourless pungent liquid, HCOOH; methylbenzene (toluene) A ° ° r.d. 1.2; m.p. 8 C; b.p. 101 C. It can be colourless liquid, CH3C6H5; r.d. 0.9; m made by the action of concentrated m.p. –95°C; b.p. 111°C. Methylben- sulphuric acid on the sodium salt zene is derived from benzene by re- (sodium methanoate), and occurs nat- placement of a hydrogen atom by a urally in ants and stinging nettles. methyl group. It can be obtained Methanoic acid is the simplest of the from coal tar or made from methyl- *carboxylic acids. cyclohexane (extracted from crude methanol (methyl alcohol) A oil) by catalytic dehydrogenation. Its main uses are as a solvent and as a colourless liquid, CH3OH; r.d. 0.79; m.p. –93.9°C; b.p. 64.96°C. It is made raw material for producing TNT. by catalytic oxidation of methane methyl bromide See bro- (from natural gas) using air. momethane. Methanol is used as a solvent (see iso- methylated spirits) and as a raw 2-methylbuta-1,3-diene See material for making methanal prene. (mainly for urea–formaldehyde methyl chloride See resins). It was formerly made by the chloromethane. dry distillation of wood (hence the ethane- name wood alcohol). methyl cyanide See nitrile. methionine See amino acid. methylene The highly reactive methoxy group The organic *carbene, :CH2. The divalent CH2 group CH3O–. group in a compound is the methyl- methyl acetate See methyl ene group. ethanoate . methylene chloride See methyl alcohol See methanol. dichloromethane. 353 metre

Methylxanthines methylenedioxymethampheta- 2-methylpropenoic acid mine (MDMA) See ecstasy. (methacrylic acid) A white crystalline methyl ethanoate (methyl ac- unsaturated carboxylic acid, etate) A colourless volatile fragrant CH2:C(CH3)COOH, used in making *methacrylate resins. liquid, CH3COOCH3; r.d. 0.92; m.p. –98°C; b.p. 54°C. A typical *ester, it methyl red An organic dye similar can be made from methanol and in structure and use to methyl or- methanoic acid and is used mainly as ange. It changes from red below pH a solvent. 4.4 to yellow above pH 6.0 (at 25°C). bu- methyl ethyl ketone See methyl violet A violet dye used as tanone . a chemical indicator and as a biologi- m methyl group (methyl radical) The cal stain. It is also the colouring mat- organic group CH3–. ter in methylated spirits. It is a mixture of compounds of rosaniline, methylidyne See carbyne. made by oxidizing dimethylpheny- methyl methacrylate An ester of lamine with copper(II) chloride. methacrylic acid (2-methylpropenoic methylxanthines Derivatives of acid), CH :C(CH )COOCH , used in 2 3 3 xanthine in which one or more hy- making *methacrylate resins. drogen atoms have been substituted methyl orange An organic dye by methyl groups. The common ones used as an acid–base *indicator. It are the trimethylxanthine *caffeine changes from red below pH 3.1 to and the dimethylxanthines *theo- yellow above pH 4.4 (at 25°C) and is phylline and *theobromine. used for titrations involving weak aminophenol bases. metol See . methylphenols (cresols) Organic metre Symbol m. The SI unit of compounds having a methyl group length, being the length of the path and a hydroxyl group bound directly travelled by light in vacuum during a to a benzene ring. There are three time interval of 1/(2.99 792 458 × 108) isomeric methylphenols with the for- second. This deÜnition, adopted by mula CH3C6H4OH, differing in the the General Conference on Weights relative positions of the methyl and and Measures in October, 1983, re- hydroxyl groups. A mixture of the placed the 1967 deÜnition based on three can be obtained by distilling the krypton lamp, i.e. 1 650 763.73 coal tar and is used as a germicide wavelengths in a vacuum of the radi- and antiseptic. ation corresponding to the transition metric system 354

between the levels 2p10 and 5d5 of Z = Si,Al. The three main mica miner- the nuclide krypton–86. This deÜni- als are: tion (in 1958) replaced the older deÜ- *muscovite, K2Al4(Si6Al2O20)(OH,F)4; 2+ 3+ nition of a metre based on a *biotite, K2(Mg,Fe )6–4(Fe ,Al,Ti)0–2- platinum–iridium bar of standard (Si6–5Al2–3O20)(OH,F)4; length. When the *metric system lepidolite, K2(Li,Al)5–6(Si6–7Al2–1O20)- was introduced in 1791 in France, (OH,F)4. the metre was intended to be one Micas have perfect basal cleavage and Û Û ten-millionth of the earth’s meridian the thin cleavage akes are exible quadrant passing through Paris. How- and elastic. Flakes of mica are used as ever, the original geodetic surveys electrical insulators and as the dielectric in capacitors. proved the impractibility of such a standard and the original platinum micelle An aggregate of molecules metre bar, the mètre des archives, was in a *colloid. For example, when constructed in 1793. soap or other *detergents dissolve in water they do so as micelles – small metric system A decimal system clusters of molecules in which the of units originally devised by a com- nonpolar hydrocarbon groups are in mittee of the French Academy, the centre and the hydrophilic polar which included J. L. Lagrange and groups are on the outside solvated by P. S. Laplace, in 1791. It was based on the water molecules. the *metre, the gram deÜned in terms of the mass of a cubic centime- Michaelis–Menten curve A graph tre of water, and the second. This that shows the relationship between m the concentration of a substrate and centimetre-gram-second system (see the rate of the corresponding en- c.g.s. units) later gave way for zyme-controlled reaction. The curve scientiÜc work to the metre-kilo- m.k.s. units only applies to enzyme reactions in- gram-second system (see ) volving a single substrate. It was de- on which *SI units are based. vised by Leonor Michaelis (1875– Meyer, Viktor (1848–97) German 1949) and Maud Menten (1879–1960). chemist who worked at Zürich and The graph can be used to calculate later at Heidelberg on a wide range the Michaelis constant (Km), which is of chemical topics. He was the Ürst to the concentration of a substrate re- prepare oximes and the sulphur com- quired in order for an enzyme to act pound theophene. He is known for at half of its maximum velocity his method of measuring relative (Vmax). The Michaelis constant is a Ü molecular mass by determining measure of the af nity of an enzyme for a substrate. A low value corre- vapour density. Meyer also worked Ü on stereochemistry and was the Ürst sponds to a high af nity, and vice versa. See also enzyme kinetics. to identify steric hindrance in chemical reactions. A • Original paper on Michaelis–Menten mica Any of a group of silicate min- kinetics erals with a layered structure. Micas micro- Symbol µ. A preÜx used in are composed of linked SiO4 tetrahedra with cations and hydroxyl group- the metric system to denote one mil- –6 ings between the layers. The general lionth. For example, 10 metre = 1 micrometre (µm). formula is X2Y4–6Z8O20(OH,F)4, where X = K,Na,Ca; Y = Al,Mg,Fe,Li; and microbalance A sensitive *balance 355 mineral capable of weighing masses of the milk sugar See lactose. order 10–6 to 10–9 kg. Miller indices A set of three num- microcrystal test See crystal test. bers that characterize a face of a crys- microscopic Designating a size tal. The French mineralogist René scale comparable to the subatomic Just Haüy (1743–1822) proposed the particles, atoms, and molecules. law of rational intercepts, which Microscopic objects and systems are states that there is always a set of described by *quantum mechanics. axes, known as crystal axes, that al- Compare macroscopic; mesoscopic. lows a crystal face to be characterized in terms of intercepts of the face microscopic reversibility The with these axes. The reciprocals of principle that in a reversible reaction these intercepts are small rational the mechanism in one direction is numbers. When the fractions are the exact reverse of the mechanism cleared there is a set of three inte- in the other direction. See also de- gers. These integers are known as the tailed balance. Miller indices of the crystal face after microwaves Electromagnetic the British mineralogist William Hal- waves with wavelengths in the range lowes Miller (1810–80), who pointed 10–3 to 0.03 m. out that crystal faces could be characterized by these indices. microwave spectroscopy A sen- If a plane is parallel to one of the sitive technique for chemical analysis crystal axes then its intercept is at and the determination of molecular inÜnity and hence its reciprocal is 0. structure (bond lengths, bond angles, If a face cuts a crystal axis on the m and dipole moments), and also rela- negative side of the origin then the tive atomic masses. It is based on the intercept, and hence its reciprocal, principle that microwave radiation microwaves i.e. the Miller index for that axis, are (see ) causes changes in negative. This is indicated by a bar the rotational energy levels of mol- over the Miller index. For example, ecules and absorption consequently the Miller indices for the eight_ faces_ occurs at characteristic frequencies. of_ an_ octahedron_ __ _ _ are (III),__ _(III), (III), In a microwave spectrometer a mi- (III ), (II I), (III ), (III) and (II I ). crowave source, usually a klystron valve, produces a beam that is passed milli- Symbol m. A preÜx used in through a gaseous sample. The beam the metric system to denote one then impinges on the detector, usu- thousandth. For example, 0.001 volt ally a crystal detector, and the signal = 1 millivolt (mV). (wavelength against intensity) is dis- Millon’s reagent A solution of played, either as a printed plot or on mercury(II) nitrate and nitrous acid an oscilloscope. As microwaves are used to test for proteins. The sample absorbed by air the instrument is is added to the reagent and heated evacuated. for two minutes at 95°C; the forma- migration 1. The movement of a tion of a red precipitate indicates the group, atom, or double bond from presence of protein in the sample. one part of a molecule to another. The reagent is named after French 2. The movement of ions under the chemist Auguste Millon (1812–67). Û Ü in uence of an electric eld. mineral A naturally occurring sub- milk of magnesia See magnesium stance that has a characteristic chem- hydroxide. ical composition and, in general, a mineral acid 356

crystalline structure. The term is also mixture of iron Ülings and sulphur. often applied generally to organic In a mixture there is no redistribu- substances that are obtained by min- tion of valence electrons, and the ing (e.g. coal, petroleum, and natural components retain their individual gas) but strictly speaking these are chemical properties. Unlike com- not minerals, being complex mix- pounds, mixtures can be separated tures without deÜnite chemical for- by physical means (distillation, crys- mulas. Rocks are composed of tallization, etc.). mixtures of minerals. Minerals may m.k.s. units A *metric system of be identiÜed by the properties of their crystal system, hardness (meas- units devised by A. Giorgi (and some- ured on the Mohs’ scale), relative times known as Giorgi units) in 1901. density, lustre, colour, cleavage, and It is based on the metre, kilogram, fracture. Many names of minerals and second and grew from the ear- end in -ite. lier *c.g.s. units. The electrical unit chosen to augment these three basic mineral acid A common inorganic units was the ampere and the perme- acid, such as hydrochloric acid, sul- ability of space (magnetic constant) phuric acid, or nitric acid. was taken as 10–7 Hm–1. To simplify mirabilite A mineral form of electromagnetic calculations the magnetic constant was later changed *sodium sulphate, Na2SO4.10H2O. to 4π×10–7 Hm–1 to give the rational- misch metal An alloy of cerium ized MKSA system. This system, with (50%), lanthanum (25%), neodymium some modiÜcations, formed the basis m (18%), praseodymium (5%), and other of *SI units, now used in all scientiÜc rare earths. It is used alloyed with work. iron (up to 30%) in lighter Ûints, and in small quantities to improve the mmHg A unit of pressure equal to malleability of iron. It is also added that exerted under standard gravity to copper alloys to make them by a height of one millimetre of mer- harder, to aluminium alloys to make cury, or 133.322 pascals. them stronger, to magnesium alloys mobility (of an ion) Symbol u. The to reduce creep, and to nickel alloys terminal speed of an ion in an elec- to reduce oxidation. tric Üeld divided by the Üeld strength. Mitscherlich’s law (law of isomor- mode The pattern of motion in a vi- phism) Substances that have the brating body. If the body has several same crystal structure have similar component particles, such as a mol- chemical formulae. The law can be ecule consisting of several atoms, the used to determine the formula of an modes of vibration are the different unknown compound if it is isomor- types of molecular vibrations possi- phous with a compound of known ble. formula. It is named after Eilhard Mitscherlich (1794–1863). moiety A characteristic part of a molecule. For example, in the ester mixture A system of two or more C H COOCH , the OCH can be re- distinct chemical substances. Homo- 2 5 3 3 garded as the alcohol (methanol) geneous mixtures are those in which moiety. the atoms or molecules are inter- spersed, as in a mixture of gases or in moissanite A clear form of silicon a solution. Heterogeneous mixtures carbide used as an inexpensive sub- have distinguishable phases, e.g. a stitute for diamond. It is named after 357 molecularity the French chemist Henri Moissan acts as a collimator by containing (1852–1907), who discovered it in several additional apertures and vac- 1893. uum pumps to remove any particles that do not pass through the aper- molal concentration See concen- tures. Molecular beams are used in tration. studies of surfaces and chemical reac- molality See concentration. tions and in spectroscopy. molar 1. Denoting that an exten- molecular chaperone Any of a sive physical property is being ex- group of proteins in living cells that pressed per *amount of substance, assist newly synthesized or dena- usually per mole. For example, the tured proteins to fold into their func- molar heat capacity of a compound is tional three-dimensional structures. the heat capacity of that compound The chaperones bind to the protein per unit amount of substance; in SI and prevent improper interactions units it would be expressed in J K–1 within the polypeptide chain, so that mol–1. 2. Having a concentration of it assumes the correct folded orienta- one mole per dm3. tion. This process requires energy in molar conductivity Symbol Λ. the form of ATP. The conductivity of that volume of molecular distillation Distillation an electrolyte that contains one mole in high vacuum (about 0.1 pascal) of solution between electrodes with the condensing surface so close placed one metre apart. to the surface of the evaporating liquid that the molecules of the liquid molar heat capacity See heat ca- travel to the condensing surface m pacity. without collisions. This technique en- molarity See concentration. ables very much lower temperatures molar volume (molecular volume) to be used than are used with distilla- The volume occupied by a substance tion at atmospheric pressure and per unit amount of substance. therefore heat-sensitive substances can be distilled. Oxidation of the dis- mole Symbol mol. The SI unit of tillate is also eliminated as there is *amount of substance. It is equal to no oxygen present. the amount of substance that con- molecular Ûow (Knudsen Ûow) tains as many elementary units as Û there are atoms in 0.012 kg of car- The ow of a gas through a pipe in bon–12. The elementary units may which the mean free path of gas mol- be atoms, molecules, ions, radicals, ecules is large compared to the di- electrons, etc., and must be speciÜed. mensions of the pipe. This occurs at 1 mole of a compound has a mass low pressures; because most colli- equal to its *relative molecular mass sions are with the walls of the pipe expressed in grams. rather than other gas molecules, the Ûow characteristics depend on the molecular beam A beam of relative molecular mass of the gas atoms, ions, or molecules at low pres- rather than its viscosity. The effect sure, in which all the particles are was studied by M. H. C. Knudsen travelling in the same direction and (1871–1949). there are few collisions between molecular formula See formula. them. They are formed by allowing a gas or vapour to pass through an molecularity The number of mol- aperture into an enclosure, which ecules involved in forming the acti- molecular knot 358

vated complex in a step of a chemical the whole molecule. The molecule reaction. Reactions are said to be uni- has a set of molecular orbitals (see or- molecular, bimolecular, or trimolecu- bital). The usual technique is to ob- lar according to whether 1, 2, or 3 tain molecular orbitals by a linear molecules are involved. combination of atomic orbitals (LCAO). See also density-function molecular knot (knotane) A type theory; valence-bond theory. of compound in which one or more chains of atoms in the molecule are molecular recognition The way looped in the conÜguration of a knot. in which a molecule may have a The molecule may have only one highly speciÜc response to another closed chain forming the knot. If the molecule or atom. It is a feature of knot is a trefoil knot the compound host–guest chemistry. See supramol- is chiral. Alternatively, molecular ecular chemistry. knots may have two or more sepa- molecular sieve Porous crystalline rate loops tied together in a knot. In substances, especially aluminosili- such cases there is no formal chemi- cates (see zeolite), that can be dehy- cal bonding between the rings and drated with little change in crystal they are held by *mechanical bond- structure. As they form regularly ing. Molecular knots can be produced spaced cavities, they provide a high synthetically and also occur naturally surface area for the adsorption of in certain proteins. smaller molecules. molecular modelling The use of The general formula of these sub- computer software to produce simu- stances is MnO.Al2O3.xSiO2.yH2O, m lations of molecular structures. Vari- where M is a metal ion and n is twice ous chemical drawing programs exist the reciprocal of its valency. Molecu- to allow a graphic representation of lar sieves are used as drying agents Ü chemical formulas in two dimen- and in the separation and puri ca- Û sions. More sophisticated three- tion of uids. They can also be loaded dimensional modelling programs with chemical substances, which re- also exist. The information about the main separated from any reaction molecule is stored in a data Üle giv- that is taking place around them, ing the numbers and types of atoms until they are released by heating or present and the coordinates of these by displacement with a more atoms. The software converts this strongly adsorbed substance. They into an on-screen three-dimensional can thus be used as cation exchange view of the molecular structure in mediums and as catalysts and cata- some speciÜed format (e.g. ball-and- lyst supports. They are also used as stick, wireframe, etc.). The structure the stationary phase in certain types * molecular-sieve can be rotated on the screen and, de- of chromatography ( chromatography). pending on the software, calculations may be done on the molecule. molecular symmetry The set of symmetry operations (rotations, molecular orbital See orbital. reÛections, etc.) that can be applied molecular-orbital theory A to a molecule. This set forms the method of computational chemistry *point group of the molecule. For an in which the electrons are not as- isolated molecule the *SchoenÛies signed to individual bonds between system rather than the *Hermann– atoms, but are treated as moving Mauguin system is used to denote its under the inÛuence of the nuclei in symmetry. Molecular symmetry is 359 moment of inertia analysed systematically using *group found in molybdenite (MoS2), the theory. It is possible to make deÜnite metal being extracted by roasting to statements about certain properties give the oxide, followed by reduction of molecules, such as whether they with hydrogen. The element is used can have an electric dipole moment in alloy steels. Molybdenum(IV) sul- or exhibit optical activity on the phide (MoS2) is used as a lubricant. basis of symmetry. Chemically, it is unreactive, being molecular volume See molar vol- unaffected by most acids. It oxidizes ume. at high temperatures and can be dissolved in molten alkali to give a molecular weight See relative range of molybdates and polymolyb- molecular mass. dates. Molybdenum was discovered molecule One of the fundamental in 1778 by Karl *Scheele. units forming a chemical compound; A the smallest part of a chemical com- • Information from the WebElements site pound that can take part in a chemi- moment of inertia Symbol I. A cal reaction. In most covalent quantity associated with a body that compounds, molecules consist of is rotating about an axis. If a body groups of atoms held together by co- consists of i particles of mass m a per- valent or coordinate bonds. Covalent pendicular distance r from an axis of substances that form *macromolecular crystals have no discrete mol- rotation, the moment of inertia I of ecules (in a sense, the whole crystal the body about that axis of rotation ∑ 2 is a molecule). Similarly, ionic com- is given by I = miri . The moment of m pounds do not have single molecules, inertia of a body is an important being collections of oppositely quantity because it is the analogue charged ions. for rotational motion of mass for linear motion. mole fraction Symbol X. A meas- In a molecule all rotational motion ure of the amount of a component in can be analysed using three perpen- a mixture. The mole fraction of com- dicular axes of rotation. Each of these ponent A is given by XA = nA/N, where has a moment of inertia associated nA is the amount of substance of A with it. To a Ürst approximation, a (for a given entity) and N is the total molecule can be regarded as a rigid amount of substance of the mixture rotor, i.e. a body that is not distorted (for the same entity). by its rotation. There are four types molÜle format A format using a of rigid rotor: connection table and atom coordi- In a spherical top all three moments nates to hold information about of inertia are equal (e.g. SF6). chemical structures. It was produced In a symmetric top two of the mo- by Elsevier MDL and is widely used in ments of inertia are equal (e.g. NH3). chemical drawing programs. Files In an asymmetric top all three mo- have the extension .mol. ments of inertia are different (e.g. H O). Molisch’s test See alpha-naphthol 2 test In a linear rotor the moment of iner- . tia about the axis of the molecule is molybdenum Symbol Mo. A sil- zero (e.g. HCl, CO2). very hard metallic *transition el- The type of rotor depends on the ement; a.n. 42; r.a.m. 95.94; r.d. symmetry of the molecule. If a mol- 10.22; m.p. 2617°C; b.p. 4612°C. It is ecule has cubic or icosahedral sym- monatomic molecule 360

metry it is a spherical top. If it has a monomer A molecule (or com- threefold or higher axis of symmetry pound) that joins with others in it is a symmetric top. If it does not forming a dimer, trimer, or polymer. have a threefold or higher axis of monosaccharide (simple sugar) A symmetry it is an asymmetric top. carbohydrate that cannot be split All linear molecules (and hence all into smaller units by the action of di- diatomic molecules) are linear lute acids. Monosaccharides are rotors. classiÜed according to the number of In reality, molecules are not rigid carbon atoms they possess: trioses rotors because of the centrifugal have three carbon atoms; tetroses, forces associated with the rotation. four; pentoses, Üve; hexoses, six; etc. The effect of such forces can be Each of these is further divided into taken into account by adding a cor- aldoses and ketoses, depending on rection term to the rigid rotor model. whether the molecule contains an monatomic molecule A ‘mol- aldehyde group (–CHO) or a ketone ecule’ consisting of only one atom group (–CO–). For example glucose, (e.g. Ar or He), distinguished from di- having six carbon atoms and an alde- atomic and polyatomic molecules. hyde group, is an aldohexose whereas fructose is a ketohexose. Mond process A method of obtain- These aldehyde and ketone groups ing pure nickel by heating the im- confer reducing properties on mono- pure metal in a stream of carbon saccharides: they can be oxidized to ° monoxide at 50–60 C. Volatile nickel yield sugar acids. They also react m carbonyl (Ni(CO)4) is formed, and this with phosphoric acid to produce can be decomposed at higher tem- phosphate esters (e.g. in *ATP), peratures (180°C) to give pure nickel. which are important in cell metabo- The method was invented by the Ger- lism. Monosaccharides can exist as ei- man–British chemist Ludwig Mond ther straight-chain or ring-shaped (1839–1909). molecules. They also exhibit *optical Monel metal An alloy of nickel activity, giving rise to both dextro- (60–70%), copper (25–35%), and small rotatory and laevorotatory forms. quantities of iron, manganese, sili- monosodium glutamate (MSG) A con, and carbon. It is used to make white solid, C8H8NNaO4.H2O, used acid-resisting equipment in the extensively as a Ûavour enhancer, es- chemical industry. pecially in convenience foods. It is a monobasic acid An *acid that has salt of glutamic acid (an *amino only one acidic hydrogen atom in its acid), from which it is prepared. It molecules. Hydrochloric (HCl) and ni- can cause an allergic reaction in some susceptible people who con- tric (HNO3) acids are common examples. sume it. monotropy See allotropy. monoclinic See crystal system. monovalent (univalent) Having a monoethanolamine See valency of one. ethanolamine. Monte Carlo method A numeri- monoglyceride See glyceride. cal method for solving mathematical monohydrate A crystalline com- and physical problems that involves pound having one mole of water per the random sampling of numbers. mole of compound. Applications of Monte Carlo methods 361 Morse potential

Glucose (an aldohexose) 6 6 CH OH CH OH above 1 aldehyde 2 2 plane H C O group of ring 5 5 C O C O 2 H OH H C OH H H H H 1 1 3 C OH H C C OH C HO C O 4 4 H 4 HO HO H C OH C C OH C C H 3 2 3 2 5 below H C OH H OH plane H OH 6 of ring CH OH α-glucose β-glucose 2 straight-chain form ring forms

Fructose (a ketohexose) H

1 H C OH

2 ketone C O group 6 1 HOCH O CH OH 3 2 2 HO C H 5 2 C C 4 H OH H C OH HO m C C OH 5 4 3 H C OH 6 CH OH OH H 2 straight-chain form ring form

Monosaccharide include calculations in the theory of tain dyeing processes. Mordants are liquids, phase transitions, and quan- often inorganic oxides or salts, which tum mechanical systems. The name are absorbed on the fabric. The comes from the casino at Monte dyestuff then forms a coloured com- Carlo and alludes to the use of ran- plex with the mordant, the colour dom numbers in this technique. depending on the mordant used as well as the dyestuff. See also lake. montmorillonite A clay mineral of variable composition. It is a hy- morphine An opiate that is the drated aluminosilicate with a large main active constituent of opium. It capacity for exchanging cations (see is used medically in the relief of se- ion exchange) and is the principal vere pain, and can be acetylated to constituent of bentonite and fuller’s produce heroin. Morphine can be de- earth. One type of montmorillonite tected by the *Marquis test. readily absorbs water, and another Morse potential The potential en- type swells in water to form a *gel. ergy of a diatomic molecule as a mordant A substance used in cer- function of the difference (r – re), mosaic gold 362

where r is the variable interatomic tice. Since this may typically contain 10 20 distance and re is the equilibrium in- 10 –10 atoms, the recoil energy is teratomic distance. The Morse poten- negligible and the energy of the Ü tial U(r – re) is given by emitted photon is sharply de ned in D {1 – exp[–β(–r – r )]}2, a very narrow energy spread. e e The effect is exploited in Möss- where De is the dissociation energy at bauer spectroscopy in which a the minimum of the curve (i.e. when β gamma-ray source is mounted on a r = re) and is a constant. The Morse moving platform and a similar sam- potential was used by the US physi- ple is mounted nearby. A detector cist Philip M. Morse in 1929 in solv- measures gamma rays scattered by ing the Schrödinger equation. The the sample. The source is moved Morse potential is a reasonably good slowly towards the sample at a vary- representation of a potential-energy ing speed, so as to continuously function except that as r approaches change the frequency of the emitted Ü 0, U does not approach in nity as it gamma radiation by the Doppler ef- should for a true potential energy fect. A sharp decrease in the signal Ü function. Modi cations of the Morse from the detector at a particular potential have been suggested to im- speed (i.e. frequency) indicates reso- prove on this aspect. nance absorption in the sample nu- mosaic gold See tin(iv) sulphide. clei. The effect is used to investigate nuclear energy levels. In chemistry, Moseley’s law The frequencies of Mössbauer spectroscopy can also give the lines in the *X-ray spectra of the information about the bonding and m elements are related to the atomic structure of compounds because numbers of the elements. If the chemical shifts in the resonance en- square roots of the frequencies of ergy are produced by the presence of corresponding lines of a set of el- surrounding atoms. ements are plotted against the melting point atomic numbers a straight line is m.p. See . obtained. The law was discovered by MSG See monosodium glutamate. H. G. Moseley (1887–1915). mucopolysaccharide See gly- A cosaminoglycan. • Original paper on the law mufÛe furnace An insulated fur- moss agate See agate. nace, usually electrically heated, used Mössbauer effect An effect occur- for producing controlled high tem- Û ring when certain nuclides decay peratures. In the laboratory, muf e with emission of gamma radiation. furnaces are used for drying solids, For an isolated nucleus, the gamma sintering, and studying high-temper- radiation would usually have a ature reactions. They typically oper- ° spread of energies because the en- ate in the range 100–1200 C. ergy of the process is partitioned be- multicentre bond A bond formed tween the gamma-ray photon and between three, and sometimes more, the recoil energy of the nucleus. In atoms that contains only a single pair 1957 Rudolph Mössbauer (1929– ) of electrons. The structure of *bo- found that in certain solids, in which ranes can be explained by consider- the emitting nucleus is held by ing them to be *electron-deÜcient strong forces in the lattice, the recoil compounds containing multicentre energy is taken up by the whole lat- bonds. 363 muscovite multidecker sandwich See sand- 2m-pole interacts with a 2n-pole, the wich compound. interaction potential energy, V, multiple bond A bond between varies with distance r as V = m + n –1 two atoms that contains more than c/(r ), where c is a constant. The one pair of electrons. Such bonds pri- rapid fall-off with distance, as the marily involve sigma bonding with order increases, is due to the set of secondary contribution from pi bond- charges appearing to tend towards ing (or, sometimes, delta bonding). neutrality (as seen from outside). The See orbital. multipole interactions with the largest interactions are: ion–ion, multiple proportions See chemi- ion–dipole, dipole–dipole, and disper- cal combination. sion interactions. multiplet 1. A spectral line formed by more than two (see doublet) Mumetal The original trade name closely spaced lines. 2. A group of el- for a ferromagnetic alloy, containing ementary particles that are identical 78% nickel, 17% iron, and 5% copper, in all respects except that of electric that had a high permeability and a charge. low coercive force. More modern ver- sions also contain chromium and multiplicity A quantity used in molybdenum. These alloys are used * atomic spectra to describe the en- in some transformer cores and for ergy levels of many-electron atoms shielding various devices from exter- characterized by *Russell–Saunders nal magnetic Üelds. coupling given by 2S + 1, where S is the total electron *spin quantum Muntz metal A form of *brass con- m number. The multiplicity of an en- taining 60% copper, 39% zinc, and ergy level is indicated by a left super- small amounts of lead and iron. script to the value of L, where L is the Stronger than alpha-brass, it is used resultant electron *orbital angular for hot forgings, brazing rods, and momentum of the individual elec- large nuts and bolts. It is named after tron orbital angular momenta l. G. F. Muntz (1794–1857). multipole interactions Interac- muscovite (white mica; potash tions between arrays of point charges mica) A mineral form of potassium (multipoles) associated with potential aluminosilicate, K2Al4(Si6Al2)O20- energies that depend on distance. (OH,F) ; one of the most important Multipole interactions are an impor- 4 members of the *mica group of min- tant feature of intermolecular forces. erals. It is chemically complex and An n-pole is a set of n charges having has a sheetlike crystal structure (see an n-pole moment but not a lower intercalation compound moment. A monopole is a single ). It is usu- charge and the monopole moment is ally silvery-grey in colour, sometimes the charge. A dipole consists of two tinted with green, brown, or pink. opposite charges and thus has no Muscovite is a common constituent overall charge (and hence no mono- of certain granites and pegmatites. It pole moment). Higher multipoles are is also common in metamorphic and the *quadrupole and the *octupole. sedimentary rocks. It is widely used The interaction potential energy be- in industry, for example in the man- tween multipoles falls off with dis- ufacture of electrical equipment and tance increasingly rapidly as the as a Üller in rooÜng materials, wall- order of the multipoles increases. If a papers, and paint. mustard gas 364

mustard gas See sulphur group, which reversibly binds a mol- mustard; nitrogen mustard. ecule of oxygen. This is only relin- quished at relatively low external mutarotation Change of optical oxygen concentrations, e.g. during activity with time as a result of spon- strenuous exercise when muscle oxy- taneous chemical reaction. gen demand outpaces supply from myoglobin A globular protein oc- the blood. Myoglobin thus acts as an curring widely in muscle tissue as an emergency oxygen store. oxygen carrier. It comprises a single myristic acid See tetradecanoic polypeptide chain and a *haem acid.

m N

NAD (nicotinamide adenine dinu- zymes tend to be speciÜc for either cleotide) A *coenzyme, derived from NAD or NADP as coenzyme. the B vitamin *nicotinic acid, that nano- Symbol n. A preÜx used in participates in many biological dehy- the metric system to denote 10–9. For drogenation reactions. NAD is charac- example, 10–9 second = 1 nanosecond teristically loosely bound to the (ns). enzymes concerned. It normally car- nanotechnology The develop- ries a positive charge and can accept ment and use of devices that have a one hydrogen atom and two elec- size of only a few nanometres. Re- trons to become the reduced form, search has been carried out into very NADH. NADH is generated during the small components, which depend on oxidation of food; it then gives up its electronic effects and may involve two electrons (and single proton) to movement of a countable number of the electron transport chain, thereby electrons in their action. Such de- reverting to NAD+ and generating vices would act much faster than three molecules of ATP per molecule larger components. Considerable in- of NADH. terest has been shown in the produc- NADP (nicotinamide adenine dinu- tion of structures on a molecular cleotide phosphate) differs from NAD level by suitable sequences of chemi- only in possessing an additional cal reactions. It is also possible to ma- phosphate group. It functions in the nipulate individual atoms on surfaces same way as NAD although anabolic using a variant of the *atomic force reactions (see anabolism) generally microscope. use NADPH (reduced NADP) as a hy- nanotube See buckminster- drogen donor rather than NADH. En- fullerene.

CONH 2

O CH O + 2 N site of reduction to NADH

O PPO– HH H NH O 2 OH OH N O PPO– N

O CH O N 2 N

H H H H phosphate group attached here to OH OH form NADP

NAD napalm 366

napalm A substance used in incen- obtained by removing a hydrogen diary bombs and Ûame throwers, atom from naphthalene. There are made by forming a gel of petrol with two forms depending on whether the aluminium soaps (aluminium salts of hydrogen is removed from the 1- or long-chain carboxylic acids, such as 2-position. palmitic acid). nascent hydrogen A reactive naphtha Any liquid hydrocarbon or form of hydrogen generated in situ in mixture obtained by the fractional the reaction mixture (e.g. by the ac- distillation of petroleum. It is gener- tion of acid on zinc). Nascent hydro- ally applied to higher *alkane frac- gen can reduce elements and tions with nine or ten carbon atoms. compounds that do not readily react Naphtha is used as a solvent and as a with ‘normal’ hydrogen. It was once starting material for *cracking into thought that the hydrogen was pre- more volatile products, such as sent as atoms, but this is not the petrol. case. Probably hydrogen molecules naphthalene A white volatile are formed in an excited state and solid, C10H8 (see formula); r.d. 1.025; react before they revert to the m.p. 80.55°C; b.p. 218°C. Naphtha- ground state. lene is an aromatic hydrocarbon with natron A mineral form of hydrated an odour of mothballs and is ob- sodium carbonate, Na CO .H O. tained from crude oil. It is a raw ma- 2 3 2 terial for making certain synthetic Natta process See ziegler process. resins. natural abundance See abun- α dance. n natural gas β A naturally occurring mixture of gaseous hydrocarbons that is found in porous sedimentary rocks in the earth’s crust, usually in association with *petroleum deposits. It consists chieÛy of methane Naphthalene (about 85%), ethane (up to about 10%), propane (about 3%), and butane. naphthols Two phenols derived Carbon dioxide, nitrogen, oxygen, from naphthalene with the formula hydrogen sulphide, and sometimes C H OH, differing in the position of 10 7 helium may also be present. Natural the –OH group. The most important gas, like petroleum, originates in the is naphthalen-2-ol (β-naphthol), decomposition of organic matter. It is with the –OH in the 2-position. It widely used as a fuel and also to pro- is a white solid (r.d. 1.28; m.p. 123–124°C; b.p. 295°C) used in rub- duce carbon black and some organic ber as an antioxidant. Naphthalen-2- chemicals. Natural gas occurs on ol will couple with diazonium salts at every continent, the major reserves the 1-position to form red *azo com- occurring in the USA, Russia, Kazakh- pounds, a reaction used in testing for stan, Turkmenistan, Ukraine, Alge- ria, Canada, and the Middle East. See the presence of primary amines (by liquefied petroleum gas making the diazonium salt and also . adding naphthalen-2-ol). NBR See nitrile rubber.

naphthyl group The group C10H7– NBS See N-bromosuccinimide. 367 neptunium

Néel temperature The tempera- r.a.m. 20.179; d. 0.9 g dm–3; m.p. ture above which an antiferro- –248.67°C; b.p. –246.05°C. Neon oc- magnetic substance becomes curs in air (0.0018% by volume) and is paramagnetic (see magnetism). The obtained by fractional distillation of susceptibility increases with temper- liquid air. It is used in discharge ature, reaching a maximum at the tubes and neon lamps, in which it Néel temperature, after which it has a characteristic red glow. It forms abruptly declines. The phenomenon hardly any compounds (neon was discovered around 1930 by Ûuorides have been reported). The el- L. E. F. Néel (1904–2000). ement was discovered in 1898 by Sir neighbouring-group participa- William Ramsey and M. W. Travers. tion An effect in an organic chemi- A cal reaction in which the reactive • Information from the WebElements site centre interacts with a lone pair or neoprene A synthetic rubber made with electrons in other bonds in the by polymerizing the compound 2- molecule that are not conjugated with the centre. This may affect the chlorobuta-1,2-diene. Neoprene is rate or the stereochemistry of the often used in place of natural rubber products. It is sometimes called an- in applications requiring resistance chimeric assistance. to chemical attack. nematic crystal See liquid nephelometry The measurement crystal. of the turbidity (cloudiness) of a liquid. Nephalometers generally have a neodymium Symbol Nd. A soft sil- light source (often a laser) and a de- very metallic element belonging to tector to measure the amount of the *lanthanoids; a.n. 60; r.a.m. light scattered by suspended parti- n ° ° 144.24; r.d. 7.004 (20 ); m.p. 1021 C; cles. They are used in a number of ° b.p. 3068 C. It occurs in bastnasite Üelds including water quality control and monazite, from which it is recov- and blood analysis (for protein con- ered by an ion-exchange process. tent). There are seven naturally occurring isotopes, all of which are stable, ex- nephrite See jade. cept neodymium–144, which is neptunium Symbol Np. A radio- slightly radioactive (half-life active metallic transuranic element 10 15 Ü 10 –10 years). Seven arti cial ra- belonging to the *actinoids; a.n. 93; dioisotopes have been produced. The r.a.m. 237.0482. The most stable iso- metal is used to colour glass violet- tope, neptunium–237, has a half-life purple and to make it dichroic. It is of 2.2 × 106 years and is produced in also used in misch metal (18% small quantities as a by-product by neodymium) and in neodymium– nuclear reactors. Other isotopes have iron–boron alloys for magnets. It mass numbers 229–236 and 238–241. was discovered by Carl von Welsbach The only other relatively long-lived (1856–1929) in 1885. isotope is neptunium–236 (half-life 5 A × 103 years). The element was Ürst • Information from the WebElements site produced by Edwin McMillan (1907– neon Symbol Ne. A colourless 91) and Philip Abelson (1913–2004) in gaseous element belonging to group 1940. 18 (formerly group 0) of the periodic A table (the *noble gases); a.n. 10; • Information from the WebElements site neptunium series 368

neptunium series See radioactive Walter Nernst: if a chemical change series. takes place between pure crystalline * Nernst, (Hermann) Walther solids at absolute zero there is no change of entropy. (1864–1941) German physical chemist noted for his work on elec- nerve agents A class of highly trochemistry and chemical thermo- toxic compounds that act by affect- dynamics. He is remembered as the ing the action of acetylcholine, a neu- discoverer of the third law of ther- rotransmitter regulated by the modynamics. Nernst was awarded enzyme acetylcholinesterase (AChE). the 1920 Nobel prize for chemistry. The nerve agents, which are organophosphates, bind to AChE and Nernst–Einstein equation An block its action. Consequently there equation relating the limiting molar is nothing to stop the build up of conductivity Λ 0 (see kohlrausch’s m acetylcholine, and exposure to small law) to the ionic diffusion coefÜ- amounts of nerve gas can kill in min- cients, devised by Nernst and Albert utes. There are two series of agents. *Einstein. The Nernst–Einstein equa- The G-series was produced by Ger- tion is man scientists in the 1930s and 40s Λ 0 2 2 2 m = (F /RT)(v+z+ D+ + v–z– D–), (the G stands for German). The main where F is the Faraday constant, R is ones are *tabun (GA), *sarin (GB), the gas constant, T is the thermody- *soman (GD), and *cyclosarin (GF). A namic temperature, v+ and v– are the further series of nerve agents is the number of cations and anions per V-series. The most common of these formula unit of electrolyte, z+ and z– is *VX, which was discovered at Por- are the valences of the ions, and D+ ton Down in the UK in 1952. Nerve Ü n and D– are the diffusion coef cients agents are classiÜed as weapons of of the ions. An application of the mass destruction by the UN. Nernst–Einstein equation is to calcu- nesquehonite A mineral form of late the ionic diffusion coefÜcients *magnesium carbonate trihydrate, from experimental determinations of MgCO .3H O. conductivity. 3 2 Nessler’s reagent A solution of Nernst equation An equation that mercury(II) iodide (HgI ) in potassium relates the *electrode potential E of 2 iodide and potassium hydroxide an electrode that is in contact with named after Julius Nessler an ionic solution to the ionic concen- (1827–1905). It is used in testing for tration c. The equation is: ammonia, with which it forms a Š E = E – RTzFlnc, brown coloration or precipitate. where EŠ is the standard electrode neutral Describing a compound or potential, R is the gas constant, T is solution that is neither acidic nor the absolute temperature, z is the va- basic. A neutral solution is one that lence of the ion, and F is the Faraday contains equal numbers of both pro- constant. The equation was derived tonated and deprotonated forms of by Walther *Nernst in 1889. It is fun- the solvent. damental to the thermodynamics of electrochemical cells. neutralization The process in which an acid reacts with a base to Nernst heat theorem A state- form a salt and water. ment of the third law of *thermodynamics in a restricted form given by neutron A neutral hadron that is 369 Newtonian fluid stable in the atomic nucleus but de- cleus of a particular nuclide. It is cays into a proton, an electron, and equal to the difference between the an antineutrino with a mean life of *nucleon number and the *atomic 12 minutes outside the nucleus. Its number. rest mass is slightly greater than that Newlands’ law See law of oc- of the proton, being 1.674 9286(10) × taves. 10–27 kg. Neutrons occur in all atomic nuclei except normal hydrogen. The Newman projection A type of neutron was Ürst reported in 1932 by *projection in which a molecule is the British physicist James Chadwick viewed along a bond. See conforma- (1891–1974). tion. neutron activation analysis See newton Symbol N. The *SI unit of activation analysis. force, being the force required to give a mass of one kilogram an accel- neutron diffraction The scatter- –2 ing of neutrons by atoms in solids, eration of 1 m s . It is named after liquids, or gases. This process has Sir Isaac Newton (1642–1727). given rise to a technique, analogous Newtonian Ûuid A Ûuid in which to *X-ray diffraction techniques, the velocity gradient is directly pro- using a Ûux of thermal neutrons portional to the shear stress. If two from a nuclear reactor to study solid- Ûat plates of area A are separated by state phenomena. Thermal neutrons a layer of Ûuid of thickness d and have average kinetic energies of move relative to each other at a ve- about 0.025 eV (4 × 10–21 J) giving locity v, then the rate of shear is v/d them an equivalent wavelength of and the shear stress is F/A, where F is about 0.1 nanometre, which is suit- the force applied to each. For a New- n able for the study of interatomic in- tonian Ûuid F/A = µv/d, where µ is the terference. There are two types of constant of proportionality and is interaction in the scattering of neu- called the Newtonian viscosity. Many trons by atoms: one is the interaction liquids are Newtonian Ûuids over a between the neutrons and the wide range of temperatures and pres- atomic nucleus, the other is the in- sures. However, some are not; these teraction between the *magnetic mo- are called non-Newtonian Ûuids. In ments of the neutrons and the spin such Ûuids there is a departure from and orbital magnetic moments of the the simple Newtonian relationships. atoms. The latter interaction has pro- For example, in some liquids the vis- vided valuable information on anti- cosity increases as the velocity gradi- ferromagnetic and ferrimagnetic ent increases, i.e. the faster the liquid magnetism materials (see ). Interac- moves the more viscous it becomes. tion with the atomic nucleus gives Such liquids are said to be dilatant diffraction patterns that complement and the phenomenon they exhibit is those from X-rays. X-rays, which called dilatancy. It occurs in some interact with the extranuclear elec- pastes and suspensions. More com- trons, are not suitable for investigat- mon, however, is the opposite effect ing light elements (e.g. hydrogen), in which the viscosity depends not whereas neutrons do give diffraction only on the velocity gradient but also patterns from such atoms because on the time for which it has been they interact with nuclei. applied. These liquids are said to neutron number Symbol N. The exhibit thixotropy. The faster a number of neutrons in an atomic nu- thixotropic liquid moves the less vis- NHOMO 370

cous it becomes. This property is drogenation reactions (see also raney used in nondrip paints (which are nickel). The main compounds are more viscous on the brush than on formed with nickel in the +2 oxida- the wall) and in lubricating oils tion state; the +3 state also exists (e.g. (which become thinner when the the black oxide, Ni2O3). Nickel was parts they are lubricating start to discovered by Axel Cronstedt move). Another example is the non- (1722–65) in 1751. Newtonian Ûow of macromolecules A in solution or in polymer melts. In • Information from the WebElements site this case the shearing force F is not parallel to the shear planes and the nickel arsenide structure A type linear relationship does not apply. of ionic crystal structure in which In general, the many types of non- the anions have a distorted hexago- Newtonian Ûuid are somewhat com- nal close packed arrangement with plicated and no theory has been the cations occupying the octahedral developed to accommodate them holes. Each type of ion has a coordi- fully. nation number of 6. Examples of compounds with this structure are NHOMO Next-to-highest occupied NiAs, NiS, FeS, and CoS. molecular orbital. See subjacent or- A bitals. • An interactive version of the structure niacin See nicotinic acid. nickel–cadmium cell See Nichrome Tradename for a group nickel–iron accumulator. of nickel–chromium alloys used for nickel carbonyl A colourless wire in heating elements as they pos- ° volatile liquid, Ni(CO)4; m.p. –25 C; n sess good resistance to oxidation and ° have a high resistivity. Typical is b.p. 43 C. It is formed by direct combination of nickel metal with carbon Nichrome V containing 80% nickel ° and 19.5% chromium, the balance monoxide at 50–60 C. The reaction is consisting of manganese, silicon, and reversed at higher temperatures, and carbon. the reactions are the basis of the *Mond process for purifying nickel. nickel Symbol Ni. A malleable duc- The nickel in the compound has an tile silvery metallic *transition el- oxidation state of zero, and the com- ement; a.n. 28; r.a.m. 58.70; r.d. 8.9; pound is a typical example of a com- m.p. 1450°C; b.p. 2732°C. It is found plex with pi-bonding *ligands, in in the minerals pentlandite (NiS), which Ülled d-orbitals on the nickel pyrrhoite ((Fe,Ni)S), and garnierite overlap with empty p-orbitals on the ((Ni,Mg)6(OH)6Si4O11.H2O). Nickel is carbon. also present in certain iron me- nickelic compounds Compounds teorites (up to 20%). The metal is of nickel in its +3 oxidation state; e.g. extracted by roasting the ore to give nickelic oxide is nickel(III) oxide the oxide, followed by reduction (Ni O ). with carbon monoxide and puriÜca- 2 3 tion by the *Mond process. Alterna- nickel–iron accumulator (Edison tively electolysis is used. Nickel metal cell; NIFE cell) A *secondary cell de- is used in special steels, in Invar, and, vised by Thomas Edison (1847–1931) being ferromagnetic, in magnetic al- having a positive plate of nickel loys, such as *Mumetal. It is also an oxide and a negative plate of iron effective catalyst, particularly for hy- both immersed in an electrolyte of 371 ninhydrin potassium hydroxide. The reaction nicotinamide adenine dinu- on discharge is cleotide phosphate (NADP) See → nad. 2NiOOH.H2O + Fe 2Ni(OH)2 + Fe(OH)2, nicotine A colourless poisonous the reverse occurring during charg- *alkaloid present in tobacco. It is ing. Each cell gives an e.m.f. of about used as an insecticide. 1.2 volts and produces about 100 kJ nicotinic acid (niacin) A vitamin of per kilogram during each discharge. the *vitamin B complex. It can be The nickel–cadmium cell is a similar manufactured by plants and animals device with a negative cadmium elec- from the amino acid tryptophan. The trode. It is often used as a *dry cell. amide derivative, nicotinamide, is a lead acid accumulator Compare – . component of the coenzymes *NAD nickelous compounds Com- and NADP. These take part in many pounds of nickel in its +2 oxidation metabolic reactions as hydrogen ac- state; e.g. nickelous oxide is nickel(II) ceptors. DeÜciency of nicotinic acid oxide (NiO). causes the disease pellagra in humans. Good sources are liver and nickel(II) oxide A green powder, groundnut and sunÛower meals. NiO; r.d. 6.6. It can be made by heating nickel(II) nitrate or carbonate O with air excluded. nickel(III) oxide (nickel peroxide; N OH nickel sesquioxide) A black or grey powder, Ni2O3; r.d. 4.8. It is made by heating nickel(II) oxide in air and n used in *nickel–iron accumulators. Nicotinic acid nickel silver See german silver. nido-structure See borane. Nicol prism A device for producing transactinide plane-polarized light (see polarizer). nielsbohrium See elements It consists of two pieces of calcite cut . with a 68° angle and stuck together NIFE cell See nickel–iron accumu- with Canada balsam. The extraordi- lator. nary ray (see double refraction) ninhydrin A brown crystalline sub- passes through the prism while the stance, C H O .H O (see formula). It ordinary ray suffers total internal 9 4 3 2 reacts with amino acids to give a blue reÛection at the interface between colour. Ninhydrin is commonly used the two crystals, as the refractive in chromatography to analyse the index of the calcite is 1.66 for the or- amino-acid content of proteins. It is dinary ray and that of the Canada Ü balsam is 1.53. Modi cations of the O prism using different shapes and cements are used for special purposes. It was devised in 1828 by William OH Nicol (1768–1851). OH nicotinamide See nicotinic acid. nicotinamide adenine dinu- O cleotide See nad. Ninhydrin niobium 372

also used in forensic science to de- sulphuric acids (see nitrating mix- velop latent Üngerprints. ture). niobium Symbol Nb. A soft ductile nitre (saltpetre) Commercial *potas- grey-blue metallic transition el- sium nitrate; the name was formerly ement; a.n. 41; r.a.m. 92.91; r.d. 8.57; applied to natural crustlike efÛores- m.p. 2468°C; b.p. 4742°C. It occurs in cences, occurring in some arid re- several minerals, including niobite gions. (Fe(NbO ) ), and is extracted by sev- 3 2 nitre cake See sodium hydrogen- eral methods including reduction of sulphate Û . the complex uoride K2NbF7 using sodium. It is used in special steels nitrene A species of the type HN: or and in welded joints (to increase RN:, analogous to a *carbene. The ni- strength). Niobium–zirconium alloys trogen atom has four nonbonding are used in superconductors. Chemi- electrons, two of which are a lone cally, the element combines with the pair. The other two may have paral- halogens and oxidizes in air at 200°C. lel spins (a triplet state) or antiparal- It forms a number of compounds and lel spins (a singlet state). complexes with the metal in oxida- nitric acid A colourless corrosive tion states 2, 3, or 5. The element poisonous liquid, HNO ; r.d. 1.50; was discovered by Charles Hatchett 3 m.p. –42°C; b.p. 83°C. Nitric acid may (c. 1765–1847) in 1801 and Ürst iso- be prepared in the laboratory by the lated by Christian Blomstrand distillation of a mixture of an alkali- (1826–97) in 1864. Formerly, it was metal nitrate and concentrated called columbium. sulphuric acid. The industrial pro- A duction is by the oxidation of ammo- n • Information from the WebElements site nia to nitrogen monoxide, the nitrate A salt or ester of nitric acid. oxidation of this to nitrogen dioxide, and the reaction of nitrogen dioxide nitrating mixture A mixture of with water to form nitric acid and ni- concentrated sulphuric and nitric trogen monoxide (which is recycled). acids, used to introduce a nitro group Ü The rst reaction (NH3 to NO) is (–NO2) into an organic compound. Its catalysed by platinum or platinum/ action depends on the presence of Ü + rhodium in the form of ne wire the nitronium ion, NO2 . It is mainly gauze. The oxidation of NO and the used to introduce groups into the absorption of NO2 to form the prod- molecules of *aromatic compounds uct are noncatalytic and proceed (the nitro group can subsequently be with high yields but both reactions converted into or replaced by others) are second-order and slow. Increases and to make commercial *nitro com- in pressure reduce the selectivity of pounds, such as the explosives cellu- the reaction and therefore rather lose trinitrate (nitrocellulose), large gas absorption towers are re- glyceryl trinitrate (nitroglycerine), quired. In practice the absorbing acid trinitrotoluene (TNT), and picric acid is refrigerated to around 2°C and a (trinitrophenol). commercial ‘concentrated nitric acid’ nitration A type of chemical reac- at about 67% is produced. tion in which a nitro group (–NO2) is Nitric acid is a strong acid (highly added to or substituted in a mol- dissociated in aqueous solution) and ecule. Nitration can be carried out by dilute solutions behave much like a mixture of concentrated nitric and other mineral acids. Concentrated ni- 373 nitro compounds tric acid is a strong oxidizing agent. portant synthetic rubbers because of Most metals dissolve to form nitrates their resistance to oil and many sol- but with the evolution of nitrogen vents. See also buna rubber. oxides. Concentrated nitric acid also nitriles (cyanides) Organic com- reacts with several nonmetals to give pounds containing the group –CN the oxo acid or oxide. Nitric acid is bound to an organic group. Nitriles generally stored in dark brown bot- are made by reaction between potas- tles because of the photolytic decom- sium cyanide and haloalkanes in al- position to dinitrogen tetroxide. See coholic solution, e.g. also nitration. KCN + CH Cl → CH CN + KCl nitric oxide See nitrogen monox- 3 3 An alternative method is dehydration ide. of amides nitrides Compounds of nitrogen CH CONH – H O → CH CN with a more electropositive element. 3 2 2 3 Boron nitride is a covalent compound They can be hydrolysed to amides having macromolecular crystals. Cer- and carboxylic acids and can be retain electropositive elements, such as duced to amines. lithium, magnesium, and calcium, A react directly with nitrogen to form • Information about IUPAC nomenclature 3– ionic nitrides containing the N ion. nitrite A salt or ester of nitrous Transition elements form a range of acid. The salts contain the dioxoni- interstitial nitrides (e.g. Mn4N, W2N), – trate (III) ion, NO2 , which has a bond which can be produced by heating angle of 115°. the metal in ammonia. nitroalkane (nitroparafÜn) A type nitriding The process of hardening of organic compound of general for- n the surface of steel by producing a mula CnH2n+1NO2 the nitroalkanes layer of iron nitride. One technique are colourless, pleasant-smelling liq- is to heat the metal in ammonia gas. uids made by treating a haloalkane Another is to dip the hot metal in a with silver nitrate. They can be re- bath of molten sodium cyanide. duced to amines by the action of tin nitriÜcation A chemical process in and hydrochloric acid. Lower ni- which nitrogen (mostly in the form troalkanes such as nitromethane ° of ammonia) in plant and animal (CH3NO2, b.p. 100 C) are used as wastes and dead remains is oxidized high-performance fuels (for internal at Ürst to nitrites and then to ni- combustion engines and rockets), as trates. These reactions are effected chemical intermediates, and as polar mainly by the bacteria Nitrosomonas solvents. and Nitrobacter respectively. Unlike nitrobenzene A yellow oily liquid, ammonia, nitrates are readily taken C H NO ; r.d. 1.2; m.p. 6°C; b.p. up by plant roots; nitriÜcation is 6 5 2 211°C. It is made by the *nitration of therefore a crucial part of the *nitrobenzene using a mixture of nitric gen cycle. Nitrogen-containing com- and sulphuric acids. pounds are often applied to soils deÜcient in this element, as fertilizer. nitrocellulose See cellulose ni- Compare denitrification. trate. nitrile rubber A copolymer of nitro compounds Organic com- buta-1,3-diene and propenonitrile. Ni- pounds containing the group –NO2 trile rubbers are commercially im- (the nitro group) bound to a carbon nitrogen 374

atom. Nitro compounds are made by metal azide. There are two natural *nitration reactions. They can be re- isotopes: nitrogen–14 and nitro- duced to aromatic amines (e.g. ni- gen–15 (about 3%). The element is trobenzene can be reduced to used in the *Haber process for mak- phenylamine). See also explosive. ing ammonia and is also used to provide an inert atmosphere in welding nitrogen Symbol N. A colourless and metallurgy. The gas is diatomic gaseous element belonging to *group and relatively inert – it reacts with 15 (formerly VB) of the periodic hydrogen at high temperatures and table; a.n. 7; r.a.m. 14.0067; d. 1.2506 with oxygen in electric discharges. It –3 ° ° gdm ; m.p. –209.86 C; b.p. –195.8 C. also forms *nitrides with certain It occurs in air (about 78% by volume) metals. Nitrogen was discovered in and is an essential constituent of pro- 1772 by Daniel Rutherford (1749– teins and nucleic acids in living or- 1819). ganisms (see nitrogen cycle). A Nitrogen is obtained for industrial purposes by fractional distillation of • Information from the WebElements site liquid air. Pure nitrogen can be ob- nitrogen cycle One of the major tained in the laboratory by heating a cycles of chemical elements in the

nitrogen in nitrogen fixation the nitrogen fixation by lightning atmosphere by bacteria

oxides of n denitrification nitrogen in nitrogen in bacteria the atmosphere

nitrification nitrates in the soil

uptake by roots

nitrification

protein in feeding protein in plants animals

death death nitrites ammonia in dead organic matter

nitrification

Nitrogen cycle 375 nitrogen mustards environment. Nitrates in the soil are calcium dicarbide), and the *Haber taken up by plant roots and may process. then pass along food chains into ani- nitrogen monoxide (nitric oxide) mals. Decomposing bacteria convert A colourless gas, NO; m.p. –163.6°C; nitrogen-containing compounds (es- b.p. –151.8°C. It is soluble in water, pecially ammonia) in plant and ani- ethanol, and ether. In the liquid state mal wastes and dead remains back nitrogen monoxide is blue in colour into nitrates, which are released into (r.d. 1.26). It is formed in many reac- the soil and can again be taken up by tions involving the reduction of ni- nitrification plants (see ). Though nitric acid, but more convenient trogen is essential to all forms of life, reactions for the preparation of rea- the huge amount present in the at- sonably pure NO are reactions of mosphere is not directly available to sodium nitrite, sulphuric acid, and ei- most organisms (compare carbon ther sodium iodide or iron(II) sul- cycle). It can, however, be assimi- phate. Nitrogen monoxide reacts lated by some specialized bacteria readily with oxygen to give nitrogen (see nitrogen fixation) and is thus dioxide and with the halogens to give made available to other organisms the nitrosyl halides XNO (X = F,Cl,Br). indirectly. Lightning Ûashes also It is oxidized to nitric acid by strong make some nitrogen available to oxidizing agents and reduced to dini- plants by causing the combination of trogen oxide by reducing agents. The atmospheric nitrogen and oxygen to molecule has one unpaired electron, form oxides of nitrogen, which enter which accounts for its paramagnet- the soil and form nitrates. Some ni- ism and for the blue colour in the liq- trogen is returned from the soil to uid state. This electron is relatively the atmosphere by denitrifying bac- easily removed to give the nitrosyl n teria (see denitrification). ion NO+, which is the ion present in such compounds as NOClO , NOBF , nitrogen dioxide dinitrogen 4 4 See NOFeCl , (NO) PtCl and a ligand in tetroxide. 4 2 6 complexes, such as Co(CO)3NO. nitrogen Üxation A chemical In mammals and other vertebrates, process in which atmospheric nitro- nitrogen monoxide is now known to gen is assimilated into organic com- play several important roles. For ex- pounds in living organisms and ample, it acts as a gaseous mediator hence into the *nitrogen cycle. The in producing such responses as dila- ability to Üx nitrogen is limited to tion of blood vessels, relaxation of certain bacteria (e.g. Azotobacter, Ana- smooth muscle, and inhibition of baena). Rhizobium bacteria are able to platelet aggregation. In certain cells Üx nitrogen in association with cells of the immune system it is converted – in the roots of leguminous plants, to the peroxynitrite ion ( O–O–N=O), such as peas and beans, in which which has activity against tumour they form characteristic root nod- cells and pathogens. ules; cultivation of legumes is there- nitrogen mustards A group of ni- fore one way of increasing soil trogen compounds similar to *sul- nitrogen. Various chemical processes phur mustard. They were used as are used to Üx atmospheric nitrogen chemotherapy agents in cancer treat- in the manufacture of *fertilizers. ment. Like sulphur mustard, they are These include the *Birkeland–Eyde powerful blistering agents. Large process, the cyanamide process (see quantities were made during World nitrogenous base 376

War II, although none were used in heating to give nitric acid and nitro- combat. gen monoxide. Nitrous acid can func- nitrogenous base A basic com- tion both as an oxidizing agent (forms NO) with I– and Fe2+, or as a pound containing nitrogen. The term – reducing agent (forms NO3 ) with, for is used especially of organic ring 2+ compounds, such as adenine, gua- example, Cu ; the latter is most nine, cytosine, and thymine, which common. It is widely used (prepared are constituents of nucleic acids. See in situ) for the preparation of diazo- amine salts. nium compounds in organic chemistry. The full systematic name is nitroglycerine An explosive made dioxonitric(III) acid. by reacting 1,2,3-trihydroxypropane dinitrogen (glycerol) with a mixture of concen- nitrous oxide See oxide trated sulphuric and nitric acids. . Despite its name and method of nitryl ion (nitronium ion) The ion + preparation, it is not a nitro com- NO2 , found in mixtures of nitric acid pound, but an ester of nitric acid, and sulphuric acid and solutions of CH2(NO3)CH(NO3)CH2(NO3). It is used nitrogen oxides in nitric acid. Nitryl in dynamites. + – salts, such as NO2 ClO4 , can be iso- nitro group See nitro compounds. lated but are extremely reactive. Nit- ryl ions generated in situ are used for nitryl ion nitronium ion See . *nitration in organic chemistry. Ü nitroalkane nitroparaf n See . NMR See nuclear magnetic reso- nitrosamines A group of carcino- nance. genic compounds with the general nobelium n ′ ′ Symbol No. A radioactive formula RR NNO, where R and R are metallic transuranic element belong- side groups with a variety of possible ing to the *actinoids; a.n. 102; mass structures. Nitrosamines, which are a number of most stable element 254 component of cigarette smoke, cause (half-life 55 seconds). Seven isotopes cancer in a number of organs, partic- are known. The element was Ürst ularly in the liver, kidneys, and identiÜed with certainty by Albert lungs. An example of a nitrosamine Ghiorso and Glenn Seaborg (1912–99) is dimethylnitrosamine, which has in 1966. two methyl side groups (CH3–). A nitrosonium ion The positive ion • Information from the WebElements site NO+, present in certain salts such as + noble gases (inert gases; rare the chlorate (NO ClO4) and the Û + gases; group 18 elements) A group boro uoride (NO BF4). of monatomic gaseous elements nitrosyl ion The ion NO+. See nitrogen monoxide forming group 18 (formerly group 0) . of the *periodic table: helium (He),

nitrous acid A weak acid, HNO2, neon (Ne), argon (Ar), krypton (Kr), known only in solution and in the xenon (Xe), and radon (Rn). The elec- gas phase. It is prepared by the ac- tron conÜguration of helium is 1s2. tion of acids upon nitrites, preferably The conÜgurations of the others ter- using a combination that removes minate in ns2np6 and all inner shells the salt as an insoluble precipitate are fully occupied. The elements thus (e.g. Ba(NO2)2 and H2SO4). The solu- represent the termination of a period tions are unstable and decompose on and have closed-shell conÜguration 377 nonequilibrium thermodynamics and associated high ionization ener- by it lack of chemical reactivity, par- gies (He 2370 to Rn 1040 kJ mol–1) ticularly to acids and atmospheric and lack of chemical reactivity. Being corrosion. Examples include gold, monatomic the noble gases are palladium, platinum, and rhodium. spherically symmetrical and have no-bond resonance See hypercon- very weak interatomic interactions jugation. and consequent low enthalpies of vaporization. The behaviour of the NOE See nuclear overhauser ef- lighter members approaches that of fect. an ideal gas at normal temperatures; nonahydrate A crystalline com- with the heavier members increasing pound that has nine moles of water polarizability and dispersion forces per mole of compound. lead to easier liquefaction under pressure. Four types of ‘compound’ nonanoic acid (perargonic acid) A have been described for the noble clear oily liquid carboxylic acid, CH3 ° gases but of these only one can be (CH2)7 COOH; r.d. 0.9; m.p. 12.5 C; ° correctly described as compounds in b.p. 254 C. It is found as esters in oil the normal sense. One type consists of pelargonium and certain esters are + + + used as Ûavourings. of such species as HHe , He2 , Ar2 , HeLi+, which form under highly ener- nonbenzenoid aromatics Aro- getic conditions, such as those in arcs matic compounds that have rings and sparks. They are short-lived and other than benzene rings. Examples only detected spectroscopically. A are the cyclopentadienyl anion, second group of materials described – C5H5 , and the tropylium cation, + annulenes as inert-gas–metal compounds do not C7H7 . See also . have deÜned compositions and are n noncompetitive inhibition See simply noble gases adsorbed onto the inhibition surface of dispersed metal. The third . type, previously described as ‘hy- nonequilibrium statistical me- drates’ are in fact clathrate com- chanics The statistical mechanics of pounds with the noble gas molecule systems not in thermal equilibrium. trapped in a water lattice. True com- One of the main purposes of non- pounds of the noble gases were Ürst equilibrium statistical mechanics is described in 1962 and several to calculate *transport coefÜcients Ûuorides, oxyÛuorides, Ûuoroplati- and inverse transport coefÜcients, nates, and Ûuoroantimonates of such as conductivity and viscosity, *xenon are known. A few krypton from Ürst principles and to provide a Ûuorides and a radon Ûuoride are basis for transport theory. The non- also known although the short half- equilibrium systems easiest to under- life of radon and its intense alpha ac- stand are those near thermal tivity restrict the availability of equilibrium. For systems far from information. Apart from argon, the equilibrium, such possibilities as noble gases are present in the atmos- *chaos and self-organization can phere at only trace levels. Helium arise due to nonlinearity. may be found along with natural gas nonequilibrium thermodynam- (up to 7%), arising from the radio- ics The thermodynamics of systems active decay of heavier elements (via not in thermal equilibrium. The non- alpha particles). equilibrium systems easiest to under- noble metal A metal characterized stand are those near thermal nonferrous metal 378

equilibrium; these systems are de- most common nonreducing sugar. scribed by the *Onsager relations. The linkage between the glucose and For systems far from equilibrium, fructose units in sucrose, which in- more complicated patterns, such as volves aldehyde and ketone groups, *chaos and *self-organization, can is responsible for the inability of su- arise due to nonlinearity. Which be- crose to act as a *reducing sugar. haviour is observed depends on the nonrelativistic quantum theory value of certain parameters in the quantum theory system. The transition from one type See . of behaviour to another as the para- nonrenewable energy sources meters are altered occurs at *bifurca- See renewable energy sources. tions. nonstoichiometric compound nonferrous metal Any metal (Berthollide compound) A chemical other than iron or any alloy that does compound in which the elements do not contain iron. In commercial not combine in simple ratios. For ex- terms this usually means aluminium, ample, rutile (titanium(IV) oxide) is copper, lead, nickel, tin, zinc, or often deÜcient in oxygen, typically their alloys. having a formula TiO1.8. nonmetal An element that is not a noradrenaline (norepinephrine) A *metal. Nonmetals can either be in- hormone produced by the adrenal sulators or semiconductors. At low glands and also secreted from nerve temperatures nonmetals are poor endings in the sympathetic nervous conductors of both electricity and system as a chemical transmitter of heat as few free electrons move nerve impulses. Many of its general n through the material. If the conduc- actions are similar to those of tion band is near to the valence band *adrenaline, but it is more concerned energy bands (see ) it is possible for with maintaining normal body activ- nonmetals to conduct electricity at ity than with preparing the body for high temperatures but, in contrast to emergencies. metals, the conductivity increases with increasing temperature. Non- Nordhausen sulphuric acid See metals are electronegative elements, disulphuric(vi) acid. such as carbon, nitrogen, oxygen, norepinephrine See noradrena- phosphorus, sulphur, and the halo- line. gens. They form compounds that contain negative ions or covalent normal Having a concentration of 3 bonds. Their oxides are either neu- one gram equivalent per dm . tral or acidic. normal mode of vibration A nonpolar compound A com- basic vibration of a polyatomic mol- pound that has covalent molecules ecule. All vibrational motion of a with no permanent dipole moment. polyatomic molecule can be treated Examples of nonpolar compounds as a superposition of the normal are methane and benzene. modes of vibration of the molecule. solvent If N is the number of atoms in a mol- nonpolar solvent See . ecule, the number of modes of vibra- nonreducing sugar A sugar that tion is 3N – 5 for a linear molecule cannot donate electrons to other and 3N – 6 for a nonlinear molecule. molecules and therefore cannot act Each of these vibrational modes has a as a reducing agent. Sucrose is the characteristic frequency, although it 379 nuclear Overhauser effect is possible for some of them to be de- are produced by magnetic-resonance generate. For example, in a linear tri- techniques. atomic molecule there are four The main application of NMR is as normal modes of vibration since a technique for chemical analysis 3N – 5 = 4 for N = 3. These vibrational and structure determination, known modes are: (a) symmetric stretching as NMR spectroscopy. It depends on (breathing) vibrations; (b) antisym- the fact that the electrons in a mol- metric stretching vibrations; and (c) ecule shield the nucleus to some ex- two bending vibrations, which are tent from the Üeld, causing different degenerate. atoms to absorb at slightly different frequencies (or at slightly different N.T.P. s.t.p See . Üelds for a Üxed frequency). Such n-type semiconductor See semi- effects are known as chemical shifts. conductor. There are two methods of NMR spectroscopy. In continuous wave (CW) nuclear magnetic resonance NMR, the sample is subjected to a (NMR) The absorption of electromag- strong Üeld, which can be varied in a netic radiation at a suitable precise controlled way over a small region. It frequency by a nucleus with a is irradiated with radiation at a Üxed nonzero magnetic moment in an ex- Ü frequency, and a detector monitors ternal magnetic eld. The phenome- the Üeld at the sample. As the Üeld non occurs if the nucleus has changes, absorption corresponding to nonzero *spin, in which case it be- transitions occurs at certain values, haves as a small magnet. In an exter- Ü and this causes oscillations in the nal magnetic eld, the nucleus’s Üeld, which induce a signal in the de- magnetic moment vector precesses Fourier transform Ü tector. (FT) NMR n about the eld direction but only cer- uses a Üxed magnetic Üeld and tain orientations are allowed by the sample is subjected to a high- quantum rules. Thus, for hydrogen intensity pulse of radiation covering ½ (spin of ) there are two possible a range of frequencies. The signal Ü states in the presence of a eld, each produced is analysed mathematically with a slightly different energy. Nu- to give the NMR spectrum. The most clear magnetic resonance is the ab- common nucleus studied is 1H. For sorption of radiation at a photon instance, an NMR spectrum of energy equal to the difference be- ethanol (CH3CH2OH) has three peaks tween these levels, causing a transi- in the ratio 3:2:1, corresponding to tion from a lower to a higher energy the three different hydrogen-atom state. For practical purposes, the dif- environments. The peaks also have a ference in energy levels is small and Üne structure caused by interaction the radiation is in the radiofrequency between spins in the molecule. Other region of the electromagnetic spec- nuclei can also be used for NMR spectrum. It depends on the Üeld troscopy (e.g. 13C, 14N, 19F) although strength. these generally have lower magnetic NMR can be used for the accurate moment and natural abundance than determination of nuclear moments. hydrogen. See also electron paramag- It can also be used in a sensitive form netic resonance; endor. of magnetometer to measure mag- nuclear magneton magneton netic Üelds. In medicine, magnetic See . resonance imaging (MRI) has been de- nuclear Overhauser effect (NOE) veloped, in which images of tissue An effect in *nuclear magnetic reso- nucleic acid 380

nance (NMR) used to increase the in- to a positive (electron-deÜcient) part tensities of resonance lines. The large of the molecule. *Aldehydes and *ke- population differences between the tones undergo reactions of this type states given by the Boltzmann distri- because of polarization of the car- bution gives rise to strong intensities bonyl group (carbon positive). of resonance lines. In the nuclear nucleophilic substitution A type Overhauser effect, spin relaxation is of substitution reaction in which a used to transfer population differ- *nucleophile displaces another group ence from one type of nucleus to an- or atom from a compound. For exam- other type of nucleus so that the ple, in intensities of the resonance lines of CR Cl + OH– → CR OH + Cl– the second type of nucleus are in- 3 3 – creased. An example of such an en- the nucleophile is the OH ion. There hancement, which is widely used, is are two possible mechanisms of nu- between protons and 13C, in which cleophilic substitution. In SN1 reac- case it is possible to attain enhance- tions, a positive carbonium ion is Ü ment of about three times. In the rst formed: → + – NOE proton, irradiation is used to CR3Cl CR3 + Cl produce the enhancement of the 13C This then reacts with the nucleophile line. Another use of the NOE is to de- + – → CR3 + OH CR3OH termine the distance between pro- + – tons. The CR3 ion is planar and the OH ion can attack from either side. Con- nucleic acid A complex organic sequently, if the original molecule is compound in living cells that con- optically active (the three R groups sists of a chain of *nucleotides. There are different) then a racemic mixture n are two types: *DNA (deoxyribonu- of products results. cleic acid) and *RNA (ribonucleic The alternative mechanism, the acid). SN2 reaction, is a concerted reaction A in which the nucleophile approaches • Information on IUPAC nomenclature from the side of the R groups as the other group (Cl in the example) nucleon A *proton or a *neutron. leaves. In this case the conÜguration nucleon number (mass number) of the molecule is inverted. If the Symbol A. The number of *nucleons original molecule is optically active, in an atomic nucleus of a particular the product has the opposite activity, nuclide. an effect known as Walden inversion. The notations SN1 and SN2 refer to nucleophile An ion or molecule the kinetics of the reactions. In the that can donate electrons. Nucle- SN1 mechanism, the slow step is the ophiles are often oxidizing agents Ürst one, which is unimolecular (and Ü and Lewis bases. They are either neg- rst order in CR3Cl). In the SN2 reac- ative ions (e.g. Cl–) or molecules that tion, the process is bimolecular (and have electron pairs (e.g. NH3). In or- second order overall). ganic reactions they tend to attack nucleoside An organic compound positively charged parts of a mol- consisting of a nitrogen-containing ecule. Compare electrophile. *purine or *pyrimidine base linked nucleophilic addition A type of to a sugar (ribose or deoxyribose). An addition reaction in which the Ürst example is *adenosine. Compare nu- step is attachment of a *nucleophile cleotide. 381 nylon nucleosynthesis The synthesis of neutrons contribute to the atomic chemical elements by nuclear mass (see nucleon number) but not processes. There are several ways in to the nuclear charge. The most mas- which nucleosynthesis can take sive nucleus that occurs in nature is place. Primordial nucleosynthesis uranium–238, containing 92 protons took place very soon after the big and 146 neutrons. The symbol used 238 bang, when the universe was ex- for this *nuclide is 92U, the upper tremely hot. This process was respon- Ügure being the nucleon number and sible for the cosmic abundances the lower Ügure the atomic number. observed for light elements, such as In all nuclei the nucleon number (A) helium. Explosive nucleosynthesis is equal to the sum of the atomic can also occur during the explosion number (Z) and the neutron number of a supernova. However, stellar nu- (N), i.e. A = Z + N. cleosynthesis, which takes place in the centre of stars at very high tem- nuclide A type of atom as charac- peratures, is now the principal form terized by its *atomic number and its of nucleosynthesis. The exact process *neutron number. An *isotope refers occurring in stellar nucleosynthesis to a series of different atoms that depends on the temperature, density, have the same atomic number but and chemical composition of the different neutron numbers (e.g. ura- star. The synthesis of helium from nium–238 and uranium–235 are iso- protons and of carbon from helium topes of uranium), whereas a nuclide can both occur in stellar nucleosyn- refers only to a particular nuclear thesis. species (e.g. the nuclides uranium–235 and plutonium–239 are Üs- nucleotide An organic compound sile). The term is also used for the consisting of a nitrogen-containing n *purine or *pyrimidine base linked type of nucleus. to a sugar (ribose or deoxyribose) and nylon Any of various synthetic a phosphate group. *DNA and *RNA polyamide Übres having a protein- are made up of long chains of nu- like structure formed by the conden- cleotides (i.e. polynucleotides). Com- sation between an amino group of pare nucleoside. one molecule and a carboxylic acid nucleus The central core of an group of another. There are three Ü atom that contains most of its mass. main nylon bres, nylon 6, nylon 6,6, It is positively charged and consists and nylon 6,10. Nylon 6, for example of one or more nucleons (protons or Enkalon and Celon, is formed by the neutrons). The positive charge of the self-condensation of 6-aminohexa- nucleus is determined by the number noic acid. Nylon 6,6, for example Bri of protons it contains (see atomic nylon, is made by polycondensation number) and in the neutral atom this of hexanedioic acid (adipic acid) and is balanced by an equal number of 1,6-diaminohexane (hexamethylene- electrons, which move around the diamine) having an average formula nucleus. The simplest nucleus is the weight between 12 000 and 15 000. hydrogen nucleus, consisting of one Nylon 6,10 is prepared by polymeriz- proton only. All other nuclei also ing decanedioic acid and 1,6-diamino- contain one or more neutrons. The hexane. O

occlusion 1. The trapping of small behaviour of the fuel being tested in pockets of liquid in a crystal during a single cylinder four-stroke engine crystallization. 2. The absorption of of standard design. Compare cetane a gas by a solid such that atoms or number. molecules of the gas occupy intersti- octanitrocubane See cubane. tial positions in the solid lattice. Pal- ladium, for example, can occlude octanoic acid (caprylic acid) A hydrogen. colourless liquid straight-chain saturated *carboxylic acid, ochre A yellow or red mineral form ° CH3(CH2)6COOH; b.p. 239.3 C. of iron(III) oxide, Fe2O3, used as a pigment. octavalent Having a valency of octadecanoate See stearate. eight. law of octaves octadecanoic acid See stearic octave See . acid. octet A stable group of eight elec- octadecenoic acid A straight- trons in the outer shell of an atom chain unsaturated fatty acid with the (as in an atom of a noble gas). formula C17H33COOH. Cis-octadec-9- octogen See hmx. enoic acid (see oleic acid) has the for- octupole A set of eight point mula CH3(CH2)7CH:CH(CH2)7COOH. The glycerides of this acid are found charges that has zero net charge and in many natural fats and oils. does not have either a dipole moment or a quadrupole moment. An complex octahedral See . example of an octupole is a methane octahydrate A crystalline hydrate molecule (CH4). Octupole interactions that has eight moles of water per are much smaller than quadrupole mole of compound. interactions and very much smaller than dipole interactions. octane A straight-chain liquid Ω *alkane, C8H18; r.d. 0.7; m.p. ohm Symbol . The derived *SI –56.79°C; b.p. 125.66°C. It is present unit of electrical resistance, being in petroleum. The compound is iso- the resistance between two points on meric with 2,2,4-trimethylpentane, a conductor when a constant poten- ((CH3)3CCH2CH(CH3)2, iso-octane). See tial difference of one volt, applied be- octane number. tween these points, produces a current of one ampere in the conduc- octane number A number that tor. The former international ohm provides a measure of the ability of a (sometimes called the ‘mercury fuel to resist knocking when it is Ü burnt in a spark-ignition engine. It is ohm’) was de ned in terms of the re- the percentage by volume of iso- sistance of a column of mercury. The unit is named after Georg Ohm octane (C8H18; 2,2,4-trimethylpentane) in a blend with normal heptane (1787–1854). (C7H16) that matches the knocking oil Any of various viscous liquids 383 one-pot synthesis that are generally immiscible with ganisms, which normally contain water. Natural plant and animal oils 20–25% oil, are of interest in biotech- are either volatile mixtures of ter- nology as alternative sources of con- penes and simple esters (e.g. *essen- ventional oils or as possible sources tial oils) or are *glycerides of fatty for novel oils. The majority of the acids. Mineral oils are mixtures of hy- oils produced by oleaginous eukary- drocarbons (e.g. *petroleum). otic microorganisms are similar to oil of vitriol See sulphuric acid. plant oils. One possibility under con- sideration is the production of oils oil of wintergreen Methyl salicy- and fats from waste material, to be late (methyl 2-hydroxybenzoate, used in animal feed. C8H8O3), a colourless aromatic liquid ester, b.p. 223°C. It occurs in the es- oleate A salt or ester of *oleic acid. sential oils of some plants, and is oleÜnes See alkenes. manufactured from salicylic acid. It is oleic acid An unsaturated *fatty easily absorbed through the skin and acid with one double bond, used in medicine for treating muscu- CH (CH ) CH:CH(CH ) COOH; r.d. 0.9; lar and sciatic pain. Because of its at- 3 2 7 2 7 m.p. 13°C. Oleic acid is one of the tractive smell it is also used in most abundant constituent fatty perfumes and food Ûavourings. acids of animal and plant fats, oc- oil sand (tar sand; bituminous sand) curring in butterfat, lard, tallow, A sandstone or porous carbonate groundnut oil, soya-bean oil, etc. rock that is impregnated with hydro- Its systematic chemical name is cis- carbons. The largest deposit of oil octadec-9-enoic acid. sand occurs in Alberta, Canada (the oleum See disulphuric(vi) acid. Athabasca tar sands); there are also deposits in the Orinoco Basin of oligonucleotide A short polymer Venezuela, Russia, USA, Madagascar, of *nucleotides. o Albania, Trinidad, and Romania. oligopeptide See peptide. oil shale A Üne-grained carbona- oligosaccharide A carbohydrate (a ceous sedimentary rock from which type of *sugar) whose molecules con- oil can be extracted. The rock contain a chain of up to 20 united mono- tains organic matter – kerogen – saccharides. Oligosaccharides are which decomposes to yield oil when formed as intermediates during the heated. Deposits of oil shale occur on digestion of *polysaccharides, such every continent, the largest known as cellulose and starch. reserves occurring in Colorado, Utah, and Wyoming in the USA. Commer- olivine An important group of cial production of oil from oil shale is rock-forming silicate minerals crys- generally considered to be uneco- tallizing in the orthorhombic system. nomic unless the price of petroleum Olivine conforms to the general for- rises above the recovery costs for oil mula (Mg,Fe)2SiO4 and comprises a from oil shale. However, threats of complete series from pure magne- declining conventional oil resources sium silicate (forsterite, Mg2SiO4) to have resulted in considerable interest pure iron silicate (fayalite, Fe2SiO4). It and developments in recovery tech- is green, brown-green, or yellow- niques. green in colour. A gem variety of olivine is peridot. oleaginous Producing or containing oil or lipids. Oleaginous microor- one-pot synthesis A method of onium ion 384

synthesizing organic compounds in stones, display the property of which the materials used are mixed opalescence – a characteristic inter- together in a single vessel and al- nal play of colours resulting from the lowed to react, rather than conduct- interference of light rays within the ing the reaction in a sequence of stone. Black opal has a black back- separate stages. ground against which the colours are displayed. The chief sources of pre- onium ion An ion formed by cious opals are Australia and Mexico. adding a proton to a neutral molecule, e.g. the hydroxonium ion Geyserite is a variety deposited by (H O+) or the ammonium ion (NH +). geysers or hot springs. Another vari- 3 4 ety, diatomite, is made up of the Onsager relations In a system skeletons of diatoms. that is not at equilibrium, various open chain chain changes are occurring. For example, See . there may be a Ûow of energy from open-hearth process A tradi- one part of the system to another tional but now obsolete method for and, at the same time, a Ûow of mass manufacturing steel by heating to- (diffusion). Flows of this type are cou- gether scrap, pig iron, etc., in a re- pled, i.e. each depends on the other. fractory-lined shallow open furnace Equations exist of the type heated by burning producer gas in air. It has been replaced by the J1 = L11X1 + L12X2 *basic-oxygen process. J2 = L21X1 + L22X2 operator A mathematical entity Here, J is the Ûow of energy and J 1 2 that performs a speciÜc operation on the Ûow of matter. X is the ‘force’ 1 a function to transform the function producing energy Ûow and X that 2 into another function. For example, producing matter Ûow. L is the 11 the square root sign √ and the differ- coefÜcient for thermal conductance o Ü entiation symbol d/dx are operators. and L22 the coef cient for diffusion. Ü The coef cients L12 and L21 represent opiate One of a group of drugs de- coupling of the Ûows with each rived from *opium, which depress other. Equations of this type can be brain function (a narcotic action). generalized to any number of Ûows Opiates include *morphine and its and are known as the phenomeno- synthetic derivatives, such as *heroin logical relations. In Onsager’s theory and codeine. They are used in medi- the coupling coefÜcients are equal, cine chieÛy to relieve pain, but the i.e. L12 = L21, etc. These are known as use of morphine and heroin is reciprocal relations. It follows that: strictly controlled since they can ∂ ∂ ∂ ∂ cause drug dependence and toler- ( J1/ X2)X = ( J2/ X1)X . 1 2 ance. The theory was developed by the Swedish physicist Lars Onsager in opioid Any one of a group of sub- 1931. stances that produce pharmacological and physiological effects similar opal A hydrous amorphous form of to those of morphine. Opioids are silica. Many varieties of opal occur, not necessarily structurally similar to some being prized as gemstones. morphine, although a subgroup of Common opal is usually milk white opioids, the *opiates, are morphine- but the presence of impurities may derived compounds. colour it yellow, green, or red. Pre- cious opals, which are used as gem- opium A substance obtained from 385 optical activity the unripe seed head of the opium COOH COOH poppy (Papaver somniferum). It contains a number of alkaloids, includ- C C ing *morphine and *codeine. See opiate. H C H H CH 3 3 optical activity The ability of cer- HO OH tain substances to rotate the plane of plane-polarized light as it passes D-form L-form through a crystal, liquid, or solution. Optical activity It occurs when the molecules of the substance are asymmetric, so that which one part of the molecule is a they can exist in two different struc- mirror image of the other. Such mol- tural forms each being a mirror ecules are not optically active. image of the other (see chirality el- Molecules that show optical activ- ement). The two forms are optical isomers or enantiomers. The exis- ity have no plane of symmetry. The tence of such forms is also known as commonest case of this is in organic enantiomorphism (the mirror images compounds in which a carbon atom being enantiomorphs). One form will is linked to four different groups. An rotate the light in one direction and atom of this type is said to be a chiral the other will rotate it by an equal centre. Asymmetric molecules show- amount in the other. The two possi- ing optical activity can also occur in ble forms are described as *dextroro- inorganic compounds. For example, tatory or *laevorotatory according to an octahedral complex in which the the direction of rotation. PreÜxes are central ion coordinates to six differ- used to designate the isomer: (+)- ent ligands would be optically active. (dextrorotatory), (–)- (laevorotatory), Many naturally occurring compounds and (±)- (racemic mixture) are now show optical isomerism and usually o preferred to, and increasingly used only one isomer occurs naturally. For for, the former d-, l-, and dl-, respec- instance, glucose is found in the dex- tively. An equimolar mixture of the trorotatory form. The other isomer, two forms is not optically active. (–)- or l-glucose, can be synthesized in It is called a racemic mixture (or ra- the laboratory, but cannot be synthe- cemate). In addition, certain mol- sized by living organisms. See also ab- ecules can have a meso form in solute configuration.

Opioid optical brightener 386

optical brightener See brighten- orbital A region in which an elec- ers. tron may be found in an atom or molecule. In the original *Bohr optical glass Glass used in the theory of the atom the electrons manufacture of lenses, prisms, and were assumed to move around the other optical parts. It must be homo- nucleus in circular orbits, but further geneous and free from bubbles and advances in quantum mechanics led strain. Optical crown glass may con- to the view that it is not possible to tain potassium or barium in place of give a deÜnite path for an electron. the sodium of ordinary crown glass According to *wave mechanics, the and has a refractive index in the electron has a certain probability of range 1.51 to 1.54. Flint glass con- being in a given element of space. tains lead oxide and has a refractive Thus for a hydrogen atom the elec- index between 1.58 and 1.72. Higher tron can be anywhere from close to refractive indexes are obtained by the nucleus to out in space but the adding lanthanoid oxides to glasses; maximum probability in spherical these are now known as lanthanum Û shells of equal thickness occurs in a crowns and ints. spherical shell around the nucleus optical isomers See optical activ- with a radius equal to the Bohr ra- ity. dius of the atom. The probabilities of Ünding an electron in different re- optical maser See laser. gions can be obtained by solving the optical pumping See laser. Schrödinger wave equation to give ψ optical rotary dispersion (ORD) the wave function , and the probability of location per unit volume is The effect in which the amount of then proportional to |ψ|2. Thus the rotation of plane-polarized light by idea of electrons in Üxed orbits has an optically active compound de- been replaced by that of a probability o pends on the wavelength. A graph of distribution around the nucleus – an rotation against wavelength has a atomic orbital (see illustration). Alter- characteristic shape showing peaks natively, the orbital can be thought or troughs. of as an electric charge distribution optical rotation Rotation of plane- (averaged over time). In representing polarized light. See optical activity. orbitals it is convenient to take a sur- optoacoustic spectroscopy A face enclosing the space in which the spectroscopic technique in which electron is likely to be found with a high probability. electromagnetic radiation is absorbed The possible atomic orbitals corre- by materials that generate sound spond to subshells of the atom. Thus waves. This technique has been used there is one s-orbital for each shell particularly in gases. The principle (orbital quantum number l = 0). underlying optoacoustic spectroscopy This is spherical. There are three p- is that the absorbed electromagnetic orbitals (corresponding to the three radiation is converted into motion, values of l) and Üve d-orbitals. The which is associated with the produc- shapes of orbitals depend on the tion of sound waves. See also photo- value of l. For instance, p-orbitals acoustic spectroscopy. each have two lobes; most d-orbitals orbit The path of an electron as it have four lobes. travels round the nucleus of an atom. In molecules, the valence electrons See orbital. move under the inÛuence of two nu- 387 orbital clei (in a bond involving two atoms) lecular orbital between the two nu- and there are corresponding molecu- clei. This is an example of a sigma or- lar orbitals for electrons (see illustra- bital. In a double bond, as in ethene, tion). It is convenient in considering one bond is produced by overlap these to regard them as formed by along the line of axes to form a overlap of atomic orbitals. In a hydro- sigma orbital. The other is produced gen molecule the s-orbitals on the by sideways overlap of the lobes of two atoms overlap and form a mo- the p-orbitals (see illustration). The z

symmetrical s-orbital

zz z

x x x y y y o

p p p x y z

three equivalent p-orbitals, each having 2 lobes

Atomic orbitals

p-orbitals pi orbital

H H H H C C CC

H H H H

hybrid sp -orbitals 3 sigma orbital Molecular orbitals: formation of the double bond in ethene

Orbital orbital quantum number 388

resulting molecular orbital has two the powers of the concentrations is parts, one on each side of the sigma the overall order of the reaction. For orbital – this is a pi orbital. It is also instance, in a reaction possible for a delta orbital to form by A + B → C lateral overlap of two d-orbitals. In fact, the combination of two atomic the rate equation may have the form orbitals produces two molecular or- R = k[A][B]2 bitals with different energies. The This reaction would be described as one of lower energy is the bonding Ürst order in A and second order in B. orbital, holding the atoms together; The overall order is three. The order the other is the antibonding orbital, of a reaction depends on the mecha- which would tend to push the atoms nism and it is possible for the rate to apart. In the case of valence elec- be independent of concentration trons, only the lower (bonding) or- (zero order) or for the order to be a Ü bital is lled. fraction. See also molecularity; In considering the formation of pseudo order. molecular orbitals it is often useful to think in terms of hybrid atomic or- ore A naturally occurring mineral bitals. For instance, carbon has in its from which a metal and certain outer shell one s-orbital and three p- other elements (e.g. phosphorus) can orbitals. In forming methane (or be extracted, usually on a commer- other tetrahedral molecules) these cial basis. Metals may be present in can be regarded as combining to give ores in the native form, but more four equivalent sp3 hybrid orbitals, commonly they occur combined as each with a lobe directed to a corner oxides, sulphides, sulphates, silicates, of a tetrahedron. It is these that over- etc. lap with the s-orbitals on the hydro- ore dressing See beneficiation. gen atoms. In ethene, two p-orbitals o combine with the s-orbital to give oregonator A type of chemical re- three sp2 hybrids with lobes in a action mechanism that causes an plane pointing to the corners of an *oscillating reaction. It is the type of equilateral triangle. These form the mechanism responsible for the *B–Z sigma orbitals in the C–H and C–C reaction, and involves Üve steps of bonds. The remaining p-orbitals (one the form: on each carbon) form the pi orbital. → 2 A + Y X In ethyne, sp hybridization occurs to → give two hybrid orbitals on each X + Y C atom with lobes pointing along the A + X → 2X + Z axis. The two remaining p-orbitals on 2X → D each carbon form two pi orbitals. Hy- Z → Y brid atomic orbitals can also involve d-orbitals. For instance, square-planar Autocatalysis occurs as in the complexes use sp2d hybrids; octa- *Lotka–Volterra mechanism and the hedral complexes use sp3d2. *brusselator. The mechanism was named after Oregon in America, orbital quantum number See atom where the research group that dis- . covered it is based. optical rotary dispersion ORD See . organic chemistry The branch of order In the expression for the rate chemistry concerned with com- of a chemical reaction, the sum of pounds of carbon. 389 orpiment organo- PreÜx used before the named after Leslie Orgel, who devel- name of an element to indicate com- oped them in 1955. pounds of the elements containing origin of elements The nuclear organic groups (with the element processes that give rise to chemical bound to carbon atoms). For exam- elements. There is not one single ple, lead(IV) tetraethyl is an organo- process that can account for all the lead compound. elements. The abundance of the organometallic compound A chemical elements is determined not compound in which a metal atom or just by the stability of the nuclei of ion is bound to an organic group. the atoms but also how readily the Organometallic compounds may nuclear processes leading to the exis- have single metal–carbon bonds, tence of these atoms occur. as in the aluminium alkyls (e.g. Most of the helium in the universe Al(CH3)3). In some cases, the bonding was produced by fusion in the early is to the pi electrons of a double universe when the temperature and bond, as in complexes formed be- the pressure were very high. Most of tween platinum and ethene, or to the elements between helium and the pi electrons of a ring, as in *fer- iron were made in nuclear fusion re- rocene. actions inside stars. Since iron is at A the bottom of the energy valley of stability • Information about IUPAC nomenclature , energy needs to be put into • Further information about a nucleus heavier than iron for a fu- nomenclature sion reaction to occur. Inside stars some heavy elements are built up by organophosphorus compound the s-process, where s stands for A compound that has at least one slow, in which high-energy neutrons carbon–phosphorus bond. Organo- are absorbed by a nucleus, with the phosphorus compounds are used as resulting nucleus undergoing beta o pesticides (as in malathion and decay to produce a nucleus with a parathion) and as catalysts and sol- higher atomic number. Some heavy vents. Phosphate esters, sometimes elements are produced by the r- included in this category, are em- process, where r stands for rapid, ployed for Üre-prooÜng textiles. which occurs in supernova explo- Orgel diagram A diagram showing sions when a great deal of gravita- how the energy levels of a transition- tional energy is released when a metal atom split when it is placed in large star that has exhausted its nu- a ligand Üeld. The vertical axis shows clear fuel collapses to a neutron star, the energy and the horizontal axis with the release of a very large num- shows the strength of the ligand ber of neutrons. Üeld, with zero ligand Üeld strength ornithine (Orn) An *amino acid, at the centre of the horizontal axis. H2N(CH2)3CH(NH2)COOH, that is not That the splitting for dn is the same a constituent of proteins but is im- as dn +5 and the opposite of d10 – n is portant in living organisms as an in- readily seen on an Orgel diagram, termediate in the reactions of the both for octahedral and tetrahedral *urea cycle and in arginine synthesis. Üelds. The spectroscopic, optical, and ornithine cycle See urea cycle. magnetic properties of complexes of transition metals are made clear in orpiment A natural yellow mineral such diagrams. Orgel diagrams are form of arsenic(III) sulphide, As2S3. ortho- 390

The name is also used for the syn- which the concentrations of the thetic compound, which is used as a products and reactants change peri- pigment. odically, either with time or with po- ortho- 1. PreÜx indicating that a sition in the reacting medium. Thus, benzene compound has two substi- the concentration of a component tuted groups in the 1,2 positions (i.e. may increase with time to a maxi- on adjacent carbon atoms). The ab- mum, decrease to a minimum, then breviation o- is used; for example increase again, and so on, continuing o-dichlorobenzene is 1,2-dichloro- the oscillation over a period of time. benzene. Compare meta-; para-. Systems are also known in which spi- 2. PreÜx formerly used to indicate rals and other patterns spread the most hydrated form of an acid. through the reacting medium, For example, phosphoric(V) acid, demonstrating a periodic spatial vari- H3PO4, was called orthophosphoric ation. Oscillating chemical reactions acid to distinguish it from the lower have certain features in common. metaphosphoric acid, HPO3 (which is They all occur under conditions far Ü actually (HPO3)n). 3. Pre x denoting from chemical equilibrium and all in- the form of diatomic molecules in volve *autocatalysis, i.e. a product of which nuclei have parallel spins, e.g. a reaction step acts as a catalyst for orthohydrogen (see hydrogen). Com- that step. This autocatalysis drives pare para-. the oscillation by a process of positive feedback. Moreover, oscillating orthoboric acid See boric acid. chemical reactions are associated orthoclase See feldspars. with the phenomenon known as orthohelium A form of helium bistability. In this, a reaction may be once thought to exist as of one of in a steady-state condition, with reac- Û two species of the element. Because tants owing into a reaction zone Û o the spectrum of atomic helium con- while products are owing out of it. sists of transitions between singlet Under these conditions, the concen- states (including transitions from the trations in the reaction zone may not ground state) and transitions be- change with time, although the reac- tween triplet states but not transition is not in a state of chemical equi- tions between singlet and triplet librium. Bistable systems have two states, early spectroscopists thought possible stable steady states. Interac- that two species of helium existed. tion with an additional substance in The other form was called para- the reaction medium causes the sys- helium. tem to oscillate between the states as the concentrations change. Oscillat- orthohydrogen See hydrogen. ing chemical reactions are thought to orthophosphoric acid See phos- occur in a number of biochemical phoric(v) acid. processes. For example, they occur in orthoplumbate See plumbate. glycolysis, in which ATP is produced by enzyme-catalysed reactions. They crystal system orthorhombic See . are also known to regulate the orthosilicate See silicate. rhythm of the heartbeat. Most have stannate highly complex reaction mecha- orthostannate See . nisms. See also lotka–volterra mech- oscillating reaction (clock reac- anism; brusselator; oregonator; tion) A type of chemical reaction in chaotic reaction. 391 Ostwald’s dilution law osmiridium A hard white naturally istic of the solution, and is called the occurring alloy consisting principally osmotic pressure (symbol Π). Osmotic of osmium (17–48%) and iridium pressure depends only on the con- (49%). It also contains small quanti- centration of particles in the solu- ties of platinum, rhodium, and ruthe- tion, not on their nature (i.e. it is a nium. It is used for making small *colligative property). For a solution items subject to wear, e.g. electrical of n moles in volume V at thermody- contacts or the tips of pen nibs. namic temperature T, the osmotic pressure is given by ΠV = nRT, where osmium Symbol Os. A hard blue- R is the gas constant. Osmotic- white metallic *transition element; pressure measurements are used in a.n. 76; r.a.m. 190.2; r.d. 22.57; m.p. Ünding the relative molecular masses 3045°C; b.p. 5027°C. It is found asso- of compounds, particularly macro- ciated with platinum and is used in molecules. A device used to measure certain alloys with platinum and irid- osmotic pressure is called an os- ium (see osmiridium). Osmium forms mometer. a number of complexes in a range of oxidation states. It was discovered by osmotic pressure See osmosis. Smithson Tennant (1761–1815) in Ostwald, Friedrich Wilhelm 1804. (1853–1932) German physical A chemist. In 1887 he went to Leipzig • Information from the WebElements site where he worked on a wide range of osmium(IV) oxide (osmium tetrox- topics including hydrolysis, viscosity, ide) A yellow solid, OsO , made by ionization, and catalysis. Ostwald was 4 Û heating osmium in air. It is used as highly in uential in the development an oxidizing agent in organic chem- of physical chemistry as a subject. He istry, as a catalyst, and as a Üxative in was awarded the 1909 Nobel Prize electron microscopy. for chemistry. o osmometer See osmosis. Ostwald ripening A process used in crystal growth in which a mixture osmosis The passage of a solvent of large and small crystals are both in through a semipermeable membrane contact with a solvent. The large separating two solutions of different crystals grow and the small crystals concentrations. A semipermeable disappear. This occurs because there membrane is one through which the is a higher energy associated with molecules of a solvent can pass but the smaller crystals. When they dis- the molecules of most solutes can- solve the heat associated with this not. There is a thermodynamic ten- higher energy is released, enabling dency for solutions separated by such recrystallization to occur on the large a membrane to become equal in con- crystals. Ostwald ripening is used in centration, the water (or other sol- such applications as photography, re- Û vent) owing from the weaker to the quiring crystals with speciÜc proper- stronger solution. Osmosis will stop ties. when the two solutions reach equal concentration, and can also be Ostwald’s dilution law An ex- stopped by applying a pressure to the pression for the degree of dissocia- liquid on the stronger-solution side tion of a weak electrolyte. For of the membrane. The pressure re- example, if a weak acid dissociates in quired to stop the Ûow from a pure water solvent into a solution is a character- HA ˆ H+ + A– outer-sphere mechanism 392

the dissociation constant Ka is given It occurs in certain plants, e.g. sorrel by and the leaf blades of rhubarb. α2 α Ka = n/(1 – )V oxaloacetic acid A compound, where α is the degree of dissociation, HO2CCH2COCO2H, that plays an inte- n the initial amount of substance (be- gral role in the *Krebs cycle. The fore dissociation), and V the volume. anion, oxaloacetate, reacts with the If α is small compared with 1, then acetyl group from acetyl coenzyme A α2 = KV/n; i.e. the degree of dissocia- to form citrate. tion is proportional to the square oxazole A heterocyclic compound root of the dilution. The law was Ürst having a nitrogen atom and an oxy- put forward by Wilhelm Ostwald to gen atom in a Üve-membered ring, account for electrical conductivities C3H3NO. of electrolyte solutions. O outer-sphere mechanism See electron-transfer reaction. overpotential (overvoltage) A po- N tential that must be applied in an Oxazole electrolytic cell in addition to the theoretical potential required to liberate a given substance at an elec- oxfuel A liquid fuel containing trode. The value depends on the added alcohols or ethers to act as an electrode material and on the cur- additional source of oxygen during rent density. It is a kinetic effect oc- combustion of the fuel. It has been curring because of the signiÜcant claimed that such additives help to activation energy for electron trans- lower the concentration of carbon o fer at the electrodes, and is particu- monoxide in engine emissions. larly important for the liberation of oxidant See oxidizing agent. such gases as hydrogen and oxygen. oxidation See oxidation– For example, in the electrolysis of a reduction. solution of zinc ions, hydrogen (EŠ = 0.00 V) would be expected to be liber- oxidation number (oxidation ated at the cathode in preference to state) See oxidation–reduction. zinc (EŠ = –0.76 V). In fact, the high oxidation–reduction (redox) Orig- overpotential of hydrogen on zinc inally, oxidation was simply regarded (about 1 V under suitable conditions) as a chemical reaction with oxygen. means that zinc can be deposited in- The reverse process – loss of oxygen stead. The relation between the cur- – was called reduction. Reaction with rent and the overpotential is given by hydrogen also came to be regarded as the *Butler–Volmer equation. reduction. Later, a more general idea overtones See harmonic. of oxidation and reduction was developed in which oxidation was loss of oxalate A salt or ester of *oxalic electrons and reduction was gain of acid. electrons. This wider deÜnition cov- oxalic acid (ethanedioic acid) A ered the original one. For example, crystalline solid, (COOH) , that is in the reaction 2 → slightly soluble in water. Oxalic acid 4Na(s) + O2(g) 2Na2O(s) is strongly acidic and very poisonous. the sodium atoms lose electrons to 393 oxidizing acid give Na+ ions and are oxidized. At the naming inorganic compounds. Thus same time, the oxygen atoms gain in H2SO4, sulphuric(VI) acid, the sul- electrons and are reduced. These phur has an oxidation number of +6. deÜnitions of oxidation and reduc- Compounds that tend to undergo re- tion also apply to reactions that do duction readily are *oxidizing agents; not involve oxygen. For instance in those that undergo oxidation are *re- → ducing agents. 2Na(s) + Cl2(g) 2NaCl(s) the sodium is oxidized and the chlo- oxidation state See oxidation– rine reduced. Oxidation and reduc- reduction. tion also occurs at the electrodes in oxidative deamination A reac- *cells. tion involved in the catabolism of Ü This de nition of oxidation and re- amino acids that assists their excre- duction applies only to reactions in tion from the body. An example which electron transfer occurs – i.e. of an oxidative deamination is to reactions involving ions. It can be the conversion of glutamate to α- extended to reactions between cova- ketoglutarate, a reaction catalysed lent compounds by using the concept by the enzyme glutamate dehydroge- of oxidation number (or state). This is nase. a measure of the electron control that an atom has in a compound oxidative decarboxylation The compared to the atom in the pure el- reaction in the *Krebs cycle in which ement. An oxidation number consists oxygen, derived from two water mol- of two parts: ecules, is used to oxidize two carbon (1) Its sign, which indicates whether atoms to two molecules of carbon the control has increased (negative) dioxide. The two carbon atoms result or decreased (positive). from the decarboxylation reactions (2) Its value, which gives the number that occur during the Krebs cycle as of electrons over which control has the six-carbon compound citrate is o changed. converted to the four-carbon com- The change of electron control may pound oxaloacetate. be complete (in ionic compounds) or oxides Binary compounds formed partial (in covalent compounds). For between elements and oxygen. Ox- example, in SO2 the sulphur has an ides of nonmetals are covalent com- oxidation number +4, having gained pounds having simple molecules (e.g. partial control over 4 electrons com- CO, CO2, SO2) or giant molecular lat- pared to sulphur atoms in pure sul- tices (e.g. SiO2). They are typically phur. The oxygen has an oxidation acidic or neutral. Oxides of metals number –2, each oxygen having lost are ionic, containing the O2– ion. partial control over 2 electrons com- They are generally basic or *ampho- pared to oxygen atoms in gaseous teric. Various other types of ionic oxygen. Oxidation is a reaction in- oxide exist (see ozonides; peroxides; volving an increase in oxidation superoxides). number and reduction involves a deoxidizing acid An acid that can act crease. Thus in as a strong oxidizing agent as well as → 2H2 + O2 2H2O an acid. Nitric acid is a common ex- the hydrogen in water is +1 and the ample. It is able to attack metals, oxygen –2. The hydrogen is oxidized such as copper, that are below hydro- and the oxygen is reduced. gen in the electromotive series, by The oxidation number is used in oxidizing the metal: oxidizing agent 394

→ 2HNO3 + Cu CuO + H2O + 2NO2 called the hydroxonium ion or the This is followed by reaction between hydronium ion. the acid and the oxide: oxo process An industrial process → 2HNO3 + CuO Cu(NO3)2 + H2O for making aldehydes by reaction between alkanes, carbon monoxide, oxidizing agent (oxidant) A sub- and hydrogen (cobalt catalyst using stance that brings about oxidation in high pressure and temperature). other substances. It achieves this by being itself reduced. Oxidizing agents oxyacetylene burner A welding contain atoms with high oxidation or cutting torch that burns a mixture numbers; that is the atoms have suf- of oxygen and acetylene (ethyne) in a Û fered electron loss. In oxidizing other specially designed jet. The ame tem- ° substances these atoms gain elec- perature of about 3300 C enables all trons. ferrous metals to be welded. For cutting, the point at which the steel is oximes Compounds containing the to be cut is preheated with the oxy- group C:NOH, formed by reaction of acetylene Ûame and a powerful jet of an aldehyde or ketone with hydroxy- oxygen is then directed onto the lamine (H2NOH) (see illustration). steel. The oxygen reacts with the hot Ethanal (CH3CHO), for example, steel to form iron oxide and the heat forms the oxime CH3CH:NOH. of this reaction melts more iron, A which is blown away by the force of • Information about IUPAC nomenclature the jet.

oxo- PreÜx indicating the presence oxycodone An opioid, C18H21N2, of oxygen in a chemical compound. similar in structure to *codeine but with a –OH group in codeine re- oxoacid An acid in which the placed by a carbonyl group. It is an acidic hydrogen is part of a hydroxyl analgesic often used for the treat- o group bound to an atom that is ment of chronic pain. It is also used bound to an oxo group (=O). Sul- illegally and is a controlled substance phuric acid is an example. Compare in most countries. hydroxoacid. oxygen Symbol O. A colourless 3-oxobutanoic acid (acetoacetic odourless gaseous element belonging acid) A colourless syrupy liquid, to *group 16 (formerly VIB) of the pe- CH COCH COOH. It is an unstable 3 2 riodic table; a.n. 8; r.a.m. 15.9994; d. compound, decomposing into 1.429 g dm–3; m.p. –218.4°C; b.p. propanone and carbon dioxide. The –183°C. It is the most abundant el- acid can be prepared from its ester, ement in the earth’s crust (49.2% by *ethyl 3-oxobutanoate. weight) and is present in the atmos- oxonium ion An ion of the type phere (28% by volume). Atmospheric + R3O , in which R indicates hydrogen oxygen is of vital importance for all or an organic group especially the organisms that carry out aerobic res- + ion H3O , which is formed when piration. For industrial purposes it is *acids dissociate in water. This is also obtained by fractional distillation of

– H O O H 2 NOH R C + NOH R C R′ H R′ ketone hydroxylamine oxime Oximes 395 ozonides liquid air. It is used in metallurgical the greenhouse gases (see green- processes, in high-temperature house effect). It is a powerful oxi- Ûames (e.g. for welding), and in dizing agent and is used to form breathing apparatus. The common ozonides by reaction with alkenes form is diatomic (dioxygen, O2); and subsequently by hydrolysis to there is also a reactive allotrope carbonyl compounds. *ozone (O ). Chemically, oxygen re- 3 ozone hole See ozone layer. acts with most other elements forming *oxides. The element was ozone layer (ozonosphere) A layer discovered by Joseph Priestley in of the *earth’s atmosphere in which 1774. most of the atmosphere’s ozone is A concentrated. It occurs 15–50 km • Information from the WebElements site above the earth’s surface and is virtually synonymous with the stratos- oxygenates Oxygen-containing phere. In this layer most of the sun’s organic compounds, such as ethanol ultraviolet radiation is absorbed by and acetone, present in motor the ozone molecules, causing a rise fuels. in the temperature of the stratos- oxyhaemoglobin See haemoglo- phere and preventing vertical mixing bin. so that the stratosphere forms a stable layer. By absorbing most of the ozonation The formation of solar ultraviolet radiation the ozone *ozone (O3) in the earth’s atmosphere. In the upper atmosphere layer protects living organisms on (stratosphere) about 20–50 km above earth. The fact that the ozone layer is the surface of the earth, oxygen mol- thinnest at the equator is believed to account for the high equatorial inci- ecules (O2) dissociate into their constituent atoms under the inÛuence of dence of skin cancer as a result of ex- ultraviolet light of short wavelength posure to unabsorbed solar o (below about 240 nm). These atoms ultraviolet radiation. In the 1980s it combine with oxygen molecules to was found that depletion of the form ozone (see ozone layer). Ozone ozone layer was occurring over both is also formed in the lower atmos- the poles, creating ozone holes. This phere from nitrogen oxides and is thought to have been caused by a other pollutants by photochemical series of complex photochemical re- reactions (see photochemical smog). actions involving nitrogen oxides produced from aircraft and, more se- ozone (trioxygen) A colourless gas, riously, *chloroÛuorocarbons (CFCs) O3, soluble in cold water and in alka- * ° ° and halons. CFCs rise to the stratos- lis; m.p. –192.7 C; b.p. –111.9 C. Liq- phere, where they react with ultravi- uid ozone is dark blue in colour and olet light to release chlorine atoms; is diamagnetic (dioxygen, O2, is para- these atoms, which are highly reac- magnetic). The gas is made by pass- tive, catalyse the destruction of ing oxygen through a silent electric ozone. Use of CFCs is now much re- discharge and is usually used in mix- duced in an effort to reverse this tures with oxygen. It is produced in human-induced damage to the ozone the stratosphere by the action of layer. high-energy ultraviolet radiation on oxygen and its presence there acts as ozonides 1. A group of compounds a screen for ultraviolet radiation (see formed by reaction of ozone with al- ozone layer). Ozone is also one of kali metal hydroxides and formally ozonolysis 396

– containing the ion O3 . 2. Unstable structure of alkenes by hydrolysing compounds formed by the addition the ozonide to give aldehydes or ke- of ozone to the C=C double bond in tones. For instance alkenes. See ozonolysis. ′→ ′ R2C:CHR R2CO + R CHO ozonolysis A reaction of alkenes These could be identiÜed, and the with ozone to form an ozonide. It structure of the original alkene deter- was once used to investigate the mined.

o P

PAGE (polyacrylamide gel electro- palmitic acid occur widely in plant phoresis) A type of *electrophore- and animal oils and fats. sis used to determine the size and pantothenic acid A vitamin of the composition of proteins. Proteins are *vitamin B complex. It is a con- placed on a matrix of polyacrylamide stituent of coenzyme A, which per- gel and an electric Üeld is applied. forms a crucial role in the oxidation The protein molecules migrate to- of fats, carbohydrates, and certain wards the positive pole, the smaller amino acids. DeÜciency rarely occurs molecules moving at a faster rate because the vitamin occurs in many through the pores of the gel. The pro- foods, especially cereal grains, peas, teins are then detected by applying a egg yolk, liver, and yeast. stain. H OH H H PAH Polyaromatic hydrocarbon. See 2 H 2 particulate matter. C N C OH OH C H palladium Symbol Pd. A soft white 2 CH CH ductile *transition element (see also 3 3 O O platinum metals); a.n. 46; r.a.m. Pantothenic acid 106.4; r.d. 12.02; m.p. 1552°C; b.p. papain 3140±1°C. It occurs in some copper A protein-digesting enzyme occurring in the fruit of the West In- and nickel ores and is used in jew- dian papaya tree (Carica papaya). It is ellery and as a catalyst for hydro- used as a digestant and in the manu- genation reactions. Chemically, it facture of meat tenderizers. does not react with oxygen at normal temperatures. It dissolves slowly in paper chromatography A tech- hydrochloric acid. Palladium is ca- nique for analysing mixtures by pable of occluding 900 times its own *chromatography, in which the sta- volume of hydrogen. It forms few tionary phase is absorbent paper. A simple salts, most compounds being spot of the mixture to be investigated complexes of palladium(II) with some is placed near one edge of the paper palladium(IV). It was discovered by and the sheet is suspended vertically in a solvent, which rises through the William Woolaston (1766–1828) in paper by capillary action carrying the 1803. components with it. The components A move at different rates, partly be- • Information from the WebElements site cause they absorb to different extents palmitate (hexadecanoate) A salt on the cellulose and partly because of or ester of palmitic acid. partition between the solvent and the moisture in the paper. The paper palmitic acid (hexadecanoic acid) is removed and dried, and the differ- A 16-carbon saturated fatty acid, ent components form a line of spots CH3(CH2)14COOH; r.d. 0.85; m.p. along the paper. Colourless sub- 63°C; b.p. 390°C. Glycerides of stances are detected by using ultra- para- 398

violet radiation or by spraying with a A membrane that is permeable to the substance that reacts to give a small molecules of water and certain coloured spot (e.g. ninhydrin gives a solutes but does not allow the pas- blue coloration with amino acids). sage of large solute molecules. This The components can be identiÜed by term is preferred to semipermeable the distance they move in a given membrane when describing biologi- time. cal membranes. See osmosis. para- 1. PreÜx designating a ben- partial pressure See dalton’s law. zene compound in which two sub- particle in a box A system in stituents are in the 1,4 positions, i.e. quantum mechanics used to illus- directly opposite each other, on the trate important features of quantum benzene ring. The abbreviation p- is mechanics, such as quantization of used; for example, p-xylene is 1,4- energy levels and the existence of ortho- dimethylbenzene. Compare ; *zero-point energy. A particle with a meta- Ü . 2. Pre x denoting the form of mass m is allowed to move between diatomic molecules in which the nu- two walls having the coordinates x = clei have opposite spins, e.g. parahy- 0 and x = L. The potential energy of hydrogen drogen (see ). Compare the particle is taken to be zero be- ortho- . tween the walls and inÜnite outside parafÜn See petroleum. the walls. The time-independent *Schrödinger equation is exactly sol- Ü alkanes paraf ns See . uble for this problem, with the ener- 2 2 2 parafÜn wax See petroleum. gies En being given by n h /8mL , n = ψ 1,2,…, and the wave functions n paraformaldehyde See methanal. ψ ½ π being given by n = (2/L) sin(n x/L), parahelium See orthohelium. where n are the quantum numbers that label the energy levels and h is parahydrogen See hydrogen. the *Planck constant. The particle in p paraldehyde See ethanal. a box can be used as a rough model for delocalized electrons in a mol- parallel spins Neighbouring spin- ecule or solid. Thus it can be used to ning electrons in which the *spins, explain the colours of molecules and hence the magnetic moments, of with conjugated double bonds. It can the electrons are aligned in the same also explain the colour imparted to direction. crystals by F-centres. The problem of paramagnetism See magnetism. a particle in a box can also be solved Paraquat Tradename for an or- in two and three dimensions, enabling relations between symmetry ganic herbicide (see dipyridyl) used and degenerate energy levels to be to control broadleaved weeds and seen. grasses. It is poisonous to humans, having toxic effects on the liver, particulate matter (PM) Matter lungs, and kidneys if ingested. present as small particles. Particulate Paraquat is not easily broken down matter, also known as particulates, is and can persist in the environment produced as an airborne pollutant in adsorbed to soil particles. See also pes- many processes. It may be inorganic, ticide. e.g. silicate or carbon, or organic, e.g. polyaromatic hydrocarbons (PAHs). parent See daughter. Particulates are classiÜed by size; e.g. partially permeable membrane PM10, with particles less than 10 µm, 399 Pasteur, Louis and PM2.5, with particles less than mathematician Blaise Pascal 2.5 µm. (1623–62). parting agent See release agent. Paschen–Back effect An effect on atomic line spectra that occurs when partition If a substance is in con- the atoms are placed in a strong mag- tact with two different phases then, netic Üeld. Spectral lines that give in general, it will have a different the anomalous *Zeeman effect when afÜnity for each phase. Part of the the atoms are placed in a weaker substance will be absorbed or dis- magnetic Üeld have a splitting pat- solved by one and part by the other, tern in a very strong magnetic Üeld. the relative amounts depending on The Paschen–Back effect is named the relative afÜnities. The substance after the German physicists Louis is said to be partitioned between the Carl Heinrich Friedrich Paschen two phases. For example, if two im- (1865–1947) and Ernest E. A. Back miscible liquids are taken and a third (1881–1959), who discovered it in compound is shaken up with them, 1912. In the quantum theory of then an equilibrium is reached in atoms the Paschen–Back effect is ex- which the concentration in one sol- plained by the fact that the energies vent differs from that in the other. of precession of the electron’s orbital The ratio of the concentrations is angular momentum l and the spin the partition coefÜcient of the sys- angular momentum s about the di- tem. The partition law states that rection of the magnetic Üeld H are this ratio is a constant for given liq- greater than the energies of coupling uids. between l and s. In the Paschen–Back partition coefÜcient See parti- effect the orbital magnetic moment tion. and the spin magnetic moment pre- partition function The quantity Z cess independently about the direc- deÜned by tion of H. Z = ∑exp(–E /kT), Paschen series See hydrogen spec- p i trum where the sum is taken over all . states i of the system. Ei is the energy passive Describing a solid that has of the ith state, k is the Boltzmann reacted with another substance to constant, and T is the thermody- form a protective layer, so that fur- namic temperature. Z is a quantity of ther reaction stops. The solid is said fundamental importance in equilib- to have been ‘rendered passive’. For rium statistical mechanics. For a sys- example, aluminium reacts sponta- tem in which there are nontrivial neously with oxygen in air to form a interactions, it is very difÜcult to cal- thin layer of *aluminium oxide, culate the partition function exactly. which prevents further oxidation. For such systems it is necessary to Similarly, pure iron forms a protec- use approximation techniques. The tive oxide layer with concentrated ni- partition function links results at the tric acid and is not dissolved further. atomic level to thermodynamics, Pasteur, Louis (1822–95) French since Z is related to the Helmholtz chemist and microbiologist, who free energy F by F = kTlnZ. held appointments in Strasbourg pascal The *SI unit of pressure (1849–54) and Lille (1854–57), before equal to one newton per square returning to Paris to the Ecole Nor- metre. It is named after the French male and the Sorbonne. From 1888 Patterson function 400

to his death he was director of the principle that no two electrons in an Pasteur Institute. In 1848 he discov- atom can have all four quantum ered *optical activity, in 1860 relat- numbers the same. It was Ürst formu- ing it to molecular structure. In 1856 lated in 1925 by Wolfgang *Pauli and he began work on fermentation, and more generally applies to the quan- by 1862 was able to disprove the exis- tum states of all elementary particles tence of spontaneous generation. He with half-integral spin. introduced pasteurization (originally for wine) in 1863. He went on to Pauling, Linus Carl (1901–94) US study disease and developed vaccines chemist. After spending two years in against cholera (1880), anthrax Europe, he became a professor at the (1882), and rabies (1885). Californian Institute of Technology. His original work was on chemical Patterson function A function, bonding; in the mid-1930s he turned denoted P(x,y,z), used in the analysis to the structure of proteins, for of the results of *X-ray crystallogra- which he was awarded the 1954 Ü phy. P(x,y,z) is de ned by Nobel Prize for chemistry. He was Σ π P(x,y,z) = |Fhkl|cos2 (hx + ky + lz), also an active campaigner against nu- hkl where h, k, and l are the Miller in- clear weapons and in 1962 was dices of the crystal. This function is awarded the Nobel Peace Prize. plotted as a contour map with max- Pauling’s rules A set of rules that ima of the contours at vector dis- enables the structures of many com- tances from the origin, i.e. vectors plex ionic crystals to be understood. from the origin to the point (x,y,z), These rules involve the sizes and where the maximum occurs, corre- electrical charges of the ions and sponding to vector distances between were proposed by Linus *Pauling in pairs of maxima in electron density. 1929. They work well for mainly Thus, the Patterson function enables ionic crystals, such as silicates, but interatomic vectors, which give the less well for crystals, such as sul- lengths and directions between p phides, in which the bonding is atomic centres, to be determined. mainly covalent. Pauling put forward This technique, introduced by A. L. his rules on the basis of empirical ob- Patterson in 1934, is called Patterson synthesis and is most useful if the servations and calculations of crystal unit cell contains heavy atoms. energies. A number of extensions and modiÜcations have been proposed. Patterson synthesis See patterson function. p-block elements The block of elements in the periodic table consist- Pauli, Wolfgang Ernst (1900–58) ing of the main groups 13 (B to Tl), Austrian-born Swiss physicist. After 14 (C to Pb), 15 (N to Bi), 16 (O to Po), studying with Niels *Bohr and Max 17 (F to At) and 18 (He to Rn). The *Born, he taught at Heidelberg and, outer electronic conÜgurations of Ü nally Zurich. His formulation in these elements all have the form 1924 of the *Pauli exclusion princi- np2npx where x = 1 to 6. Members at ple explained the electronic make-up the top and on the right of the p- of atoms. For this work he was block are nonmetals (C, N, P, O, F, S, awarded the 1945 Nobel Prize for Cl, Br, I, At). Those on the left and at physics. In 1931 he predicted the ex- the bottom are metals (Al, Ga, In, Tl, istence of the neutrino. Sn, Pb, Sb, Bi, Po). Between the two, Pauli exclusion principle The from the top left to bottom right, lie 401 pentavalent an ill-deÜned group of metalloid el- mercury. The process of Penning ion- ements (B, Si, Ge, As, Te). ization was discovered by F. M. Pen- PCB See polychlorinated biphenyl. ning in 1927. PCP See phencyclidine. Penrose tiling A method of tiling a two-dimensional plane using tiles peacock ore See bornite. with Üvefold symmetry. Since this pearl ash See potassium type of symmetry is not allowed in carbonate. crystallography, two types of tile are needed, which are called ‘fat’ and steel pearlite See . ‘skinny’. Adjacent tiles have to obey pelargonic acid See nonanoic certain matching rules. Penrose acid. tiling, named after the British mathematician and physicist Sir Roger Pen- penicillin An antibiotic derived rose (1931– ), who put forward this from the mould Penicillium notatum; idea in 1974, is the two-dimensional speciÜcally it is known as penicillin G analogue of a *quasicrystal. and belongs to a class of similar substances called penicillins. They pro- pentaerythritol A white crys- duce their effects by disrupting talline compound, C(CH2OH)4; m.p. synthesis of the bacterial cell wall, 260°C; b.p. 276°C (30 mmHg). It is and are used to treat a variety of in- used in making the explosive pen- fections caused by bacteria. taerythritol trinitrate and in producing resins and other organic H products. S N R

3CH pentaerythritol tetranitrate CH N O (PETN) A powerful high explosive 3 made from pentaerythritol, O C(CH2ONO2)4. O pentahydrate A crystalline hy- p OH drate that has Üve moles of water per Penicillin mole of compound. Penning ionization A photochem- pentane A straight-chain alkane ical process that produces a posi- hydrocarbon, C5H12; r.d. 0.63; m.p. tively charged ion. For atoms A and –129.7°C; b.p. 36.1°C. It is obtained B, the process of Penning ionization by distillation of petroleum. is written: pentanedioic acid (glutaric acid) A* + B → A + B+ + e–, A simple dicarboxylic acid, ° where A* denotes that the atom A HOOC(CH2)3COOH; m.p. 96 C; b.p. ° has absorbed a photon, thus acquir- 200 C. It is used in the production of ing enough energy for the process to certain polymers. take place, and e– is an electron. An pentanoic acid (valeric acid) A example of Penning ionization is the colourless liquid *carboxylic acid, ° ionization of mercury by argon: CH3(CH2)3COOH; r.d. 0.9; m.p. –34 C; Ar* + Hg → Ar + Hg+ + e–, b.p. 186.05°C. It is used in the per- which occurs because the energy of fume industry. the metastable state of argon is pentavalent (quinquevalent) Hav- higher than the ionization energy of ing a valency of Üve. pentlandite 402

pentlandite A mineral consisting These bonds are formed by the reac- of a mixed iron–nickel sulphide, tion between adjacent carboxyl (Fe,Ni)9S8, crystallizing in the cubic (–COOH) and amino (–NH2) groups system; the chief ore of nickel. It is with the elimination of water (see il- yellowish-bronze in colour with a lustration). Dipeptides contain two metallic lustre. The chief occurrence amino acids, tripeptides three, and so of the mineral is at Sudbury in On- on. *Polypeptides contain more than tario, Canada. ten and usually 100–300. Naturally occurring oligopeptides (of less than pentose A sugar that has Üve car- ten amino acids) include the tripep- bon atoms per molecule. See monosaccharide tide glutathione and the pituitary . hormones vasopressin and oxytocin, pentose phosphate pathway which are octapeptides. Peptides also (pentose shunt) A series of biochemi- result from protein breakdown, e.g. cal reactions that results in the con- during digestion. version of glucose 6-phosphate to A ribose 5-phosphate and generates • Information about IUPAC nomenclature NADPH, which provides reducing of peptides power for other metabolic reactions, Ü such as synthesis of fatty acids. Ri- peptide mapping (peptide nger- bose 5-phosphate and its derivatives printing) The technique of forming are components of such molecules as two-dimensional patterns of peptides ATP, coenzyme A, NAD, FAD, DNA, (on paper or gel) by partial hydrolysis and RNA. In plants the pentose phos- of a protein followed by electrophoresis and chromatography. The phate pathway also plays a role in Ü the synthesis of sugars from carbon peptide pattern (or ngerprint) pro- dioxide. In animals the pathway oc- duced is characteristic for a particu- curs at various sites, including the lar protein and the technique can be used to separate a mixture of pep- liver and adipose tissue. p tides. pentyl group (pentyl radical) The peptidoglycan A macromolecule organic group CH CH CH CH CH –, 3 2 2 2 2 that is a component of the cell wall derived from pentane. of bacteria; it is not found in eukary- pepsin An enzyme that catalyses otes. Consisting of chains of amino the breakdown of proteins to sugars (N-acetylglucosamine and N- polypeptides in the vertebrate stom- acetylmuramic acid) linked to a ach. It is secreted as an inactive pre- tripeptide (of alanine, glutamic acid, cursor, pepsinogen. and lysine or diaminopimelic acid), it peptide Any of a group of organic confers strength and shape to the cell wall. compounds comprising two or more amino acids linked by peptide bonds. per- PreÜx indicating that a chemi-

HRO HR′ O HRO HR′ O

HNCCOH+ HNCCOH HNCC NC COH + H O 2 H H H H amino acid 1 amino acid 2 dipeptide water peptide bond

Peptide bond 403 periodic table cal compound contains an excess of baum in the early 1980s. Routes to an element, e.g. a peroxide. chaos other than period doubling also exist. perchlorate See chlorates. periodic acid See iodic(vii) acid. perchloric acid See chloric(vii) acid. periodic law The principle that the physical and chemical properties perdisulphuric acid See peroxo- of elements are a periodic function of sulphuric(vi) acid. their proton number. The concept perfect gas See ideal gas; gas. was Ürst proposed in 1869 by Dimitri perfect solution See raoult’s law. *Mendeleev, using relative atomic mass rather than proton number, as pericyclic reaction A type of con- a culmination of efforts to rationalize certed chemical reaction that pro- chemical properties by Johann ceeds through a cyclic conjugated Döbereiner (1817), John Newlands transition state. Pericyclic reactions (1863), and Lothar Meyer (1864). One include *cheletropic reactions, some of the major successes of the periodic *cycloadditions, *sigmatropic reac- law was its ability to predict chemi- tions, and *electrocyclic reactions. cal and physical properties of undis- They can be understood using *fron- covered elements and unknown tier orbital theory or the *Woodward– compounds that were later conÜrmed Hoffmann rules. experimentally. See periodic table. peridot See olivine. periodic table A table of elements period 1. The time taken for one arranged in order of increasing pro- complete cycle of an oscillating sys- ton number to show the similarities tem or wave. 2. See periodic table. of chemical elements with related electronic conÜgurations. (The origi- period doubling A mechanism for nal form was proposed by Dimitri describing the transition to *chaos in Mendeleev in 1869 using relative certain dynamical systems. If the atomic masses.) In the modern short p force on a body produces a regular form, the *lanthanoids and *acti- orbit with a speciÜc *period a sudden noids are not shown. The elements increase in the force can suddenly fall into vertical columns, known as double the period of the orbit and groups. Going down a group, the the motion becomes more complex. atoms of the elements all have the The original simple motion is called same outer shell structure, but an in- a one-cycle, while the more compli- creasing number of inner shells. Tra- cated motion after the period dou- ditionally, the alkali metals were bling is called a two-cycle. The shown on the left of the table and process of period doubling can con- the groups were numbered IA to tinue until a motion called an n-cycle VIIA, IB to VIIB, and 0 (for the noble is produced. As n increases to inÜnity gases). All the elements in the middle the motion becomes non-periodic. of the table are classiÜed as *transi- The period-doubling route to chaos tion elements and the nontransition occurs in many systems involving elements are regarded as main-group nonlinearity, including lasers and elements. Because of confusion in certain chaotic chemical reactions. the past regarding the numbering of The period-doubling route to chaos groups and the designations of sub- was postulated and investigated by groups, modern practice is to num- the US physicist Mitchell Feigen- ber the groups across the table from periplanar 404

1 to 18 (see Appendix). Horizontal Mark Winter at the University of rows in the table are periods. The ShefÜeld Ürst three are called short periods; • Over 50 different forms of the periodic the next four (which include transi- table in the Chemogenesis web book by tion elements) are long periods. Mark R. Leach Within a period, the atoms of all the periplanar torsion angle elements have the same number of See . shells, but with a steadily increasing Perkin, Sir William Henry number of electrons in the outer (1838–1907) British chemist, who shell. The periodic table can also be while still a student accidentally pro- divided into four blocks depending duced mauvine, the Ürst aniline dye on the type of shell being Ülled: the and the Ürst dyestuff to be synthe- *s-block, the *p-block, the *d-block, sized. Perkin built a factory to pro- and the *f-block. duce it, and made a fortune. There are certain general features of chemical behaviour shown in the Permalloys A group of alloys of periodic table. In moving down a high magnetic permeability consist- group, there is an increase in metal- ing of iron and nickel (usually lic character because of the increased 40–80%) often with small amounts of size of the atom. In going across a pe- other elements (e.g. 3–5% molybde- riod, there is a change from metallic num, copper, chromium, or tung- (electropositive) behaviour to non- sten). They are used in thin foils in metallic (electronegative) because of electronic transformers, for magnetic the increasing number of electrons shielding, and in computer memo- in the outer shell. Consequently, ries. metallic elements tend to be those permanent gas A gas, such as oxy- on the left and towards the bottom gen or nitrogen, that was formerly of the table; nonmetallic elements thought to be impossible to liquefy. are towards the top and the right. A permanent gas is now regarded as There is also a signiÜcant differ- p one that cannot be liqueÜed by presence between the elements of the sure alone at normal temperatures second short period (lithium to Ûuorine) and the other elements in (i.e. a gas that has a critical tempera- their respective groups. This is be- ture below room temperature). cause the atoms in the second period permanent hardness See hard- are smaller and their valence elec- ness of water. trons are shielded by a small 1s2 permanganate See manga- inner shell. Atoms in the other peri- nate(vii) ods have inner s- and p-electrons . shielding the outer electrons from permonosulphuric(VI) acid See the nucleus. Moreover, those in the peroxosulphuric(vi) acid. second period only have s- and p- orbitals available for bonding. Heav- Permutit Tradename for a *zeolite ier atoms can also promote electrons used for water softening. to vacant d-orbitals in their outer perovskite structure A type of shell and use these for bonding. See ionic crystal structure shown by com- also diagonal relationship; inert- pounds of the type ABO3. There is a pair effect. cubic arrangement with A atoms sur- A rounded by 12 O atoms and B atoms • The WebElements table produced by surrounded by 6 O atoms. Examples 405 PESM of the perovskite structure include (POP) A toxic substance that is not CaTiO3, BaTiO3, and SrTiO3. biodegradable and persists in the en- A vironment. POPs can concentrate as • An interactive version of the structure they move up the food chain and are generally deleterious to health. In peroxides A group of inorganic 2– 2001, an international conference in compounds that contain the O2 ion. Sweden resulted in an agreement They are notionally derived from hy- (the Stockholm Convention) to reduce drogen peroxide, H O , but these 2 2 or eliminate the 12 POPs of greatest ions do not exist in aqueous solution concern, and to add other chemicals due to extremely rapid hydrolysis to to the list in the future. The 12 chem- OH–. A icals were the pesticides aldrin, chlordane, *DDT, dieldrin, endrin, • Information about IUPAC nomenclature heptachlor, mirex, and toxaphene, peroxodisulphuric acid See per- together with the industrial chemical oxosulphuric(vi) acid. hexachlorobenzene and the groups *polychlorinated biphenyls, *diox- peroxomonosulphuric(VI) acid ins, and *furans. See peroxosulphuric(vi) acid. A peroxosulphuric(VI) acid The •OfÜcial Stockholm Convention site term commonly refers to peroxomonosulphuric(VI) acid, H2SO5, Perspex Tradename for a form of which is also called permonosul- *polymethylmethacrylate. phuric(VI) acid and Caro’s acid. It is a perturbation theory A method crystalline compound made by the used in calculations in both classical action of hydrogen peroxide on con- physics (e.g. planetary orbits) and centrated sulphuric acid. It decom- quantum mechanics (e.g. atomic poses in water and the crystals structure), in which the system is di- decompose, with melting, above vided into a part that is exactly calcu- 45°C. The compound peroxodisul- p lable and a small term, which phuric acid, H S O , also exists (for- 2 2 8 prevents the whole system from merly called perdisulphuric acid). It is made by the high-current electrolysis being exactly calculable. The tech- of sulphate solutions. It decomposes nique of perturbation theory enables ° the effects of the small term to be at 65 C (with melting) and is hydrol- Ü ysed in water to give the mono acid calculated by an in nite series (which and sulphuric acid. Both peroxo acids in general is an asymptotic series). are very powerful oxidizing agents. Each term in the series is a ‘correc- See also sulphuric acid (for structural tion term’ to the solutions of the ex- formulas). actly calculable system. In classical physics, perturbation theory can be persistent Describing a pesticide used for calculating planetary orbits. or other pollutant that is not readily In quantum mechanics, it can be broken down and can persist for long used to calculate the energy levels in periods, causing damage in the envi- molecules. ronment. For example, the herbicides *Paraquat and *DDT can persist PES See photoelectron spec- in the soil for many years after their troscopy. application. PESM See photoelectron spectro- persistent organic pollutant microscopy. pesticide 406

pesticide Any chemical compound chemical and chemical precipitates, used to kill pests that destroy agricul- in shallow depressions, chieÛy in ma- tural production or are in some way rine conditions. Under burial and harmful to humans. Pesticides in- compaction the organic matter went clude herbicides (such as *2,4-D and through a series of processes before *Paraquat), which kill unwanted being transformed into petroleum, plants or weeds; insecticides (such as which migrated from the source rock pyrethrum), which kill insect pests; to become trapped in large under- fungicides, which kill fungi; and ro- ground reservoirs beneath a layer of denticides (such as *warfarin), which impermeable rock. The petroleum kill rodents. The problems associated often Ûoats above a layer of water with pesticides are that they are very and is held under pressure beneath a often nonspeciÜc and may there- layer of *natural gas. fore be toxic to organisms that are Petroleum reservoirs are discov- not pests; they may also be non- ered through geological exploration: biodegradable, so that they persist in commercially important oil reserves the environment and may accumu- are detected by exploratory narrow- late in living organisms (see bioaccu- bore drilling. The major known re- mulation). Organophosphorus serves of petroleum are in Saudi insecticides, such as malathion and Arabia, Russia, China, Kuwait, Iran, parathion, are biodegradable but can Iraq, Mexico, USA, United Arab Emi- also damage the respiratory and ner- rates, Libya, and Venezuela. The oil is vous systems in humans as well as actually obtained by the sinking of killing useful insects, such as bees. an oil well. Before it can be used it is They act by inhibiting the action of separated by fractional distillation in the enzyme cholinesterase. Organo- oil reÜneries. The main fractions ob- chlorine insecticides, such as dield- tained are: rin, aldrin, and *DDT, are very (1) ReÜnery gas A mixture of persistent and not easily biodegrad- methane, ethane, butane, and p able. propane used as a fuel and for mak- Ü ing other organic chemicals. peta- Symbol P. A pre x used in the (2) Gasoline A mixture of hydrocar- metric system to denote one thou- bons containing 5 to 8 carbon atoms, sand million million times. For exam- ° 15 boiling in the range 40–180 C. It is ple, 10 metres = 1 petametre (Pm). used for motor fuels and for making petrochemicals Organic chemicals other chemicals. obtained from petroleum or natural (3) Kerosine (or parafÜn oil) A mix- gas. ture of hydrocarbons having 11 or 12 carbon atoms, boiling in the range petrolatum See petroleum jelly. 160–250°C. Kerosine is a fuel for jet petroleum A naturally occurring aircraft and for oil-Üred domestic oil that consists chieÛy of hydrocar- heating. It is also cracked to produce bons with some other elements, such smaller hydrocarbons for use in as sulphur, oxygen, and nitrogen. In motor fuels. its unreÜned form petroleum is (4) Diesel oil (or gas oil) A mixture known as crude oil (sometimes rock of hydrocarbons having 13 to 25 oil). Petroleum is believed to have carbon atoms, boiling in the range been formed from the remains of liv- 220–350°C. It is a fuel for diesel ening organisms that were deposited, gines. together with rock particles and bio- The residue is a mixture of higher 407 phase transition hydrocarbons. The liquid compo- uid, and gaseous *phases over a nents are obtained by vacuum distil- range of conditions (e.g. temperature lation and used in lubricating oils. and pressure). See steel (illustra- The solid components (parafÜn wax) tion). are obtained by solvent extraction. phase rule For any system at equi- The Ünal residue is a black tar con- librium, the relationship P + F = C + 2 taining free carbon (asphalt or bitu- holds, where P is the number of dis- men). tinct phases, C the number of compo- petroleum ether A colourless nents, and F the number of degrees volatile Ûammable mixture of hydro- of freedom of the system. The rela- carbons (not an ether), mainly pen- tionship derived by Josiah Willard tane and hexane. It boils in the range *Gibbs in 1876, is often called the 30–70°C and is used as a solvent. Gibbs phase rule. petroleum jelly (petrolatum) A phase space For a system with n semi-solid mixture of hydrocarbons degrees of freedom, the 2n-dimen- extracted from petroleum. It is used sional space with coordinates (q1, q2, in skincare products and cosmetics, …, qn, p1, p2, …, pn), where the qs de- and as a lubricant. Petroleum is scribe the degrees of freedom of the widely available under the trade- system and the ps are the corre- name Vaseline. sponding momenta. Each point rep- pewter An alloy of lead and tin. It resents a state of the system. In a gas usually contains 63% tin; pewter of N point particles, each particle has tankards and food containers should three positional coordinates and have less than 35% of lead so that the three corresponding momentum co- lead remains in solid solution with ordinates, so that the phase space the tin in the presence of weak acids has 6N-dimensions. If the particles in the food and drink. Copper is have internal degrees of freedom, sometimes added to increase ductil- such as the vibrations and rotations p ity and antimony is added if a hard of molecules, then these must be in- alloy is required. cluded in the phase space, which is consequently of higher dimension peyote See mescaline. than that for point particles. As the Pfund series See hydrogen spec- system changes with time the repre- trum. sentative points trace out a curve in phase space known as a trajectory. phosphoglyceric acid PGA See . See also attractor; configuration pH See ph scale. space; statistical mechanics. phane See cyclophane. phase transition A change in a feature that characterizes a system. phase A homogeneous part of a Examples of phase transitions are heterogeneous system that is sepa- changes from solid to liquid, liquid to rated from other parts by a distin- gas, and the reverse changes. Other guishable boundary. A mixture of ice examples of phase transitions in- and water is a two-phase system. A clude the transition from a paramag- solution of salt in water is a single- net to a ferromagnet (see magnetism) phase system. and the transition from a normally phase diagram A graph showing conducting metal to a superconduc- the relationship between solid, liq- tor. Phase transitions can occur by al- phencyclidine 408

tering such variables as temperature thermosetting materials. *Bakelite is and pressure. the original example. Phase transitions can be classiÜed phenolic resins Synthetic resins by their order. If there is non-zero made by copolymerizing phenols *latent heat at the transition it is with aldehydes. *Phenol–formalde- said to be a Ürst-order transition. If hyde resins are the commonest type. the latent heat is zero it is said to be a second-order transition. phenolphthalein A dye used as an Some models describing phase acid-base *indicator. It is colourless transitions, particularly in low- below pH 8 and red above pH 9.6. It dimensional systems, are amenable is used in titrations involving weak to exact mathematical solutions. acids and strong bases. It is also used Techniques for investigating transi- as a laxative. tions may include the feature of uni- phenolphthalein test See kastle versality, in which very different meyer test. physical systems behave in the same way near a phase transition. phenols Organic compounds that contain a hydroxyl group (–OH) phencyclidine (PCP) A hallucino- bound directly to a carbon atom in a genic drug, originally used as a veti- benzene ring. Unlike normal alco- nary anaesthetic. It is usually used as hols, phenols are acidic because of a powder (known as ‘angel dust’). the inÛuence of the aromatic ring. Thus, phenol itself (C6H5OH) ionizes in water: → – + C6H5OH C6H5O + H N Phenols are made by fusing a sulphonic acid salt with sodium hydroxide to form the sodium salt of the phenol. The free phenol is liberated p by adding sulphuric acid. phenomenological relations See Phencyclidine onsager relations. phenol (carbolic acid) A white crys- phenoxy resins Thermoplastic

talline solid, C6H5OH; r.d. 1.1; m.p. materials made by condensation of 43°C; b.p. 182°C. It is made by the phenols, used mainly for packaging. *cumene process or by the *Raschig phenylalanine See amino acid. process. Phenol reacts readily to form phenylamine (aniline; aminoben- substituted derivatives. It is an im- zene) A colourless oily liquid aro- portant industrial chemical used in matic *amine, C6H5NH2, with an making Nylon, phenolic and epoxy ‘earthy’ smell; r.d. 1.0217; m.p. resins, dyestuffs, explosives, and –6.3°C; b.p. 184.1°C. The compound pharmaceuticals. See also phenols. turns brown on exposure to sunlight. It is basic, forming the phenylammo- phenol–formaldehyde resin A + nium (or anilinium) ion, C6H5NH3 , class of resins produced by polymer- with strong acids. It is manufactured izing phenols with formaldehyde by the reduction of nitrobenzene or (methanol) using acid or basic cata- by the addition of ammonia to lysts. They are generally cross-linked chlorobenzene using a copper(II) salt 409 phlogiston theory catalyst at 200°C and 55 atm. The phenyl methyl ketone (aceto- compound is used extensively in the phenone) A colourless, crystalline rubber industry and in the manufac- ketone with an odour of bitter al- ° ture of drugs and dyes. monds, C6H5COCH3, m.p. 20 C. It is made by treating benzene with phenylammonium ion The ion + ethanoyl chloride in the presence of C6H5NH3 , derived from *pheny- aluminium chloride. It is used as a lamine. solvent and in making perfumes. ac- N-phenylethanamide See 3-phenylpropenoic acid See cin- etanilide . namic acid. di- phenylenediamine See Phillips process A process for aminobenzene . making high-density polyethene by phenylethene (styrene) A liquid polymerizing ethene at high pressure (30 atmospheres) and 150°C. The hydrocarbon, C6H5CH:CH2; r.d. 0.9; m.p. –31°C; b.p. 145°C. It can be catalyst is chromium(III) oxide sup- made by dehydrogenating ethylben- ported on silica and alumina. zene and is used in making poly- phlogiston theory A former styrene. theory of combustion in which all Û phenyl group The organic group ammable objects were supposed C H –, present in benzene. to contain a substance called phlogis- 6 5 ton, which was released when the phenylhydrazine A toxic colour- object burned. The existence of this less dense liquid, C6H5NHNH2, b.p. hypothetical substance was proposed 240°C, which turns brown on expo- in 1669 by Johann Becher, who called sure to air. It is a powerful reducing it ‘combustible earth’ (terra pinguis: agent, made from *diazonium salts literally ‘fat earth’). For example, ac- of benzene. It is used to identify alde- cording to Becher, the conversion of hydes and ketones, with which it wood to ashes by burning was ex- forms condensation products called plained on the assumption that the p *hydrazones. With glucose and simi- original wood consisted of ash and lar sugars it forms osazones. For terra pinguis, which was released on such tests, the nitro derivative 2,4- burning. In the early 18th century dinitrophenylhydrazine (DNP) is Georg Stahl renamed the substance often preferred as this generally phlogiston (from the Greek for forms crystalline derivatives that can ‘burned’) and extended the theory to be identiÜed by their melting points. include the calcination (and corro- Phenylhydrazine is also used to make sion) of metals. Thus, metals were dyes and derivatives of *indole. thought to be composed of calx (a powdery residue) and phlogiston; phenylhydrazones See hydra- when a metal was heated, phlogiston zones. was set free and the calx remained. phenylmethanol (benzyl al- The process could be reversed by cohol) A liquid aromatic alcohol, heating the metal over charcoal (a C H CH OH; r.d. 1.04; m.p. –15.3°C; substance believed to be rich in phlo- 6 5 2 giston, because combustion almost b.p. 205.4°C. It is used mainly as a totally consumed it). The calx would solvent. absorb the phlogiston released by the phenylmethylamine See benzyl- burning charcoal and become metal- amine. lic again. phonochemistry 410

The theory was Ünally demolished polyphosphates and cross-linked by Antoine Lavoisier, who showed by polyphosphates or ultraphosphates careful experiments with reactions are known. in closed containers that there was phosphatides See phospholipids. no absolute gain in mass – the gain in mass of the substance was phosphide A binary compound of matched by a corresponding loss in phosphorus with a more electroposi- mass of the air used in combustion. tive element. Phosphides show a After experiments with Priestley’s de- wide range of properties. Alkali and phlogisticated air, Lavoisier realized alkaline earth metals form ionic that this gas, which he named oxy- phosphides, such as Na3P and Ca3P2, gen, was taken up to form a calx which are readily hydrolysed by (now called an oxide). The role of water. The other transition-metal oxygen in the new theory was almost phosphides are inert metallic-looking exactly the opposite of phlogiston’s solids with high melting points and role in the old. In combustion and electrical conductivities. corrosion phlogiston was released; in phosphine A colourless highly the modern theory, oxygen is taken ° toxic gas, PH3; m.p. –133 C; b.p. up to form an oxide. –87.7°C; slightly soluble in water. phonochemistry The use of high- Phosphine may be prepared by react- frequency sound (ultrasound, with a ing water or dilute acids with cal- frequency greater than about 20 kHz) cium phosphide or by reaction to induce or accelerate certain types between yellow phosphorus and con- of chemical reaction. centrated alkali. Solutions of phosphine are neutral but phosphine phosgene See carbonyl chloride. does react with some acids to give + phosphagen A compound found phosphonium salts containing PH4 in animal tissues that provides a re- ions, analogous to the ammonium serve of chemical energy in the form ions. Phosphine prepared in the labo- p of high-energy phosphate bonds. The ratory is usually contaminated with most common phosphagens are diphosphine and is spontaneously creatine phosphate, occurring in ver- Ûammable but the pure compound is tebrate muscle and nerves, and argi- not so. Phosphine can function as a nine phosphate, found in most ligand in binding to transition-metal invertebrates. During tissue activity ions. Dilute gas mixtures of very pure (e.g. muscle contraction) phospha- phosphine and the rare gases are gens give up their phosphate groups, used for doping semiconductors. thereby generating *ATP from ADP. phosphinic acid (hypophosphorus The phosphagens are then reformed acid) A white crystalline solid, when ATP is available. ° H3PO2; r.d. 1.493; m.p. 26.5 C; decom- phosphates Salts based formally poses above 130°C. It is soluble in on phosphorus(V) oxoacids and in water, ethanol, and ethoxyethane. particular salts of *phosphoric(V) Salts of phosphinic acid may be pre- acid, H3PO4. A large number of poly- pared by boiling white phosphorus meric phosphates also exist, contain- with the hydroxides of group I or ing P–O–P bridges. These are formed group II metals. The free acid is made by heating the free acid and its salts by the oxidation of phosphine with under a variety of conditions; as well iodine. It is a weak monobasic acid in as linear polyphosphates, cyclic which it is the –O–H group that is 411 phosphor

– phosphonic ionized to give the ion H2PO2 . The phosphonate See acid and its salts are readily oxidized acid. to the orthophosphate and conse- phosphonic acid (phosphorous quently are good reducing agents. acid; orthophosphorous acid) A phosphite See phosphonic acid. colourless to pale-yellow deliquescent crystalline solid, H PO ; r.d. phosphodiester bond The cova- 3 3 1.65; m.p. 73.6°C; decomposes at lent bond that links a phosphate 200°C; very soluble in water and sol- group and a sugar group, by means uble in alcohol. Phosphonic acid may of an oxygen bridge, in the sugar– be crystallized from the solution ob- phosphate backbone of a nucleic acid tained by adding ice-cold water to molecule. See dna. phosphorus(III) oxide or phosphorus phosphoglyceric acid (PGA; trichloride. The structure of this ma- 3-phosphoglycerate) See glycerate terial is unusual in that it contains 3-phosphate. one direct P–H bond and is more cor- phospholipids (phosphatides) rectly written (HO)2HPO. The acid is A – group of lipids having both a phos- dibasic, giving rise to the ions H2PO3 2– phate group and one or more fatty and HPO3 (phosphonates; formerly acids. Glycerophospholipids (or phos- phosphites), and has moderate reduc- phoglycerides) are based on *glyc- ing properties. On heating it gives erol; the three hydroxyl groups are phosphine and phosphoric(V) acid. Ü + esteri ed with two fatty acids and a phosphonium ion The ion PH4 , phosphate group, which may itself or the corresponding organic deriva- + + be bound to one of a variety of sim- tives of the type R3PH , RPH3 . The + ple organic groups (see illustration). phosphonium ion PH4 is formally Sphingolipids are based on the alco- analogous to the ammonium ion + hol sphingosine and contain only one NH4 but PH3 has a much lower pro- fatty acid linked to an amino group. Ü ton af nity than NH3 and reaction of p With their hydrophilic polar phos- PH3 with acids is necessary for the phate groups and long hydrophobic production of phosphonium salts. hydrocarbon ‘tails’, phospholipids phosphonium salts See phospho- readily form membrane-like struc- nium ion. tures in water. They are a major component of cell membranes. phosphor A substance that is ca-

fatty acid

H C (CH ) C O CH 3 2 14 2

H CC(CH ) C (CH ) C O C H 3 2 7 H H 2 7 O O CH 3 H C O P O CH CH N CH fatty acid 2 2 2 3 – CH O 3

glycerol phosphate group choline

Structure of phosphatidylcholine (lecithin), a glycerophospholipid phosphor bronze 412

pable of *luminescence (including acter respectively and phosphate ions phosphorescence). Phosphors that re- often feature in buffer systems. lease their energy after a short delay There is also a wide range of higher of between 10–10 and 10–4 second are acids and acid anions in which there sometimes called scintillators. is some P–O–P chain formation. The phosphor bronze An alloy of cop- simplest of these is pyrophosphoric per containing 4% to 10% of tin and acid (technically heptaoxodiphos- 0.05% to 1% of phosphorus as a deoxi- phoric(V) acid), H4P2O7, produced by dizing agent. It is used particularly heating phosphoric(V) acid (solid) and for marine purposes and where it is phosphorus(III) chloride oxide. exposed to heavy wear, as in gear Metaphosphoric acid is a glassy poly- wheels. See also bronze. meric solid (HPO2)x. phosphonic phosphorescence See lumines- phosphorous acid See acid cence. . phosphoric(V) acid (orthophos- phosphorus Symbol P. A non- phoric acid) A white rhombic solid, metallic element belonging to ° *group 15 (formerly VB) of the peri- H3PO4; r.d. 1.834; m.p. 42.35 C; loses water at 213°C; very soluble in water odic table; a.n. 15; r.a.m. 30.9738; r.d. ° and soluble in ethanol. Phosphoric(V) 1.82 (white), 2.34 (red); m.p. 44.1 C α ° α acid is very deliquescent and is gen- ( -white); b.p. 280 C ( -white). It oc- erally supplied as a concentrated curs in various phosphate rocks, aqueous solution. It is the most com- from which it is extracted by heating mercially important derivative of with carbon (coke) and silicon(IV) ° phosphorus, accounting for over 90% oxide in an electric furnace (1500 C). of the phosphate rock mined. It is Calcium silicate and carbon monox- manufactured by two methods; the ide are also produced. Phosphorus wet process, in which the product has a number of allotropic forms. α contains some of the impurities origi- The -white form consists of P4 tetra- p nally present in the rock and applica- hedra (there is also a β-white form tions are largely in the fertilizer stable below –77°C). If α-white phos- industry, and the thermal process, phorus is dissolved in lead and which produces a much purer prod- heated at 500°C a violet form is ob- uct suitable for the foodstuffs and de- tained. Red phosphorus, which is a tergent industries. In the wet process combination of violet and white the phosphate rock, Ca3(PO4)2, is phosphorus, is obtained by heating treated with sulphuric acid and the α-white phosphorus at 250°C with air calcium sulphate removed either as excluded. There is also a black al- gypsum or the hemihydrate. In the lotrope, which has a graphite-like thermal process, molten phosphorus structure, made by heating white is sprayed and burned in a mixture phosphorus at 300°C with a mercury of air and steam. Phosphoric(V) acid catalyst. The element is highly reac- is a weak tribasic acid, which is best tive. It forms metal *phosphides and visualized as (HO)3PO. Its full system- covalently bonded phosphorus(III) atic name is tetraoxo-phosphoric(V) and phosphorus(V) compounds. Phos- acid. It gives rise to three series of phorus is an *essential element for salts containing phosphate(V) ions living organisms. It is an important – based on the anions [(HO)2PO2] , constituent of tissues (especially 2– 3– [(HO)PO3] , and PO4 . These salts are bones and teeth) and of cells, being acidic, neutral, and alkaline in char- required for the formation of nucleic 413 phosphorus(III) chloride oxide acids and energy-carrying molecules ether and in carbon tetrachloride but (e.g. ATP) and also involved in various reacts with water and with ethanol. metabolic reactions. The element It may be prepared by passing chlo- was discovered by Hennig Brand rine over excess phosphorus (excess (c. 1630–92) in 1669. chlorine contaminates the product A with phosphorus(V) chloride). The • Information from the WebElements site molecule is pyramidal in the gas phase and possesses weak electron- phosphorus(III) bromide (phos- pair donor properties. It is hy- phorus tribromide) A colourless fum- drolysed violently by water to ° ing liquid, PBr3; r.d. 2.85; m.p. –40 C; phosphonic acid and hydrogen chlo- b.p. 173°C. It is prepared by passing ride. Phosphorus(III) chloride is an bromine vapour over phosphorus but important starting point for the syn- avoiding an excess, which would lead thesis of a variety of inorganic and to the phosphorus(V) bromide. Like organic derivatives of phosphorus. the other phosphorus(III) halides, phosphorus(V) chloride (phos- PBr3 is pyramidal in the gas phase. In the liquid phase the P–Br bonds are phorus pentachloride) A yellow- white rhombic solid, PCl5, which labile; for example, PBr3 will react ° with PCl to give a mixture of prod- fumes in air; r.d. 4.65; m.p. 166.8 C 3 (under pressure); sublimes at ucts in which the halogen atoms ° have been redistributed. Phospho- 160–162 C. It is decomposed by rus(III) bromide is rapidly hydrolysed water to give hydrogen chloride and by water to give phosphonic acid and phosphoric(V) acid. It is soluble in or- hydrogen bromide. It reacts readily ganic solvents. The compound may with many organic hydroxyl groups be prepared by the reaction of chlo- and is used as a reagent for introduc- rine with phosphorus(III) chloride. ing bromine atoms into organic mol- Phosphorus(V) chloride is structurally ecules. interesting in that in the gas phase it has the expected trigonal bipyrami- phosphorus(V) bromide (phos- dal form but in the solid phase it con- p + – phorus pentabromide) A yellow sists of the ions [PCl4] [PCl6] . The readily sublimable solid, PBr5, which same ions are detected when phos- decomposes below 100°C and is solu- phorus(V) chloride is dissolved in ble in benzene and carbon tetrachlo- polar solvents. It is used in organic ride (tetrachloromethane). It may be chemisty as a chlorinating agent. prepared by the reaction of phospho- phosphorus(III) chloride oxide rus(III) bromide with bromine or the (phosphorus oxychloride; phosphoryl direct reaction of phosphorus with chloride) A colourless fuming liquid, excess bromine. It is very readily hy- POCl ; r.d. 1.67; m.p. 2°C; b.p. drolysed to give hydrogen bromide 3 105.3°C. It may be prepared by the and phosphoric(V) acid. An interest- reaction of phosphorus(III) chloride ing feature of this material is that in with oxygen or by the reaction of the solid state it has the structure phosphorus(V) oxide with phospho- [PBr ]+Br–. It is used in organic chem- 4 rus(V) chloride. Its reactions are very istry as a brominating agent. similar to those of phosphorus(III) phosphorus(III) chloride (phos- chloride. Hydrolysis with water gives phorus trichloride) A colourless fum- phosphoric(V) acid. Phosphorus(III) ing liquid, PCl3; r.d. 1.57; m.p. chloride oxide has a distorted tetra- –112°C; b.p. 75.5°C. It is soluble in hedral shape and can act as a donor phosphorus cycle 414

towards metal ions, thus giving rise phosphorus atom is linked to the to a series of complexes. three others by an oxygen bridge. The chemistry is very complex. phosphorus cycle The cycling of Above 210°C it decomposes into red phosphorus between the biotic and phosphorus and polymeric oxides. It abiotic components of the environ- reacts with chlorine and bromine to ment. Inorganic phosphates (PO 3–, 4 give oxo-halides and with alkalis to HPO 2–, or H PO –) are absorbed by 4 2 4 give phosphonates (see phosphonic plants from the soil and bodies of acid). water and eventually pass into animals through food chains. Within liv- phosphorus(V) oxide (phosphorus ing organisms phosphates are built pentoxide; phosphoric anhydride) A up into nucleic acids and other or- white powdery and extremely deli- ganic molecules. When plants and quescent solid, P4O10; r.d. 2.39; m.p. animals die, phosphates are released 580°C (under pressure); sublimes at and returned to the abiotic environ- 300°C. It reacts violently with water ment through the action of bacteria. to give phosphoric(V) acid. It is pre- On a geological time scale, phos- pared by burning elemental phosphates in aquatic environments even- phorus in a plentiful supply of tually become incorporated into and oxygen, then puriÜed by sublimation. form part of rocks; through a gradual The hexagonal crystalline form con- process of erosion, these phosphates sists of P4O10 molecular units; these are returned to the soil, seas, rivers, have the phosphorus atoms arranged and lakes. Phosphorus-containing tetrahedrally, each P atom linked to rocks are mined for the manufacture three others by oxygen bridges and of fertilizers, which provide an addi- having in addition one terminal oxy- tional supply of inorganic phosphate gen atom. The compound is used as a to the abiotic environment. drying agent and as a dehydrating agent; for example, amides are con- phosphorus(III) oxide (phospho- verted into nitrites and sulphuric p rus trioxide) A white or colourless acid is converted to sulphur trioxide. waxy solid, P4O6; r.d. 2.13; m.p. 23.8°C; b.p. 173.8°C. It is soluble in phosphorus oxychloride See ether, chloroform, and benzene but phosphorus(iii) chloride oxide. reacts with cold water to give phos- phosphorus pentabromide See phonic acid, H3PO3, and with hot phosphorus(v) bromide. water to give phosphine and phosphoric(V) acid. The compound is phosphorus pentachloride See formed when phosphorus is burned phosphorus(v) chloride. in an oxygen-deÜcient atmosphere phosphorus tribromide See phos- (about 50% yield). As it is difÜcult to phorus(iii) bromide. separate from white phosphorus by phos- distillation, the mixture is irradiated phosphorus trichloride See phorus(iii) chloride with ultraviolet radiation to convert . excess white phosphorus into the red phosphorus trioxide See phos- form, after which the oxide can be phorus(iii) oxide. separated by dissolution in organic phosphoryl chloride See phospho- solvents. Although called a trioxide rus(iii) chloride oxide. for historical reasons, phosphorus(III) oxide consists of P4O6 molecules of photoacoustic spectroscopy A tetrahedral symmetry in which each spectroscopic technique in which 415 photoelectron spectromicroscopy spectra of opaque materials, such as ceeds the *work function, φ, of the φ powders, are obtained by exposing solid, i.e. if hf0 = an electron will be the material to light that is modu- ejected; f0 is the minimum frequency lated at acoustic frequencies. This (or threshold frequency) at which produces a photoacoustic signal, the ejection will occur. For many solids magnitude of which can be related to the photoelectric effect occurs at ul- the ultraviolet or infrared absorption traviolet frequencies or above, but coefÜcient of the material. See also for some materials (having low work optoacoustic spectroscopy. functions) it occurs with light. The maximum kinetic energy, E , of the photochemical reaction A chem- m photoelectron is given by *Einstein’s ical reaction caused by light or ultra- equation: E = hf – φ. violet radiation. The incident m Apart from the liberation of elec- photons are absorbed by reactant trons from atoms, other phenomena molecules to give excited molecules are also referred to as photoelectric or free radicals, which undergo fur- effects. These are the photoconduc- ther reaction. tive effect and the photovoltaic ef- photochemical smog A noxious fect. In the photoconductive effect, smog produced by the reaction of ni- an increase in the electrical conduc- trogen oxides with hydrocarbons in tivity of a semiconductor is caused by the presence of ultraviolet light from radiation as a result of the excitation the sun. The reaction is very complex of additional free charge carriers by and one of the products is ozone. the incident photons. Photoconduc- tive cells, using such photosensitive photochemistry The branch of materials as cadmium sulphide, are chemistry concerned with *photo- widely used as radiation detectors chemical reactions. and light switches (e.g. to switch on photochromism A change of col- street lighting). our occurring in certain substances In the photovoltaic effect, an e.m.f. when exposed to light. Photochromic is produced between two layers of p materials are used in sunglasses that different materials as a result of irra- darken in bright sunlight. diation. The effect is made use of in photoconductive effect See pho- photovoltaic cells, most of which toelectric effect. consist of p–n semiconductor junctions. When photons are absorbed photoelectric effect The libera- near a p–n junction new free charge tion of electrons from a substance ex- carriers are produced (as in photo- posed to electromagnetic radiation. conductivity); however, in the photo- The number of such electrons (pho- voltaic effect the electric Üeld in the toelectrons) emitted depends on the junction region causes the new intensity of the radiation. The kinetic charge carriers to move, creating a energy of the electrons emitted de- Ûow of current in an external circuit pends on the frequency of the radia- without the need for a battery. tion. The effect is a quantum process photoelectron in which the radiation is regarded as An electron emit- a stream of *photons, each having an ted from a substance by irradiation as a result of the *photoelectric ef- energy hf, where h is the Planck con- fect or *photoionization. stant and f is the frequency of the radiation. A photon can only eject an photoelectron spectromicros- electron if the photon energy ex- copy (PESM) A technique for inves- photoelectron spectroscopy 416

tigating the composition of surfaces, on specially treated Ülm or paper. In based on ionization of atoms at the normal black-and-white photography surfaces. Ionizing radiation (such as a camera is used to expose a Ülm or X-rays or ultraviolet radiation) is used plate to a focused image of the scene to eject electrons from atoms in the for a speciÜed time. The Ülm or plate surface. The ejected electrons are is coated with an emulsion contain- then focused, thus enabling an image ing silver salts and the exposure to of the surface to be constructed. light causes the silver salts to break photoelectron spectroscopy down into silver atoms; where the light is bright dark areas of silver are (PES) A technique for determining formed on the Ülm after develop- the *ionization potentials of mol- ment (by a mild reducing agent) and ecules. In ultraviolet photoelectron Üxing. The negative so formed is spectroscopy (UPS) the sample is a printed, either by a contact process gas or vapour irradiated with a nar- or by projection. In either case light row beam of ultraviolet radiation passing through the negative Ülm (usually from a helium source at 58.4 falls on a sheet of paper also coated nm, 21.21 eV photon energy). The with emulsion. Where the negative is photoelectrons produced in accor- dark, less light passes through and dance with *Einstein’s equation are the resulting positive is light in this passed through a slit into a vacuum area, corresponding with a light area region, where they are deÛected by in the original scene. As photo- magnetic or electrostatic Üelds to graphic emulsions are sensitive to ul- give an energy spectrum. The photo- traviolet and X-rays, they are widely electron spectrum obtained has used in studies involving these forms peaks corresponding to the ioniza- of electromagnetic radiation. See also tion potentials of the molecule (and colour photography. hence the orbital energies). The technique also gives information on the photoionization The *ionization vibrational energy levels of the ions of an atom or molecule as a result of p formed. X-ray photoelectron spec- irradiation by electromagnetic radia- troscopy (XPS), also known as ESCA tion. For a photoionization to occur (electron spectroscopy for chemical the incident photon of the radiation analysis), is a similar analytical tech- must have an energy in excess of the nique in which a beam of X-rays is *ionization potential of the species used. In this case, the electrons being irradiated. The ejected photo- ejected are from the inner shells of electron will have an energy, E, given the atoms. Peaks in the electron spec- by E = hf – I, where h is the Planck trum for a particular element show constant, f is the frequency of the in- characteristic chemical shifts, which cident radiation, and I is the ioniza- depend on the presence of other tion potential of the irradiated atoms in the molecule. See also koop- species. mans’ theorem. photoluminescence See lumines- photoemission The process in cence. which electrons are emitted by a sub- photolysis A chemical reaction stance as a result of irradiation. See produced by exposure to light or ul- photoelectric effect; photoioniza- traviolet radiation. Photolytic reaction. tions often involve free radicals, the photography The process of form- Ürst step being homolytic Üssion of a ing a permanent record of an image chemical bond. (See flash photoly- 417 photosynthesis sis.) The photolysis of water, using magnetic radiation produces a chem- energy from sunlight absorbed by ical change. chlorophyll, produces gaseous oxy- photosynthesis The chemical gen, electrons, and hydrogen ions process by which green plants syn- and is a key reaction in *photosyn- thesize organic compounds from thesis. carbon dioxide and water in the pres- photon A particle with zero rest ence of sunlight. It occurs in the mass consisting of a *quantum of chloroplasts (most of which are in electromagnetic radiation. The pho- the leaves) and there are two princi- ton may also be regarded as a unit of pal types of reactions. In the light- energy equal to hf, where h is the dependent reactions, which require the presence of light, energy from *Planck constant and f is the fre- sunlight is absorbed by *photosyn- quency of the radiation in hertz. Pho- thetic pigments (chieÛy the green tons travel at the speed of light. They pigment *chlorophyll) and used to are required to explain the photo- bring about the photolysis of water: electric effect and other phenomena → + – ½ that require light to have particle H2O 2H + 2e + O2. character. The electrons released by this reaction pass along a series of electron photosensitive substance 1. Any carriers (see electron transport substance that when exposed to elec- chain); as they do so they lose their tromagnetic radiation produces a energy, which is used to convert ADP photoconductive, photoelectric, or to ATP in the process of photophos- photovoltaic effect. 2. Any sub- phorylation. The electrons and pro- stance, such as the emulsion of a tons produced by the photolysis of photographic Ülm, in which electro- water are used to reduce NADP:

Light-dependent reactions Light-independent reactions ADP NADP+ p ATP electrons ADP + e– e– 2H NADPH+H 2H ATP Calvin cycle glyceraldehyde electron + water transport 2H 3-phosphate system H O glycerate Noncyclic 2 3C 3-phosphate Cyclic photophos- ATP photophos- phorylation CO phorylation – Photolysis 2 2OH H O of water 2 carbon ADP dioxide ATP 2OH chlorophyll ADP ribulose bisphosphate 1 O 2 2 5C ATP oxygen ADP light

glucose

Photosynthesis photosynthetic pigments 418

2H+ + 2e– + NADP+ → NADPH + H+. has a hydrogen-ion concentration of –7 –3 The ATP and NADPH produced dur- 10 mol dm , so the pH is 7. A pH ing the light-dependent reactions below 7 indicates an acid solution; provide energy and reducing power, one above 7 indicates an alkaline so- respectively, for the ensuing light- lution. More accurately, the pH de- independent reactions (formerly pends not on the concentration of called the ‘dark reaction’), which hydrogen ions but on their *activity, nevertheless cannot be sustained which cannot be measured experi- without the ATP generated by the mentally. For practical purposes, the Ü light-dependent reactions. During pH scale is de ned by using a hydro- these reactions carbon dioxide is regen electrode in the solution of inter- duced to carbohydrate in a metabolic est as one half of a cell, with a pathway known as the *Calvin cycle. reference electrode (e.g. a calomel Photosynthesis can be summarized electrode) as the other half cell. The by the equation: pH is then given by (E – ER)F/2.303RT, → where E is the e.m.f. of the cell and CO2 + 2H2O [CH2O] + H2O + O2. ER the standard electrode potential of Since virtually all other forms of the reference electrode, and F the life are directly or indirectly depend- Faraday constant. In practice, a glass ent on plants for food, photosynthe- electrode is more convenient than a sis is the basis for all life on earth. hydrogen electrode. Furthermore virtually all the atmos- pH stands for ‘potential of hydro- pheric oxygen has originated from gen’. The scale was introduced by oxygen released during photosynthe- Søren Sørensen (1868–1939) in 1909. sis. phthalic acid A colourless photosynthetic pigments The crystalline dicarboxylic acid, ° plant pigments responsible for the C6H4(COOH)2; r.d. 1.6; m.p. 207 C. capture of light energy during the The two –COOH groups are substi- light-dependent reactions of *photo- tuted on adjacent carbon atoms of p synthesis. The green pigment the ring, the technical name being *chlorophyll is the principal light re- benzene-1,2-dicarboxylic acid. The ceptor, absorbing blue and red light. acid is made from phthalic anhydride pH scale A logarithmic scale for ex- (benzene-1,2-dicarboxylic anhydride, pressing the acidity or alkalinity of a C8H4O3), which is made by the cata- solution. To a Ürst approximation, lytic oxidation of naphthalene. The the pH of a solution can be deÜned as anhydride is used in making plasticizers and polyester resins. –log10c, where c is the concentration of hydrogen ions in moles per cubic phthalic anhydride See phthalic decimetre. A neutral solution at 25°C acid.

O O

OH O OH

O O phthalic acid phthalic anhydride Phthalic acid 419 pi orbital phthalic ester An ester of phthalic by donation of an electron pair be- acid. Phthalic esters are made by re- tween a pi orbital and a sigma or pi acting phthalic anhydride with an al- orbital. cohol in the presence of sulphuric piano stool See sandwich com- acid. They are high boiling-point liq- pound uids used as plasticizers and in the . manufacture of polymers and alkyd pi bond See orbital. resins. The dimethyl ester is used as pico- Symbol p. A preÜx used in the an insect repellent. metric system to denote 10–12. For ex- phthalocyanine A synthetic com- ample, 10–12 farad = 1 picofarad (pF). pound having molecules with four isoindole rings linked by –N= bridges. picrate A salt or ester of picric acid. The structure is similar to that of the picric acid (2,4,6-trinitrophenol) A *porphyrins. It can form complexes yellow highly explosive nitro com- with central metal ions. Copper pound, C6H2(NO2)3; r.d. 1.8; m.p. phthalocyanines are used as dyes. 122°C. pi electron An electron in a pi orbital. See orbital. N pig iron The impure form of iron produced by a blast furnace, which is NH N cast into pigs (blocks) for converting at a later date into cast iron, steel, N N etc. The composition depends on the ores used, the smelting procedure, N NH and the use to which the pigs will later be put.

N pinacol rearrangement A reaction in which the diol pinacol, (CH3)2 p COH(CH3)2COH, converts to the ke- Phthalocyanine tone pinacolone, CH3COC(CH3)3, under acid conditions, with loss of a physical chemistry The branch of molecule of water. A methyl group chemistry concerned with the effect moves from one carbon atom to an of chemical structure on physical adjacent one in order to stabilize properties. It includes chemical ther- an intermediate carbocation. The modynamics and electrochemistry. reaction, also called the pinacol- adsorption pinacolone rearrangement, gives its physisorption See . name to a class of similar rearrange- phytostigmine See eserine. ments. pi-adduct An adduct that is formed pi orbital See orbital. Pinacol rearrangement piperidine 420

piperidine A saturated heterocyclic troleum residue (asphalt). Pitch is compound having a nitrogen atom in used as a binding agent (e.g. in road Ü a six-membered ring, C5H11N; r.d. tars), for waterproo ng (e.g. in 0.86; m.p. –7°C; b.p. 106°C. The struc- rooÜng felts), and as a fuel. ture is present in many alkaloids pitchblende See uraninite. H N pK value A measure of the CH CH strength of an acid on a logarithmic 2 2 scale. The pK value is given by

2CH CH2 log10(1/Ka), where Ka is the acid disso- C ciation constant. pK values are often H 2 used to compare the strengths of dif- Piperidine ferent acids. Planck, Max Karl Ernst Ludwig pipette A graduated tube used for (1858–1947) German physicist, who transferring measured volumes of became a professor at Berlin Univer- liquid or, sometimes, gases. sity in 1892. Here he formulated the * Pirani gauge An instrument used quantum theory, which had its basis in a paper of 1900. One of the to measure low pressures (1–10–4 most important scientiÜc discoveries torr; 100–0.01 Pa). It consists of an of the century, this work earned him electrically heated Ülament, which is the 1918 Nobel Prize for physics. exposed to the gas whose pressure is to be measured. The extent to which Planck constant Symbol h. The heat is conducted away from the Üla- fundamental constant equal to the ment depends on the gas pressure, ratio of the energy of a quantum of which thus controls its equilibrium energy to its frequency. It has the temperature. Since the resistance of value 6.626 0755(40) × 10–34 J s. It is the Ülament is dependent on its tem- named after Max Planck. In quan- p perature, the pressure is related to tum-mechanical calculations the ra- the resistance of the Ülament. The tionalized Planck constant (or Dirac Ülament is arranged to be part of a constant) ប = h/2π = 1.054 589 × Wheatstone bridge circuit and the 10–34 Js is frequently used. pressure is read from a microamme- plane-polarized light See polar- ter calibrated in pressure units. As ization of light. the effect depends on the thermal conductivity of the gas, the calibra- plaster of Paris The hemihydrate tion has to be made each time the of *calcium sulphate, 2CaSO4.H2O, prepared by heating the mineral gyp- pressure of a different gas is meas- Ü ured. sum. When ground to a ne powder and mixed with water, plaster of pirsonnite A mineral consisting Paris sets hard, forming interlocking of a hydrated mixed carbonate of crystals of gypsum. The setting re- sodium and calcium, Na2CO3.CaCO3. sults in an increase in volume and so 2H2O. the plaster Üts tightly into a mould. It pitch A black or dark-brown residue is used in pottery making, as a cast resulting from the distillation of coal for setting broken bones, and as a tar, wood tar, or petroleum (bitu- constituent of the plaster used in the men). The term is also sometimes building industry. used for the naturally occurring pe- plasticizer A substance added to a 421 Platonic hydrocarbons synthetic resin to make it Ûexible. See ium or rhodium). It is also a hy- plastics. drogenation catalyst. The element does not oxidize nor dissolve in plastics Materials that can be hydrochloric acid. Most of its com- shaped by applying heat or pressure. Most plastics are made from poly- pounds are platinum(II) or meric synthetic *resins, although a platinum(IV) complexes. few are based on natural substances A (e.g. cellulose derivatives or shellac). • Information from the WebElements site They fall into two main classes. Ther- platinum black Black Ünely di- moplastic materials can be repeatedly vided platinum metal produced by softened by heating and hardened vacuum evaporation and used as an again on cooling. Thermosetting ma- absorbent and a catalyst. See also terials are initially soft, but change ir- adams catalyst. reversibly to a hard rigid form on heating. Plastics contain the syn- platinum metals The three mem- thetic resin mixed with such addi- bers of the second and third transi- tives as pigments, plasticizers (to tion series immediately proceeding improve Ûexibility), antioxidants and silver and gold: ruthenium (Ru), other stabilizers, and Üllers. See rhodium (Rh), and palladium (Pd); Chronology overleaf. and osmium (Os), iridium (Ir), and platinum (Pt). These elements, to- plastocyanin A blue copper- gether with iron, cobalt, and nickel, containing protein that is found were formerly classed as group VIII in chloroplasts and acts as an elec- of the periodic table. The platinum- tron carrier molecule in the light- group metals are relatively hard and dependent reactions of *photo- resistant to corrosion and are used in synthesis. Plastocyanin consists of jewellery and in some industrial ap- amino acid groups in association plications (e.g. electrical contacts). with a copper molecule which gives They have certain chemical similari- this compound a blue colour. ties that justify classifying them to- p plastoquinone A quinone, found gether. All are resistant to chemical in chloroplasts, that functions as attack. In solution they form a vast one of the carrier molecules of range of complex ions. They also the electron transport chain in the form coordination compounds with light-dependent reactions of *photo- carbon monoxide and other pi- synthesis. bonding ligands. A number of complexes can be made in which a hy- platinum Symbol Pt. A silvery drogen atom is linked directly to the white metallic *transition element metal. The metals and their organic (see also platinum metals); a.n. 78; compounds have considerable cata- r.a.m. 195.09; r.d. 21.45; m.p. 1772°C; lytic activity. See also transition el- b.p. 3827±100°C. It occurs in some ements. nickel and copper ores and is also found native in some deposits. The Platonic hydrocarbons Hydrocar- main source is the anode sludge ob- bons that have the same molecular tained in copper–nickel reÜning. The geometry as the Üve Platonic solids, element is used in jewellery, labora- i.e. the tetrahedron, the cube, the oc- tory apparatus (e.g. thermocouples, tahedron, the dodecahedron and the electrodes, etc.), electrical contacts, icosahedron. These hydrocarbons and in certain alloys (e.g. with irid- have carbon atoms at the vertices of 422

PLASTICS

1851 Scottish chemist Charles Macintosh (1766–1843) makes ebonite (from rubber). 1855 British chemist Alexander Parkes (1813–90) patents Parkesine, a plastic made from nitrocellulose, methanol, and wood pulp; it is later called ‘celluloid’. 1860 British chemist Charles Williams (1829–1910) prepares isoprene (synthetic rubber). 1868 US printer John Hyatt (1837–1920) develops commercial process for making celluloid. 1884 French chemist Hilaire de Chardonnet (1839–1924) develops process for making rayon. 1892 British chemists Edward Bevan (1856–1921) and Charles Cross (1855–1935) develop the viscose process for making rayon. 1899 British chemist Frederick Kipping (1863–1949) discovers silicone plastics. 1901 German chemists Krische and Spitteler make formaldehyde–casein plastic (Galalith). 1905 Belgian-born US chemist Leo Baekland (1863–1944) invents Bakelite. 1912 Swiss chemist Jacques Brandenberger produces Cellophane (viscose cellulose film). 1913 US Formica Insulation company markets plastic laminate made from formaldehyde resins. 1918 Hans John prepares urea–formaldehyde resin. 1926 German chemist Hermann Staudinger (1881–1965) discovers the polymeric nature of plastics. p 1930 US chemist Waldo Semon develops PVC (polyvinyl chloride). 1930 Canadian chemist William Chalmers discovers polymethylmethacrylate (Perspex and Plexiglass). 1930 German chemists at IG Farbenindustrie produce polystyrene. 1931 Wallace Carothers invents nylon. 1938 US chemist Roy Plunkett produces polytetrafluoroethene (PTFE). 1939 British company ICI develops commercial process for making polyethene. 1941 British chemists John Whinfield (1901–66) and J. Dickson develop Terylene (Dacron). 1941 German company IG Farbenindustrie produces polyurethane. 1943 US Dow Corning company produces silicone plastics. 1947 British chemists produce acrylic fibres. 1953 German chemist Karl Ziegler (1896–1973) discovers catalyst for making high-density polyethene. 1954 Italian chemist Giulio Natta (1903–79) develops industrial process for making high-density polyethene (using Ziegler catalyst). 1989 Italian company Ferruzzi produces biodegradable plastic (based on starch). 423 pnictogens

– → the polyhedra and single bonds along PbO(s) + OH (aq) + H2O(l) 2– the edges. In fact, there are only two Pb(OH)3 (aq) known Platonic hydrocarbons: Plumbate(IV) compounds were for- cubane (C8H8) and dodecahedrane merly referred to as orthoplumbates 4– 2– (C20H20). The hydrocarbon based on a (PbO4 ) or metaplumbates (PbO3 ). tetrahedron (C4H4) does not exist be- Plumbate(II) compounds were called cause of angle strain, but substituted plumbites. derivatives C X are known. Angle 4 4 plumbic compounds Compounds strain is also the reason for the of lead in its higher (+4) oxidation nonexistence of octahedrane (C6H6) state; e.g. plumbic oxide is lead(IV) and icosahedrane (C12H12). oxide, PbO2. pleochroic Denoting a crystal that plumbite See plumbate. appears to be of different colours, depending on the direction from which plumbous compounds Com- it is viewed. It is caused by polariza- pounds of lead in its lower (+2) oxidation of light as it passes through an tion state; e.g. plumbous oxide is lead(II) oxide, PbO. anisotropic medium. plutonium Symbol Pu. A dense sil- plumbago See carbon. very radioactive metallic transuranic plumbane (lead(IV) hydride) An ex- element belonging to the *actinoids; tremely unstable gas, PbH4, said to be a.n. 94; mass number of most stable formed by the action of acids on isotope 244 (half-life 7.6 × 107 years); magnesium–lead alloys. It was Ürst r.d. 19.84; m.p. 641°C; b.p. 3232°C. reported in 1924, although doubts Thirteen isotopes are known, by far have since been expressed about the most important being pluto- × 4 the existence of the compound. It nium–239 (half-life 2.44 10 years), Ü demonstrates the declining stability which undergoes nuclear ssion with of the hydrides in group 14. More sta- slow neutrons and is therefore a vital ble organic derivatives are known; power source for nuclear weapons p and some nuclear reactors. About 20 e.g. trimethyl plumbane, (CH ) PbH. 3 3 tonnes of plutonium are produced plumbate A compound formed by annually by the world’s nuclear reac- reaction of lead oxides (or hydrox- tors. The element was Ürst produced ides) with alkali. The oxides of lead by Seaborg, McMillan, Kennedy, and are amphoteric (weakly acidic) and Wahl in 1940. react to give plumbate ions. With the A lead(IV) oxide, reaction with molten • Information from the WebElements site alkali gives the plumbate(IV) ion PM See particulate matter. – → 2– PbO2 + 2OH PbO3 + H2O pnicogens See pnictogens. In fact, various ions are present in which the lead is bound to hydroxide pnictides See pnictogens. groups, the principal one being the pnictogens (pnicogens) The el- hexahydroxoplumbate(IV) ion ements of group 15 of the periodic 2– Pb(OH)6 . This is the negative ion table, i.e. nitrogen (N), arsenic (As), present in crystalline ‘trihydrates’ of antimony (Sb), and bismuth (Bi). The the type K2PbO3.3H2O. Lead(II) oxide elements As, Sb, and Bi form a range gives the trihydroxoplumbate(II) ion of solid-state ternary compounds in alkaline solutions with interesting electrical and mag- Pockels cell 424

netic properties. Examples are LaSiP3, polar compound A compound K6Bi2Sn23, and Yb14MnSb11. Com- that is either ionic (e.g. sodium chlo- pounds of this type are known as ride) or that has molecules with a pnictides. large permanent dipole moment (e.g. water). Pockels cell An electro-optical device used to produce population in- polarimeter (polariscope) An inversion in lasers and a pulse of strument used to determine the radiation. In a Pockels cell, crystals of angle through which the plane of po- ammonium dihydrogenphosphate larization of plane-polarized light is are used to convert plane-polarized rotated on passing through an opti- light to circularly polarized light cally active substance. Essentially, a when a potential difference is ap- polarimeter consists of a light source, plied. A Pockels cell can be made a polarizer (e.g. a sheet of Polaroid) part of a laser cavity. When this hap- for producing plane-polarized light, a pens, a change of polarization occurs transparent cell containing the sam- when light is reÛected from a mirror ple, and an analyser. The analyser is and the light that is polarized in one a polarizing material that can be ro- plane is converted into light polar- tated. Light from the source is plane- ized in the perpendicular plane. The polarized by the polarizer and passes effect is that the reÛected light does through the sample, then through not stimulate emission. When the the analyser into the eye or onto a Pockels cell is turned off the polariza- light-detector. The angle of polarization does not occur and the energy in tion is determined by rotating the the cavity can be released in the analyser until the maximum transform of a pulse of stimulated radia- mission of light occurs. The angle of tion. See also laser. rotation is read off a scale. Simple portable polarimeters are used for es- point group A group of symmetry timating the concentrations of sugar elements that leave a point un- solutions in confectionary manufac- p changed, used in classifying mol- ture. ecules. Compare space group. polariscope (polarimeter) A device poise A *c.g.s. unit of viscosity used to study optically active sub- equal to the tangential force in dynes stances (see optical activity). The per square centimetre required to simplest type of instrument consists maintain a difference in velocity of of a light source, collimator, polar- one centimetre per second between izer, and analyser. The specimen two parallel planes of a Ûuid sepa- is placed between polarizer and rated by one centimetre. 1 poise is analyser, so that any rotation of the equal to 10–1 Nsm–2. plane of polarization of the light can poison 1. Any substance that is in- be assessed by turning the analyser. jurious to the health of a living or- polarizability Symbol α. A meas- ganism. 2. A substance that prevents ure of the response of a molecule to the activity of a catalyst. 3. A sub- an external electric Üeld. When a stance that absorbs neutrons in a nu- molecule is placed in an external clear reactor and therefore slows electric Üeld, the displacement of down the reaction. It may be added electric charge induces a dipole in intentionally for this purpose or may the molecule. If the electric Üeld be formed as a Üssion product and strength is denoted E and the electri- need to be periodically removed. cal dipole moment induced by this 425 polarography electric Üeld p, the polarizability α is the direction of propagation de- deÜned by p = αE. To calculate the scribes an ellipse. Circularly and el- polarizability from Ürst principles it liptically polarized light are produced is necessary to use the quantum me- using a retardation plate. chanics of molecules. However, if polarizer A device used to plane- regarded as a parameter, the polariz- polarization of α polarize light (see ability provides a link between light). *Nicol prisms or *Polaroid microscopic and macroscopic theo- can be used as polarizers. If a polar- ries as in the *Clausius–Mossotti izer is placed in front of a source of equation and the *Lorentz–Lorenz unpolarized light, the transmitted equation. light is plane-polarized in a speciÜc polarization 1. See polarization direction. As the human eye is un- of light. 2. The formation of prod- able to detect that light is polarized, ucts of the chemical reaction in a it is necessary to use an *analyser to *voltaic cell in the vicinity of the detect the direction of polarization. electrodes resulting in increased re- Crossing a polarizer and analyser sistance to current Ûow and, fre- causes extinction of the light, i.e. if quently, to a reduction in the e.m.f. the plane of polarization of the polar- of the cell. See also depolarization. izer and the plane of the analyser are 3. The partial separation of electric perpendicular, no light is transmitted charges in an insulator subjected to when the polarizer and analyser are an electric Üeld. 4. The separation of combined. Both a polarizer and an charge in a polar *chemical bond. analyser are components of a *polarimeter. polarization of light The process of conÜning the vibrations of the polar molecule A molecule that electric vector of light waves to one has a dipole moment; i.e. one in direction. In unpolarized light the which there is some separation of electric Üeld vibrates in all directions charge in the *chemical bonds, so perpendicular to the direction of that one part of the molecule has a p propagation. After reÛection or trans- positive charge and the other a nega- mission through certain substances tive charge. (see polaroid) the electric Üeld is polarography An analytical tech- conÜned to one direction and the ra- nique having an electrochemical diation is said to be plane-polarized basis. A dropping-mercury electrode light. The plane of plane-polarized is used as the cathode along with a light can be rotated when it passes large nonpolarizable anode, and a di- through certain substances (see opti- lute solution of the sample. The drop- cal activity). ping-mercury electrode consists of a In circularly polarized light, the tip narrow tube through which mercury of the electric vector describes a cir- is slowly passed into the solution so cular helix about the direction of as to form small drops at the end of propagation with a frequency equal the tube, which fall away. In this way to the frequency of the light. The the cathode can have a low surface magnitude of the vector remains con- area and be kept clean. A variable po- stant. In elliptically polarized light, tential is applied to the cell and a the vector also rotates about the di- plot of current against potential (a rection of propagation but the ampli- polarogram) made. As each chemical tude changes; a projection of the species is reduced at the cathode (in vector on a plane at right angles to order of their electrode potentials) a Polaroid 426

step-wise increase in current is ob- centrated in food chains. Other forms tained. The height of each step is proof pollution in the environment in- portional to the concentration of the clude noise (e.g. from jet aircraft, component. The technique is useful trafÜc, and industrial processes) and for detecting trace amounts of metals thermal pollution (e.g. the release of and for the investigation of solvated excessive waste heat into lakes or complexes. rivers causing harm to wildlife). Re- cent pollution problems include the Polaroid A doubly refracting ma- disposal of radioactive waste; *acid terial that plane-polarizes unpolar- rain; *photochemical smog; increas- ized light passed through it. It ing levels of human waste; high lev- consists of a plastic sheet strained in els of carbon dioxide and other a manner that makes it birefringent greenhouse gases in the atmosphere by aligning its molecules. Sunglasses (see greenhouse effect); damage to incorporating a Polaroid material ab- the *ozone layer by nitrogen oxides, sorb light that is vibrating horizon- Û Û *chloro uorocarbons (CFCs), and tally – produced by re ection from *halons; and pollution of inland wa- horizontal surfaces – and thus reduce ters by agricultural *fertilizers and glare. sewage efÛuent, causing eutrophica- polar solvent See solvent. tion. Attempts to contain or prevent pollution include strict regulations pollutant Any substance, produced concerning factory emissions, the and released into the environment as use of smokeless fuels, the banning a result of human activities, that has of certain pesticides, the increasing damaging effects on living organ- use of lead-free petrol, restrictions on isms. Pollutants may be toxic sub- the use of chloroÛuorocarbons, and * stances (e.g. pesticides) or natural the introduction, in some countries, constituents of the atmosphere (e.g. of catalytic converters to cut pollu- carbon dioxide) that are present in tants in car exhausts. excessive amounts. See pollution. p polonium Symbol Po. A rare radio- pollution An undesirable change in active metallic element of group 16 the physical, chemical, or biological (formerly VIB) of the periodic table; characteristics of the natural environ- a.n. 84; r.a.m. 210; r.d. 9.32; m.p. ment, brought about by man’s activi- 254°C; b.p. 962°C. The element oc- ties. It may be harmful to human or curs in uranium ores to an extent of nonhuman life. Pollution may affect about 100 micrograms per 1000 kilo- the soil, rivers, seas, or the atmos- grams. It has over 30 isotopes, more phere. There are two main classes than any other element. The longest- of *pollutants: those that are bio- lived isotope is polonium–209 (half- degradable (e.g. sewage), i.e. can life 103 years). Polonium has be rendered harmless by natural attracted attention as a possible heat processes and need therefore cause source for spacecraft as the energy no permanent harm if adequately released as it decays is 1.4 × 105 dispersed or treated; and those that Jkg–1 s–1. It was discovered by Marie are nonbiodegradable (e.g. *heavy Curie in 1898 in a sample of pitch- metals (such as lead) in industrial blende. efÛuents and *DDT and other chlori- A nated hydrocarbons used as pesti- • Information from the WebElements site cides), which eventually accumulate in the environment and may be con- poly- PreÜx indicating a polymer, 427 polyethene e.g. polyethene. Sometimes brackets cern because they have been shown are used in polymer names to in- to accumulate in the food chain. dicate the repeated unit, e.g. polychloroethene (PVC; polyvinyl poly(ethene). chloride) A tough white solid ma- polyacrylamide A polymer terial, which softens with the appli- formed from 2-propenamide cation of a plasticizer, manufactured (CH2:CHCONH2). Polyacrylamide gels from chloroethene by heating in an are made using a cross-linking agent inert solvent using benzoyl peroxide to form three-dimensional matrices. as an initiator, or by the free-radical They are used in gel electrophoresis mechanism initiated by heating (see page). chloroethene in water with potassium persulphate or hydrogen perox- polyacrylamide gel electro- ide. The polymer is used in a variety page phoresis See . of ways, being easy to colour and re- polyamide A type of condensation sistant to Üre, chemicals, and polymer produced by the interaction weather. of an amino group of one molecule polycyclic Denoting a compound and a carboxylic acid group of an- that has two or more rings in its mol- other molecule to give a protein-like ecules. Polycyclic compounds may structure. The polyamide chains are contain single rings (as in phenylben- linked together by hydrogen bond- zene, C6H5.C6H5) or fused rings (as in ing. naphthalene, C10H8). polyatomic molecule A molecule polydioxoboric(III) acid See boric formed from several atoms (e.g. pyri- acid. dine, C H N, or dinitrogen tetroxide, 5 5 polyene An unsaturated hydrocar- N O ), as distinguished from di- 2 4 bon that contains two or more dou- atomic and monatomic molecules. ble carbon–carbon bonds in its polybasic acid An acid with more molecule. p than one replaceable hydrogen atom. polyester A condensation polymer Examples include the dibasic sul- formed by the interaction of polyhy- phuric acid (H2SO4) and tribasic phos- dric alcohols and polybasic acids. phoric(V) (orthophosphoric) acid Linear polyesters are saturated ther- (H3PO4). Replacement of all the hy- moplastics and linked by dipole– drogens by metal atoms forms nor- dipole attraction as the carbonyl mal salts. If not all the hydrogens are groups are polarized. They are exten- replaced, *acid salts are formed. sively used as Übres (e.g. Terylene). polychlorinated biphenyl (PCB) Unsaturated polyesters readily Any of a number of derivatives of copolymerize to give thermosetting biphenyl (C H C H ) in which some products. They are used in the manu- 6 5 6 5 Ü of the hydrogen atoms on the ben- facture of glass- bre products. See also alkyd resin zene rings have been replaced by . chlorine atoms. Polychlorinated polyethene (polyethylene; poly- biphenyls are used in the manufac- thene) A Ûexible waxy translucent ture of certain polymers as electrical polyalkene thermoplastic made in a insulators. They are highly toxic and variety of ways producing a polymer are suspected to be carcinogenic; of varying characteristics. In the ICI their increasing use has caused con- process, ethene containing a trace of polyethylene 428

oxygen is subjected to a pressure in polymorphism The existence of excess of 1500 atmospheres and a chemical substances in two (dimor- temperature of 200°C. Low-density phism) or more physical forms. See polyethene (r.d. 0.92) has a formula allotropy. weight between 50 000 and 300 000, polyol See polyhydric alcohol. softening at a temperature around 110°C, while the high-density poly- polypeptide A *peptide compris- thene (r.d. 0.945–0.96) has a formula ing ten or more amino acids. weight up to 3 000 000, softening Polypeptides that constitute proteins around 130°C. The low-density poly- usually contain 100–300 amino acids. mer is less crystalline, being more at- Shorter ones include certain antibiot- actic. Polyethene is used as an ics, e.g. gramicidin, and some hor- insulator; it is acid resistant and is mones, e.g. ACTH, which has 39 easily moulded and blown. See amino acids. The properties of a polypeptide are determined by the phillips process; ziegler process. type and sequence of its constituent polyethylene See polyethene. amino acids. polyhydric alcohol (polyol) An polypropene (polypropylene) An *alcohol that has several hydroxyl isotactic polymer existing in both groups per molecule. low and high formula-weight forms. The lower-formula-weight polymer is polymer A substance having large made by passing propene at moder- molecules consisting of repeated ate pressure over a heated phos- units (the monomers). See Feature phoric acid catalyst spread on an pp. 430–431. inert material at 200°C. The reaction A yields the trimer and tetramer. The • Information about IUPAC nomenclature higher-formula-weight polymer is polymerization A chemical reac- produced by passing propene into an inert solvent, heptane, which con- tion in which molecules join to- p tains a trialkyl aluminium and a tita- gether to form a polymer. If the nium compound. The product is a reaction is an addition reaction, the mixture of isotactic and atactic process is addition polymerization; polypropene, the former being the condensation reactions cause con- major constituent. Polypropene is densation polymerization, in which a used as a thermoplastic moulding small molecule is eliminated during material. the reaction. Polymers consisting of a single monomer are homopolymers; polypropylene See polypropene. those formed from two different polysaccharide Any of a group of monomers are copolymers. carbohydrates comprising long polymethanal A solid polymer of chains of monosaccharide (simple- methanal, formed by evaporation of sugar) molecules. Homopolysaccha- an aqueous solution of methanal. rides consist of only one type of monosaccharide; heteropolysaccha- polymethylmethacrylate A clear rides contain two or more different thermoplastic acrylic material made types. Polysaccharides may have mo- by polymerizing methyl meth- lecular weights of up to several mil- acrylate. The technical name is lion and are often highly branched. poly(methyl 2-methylpropenoate). It Some important examples are starch, is used in such materials as Perspex. glycogen, and cellulose. 429 potash polystyrene A clear glasslike ma- (PVA) by hydrolysis with sodium hy- terial manufactured by free-radical droxide. It is used for making adhe- polymerization of phenylethene sives that are miscible with water, as (styrene) using benzoyl peroxide as a size for textiles, and for making an initiator. It is used as both a ther- synthetic Übres. mal and electrical insulator and for polyvinyl chloride See poly- packing and decorative purposes. chloroethene. sulphides polysulphides See . polyyne An unsaturated hydrocar- polytetraÛuoroethene (PTFE) A bon that contains two or more triple thermosetting plastic with a high carbon–carbon bonds in its molecule. ° softening point (327 C) prepared POP See persistent organic pollu- by the polymerization of tetra- tant. Ûuoroethene under pressure (45–50 laser atmospheres). The reaction requires population inversion See . an initiator, ammonium peroxosul- porphyrin Any of a group of or- phate. The polymer has a low coefÜ- ganic pigments characterized by the cient of friction and its ‘anti-stick’ possession of a cyclic group of four properties are probably due to its he- linked nitrogen-containing rings (a lical structure with the Ûuorine tetrapyrrole nucleus), the nitrogen atoms on the surface of an inner ring atoms of which are often coordinated of carbon atoms. It is used for coating to metal ions. Porphyrins differ in cooking utensils and nonlubricated the nature of their side-chain groups. bearings. They include the *chlorophylls, which contain magnesium; and polythene See polyethene. *haem, which contains iron and polythionate A salt of a poly- forms the *prosthetic group of thionic acid. haemoglobin, myoglobin, and the cy- polythionic acids Oxo acids of sul- tochromes. Porphyrins are also used in host–guest chemistry. See phur with the general formula p supramolecular chemistry. HO.SO2.Sn.SO2.OH, where n = 0–4. See also sulphuric acid. polyurethane A polymer containing the urethane group –NH.CO.O–, prepared by reacting di-isocyanates NH N with appropriate diols or triols. A wide range of polyurethanes can be made, and they are used in adhesives, durable paints and varnishes, N NH plastics, and rubbers. Addition of water to the polyurethane plastics turns them into foams. Porphyrin polyvinylacetate (PVA) A thermoplastic polymer used in adhesives positronium exotic atom and coatings. It is made by polymer- See . izing vinyl acetate (CH2:CHCOOCH3). potash Any of a number of potas- polyvinyl alcohol A water-soluble sium compounds, such as the carbon- polymer made from polyvinylacetate ate or the hydroxide. 430

POLYMERS

Polymers are substances that have *macromolecules composed of many repeating units (known as ‘mers’). A large number of naturally occurring substances are polymers including *rubber and many substances based on glucose, such as the polysaccharides *cellulose and *starch (in plants) and *glycogen (in animals). *Proteins, nucleic acids, and inorganic macromolecular substances, such as *silicates, are other examples. Synthetic polymers One of the unique features of the chemistry of carbon is its ability to form long chains of atoms. This property is the basis of an important area of industrial chemistry concerned with the manufacture of polymeric materials with a variety of properties (see plastics). The molecules in these materials are essentially long chains of atoms of various lengths. In some polymers, cross-linkage occurs between the chains. Synthetic polymers are formed by chemical reactions in which individual molecules (monomers) join together to form larger units (see polymerization). Two types of polymer, homopolymers and heteropolymers, can be distinguished Homopolymers These are polymers formed from a single monomer. An example is *polyethene (polyethylene), which is made by polymerization of ethene (CH2:CH2). Typically such polymers are formed by *addition reactions involving unsaturated molecules. Other similar examples are *polypropene (polypropylene), polystyrene, and *polytetrafluoroethene (PTFE). Homopolymers may also be made by *condensation reactions (as in the case of *polyurethane).

HH H H H H H H p CC C C C C C C

HH H H H H H H

Addition polymerization of ethene to form polyethene: a homopolymer

O O

H N (CH ) NH + C (CH ) C 2 2 6 2 2 4 HO OH 1,6-diaminohexane hexanedioic acid

H HO OH H

N(CH) NC(CH ) CN(CH ) N n H O+ 2 6 2 4 2 6 2

Condensation polymerization to form nylon: a heteropolymer

Formation of polymers 431

Heteropolymers These are also known as copolymers. They are made from two (or more) different monomers, which usually undergo a condensation reaction with the elimination of a simple molecule, such as water. A typical example is the condensation of 1,6-diaminohexane (hexamethylenediamine) with hexanedioic acid (adipic acid) to form nylon 6,6. The reaction occurs between the amine groups on the diaminohexane and the carboxyl groups on the hexanedioic acid, with elimination of water molecules (see diagram opposite). The properties of a polymeric plastic can most easily be modified if it is a copolymer of two or more different monomers. A well-known example is ABS (acrylonitrile–butadiene–styrene) copolymer, commonly used for the body shells of computers and other electronic apparatus. Its properties can be preselected by varying the proportions of the component monomers. Stereospecific polymers In both normal polyethene and nylon the polymer molecules take the form of long chains of various lengths with no regular arrangement of the subunits. Such polymers are said to be atactic. If the constituent subunits repeat along the chain in a regular way, a stereospecific polymer may result. The polymer may be isotactic, with a particular group always along the same side of the main chain, or syndiotactic, with the group alternating from side to side of the chain. Stereospecific polymerization can be performed by use of certain catalytic agents (see ziegler process).

alternating A B A B A B A B

random AABABBBA block AABBBBAA p graft AAAAAAAAA

BBB

Types of copolymer depending on the arrangement of the monomers A and B

HHH HHH HHH HHH

HHH H X H X

XXX X H X H

isotactic syndiotactic

Types of stereospecific polymer

Structure of polymers potash alum 432

potash alum See aluminium potassium–40. The mass of argon–40 and sium sulphate; alums. potassium–40 in the sample is estimated and the sample is then dated potash mica See muscovite. from the equation: potassium λ potassamide See 40Ar = 0.1102 40K(e t – 1), monoxide. where λ is the decay constant and t is potassium Symbol K. A soft silvery the time in years since the mineral metallic element belonging to group cooled to about 300°C, when the 40Ar 1 (formerly IA) of the periodic table became trapped in the crystal lattice. (see alkali metals); a.n. 19; r.a.m. The method is effective for micas, 39.098; r.d. 0.86; m.p. 63.7°C; b.p. feldspar, and some other minerals. 774°C. The element occurs in seawa- potassium bicarbonate See ter and in a number of minerals, potassium hydrogencarbonate. such as sylvite (KCl), carnallite potas- (KCl.MgCl2.6H2O), and kainite potassium bichromate See sium dichromate (MgSO4.KCl.3H2O). It is obtained by . electrolysis. The metal has few uses potassium bromide A white or but potassium salts are used for a colourless crystalline solid, KBr, wide range of applications. Potas- slightly hygroscopic and soluble in sium is an *essential element for liv- water and very slightly soluble in ing organisms. The potassium ion, ° + ethanol; cubic; r.d. 2.75; m.p. 734 C; K , is the most abundant cation in b.p. 1435°C. Potassium bromide may plant tissues, being absorbed through be prepared by the action of bromine the roots and being used in such on hot potassium hydroxide solution processes as protein synthesis. In ani- or by the action of iron(III) bromide mals the passage of potassium and or hydrogen bromide on potassium sodium ions across the nerve-cell carbonate solution. It is used widely membrane is responsible for the in the photographic industry and is p changes of electrical potential that also used as a sedative. Because of its accompany the transmission of im- range of transparency to infrared ra- pulses. Chemically, it is highly reac- diation, KBr is used both as a matrix tive, resembling sodium in its for solid samples and as a prism ma- behaviour and compounds. It also terial in infrared spectroscopy. forms an orange-coloured superox- – potassium carbonate (pearl ash; ide, KO2, which contains the O2 ion. Potassium was discovered by Sir potash) A translucent (granular) or Humphry *Davy in 1807. white (powder) deliquescent solid A known in the anhydrous and hydrated forms. K CO (monoclinic; r.d. • Information from the WebElements site 2 3 2.4; m.p. 891°C) decomposes without potassium–argon dating A *dat- boiling. 2K2CO3.3H2O (monoclinic; ing technique for certain rocks that r.d. 2.04) dehydrates to K2CO3.H2O ° depends on the decay of the radioiso- above 100 C and to K2CO3 above tope potassium–40 to argon–40, a 130°C. It is prepared by the Engel– process with a half-life of about 1.27 Precht process in which potas- × 1010 years. It assumes that all the sium chloride and magnesium argon–40 formed in the potassium- oxide react with carbon dioxide to bearing mineral accumulates within give the compound Engel’s salt, it and that all the argon present is MgCO3.KHCO3.4H2O. This is decom- formed by the decay of potas- posed in solution to give the hydro- 433 potassium cyanide gencarbonate, which can then be cal- composes without boiling. It is pro- cined to K2CO3. Potassium carbonate duced industrially by roasting pow- is soluble in water (insoluble in alco- dered chromite ore with potassium hol) with signiÜcant hydrolysis to hydroxide and limestone and leach- produce basic solutions. Industrial ing the resulting cinder with hot uses include glasses and glazes, the potassium sulphate solution. Potas- manufacture of soft soaps, and in sium chromate is used in leather dyeing and wool Ünishing. It is used Ünishing, as a textile mordant, and in in the laboratory as a drying agent. enamels and pigments. In the labora- potassium chlorate A colourless tory it is used as an analytical reagent and as an indicator. Like crystalline compound, KClO3, which is soluble in water and moderately other chromium(III) compounds it is soluble in ethanol; monoclinic; r.d. toxic when ingested or inhaled. 2.32; m.p. 356°C; decomposes above potassium chromium sulphate 400°C giving off oxygen. The indus- (chrome alum) A violet or ruby-red trial route to potassium chlorate in- crystalline solid, K2SO4.Cr2(SO4)3. volves the fractional crystallization of 24H2O, that is soluble in water and a solution of potassium chloride and insoluble in ethanol; cubic or octahe- sodium chlorate but it may also be dral; r.d. 1.826; m.p. 89°C; loses ° ° prepared by electrolysis of hot con- 10H2O at 100 C, 12H2O at 400 C. Six centrated solutions of potassium water molecules surround each of chloride. It is a powerful oxidizing the chromium(III) ions and the reagent Ünding applications in weed- maining ones are hydrogen bonded killers and disinfectants and, because to the sulphate ions. Like all alums, of its ability to produce oxygen, it is the compound may be prepared by used in explosives, pyrotechnics, and mixing equimolar quantities of the matches. constituent sulphates. See alums. potassium chloride A white crys- potassium cyanide (cyanide) A talline solid, KCl, which is soluble in white crystalline or granular deli- p water and very slightly soluble in quescent solid, KCN, soluble in water ° ethanol; cubic; r.d. 1.98; m.p. 772 C; and in ethanol and having a faint ° sublimes at 1500 C. Potassium chlo- characteristic odour of almonds (due ride occurs naturally as the mineral to hydrolysis forming hydrogen sylvite (KCl) and as carnallite cyanide at the surface); cubic; r.d. (KCl.MgCl2.6H2O); it is produced in- 1.52; m.p. 634°C. It is prepared indus- dustrially by fractional crystallization trially by the absorption of hydrogen of these deposits or of solutions from cyanide in potassium hydroxide. The lake brines. It has the interesting compound is used in the extraction property of being more soluble than of silver and gold, in some metal- sodium chloride in hot water but less Ünishing processes and electroplat- soluble in cold. It is used as a fertil- ing, as an insecticide and fumigant izer, in photography, and as a source (source of HCN), and in the prepara- of other potassium salts, such as the tion of cyanogen derivatives. In the chlorate and the hydroxide. It has laboratory it is used in analysis, as a low toxicity. reducing agent, and as a stabilizing potassium chromate A bright yel- *ligand for low oxidation states. The low crystalline solid, K2CrO4, soluble salt itself is highly toxic and aqueous in water and insoluble in alcohol; solutions of potassium cyanide are rhombic; r.d. 2.73; m.p. 968.3°C; de- strongly hydrolysed to give rise to potassium dichromate 434

the slow release of equally toxic hy- guishers. Because of its buffering ca- drogen cyanide gas. pacity, it is added to some detergents and also used as a laboratory reagent. potassium dichromate (potassium bichromate) An orange-red potassium hydrogentartrate crystalline solid, K2Cr2O7, soluble in (cream of tartar) A white crystalline water and insoluble in alcohol; mon- acid salt, HOOC(CHOH)2COOK. It is oclinic or triclinic; r.d. 2.68; mono- obtained from deposits on wine vats clinic changes to triclinic at 241.6°C; (argol) and used in baking powders. m.p. 396°C; decomposes above potassium hydroxide (caustic 500°C. It is prepared by acidiÜcation potash; lye) A white deliquescent of crude potassium chromate solu- solid, KOH, often sold as pellets, tion (the addition of a base to solu- Ûakes, or sticks, soluble in water and tions of potassium dichromate in ethanol and very slightly soluble reverses this process). The compound in ether; rhombic; r.d. 2.044; m.p. is used industrially as an oxidizing 360.4°C; b.p. 1320°C. It is prepared agent in the chemical industry and in industrially by the electrolysis of con- dyestuffs manufacture, in electroplat- centrated potassium chloride solu- ing, pyrotechnics, glass manufacture, tion but it can also be made by glues, tanning, photography and lith- heating potassium carbonate or sul- ography, and in ceramic products. phate with slaked lime, Ca(OH) . It Laboratory uses include application 2 closely resembles sodium hydroxide as an analytical reagent and as an ox- but is more soluble and is therefore idizng agent. Potassium dichromate preferred as an absorber for carbon is toxic and considered a Üre risk on dioxide and sulphur dioxide. It is also account of its oxidizing properties. used in the manufacture of soft soap, potassium dioxide See potassium other potassium salts, and in Ni–Fe superoxide. and alkaline storage cells. Potassium potassium hydride A white or hydroxide is extremely corrosive to p greyish white crystalline solid, KH; body tissues and especially damaging r.d. 1.43–1.47. It is prepared by pass- to the eyes. ing hydrogen over heated potassium potassium iodate A white crys- and marketed as a light grey powder talline solid, KIO3, soluble in water dispersed in oil. The solid decom- and insoluble in ethanol; monoclinic; poses on heating and in contact with r.d. 3.9; m.p. 560°C. It may be pre- moisture and is an excellent reduc- pared by the reaction of iodine with ing agent. Potassium hydride is a Üre hot concentrated potassium hydrox- hazard because it produces hydrogen ide or by careful electrolysis of on reaction with water. potassium iodide solution. It is an potassium hydrogencarbonate oxidizing agent and is used as an (potassium bicarbonate) A white analytical reagent. Some potassium iodate is used as a food additive. crystalline solid, KHCO3, soluble in water and insoluble in ethanol; r.d. potassium iodide A white crys- 2.17; decomposes about 120°C. It oc- talline solid, KI, with a strong bitter curs naturally as calcinite and is pre- taste, soluble in water, ethanol, and pared by passing carbon dioxide into acetone; cubic; r.d. 3.13; m.p. 681°C; saturated potassium carbonate solu- b.p. 1330°C. It may be prepared by tion. It is used in baking, soft-drinks the reaction of iodine with hot potas- Ü manufacture, and in CO2 re extin- sium hydroxide solution followed by 435 potassium sulphide separation from the iodate (which is decomposes at 400°C. It occurs natu- also formed) by fractional crystalliza- rally as nitre and may be prepared by tion. In solution it has the interesting the reaction of sodium nitrate with property of dissolving iodine to form potassium chloride followed by frac- – the triiodide ion I3 , which is brown. tional crystallization. It is a powerful Potassium iodide is widely used as an oxidizing agent (releases oxygen on analytical reagent, in photography, heating) and is used in gunpowder and also as an additive to table salt to and fertilizers. prevent goitre and other disorders potassium nitrite A white or due to iodine deÜciency. slightly yellow deliquescent solid, potassium manganate(VII) KNO2, soluble in water and insoluble (potassium permanganate) A com- in ethanol; r.d. 1.91; m.p. 440°C; may pound, KMnO4, forming purple crys- explode at 600°C. Potassium nitrite is tals with a metallic sheen, soluble in prepared by the reduction of potas- water (intense purple solution), ace- sium nitrate. It reacts with cold di- tone, and methanol, but decomposed lute mineral acids to give nitrous by ethanol; r.d. 2.70; decomposition acid and is also able to behave as a begins slightly above 100°C and is reducing agent (if oxidized to the ni- complete at 240°C. The compound is trate) or as an oxidizing agent (if re- prepared by fusing manganese(IV) duced to nitrogen). It is used in oxide with potassium hydroxide to organic synthesis because of its part form the manganate and elec- in diazotization, and in detecting the trolysing the manganate solution presence of the amino groups in or- using iron electrodes at about 60°C. ganic compounds. An alternative route employs production of sodium manganate by a simi- potassium permanganate See potassium manganate(vii) lar fusion process, oxidation with . chlorine and sulphuric acid, then potassium sulphate A white crys- treatment with potassium chloride to talline powder, K2SO4, soluble in crystallize the required product. water and insoluble in ethanol; p Potassium manganate(VII) is widely rhombic or hexagonal; r.d. 2.66; m.p. used as an oxidizing agent and as a 1069°C. It occurs naturally as disinfectant in a variety of applica- schönite (Strassfurt deposits) and in tions, and as an analytical reagent. lake brines, from which it is sepa- potassium monoxide A grey crys- rated by fractional crystallization. It has also been produced by the Har- talline solid, K2O; cubic; r.d. 2.32; decomposition occurs at 350°C. It may greaves process, which involves the be prepared by the oxidation of oxidation of potassium chloride with potassium metal with potassium ni- sulphuric acid. In the laboratory it trate. It reacts with ethanol to form may be obtained by the reaction of potassium ethoxide (KOC2H5), and either potassium hydroxide or potas- with liquid ammonia to form potassium carbonate with sulphuric acid. sium hydroxide and potassamide Potassium sulphate is used in ce- (KNH2). ments, in glass manufacture, as a food additive, and as a fertilizer potassium nitrate (saltpetre) A (source of K+) for chloride-sensitive colourless rhombohedral or trigonal plants, such as tobacco and citrus. solid, KNO3, soluble in water, insoluble in alcohol; r.d. 2.109; transition to potassium sulphide A yellow-red trigonal form at 129°C; m.p. 334°C; or brown-red deliquescent solid, K2S, potassium sulphite 436

which is soluble in water and in tential barrier surrounds the atomic ethanol but insoluble in diethyl nucleus and is important in nuclear ether; cubic; r.d. 1.80; m.p. 840°C. It physics; a similar but much lower is made industrially by reducing barrier exists at the interface be- potassium sulphate with carbon at tween semiconductors and metals high temperatures in the absence of and between differently doped semi- air. In the laboratory it may be pre- conductors. These barriers are impor- pared by the reaction of hydrogen tant in the design of electronic sulphide with potassium hydroxide. devices. The pentahydrate is obtained on crys- potential energy See energy. tallization. Solutions are strongly alkaline due to hydrolysis. It is used as potential-energy curve A graph an analytical reagent and as a depila- of the potential energy of electrons tory. Potassium sulphide is generally in a diatomic molecule, in which the regarded as a hazardous chemical potential energy of an electronic Ü with a re risk; dusts of K2S have state is plotted vertically and the in- been known to explode. teratomic distance is plotted horizon- tally, with the minimum of the curve potassium sulphite A white crys- being the average internuclear dis- talline solid, K SO , soluble in water 2 3 tance. The *Morse potential gives a and very sparingly soluble in good analytic description of a poten- ethanol; r.d. 1.51; decomposes on tial-energy curve. The dissociation heating. It is a reducing agent and is energy and certain quantities of in- used as such in photography and in terest in vibrational spectroscopy can the food and brewing industries, be found from the potential-energy where it prevents oxidation. curve. potassium superoxide (potassium potential-energy surface A mul- dioxide) A yellow paramagnetic tidimensional surface used in the solid, KO , produced by burning 2 theory of electronic states of poly- potassium in an excess of oxygen; it p atomic molecules and chemical reac- is very soluble (by reaction) in water, tions. A *potential-energy curve is soluble in ethanol, and slightly solu- the simplest type of potential-energy ble in diethyl ether; m.p. 380°C. surface. Each internuclear distance in When treated with cold water or di- the molecule or reacting system is lute mineral acids, hydrogen perox- represented by one dimension, as is ide is obtained. The compound is a the potential energy. This means that powerful oxidizing agent and on if a nonlinear molecule has N atoms strong heating releases oxygen with its potential-energy surface is a the formation of the monoxide, K O. 2 (3N – 6)-dimensional surface in a potential barrier A region con- (3N – 5)-dimensional space. In the taining a maximum of potential that case of a linear molecule the poten- prevents a particle on one side of it tial-energy surface is a (3N – 5)-dimen- from passing to the other side. Ac- sional space. If the surface has a cording to classical theory a particle minimum then that electronic state must possess energy in excess of the is stable. It is possible for there to be height of the potential barrier to pass more than one minimum. In poten- it. However, in quantum theory there tial-energy surfaces for chemical re- is a Ünite probability that a particle actions the reactants and products with less energy will pass through are frequently ‘valleys’ connected by the barrier (see tunnel effect). A po- a region of higher energy, which is 437 pressure associated with the *activation en- cracking crude oil. The element was ergy of the reaction. discovered by C. A. von Welsbach in potentiometric titration A titra- 1885. tion in which the end point is found A by measuring the potential on an • Information from the WebElements site electrode immersed in the reaction precessional motion A form of mixture. motion that occurs when a torque is Pourbaix diagram A diagram applied to a rotating body in such a showing how oxidation–reduction way that it tends to change the direc- behaviour for compounds of a given tion of its axis of rotation. It arises element depends on pH. because the resultant of the angular powder metallurgy A process in velocity of rotation and the incre- which powdered metals or alloys are ment of angular velocity produced by pressed into a variety of shapes at the torque is an angular velocity high temperatures. The process about a new direction; this com- started with the pressing of pow- monly changes the axis of the ap- dered tungsten into incandescent plied torque and leads to sustained lamp Ülaments in the Ürst decade of rotation of the original axis of rota- this century and is now widely used tion. for making self-lubricating bearings A spinning top, the axis of which is and cemented tungsten carbide cut- not exactly vertical, has a torque act- ting tools. ing on it as a result of gravity. Instead The powders are produced by at- of falling over, the top precesses omization of molten metals, chemi- about a vertical line through the cal decomposition of a compound of pivot. Precessional effects occur in the metal, or crushing and grinding atoms in magnetic Üelds. of the metal or alloy. The parts are precipitate pressed into moulds at pressures A suspension of small ranging from 140 × 106 Pa to 830 × solid particles produced in a liquid by p 106 Pa after which they are heated in chemical reaction. a controlled atmosphere to bond the precipitation 1. All liquid and particles together (see sintering). solid forms of water that are de- praseodymium Symbol Pr. A soft posited from the atmosphere; it in- silvery metallic element belonging to cludes rain, drizzle, snow, hail, dew, the *lanthanoids; a.n. 59; r.a.m. and hoar frost. 2. The formation of a 140.91; r.d. 6.773; m.p. 931°C; b.p. precipitate. ° 3512 C. It occurs in bastnasite and precursor A compound that leads monazite, from which it is recovered to another compound in a series of by an ion-exchange process. The only chemical reactions. naturally occurring isotope is praseodymium–141, which is not pressure The force acting normally radioactive; however, fourteen ra- on unit area of a surface or the ratio dioisotopes have been produced. It is of force to area. It is measured in used in mischmetal, a rare-earth *pascals in SI units. Absolute pres- alloy containing 5% praseodymium, sure is pressure measured on a gauge for use in lighter Ûints. Another rare- that reads zero at zero pressure earth mixture containing 30% rather than at atmospheric pressure. praseodymium is used as a catalyst in Gauge pressure is measured on a pressure gauge 438

gauge that reads zero at atmospheric primary cell A *voltaic cell in pressure. which the chemical reaction produc- pressure gauge Any device used ing the e.m.f. is not satisfactorily re- to measure *pressure. Three basic versible and the cell cannot therefore be recharged by the application of a types are in use: the liquid-column daniell cell gauge (e.g. the mercury barometer charging current. See ; leclanché cell; weston cell; mer- and the manometer), the expanding- cury cell. Compare secondary cell. element gauge (e.g. the Bourdon gauge and the aneroid barometer), principal quantum number See and the electrical transducer. In the atom. last category the strain gauge is an prismane A saturated hydrocarbon, example. Capacitor pressure gauges C H , in which the six carbon atoms also come into this category. In these 6 6 are arranged at the corners of a trian- devices, the pressure to be measured gular prism. The structure was sug- displaces one plate of a capacitor and gested in 1869 by Albert Ladenburg thus alters its capacitance. as a possible structure for benzene presumptive test A simple test (since referred to as Ladenburg ben- for a given substance using a reagent zene). The actual compound was syn- that changes colour when mixed thesized by T.J. Katz and N. Acton in with the substance under investiga- 1973. Hexamethylprismane, in which tion. Presumptive tests are not deÜni- the carbon atoms are linked to tive and further conÜrmatory tests methyl groups rather than hydrogen are always required. They are used atoms, was synthesized in 1966. extensively in forensic science. Ex- amples are the Duquenois–Levine H test for marijuana and Scott’s test for C cocaine. In general analytical chem- CH istry, presumptive tests are often called spot tests. C CH p H Priestley, Joseph (1733–1804) CH C British chemist, who in 1755 became H a Presbyterian minister. In Leeds, in Prismane 1767, he experimented with carbon dioxide (‘Üxed air’) from a nearby prochiral Denoting an organic mol- brewery; with it he invented soda ecule lacking a chiral centre but con- water. He moved to a ministry in taining one or more carbon atoms Birmingham in 1780, and in 1791 his attached to two identical ligands and revolutionary views caused a mob to two other different ligands (Caabc). burn his house, as a result of which The carbon atom is said to be a he emigrated to the USA in 1794. In prochiral centre. The name prochiral the early 1770s he experimented is used because if a is replaced by a with combustion and produced the ligand d, different from either a, b, or gases hydrogen chloride, sulphur c, there is a chiral centre in the mol- dioxide, and dinitrogen oxide (ni- ecule C . trous oxide). In 1774 he isolated oxy- abcd gen (see also lavoisier, antoine). producer gas (air gas) A mixture of carbon monoxide and nitrogen primary alcohol alcohols See . made by passing air over very hot primary amine See amines. carbon. Usually some steam is added 439 propane to the air and the mixture contains duce milk and the corpus luteum of hydrogen. The gas is used as a fuel in the ovary to secrete the hormone some industrial processes. *progesterone. product See chemical reaction. proline See amino acid. progesterone A hormone, pro- promethium Symbol Pm. A soft sil- duced primarily by the corpus lu- very metallic element belonging to teum of the ovary but also by the the *lanthanoids; a.n. 61; r.a.m. 145; placenta, that prepares the inner lin- r.d. 7.26 (20°C); m.p. 1080°C; b.p. ing of the uterus for implantation of 2460°C. The only naturally occurring a fertilized egg cell. If implantation isotope, promethium–147, has a half- fails, the corpus luteum degenerates life of only 2.52 years. Eighteen other and progesterone production ceases radioisotopes have been produced, accordingly. If implantation occurs, but they have very short half-lives. the corpus luteum continues to se- The only known source of the el- crete progesterone, under the in- ement is nuclear-waste material. Û uence of luteinizing hormone and Promethium–147 is of interest as a prolactin, for several months of preg- beta-decay power source but the nancy, by which time the placenta promethium–146 and –148, which has taken over this function. During emit penetrating gamma radiation, pregnancy, progesterone maintains must Ürst be removed. It was discov- the constitution of the uterus and ered by J. A. Marinsky, L. E. Glen- prevents further release of eggs from denin, and C. D. Coryell in 1947. the ovary. Small amounts of proges- A terone are produced by the testes. See also progestogen. • Information from the WebElements site progestogen One of a group of promoter A substance added to a naturally occurring or synthetic hor- catalyst to increase its activity. mones that maintain the normal proof A measure of the amount of course of pregnancy. The best known alcohol (ethanol) in drinks. Proof p is *progesterone. In high doses spirit contains 49.28% ethanol by progestogens inhibit secretion of weight (about 57% by volume). De- luteinizing hormone, thereby pre- grees of proof express the percentage venting ovulation, and alter the con- of proof spirit present, so 70° proof sistency of mucus in the vagina so spirit contains 0.7 × 57% alcohol. that conception tends not to occur. They are therefore used as major 1,2-propadiene (allene) A colour- constituents of oral contraceptives. less gas, CH2CCH2; r.d. 1.79; m.p. –136°C; b.p. –34.5°C. Propadiene may projection formula A conven- be prepared from 1,3-dibromo- tion for representing the three- propane (CH2BrCHCH2Br) by the ac- dimensional structure of a molecule tion of zinc dust. See also allenes. in two dimensions. See fischer projection; newman projection; propanal (propionaldehyde) A sawhorse projection. colourless liquid *aldehyde, C H CHO; m.p. –81°C; b.p. 48.8°C. prolactin (lactogenic hormone; lu- 2 5 teotrophic hormone; luteotrophin) A propane A colourless gaseous hy- ° hormone produced by the anterior drocarbon, C3H8; m.p. –190 C; b.p. pituitary gland. In mammals it stimu- –42°C. It is the third member of the lates the mammary glands to pro- *alkane series and is obtained from propanedioic acid 440

petroleum. Its main use is as bottled producing the spray. Formerly gas for fuel. chloroÛuorocarbons were used but propanedioic acid (malonic acid) their use has been discontinued be- A white crystalline dicarboxylic acid, cause of their effect on the ozone ° layer. Most aerosol cans use liquid HOOCCH2COOH; m.p. 132 C. It decomposes above its melting point to hydrocarbon mixtures as the propel- ethanoic acid. Propanedioic acid is lant. used in the synthesis of other dicar- propenal (acrolein) A colourless boxylic acids. pungent liquid unsaturated aldehyde, ° propanoic acid (propionic acid) A CH2:CHCHO; r.d. 0.84; m.p. –87 C; ° colourless liquid *carboxylic acid, b.p. 53 C. It is made from propene and is used in producing polyester CH3CH2COOH; r.d. 0.99; m.p. –20.8°C; b.p. 141°C. It is used to and polyurethane resins. make calcium propanate – an addi- propene (propylene) A colourless tive in bread. gaseous hydrocarbon, CH3CH:CH2; ° ° propanol Either of two *alcohols m.p. –185.25 C; b.p. –47.4 C. It is an *alkene obtained from petroleum by with the formula C3H7OH. Propan-1- cracking alkanes. Its main use is in ol is CH3CH2CH2OH and propan-2-ol the manufacture of polypropene. is CH3CH(OH)CH3. Both are colourless volatile liquids. Propan-2-ol is propenoate (acrylate) A salt or used in making propanone (acetone). ester of *propenoic acid. propanone (acetone) A colourless propenoic acid (acrylic acid) An Û ammable volatile compound, unsaturated liquid *carboxylic acid, ° CH3COCH3; r.d. 0.79; m.p. –95.4 C; CH :CHCOOH; m.p. 13°C; b.p. ° 2 b.p. 56.2 C. The simplest *ketone, 141.6°C. It readily polymerizes and it propanone is miscible with water. It is used in the manufacture of is made by oxidation of propan-2-ol *acrylic resins. p (see propanol) or is obtained as a by- product in the manufacture of phe- propenol (allyl alcohol) A pungent- nol from cumene; it is used as a smelling colourless unsaturated liq- solvent and as a raw material for uid alcohol, CH2=CHCH2OH; b.p. making plastics. 97°C. It is found in wood alcohol and can be prepared in the laboratory by propellant 1. A substance that reducing propenal or by heating glyc- burns rapidly in a controlled way, erol with oxalic acid. Industrially, it used to propel a projectile (e.g. from Ü is made by the high-temperature cat- a gun). In rearms, gunpowder and alytic reaction between propenal and cordite are common examples. 2. A 2-propanol or by the hydrolysis of 2- fuel used in a rocket engine. Usually chloropropene, which is made by the propellant is a fuel and an oxi- chlorinating propene. Potassium per- dizer; for example kerosene or liquid manganate oxidizes propenol to glyc- hydrogen with a liquid oxygen pro- erol and silver oxide converts it to a pellant. 3. A substance used to pro- mixture of propenoic acid and prope- duce the spray in an aerosol can. nal. Aerosol propellants are volatile substances that can be liqueÜed under propenonitrile (acrylonitrile; vinyl pressure and are able to dissolve the cyanide) A colourless liquid, ° working substance. When the pres- H2C:CHCN; r.d. 0.81; m.p. –83.5 C. It sure is released, the liquid vaporizes is an unsaturated nitrile, made from 441 protein propene and used to make acrylic ered by Lise Meitner and Otto Hahn resins. in 1917. propenyl group (allyl group) The A • Information from the WebElements site organic group H2C=CHCH2–. propionaldehyde See propanal. protamine Any of a group of proteins of relatively low molecular propene propylene See . weight found in association with the propyl group The organic group chromosomal *DNA of vertebrate sperm cells. They contain a single CH3CH2CH2–. polypeptide chain comprising about prostaglandin Any of a group of 67% arginine. Protamines are thought organic compounds derived from to protect and support the chromo- *essential fatty acids and causing a somes. range of physiological effects in animals. Prostaglandins have been de- protease (peptidase; proteinase; tected in most body tissues. They act proteolytic enzyme) Any enzyme at very low concentrations to cause that catalyses the hydrolysis of pro- the contraction of smooth muscle; teins into smaller *peptide fractions natural and synthetic prostaglandins and amino acids, a process known as proteolysis * are used to induce abortion or labour . Examples are pepsin and *trypsin. Several proteases, actin humans and domestic animals. ing sequentially, are normally re- Two prostaglandin derivatives have quired for the complete digestion of antagonistic effects on blood circula- a protein to its constituent amino tion: thromboxane A causes blood 2 acids. clotting while prostacyclin causes blood vessels to dilate. InÛammation protecting group A group used to in allergic reactions and other dis- protect a certain functional group in eases is also thought to involve a chemical synthesis. For example, prostaglandins. a hydroxyl group (–OH) can be p converted into an acetyl group prosthetic group A tightly bound (–OOCCH3) to protect it taking part in nonpeptide inorganic or organic a certain step of the synthesis. In this component of a protein. Prosthetic case, the acetyl is the protecting groups may be lipids, carbohydrates, group. Later it can easily be changed metal ions, phosphate groups, etc. back into the original hydroxyl Some *coenzymes are more correctly group. regarded as prosthetic groups. protein Any of a large group of or- protactinium Symbol Pa. A radio- ganic compounds found in all living active metallic element belonging to organisms. Proteins comprise carbon, the *actinoids; a.n. 91; r.a.m. hydrogen, oxygen, and nitrogen and 231.036; r.d. 15.37 (calculated); m.p. most also contain sulphur; molecular <1600°C (estimated). The most stable weights range from 6000 to several isotope, protactinium–231, has a million. Protein molecules consist of half-life of 3.43 × 104 years; at least one or several long chains (*polypep- ten other radioisotopes are known. tides) of *amino acids linked in a Protactinium–231 occurs in all ura- characteristic sequence. This se- nium ores as it is derived from ura- quence is called the primary structure nium–235. Protactinium has no of the protein. These polypeptides practical applications; it was discov- may undergo coiling or pleating, the protein engineering 442

nature and extent of which is de- new proteins by cells or other sys- scribed as the secondary structure. tems containing the necessary fac- The three-dimensional shape of the tors for transcription and translation. coiled or pleated polypeptides is Alternatively, new proteins can be called the tertiary structure. Quater- synthesized by solid state synthesis, nary structure speciÜes the structural in which polypeptide chains are as- relationship of the component sembled under the control of chemi- polypeptides. cals. One end of the chain is Proteins may be broadly classiÜed anchored to a solid support and the into globular proteins and Übrous chemicals selectively determine proteins. Globular proteins have which amino acids are added to the compact rounded molecules and are free end. The appropriate chemicals usually water-soluble. Of prime im- can be renewed during the process; portance are the *enzymes, proteins when synthesized, the polypeptide is that catalyse biochemical reactions. removed and puriÜed. Protein engi- Other globular proteins include the neering is used to synthesize en- antibodies, which combine with for- zymes (so-called ‘designer enzymes’) eign substances in the body; the car- used in biotechnology. The three- rier proteins, such as *haemoglobin; dimensional tertiary structure of pro- the storage proteins (e.g. casein in teins is crucially important for their milk and albumin in egg white), and function, and this can be investigated certain hormones (e.g. insulin). Fi- using computer-aided modelling. brous proteins are generally insolu- protein synthesis The process by ble in water and consist of long which living cells manufacture pro- Û coiled strands or at sheets, which teins from their constituent amino confer strength and elasticity. In this acids, in accordance with the genetic category are keratin and collagen. information carried in the DNA of Actin and myosin are the principal the chromosomes. This information Ü brous proteins of muscle, the inter- is encoded in messenger *RNA, p action of which brings about muscle which is transcribed from DNA in contraction. Blood clotting involves the nucleus of the cell: the sequence the Übrous protein called Übrin. of amino acids in a particular protein When heated over 50°C or sub- is determined by the sequence of nu- jected to strong acids or alkalis, pro- cleotides in messenger RNA. At the teins lose their speciÜc tertiary ribosomes the information carried by structure and may form insoluble co- messenger RNA is translated into the agulates (e.g. egg white). This usually sequence of amino acids of the pro- inactivates their biological proper- tein in the process of translation. ties. proteolysis The enzymic splitting protein engineering The tech- of proteins. See protease. niques used to alter the structure of protease proteins (especially enzymes) in proteolytic enzyme See . order to improve their use to hu- proton An elementary particle that mans. This involves artiÜcially modi- is stable, bears a positive charge fying the DNA sequences that encode equal in magnitude to that of the them so that, for example, new *electron, and has a mass of amino acids are inserted into existing 1.672 614 × 10–27 kg, which is proteins. Synthesized lengths of 1836.12 times that of the electron. novel DNA can be used to produce The proton is a hydrogen ion and oc- 443 pullulan curs in all atomic nuclei. See also hy- pseudo-axial See ring conforma- dron. tions. protonic acid An *acid that forms pseudoephedrine See ephedrine. positive hydrogen ions (or, strictly, pseudo-equatorial See ring con- oxonium ions) in aqueous solution. formations. The term is used to distinguish ‘traditional’ acids from Lewis acids or from pseudohalogens A group of com- Lowry–Brønsted acids in nonaqueous pounds, including cyanogen (CN)2 solvents. and thiocyanogen (SCN)2, that have some resemblance to the halogens. proton number See atomic num- Thus, they form hydrogen acids ber. (HCN and HSCN) and ionic salts con- Prout’s hypothesis The hypothe- taining such ions as CN– and SCN–. sis put forward by the British pseudo order An order of a chemi- chemist William Prout (1785–1850) cal reaction that appears to be less in 1815 that all atomic weights are than the true order because of the integer multiples of the atomic experimental conditions used. weight of hydrogen and hence that Pseudo orders occur when one reac- all atoms are made out of hydrogen. tant is present in large excess. For ex- Subsequent work on atomic weights ample, a reaction of substance A in the 19th century showed that this undergoing hydrolysis may appear to hypothesis is incorrect (with chlorine be proportional only to [A] because having an atomic weight of 35.5 the amount of water present is so being a glaring example of this). The large. understanding of atomic structure that emerged in the 20th century, psilocin See psilocybin. with atomic number being the num- psilocybin A hallucinogen, similar ber of protons in an atom and non- in effect to psi*mescaline, found in integer atomic weights being due to certain species of mushroom. It is ac- p mixtures of isotopes, has vindicated companied by a related, more active, the spirit of Prout’s hypothesis. compound psilocin. Both are clas- A siÜed as class A drugs in the UK. They • William Prout’s original paper can be detected by the Marquis test prussic acid See hydrogen or the Froehde test. cyanide. PTFE See polytetrafluoroethene. pseudoaromatic (antiaromatic) A ptyalin An enzyme that digests car- compound that has a ring of atoms bohydrates (see amylase). It is present containing alternating double and in mammalian saliva and is responsi- single bonds, yet does not have the ble for the initial stages of starch di- characteristic properties of *aromatic gestion. compounds. Such compounds do not p-type semiconductor See semi- obey the Hückel rule. Cyclooctate- conductor. traene (C8H8), for instance, has a ring of eight carbon atoms with conju- pullulan A water-soluble polysac- gated double bonds, but the ring is charide composed of glucose units not planar and the compound acts that are polymerized in such a way like an alkene, undergoing addition as to make it viscous and imperme- reactions. able to oxygen. Pullulan is used in pumice 444

adhesives, food packaging, and O

moulded articles. It is derived from 2CH CH2 the fungus Aureobasidium pullulans. 2CH CH2 pumice A porous volcanic rock that C is light and full of cavities due to ex- H2 panding gases that were liberated Pyran from solution in the lava while it so- lidiÜed. Pumice is often light enough pyranose A *sugar having a six- to Ûoat on water. It is usually acid membered ring containing Üve car- (siliceous) in composition, and is used bon atoms and one oxygen atom. as an abrasive and for polishing. pyrazine An unsaturated hetero- pump A device that imparts energy cyclic compound having two nitro- to a Ûuid in order to move it from gen atoms in a six-membered ring, one place or level to another or to C H N ; r.d. 1.03; m.p. 52°C; b.p. vacuum 4 4 2 raise its pressure (compare 115°C. Pyrazine is a symmetric di- pump ). Centrifugal pumps and tur- azine. bines have rotating impellers, which increase the velocity of the Ûuid, part N of the energy so acquired by the Ûuid then being converted to pressure energy. Displacement pumps act directly on the Ûuid, forcing it to Ûow N against a pressure. They include pis- Pyrazine ton, plunger, gear, screw, and cam pumps. pyrazole An unsaturated heterocyclic compound having two nitro- purine An organic nitrogenous base gen atoms in a Üve-membered ring, (see formula), sparingly soluble in ° C3H4N2; m.p. 66–70 C; b.p. 168– water, that gives rise to a group of ° p biologically important derivatives, 188 C. Pyrazine is a symmetric notably *adenine and *guanine, diazine. which occur in nucleotides and nu- H cleic acids (DNA and RNA). N N N N Pyrazole

N N pyridine A colourless liquid with a Purine strong unpleasant smell, C5H5N (see formula); r.d. 0.98; m.p. –42°C; b.p. PVA See polyvinylacetate. 115°C. Pyridine is an aromatic hete- polychloroethene PVC See . rocyclic compound present in coal pyran A heterocyclic compound having an oxygen atom and two double bonds in a six-membered ring, C5H6O. There are two isomers de- N pending on the position of the CH2 group. Pyridine 445 pyrometry tar. It is used in making other or- produces a polarization of some 10–5 ganic chemicals. Cm–2. pyridoxine See vitamin b complex. pyrogallol 1,2,3-trihydroxyben- pyrimidine zene, C6H3(OH)3, a white crystalline An organic nitrog- ° enous base (see formula), sparingly solid, m.p. 132 C. Alkaline solutions soluble in water, that gives rise to a turn dark brown on exposure to air group of biologically important deriv- through reaction with oxygen. It is a atives, notably *uracil, *thymine, powerful reducing agent, employed and *cytosine, which occur in *nu- in photographic developers. It is also cleotides and nucleic acids (DNA and used in volumetric gas analysis as an RNA). absorber of oxygen.

OH N OH OH N

Pyrimidine Pyrogallol pyrite (iron pyrites) A mineral form Ü of iron(II) sulphide, FeS2. Super cially pyrolusite See manganese(iv) it resembles gold in appearance, oxide. hence it is also known as fool’s gold, pyrolysis Chemical decomposition but it is harder and more brittle than occurring as a result of high tempera- gold (which may be cut with a knife). ture. Pyrite crystallizes in the cubic system, is brass yellow in colour, has a pyrometric cones See seger metallic lustre, and a hardness of cones. 6–6.5 on the Mohs’ scale. It is the pyrometry The measurement of p most common and widespread of the high temperatures from the amount sulphide minerals and is used as a of radiation emitted, using a pyrom- source of sulphur for the production eter. Modern narrow-band or spectral of sulphuric acid. Sources include the pyrometers use infrared-sensitive Rio Tinto mines in Spain. photoelectric cells behind Ülters that exclude visible light. In the optical pyro- PreÜx denoting an oxo acid pyrometer (or disappearing Ülament that could be obtained from a lower pyrometer) the image of the incan- acid by dehydration of two mol- descent source is focused in the ecules. For example, pyrosulphuric plane of a tungsten Ülament that is acid is H2S2O7 (i.e. 2H2SO4 minus heated electrically. A variable resistor H2O). is used to adjust the current through Ü pyroboric acid See boric acid. the lament until it blends into the image of the source, when viewed pyroelectricity The property of through a red Ülter and an eyepiece. certain crystals, such as tourmaline, The temperature is then read from a of acquiring opposite electrical calibrated ammeter or a calibrated charges on opposite faces when dial on the variable resistor. In the heated. In tourmaline a rise in tem- total-radiation pyrometer radiation perature of 1 K at room temperature emitted by the source is focused by a pyrones 446

concave mirror onto a blackened foil The general formula is X1–pY1+pZ2O6, to which a thermopile is attached. where X = Ca,Na; Y = Mg,Fe2+,Mn, From the e.m.f. produced by the ther- Li,Al,Fe3+,Ti; and Z = Si,Al. mopile the temperature of the source Orthorhombic pyroxenes (ortho- can be calculated. pyroxenes), (Mg,Fe)2Si2O6, vary in composition between the end- pyrones Compounds containing a members enstatite (Mg2Si2O6) and six-membered ring system having an orthoferrosilite (Fe2Si2O6). Mono- oxygen hetero atom and a carbonyl clinic pyroxenes (clinopyroxenes), group attached to the ring. There are the larger group, include: two forms depending on whether the diopside, CaMgSi2O6; 2+ carbonyl is in the 1 or 3 position. The hedenbergite, CaFe Si2O6; pyrone ring system occurs in many johannsenite, CaMnSi2O6; naturally occurring compounds. augite, (Ca,Mg,Fe,Ti,Al)2(Si,Al)2O6; 3+ aegirine, NaFe Si2O6; jade O O O jadeite (see ); 2+ 2+ pigeonite (Mg,Fe ,Ca)(Mg,Fe )Si2O6. pyrrole An organic nitrogen- containing compound that forms part of the structure of *porphyrins. O pyrrolidine A saturated hetero- Pyrones cyclic compound having one nitrogen atom in a Üve-membered ring, ° pyrophoric Igniting spontaneously C4H9N; r.d. 0.87; m.p. –63 C b.p. in air. Pyrophoric alloys are alloys 87°C. It is found in certain plants and that give sparks when struck. See the ring structure is present in many misch metal. alkaloids. pyrophosphoric acid See phos- p phoric(v) acid. N CH CH pyrosilicate See silicate. 2 2 disul- C C pyrosulphuric acid See H H phuric(vi) acid. 2 2 Pyrrolidine pyroxenes A group of ferromagne- sian rock-forming silicate minerals. They are common in basic igneous pyruvic acid (2-oxopropanoic acid) rocks but may also be developed by A colourless liquid organic acid, metamorphic processes in gneisses, CH3COCOOH. Pyruvate is an impor- schists, and marbles. Pyroxenes have tant intermediate compound in me- a complex crystal chemistry; they are tabolism, being produced during composed of continuous chains of sil- *glycolysis and converted to acetyl icon and oxygen atoms linked by a coenzyme A, required for the *Krebs variety of other elements. They are cycle. Under anaerobic conditions related to the *amphiboles, from pyruvate is converted to lactate or which they differ in cleavage angles. ethanol. Q

QSAR See quantitative quantum (pl. quanta) The mini- structure–activity relationship. mum amount by which certain properties, such as energy or angular QSMR See quantitative momentum, of a system can change. structure–metabolism relation- Such properties do not, therefore, ship. vary continuously, but in integral quadrivalent Having a valency of multiples of the relevant quantum, four. and are described as quantized. This concept forms the basis of the *quan- quadrupole A set of four point tum theory. In waves and Üelds the charges that has zero net charge and quantum can be regarded as an exci- dipole moment. An example of a tation, giving a particle-like interpre- quadrupole is a carbon dioxide (CO ) 2 tation to the wave or Üeld. Thus, the molecule. Quadrupole interactions quantum of the electromagnetic Üeld are much smaller than dipole inter- is the *photon and the graviton is actions. Transitions involving the quantum of the gravitational quadrupole moments are much Üeld. weaker than transitions involving dipole moments, but can allow transi- quantum chaos The *quantum tions forbidden in dipole moment mechanics of systems for which the transitions. corresponding classical system can exhibit *chaos. This subject was initi- quadrupole mass spectrometer ated by Einstein in 1917, who mass spectroscopy See . showed that the quantization condi- qualitative analysis See analysis. tions associated with the *Bohr theory need to be modiÜed for sys- analy- quantitative analysis See tems that show chaos in classical me- sis . chanics. The subject of quantum quantitative structure–activity chaos is an active Üeld of research in relationship (QSAR) A statistical al- which many basic issues still require gorithm that quantitatively deÜnes clariÜcation. It appears that systems the relationship between the chemi- exhibiting chaos in classical mechan- cal structure of a drug and its effect ics do not necessarily exhibit chaos on an organism. QSAR studies are in quantum mechanics. often used to predict the activity or quantum chemistry The applica- toxicity of new drugs. Similar meth- tion of quantum mechanics to chem- ods can be used to predict the metab- istry. Quantum chemistry mostly olism of new drugs (quantitative used empirical methods at Ürst but as structure–metabolism relationships). computing power has developed it has been increasingly based on the quantitative structure–metabo- Ü lism relationship (QSMR) See rst principles of quantum mechan- quantitative structure–activity ics. relationship. quantum entanglement A phe- quantum jump 448

nomenon in quantum mechanics in quantized. If the particles are treated which a particle or system does not as indistinguishable, Bose–Einstein have a deÜnite state but exists as an statistics apply if any number of par- intermediate form of two ‘entangled’ ticles can occupy a given quantum states. One of these states is realized state. Such particles are called when a ‘measurement’ is made. bosons. All known bosons have an angular momentum nh, where n is quantum jump A change in a sys- zero or an integer and h is the Planck tem (e.g. an atom or molecule) from constant. For identical bosons the one quantum state to another. *wave function is always symmetric. quantum mechanics A system of If only one particle may occupy each mechanics that was developed from quantum state, Fermi–Dirac statistics *quantum theory and is used to ex- apply and the particles are called plain the properties of atoms and fermions. All known fermions have a molecules. Using the energy *quan- total angular momentum (n + ½)h/2π tum as a starting point it incorpo- and any wave function that involves rates Heisenberg’s *uncertainty identical fermions is always antisym- principle and the *de Broglie wave- metric. length to establish the wave–particle quantum theory The theory de- duality on which *Schrödinger’s vised by Max *Planck in 1900 to ac- equation is based. This form of quan- count for the emission of the tum mechanics is called wave me- black-body radiation from hot bodies. chanics. An alternative but equivalent According to this theory energy is formalism, *matrix mechanics, is emitted in quanta (see quantum), based on mathematical operators. each of which has an energy equal to quantum number See atom; spin. hν, where h is the *Planck constant and ν is the frequency of the radia- quantum simulation The mathe- tion. This theory led to the modern matical modelling of systems of large theory of the interaction between numbers of atoms or molecules by matter and radiation known as computer studies of relatively small *quantum mechanics, which gener- clusters. It is possible to obtain infor- q alizes and replaces classical mechan- mation about solids and liquids in ics and Maxwell’s electromagnetic this way and to study surface proper- theory. In nonrelativistic quantum ties and reactions. theory particles are assumed to be quantum state The state of a neither created nor destroyed, to quantized system as described by its move slowly relative to the speed of quantum numbers. For instance, the light, and to have a mass that does state of a hydrogen *atom is denot change with velocity. These as- scribed by the four quantum num- sumptions apply to atomic and mo- bers n, l, m, ms. In the ground state lecular phenomena and to some they have values 1, 0, 0, and ½ re- aspects of nuclear physics. Relativis- spectively. tic quantum theory applies to parti- quantum statistics A statistical cles that have zero rest mass or description of a system of particles travel at or near the speed of light. that obeys the rules of *quantum quantum theory of radiation mechanics rather than classical me- The theory that describes the emis- chanics. In quantum statistics, en- sion and absorption of electromag- ergy states are considered to be netic radiation. Since it is a quantum 449 quicklime theory the processes of emission and mentary rocks such as sandstone and absorption are described in terms of limestone. The mineral is piezoelec- the creation and disappearance of tric and is used in oscillators. It is photons, with photons being created also used in optical instruments and in emission processes and disappear- in glass, glaze, and abrasives. ing in absorption. Absorption occurs quasicrystal A solid structure in when electromagnetic radiation that which there is (1) long-range incom- impinges on a quantum mechanical mensurate translational order (see in- system, such as an atom or mol- commensurate lattice) and (2) ecules, induces a transition from the long-range orientational order with a ground state of the system to an ex- point group, which is not allowed in cited state by virtue of a photon of *crystallography. Condition (1) is the radiation being absorbed. Simi- called quasiperiodicity. In two dimen- larly, emission of a photon occurs sions, the Üvefold symmetry of a pen- when an atom returns to the ground tagon is an example of a point state from an excited state, i.e. symmetry, which is not allowed in *spontaneous emission occurs. It is crystallography, but for which qua- also possible for an atom in an ex- sicrystals exist. In three dimensions cited state to emit a photon by *in- the symmetry of an icosahedron is duced emission, a process that is not allowed in crystallography, but used in *lasers. quasicrystals with this symmetry quantum yield In a photochemi- exist (e.g. AlMn). Diffraction patterns cal reaction, the number of events for quasicrystals have Bragg peaks, occurring per photon absorbed. It is with the density of Bragg peaks in given by the number of moles of each plane being higher than would product formed (or reactant con- be expected for a perfect periodic sumed) divided by the number of crystal. moles of photons absorbed. quasiperiodicity See quasicrystal. quartz The most abundant and quaternary ammonium com- common mineral, consisting of crys- pounds See amine salts. talline silica (silicon dioxide, SiO2), q crystallizing in the trigonal system. It quenching 1. (in metallurgy) The has a hardness of 7 on the Mohs’ rapid cooling of a metal by immers- scale. Well-formed crystals of quartz ing it in a bath of liquid in order to are six-sided prisms terminating in improve its properties. Steels are six-sided pyramids. Quartz is ordinar- quenched to make them harder ily colourless and transparent, in but some nonferrous metals are which form it is known as rock crys- quenched for other reasons (copper, tal. Coloured varieties, a number of for example, is made softer by which are used as gemstones, in- quenching). 2. (in physics) The clude *amethyst, citrine quartz (yel- process of inhibiting a continuous low), rose quartz (pink), milk quartz discharge in a *Geiger counter so (white), smoky quartz (grey-brown), that the incidence of further ionizing *chalcedony, *agate, and *jasper. radiation can cause a new discharge. Quartz occurs in many rocks, espe- This is achieved by introducing a cially igneous rocks such as granite quenching vapour, such as methane and quartzite (of which it is the chief mixed with argon or with neon, into the tube. constituent), metamorphic rocks such as gneisses and schists, and sedi- quicklime See calcium oxide. quinhydrone electrode 450

OH O to treat malaria; in small doses it may be prescribed for colds and in- Ûuenza. In dilute solutions it has a + pleasant astringent taste and is added + 2H to some types of tonic water. quinol See benzene-1,4-diol.

OH O quinoline A hygroscopic unpleasant-smelling colourless oily liquid, ° Quinhydrone electrode C9H7N; b.p. 240 C. Its molecules consist of a benzene ring fused to a pyri- quinhydrone electrode A *half dine ring. It occurs in coal tar and cell consisting of a platinum elec- bone oil, and is made from phenyl- trode in an equimolar solution of amine and nitrobenzene. Quinoline quinone (cyclohexadiene-1,4-dione) is a basic compound, forming salts and hydroquinone (benzene-1,4-diol). with mineral acids and forming qua- It depends on the oxidation–reduc- ternary ammonium compounds with tion reaction haloalkanes. It is used for making medicines and dyes. In quinoline, the C H (OH) ˆ C H O + 2H+ + 2e. 6 4 2 6 4 2 nitrogen atom is one atom away quinine A white solid, from the position at which the rings ° are fused. In an isomer, isoquinoline, C20H24N2O2.3H2O, m.p. 57 C. It is a poisonous alkaloid occurring in the the nitrogen atom is positioned two bark of the South American cinchona atoms away from the fused ring. tree, although it is now usually pro- quinone 1. See cyclohexadiene-1,4- duced synthetically. It forms salts dione. 2. Any similar compound and is toxic to the malarial parasite, containing C=O groups in an unsatu- and so quinine and its salts are used rated ring.

Quinoline R

racemate See racemic mixture. radionuclide and the dose of ionizing radiation. The units curie, roentgen, racemic mixture (racemate) A rad, and rem are not coherent with mixture of equal quantities of the d- SI units but their temporary use with and l-forms of an optically active SI units has been approved while the compound. Racemic mixtures are de- derived SI units becquerel, gray, and noted by the preÜx dl- (e.g. dl-lactic sievert become familiar. acid). A racemic mixture shows no The becquerel (Bq), the SI unit of *optical activity. activity, is the activity of a radionu- racemization A chemical reaction clide decaying at a rate, on average, in which an optically active com- of one spontaneous nuclear transi- pound is converted into a *racemic tion per second. Thus 1 Bq = 1 s–1. mixture. The former unit, the curie (Ci), is × 10 rad See radiation units. equal to 3.7 10 Bq. The curie was originally chosen to approximate the radial distribution function The activity of 1 gram of radium–226. average number of atoms found at a The gray (Gy), the SI unit of ab- distance r from a central atom. In the sorbed dose, is the absorbed dose case of a crystal, the radial distribu- when the energy per unit mass im- tion function is a series of regular parted to matter by ionizing radia- spikes, corresponding to the atomic tion is 1 joule per kilogram. The positions in the lattice. Calculating former unit, the rad (rd), is equal to the radial distribution function is of 10–2 Gy. fundamental importance in the The sievert (Sv), the SI unit of dose theory of liquids. equivalent, is the dose equivalent radiation 1. Energy travelling in when the absorbed dose of ionizing the form of electromagnetic waves or radiation multiplied by the stipulated photons. 2. A stream of particles, es- dimensionless factors is 1 J kg–1. As pecially alpha- or beta-particles from different types of radiation cause dif- a radioactive source or neutrons ferent effects in biological tissue a from a nuclear reactor. weighted absorbed dose, called the dose equivalent, is used in which the radiationless decay Decay of an absorbed dose is modiÜed by multi- atom or molecule from an excited plying it by dimensionless factors state to a lower energy state without stipulated by the International Com- the emission of electromagnetic radi- mission on Radiological Protection. ation. A common example of a radia- The former unit of dose equivalent, * tionless process is the Auger effect, the rem (originally an acronym for in which an electron rather than a roentgen equivalent man), is equal to photon is emitted as a result of 10–2 Sv. decay. In SI units, exposure to ionizing ra- radiation units Units of measure- diation is expressed in coulombs per ment used to express the activity of a kilogram, the quantity of X- or radiative capture 452

gamma-radiation that produces ion tunium–237, and ends with bis- pairs carrying 1 coulomb of charge of muth–209. either sign in 1 kilogram of pure dry radioactive tracing See labelling. air. The former unit, the roentgen (R), is equal to 2.58 × 10–4 Ckg–1. radioactivity The spontaneous dis- capture integration of certain atomic nuclei radiative capture See . accompanied by the emission of radical A group of atoms, either in alpha-particles (helium nuclei), beta- a compound or existing alone. See particles (electrons or positrons), or free radical; functional group. gamma radiation (short-wavelength A electromagnetic waves). Natural radioactivity is the result of • Information about IUPAC nomenclature the spontaneous disintegration of • List of radical names naturally occurring radioisotopes. radical ion A radical that has a pos- Many radioisotopes can be arranged itive or negative charge. An example in three *radioactive series. The rate .+ is the benzene radical cation C6H6 . of disintegration is uninÛuenced by In radical ions, the odd electron and chemical changes or any normal the charge are usually (but not neces- changes in their environment. How- sarily) on the same atom. ever, radioactivity can be induced in many nuclides by bombarding them radioactive age The age of an ar- with neutrons or other particles. See chaeological or geological specimen also decay; ionizing radiation; radi- as determined by a process that de- ation units. pends on a radioactive decay. See carbon dating; fission-track dating; radiocarbon dating See carbon potassium–argon dating; rubidium– dating. strontium dating uranium lead ; – radiochemistry The branch of dating . chemistry concerned with radio- radioactive dating See radiomet- active compounds and with ioni- ric dating. zation. It includes the study of compounds of radioactive elements radioiso- radioactive isotope See and the preparation and use of com- tope r . pounds containing radioactive atoms. radioactive nuclide See radionu- See labelling; radiolysis. clide . radiogenic Resulting from radio- radioactive series A series of active decay. radioactive nuclides in which each radioimmunoassay (RIA) A sensi- member of the series is formed by tive quantitative method for detect- the decay of the nuclide before it. ing trace amounts of a biomolecule, The series ends with a stable nuclide. based on its capacity to displace a ra- Three radioactive series occur natu- dioactively labelled form of the mol- rally, those headed by thorium–232 ecule from combination with its (thorium series), uranium–235 (ac- antibody. tinium series), and uranium–238 (uranium series). All three series end with radioisotope (radioactive isotope) An isotope of an element that is an isotope of lead. The neptunium se- labelling ries starts with the artiÜcial isotope radioactive. See . plutonium–241, which decays to nep- radiolysis The use of ionizing radi- 453 Raman effect ation to produce chemical reactions. undergoes alpha decay. It is used in The radiation used includes alpha radiotherapy. Radon occurs naturally, particles, electrons, neutrons, X-rays, particularly in areas underlain by and gamma rays from radioactive granite, where it is thought to be a materials or from accelerators. En- health hazard. As a noble gas, radon ergy transfer produces ions and exis practically inert, although a few cited species, which undergo further compounds, e.g. radon Ûuoride, reaction. A particular feature of radi- can be made. It was Ürst isolated olysis is the formation of short-lived by William Ramsey and Robert solvated electrons in water and other Whytlaw-Gray (1877–1958) in 1908. polar solvents. A radiometric dating (radioactive • Information from the WebElements site dating) See dating techniques; Ü Ü radioactive age raf nate A liquid puri ed by sol- . vent extraction. radionuclide (radioactive nuclide) rafÜnose A white solid carbohy- A *nuclide that is radioactive. ° drate, C18H32O16, m.p. 80 C. It is a radiopharmaceuticals Com- trisaccharide (a type of *sugar) con- pounds used in medicine that have a sisting of fructose, galactose and glu- radioactive atom in the molecule. Ra- cose. It occurs naturally in sugar-beet diopharmaceuticals are both for diag- and cotton-seed residues. nostic purposes (as in radionuclide r.a.m. See relative atomic mass. imaging) or for therapy (e.g. certain cancer treatments). Raman effect A type of scattering of electromagnetic radiation in radium Symbol Ra. A radioactive which light suffers a change in fre- metallic element belonging to quency and a change in phase as it *group 2 (formerly IIA) of the peri- passes through a material medium. odic table; a.n. 88; r.a.m. 226.0254; The intensity of Raman scattering is r.d. ∼5; m.p. 700°C; b.p. 1140°C. It oc- about one-thousandth of that in curs in uranium ores (e.g. pitchblende). The most stable isotope is Rayleigh scattering in liquids; for this radium–226 (half-life 1602 years), reason it was not discovered until which decays to radon. It is used as a 1928. However, it was not until the r radioactive source in research and, to development of the laser that the ef- some extent, in radiotherapy. The el- fect was put to use. ement was isolated from pitchblende In Raman spectroscopy light from a in 1898 by Marie and Pierre Curie. laser is passed through a substance A and the scattering is analysed spectroscopically. The new frequencies in • Information from the WebElements site the Raman spectrum of monochro- radon Symbol Rn. A colourless matic light scattered by a substance radioactive gaseous element belong- are characteristic of the substance. ing to group 18 of the periodic table Both inelastic and superelastic scat- (the *noble gases); a.n. 86; r.a.m. 222; tering occurs. The technique is used d. 9.73 g dm–3; m.p. –71°C; b.p. as a means of determining molecular –61.8°C. At least 20 isotopes are structure and as a tool in chemical known, the most stable being analysis. The effect was discovered by radon–222 (half-life 3.8 days). It is the Indian scientist Sir C. V. Raman formed by decay of radium–226 and (1888–1970). Ramsay, Sir William 454

A efÜcient catalyst, especially for hy- • The original paper by Raman and drogenation reactions at room tem- Krishnan perature. It was discovered in 1927 by the American chemist M. Raney Ramsay, Sir William (1852–1916) (1885–1966). British chemist, born in Glasgow. After working under Robert *Bun- ranksite A mineral consisting of sen, he returned to Glasgow before a mixed sodium carbonate, sodium taking up professorships at Bristol sulphate, and potassium chloride, (1880–87) and London (1887–1912). In 2Na2CO3.9Na2SO4.KCl. the early 1890s he worked with Lord Raoult’s law The partial vapour *Rayleigh on the gases in air and in pressure of a solvent is proportional 1894 they discovered argon. In 1898, to its mole fraction. If p is the vapour with Morris Travers (1872–1961), he pressure of the solvent (with a sub- discovered neon, krypton, and stance dissolved in it) and X the mole xenon. Six years later he discovered fraction of solvent (number of moles the last of the noble gases, radon. He of solvent divided by total number of was awarded the Nobel Prize for moles) then p = p0X, where p0 is the chemistry in 1904, the year in which vapour pressure of the pure solvent. Rayleigh received the physics prize. A solution that obeys Raoult’s law is random alloy See disordered said to be an ideal solution. In gen- solid. eral the law holds only for dilute solutions, although some mixtures of random walk The problem of de- liquids obey it over a whole range of termining the distance from a start- concentrations. Such solutions are ing position made by a walker, who perfect solutions and occur when the can either move forward (toward +x) or backwards (toward –x) with the intermolecular forces between mol- choice being made randomly (e.g. by ecules of the pure substances are tossing a coin). The progress of the similar to the forces between mol- walker is characterized by the net ecules of one and molecules of the other. Deviations in Raoult’s law for distance DN travelled in N steps. After N steps the root mean square value mixtures of liquids cause the formation of *azeotropes. The law was dis- r Drms, which is the average distance away from the starting position, is covered by the French chemist √ François Raoult (1830–1901). given by Drms = N. Physical applications of the random walk include dif- A fusion and the related problem of • Raoult’s original paper Brownian motion as well as prob- rapeseed methyl ester See bio- lems involving the structures of poly- fuel. mers and disordered solids. rare-earth elements See lan- Raney nickel A spongy form of thanoids. nickel made by the action of sodium hydroxide on a nickel–aluminium rarefaction A reduction in the alloy. The sodium hydroxide dis- pressure of a Ûuid and therefore of solves the aluminium leaving a its density. highly active form of nickel with a rare gases See noble gases. large surface area. The material is a black pyrophoric powder saturated Raschig process An industrial with hydrogen. It is an extremely process for making chlorobenzene 455 rayon

(and phenol) by a gas-phase reaction rationalized Planck constant See between benzene vapour, hydrogen planck constant. chloride, and oxygen (air) at 230°C: rationalized units A system of → 2C6H6 + 2HCl + O2 2H2O + units in which the deÜning equations 2C6H5Cl have been made to conform to the The catalyst is copper(II) chloride. geometry of the system in a logical The chlorobenzene is mainly used way. Thus equations that involve cir- for making phenol by the reaction cular symmetry contain the factor → 2π, while those involving spherical C6H5Cl + H2O HCl + C6H5OH π * This reaction proceeds at 430°C with symmetry contain the factor 4 . SI a silicon catalyst. The process was in- units are rationalized; c.g.s. units are vented by the German chemist Fritz unrationalized. Raschig (1863–1928). Rayleigh, Lord (John William Raschig synthesis See hydrazine. Strutt; 1842–1919) British physicist, who built a private laboratory after RasMol A commonly used program working at Cambridge University. for visualizing molecules. It can be His work in this laboratory included used with a range of Üle formats and the discovery of *Rayleigh scattering the display can be choosen (e.g. wire- of electromagnetic radiation. He also frame, ball-and-stick, space Ülling, worked in acoustics, electricity, and etc.). The original version was writ- optics, as well as collaborating with ten by Roger Sayle of Glaxo around William *Ramsay on the discovery of 1990. It is freely available. argon. He was awarded the 1904 A Nobel Prize for physics. • A downloadable version of RasMol from Rayleigh scattering Scattering of the website of the University of electromagnetic radiation by mol- Massachusetts Amherst ecules in which the frequency of the rate constant (velocity constant) scattered radiation is unchanged. Symbol k. The constant in an expres- This type of scattering was analysed sion for the rate of a chemical reac- by Lord *Rayleigh in his papers in tion in terms of concentrations (or the late 19th century, which showed activities). For instance, in a simple that the blue colour of the sky is a re- r unimolecular reaction A → B, the sult of this type of light scattering, rate is proportional to the concentra- with molecules of the atmosphere of tion of A, i.e. rate = k[A], where k is the earth scattering light from the the rate constant, which depends on sun. the temperature. The equation is the rayon A textile made from cellu- rate equation of the reaction, and its lose. There are two types, both made form depends on the reaction mecha- from wood pulp. In the viscose nism. process, the pulp is dissolved in car- rate-determining step The slow- bon disulphide and sodium hydrox- est step in a chemical reaction that ide to give a thick brown liquid involves a number of steps. In such containing cellulose xanthate. The reactions, there is often a single step liquid is then forced through Üne that is appreciably slower than the nozzles into acid, where the xanthate other steps, and the rate of this de- is decomposed and a cellulose Üla- termines the overall rate of the reac- ment is produced. The product is vis- tion. cose rayon. In the acetate process RBS 456

cellulose acetate is made and dis- iprocal lattice than the real-space lat- solved in a solvent. The solution is tice. forced through nozzles into air, reciprocal proportions See chemi- where the solvent quickly evaporates cal combination. leaving a Ülament of acetate rayon. recombination process The RBS See rutherford backscattering spectrometry process in which a neutral atom or . molecule is formed by the combina- RDX See cyclonite. tion of a positive ion and a negative ion or electron; i.e. a process of the reactant See chemical reaction. type: reaction See chemical reaction. A+ + B– → AB reactive dye See dyes. or reagent A substance reacting with A+ + e– → A another substance. Laboratory In recombination, the neutral reagents are compounds, such as sul- species formed is usually in an ex- phuric acid, hydrochloric acid, cited state, from which it can decay sodium hydroxide, etc., used in with emission of light or other elec- chemical analysis or experiments. tromagnetic radiation. realgar A red mineral form of ar- recreational drug A drug used for senic(II) sulphide, As2S2. recreation, as opposed to medical use. Strictly, recreational drugs in- real gas A gas that does not have clude alcohol and nicotine, but the the properties assigned to an *ideal Ü term is often understood to mean gas. Its molecules have a nite size substances such as marijuana, co- and there are forces between them caine, and amphetamines. (see equation of state). recrystallization A process of re- rearrangement A type of chemical peated crystallization in order to pu- reaction in which the atoms in a rify a substance or to obtain more molecule rearrange to form a new regular crystals of a puriÜed sub- molecule. Examples are the *pinacol stance. rearrangement and the *Wagner– r Meerwein rearrangement. rectiÜcation The process of purifying a liquid by *distillation. See frac- reciprocal lattice A lattice for a tional distillation. crystal that can be deÜned from the lattice in real space. If the primitive rectiÜed spirit A constant-boiling ° translation vectors of the lattice in mixture of *ethanol (95.6 ) and real space are denoted by a, b, c then water; it is obtained by distillation. the primitive translations a′, b′, c′ in red lead See dilead(ii) lead(iv) the reciprocal lattice are deÜned by oxide. a′ b × c abc b′ c × a abc c′ = [ ], = [ ], = oxidation reduction a × b[abc], where [abc] denotes the redox See – . scalar triple product a.(b × c). reducing agent (reductant) A sub- The reciprocal lattice is a funda- stance that brings about reduction in mental concept in the theory of *X- other substances. It achieves this by ray diffraction and energy bands being itself oxidized. Reducing with a diffraction pattern being agents contain atoms with low oxida- much more closely related to the rection numbers; that is the atoms have 457 relative density gained electrons. In reducing other refractory 1. Having a high melt- substances, these atoms lose elec- ing point. Metal oxides, carbides, and trons. silicides tend to be refractory, and reducing sugar A monosaccharide are extensively used for lining fur- or disaccharide sugar that can donate naces. 2. A refractory material. electrons to other molecules and can regioselectivity The effect in therefore act as a reducing agent. which certain positions in the mol- The possession of a free ketone ecule are favoured over others in a (–CO–) or aldehyde (–CHO) group en- reaction. An example is the way in ables most monosaccharides and dis- which certain groups on the benzene accharides to act as reducing sugars. ring direct further substituents to the Reducing sugars can be detected by meta position or to the ortho and *Benedict’s test. Compare nonreduc- para positions. *Markovnikoff’s rule ing sugar. for electrophilic additions to alkenes reductant See reducing agent. is another example. reduction See oxidation– Regnault’s method A technique reduction. for measuring gas density by evacuat- ing and weighing a glass bulb of Ü petroleum re nery gas See . known volume, admitting gas at reÜning The process of purifying known pressure, and reweighing. substances or extracting substances The determination must be carried from mixtures. out at constant known temperature and the result corrected to standard reÛuxing A laboratory technique temperature and pressure. The in which a liquid is boiled in a con- method is named after the French tainer attached to a condenser (reÛux chemist Henri Victor Regnault condenser), so that the liquid contin- (1810–78). uously Ûows back into the container. It is used for carrying out reactions relative atomic mass (atomic over long periods in organic synthe- weight; r.a.m.) Symbol Ar. The ratio sis. of the average mass per atom of the naturally occurring form of an el- reforming The conversion of straight-chain alkanes into branched- ement to 1/12 of the mass of a car- r chain alkanes by *cracking or by bon–12 atom. catalytic reaction. It is used in pe- A troleum reÜning to produce hydro- • Current values for relative atomic carbons suitable for use in gasoline. masses Benzene is also manufactured from relative density (r.d.) The ratio of alkane hydrocarbons by catalytic re- the *density of a substance to the forming. Steam reforming is a density of some reference substance. process used to convert methane For liquids or solids it is the ratio of (from natural gas) into a mixture of the density (usually at 20°C) to the carbon monoxide and hydrogen, density of water (at its maximum which is used to synthesize organic density). This quantity was formerly chemicals. The reaction called speciÜc gravity. Sometimes rel- → CH4 + H2O CO + 3H2 ative densities of gases are used; for occurs at about 900°C using a nickel example, relative to dry air, both catalyst. gases being at s.t.p. relative molecular mass 458

relative molecular mass (molecu- from sticking together. Release lar weight) Symbol Mr. The ratio of agents are used in many industrial the average mass per molecule of the processes, including the manufacture naturally occurring form of an el- of food, glass, paper, and plastics. ement or compound to 1/12 of the They include polyethene, polyte- mass of a carbon–12 atom. It is equal traÛuoroethene (PTFE), polyvinyl al- to the sum of the relative atomic cohol (PVA), silicones, and waxes, as masses of all the atoms that com- well as stearates and other glyc- prise a molecule. erides. They are also known as abher- ents or parting agents. relative permittivity See permittivity. rem See radiation units. relativistic quantum mechanics renaturation The reconstruction Quantum mechanics that is in accord of a protein or nucleic acid that has with special relativity theory. The been denatured such that the mol- main equation of relativistic quan- ecule resumes its original function. tum mechanics is the *Dirac equa- Some proteins can be renatured by tion. It is necessary to use relativistic reversing the conditions (of tempera- quantum mechanics to describe the ture, pH, etc.) that brought about de- electronic properties of heavy atoms naturation. Ü and all the * ne structure of atomic renewable energy sources spectra. The colour of solid gold and Sources of energy that do not use up mercury existing as a liquid are both the earth’s Ünite mineral resources. due to relativistic effects in quantum Nonrenewable energy sources are mechanics. *fossil fuels and Üssion fuels. Various relativistic quantum theory See renewable energy sources are being quantum theory. used or investigated, including geothermal energy, hydroelectric power, relaxation The return to the equi- nuclear fusion, solar energy, tides, librium state of a system after it has wind power, and wave power. experienced a sudden change due to an external inÛuence. The time for rennin An enzyme secreted by cells relaxation to take place is called the lining the stomach in mammals that r relaxation time. An example is the is responsible for clotting milk. It average time that a system remains acts on a soluble milk protein (ca- in the higher energy state before it seinogen), which it converts to the falls to the lower energy state in nu- insoluble form casein. This ensures clear magnetic resonance (NMR). that milk remains in the stomach In general, any process involving long enough to be acted on by pro- decay is assumed to have exponential tein-digesting enzymes. decay, with the relaxation time being Reppe processes A set of related the time it takes for the variable to industrial reactions of acetylene to fall from its initial value to 1/e of its produce vinyl compounds, such as initial value. Another example of re- ≡ → laxation time is the time needed for HC CH + ROH H2C=CHR a gas to return to the Maxwell distri- ≡ → HC CH + RCN H2C=CR(CN) bution of velocities, after it has been They take place at high pressure suddenly disturbed from that state. using metal acelylide catalysts. The release agent A substance that is processes are named after the Ger- applied to surfaces to prevent them man chemist Walter Reppe (1892– 459 resonance ionization spectroscopy

1969), who pioneered techniques for be separated and reconverted into the safe industrial use of acetylene. the pure l-A and d-A. Biological tech- reptation A motion describing the niques using bacteria that convert dynamics of a polymer in a highly one form but not the other can also entangled state, such as a network. be used. Regarding the entangled state as a resonance The representation of set of chains between crosslinks it is the structure of a molecule by two or possible to regard the chain as being more conventional formulae. For ex- in a ‘tube’, with the tube being ample, the formula of methanal can formed by topological constraints. be represented by a covalent struc- The chain is longer than the tube so ture H2C=O, in which there is a dou- that the ‘slack’ of the chain moves ble bond in the carbonyl group. It is through the tube, which causes the known that in such compounds the tube itself to change with time. This oxygen has some negative charge motion was called reptation (from and the carbon some positive charge. the Latin reptare, to creep), by the The true bonding in the molecule is French physicist P. G. de Gennes, somewhere between H2C=O and the who postulated it in 1971. Many ex- + – periments indicate that reptation ionic compound H2C O . It is said to dominates the dynamics of polymer be a resonance hybrid of the two, in- chains when they are entangled. dicated by H C=O ↔ H C+O– resin A synthetic or naturally occur- 2 2 ring *polymer. Synthetic resins are The two possible structures are called used in making *plastics. Natural canonical forms, and they need not resins are acidic chemicals secreted contribute equally to the actual form. by many trees (especially conifers) Note that the double-headed arrow into ducts or canals. They are found does not imply that the two forms either as brittle glassy substances or are in equilibrium. dissolved in essential oils. Their func- resonance effect See electronic tions are probably similar to those of effects. gums and mucilages. resonance ionization spec- resolution The process of separat- troscopy (RIS) A spectroscopic tech- ing a racemic mixture into its opti- r nique in which single atoms in a gas cally active constituents. In some are detected using a laser to ionize cases the crystals of the two forms have a different appearance, and the that atom. A sample containing the separation can be done by hand. In atoms to be excited is subjected to general, however, physical methods light from a laser, tuned so that only (distillation, crystallization, etc.) can- that type of atom is excited by the not be used because the optical iso- light. If the frequency of light at ν mers have identical physical which the atom is excited is , the properties. The most common tech- atoms in the excited state can be ion- nique is to react the mixture with a ized if the ionization potential of the compound that is itself optically ac- atom is less than 2ν. In contrast to tive, and then separate the two. For other techniques of ionization, this instance, a racemic mixture of l-A type of ionization only occurs for and d-A reacted with l-B, gives two atoms that are ‘in tune’ with the fre- compounds AB that are not optical quency of light. Because RIS is very isomers but diastereoisomers and can selective in determining which atom resorcinol 460

is ionized for a given frequency it has to be heated is kept separate from many applications in chemistry. the fuel. It consists of a shallow hearth on which the charge is heated resorcinol See 1,3-dihydroxyben- by Ûames that pass over it and by ra- zene. diation reÛected onto it from a low retinol See vitamin a. roof. retort 1. A laboratory apparatus reverse osmosis A method of ob- consisting of a glass bulb with a long taining pure water from water con- neck. 2. A vessel used for reaction or taining a salt, as in *desalination. distillation in industrial chemical Pure water and the salt water are processes. separated by a semipermeable membrane and the pressure of the salt retrosynthetic analysis A tech- water is raised above the osmotic nique for planning the series of reac- pressure, causing water from the tions required to synthesize a given brine to pass through the membrane molecule. It involves working back- into the pure water. This process re- wards from the known structure. The quires a pressure of some 25 atmos- tar- molecule to be synthesized (the pheres, which makes it difÜcult to get molecule) is considered to be split apply on a large scale. into two parts (a process called disconnection). The parts are not them- reversible process Any process in selves molecules, but are fragments which the variables that deÜne the (called synthons) that correspond to state of the system can be made to actual reagents. A trivial example change in such a way that they pass would be the synthesis of the target through the same values in the re- molecule verse order when the process is reversed. It is also a condition of a R1–O–CO–R2 reversible process that any ex- This is an ester. A suitable disconnec- changes of energy, work, or matter tion is at the O–CO bond, giving two with the surroundings should be re- synthons, versed in direction and order when R1–O– and R2C(O)–. the process is reversed. Any process The convention is to write this in the that does not comply with these con- r form ditions when it is reversed is said to be an irreversible process. All natural R1–O–C(O)– R2 ⇒ R1–O– + R2–C(O)–. processes are irreversible, although Here the symbol is a logic symbol for some processes can be made to ap- ‘implies’; it is called the retrosyn- proach closely to a reversible process. thetic arrow. In this example, suitable reagents corresponding to these RF value (in chromatography) The synthons would be an alcohol R1OH distance travelled by a given compo- and an acyl chloride R2COCl. Note nent divided by the distance trav- that the disconnection is always elled by the solvent front. For a given made at a point corresponding to a system at a known temperature, it is known reaction. In actual schemes, a characteristic of the component the target molecules are complex and can be used to identify compo- and a number of retrosynthetic steps nents. are needed. rhe A unit of Ûuidity equal to the reciprocal of the *poise. reverberatory furnace A metallurgical furnace in which the charge rhenium Symbol Re. A silvery- 461 Rice–Ramsperger–Kassel theory white metallic *transition element; of *RNA (ribonucleic acid). Its deriva- a.n. 75; r.a.m. 186.2; r.d. 20.53; m.p. tive deoxyribose, C5H10O4, is equally 3180°C; b.p. 5627 (estimated)°C. The important as a constituent of *DNA element is obtained as a by-product (deoxyribonucleic acid), which car- in reÜning molybdenum, and is used ries the genetic code in chromo- in certain alloys (e.g. rhenium– somes. molybdenum alloys are supercon- ribulose A ketopentose sugar (see ducting). The element forms a num- monosaccharide), C H O , that is ber of complexes with oxidation 5 11 5 involved in carbon dioxide Üxation in states in the range 1–7. It was discov- photosynthesis as a component of ered by Walter Noddack (1893–1960) *ribulose bisphosphate. and Ida Tacke (1896–1978) in 1925. A ribulose bisphosphate (RuBP) A Ü • Information from the WebElements site ve-carbon sugar that is combined with carbon dioxide to form two rheology The study of the defor- three-carbon intermediates in the Û mation and ow of matter. Ürst stage of the light-dependent re- rheopexy The process by which actions of *photosynthesis (see certain thixotropic substances set calvin cycle). The enzyme that me- more rapidly when they are stirred, diates the carboxylation of ribulose shaken, or tapped. Gypsum in water bisphosphate is ribulose bisphos- is such a rheopectic substance. phate carboxylase. rhodinol See aminophenol. Rice–Herzfeld mechanism A mechanism enabling complex chain rhodium Symbol Rh. A silvery- reactions to give simple rate laws in white metallic *transition element; chemical kinetics. An example of a a.n. 45; r.a.m. 102.9; r.d. 12.4; m.p. Rice–Herzfeld mechanism occurs 1966°C; b.p. 3727°C. It occurs with with the pyrolysis of acetaldehyde, platinum and is used in certain plat- the mechanism consisting of initia- inum alloys (e.g. for thermocouples) tion, propagation (in two steps), and and in plating jewellery and optical termination. This leads to the experi- reÛectors. Chemically, it is not at- mental result that the overall reac- tacked by acids (dissolves only slowly tion is three-halves order in CH CHO. in aqua regia) and reacts with non- 3 r To ascertain that such a mechanism metals (e.g. oxygen and chlorine) at is correct, either the steady-state ap- red heat. Its main oxidation state is proximation is used or the differen- +3 although it also forms complexes in the +4 state. The element was dis- tial equations for the reaction rates covered in 1803 by William Wolla- are solved numerically. It is pos- ston (1766–1828). sible, as in this example, for the A Rice–Herzfeld mechanism to give a correct description of the main part • Information from the WebElements site of the reaction but not to take ac- RIA See radioimmunoassay. count of reactions that produce Û vitamin b complex by-products. The Rice–Herzfeld ribo avin See . mechanism is named after F. O. Rice ribonucleic acid See rna. and K. F. Herzfeld, who put forward the scheme in 1934. ribose A *monosaccharide, C5H10O5, rarely occurring free in na- Rice–Ramsperger–Kassel theory ture but important as a component (RRK theory) A statistical theory used ricin 462

to describe unimolecular reactions. strain. In the boat conformation, the The RRK theory, put forward by O. K. 1 and 4 carbon atoms lie on the same Rice, H. C. Ramsperger, and indepen- side of the plane. Other conformers dently by L. S. Kassel in the late are the twist conformation and the 1920s, assumes that a molecule is a half chair conformation (see dia- system of loosely coupled oscillators. gram). The energy difference be- Regarding the coupling of the oscilla- tween the chair conformation and tors as loose enables calculations to the (least stable) half chair is 10.8 be made on the statistical distribu- kcal/mol and the forms interconvert Û tion of energy freely owing between quickly at normal temperatures the vibrational modes. In RRK theory (about 99% of molecules are in the the rate constant for the decomposi- chair form). tion of a molecule increases with the Other rings also have different con- energy of the molecule. The RRK formations. In a Üve-membered ring, theory has been applied with success to some chemical reactions but is such as that of cyclopentane, the limited in its predictive power. The twist conformation has three adja- unsatisfactory features of the RRK cent carbon atoms in a plane with theory are improved by the *RRKM one of the carbons above this plane theory (Rice–Ramsperger–Kassel– and the other below it. The less sta- Marcus theory), in which the individ- ble envelope conformation has four ual vibrational modes of the system atoms in a plane with one atom out are taken into account. of the plane. In an eight-membered ring, such as that of cyclooctane, the ricin A highly toxic protein present tub conformation has four atoms cor- in the castor oil plant (Ricinus commu- responding to a pair of diametrically nis), in particular in the seeds of the plant (castor beans). It is probably the opposite bonds in one plane, with most poisonous substance present in the other four atoms on the same plants – the fatal dose is about 1 mil- side of this plane. It is analogous to ligram per kilogram of body weight. the boat form of cyclohexane. The crown conformation has atoms alter- ring A closed chain of atoms in a natively above and below the average molecule. In compounds, such as plane of the molecule. r naphthalene, in which two rings Various names are given to posi- share a common side, the rings are tions attached to atoms in different fused rings. Ring closures are chemi- ring conformations. In the chair con- cal reactions in which one part of a formation of cyclohexane, an axial chain reacts with another to form a bond is one that makes a large angle ring, as in the formation of *lactams and *lactones. with the average plane of the ring. An equatorial bond is one that makes ring conformations Shapes that a small angle. In a cyclopentane ring, can be taken by nonplanar rings and the terms pseudo-axial and pseudo- can be interconverted by formal rota- equatorial are used. In the boat con- tions about single bonds. In cyclo- formation of cyclohexane the bond hexane, for example, the most stable roughly parallel to the plane of four conformation is the chair conforma- atoms is the bowsprit, the other tion in which the 2, 3, 5 and 6 atoms bond being the Ûagpole. lie in a plane and the 1 and 4 carbon atoms lie on opposite sides of the RIS See resonance ionization spec- plane. In this form, there is no ring troscopy. 463

chair boat

twist half chair

flagpole axial

bowsprit equatorial

Conformations of of a six-membereda six-membered ring ring

4 5

5 1 3 1 3 2 4 2 r twist (1, 2, 3, are coplanar) envelope (1, 2, 3, 5, are coplanar)

Conformations of of a five-membereda five-membered ring ring

crown tub

Conformations of ofan aneight-membered eight-membered ring ring R-isomer 464

O–

O P O CH2 nucleotide O– O

OOH

O P O CH2 O– O ribose

OOH G U A G C O P O CH2 G G A C bases A O– O C C U A A OOH A U G OOCHP U C 2 O– O G U A A U U G OH OH G G G Detail of molecular structure of sugar– A phosphate backbone. Each ribose unit is A U G attached to a phosphate group and a base, A bases forming a nucleotide.

sugar–phosphate backbone

Single-stranded structure of RNA

NH2 O adenine N C (A) N C C N C NH HC HC C CH C C N guanine N N N NH2 H (G) H

NH2 O C C HC N cytosine HC NH uracil (U) HC C (C) HC C r N O N O H H

The four bases of RNA

RNA

R-isomer See absolute configura- cleus and then distributed to various tion. parts of the cytoplasm. An RNA mol- biofuel ecule consists of a long chain of *nu- RME See . cleotides in which the sugar is RNA (ribonucleic acid) A complex or- *ribose and the bases are adenine, ganic compound (a nucleic acid) in cytosine, guanine, and uracil (see il- dna living cells that is concerned with lustration). Compare . *protein synthesis. In some viruses, roasting The heating of a Ünely RNA is also the hereditary material. ground ore, especially a sulphide, in Most RNA is synthesized in the nu- air prior to *smelting. The roasting 465 rosin process expels moisture, chemically named after the discoverer of X-rays, combined water, and volatile matter; W. K. Roentgen (1845–1923). in the case of sulphides, the sulphur roentgenium Symbol Rg. A radio- is expelled as sulphur dioxide and active transactinide; a.n. 111. It was the ore is converted into an oxide. made by fusion of 209Bi with 64Ni. Rochelle salt Potassium sodium Only a few atoms have been de- tartrate tetrahydrate, KNaC4H4O6. tected. 4H2O. A colourless crystalline salt A used for its piezoelectric properties. • Information from the WebElements Rochon prism An optical device site consisting of two quartz prisms; the Rohypnol See flunitrazepam. Ürst, cut parallel to the optic axis, re- ceives the light; the second, with the root-mean-square value (RMS value) 1. optic axis at right angles, transmits (in statistics) A typical value of a number (n) of values of a quan- the ordinary ray without deviation tity (x ,x ,x …) equal to the square but the extraordinary ray is deÛected 1 2 3 root of the sum of the squares of the and can be absorbed by a screen. The values divided by n, i.e. device can be used to produce plane- √ 2 2 2 polarized light and it can also be RMS value = [(x1 + x2 + x3 …)/n] used with ultraviolet radiation. 2. (in physics) A typical value of a continuously varying quantity, such rock An aggregate of mineral parti- as an alternating electric current, ob- cles that makes up part of the earth’s tained similarly from many samples crust. It may be consolidated or un- taken at regular time intervals dur- consolidated (e.g. sand, gravel, mud, ing a cycle. Theoretically this can be shells, coral, and clay). shown to be the effective value, i.e. rock crystal See quartz. the value of the equivalent direct current that would produce the same rocking-chair cell See intercalation cell power dissipation. For a sinusoidal . √ current this is equal to Im/ 2, where halite rock salt See . Im is the maximum value of the cur- rock salt structure (sodium chlo- rent. r ride structure) A type of ionic crystal ROSDAL A type of *line notation structure in which the cations have a used in databases. It is based on a face-centred cubic arrangement, with *connection table for the molecule anions occupying all the octahedral written in a single text line. ROSDAL holes. It can equally be described as a is an acronym for Representation of fcc array of anions with cations in Organic Structure Descriptions the octahedral holes. Each type of Arranged Linearly. ion has a coordination number of 6. Rose’s metal An alloy of low melt- Examples of compounds that have ing point (about 100°C) consisting of the structure are NaCl, KBr, AgCl, 50% bismuth, 25–28% lead, and AAgBr, HgO, CaO, FeO, NiO, and SnAs. 22–25% tin. rosin A hard natural resin obtained • An interactive version of the structure from pine tree oil and the wastes roentgen The former unit of dose from processing wood pulp. It may equivalent (see radiation units). It is be colourless, yellow, brown, or rotamer 466

black. It is used as a Ûotation agent, is no formal chemical bonding be- solder Ûux, sizing compound, and in tween the dumbbell and the ring. Ro- lacquers and plasticizers. It is also taxanes are examples of compounds used to provide ‘grip’ to violinists’ with *mechanical bonding. A num- bows (when it may be called ber of natural peptide rotaxanes have colophony) and dancers’ and boxers’ been identiÜed. Synthesis of new ro- shoes. taxanes is a matter of interest be- rotamer One of a set of *conforma- cause of their possible use as tional isomers that differ from each ‘molecular machines’ in nanotech- other by restricted rotation about nology (e.g. as molecular switches or one or more single bonds. information storage units). RRK theory See rice–ramsperger– rotational spectroscopy The kassel theory spectroscopic study of the rotational . motion of molecules. Rotational spec- RRKM theory (Rice–Ramsperger– troscopy gives information about in- Kassel–Marcus theory) A theory of teratomic distances. The transitions unimolecular chemical reactions in between different rotational energy which the *Rice–Ramsperger–Kassel levels in molecules correspond to the theory was improved by the US microwave and far-infrared regions chemists O. K. Rice and R. A. Marcus of the electromagnetic spectrum. It is in 1950; it was further improved sub- only possible for there to be transi- sequently in several papers by Mar- tions between rotational energy lev- cus and associates by taking into els in pure rotational spectra if the account the individual vibrational molecule has a permanent dipole frequencies, rotations, and zero-point moment. In the near-infrared region energies of the energized species and rotational transitions are superim- activated complexes taking part. The posed on vibrational transitions, re- RRKM theory is very successful in ex- sulting in a vibrational–rotational plaining the results of experiments spectrum. This type of spectrum is for unimolecular reactions for a vari- considerably more complicated than ety of reactions. vi- a purely rotational spectrum. See absolute con- brational spectroscopy. R–S convention See r figuration. rotation group A group formed by A the set of all rotations about a point. • Information about IUPAC nomenclature The rotation group is associated with the angular momentum of a system rubber A polymeric substance ob- and is important in the theory of the tained from the sap of the tree Hevea rotational motion of molecules. The brasiliensis. Crude natural rubber is group representations of the rotation obtained by coagulating and drying group are closely associated with the the sap (latex), and is then modi- quantum theory of angular momen- Üed by *vulcanization and com- tum. pounding with Üllers. It is a polymer of *isoprene containing the unit rotaxane A type of compound that –CH C(CH ):CHCH –. Various syn- has a dumbbell-shaped molecule 2 3 2 thetic rubbers can also be made. with a cyclic molecule around its See neoprene; nitrile rubber; sili- axis. The dumbbell has a chain with cones. large groups at each end, these being large enough to trap the ring. There rubidium Symbol Rb. A soft silvery- 467 ruthenium white metallic element belonging to atoms and nucleons in nuclei, in *group 1 (formerly IA) of the periodic which the energies associated with table; a.n. 37; r.a.m. 85.47; r.d. 1.53; electrostatic repulsion are much m.p. 38.89°C; b.p. 688°C. It is found greater than the energies associated in a number of minerals (e.g. lepido- with *spin–orbit coupling. *Multi- lite) and in certain brines. The metal plets of many-electron atoms with a is obtained by electrolysis of molten low atomic number are characterized rubidium chloride. The naturally oc- by Russell–Saunders coupling. It is curring isotope rubidium–87 is radio- named after the US physicists Henry active (see rubidium–strontium Norris Russell (1877–1957) and Fred- dating). The metal is highly reactive, erick Saunders (1875–1963) who pos- with properties similar to those of tulated this type of coupling to other group 1 elements, igniting explain the spectra of many-electron spontaneously in air. It was discov- atoms with low atomic number in ered spectroscopically by Robert Bun- 1925. The multiplets of heavy atoms sen and Gustav Kirchhoff in 1861. and nuclei are better described by *j- A j coupling or intermediate coupling, • Information from the WebElements site i.e. a coupling in which the energies of electrostatic repulsion and rubidium–strontium dating A spin–orbit coupling are similar in method of dating geological speci- size. mens based on the decay of the radioisotope rubidium–87 into the rusting Corrosion of iron (or steel) stable isotope strontium–87. Natural to form a hydrated iron(III) oxide rubidium contains 27.85% of rubid- Fe2O3.xH2O. Rusting occurs only in ium–87, which has a half-life of 4.7 × the presence of both water and oxy- 1010 years. The ratio 87Rb/87Sr in a gen. It is an electrochemical process specimen gives an estimate of its age in which different parts of the iron (up to several thousand million surface act as electrodes in a cell re- years). action. At the anode, iron atoms dissolve as Fe2+ ions: ruby The transparent red variety of Fe(s) → Fe2+(aq) + 2e the mineral *corundum, the colour being due to the presence of traces of At the cathode, hydroxide ions are r chromium. It is a valuable gemstone, formed: → – more precious than diamonds. The O2(aq) + 2H2O(l) + 4e 4OH (aq) Ü nest rubies are obtained from The Fe(OH)2 in solution is oxidized to Mogok in Burma, where they occur Fe2O3. Rusting is accelerated by im- in metamorphic limestones; Sri purities in the iron and by the pres- Lanka and Thailand are the only ence of acids or other electrolytes in other important sources. Rubies have the water. been produced synthetically by the ruthenium Symbol Ru. A hard Verneuil Ûame-fusion process. Indus- white metallic *transition element; trial rubies are used in lasers, a.n. 44; r.a.m. 101.07; r.d. 12.3; m.p. watches, and other precision instru- 2310°C; b.p. 3900°C. It is found asso- ments. ciated with platinum and is used as a Russell–Saunders coupling (L–S catalyst and in certain platinum al- coupling) A type of coupling in sys- loys. Chemically, it dissolves in fused tems involving many *fermions. alkalis but is not attacked by acids. It These systems include electrons in reacts with oxygen and halogens at Rutherford, Ernest, Lord 468

high temperatures. It also forms was detected by A. Ghiorso and a complexes with a range of oxidation team at Berkeley, California. It can states. The element was isolated by be made by bombarding californium- AK. K. Klaus in 1844. A249 nuclei with carbon-12 nuclei. • Information from the WebElements site • Information from the WebElements site Rutherford, Ernest, Lord (1871– Rutherford model The model of 1937) New Zealand-born British an atom put forward by Ernest physicist, who worked under Sir J. J. *Rutherford in 1911 on the basis of *Thomson at Cambridge University experiments on the scattering of (1895–98). He then took up a profes- alpha particles. The model consisted sorship at McGill University, Canada, of a very dense positively charged nu- and collaborated with Frederick cleus, with electrons orbiting round *Soddy in studying radioactivity. In the nucleus. This model presented a 1899 he discovered *alpha particles serious difÜculty for the classical and beta particles, followed by the theory of electricity and magnetism, discovery of *gamma radiation the which predicts that the electron following year. In 1905, with Soddy, should spiral into the nucleus in a he announced that radioactive fraction of a second, radiating elec- *decay involves a series of transfor- tromagnetic energy while doing so. mations. He moved to Manchester This difÜculty led to the development University in 1907 and there, with of the *Bohr theory in 1913 and was Hans Geiger and E. Marsden, devised deÜnitively solved by the develop- the alpha-particle scattering experiment of quantum mechanics and its ment that led in 1911 to the discov- application to atomic structure in the ery of the atomic nucleus After mid-1920s. moving to Cambridge in 1919 he rutile A mineral form of Ü achieved the arti cial splitting of titanium(IV) oxide, TiO . light atoms. In 1908 he was awarded 2 the Nobel Prize for chemistry. rutile structure A type of ionic crystal structure in which the anions Rutherford backscattering spec- have a hexagonal close packed r trometry (RBS) A technique for arrangement with cations in half the analysing samples of material by octahedral holes. The coordination irradiation with a beam of alpha number of the anions is 6 and the co- particles and measurement of the ordination number of the cations is energies of the alpha particles after 3. compounds with this structures in- they have been scattered by the clude TiO2, MnO2, SnO2, MgF2, and sample. This enables the elements NiF2. present and their amounts to be de- A termined because the energy of a scattered alpha particle depends on • An interactive version of the structure the mass of the nucleus with which Ruybal test See scott’s test. it collides. RBS is used extensively in Rydberg constant Symbol R. A medicine and industry. constant that occurs in the formulae rutherfordium Symbol Rf. A radio- for atomic spectra and is related to active *transactinide element; a.n. the binding energy between an elec- 104. It was Ürst reported in 1964 at tron and a nucleon. It is connected to Dubna, near Moscow, and in 1969 it other constants by the relationship R 469 Rydberg spectrum

µ 2 4 3 3 µ = 0 me c /8h , where 0 is the mag- Rydberg spectrum An absorption netic constant, m and e are the mass spectrum of a gas in the ultraviolet and charge of an electron, c is the region, consisting of a series of lines speed of light, and h is the *Planck that become closer together towards shorter wavelengths, merging into a constant. It has the value 1.097 × 107 continuous absorption region. The –1 m . It is named after the Swedish absorption lines correspond to elec- physicist Johannes Robert Rydberg tronic transitions to successively (1854–1919), who developed a for- higher energy levels. The onset of mula for the spectrum of hydrogen. the continuum corresponds to pho-

r S

Sabatier–Senderens process A Sackur–Tetrode equation An method of organic synthesis employ- equation for the entropy of a perfect ing hydrogenation and a heated monatomic gas. The entropy S is nickel catalyst. It is employed com- given by: mercially for hydrogenating unsatu- 5/2 Λ3 S = nRln(e V/nNA ), rated vegetable oils to make Λ π ½ margarine. It is named after Paul where = h/(2 mkT) , where n is the Sabatier (1854–1941) and Jean-Bap- amount of the gas, R is the gas con- tiste Senderens (1856–1937). stant, e is the base of natural loga- rithms, V is the volume of the sugar saccharide See . system, NA is the Avogadro constant, A h is the Planck constant, m is the • Information about IUPAC nomenclature mass of each atom, k is the Boltz- mann constant, and T is the thermo- saccharin A white crystalline solid, dynamic temperature. To calculate C H NO S, m.p. 224°C. It is made 7 5 3 the molar entropy of the gas both from a compound of toluene, derived sides are divided by n. The Sackur– from petroleum or coal tar. It is a Tetrode equation can be used to well-known artiÜcial sweetener, show that the entropy change ∆S, being some 500 times as sweet as when a perfect gas expands isother- sugar (sucrose), and is usually mar- mally from V to V , is given by: keted as its sodium salt. Because of i f ∆ an association with cancer in labora- S = nRln(aVf) – nRln(aVi) = tory animals, its use is restricted in nRln(Vf/Vi), some countries. where aV is the quantity inside the logarithm bracket in the Sackur– O Tetrode equation. sacriÜcial protection (cathodic protection) The protection of iron or NH steel against corrosion (see rusting)

SO2 by using a more reactive metal. A common form is galvanizing (see gal- Saccharin vanized iron), in which the iron surface is coated with a layer of zinc. sucrose saccharose See . Even if the zinc layer is scratched, Sachse reaction A reaction of the iron does not rust because zinc methane at high temperature to pro- ions are formed in solution in prefer- duce ethyne: ence to iron ions. Pieces of magnesium alloy are similarly used in 2CH → C H + 3H 4 2 2 2 protecting pipelines, etc. The reaction occurs at about 1500°C, sal ammoniac See ammonium the high temperature being obtained chloride by burning part of the methane in . air. SALC (symmetry-adapted linear 471 Sandmeyer reaction combinations) A linear combination salting in See salting out. of atomic orbitals (LCAO), which is a salting out The effect in which the ‘building block’ of the LCAO orbitals solubility of a substance in a certain making up a *molecular orbital. The solvent is reduced by the presence of SALC are constructed by group a second solute dissolved in the sol- theory appropriate for the symmetry vent. For example, certain substances group of the molecule. SALC are used dissolved in water can be precipi- in the construction of molecular or- tated (or evolved as a gas) by addition bitals. of an ionic salt. The substance is salicylic acid (1-hydroxybenzoic more soluble in pure water than in acid) A naturally occurring car- the salt solution. The opposite effect boxylic acid, HOC6H4COOH, found in involving an increase in solubility certain plants; r.d. 1.44; m.p. 159°C; may occur. This is known as salting sublimes at 211°C. It is used in the in. foodstuffs and dyestuffs industries. saltpetre See nitre. See also aspirin. samarium Symbol Sm. A soft sil- saline Describing a chemical com- very metallic element belonging to pound that is a salt, or a solution the *lanthanoids; a.n. 62; r.a.m. containing a salt. 150.35; r.d. 7.52 (20°C); m.p. 1077°C; salinometer An instrument for b.p. 1791°C. It occurs in monazite measuring the salinity of a solution. and bastnatite. There are seven natu- There are two main types: one is a rally occurring isotopes, all of which type of hydrometer to measure den- are stable except samarium–147, sity; the other is an apparatus for which is weakly radioactive (half-life measuring the electrical conductivity 2.5 × 1011 years). The metal is used of the solution. in special alloys for making nuclear- reactor parts as it is a neutron ab- sal soda Anhydrous *sodium car- sorber. Samarium oxide (Sm2O3) is bonate, Na2CO3. used in small quantities in special op- salt A compound formed by reac- tical glasses. The largest use of the el- tion of an acid with a base, in which ement is in the ferromagnetic alloy the hydrogen of the acid has been re- SmCo5, which produces permanent placed by metal or other positive magnets Üve times stronger than any ions. Typically, salts are crystalline other material. The element was dis- s ionic compounds such as Na+Cl– and covered by François Lecoq de Bois- + – baudran in 1879. NH4 NO3 . Covalent metal com- A pounds, such as TiCl4, are also often regarded as salts. • Information from the WebElements site salt bridge An electrical connec- sand Particles of rock with diame- tion made between two half cells. It ters in the range 0.06–2.00 mm. Most usually consists of a glass U-tube sands are composed chieÛy of parti- Ülled with agar jelly containing a cles of quartz, which are derived salt, such as potassium chloride. A from the weathering of quartz- strip of Ülter paper soaked in the salt bearing rocks. solution can also be used. Sandmeyer reaction A reaction of salt cake Industrial *sodium sul- diazonium salts used to prepare phate. chloro- or bromo-substituted aro- sandwich compound 472

matic compounds. The method is to ters with alkalis to give alcohols and diazotize an aromatic amide at low salts of carboxylic acids: temperature and add an equimolar RCOOR′ + OH– → RCOO– + R′OH solution of the halogen acid and cop- See esterification; soap. per(I) halide. A complex of the diazonium salt and copper halide forms, saponin A type of toxic *glycoside which decomposes when the temper- that forms a frothy colloidal solution ature is raised. The copper halide acts on shaking with water. Saponins as a catalyst in the reaction of the occur in many plants (such as horse halide ions from the acid, for exam- chestnut). They break down red ple blood cells and have been used for poisoning Üsh. On hydrolysis they C H N +(aq) + Cl–(aq) + CuCl(aq) → 6 5 2 yield a variety of sugars. C6H5Cl(l) + N2(g) + CuCl(aq) The reaction was discovered in 1884 sapphire Any of the gem varieties by the German chemist Traugott of *corundum except ruby, especially Sandmeyer (1854–1922). See also gat- the blue variety, but other colours of termann reaction. sapphire include yellow, brown, green, pink, orange, and purple. Sap- sandwich compound A transi- phires are obtained from igneous and tion-metal complex in which a metal metamorphic rocks and from alluvial atom or ion is ‘sandwiched’ between deposits. The chief sources are Sri two rings of atoms. *Ferrocene was Lanka, Kashmir, Burma, Thailand, the Ürst such compound to be pre- East Africa, the USA, and Australia. pared, having two parallel cyclopen- Sapphires are used as gemstones and tadienyl rings with an iron ion in record-player styluses and some between them. In such compounds types of laser. They are synthesized (also known as metallocenes) the by the Verneuil Ûame-fusion process. metal coordinates to the pi electron sarin A highly toxic colourless liq- system of the ring, rather than to in- ° uid, C4H10FO2P; r.d. 1.09; m.p. –56 C; dividual atoms. A wide variety of b.p. 158°C. It is an organophosphorus these compounds are known, having compound, O-isopropyl methylphos- Ü ve-, six-, seven-, or eight-membered phonoÛuoridate. Sarin was discov- rings and involving such metals as ered in 1938 and belongs to the Cr, Mn, Co, Ni, and Fe. Other similar G-series of *nerve agents (GB). It was s compounds are known. A multi- used by Iraq in the Iran–Iraq war decker sandwich has three or more (1980–88). In 1988, sarin was also parallel rings with metal atoms be- used by Iraq in a poison gas attack on tween them. In a bent sandwich, the the Kurd city of Halabja in the north rings are not parallel. A half sand- of Iraq. About 5000 people died. In wich (or piano stool) has one ring, 1994, a Japanese religious sect re- with single ligands on the other side leased sarin in Matsumoto and in of the metal. 1995 released it in the Tokyo subway. Sanger’s reagent 2,4-dinitro- saturable laser See dye laser. Ûuorobenzene, C H F(NO ) , used to 6 3 2 2 saturated 1. (of a compound) Con- identify the end *amino acid in a sisting of molecules that have only protein chain. It is named after Fred- single bonds (i.e. no double or triple erick Sanger (1918– ). bonds). Saturated compounds can un- saponiÜcation The reaction of es- dergo substitution reactions but not 473 scanning tunnelling microscope addition reactions. Compare unsatu- table; i.e. groups 1 (Li, Na, K, Rb, Cs, rated. 2. (of a solution) Containing Fr) and 2 (Be, Mg, Ca, Sr, Ba, Ra). The the maximum equilibrium amount outer electronic conÜgurations of of solute at a given temperature. In a these elements all have inert-gas saturated solution the dissolved sub- structures plus outer ns1 (group 1) or stance is in equilibrium with undis- ns2 (group 2) electrons. The term thus solved substance; i.e. the rate at excludes elements with incomplete which solute particles leave the solu- inner d-levels (transition metals) or tion is exactly balanced by the rate at with incomplete inner f-levels (lan- which they dissolve. A solution con- thanoids and actinoids) even though taining less than the equilibrium these often have outer ns2 or occa- amount is said to be unsaturated. sionally ns1 conÜgurations. Typically, One containing more than the equi- the s-block elements are reactive librium amount is supersaturated. metals forming stable ionic com- Supersaturated solutions can be pounds containing M+ or M2+ ions. made by slowly cooling a saturated See alkali metals; alkaline-earth solution. Such solutions are meta- metals. stable; if a small crystal seed is added sc Synclinal. See torsion angle. the excess solute crystallizes out of solution. 3. (of a vapour) See vapour scandium Symbol Sc. A rare soft pressure. silvery metallic element belonging to group 3 (formerly IIIA) of the peri- supersaturation saturation See . odic table; a.n. 21; r.a.m. 44.956; r.d. saturation spectroscopy A spec- 2.989 (alpha form), 3.19 (beta form); troscopic technique using lasers to lo- m.p. 1541°C; b.p. 2831°C. Scandium cate absorption maxima with great often occurs in *lanthanoid ores, precision. In saturation spectroscopy from which it can be separated on ac- the laser beam is split into an intense count of the greater solubility of its saturating beam and a less intense thiocyanate in ether. The only nat- beam that pass through the cavity ural isotope, which is not radioactive, containing the sample in almost op- is scandium–45, and there are nine posite directions. The saturating radioactive isotopes, all with rela- beam sometimes excites molecules, tively short half-lives. Because of the which are shifted to its frequency by metal’s high reactivity and high cost the Doppler effect. The other beam no substantial uses have been found gives a modulated signal at the detec- for either the metal or its com- s tor if it is interacting with the same pounds. Predicted in 1869 by Dmitri Doppler-shifted molecules. These *Mendeleev, and then called eka- molecules are not moving parallel to boron, the oxide (called scandia) was the beams and have an extremely isolated by Lars Nilson (1840–99) in small Doppler shift, thus providing 1879. very high resolution. See also lamb-dip A spectroscopy. • Information from the WebElements site sawhorse projection A type scanning electron microscope of *projection in which a three- See electron microscope. con- dimensional view is drawn. See scanning tunnelling microscope formation. (STM) A type of electron microscope s-block elements The elements of that uses the quantum-mechanical the Ürst two groups of the *periodic *tunnel effect to study atomic struc- scavenger 474

tures and observe single atoms. A named after the German chemist Üne-pointed conducting tip near a Hugo Schiff (1834–1915). surface of a sample causes electrons Schiff’s reagent A reagent, de- to tunnel from the surface to the tip. vised by Hugo Schiff, used for testing Its occurrence depends on the elec- for aldehydes and ketones; it consists tron density of the surface and the of a solution of fuchsin dye that has distance between the tip and the sur- been decolorized by sulphur dioxide. face itself. The electric current pro- Aliphatic aldehydes restore the pink duced is kept constant by moving the immediately, whereas aromatic ke- tip up or down as moves across the tones have no effect on the reagent. surface. Aromatic aldehydes and aliphatic ke- scavenger A reagent that removes tones restore the colour slowly. a trace component from a system or SchoenÛies system A system for that removes a reactive intermediate categorizing symmetries of mol- from a reaction. ecules. C groups contain only an n- self-consistent field n SCF See . fold rotation axis. Cnv groups, in Scheele, Karl Wilhelm (1742–86) addition to the n-fold rotation axis, Swedish chemist, who became an have a mirror plane that contains the apothecary and in 1775 set up his axis of rotation (and mirror planes as- own pharmacy at Köping. He made sociated with the existence of the n- many chemical discoveries. In fold axis). Cnh groups, in addition to 1772 he prepared oxygen (see also the n-fold rotation axis, have a mirror lavoisier, antoine; *Priestley, plane perpendicular to the axis. Sn Û Joseph) and in 1774 he isolated chlo- groups have an n-fold rotation–re ec- rine. He also discovered manganese, tion axis. Dn groups have an n-fold ro- glycerol, hydrocyanic (prussic) acid, tation axis and a two-fold axis citric acid, and many other sub- perpendicular to the n-fold axis (and stances. two-fold axes associated with the existence of the n-fold axis). Dnh groups scheelite A mineral form of cal- have all the symmetry operations of cium tungstate, CaWO , used as an 4 Dn and also a mirror plane perpendic- ore of tungsten. It occurs in contact ular to the n-fold axis. Dnd groups con- metamorphosed deposits and vein tain all the symmetry operations of deposits as colourless or white tetrag- Dn and also mirror planes that con- s onal crystals. tain the n-fold axis and bisect the an- Schiff base An imine with the gen- gles between the two-fold axes. In eral formula R1R2C = NR3, where R1, the SchoenÛies notation C stands for R2, and R3 are alkyl or aryl groups. ‘cyclic’, S stands for ‘spiegel’ (mirror), and D stands for ‘dihedral’. The Schiff’s base A compound formed subscripts h, v, and d stand for hori- by a condensation reaction between zontal, vertical, and diagonal respec- an aromatic amine and an aldehyde tively, where these words refer to or ketone, for example the position of the mirror planes ′ → ′ RNH2 + R CHO RN:CHR + H2O with respect to the n-fold axis (con- The compounds are often crystalline sidered to be vertical). In addition to and are used in organic chemistry for the noncubic groups referred to so characterizing aromatic amines (by far, there are cubic groups, which preparing the Schiff’s base and meas- have several rotation axes with the uring the melting point). They are same value of n. These are the tetra- 475 secondary emission hedral groups T, Th, and Td, the octa- rayon. When the cellulose solution is hedral groups O and Oh, and the forced through spinnarets into an icosahedral group I. The SchoenÛies acid bath, Übres of cellulose are re- system is commonly used for isolated formed. molecules, while the *Hermann– Scott’s test (cobalt thiocyanate Mauguin system is commonly used test; Ruybal test) A *presumptive in crystallography. test for cocaine. Scott’s reagent has schönite A mineral form of potas- an initial solution of 2% cobalt thio- sium sulphate, K2SO4. cyanate (Co(SCN)2) in glycerine and Schottky defect See crystal de- water. This is followed by concen- fect. trated hydrochloric acid, and then chloroform. A positive test is indi- Schrödinger, Erwin (1887–1961) cated by a blue colour in the chloro- Austrian physicist, who became pro- form layer. fessor of physics at Berlin University absorption tower in 1927. He left for Oxford to escape scrubber See . the Nazis in 1933, returned to Graz seaborgium Symbol Sg. A radio- in Austria in 1936, and then left active *transactinide element; a.n. again in 1938 for Dublin’s Institute of 106. It was Ürst detected in 1974 by Advanced Studies. He Ünally returned Albert Ghiorso and a team in Califor- to Austria in 1956. He is best known nia. It can be produced by bombard- for the development of *wave me- ing californium-249 nuclei with chanics and the *Schrödinger equa- oxygen-18 nuclei. It is named after tion, work that earned him a share of the US physicist Glenn Seaborg the 1933 Nobel Prize for physics with (1912–99). Paul *Dirac. A Schrödinger equation An equa- • Information from the WebElements site tion used in wave mechanics (see sebacic acid See decanedioic acid. quantum mechanics) for the wave function of a particle. The time- secondary alcohol See alcohols. independent Schrödinger equation is: secondary amine See amines. ∇2ψ + 8π2m(E – U)ψ/h2 = 0, secondary cell A *voltaic cell in ψ ∇2 where is the wave function, the which the chemical reaction produc- Laplace operator, h the Planck con- ing the e.m.f. is reversible and the s stant, m the particle’s mass, E its total cell can therefore be charged by pass- energy, and U its potential energy. It ing a current through it. See accumu- can also be written as: lator; intercalation cell. Compare Hψ = Eψ, primary cell. where H is the *Hamiltonian opera- secondary emission The emission tor. It was devised by Erwin of electrons from a surface as a result Schrödinger, who was mainly re- of the impact of other charged parti- sponsible for wave mechanics. cles, especially as a result of bom- Schweizer’s reagent A solution bardment with (primary) electrons. made by dissolving copper(II) hydrox- As the number of secondary elec- ide in concentrated ammonia solu- trons can exceed the number of pri- tion. It has a deep blue colour and is mary electrons, the process is used as a solvent for cellulose in the important in photomultipliers. See cuprammonium process for making also auger effect. secondary-ion mass spectrometry 476

secondary-ion mass spectrom- been reached and thus the furnace etry (SIMS) A technique for ana- temperature can be estimated. lysing the chemical structure of a selection rules Rules that deter- solid by bombarding it with ions. A mine which transitions between dif- sample of the solid to be analysed is ferent energy levels are possible bombarded with ions, referred to as in a system, such as an elementary primary ions, having energies be- particle, nucleus, atom, molecule, or tween 5 and 20 keV. A number of dif- crystal, described by quantum me- ferent types of entity are detached chanics. Transitions cannot take from the surface of the solid, includ- place between any two energy levels. ing neutral atoms and molecules, Group theory, associated with the electrons, photons, and negative and symmetry of the system, determines positive ions. The negative and posi- which transitions, called *allowed tive ions given off, called the sec- transitions, can take place and ondary ions, can be identiÜed using * mass spectrometry. Using this tech- which transitions, called forbidden nique solid samples can be character- transitions, cannot take place. Se- ized with great accuracy. lection rules determined in this way are very useful in analysing the second-order reaction See order. spectra of quantum-mechanical sys- sedimentation The settling of the tems. solid particles through a liquid either selenides Binary compounds of to produce a concentrated slurry selenium with other more elec- from a dilute suspension or to clarify tropositive elements. Selenides of a liquid containing solid particles. nonmetals are covalent (e.g. H2Se). Usually this relies on the force of Most metal selenides can be pre- gravity, but if the particles are too pared by direct combination of the small or the difference in density be- elements. Some are well-deÜned tween the solid and liquid phases is ionic compounds (containing Se2–), too small, a *centrifuge may be used. while others are nonstoichiometric In the simplest case the rate of sedi- interstitial compounds (e.g. Pd4Se, mentation is determined by Stokes’s PdSe2). law, but in practice the predicted rate is rarely reached. Measurement selenium Symbol Se. A metalloid of the rate of sedimentation in an element belonging to group 16 (for- s *ultracentrifuge can be used to esti- merly VIB) of the periodic table; a.n. mate the size of macromolecules. 34; r.a.m. 78.96; r.d. 4.81 (grey); m.p. 217°C (grey); b.p. 684.9°C. There are a seed A crystal used to induce other number of allotropic forms, includ- crystals to form from a gas, liquid, or ing grey, red, and black selenium. It solution. occurs in sulphide ores of other met- Seger cones (pyrometric cones) A als and is obtained as a by-product series of cones used to indicate the (e.g. from the anode sludge in elec- temperature inside a furnace or kiln. trolytic reÜning). The element is a The cones are made from different semiconductor; the grey allotrope is mixtures of clay, limestone, feld- light-sensitive and is used in photo- spars, etc., and each one softens at a cells, xerography, and similar appli- different temperature. The drooping cations. Chemically, it resembles of the vertex is an indication that the sulphur, and forms compounds with known softening temperature has selenium in the +2, +4, and +6 oxida- 477 semicarbazones tion states. Selenium was discovered *Hartree–Fock procedure makes use in 1817 by Jöns Berzelius. of self-consistent Üelds. A self-organization The sponta- • Information from the WebElements site neous order arising in a system when selenium cell Either of two types certain parameters of the system reach critical values. Self-organiza- of photoelectric cell; one type relies tion occurs in many systems in on the photoconductive effect, the physics, chemistry, and biology. It other on the photovoltaic effect (see photoelectric effect can occur when a system is driven ). In the photo- far from thermal *equilibrium. Since conductive selenium cell an external a self-organizing system is open to its e.m.f. must be applied; as the sele- environment, the second law of nium changes its resistance on expo- *thermodynamics is not violated by sure to light, the current produced is the formation of an ordered phase, as a measure of the light energy falling entropy can be transferred to the enon the selenium. In the photovoltaic vironment. Self-organization is re- selenium cell, the e.m.f. is generated lated to the concepts of broken within the cell. In this type of cell, a symmetry, complexity, nonlinearity, thin Ülm of vitreous or metallic sele- and *nonequilibrium statistical me- nium is applied to a metal surface, a chanics. Many systems that undergo transparent Ülm of another metal, transitions to self-organization can usually gold or platinum, being also undergo transitions to *chaos. In placed over the selenium. Both types chemistry, self-assembly is one of the of cell are used as light meters in features of *supramolecular chem- photography. istry. self-assembly See self-organiza- Seliwanoff’s test A biochemical tion. test to identify the presence of ke- tonic sugars, such as fructose, in so- Ü self-consistent eld (SCF) A con- lution. It was devised by the Russian Ü cept used to nd approximate solu- chemist F. F. Seliwanoff. A few drops tions to the many-body problem in of the reagent, consisting of resorci- *quantum mechanics. The procedure nol crystals dissolved in equal starts with an approximate solution amounts of water and hydrochloric for a particle moving in a single- acid, are heated with the test solu- particle potential, which derives tion and the formation of a red pre- s from its average interaction with all cipitate indicates a positive result. the other particles. This average in- semicarbazones Organic com- teraction is determined by the *wave pounds containing the unsaturated functions of all the other particles. group =C:N.NH.CO.NH2. They are The equation describing this average formed when aldehydes or ke- interaction is solved and the im- tones react with a semicarbazide proved solution obtained is used in (H2N.NH.CO.NH2). Semicarbazones the calculation of the interaction are crystalline compounds with rela- term. This procedure is repeated for tively high melting points. They are wave functions until the wave func- used to identify aldehydes and ke- tions and associated energies are tones in quantitative analysis: the not signiÜcantly changed in the semicarbazone derivative is made cycle, self-consistency having been and identiÜed by its melting point. attained. In atomic theory the Semicarbazones are also used in sep- semiclassical approximation 478

arating ketones from reaction mix- semiconductor. The type of charge tures: the derivative is crystallized carrier that predominates in a partic- out and hydrolysed to give the ke- ular region or material is called the tone. majority carrier and that with the A lower concentration is the minority • Information about IUPAC nomenclature carrier. An intrinsic semiconductor is one in which the concentration of semiclassical approximation An charge carriers is a characteristic of approximation technique used to cal- the material itself; electrons jump to culate quantities in quantum me- the conduction band from the va- chanics. This technique is called the lence band as a result of thermal ex- semiclassical approximation because citation, each electron that makes * the wave function is written as an the jump leaving behind a hole in asymptotic series with ascending the valence band. Therefore, in an in- powers of the Planck constant, h, trinsic semiconductor the charge car- with the Ürst term being purely riers are equally divided between classical. It is also known as the electrons and holes. In extrinsic semi- Wentzel–Kramers–Brillouin (WKB) conductors the type of conduction approximation, named after Gregor that predominates depends on the Wentzel (1898–1978), Hendrik Anton number and valence of the impurity Kramers (1894–1952), and Léon Bril- louin (1889–1969), who invented it atoms present. Germanium and sili- independently in 1926. The semicon atoms have a valence of four. If impurity atoms with a valence of classical approximation is particu- Ü larly successful for calculations ve, such as arsenic, antimony, or involving the *tunnel effect, such as phosphorus, are added to the lattice, Üeld emission, and radioactive decay there will be an extra electron per producing alpha particles. atom available for conduction, i.e. one that is not required to pair with semiconductor A crystalline solid the four valence electrons of the ger- with an electrical conductivity (typi- manium or silicon. Thus extrinsic 5 –7 cally 10 –10 siemens per metre) semiconductors doped with atoms of intermediate between that of a valence Üve give rise to crystals with 9 –1 conductor (up to 10 Sm ) and an in- electrons as majority carriers, the so- –15 –1 sulator (as low as 10 Sm ). Semi- called n-type conductors. Similarly, if conducting properties are a feature the impurity atoms have a valence of s of *metalloid elements, such as sili- three, such as boron, aluminium, in- con and germanium. As the atoms in dium, or gallium, one hole per atom a crystalline solid are close together, is created by an unsatisÜed bond. The the orbitals of their electrons overlap majority carriers are therefore holes, and their individual energy levels are i.e. p-type conductors. spread out into energy bands. Con- duction occurs in semiconductors as semiconductor laser A type of the result of a net movement, under *laser in which semiconductors pro- the inÛuence of an electric Üeld, of vide the excitation. The laser action electrons in the conduction band and results from electrons in the conduc- empty states, called holes, in the vation band (see energy bands) being lence band. A hole behaves as if it stimulated to recombine with holes was an electron with a positive in the valence band. When this oc- charge. Electrons and holes are curs the electrons give up the energy known as the charge carriers in a corresponding to the band gap. Ma- 479 SEXAFS terials, such as gallium arsenide, are often involves the formation of suitable for this purpose. A junction chelate complexes, and is used to between p-type and n-type semicon- prevent the chemical effect of an ion ductors can be used with the light without removing it from the solu- passing along the plane of the junction (e.g. the sequestration of Ca2+ tion. Mirrors for the laser action are ions in water softening). It is also a provided by the ends of the crystals. way of supplying ions in a protected Semiconductor lasers can be as small form, e.g. sequestered iron solutions as 1 mm in length. for plants in regions having alkaline soil. semi-empirical calculations A technique for calculating atomic and serine See amino acid. molecular quantities in which the serpentine Any of a group of hy- values of integrals are found using drous magnesium silicate minerals quantities derived from experiment with the general composition (such as ionization energies obtained Mg3Si2O5(OH)4. Serpentine is mono- spectroscopically). Semi-empirical clinic and occurs in two main forms: calculations were formerly widely chrysotile, which is Übrous and the used, particularly for large molecules chief source of *asbestos; and antig- involving extensive computation to orite, which occurs as platy masses. It calculate all the integrals using *ab- is generally green or white with a initio calculations. As computing mottled appearance, sometimes re- power has increased, the properties sembling a snakeskin – hence the of large molecules can be calculated name. It is formed through the meta- by ab-initio calculations, therefore morphic alteration of ultrabasic the use of semi-empirical calcula- rocks rich in olivine, pyroxene, and tions has steadily declined, Ürst for amphibole. Serpentinite is a rock small molecules and more recently consisting mainly of serpentine; it is for large molecules. used as an ornamental stone. semimetal See metalloid. sesqui- PreÜx indicating a ratio of semipermeable membrane A 2:3 in a chemical compound. For ex- membrane that is permeable to mol- ample, a sesquioxide has the formula ecules of the solvent but not the M2O3. solute in *osmosis. Semipermeable sesquiterpene See terpenes. membranes can be made by support- s Ü SEXAFS (surface-extended X-ray ab- ing a lm of material (e.g. cellulose) sorption Üne-structure spectroscopy) on a wire gauze or porous pot. A technique that makes use of the os- semipolar bond See chemical cillations in X-ray absorbance found bond. in the high-frequency side of the absorption edge to investigate the Semtex A nitrogen-based stable structure of surfaces. The physical odourless plastic *explosive. principle behind SEXAFS is quantum- septivalent (heptavalent) Having a mechanical interference between the valency of seven. wave function of a photoelectron and the wave function associated with sequence rule See cip system. scattering by surrounding atoms. In sequestration The process of the case of constructive interference, forming coordination complexes of the photoelectron has a higher prob- an ion in solution. Sequestration ability of appearing, while in the case sexivalent 480

of destructive interference, the pho- sial The rocks that form the earth’s toelectron has a lower probability of continental crust. These are granite appearing. The technique therefore rock types rich in silica (SiO2) and provides information about the sur- aluminium (Al), hence the name. roundings of an atom. Analysis of re- Compare sima. sults from SEXAFS has given a great side chain See chain. deal of information about the structure of surfaces and how they re- side reaction A chemical reaction spond to adsorption. that occurs at the same time as a main reaction but to a lesser extent, sexivalent (hexavalent) Having a thus leading to other products mixed valency of six. with the main products. shale A form of *clay that occurs in siderite A brown or grey-green thin layers. Shales are very common mineral form of iron(II) carbonate, sedimentary rocks. See also oil shale. FeCO3, often with magnesium and shell See atom. manganese substituting for the iron. It occurs in sedimentary deposits or shellac A hard resin produced as a in hydrothermal veins and is an im- secretion by a plant parasite, the portant iron ore. It is found in Eng- south-east Asian lac insect Lacifer land, Greenland, Spain, N Africa, and lacca. It is used in sealing wax, var- the USA. nish (French polish), and electrical insulators. siemens Symbol S. The SI unit of electrical conductance equal to the sherardizing The process of coat- conductance of a circuit or element ing iron or steel with a zinc corro- that has a resistance of 1 ohm. 1 S = sion-resistant layer by heating it in 10–1 Ω. The unit was formerly called contact with zinc dust to a tempera- the mho or reciprocal ohm. It is ture slightly below the melting point named after Ernst Werner von of zinc. At a temperature of about Siemens (1816–92). 371°C the two metals amalgamate to sievert The SI unit of dose equiva- form internal layers of zinc–iron al- lent (see radiation units). It is loys and an external layer of pure named after the Swedish physicist zinc. The process was invented by Rolf Sievert (1896–1966). Sherard Cowper-Coles (d. 1935). sigma bond See orbital. s shock wave A very narrow region of high pressure and temperature sigma electron An electron in a formed in a Ûuid when the Ûuid sigma orbital. See orbital. Ûows supersonically over a stationary sigmatropic reaction A type of object or a projectile Ûying superson- rearrangement in which a sigma ically passes through a stationary bond is formed between two non- Ûuid. A shock wave may also be gen- linked atoms at the same time as an erated by violent disturbances in a existing sigma bond is broken. Sig- Ûuid, such as a lightning stroke or a matropic rearrangements are a type bomb blast. Shock waves are gener- of *pericyclic reaction. An example is ated experimentally to excite mol- the *Cope rearrangement. ecules for spectroscopic silane (silicane) 1. A colourless gas, investigations. SiH4, which is insoluble in water; d. short period See periodic table. 1.44 g dm–3; r.d. 0.68 (liquid); m.p. 481 silicide

° ° –185 C; b.p. –112 C. Silane is pro- unit is the tetrahedral SiO4 group. duced by reduction of silicon with This may occur as a simple discrete 4– lithium tetrahydridoaluminate(III). It SiO4 anion as in the orthosilicates, is also formed by the reaction of e.g. phenacite (Be2SiO4) and willemite magnesium silicide (Mg2Si) with (Zn2SiO4). Many larger silicate species acids, although other silicon hydrides are also found (see illustration). are also produced at the same time. These are composed of SiO4 tetrahe- Silane itself is stable in the absence dra linked by sharing oxygen atoms 6– of air but is spontaneously Ûamma- as in the pyrosilicates, Si2O7 , e.g. ble, even at low temperatures. It is a Sc2Si2O7. The linking can extend to reducing agent and has been used for such forms as bentonite, BaTiSi3O9, the removal of corrosion in inacces- or alternatively inÜnite chain anions, sible plants (e.g. pipes in nuclear re- which are single strand (*pyroxenes) actors). 2. (or silicon hydride) Any of or double strand (*amphiboles). Spo- a class of compounds of silicon and dumene, LiAl(SiO3)2, is a pyroxene hydrogen. They have the general for- and the asbestos minerals are amphi- Ü mula SinH2n+2. The rst three in the boles. Large two-dimensional sheets series are silane itself (SiH4), disilane are also possible, as in the various (Si2H6), and trisilane (Si3H8). The com- *micas (see illustration), and the pounds are analogous to the alkanes linking can extend to full three- but are much less stable and only dimensional framework structures, the lower members of the series can often with substituted trivalent be prepared in any quantity (up to atoms in the lattice. The *zeolites Si6H14). No silicon hydrides contain- are examples of this. ing double or triple bonds exist (i.e. silicate minerals A group of rock- there are no analogues of the alkenes forming minerals that make up the and alkynes). bulk of the earth’s outer crust (about silica See silicon(iv) oxide. 90%) and constitute one-third of all minerals. All silicate minerals are silica gel A rigid gel made by coag- based on a fundamental structural ulating a sol of sodium silicate and unit – the SiO tetrahedron (see sili- heating to drive off water. It is used 4 cate). They consist of a metal (e.g. as a support for catalysts and also as calcium, magnesium, aluminium) a drying agent because it readily ab- combined with silicon and oxygen. sorbs moisture from the air. The gel The silicate minerals are classiÜed on itself is colourless but, when used in s a structural basis according to how desiccators, etc., a blue cobalt salt is the tetrahedra are linked together. added. As moisture is taken up, the The six groups are: nesosilicates (e.g. salt turns pink, indicating that the olivine and *garnet); sorosilicates gel needs to be regenerated (by heat- (e.g. hemimorphite); cyclosilicates ing). (e.g. axinite, *beryl, and *tourma- silicane See silane. line); inosilicates (e.g. *amphiboles and *pyroxenes); phyllosilicates (e.g. silicate Any of a group of sub- *micas, *clay minerals, and *talc); stances containing negative ions and tektosilicates (e.g. *feldspars and composed of silicon and oxygen. The *feldspathoids). Many silicate miner- silicates are a very extensive group als are of economic importance. and natural silicates form the major component of most rocks (see sili- silicide A compound of silicon with cate minerals). The basic structural a more electropositive element. The silicon 482

Si = O O O

SiO4– as in Be SiO (phenacite) 4 2 4

Si O2– as in Sc Si O (thortveitite) 2 5 2 2 7

Si O6– as in BaTiSi O (bentonite) Si O12– as in Be Al Si O (beryl) 3 9 3 9 6 18 3 2 6 18 Structure of of some some discrete discrete silicate silicate ions ions

single chain : pyroxenes

double chain : amphiboles sheet : micas

Structure of of some some polymeric polymeric silicate silicate ions ions Silicate minerals

silicides are structurally similar to ement belonging to *group 14 (for- the interstitial carbides but the range merly IVB) of the periodic table; a.n. encountered is more diverse. They 14; r.a.m. 28.086; r.d. 2.33; m.p. react with mineral acids to form a 1410°C; b.p. 2355°C. Silicon is the range of *silanes. second most abundant element in silicon Symbol Si. A metalloid el- the earth’s crust (25.7% by weight) oc- 483 siloxanes curring in various forms of silicon(IV) cal attack than their carbon ana- oxide (e.g. *quartz) and in *silicate logues. minerals. The element is extracted silicon hydride See silane. by reducing the oxide with carbon in an electric furnace and is used exten- silicon(IV) oxide (silicon dioxide; sively for its semiconductor proper- silica) A colourless or white vitreous ties. It has a diamond-like crystal solid, SiO2, insoluble in water and Û structure; an amorphous form also soluble (by reaction) in hydro uoric acid and in strong alkali; m.p. exists. Chemically, silicon is less reac- ° ° tive than carbon. The element com- 1713 C; b.p. 2230 C. The following forms occur naturally: cristobalite bines with oxygen at red heat and is (cubic or tetragonal crystals; r.d. also dissolved by molten alkali. There 2.32); tridymite (rhombic; r.d. 2.26); is a large number of organosilicon *quartz (hexagonal; r.d. 2.63–2.66); * compounds (e.g. siloxanes) although lechatelierite (r.d. 2.19). Quartz has silicon does not form the range of two modiÜcations: α-quartz below silicon–hydrogen compounds and 575°C and β-quartz above 575°C; derivatives that carbon does (see above 870°C β-quartz is slowly trans- silane Ü ). The element was identi ed formed to tridymite and above by Antoine *Lavoisier in 1787 and 1470°C this is slowly converted to Ürst isolated in 1823 by Jöns cristobalite. Various forms of *Berzelius. silicon(IV) oxide occur widely in the A earth’s crust; yellow sand for exam- • Information from the WebElements site ple is quartz with iron(III) oxide impurities and Ûint is essentially silicon carbide (carborundum) A amorphous silica. The gemstones black solid compound, SiC, insoluble amethyst, opal, and rock crystal are in water and soluble in molten alkali; also forms of quartz. r.d. 3.217; m.p. c. 2700°C. Silicon car- Silica is an important commercial bide is made by heating silicon(IV) material in the form of silica brick, a oxide with carbon in an electric fur- highly refractive furnace lining, nace (depending on the grade re- which is also resistant to abrasion quired sand and coke may be used). It and to corrosion. Silicon(IV) oxide is is extremely hard and is widely used also the basis of both clear and as an abrasive. The solid exists in opaque silica glass, which is used on both zinc blende and wurtzite struc- account of its transparency to ultravi- s tures. There is a clear form known as olet radiation and its resistance to moissanite, used as a diamond substi- both thermal and mechanical shock. tute. A certain proportion of silicon(IV) oxide is also used in ordinary glass silicon(iv) silicon dioxide See and in some glazes and enamels. It oxide . also Ünds many applications as a dry- silicones Polymeric compounds ing agent in the form of *silica gel. containing chains of silicon atoms al- siloxanes A group of compounds ternating with oxygen atoms, with containing silicon atoms bound to the silicon atoms linked to organic oxygen atoms, with organic groups groups. A variety of silicone ma- linked to the silicon atoms, e.g. terials exist, including oils, waxes, R3SiOSiR3, where R is an organic and rubbers. They tend to be more group. *Silicones are polymers of resistant to temperature and chemi- siloxanes. silver 484

silver Symbol Ag. A white lustrous silver-mirror test See tollens soft metallic *transition element; a.n. reagent. 47; r.a.m. 107.87; r.d. 10.5; m.p. silver(I) nitrate A colourless solid, 961.93°C; b.p. 2212°C. It occurs as the ° AgNO3; r.d. 4.3; m.p. 212 C. It is an element and as the minerals argen- important silver salt because it is tite (Ag2S) and horn silver (AgCl). It is water-soluble. It is used in photogra- also present in ores of lead and cop- phy. In the laboratory, it is used as a per, and is extracted as a by-product test for chloride, bromide, and iodide Ü of smelting and re ning these met- ions and in volumetric analysis of als. The element is used in jewellery, chlorides using an *adsorption indi- tableware, etc., and silver com- cator. pounds are used in photography. Chemically, silver is less reactive silver(I) oxide A brown slightly than copper. A dark silver sulphide water-soluble amorphous powder, Ag O; r.d. 7.14. It can be made by forms when silver tarnishes in air be- 2 adding sodium hydroxide solution to cause of the presence of sulphur silver(I) nitrate solution. Silver(I) compounds. Silver(I) ionic salts exist oxide is strongly basic and is also an (e.g. AgNO , AgCl) and there are a 3 oxidizing agent. It is used in certain number of silver(II) complexes. reactions in preparative organic A chemistry; for example, moist • Information from the WebElements site silver(I) oxide converts haloalkanes silver(I) bromide A yellowish into alcohols; dry silver oxide solid compound, AgBr; r.d. 6.5; m.p. converts haloalkanes into ethers. 432°C. It can be precipitated from sil- The compound decomposes to the elements at 300°C and can be ver(I) nitrate solution by adding a so- reduced by hydrogen to silver. lution containing bromide ions. It With ozone it gives the oxide AgO dissolves in concentrated ammonia (which is diamagnetic and probably solutions (but, unlike the chloride, AgIAgIIIO ). does not dissolve in dilute ammonia). 2 The compound is used in photo- sima The rocks that form the graphic emulsions. earth’s oceanic crust and underlie the upper crust. These are basaltic silver(I) chloride A white solid rock types rich in silica (SiO ) and ° 2 compound, AgCl; r.d. 5.6; m.p. 455 C; magnesium (Mg), hence the name. s ° b.p. 1550 C. It can be precipitated The sima is denser and more plastic from silver(I) nitrate solution by than the *sial that forms the conti- adding a solution of chloride ions. It nental crust. dissolves in ammonia solution (due to formation of the complex ion Simmons–Smith reaction A reac- + tion in which a cyclopropane ring is [Ag(NH3)2] ). The compound is used in photographic emulsions. produced form an alkene. It uses the Simmons–Smith reagent, which was silver(I) iodide A yellow solid originally diiodomethane (CH2I2) compound, AgI; r.d. 6.01; m.p. 558°C; with a Zn/Cu couple. Usually, diethyl b.p. 1506°C. It can be precipitated zinc is used rather than Zn/Cu. The from silver(I) nitrate solutions by mechanism involves the formation of adding a solution of iodide ions. Un- H2C(I)(ZnI) and *carbene transfer like the chloride and bromide, it does from the zinc to the double bond of not dissolve in ammonia solutions. the alkene. 485 smelting

Simon test A *presumptive test for now replaced *c.g.s. units and *Impe- methamphetamines. The Simon rial units. The system has seven base reagent consists of 1 gram of sodium units and two dimensionless units nitroprusside (Na2Fe(CN)5 NO) and 2 (formerly called supplementary ml of acetaldehyde (ethanal) in 50 ml units), all other units being derived of water. A 2% solution of sodium from these nine units. There are 18 carbonate is then added. MDA gives a derived units with special names. turquoise colour; MDMA gives an ini- Each unit has an agreed symbol (a tial turquoise, which changes to dark capital letter or an initial capital let- blue. ter if it is named after a scientist, SIMS See secondary-ion mass spec- otherwise the symbol consists of one trometry. or two lower-case letters). Decimal multiples of the units are indicated chemical bond single bond See . by a set of preÜxes; whenever possi- singlet An atomic state in which ble a preÜx representing 10 raised to two spin angular momenta of elec- a power that is a multiple of three trons cancel each other, resulting in should be used. See Appendix. zero net spin. A singlet state usually skeletal electrons See wade’s has a higher energy than a *triplet rules. state because of the effect of correlations of spin on the Coulomb interac- skew See torsion angle. tions between electrons (as in slag Material produced during the *Hund’s rules). This can lead to sub- *smelting or reÜning of metals by re- stantial energy differences between action of the Ûux with impurities triplet and singlet states. (e.g. calcium silicate formed by reac- sintered glass Porous glass made tion of calcium oxide Ûux with sili- by sintering powdered glass, used for con dioxide impurities). The liquid Ültration of precipitates in gravimet- slag can be separated from the liquid ric analysis. metal because it Ûoats on the surface. See also basic slag. sintering The process of heating and compacting a powdered material slaked lime See calcium hydrox- at a temperature below its melting ide. point in order to weld the particles SLN See sybyl line notation. together into a single rigid shape. Materials commonly sintered include SLUMO Second-lowest unoccupied s metals and alloys, glass, and ceramic molecular orbital. See subjacent or- oxides. Sintered magnetic materials, bitals. Ü cooled in a magnetic eld, make es- slurry A paste consisting of a sus- pecially retentive permanent mag- pension of a solid in a liquid. nets. SMARTS See smiles. S-isomer See absolute configuration. smectic See liquid crystal. SI units Système International smelting The process of separating d’Unités: the international system of a metal from its ore by heating the units now recommended for all sci- ore to a high temperature in a suit- entiÜc purposes. A coherent and ra- able furnace in the presence of a re- tionalized system of units derived ducing agent, such as carbon, and a from the *m.k.s. units, SI units have Ûuxing agent, such as limestone. Iron SMILES 486

ore is smelted in this way so that the ural gas; a mixture of gaseous hydro- metal melts and, being denser than carbons produced from coal, petro- the molten *slag, sinks below the leum, etc., and suitable for use as a slag, enabling it to be removed from fuel. Before the discovery of natural the furnace separately. gas *coal gas was widely used as a do- SMILES SimpliÜed molecular input mestic and industrial fuel. This gave line entry system. A type of line nota- way to natural gas in the early part tion for representing molecules and of this century in the US and other reactions. It uses standard atom sym- countries where natural gas was bols but hydrogen atoms are omitted. plentiful. The replacement of coal A double bond is represented by an gas occurred somewhat later in the equals sign and a triple bond by a UK and other parts of Europe. More hash sign. So ethane is CC, ethene is recently, interest has developed in C=C, and ethyne is C#C. ways of manufacturing hydrocarbon Branched structures are given gas fuels. The main sources are coal using brackets. For example, triethyl- and the naphtha fraction of petroleum. In the case of coal three meth- amine ((C2H5)3N) is represented by CCN(CC)CC; ethanoic (acetic acid) ods have been used: (1) pyrolysis – i.e. Ü (CH3COOC) is CC(=O)O. Ring struc- more ef cient forms of destructive tures use numbers to show connec- distillation, often with further hydro- tions. For instance, cyclohexane is genation of the hydrocarbon prod- C1CCCCC1, where the C1 atoms are ucts; (2) heating the coal with the ones joined to form the ring. hydrogen and catalysts to give hydro- SMILES notation for aromatic mol- carbons – a process known as ecules uses lower case letters. Ben- hydroliquefaction (see also bergius zene is c1ccccc1; chlorobenzene process); (3) producing carbon could be c1c(Cl)cccc1. Rules exist for monoxide and hydrogen and obtain- expressing isotope and stereochem- ing hydrocarbons by the *Fischer– istry information and there is a nota- Tropsch process. SNG from naptha is tion for reactions. Rules also exist for made by steam *reforming. See crg choosing a unique SMILES, which is a process. Ü de nitive representation of the many soap A substance made by boiling possible valid strings that may be animal fats with sodium hydroxide. written for a given compound. An The reaction involves the hydrolysis s extension of SMILES to describe mo- of *glyceride esters of fatty acids to lecular patterns (for substructure glycerol and sodium salts of the acids searches) is called SMARTS. present (mainly the stearate, oleate, A and palmitate), giving a soft semi- • A SMILES tutorial produced by Daylight solid with *detergent action. Potas- Chemical Information Systems Inc. sium hydroxide gives a more liquid smoke A Üne suspension of solid product (soft soap). By extension, particles in a gas. other metal salts of long-chain fatty acids are also called soaps. See also nucleophilic sub- SN1 reaction See saponification. stitution. soda Any of a number of sodium nucleophilic sub- SN2 reaction See compounds, such as caustic soda stitution . (NaOH) or, especially, washing soda SNG Substitute (or synthetic) nat- (Na2CO3.10H2O). 487 sodium benzoate soda ash Anhydrous *sodium car- sodium aluminate A white solid, bonate, Na2CO3. NaA1O2 or Na2Al2O4, which is insoluble in ethanol and soluble in soda lime A mixed hydroxide of water giving strongly alkaline solu- sodium and calcium made by slaking tions; m.p. 1800°C. It is manufac- lime with caustic soda solution (to tured by heating bauxite with give NaOH + Ca(OH) ) and recovering 2 sodium carbonate and extracting greyish white granules by evapora- the residue with water, or it may be tion. The material is produced largely prepared in the laboratory by adding for industrial adsorption of carbon excess aluminium to hot concen- dioxide and water, but also Ünds trated sodium hydroxide. In solu- some applications in pollution and tion the ion Al(OH) – predominates. efÛuent control. It is also used as a 4 Sodium aluminate is used as a laboratory drying agent. mordant, in the production of zeo- sodamide See sodium amide. lites, in efÛuent treatment, in glass Soddy, Frederick (1877–1956) manufacture, and in cleansing compounds. British chemist, who worked with Ernest *Rutherford in Canada and sodium amide (sodamide) A white William *Ramsay in London before crystalline powder, NaNH2, which de- Ünally settling in Oxford in 1919. His composes in water and in warm announcement in 1913 of the exis- ethanol, and has an odour of ammo- tence of *isotopes won him the 1921 nia; m.p. 210°C; b.p. 400°C. It is pro- Nobel Prize for physics. duced by passing dry ammonia over metallic sodium at 350°C. It reacts sodium Symbol Na. A soft silvery with red-hot carbon to give sodium reactive element belonging to group cyanide and with nitrogen(I) oxide to 1 (formerly IA) of the periodic table give sodium azide. (see alkali metals); a.n. 11; r.a.m. 22.9898; r.d. 0.97; m.p. 97.8°C; b.p. sodium azide A white or colour- 882–889°C. Sodium occurs as the less crystalline solid, NaN3, soluble in chloride in sea water and in the min- water and slightly soluble in alcohol; eral halite. It is extracted by electrol- hexagonal; r.d. 1.846; decomposes on ysis in a Downs cell. The metal is heating. It is made by the action of used as a reducing agent in certain nitrogen(I) oxide on hot sodamide reactions and liquid sodium is also a (NaNH2) and is used as an organic coolant in nuclear reactors. Chemi- reagent and in the manufacture of s cally, it is highly reactive, oxidizing detonators. in air and reacting violently with sodium benzenecarboxylate water (it is kept under oil). It dis- (sodium benzoate) An either colour- solves in liquid ammonia to form less crystalline or white amorphous blue solutions containing solvated powder, C6H5COONa, soluble in electrons. Sodium is a major *essen- water and slightly soluble in ethanol. tial element required by living organ- It is made by the reaction of sodium Ü isms. The element was rst isolated hydroxide with benzoic acid and is by Humphry Davy in 1807. used in the dyestuffs industry and as A a food preservative. It was formerly • Information from the WebElements site used as an antiseptic. sodium acetate See sodium sodium benzoate See sodium ben- ethanoate. zenecarboxylate. sodium bicarbonate 488

sodium bicarbonate See sodium photography, in cleaning, in pH con- hydrogencarbonate. trol of water, in textile treatment, glasses and glazes, and as a food addi- sodium bisulphate See sodium tive and volumetric reagent. See also hydrogensulphate. sodium sesquicarbonate. sodium bisulphite See sodium hy- sodium chlorate(V) A white crys- drogensulphite. talline solid, NaClO3; cubic; r.d. 2.49; sodium bromide A white crys- m.p. 250°C. It decomposes above its talline solid, NaBr, known chieÛy as melting point to give oxygen and the dihydrate (monoclinic; r.d. 2.17), sodium chloride. The compound is and as the anhydrous salt (cubic; r.d. soluble in water and in ethanol and 3.20; m.p. 747°C; b.p. 1390°C). The di- is prepared by the reaction of chlo- hydrate loses water at about 52°C rine on hot concentrated sodium and is very slightly soluble in alcohol. hydroxide. Sodium chlorate is a pow- Sodium bromide is prepared by the erful oxidizing agent and is used in reaction of bromine on hot sodium the manufacture of matches and soft hydroxide solution or of hydrogen explosives, in calico printing, and as bromide on sodium carbonate solu- a garden weedkiller. tion. It is used in photographic pro- sodium chloride (common salt) A cessing and in analytical chemistry. colourless crystalline solid, NaCl, sol- sodium carbonate Anhydrous uble in water and very slightly solu- sodium carbonate (soda ash, sal soda) ble in ethanol; cubic; r.d. 2.17; m.p. is a white powder, which cakes and 801°C; b.p. 1413°C. It occurs as the aggregates on exposure to air due mineral *halite (rock salt) and in nat- to the formation of hydrates. The ural brines and sea water. It has the monohydrate, Na2CO3.H2O, is a white interesting property of a solubility in crystalline material, which is soluble water that changes very little with in water and insoluble in alcohol; r.d. temperature. It is used industrially as 2.532; loses water at 109°C; m.p. the starting point for a range of 851°C. sodium-based products (e.g. Solvay The decahydrate, Na2CO3.10H2O process for Na2CO3, Castner–Kellner (washing soda), is a translucent process for NaOH), and is known uni- efÛorescent crystalline solid; r.d. versally as a preservative and sea- 1.44; loses water at 32–34°C to give soner of foods. Sodium chloride has a s the monohydrate; m.p. 851°C. key role in biological systems in Sodium carbonate may be manufac- maintaining electrolyte balances. tured by the *Solvay process or by sodium chloride structure See suitable crystallization procedures rock salt structure. from any one of a number of natural deposits, such as: sodium cyanide A white or trona (Na2CO3.NaHCO3.2H2O), colourless crystalline solid, NaCN, natron (Na2CO3.10H2O), deliquescent, soluble in water and in ranksite (2Na2CO3.9Na2SO4.KCl), liquid ammonia, and slightly soluble ° pirsonnite (Na2CO3.CaCO3.2H2O), in ethanol; cubic; m.p. 564 C; b.p. ° gaylussite (Na2CO3.CaCO3.5H2O). 1496 C. Sodium cyanide is now made The method of extraction is very sen- by absorbing hydrogen cyanide in sitive to the relative energy costs and sodium hydroxide or sodium carbon- transport costs in the region in- ate solution. The compound is ex- volved. Sodium carbonate is used in tremely poisonous because it reacts 489 sodium hexaÛuoraluminate with the iron in haemoglobin in the used to study phosphate participa- blood, so preventing oxygen reaching tion in metabolic processes. the tissues of the body. It is used in sodium dioxide See sodium super- the extraction of precious metals and oxide. in electroplating industries. Aqueous solutions are alkaline due to salt hy- sodium ethanoate (sodium ac- drolysis. etate) A colourless crystalline compound, CH COONa, which is known sodium dichromate A red crys- 3 as the anhydrous salt (r.d. 1.52; m.p. talline solid, Na Cr O .2H O, soluble 2 2 7 2 324°C) or the trihydrate (r.d. 1.45; in water and insoluble in ethanol. It loses water at 58°C). Both forms are is usually known as the dihydrate soluble in water and in ethoxy- (r.d. 2.52), which starts to lose water ethane, and slightly soluble in above 100°C; the compound decom- ethanol. The compound may be pre- poses above 400°C. It is made by pared by the reaction of ethanoic melting chrome iron ore with lime acid (acetic acid) with sodium carbon- and soda ash and acidiÜcation of the ate or with sodium hydroxide. Be- chromate thus formed. Sodium cause it is a salt of a strong base and dichromate is cheaper than the cor- a weak acid, sodium ethanoate is responding potassium compound but used in buffers for pH control in has the disadvantage of being hygro- many laboratory applications, in scopic. It is used as a mordant in dye- foodstuffs, and in electroplating. It is ing, as an oxidizing agent in organic also used in dyeing, soaps, pharma- chemistry, and in analytical chem- ceuticals, and in photography. istry. sodium Ûuoride A crystalline sodium dihydrogenorthophos- compound, NaF, soluble in water and phate See sodium dihydrogenphosphate(v) very slightly soluble in ethanol; . cubic; r.d. 2.56; m.p. 993°C; b.p. sodium dihydrogenphos- 1695°C. It occurs naturally as villiau- phate(V) (sodium dihydrogen- mite and may be prepared by the re- orthophosphate) A colourless action of sodium hydroxide or of crystalline solid, NaH2PO4, which is sodium carbonate with hydrogen soluble in water and insoluble in al- Ûuoride. The reaction of sodium cohol, known as the monohydrate Ûuoride with concentrated sulphuric (r.d. 2.04) and the dihydrate (r.d. acid may be used as a source of hy- 1.91). The dihydrate loses one water drogen Ûuoride. The compound is s molecule at 60°C and the second used in ceramic enamels and as a molecule of water at 100°C, followed preservative agent for fermentation. by decomposition at 204°C. The com- It is highly toxic but in very dilute so- pound may be prepared by treating lution (less than 1 part per million) it sodium carbonate with an equimolar is used in the Ûuoridation of water quantity of phosphoric acid or by for the prevention of tooth decay on neutralizing phosphoric acid with account of its ability to replace OH sodium hydroxide. It is used in the groups with F groups in the material preparation of sodium phosphate of dental enamel. (Na3PO4), in baking powders, as a sodium formate See sodium food additive, and as a constituent of methanoate. buffering systems. Both sodium dihydrogenphosphate and trisodium sodium hexaÛuoraluminate A 32 phosphate enriched in P have been colourless monoclinic solid, Na3AlF6, sodium hydride 490

very slightly soluble in water; r.d. 2.9; medicinal applications as an antacid. m.p. 1000°C. It changes to a cubic Sodium hydrogencarbonate is also form at 580°C. The compound occurs used in baking powders (and is naturally as the mineral *cryolite but known as baking soda), dry-powder a considerable amount is manufac- Üre extinguishers, and in the textiles, tured by the reaction of aluminium tanning, paper, and ceramics indus- Ûuoride wth alumina and sodium hy- tries. The hydrogencarbonate ion has droxide or directly with sodium alu- an important biological role as an in- minate. Its most important use is in termediate between atmospheric the manufacture of aluminium in CO2/H2CO3 and the carbonate ion 2– the *Hall–Heroult cell. It is also used CO3 . For water-living organisms this in the manufacture of enamels, is the most important and in some opaque glasses, and ceramic glazes. cases the only source of carbon. sodium hydride A white crys- sodium hydrogensulphate talline solid, NaH; cubic; r.d. 0.92; de- (sodium bisulphate) A colourless ° composes above 300 C (slow); solid, NaHSO4, known in anhydrous completely decomposed at 800°C. and monohydrate forms. The anhy- Sodium hydride is prepared by the drous solid is triclinic (r.d. 2.435; m.p. reaction of pure dry hydrogen with >315°C). The monohydrate is monosodium at 350°C. Electrolysis of clinic and deliquescent (r.d. 2.103; sodium hydride in molten LiCl/KCl m.p. 59°C). Both forms are soluble in leads to the evolution of hydrogen; water and slightly soluble in alcohol. this is taken as evidence for the ionic Sodium hydrogensulphate was origi- nature of NaH and the presence of nally made by the reaction between the hydride ion (H–). It reacts vio- sodium nitrate and sulphuric acid, lently with water to give sodium hy- hence its old name of nitre cake. It droxide and hydrogen, with halogens may be manufactured by the reac- to give the halide and appropriate tion of sodium hydroxide with sul- hydrogen halide, and ignites sponta- phuric acid, or by heating equimolar neously with oxygen at 230°C. It is a proportions of sodium chloride and powerful reducing agent with several concentrated sulphuric acid. Solu- laboratory applications. tions of sodium hydrogensulphate sodium hydrogencarbonate are acidic. On heating the compound decomposes (via Na S O ) to give sul- (bicarbonate of soda; sodium bicar- 2 2 7 phur trioxide. It is used in paper s bonate) A white crystalline solid, making, glass making, and textile NaHCO , soluble in water and 3 Ünishing. slightly soluble in ethanol; monoclinic; r.d. 2.159; loses carbon dioxide sodium hydrogensulphite above 270°C. It is manufactured in (sodium bisulphite) A white solid, the *Solvay process and may be pre- NaHSO3, which is very soluble in pared in the laboratory by passing water (yellow in solution) and carbon dioxide through sodium car- slightly soluble in ethanol; mono- bonate or sodium hydroxide solution. clinic; r.d. 1.48. It decomposes on Sodium hydrogencarbonate reacts heating to give sodium sulphate, sul- with acids to give carbon dioxide phur dioxide, and sulphur. It is and, as it does not have strongly cor- formed by saturating a solution of rosive or strongly basic properties it- sodium carbonate with sulphur diox- self, it is employed in bulk for the ide. The compound is used in the treatment of acid spillage and in brewing industry and in the steriliza- 491 sodium nitrite tion of wine casks. It is a general an- studies of thyroid function and for tiseptic and bleaching agent. See also treatment of diseases of the thyroid. aldehydes. sodium methanoate (sodium for- sodium hydroxide (caustic soda) mate) A colourless deliquescent A white transluscent deliquescent solid, HCOONa, soluble in water and solid, NaOH, soluble in water and slightly soluble in ethanol; mono- ethanol but insoluble in ether; r.d. clinic; r.d. 1.92; m.p. 253°C; decom- 2.13; m.p. 318°C; b.p. 1390°C. Hy- poses on further heating. The drates containing 7, 5, 4, 3.5, 3, 2, monohydrate is also known. The and 1 molecule of water are known. compound may be produced by the Sodium hydroxide was formerly reaction of carbon monoxide with made by the treatment of sodium solid sodium hydroxide at 200°C and carbonate with lime but its main 10 atmospheres pressure; in the labo- source today is from the electrolysis ratory it can be conveniently pre- of brine using mercury cells or any of pared by the reaction of methanoic a variety of diaphragm cells. The acid and sodium hydroxide. Its uses principal product demanded from are in the production of oxalic acid these cells is chlorine (for use in plas- (ethanedioic acid) and methanoic tics) and sodium hydroxide is almost acid and in the laboratory it is a con- reduced to the status of a by-product. venient source of carbon monoxide. It is strongly alkaline and Ünds many sodium monoxide A whitish-grey applications in the chemical indus- deliquescent solid, Na O; r.d. 2.27; try, particularly in the production of 2 sublimes at 1275°C. It is manufac- soaps and paper. It is also used to ad- tured by oxidation of the metal in a sorb acidic gases, such as carbon limited supply of oxygen and puriÜed dioxide and sulphur dioxide, and is by sublimation. Reaction with water used in the treatment of efÛuent for produces sodium hydroxide. Its com- the removal of heavy metals (as hy- mercial applications are similar to droxides) and of acidity. Sodium hy- those of sodium hydroxide. droxide solutions are extremely corrosive to body tissue and are par- sodium nitrate (Chile saltpetre) A ticularly hazardous to the eyes. white solid, NaNO3, soluble in water and in ethanol; trigonal; r.d. 2.261; sodium iodide A white crystalline m.p. 306°C; decomposes at 380°C. A solid, NaI, very soluble in water and rhombohedral form is also known. It soluble in both ethanol and ethanoic s is obtained from deposits of caliche acid. It is known in both the anhy- or may be prepared by the reaction drous form (cubic; r.d. 3.67; m.p. of nitric acid with sodium hydroxide 661°C; b.p. 1304°C) and as the dihy- or sodium carbonate. It was previ- drate (monoclinic; r.d. 2.45). It is pre- ously used for the manufacture of pared by the reaction of hydrogen nitric acid by heating with concen- iodide with sodium carbonate or trated sulphuric acid. Its main use is sodium hydroxide in solution. Like in nitrate fertilizers. potassium iodide, sodium iodide in aqueous solution dissolves iodine to sodium nitrite A yellow hygro- form a brown solution containing scopic crystalline compound, NaNO2, – Ü the I3 ion. It nds applications in soluble in water, slightly soluble in photography and is also used in med- ether and in ethanol; rhombohedral; icine as an expectorant and in the ad- r.d. 2.17; m.p. 271°C; decomposes ministration of radioactive iodine for above 320°C. It is formed by the ther- sodium nitroprusside test 492

mal decomposition of sodium nitrate known as the anhydrous compound and is used in the preparation of ni- (orthorhombic; r.d. 2.67; m.p. 888°C) trous acid (reaction with cold dilute or the decahydrate (monoclinic; r.d. hydrochloric acid). Sodium nitrite is 1.46; which loses water at 100°C). used in organic *diazotization and as The decahydrate is known as a corrosion inhibitor. Glauber’s salt. A metastable heptahy- sodium nitroprusside test See drate (Na2SO4.7H2O) also exists. All simon’s test. forms are soluble in water, dissolving to give a neutral solution. The com- sodium orthophosphate See pound occurs naturally as trisodium phosphate(v). mirabilite (Na2SO4.10H2O), sodium peroxide A whitish solid threnardite (Na2SO4), and (yellow when hot), Na2O2, soluble in glauberite (Na2SO4.CaSO4). ice-water and decomposed in warm Sodium sulphate may be produced water or alcohol; r.d. 2.80; decom- industrially by the reaction of mag- poses at 460°C. A crystalline octa- nesium sulphate with sodium chlo- hydrate (hexagonal) is obtained by ride in solution followed by crystallization from ice-water. The crystallization, or by the reaction of compound is formed by the combus- concentrated sulphuric acid with tion of sodium metal in excess oxy- solid sodium chloride. The latter gen. At normal temperatures it method was used in the *Leblanc reacts with water to give sodium process for the production of alkali hydroxide and hydrogen peroxide. and has given the name salt cake to It is a powerful oxidizing agent react- impure industrial sodium sulphate. ing with iodine vapour to give the Sodium sulphate is used in the man- iodate and periodate, with carbon at ufacture of glass and soft glazes and 300°C to give the carbonate, and in dyeing to promote an even Ünish. with nitrogen(II) oxide to give the It also Ünds medicinal application as nitrate. It is used as a bleaching a purgative and in commercial aperi- agent in wool and yarn processing, ent salts. in the reÜning of oils and fats, and in the production of wood pulp. sodium sulphide A yellow-red solid, Na2S, formed by the reduction sodium sesquicarbonate A white of sodium sulphate with carbon crystalline hydrated double salt, (coke) at elevated temperatures. It is s Na2CO3.NaHCO3.2H2O, soluble in a corrosive and readily oxidized ma- water but less alkaline than sodium terial of variable composition and carbonate; r.d. 2.12; decomposes on usually contains polysulphides of the heating. It may be prepared by crys- type Na2S2, Na2S3, and Na2S4, which tallizing equimolar quantities of the cause the variety of colours. It is constituent materials; it also occurs known in an anhydrous form (r.d. naturally as trona and in Searles 1.85; m.p. 1180°C) and as a nonahy- Lake brines. It is widely used as a drate, Na S.9H O (r.d. 1.43; decom- detergent and soap builder and, 2 2 poses at 920°C). Other hydrates of because of its mild alkaline proper- sodium sulphide have been reported. ties, as a water-softening agent and The compound is deliquescent, solu- bath-salt base. See also sodium car- ble in water with extensive hydroly- bonate. sis, and slightly soluble in alcohol. It sodium sulphate A white crys- is used in wood pulping, dyestuffs talline compound, Na2SO4, usually manufacture, and metallurgy on ac- 493 solid count of its reducing properties. It ide) A whitish-yellow solid, NaO2, has also been used for the production formed by the reaction of sodium of sodium thiosulphate (for the pho- peroxide with excess oxygen at el- tographic industry) and as a depila- evated temperatures and pressures. It tory agent in leather preparation. It reacts with water to form hydrogen is a strong skin irritant. peroxide and oxygen. sodium sulphite A white solid, sodium thiosulphate (hypo) A Û Na2SO3, existing in an anhydrous colourless ef orescent solid, Na2S2O3, form (r.d. 2.63) and as a heptahydrate soluble in water but insoluble in (r.d. 1.59). Sodium sulphite is soluble ethanol, commonly encountered as in water and because it is readily oxi- the pentahydrate (monoclinic; r.d. dized it is widely used as a conve- 1.73; m.p. 42°C), which loses water at nient reducing agent. It is prepared 100°C to give the anhydrous form by reacting sulphur dioxide with ei- (r.d. 1.66). It is prepared by the reac- ther sodium carbonate or sodium hy- tion of sulphur dioxide with a sus- droxide. Dilute mineral acids reverse pension of sulphur in boiling sodium this process and release sulphur diox- hydroxide solution. Aqueous solu- ide. Sodium sulphite is used as a tions of sodium thiosulphate are bleaching agent in textiles and in readily oxidized in the presence of paper manufacture. Its use as an an- air to sodium tetrathionate and tioxidant in some canned foodstuffs sodium sulphate. The reaction with gives rise to a slightly sulphurous dilute acids gives sulphur and sul- smell immediately on opening, but phur dioxide. It is used in the photo- its use is prohibited in meats or foods graphic industry and in analytical that contain vitamin B1. Sodium sul- chemistry. phite solutions are occasionally used as biological preservatives. soft acid See hsab principle. sodium–sulphur cell A type of soft base See hsab principle. *secondary cell that has molten elec- soft soap See soap. trodes of sodium and sulphur separated by a solid electrolyte consisting soft water See hardness of water. of beta alumina (a crystalline form of sol A *colloid in which small solid aluminium oxide). When the cell is particles are dispersed in a liquid Û producing current, sodium ions ow continuous phase. through the alumina to the sulphur, s where they form sodium polysul- solder An alloy used to join metal phide. Electrons from the sodium surfaces. A soft solder melts at a tem- Ûow in the external circuit. The op- perature in the range 200–300°C and posite process takes place during consists of a tin–lead alloy. The tin charging of the cell. Sodium–sulphur content varies between 80% for the batteries have been considered for lower end of the melting range and use in electric vehicles because of 31% for the higher end. Hard solders their high peak power levels and rel- contain substantial quantities of sil- atively low weight. However, some of ver in the alloy. Brazing solders are the output has to be used to main- usually alloys of copper and zinc, tain the operating temperature which melt at over 800°C. ° (about 370 C) and the cost of sodium solid A state of matter in which is high. there is a three-dimensional regular- sodium superoxide (sodium diox- ity of structure, resulting from the solid solution 494

proximity of the component atoms, solubility product Symbol Ks. The ions, or molecules and the strength product of the concentrations of ions of the forces between them. True in a saturated solution. For instance, amor- solids are crystalline (see also if a compound AxBy is in equilibrium phous). If a crystalline solid is with its solution heated, the kinetic energy of the A B (s) ˆ xA+(aq) + yB–(aq) components increases. At a speciÜc x y temperature, called the melting the equilibrium constant is + x – y point, the forces between the compo- Kc = [A ] [B ] /[AxBy] nents become unable to contain Since the concentration of the undis- them within the crystal structure. At solved solid can be put equal to 1, this temperature, the lattice breaks the solubility product is given by down and the solid becomes a liquid. + x – y Ks = [A ] [B ] solid solution A crystalline ma- The expression is only true for spar- terial that is a mixture of two or ingly soluble salts. If the product of more components, with ions, atoms, ionic concentrations in a solution ex- or molecules of one component re- ceeds the solubility product, then placing some of the ions, atoms, or precipitation occurs. molecules of the other component in its normal crystal lattice. Solid solu- solute The substance dissolved in a tions are found in certain alloys. For solvent in forming a *solution. example, gold and copper form solid solution A homogeneous mixture solutions in which some of the cop- of a liquid (the *solvent) with a gas per atoms in the lattice are replaced or solid (the solute). In a solution, the by gold atoms. In general, the gold molecules of the solute are discrete atoms are distributed at random, and and mixed with the molecules of sol- a range of gold–copper compositions vent. There is usually some interac- is possible. At a certain composition, tion between the solvent and solute the gold and copper atoms can each molecules (see solvation). Two liq- form regular individual lattices (re- uids that can mix on the molecular ferred to as superlattices). Mixed crys- level are said to be miscible. In this tals of double salts (such as alums) case, the solvent is the major compo- are also examples of solid solutions. nent and the solute the minor com- Compounds can form solid solutions ponent. See also solid solution. if they are isomorphous (see isomor- s phism). solvation The interaction of ions of a solute with the molecules of sol- solidus A line on a phase diagram vent. For instance, when sodium below which a substance is solid. chloride is dissolved in water the solubility The quantity of solute sodium ions attract polar water mol- that dissolves in a given quantity of ecules, with the negative oxygen solvent to form a saturated solution. atoms pointing towards the positive Solubility is measured in kilograms Na+ ion. Solvation of transition-metal per metre cubed, moles per kilogram ions can also occur by formation of of solvent, etc. The solubility of a coordinate bonds, as in the hexaquo- 2+ substance in a given solvent depends copper(II) ion [Cu(H2O)6] . Solvation on the temperature. Generally, for a is the process that causes ionic solids solid in a liquid, solubility increases to dissolve, because the energy re- with temperature; for a gas, solubil- leased compensates for the energy ity decreases. See also concentration. necessary to break down the crystal 495 sorbitol lattice. It occurs only with polar sol- donate protons; tetrachloromethane vents. Solvation in which the solvent (carbon tetrachloride) is an example. is water is called hydration. solvent extraction The process of Solvay process (ammonia–soda separating one constituent from a process) An industrial method of mixture by dissolving it in a solvent making sodium carbonate from cal- in which it is soluble but in which cium carbonate and sodium chloride. the other constituents of the mixture The calcium carbonate is Ürst heated are not. The process is usually carried to give calcium oxide and carbon out in the liquid phase, in which case dioxide, which is bubbled into a solu- it is also known as liquid–liquid ex- tion of sodium chloride in ammonia. traction. In liquid–liquid extraction, Sodium hydrogencarbonate is precip- the solution containing the desired itated: constituent must be immiscible with the rest of the mixture. The process H O + CO (g) + NaCl(aq) + NH (aq) 2 2 3 is widely used in extracting oil from → NaHCO (s) + NH Cl(aq) 3 4 oil-bearing materials. The sodium hydrogencarbonate is heated to give sodium carbonate and solvolysis A reaction between a hy- carbon dioxide. The ammonium chlo- compound and its solvent. See drolysis ride is heated with calcium oxide . (from the Ürst stage) to regenerate soman A highly toxic colourless the ammonia. The process was volatile liquid, C7H16FO2P; r.d. 1.02; patented in 1861 by the Belgian m.p. –42°C; b.p. 198°C. It is an chemist Ernest Solvay (1838–1922). organophosphorus compound, O- pinacolyl methylphosphonoÛuori- solvent A liquid that dissolves an- date. Soman was discovered in 1944 other substance or substances to and belongs to the G-series of *nerve form a *solution. Polar solvents are agents (GD). compounds such as water and liquid ammonia, which have dipole mo- SOMO Singly occupied molecular ments and consequently high dielec- orbital. tric constants. These solvents are sonochemistry The study of chem- capable of dissolving ionic com- ical reactions in liquids subjected to pounds or covalent compounds that high-intensity sound or ultrasound. solvation ionize (see ). Nonpolar sol- This causes the formation, growth, s vents are compounds such as and collapse of tiny bubbles within ethoxyethane and benzene, which do the liquid, generating localized cen- not have permanent dipole mo- tres of very high temperature and ments. These do not dissolve ionic pressure, with extremely rapid cool- compounds but will dissolve nonpo- ing rates. Such conditions are suit- lar covalent compounds. Solvents can able for studying novel reactions, be further categorized according to decomposing polymers, and produc- their proton-donating and accepting ing amorphous materials. properties. Amphiprotic solvents self- sorbitol A polyhydric alcohol, ionize and can therefore act both as CH OH(CHOH) CH OH, derived from proton donators and acceptors. A typ- 2 4 2 glucose; it is isomeric with *manni- ical example is water: tol. It is found in rose hips and rowan ˆ + – 2H2O H3O + OH berries and is manufactured by the Aprotic solvents neither accept nor catalytic reduction of glucose with sorption 496

hydrogen. Sorbitol is used as a sweet- added for other reasons (e.g. speciÜc ener (in diabetic foods) and in the gravity, speciÜc resistance). These manufacture of vitamin C and vari- names are now no longer used. ous cosmetics, foodstuffs, and medi- speciÜc activity See activity. cines. speciÜc gravity See relative den- sorption *Absorption of a gas by a sity; specific. solid. speciÜc heat capacity See heat sorption pump A type of vacuum capacity. pump in which gas is removed from a system by absorption on a solid spectrochemical series A series (e.g. activated charcoal or a zeolite) at of ligands arranged in the order in low temperature. which they cause splitting of the energy levels of d-orbitals in metal com- Soxhlet apparatus An apparatus plexes (see crystal-field theory). for extracting components from a The series for some common ligands solid (e.g. extracting natural products has the form: from plant material). The material –> –> > > > –> used is placed in a thimble made of CN NO2 NH3 C5H5N H2O OH –> –> –> – thick Ülter paper and this is held in a F Cl Br I Û specially designed re ex condenser spectrograph See spectroscope. with a suitable solvent. The chamber holding the thimble Ülls with warm spectrometer Any of various in- solvent and this is led back to the struments for producing a spectrum source via a side arm. The apparatus and measuring the wavelengths, en- can be operated for long periods, ergies, etc., involved. A simple type, with components concentrating in for visible radiation, is a spectroscope the source vessel. It is named after equipped with a calibrated scale al- Franz Soxhlet, who devised it in lowing wavelengths to be read off or 1879. calculated. In the X-ray to infrared region of the electromagnetic spec- sp Synperiplanar. See torsion angle trum, the spectrum is produced by . dispersing the radiation with a prism space group A group of symmetry or diffraction grating (or crystal, in elements applying to a lattice. Com- the case of hard X-rays). Some form pare point group. of photoelectric detector is used, and s the spectrum can be obtained as a species A chemical entity, such as a graphical plot, which shows how the particular atom, ion, or molecule. intensity of the radiation varies with speciÜc 1. Denoting that an exten- wavelength. Such instruments are sive physical quantity so described is also called spectrophotometers. Spec- expressed per unit mass. For exam- trometers also exist for investigating ple, the speciÜc latent heat of a body the gamma-ray region and the mi- is its latent heat per unit mass. When crowave and radio-wave regions of the extensive physical quantity is de- the spectrum (see electron paramag- noted by a capital letter (e.g. L for la- netic resonance; nuclear mag- tent heat), the speciÜc quantity is netic resonance). Instruments for denoted by the corresponding lower- obtaining spectra of particle beams case letter (e.g. l for speciÜc latent are also called spectrometers (see heat). 2. In some older physical spectrum; photoelectron spec- quantities the adjective ‘speciÜc’ was troscopy). 497 spectrum spectrophotometer See spectrom- of the spectra so produced can be eter. used for chemical analysis, examining atomic and molecular energy lev- spectroscope An optical instru- els and molecular structures, and for ment that produces a *spectrum for determining the composition and visual observation. The Ürst such in- motions of celestial bodies. strument was made by R. W. Bunsen; in its simplest form it consists of a A hollow tube with a slit at one end by • Original paper by Kirchoff and Bunsen which the light enters and a collimat- spectrum (pl. spectra) 1. A distribu- ing lens at the other end to produce tion of entities or properties arrayed a parallel beam, a prism to disperse in order of increasing or decreasing the light, and a telescope for viewing magnitude. For example, a beam of the spectrum. In the spectrograph, ions passed through a mass spectro- the spectroscope is provided with a graph, in which they are deÛected ac- camera to record the spectrum. cording to their charge-to-mass For a broad range of spectroscopic ratios, will have a range of masses work, from the ultraviolet to the in- called a mass spectrum. A sound frared, a diffraction grating is used spectrum is the distribution of en- instead of a prism. See also spectrometer ergy over a range of frequencies of a . particular source. 2. A range of elec- spectroscopy The study of meth- tromagnetic energies arrayed in ods of producing and analysing order of increasing or decreasing *spectra using *spectroscopes, *spec- wavelength or frequency (see elec- trometers, spectrographs, and spec- tromagnetic spectrum). The emis- trophotometers. The interpretations sion spectrum of a body or substance

radio waves X-rays ultraviolet infrared microwaves visible

10-4 10-3 10-2 10-1 110102 103 104 105 106 s wavelength (micrometres)

3 × 1018 3 × 1016 3 × 1014 3 × 1012 3 × 1010 3 × 108 frequency (hertz)

inner shell outer shell molecular vibrations molecular rotations electrons (valence) electrons

Spectroscopy speculum 498

is the characteristic range of radia- cluding the USA, Canada, Mexico, tions it emits when it is heated, bom- Russia, Australia, Peru, and Poland. barded by electron or ions, or sphalerite structure (zinc-blende absorbs photons. The absorption structure) A type of ionic crystal spectrum of a substance is produced structure in which the anions have by examining, through the substance an expanded face-centred cubic and through a spectroscope, a contin- arrangement with the cations occu- uous spectrum of radiation. The ener- pying one type of tetrahedral hole. gies removed from the continuous The coordination number of each spectrum by the absorbing medium type of ion is 4. Examples of com- show up as black lines or bands. pounds with the sphalerite structure With a substance capable of emitting are ZnS, CuCl, CdS, and InAs. a spectrum, these are in exactly the A same positions in the spectrum as • An interactive version of the structure some of the lines and bands in the emission spectrum. spherical top See moment of iner- Emission and absorption spectra tia. may show a continuous spectrum, a sphingolipid See phospholipid. line spectrum, or a band spectrum. A continuous spectrum contains an un- spiegel (spiegeleisen) A form of broken sequence of frequencies over *pig iron containing 15–30% of man- a relatively wide range; it is produced ganese and 4–5% of carbon. It is by incandescent solids, liquids, and added to steel in a Bessemer con- compressed gases. Line spectra are verter as a deoxidizing agent and to discontinuous lines produced by ex- raise the manganese content of steel. cited atoms and ions as they fall back spin (intrinsic angular momentum) to a lower energy level. Band spectra Symbol s. The part of the total angu- (closely grouped bands of lines) are lar momentum of a particle, atom, characteristic of molecular gases or nucleus, etc., that can continue to chemical compounds. See also spec- exist even when the particle is appar- troscopy. ently at rest, i.e. when its transla- speculum An alloy of copper and tional motion is zero and therefore tin formerly used in reÛecting tele- its orbital angular momentum is scopes to make the main mirror as it zero. A molecule, atom, or nucleus in Ü s could be cast, ground, and polished a speci ed energy level, or a particu- to make a highly reÛective surface. It lar elementary particle, has a particu- has now been largely replaced by sil- lar spin, just as it has a particular vered glass for this purpose. charge or mass. According to *quantum theory, this is quantized and is sphalerite (zinc blende) A mineral restricted to multiples of h/2π, where form of zinc sulphide, ZnS, crystalliz- h is the *Planck constant. Spin is ing in the cubic system; the principal characterized by a quantum number ore of zinc. It is usually yellow-brown s. For example, for an electron s = to brownish-black in colour and ±½, implying a spin of + h/4π when it occurs, often with galena, in meta- is spinning in one direction and –h/4π somatic deposits and also in hydro- when it is spinning in the other. Be- thermal veins and replacement cause of their spin, particles also deposits. Sphalerite is mined on have their own intrinsic magnetic every continent, the chief sources in- moments and in a magnetic Üeld the 499 spin–orbit coupling spin of the particles lines up at an to the distribution for equilibrium angle to the direction of the Üeld, statistical mechanics after some per- precessing around this direction. See turbation, such as magnetic reso- also nuclear magnetic resonance. nance, has caused more electron spinel A group of oxide minerals spins to be in high-energy states. The 2+ 3+ excess energy of these high-energy with the general formula F R2 O4, where F2+ = Mg, Fe, Zn, Mn, or Ni and states is taken up by vibrations of the R3+ = Al, Fe, or Cr, crystallizing in the lattice of the solid. cubic system. The spinels are divided spinodal curve A curve that sepa- into three series: spinel (MgAl2O4), rates a metastable region from an un- *magnetite, and *chromite. They stable region in the coexistence occur in high-temperature igneous or region of a binary Ûuid. Above the metamorphic rocks. spinodal curve the process of moving spinel structure An ionic crystal towards equilibrium occurs by structure shown by compounds of droplet nucleation, while below the spinodal curve there are periodic the type AB2O4. There is a face- centred cubic arrangement of O2– modulations of the order parameter, which have a small amplitude at Ürst ions. The A cations occupy one spinodal decomposition eighth of the tetrahedral holes and (see ). The spinodal curve is not a sharp bound- the B cations occupy the octahedral Û holes. Example of the spinel struc- ary in real systems as a result of uctuations. ture are MgAl2O4, Fe3O4, and Mn3O4. A spinodal decomposition The • An interactive version of the structure process of moving towards equilibrium in a part of a phase diagram in spin glass An alloy of a small which the order parameter is con- amount of a magnetic metal served. Spinodal decomposition is ob- (0.1–10%) with a nonmagnetic metal, served in the quenching of binary in which the atoms of the magnetic mixtures. See also spinodal curve. element are randomly distributed through the crystal lattice of the non- spin–orbit coupling An interac- magnetic element. Examples are tion between the orbital angular mo- AuFe and CuMn. Theories of the mentum and the spin angular magnetic and other properties of momentum of an individual particle, spin glasses are complicated by the such as an electron. For light atoms, s random distribution of the magnetic spin–orbit coupling is small so that atoms. *multiplets of many-electron atoms are described by *Russell–Saunders spin label A molecule or group coupling. For heavy atoms, spin–orbit that contains an unpaired electron coupling is large so that multiplets of and can be attached to another mol- many-electron atoms are described ecule. The spin of the unpaired elec- by *j-j coupling. For medium-sized tron can be detected by electron atoms the sizes of the energies asso- paramagnetic resonance. The tech- ciated with spin–orbit coupling are nique of spin labelling is used to in- comparable to the sizes of energies vestigate proteins and biological associated with electrostatic repul- systems. sion between the electrons, the mul- spin–lattice relaxation A process tiplets in this case being described as in which electrons in a crystal return having intermediate coupling. spirits of salt 500

Spin–orbit coupling is large in many formulations such as the *Schrö- nuclei, particularly heavy nuclei. dinger equation, is responsible for spirits of salt A name formerly the limited lifetime of an excited given to hydrogen chloride because state of an atom before it emits a this compound can be made by photon. adding sulphuric acid to common spot test A simple test for a given salt (sodium chloride). substance using a reagent that spiro compound A type of com- changes colour when mixed with the pound in which there are two rings substance. In forensic science spot linked through a single atom (the tests are often called *presumptive spiro atom). In most cases, the spiro tests. atom is a carbon atom, and the rings sputtering The process by which are not in the same plane. Some some of the atoms of an electrode spiro compounds with substituents (usually a cathode) are ejected as a re- on both rings may have a chirality sult of bombardment by heavy posi- element and consequently show opti- tive ions. Although the process is cal activity. generally unwanted, it can be used to A produce a clean surface or to deposit • Information about IUPAC nomenclature a uniform Ülm of a metal on an ob- • Alternative naming system for spiro ject in an evacuated enclosure. compounds squalene An intermediate compound formed in the synthesis of cholesterol; it is a hydrocarbon containing 30 carbon atoms. The im- mediate oxidation of squalene to Spiro compound squalene 2,3-epoxide is the last common step in the synthesis of *sterols in animals, plants, and fungi. spontaneous combustion Com- bustion in which a substance pro- square-planar Describing a coordi- duces sufÜcient heat within itself, nation compound in which four lig- usually by a slow oxidation process, ands positioned at the corners of a for ignition to take place without square coordinate to a metal ion at complex s the need for an external high- the centre of the square. See . temperature energy source. stabilization energy The amount spontaneous emission The emis- by which the energy of a delocalized sion of a photon by an atom as it chemical structure is less than the makes a transition from an excited theoretical energy of a structure with state to the ground state. Sponta- localized bonds. It is obtained by sub- neous emission occurs independently tracting the experimental heat of for- –1 of any external electromagnetic radi- mation of the compound (in kJ mol ) ation; the transition is caused by in- from that calculated on the basis of a teractions between atoms and classical structure with localized vacuum Ûuctuations of the quantized bonds. electromagnetic Üeld. The process of stabilizer 1. A substance used to in- spontaneous emission, which cannot hibit a chemical reaction, i.e. a nega- be described by nonrelativistic tive catalyst. 2. A substance used to *quantum mechanics, as given by prevent a colloid from coagulating. 501 stannic compounds stable equilibrium See equilib- 101.325 kPa) or concentration (usu- rium. ally 1 M). Thermodynamic functions are designated as ‘standard’ when staggered See conformation. they refer to changes in which reac- staggered conformation See con- tants and products are all in their formation. standard and their normal physical stainless steel A form of *steel state. For example, the standard containing at least 11–12% of molar enthalpy of formation of water chromium, a low percentage of carat 298 K is the enthalpy change for bon, and often some other elements, the reaction ½ → notably nickel and molybdenum. H2(g) + O2(g) H2O(l) Stainless steel does not rust or stain ∆ Š –1 H 298 = –285.83 kJ mol . Note the and therefore has a wide variety of superscript Š is used to denote stan- uses in industrial, chemical, and do- dard state and the temperature mestic environments. A particularly should be indicated. successful alloy is the steel known as 18–8, which contains 18% Cr, 8% Ni, standard temperature and pres- and 0.08% C. sure See s.t.p. stalactites and stalagmites Ac- stannane See tin(iv) hydride. cretions of calcium carbonate in stannate A compound formed by limestone caves. Stalactites are taper- reaction of tin oxides (or hydroxides) ing cones or pendants that hang with alkali. Tin oxides are ampho- down from the roofs of caves; stalag- teric (weakly acidic) and react to give mites are upward projections from Û stannate ions. Tin(IV) oxide with the cave oor and tend to be broader molten alkali gives the stannate(IV) at their bases than stalactites. Both ion are formed from drips of water con- – → 2– taining calcium hydrogencarbonate SnO2 + 2OH SnO3 + H2O in solution and may take thousands In fact, there are various ions present of years to grow. in which the tin is bound to hydrox- standard cell A *voltaic cell, such ide groups, the main one being the hexahydroxostannate(IV) ion, as a *Clark cell, or *Weston cell, 2– used as a standard of e.m.f. Sn(OH)6 . This is the negative ion present in crystalline ‘trihydrates’ of standard electrode An electrode the type K2Sn2O3.3H2O. s (a half cell) used in measuring elec- Tin(II) oxide gives the trihydroxo- trode potential. See hydrogen half stannate(II) ion in alkaline solutions cell . – → SnO(s) + OH (aq) + H2O(l) – standard electrode potential See Sn(OH)3 (aq) electrode potential . Stannate(IV) compounds were for- standard solution A solution of merly referred to as orthostannates 4– 2– known concentration for use in volu- (SnO4 ) or metastannates (SnO3 ). metric analysis. Stannate(II) compounds were called stannites. standard state A state of a system used as a reference value in thermo- stannic compounds Compounds dynamic measurements. Standard of tin in its higher (+4) oxidation states involve a reference value of state; e.g. stannic chloride is tin(IV) pressure (usually one atmosphere, chloride. stannite 502

stannite See stannate. ing several molecules. The Stark– Einstein law is named after Johannes stannous compounds Com- Stark and Albert *Einstein. pounds of tin in its lower (+2) oxidation state; e.g. stannous chloride is state of matter One of the three tin(II) chloride. physical states in which matter can exist, i.e. *solid, *liquid, or *gas. starch A *polysaccharide consisting Plasma is sometimes regarded as the of various proportions of two glucose fourth state of matter. polymers, *amylose and *amylopectin. It occurs widely in plants, stationary phase See chromatog- especially in roots, tubers, seeds, and raphy. fruits, as a carbohydrate storage stationary state A state of a sys- product and energy source. Starch is tem when it has an energy level per- therefore a major energy source for mitted by *quantum mechanics. animals. When digested it ultimately Transitions from one stationary state yields glucose. Starch granules are in- to another can occur by the emission soluble in cold water but disrupt if or absorption of an appropriate heated to form a gelatinous solution. quanta of energy (e.g. in the form of This gives an intense blue colour photons). with iodine solutions and starch is used as an *indicator in certain titra- statistical mechanics The branch tions. of physics in which statistical methods are applied to the microscopic Stark effect The splitting of lines constituents of a system in order to in the *spectra of atoms due to the Ü predict its macroscopic properties. presence of a strong electric eld. It The earliest application of this is named after the German physicist method was Boltzmann’s attempt to Johannes Stark (1874–1957), who dis- explain the thermodynamic proper- covered it in 1913. Like the normal ties of gases on the basis of the statis- *Zeeman effect, the Stark effect can tical properties of large assemblies of be understood in terms of the classi- molecules. cal electron theory of Lorentz. The In classical statistical mechanics, Stark effect for hydrogen atoms was each particle is regarded as occupy- also described by the Bohr theory of ing a point in phase space, i.e. to the atom. In terms of quantum me- have an exact position and momen- chanics, the Stark effect is described s tum at any particular instant. The by regarding the electric Üeld as a probability that this point will oc- perturbation on the quantum states cupy any small volume of the phase and energy levels of an atom in the space is taken to be proportional to absence of an electric Üeld. This ap- the volume. The Maxwell–Boltzmann plication of perturbation theory was law gives the most probable distribu- its Ürst use in quantum mechanics. tion of the particles in phase space. Stark–Einstein law The law stat- With the advent of quantum ing that in a photochemical process theory, the exactness of these (such as a photochemical reaction) premises was disturbed (by the one photon is absorbed by each mol- Heisenberg uncertainty principle). In ecule causing the main photochemi- the *quantum statistics that evolved cal process. In some circumstances, as a result, the phase space is divided one molecule, having absorbed a into cells, each having a volume hf, photon, initiates a process involv- where h is the Planck constant and f 503 stereoisomerism is the number of degrees of freedom *basic-oxygen process (L–D process), of the particles. This new concept led which has largely replaced the to Bose–Einstein statistics, and for *Bessemer process and the *open- particles obeying the Pauli exclusion hearth process, or in electrical fur- principle, to Fermi–Dirac statistics. naces. steam distillation A method of distilling liquids that are immiscible 960 with water by bubbling steam 920 through them. It depends on the fact austenite that the vapour pressure (and hence 880 the boiling point) of a mixture of two 840 immiscible liquids is lower than the vapour pressure of either pure liquid. 800 austenite + ferrite + 760 cementite steam point The temperature at austenite which the maximum vapour pres- 720 sure of water is equal to the standard temperature (°C) 680 ferrite + cementite + atmospheric pressure (101 325 Pa). pearlite pearlite On the Celsius scale it has the value 640 100°C. 600 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 stearate (octadecanoate) A salt or carbon (%) ester of stearic acid. Steel stearic acid (octadecanoic acid) A solid saturated *fatty acid, step A single stage in a chemical re- CH (CH ) COOH; r.d. 0.94; m.p. 3 2 16 action. For example, the addition of 71.5–72°C; b.p. 360°C (with decomposition). It occurs widely (as *glyc- hydrogen chloride to ethene involves erides) in animal and vegetable fats. three steps: HCl → H+ + Cl– steel Any of a number of alloys con- + → + sisting predominantly of iron with H + C2H4 CH3CH2 varying proportions of carbon (up CH CH + + Cl– → CH CH Cl to 1.7%) and, in some cases, small 3 2 3 2 quantities of other elements (alloy steradian Symbol sr. The dimen- steels), such as manganese, silicon, sionless (supplementary) *SI unit of s chromium, molybdenum, and nickel. solid angle equal to the solid angle Steels containing over 11–12% of that encloses a surface on a sphere chromium are known as *stainless equal to the square of the radius of steels. the sphere. Carbon steels exist in three stable crystalline phases: ferrite has a body- stere A unit of volume equal to centred cubic crystal, austenite has a 1m3. It is not now used for scientiÜc face-centred cubic crystal, and ce- purposes. mentite has an orthorhombic crystal. stereochemistry The branch of Pearlite is a mixture of ferrite and ce- chemistry concerned with the struc- mentite arranged in parallel plates. ture of molecules and the way the The phase diagram shows how the arrangement of atoms and groups af- phases form at different tempera- fects the chemical properties. tures and compositions. Steels are manufactured by the stereoisomerism See isomerism. stereoregular 504

Steroid

stereoregular Describing a *poly- gular momentum is quantized, mer that has a regular pattern of side which means that the associated groups along its chain. magnet can only lie in certain discrete orientations. As expected, sharp stereospeciÜc Describing chemi- bands of atoms are observed, pro- cal reactions that give products with vided that the beam has low inten- a particular arrangement of atoms in sity. The low intensity reduces the space. An example of a stereospeciÜc blurring effects caused by collisions reaction is the *Ziegler process for between atoms. making polyethene. steroid steric effect An effect in which Any of a group of lipids de- the rate or path of a chemical reac- rived from saturated compound tion depends on the size or arrange- called cyclopentanoperhydrophenan- ment of groups in a molecule. threne, which has a nucleus of four rings (see formula). Some of the most steric hindrance An effect in important steroid derivatives are the which a chemical reaction is slowed steroid alcohols, or sterols. Other down or prevented because large steroids include the bile acids, which groups on a reactant molecule hinder aid digestion of fats in the intestine; the approach of another reactant the sex hormones (androgens and oe- molecule. strogens); and the corticosteroid hor- Stern–Gerlach experiment An mones, produced by the adrenal experiment demonstrating the space cortex. *Vitamin D is also based on quantization of rotating bodies, Ürst the steroid structure. s conducted by the German scientists A Otto Stern (1888–1969) and Walther • Information about IUPAC nomenclature Gerlach (1899–1979) in 1921. Their sterol Any of a group of *steroid- experiment involved passing a beam based alcohols having a hydrocarbon of silver atoms through an inhomo- side-chain of 8–10 carbon atoms. geneous magnetic Üeld. The purpose Sterols exist either as free sterols or of the experiment was to investigate as esters of fatty acids. Animal sterols the interaction between a charged ro- (zoosterols) include cholesterol and tating body and the Üeld. A charged lanosterol. The major plant sterol rotating body acts like a magnet. In (phytosterol) is beta-sitosterol, while classical mechanics the orientation of fungal sterols (mycosterols) include angular momentum can have any ergosterol. value so that the magnet associated with it can take any orientation. stimulated emission See induced However, in quantum mechanics an- emission; laser. 505 Stokes radiation

Stirling’s approximation An ap- stoichiometric coefÜcient See proximation for the factorial, n!, of a chemical equation. number n. The precise form of Stir- stoichiometric compound A ling’s approximation is: compound in which atoms are com- π ½ n+½ n! = (2 ) n exp(–n) bined in exact whole-number ratios. This approximation is valid for large Compare nonstoichiometric com- values of n, being accurate for n pound. greater than about 10. A simpliÜed stoichiometric mixture A mix- version of Stirling’s expression is ture of substances that can react to logen! = nlogen – n, which is an approximation to the precise form, de- give products with no excess reac- rived by dropping all those terms tant. that do not increase at least as stoichiometric sum See chemical quickly as n. An important applica- equation. tion of Stirling’s approximation is in stoichiometry The relative propor- the derivation of the Boltzmann dis- tions in which elements form com- tribution (see boltzmann equation). pounds or in which substances react. In this application n is very much greater than 10, enabling the stokes Symbol St. A c.g.s. unit of simpliÜed version of Stirling’s ap- kinematic viscosity equal to the ratio proximation to be used. of the viscosity of a Ûuid in poises to scanning tunnelling its density in grams per cubic cen- STM See –4 2 –1 microscope. timetre. 1 stokes = 10 m s . It is named after the British mathemati- stochastic process Any process in cian and physicist Sir George Gabriel which there is a random element. Stokes (1819–1903). Stochastic processes are important in *nonequilibrium statistical mechan- Stokes radiation The electromag- ics and *disordered solids. In a time- netic radiation that occurs in the dependent stochastic process, a *Raman effect when the frequency variable that changes with time does of the scattered radiation is lower so in such a way that there is no cor- than the frequency of the unscat- relation between different time inter- tered radiation, i.e. energy is trans- vals. An example of a stochastic ferred from the photon to the process is *Brownian movement. scattering molecule. The spectral Equations, such as the *Langevin Raman lines corresponding to this s equation and the *Fokker–Planck type of radiation are called Stokes equation, that describe stochastic lines, with the set of Stokes lines processes are called stochastic equa- being called the O-branch of the tions. It is necessary to use statistical Raman spectrum. methods and the theory of probabil- Similarly, *anti-Stokes radiation is ity to analyse stochastic processes the radiation when the frequency of and their equations. the scattered electromagnetic radiation is higher than the frequency of Stockholm Convention See persistent organic pollutant the unscattered radiation, i.e. energy . is transferred to the photon from the stoichiometric Describing chemi- scattering molecule. The spectral cal reactions in which the reactants Raman lines corresponding to this combine in simple whole-number ra- type of radiation are called anti- tios. Stokes lines, with the set of anti- stopped-Ûow technique 506

Stokes lines being called the S-branch lowish metallic element belonging to of the Raman spectrum. group 2 (formerly IIA) of the periodic Stokes radiation is named after the table (see alkaline-earth metals); British physicist Sir George Gabriel a.n. 38; r.a.m. 87.62; r.d. 2.6; m.p. Stokes (1819–1903). 769°C; b.p. 1384°C. The element is stopped-Ûow technique A tech- found in the minerals strontianite nique for investigating fast reactions (SrCO3) and celestine (SrSO4). It can in solution. The reactant solutions be obtained by roasting the ore to are rapidly mixed and Ûow through a give the oxide, followed by reduction tube. The composition or properties with aluminium (i.e. the *Gold- of the mixture are monitored at schmidt process). The element, some point in the tube (e.g. by pho- which is highly reactive, is used in tometry). The Ûow is suddenly certain alloys and as a vacuum getter. stopped and the change of signal The isotope strontium–90 is present with time is used to elucidate the ki- in radioactive fallout (half-life 28 netics of the process. The stopped- years), and can be metabolized with Ûow technique is useful for reactions calcium so that it collects in bone. occurring in the millisecond time Strontium was discovered by Martin range. Klaproth (1743–1817) and Thomas Hope (1766–1844) in 1798 and iso- s.t.p. Standard temperature and lated by Humphry Davy in 1808. pressure, formerly known as N.T.P. A (normal temperature and pressure). • Information from the WebElements site The standard conditions used as a basis for calculations involving quan- strontium bicarbonate See stron- tities that vary with temperature and tium hydrogencarbonate. pressure. These conditions are used strontium carbonate A white when comparing the properties of solid, SrCO ; orthorhombic; r.d. 3.7; gases. They are 273.15 K (or 0°C) and 3 decomposes at 1340°C. It occurs nat- 101 325 Pa (or 760.0 mmHg). urally as the mineral strontianite and straight chain See chain. is prepared industrially by boiling ce- strange attractor See attractor. lestine (strontium sulphate) with ammonium carbonate. It can also be streaming potential A potential prepared by passing carbon dioxide difference that occurs when a liquid over strontium oxide or hydroxide or s under pressure is forced through a by passing the gas through a solution narrow opening or diaphragm. It can of strontium salt. It is a phosphor, be measured between two electrodes used to coat the glass of cathode-ray at each end of a capillary tube made screens, and is also used in the reÜn- of the same material as the dia- ing of sugar, as a slagging agent in phragm. certain metal furnaces, and to pro- Û Ü strong acid An *acid that is com- vide a red ame in reworks. pletely dissociated in aqueous solu- strontium chloride A white com- tion. pound, SrCl2. The anhydrous salt strontia See strontium oxide. (cubic; r.d. 3.05; m.p. 872°C; b.p. 1250°C) can be prepared by passing strontianite A mineral form of chlorine over heated strontium. It is *strontium carbonate, SrCO3. deliquescent and readily forms the strontium Symbol Sr. A soft yel- hexahydrate, SrCl2.6H2O (r.d. 2.67). 507 substitution reaction

This can be made by neutralizing atom i. The structure factor is used in hydrochloric acid with strontium Patterson synthesis. carbonate, oxide, or hydroxide. strychnine A colourless poisonous Strontium chloride is used for mili- crystalline alkaloid found in certain tary Ûares. plants. strontium hydrogencarbonate styrene See phenylethene. (strontium bicarbonate) A compound, Sr(HCO3)2, which is stable subjacent orbitals The next-to- only in solution. It is formed by the highest occupied molecular orbital action of carbon dioxide on a suspen- (NHOMO) and the second-lowest un- sion of strontium carbonate in water. occupied molecular orbital (SLUMO). On heating, this process is reversed. In certain cases these subjacent orbitals are signiÜcant in *frontier- strontium oxide (strontia) A white compound, SrO; r.d. 4.7; m.p. 2430°C, orbital theory. b.p. 3000°C. It can be prepared by the sublimate A solid formed by subli- decomposition of heated strontium mation. carbonate, hydroxide, or nitrate, and sublimation A direct change of is used in the manufacture of other state from solid to gas. strontium salts, in pigments, soaps and greases, and as a drying agent. submillimetre waves Electromag- netic radiation with wavelengths strontium sulphate A white solid, below one millimetre (and therefore SrSO ; r.d. 3.96; m.p. 1605°C. It can 4 frequencies greater than 300 giga- be made by dissolving strontium hertz), extending to radiation of the oxide, hydroxide, or carbonate in sulphuric acid. It is used as a pigment in far infrared. A source of submilli- paints and ceramic glazes and to pro- metre radiation is a medium pres- vide a red colour in Üreworks. sure mercury lamp in quartz. Sub- millimetre waves can be detected by structural formula See formula. a *Golay cell. structural isomerism See iso- subshell See atom. merism. substantive dye See dyes. structure factor A quantity de- substantivity The afÜnity of a dye noted Fhkl, where h, k, and l are the Miller indices of the crystal, which for its substrate. s occurs in *X-ray crystallography and substituent 1. An atom or group other experiments involving scatter- that replaces another in a substitu- Ü ing in crystals. Fhkl is de ned by the tion reaction. 2. An atom or group equation: regarded as having replaced a hydro- ∑ π Fhkl = fiexp[2 i(hxi + kyi + lzi)], gen atom in a chemical derivative. i where the sum is over all atoms of For example, dibromobenzene (C6H4Br2) is a derivative of benzene the unit cell and fi is the scattering factor for atom i deÜned by: with bromine substituents. ∞ πΎ ρ 2 substitute natural gas sng fi = 4 0 ( sinkr/kr)r dr See . Here k = 4πsinθ/λ, where θ is the substitution reaction (displace- Bragg angle (see bragg’s law), λ is the ment reaction) A reaction in which wavelength of the X-rays, and ρ is the one atom or molecule is replaced by electron density distribution of the another atom or molecule. See elec- substrate 508

trophilic substitution; nucleo- movers, herbicides, and artiÜcial philic substitution. sweeteners. substrate 1. The substance that is sulphanes Compounds of hydro- affected by the action of a catalyst; gen and sulphur containing chains for example, the substance upon of sulphur atoms. They have the which an *enzyme acts in a biochem- general formula H2Sn. The simplest ical reaction. 2. The substance on is hydrogen sulphide, H2S; other which some other substance is ad- members of the series are H2S2, sulphides sorbed or in which it is absorbed. Ex- H2S3, H2S4, etc. See . amples include the material to which A a dye is attached, the porous solid ab- • Information about IUPAC nomenclature sorbing a gas, and the *matrix trapping isolated atoms, radicals, etc. sulphanilic acid (4-aminobenzene sulphonic acid) A colourless crys- succinic acid See butanedioic acid. talline solid, H2NC6H4SO2OH, made sucrose (cane sugar; beet sugar; sac- by prolonged heating of *phenyl- charose) A sugar comprising one amine (aniline) sulphate. It readily molecule of glucose linked to a fruc- forms *diazo compounds and is used tose molecule. It occurs widely in to make dyes and sulpha drugs. plants and is particularly abundant in sulphate A salt or ester of sul- sugar cane and sugar beet (15–20%), phuric(VI) acid. Organic sulphates from which it is extracted and have the formula R SO , where R is Ü 2 4 re ned for table sugar. If heated to an organic group. Sulphate salts con- 200°C, sucrose becomes caramel. 2– tain the ion SO4 . sugar (saccharide) Any of a group of sulphides 1. Inorganic compounds water-soluble *carbohydrates of rela- of sulphur with more electropositive tively low molecular weight and typi- elements. Compounds of sulphur cally having a sweet taste. The simple with nonmetals are covalent com- sugars are called *monosaccharides. pounds, e.g. hydrogen sulphide (H2S). More complex sugars comprise be- Metals form ionic sulphides contain- tween two and ten monosaccharides ing the S2– ion; these are salts of hy- linked together: *disaccharides con- drogen sulphide. Polysulphides can tain two, trisaccharides three, and so also be produced containing the poly- on. The name is often used to refer 2– s meric ion Sx . 2. (or thio ethers)Or- speciÜcally to *sucrose (table sugar). ganic compounds that contain the A group –S– linked to two hydrocarbon • Information about IUPAC nomenclature groups. Organic sulphides are named from the linking groups, e.g. di- sugar of lead See lead(ii) methyl sulphide (CH SCH ), ethyl ethanoate. 3 3 methyl sulphide (C2H5SCH3). They sulpha drugs See sulphonamides. are analogues of ethers in which the oxygen is replaced by sulphur (hence sulphamic acid A colourless crys- the alternative name) but are gener- talline solid, NH SO OH, which is ex- 2 2 ally more reactive than ethers. Thus tremely soluble in water and they react with halogen compounds normally exists as the *zwitterion to form *sulphonium compounds H N+.SO –. It is a strong acid, readily 3 3 and can be oxidized to *sulphoxides. forming sulphamate salts. It is used in electroplating, hard-water scale re- sulphinate (dithionite; hyposul- 509 sulphur phite) A salt that contains the nega- group. Sulphonic acids are formed by 2– tive ion S2O4 , usually formed by the reaction of aromatic hydrocarbons reduction of sulphites with excess with concentrated sulphuric acid. SO2. Solutions are not very stable and They are strong acids, ionizing com- decompose to give thiosulphate and pletely in solution to form the – hydrogensulphite ions. The structure sulphonate ion, –SO2.O . – – is O2S-SO2 . A sulphinic acid (dithionous acid; hy- • Information about IUPAC nomenclature posulphurous acid) An unstable acid, sulphonium compounds Com- H2S2O4, known in the form of its pounds containing the ion R S+ (sul- sulphuric 3 salts (sulphinates). See also phonium ion), where R is any organic acid . group. Sulphonium compounds can sulphite A salt or ester derived be formed by reaction of organic sul- from sulphurous acid. The salts con- phides with halogen compounds. 2– For example, diethyl sulphide, tain the trioxosulphate(IV) ion SO3 . Sulphites generally have reducing C2H5SC2H5, reacts with chloro- properties. methane, CH3Cl, to give diethyl- methylsulphonium chloride, sulphonamides Organic com- (C H ) .CH .S+Cl–. pounds containing the group 2 5 2 3 sulphoxides Organic compounds –SO2.NH2. The sulphonamides are amides of sulphonic acids. Many containing the group =S=O (sulphox- have antibacterial action and are also ide group) linked to two other known as sulpha drugs, including groups, e.g. dimethyl sulphoxide, (CH ) SO. sulphadiazine, NH2C6H4SO2- A3 2 NHC4H3N2, sulphathiazole, NH2C6H4SO2NHC5H2NS, and • Information about IUPAC nomenclature several others. They act by pre- sulphur Symbol S. A yellow non- venting bacteria from reproducing metallic element belonging to and are used to treat a variety of *group 16 (formerly VIB) of the peri- bacterial infections, especially of the odic table; a.n. 16; r.a.m. 32.06; r.d. gut and urinary system. 2.07 (rhombic); m.p. 112.8°C; b.p. sulphonate A salt or ester of a sul- 444.674°C. The element occurs in phonic acid. many sulphide and sulphate miner- s sulphonation A type of chemical als and native sulphur is also found in Sicily and the USA (obtained by reaction in which a –SO3H group is substituted on a benzene ring to the *Frasch process). It can also be form a *sulphonic acid. The reaction obtained from hydrogen sulphide by is carried out by reÛuxing with con- the *Claus process. Sulphur has various allotropic centrated sulphuric(VI) acid for a ° long period. It can also occur with forms. Below 95.6 C the stable crystal form is rhombic; above this tem- cold disulphuric(VI) acid (H2S2O7). Sulphonation is an example of elec- perature the element transforms into trophilic substitution in which the a triclinic form. These crystalline electrophile is a sulphur trioxide forms both contain cyclic S8 mol- molecule, SO . ecules. At temperatures just above its 3 melting point, molten sulphur is a sulphonic acids Organic com- yellow liquid containing S8 rings (as pounds containing the –SO2.OH in the solid form). At about 160°C, sulphur cycle 510

R S R sulphide (thio ether) O

R S OH sulphonic acid

RR+ O S sulphonium ion O R R S O– sulphonate ion

O R S H thiol (mercaptan)

R S NH sulphonamide R 2 S O sulphoxide R O Sulphur compounds

the sulphur atoms form chains and dioxide (SO2), produced by combus- 2– the liquid becomes more viscous and tion of fossil fuels. Sulphate (SO4 ), dark brown. If the molten sulphur is derived from the weathering and oxi- cooled quickly from this temperature dation of rocks, is taken up by plants (e.g. by pouring into cold water) a and incorporated into sulphur-con- reddish-brown solid known as plastic taining proteins. In this form sulphur sulphur is obtained. Above 200°C the is passed along food chains to ani- viscosity decreases. Sulphur vapour mals. Decomposition of dead organic contains a mixture of S2, S4, S6, and matter and faeces by anaerobic sul- S8 molecules. Flowers of sulphur is a phate-reducing bacteria returns sul- yellow powder obtained by sublim- phur to the abiotic environment in ing the vapour. It is used as a plant the form of hydrogen sulphide (H2S). fungicide. The element is also used to Hydrogen sulphide can be converted produce sulphuric acid and other sul- back to sulphate or to elemental sulphur compounds. phur by the action of different groups Sulphur is an *essential element in of photosynthetic and sulphide-oxi- living organisms, occurring in the dizing bacteria. Elemental sulphur amino acids cysteine and methionine becomes incorporated into rocks. and therefore in many proteins. It is sulphur dichloride See disulphur s also a constituent of various cell dichloride. metabolites, e.g. coenzyme A. Sul- sulphur dichloride dioxide (sulphur is absorbed by plants from the phuryl chloride) A colourless liquid, soil as the sulphate ion (SO 2–). See 4 SO Cl ; r.d. 1.67; m.p. –54.1°C; b.p. sulphur cycle. 2 2 69°C. It decomposes in water but is A soluble in benzene. The compound is • Information from the WebElements site formed by the action of chlorine on sulphur cycle The cycling of sul- sulphur dioxide in the presence of an phur between the biotic (living) and iron(III) chloride catalyst or sunlight. It is used as a chlorinating agent and abiotic (nonliving) components of the Û environment. Most of the sulphur in a source of the related uoride, the abiotic environment is found in SO2F2. rocks, although a small amount is sulphur dichloride oxide (thionyl present in the atmosphere as sulphur chloride) A colourless fuming liquid, 511 sulphuric acid

° ° SOCl2; m.p. –105 C; b.p. 78.8 C. It hy- forms a range of hydrates: H2SO4.H2O ° drolyses rapidly in water but is solu- (m.p. 8.62 C); H2SO4.2H2O (m.p. ° ° ble in benzene. It may be prepared –38/39 C); H2SO4.6H2O (m.p. –54 C); ° by the direct action of sulphur on H2SO4.8H2O (m.p. –62 C). Its full sys- chlorine monoxide or, more com- tematic name is tetraoxosulphuric(VI) monly, by the reaction of phospho- acid. rus(V) chloride with sulphur dioxide. Until the 1930s, sulphuric acid was It is used as a chlorinating agent in manufactured by the *lead-chamber synthetic organic chemistry (replac- process, but this has now been re- ing –OH groups with Cl). placed by the *contact process (catalytic oxidation of sulphur dioxide). sulphur dioxide (sulphur(IV) oxide) More sulphuric acid is made in the A colourless liquid or pungent gas, UK than any other chemical product; SO , formed by sulphur burning in 2 production levels (UK) are commonly air; r.d. 1.43 (liquid); m.p. –72.7°C; 12 000 to 13 000 tonnes per day. It is b.p. –10°C. It can be made by heating extensively used in industry, the iron sulphide (pyrites) in air. The main applications being fertilizers compound is a reducing agent and is (32%), chemicals (16%), paints and used in bleaching and as a fumigant pigments (15%), detergents (11%), and and food preservative. Large quanti- Übres (9%). ties are also used in the *contact In concentrated sulphuric acid process for manufacturing sulphuric there is extensive hydrogen bonding acid. It dissolves in water to give a and several competing equilibria, to mixture of sulphuric and sulphurous + – acid rain give species such as H3O , HSO4 , acids. See also . + H3SO4 , and H2S2O7. Apart from sulphuretted hydrogen See hy- being a powerful protonating agent drogen sulphide. (it protonates chlorides and nitrates sulphuric acid (oil of vitriol) A producing hydrogen chloride and nitric acid), the compound is a moder- colourless oily liquid, H2SO4; r.d. 1.84; m.p. 10.36°C; b.p. 338°C. The ately strong oxidizing agent. Thus, it pure acid is rarely used; it is com- will dissolve copper: → monly available as a 96–98% solution Cu(s) + H2SO4(l) CuO(s) + H2O(l) + (m.p. 3.0°C). The compound also SO2(g)

OH2SO3 sulphurous acid s HO SOH (in sulphites)

O H2SO4 O O H2S2O4 sulphuric(VI) acid sulphinic acid HO SOH HO SSOH (dithionous or O hyposulphurous)

S H2S2O3 O O H2S2O6 thiosulphuric acid dithionic acid HO SOH HO SSOH

O O O

O O H2S2O7 O O H2Sn+2O6 disulphuric(VI) acid polythionic acids HO S (S) S OH HO SOS OH (pyrosulphuric) n O O O O Sulphur acids sulphuric(IV) acid 512

→ sulphur CuO(s) + H2SO4(l) CuSO4(aq) + sulphur(IV) oxide See dioxide H2O(l) . It is also a powerful dehydrating sulphur(VI) oxide See sulphur agent, capable of removing H2O from trioxide. many organic compounds (as in the production of acid *anhydrides). In sulphur trioxide (sulphur(VI) dilute solution it is a strong dibasic oxide) A colourless fuming solid, acid forming two series of salts, the SO3, which has three crystalline Ü sulphates and the hydrogensul- modi cations. In decreasing order of α phates. stability these are: , r.d. 1.97; m.p. 16.83°C; b.p. 44.8°C; β, m.p. 16.24°C; sulphurous sulphuric(IV) acid See sublimes at 50°C; r.d. 2.29; γ, m.p. acid . 16.8°C; b.p. 44.8°C. All are polymeric, γ sulphur monochloride See disul- with linked SO4 tetrahedra: the - phur dichloride. form has an icelike structure and is obtained by rapid quenching of the sulphur mustard A chemical war- vapour; the β-form has inÜnite heli- fare agent, C4H8Cl2S; r.d. 1.27; m.p. α ° ° cal chains; and the -form has in- 14.4 C; b.p. 217 C. It is a potent blis- Ünite chains with some cross-linking tering agent, Ürst used in 1915 by of the SO tetrahedra. Even in the Germany in World War I. It is often 4 vapour, there are polymeric species, known simply as mustard gas, al- and not discrete sulphur trioxide though it is an oily liquid, which can molecules (hence the compound is be dispersed as an aerosol. It was one more correctly called by its system- of the early chemicals used in chemo- atic name sulphur(VI) oxide). therapy treatment of cancer. The sys- Sulphur trioxide is prepared by the tematic name is bis(2-chlorethyl) oxidation of sulphur dioxide with sulphide. See also nitrogen mustard. oxygen in the presence of a vana- sulphurous acid (sulphuric(IV) acid) dium(V) oxide catalyst. It may be pre- A weak dibasic acid, H2SO3, known in pared in the laboratory by distilling a the form of its salts: the sulphites mixture of concentrated sulphuric and hydrogensulphites. It is consid- acid and phosphorus(V) oxide. It re- ered to be formed (along with sul- acts violently with water to give sulphuric acid) when sulphur dioxide is phuric(VI) acid and is an important s dissolved in water. It is probable, intermediate in the preparation of however, that the molecule H2SO3 is sulphuric acid and oleum. not present and that the solution sulphuryl chloride See sulphur contains hydrated SO . It is a reduc- 2 dichloride dioxide. ing agent. The systematic name is sul- trioxosulphuric(IV) acid. See also sulphuryl group The group =SO2, phuric acid. as in *sulphur dichloride oxide.

O Cl Cl S sulphur (VI) dichloride sulphur (IV) dichloride dioxide O S O Cl oxide Cl

sulphuryl thionyl group group

Sulphuryl group 513 superphosphate sulphydryl group See thiols. place in the lattice of the solid crystal. If the liquid is seeded (see seed) superacid An *acid that has a pro- crystallization usually takes place ton-donating ability equal to or and the liquid returns to its normal greater than that of anhydrous sul- freezing point. Crystallization can phuric acid. A superacid is a particu- also be induced by the presence of lar type of Brønsted acid. Those that particles of dust, by mechanical vi- are much stronger than sulphuric bration, or by rough surfaces. 2. The acids can be made by adding certain Û analogous cooling of a vapour to penta uorides and their derivatives make it supersaturated. to such acids as Ûuorosulphuric acid Û Û (HSO3F) or hydrogen uoride (HF). super uidity The exhibition by a The pentaÛuorides, such as antimony Ûuid at extremely low temperatures, Û penta uoride (SbF5), are very strong e.g. liquid helium at 2.186 K, of very Lewis acids. The mixtures HF-SbF high thermal conductivity and fric- 5 Û and HSO3F-SbF5 are among the tionless ow. The temperature at strongest acids known; their applica- which superÛuidity occurs is called tions include the protonation of very the lambda point. weak bases in organic chemistry and superheating The heating of a liq- the abstraction of hydrogen from sat- uid to above its normal boiling point urated hydrocarbons to produce carby increasing the pressure. bonium ions. An equimolar mixture superionic conductor An ionic of HSO3F and S6F5 is known by the tradename Magic acid. Very strong solid in which the electrical conduc- bases, such as lithium diisopropyl- tivity due to the motion of ions is amide, are sometimes known as su- similar to that of a molten salt, i.e. a perbases. much higher conductivity than is usually observed in ionic solids. superbase See superacid. superlattice See solid solution. superconductivity The property possessed by some substances below supernatant liquid The clear liq- a certain temperature, the transition uid remaining when a precipitate has point, of zero resistance. Until re- settled. cently, the known superconducting superoxides A group of inorganic – materials had very low transition compounds that contain the O2 ion. points. However, synthetic organic They are formed in signiÜcant quan- s conductors and certain metal oxide tities only for sodium, potassium, ru- ceramics have now been produced bidium, and caesium. They are very that become superconducting at powerful oxidizing agents and react much higher temperatures. For ex- vigorously with water to give oxygen ample, much work has been done on gas and OH– ions. The superoxide ion ytterbium–barium–copper oxides, has an unpaired electron and is para- which have transition temperatures magnetic and coloured (orange). of about 100 K. superphosphate A commercial supercooling 1. The cooling of a phosphate mixture consisting mainly liquid to below its freezing point of monocalcium phosphate. Single- without a change from the liquid to superphosphate is made by treating solid state taking place. In this phosphate rock with sulphuric acid; metastable state the particles of the the product contains 16–20% ‘avail- liquid lose energy but do not fall into able’ P2O5: superplasticity 514

Ca10(PO4)6F2 + 7H2SO4 = supramolecular chemistry A Ü 3Ca(H2PO4)2 + 7CaSO4 + 2HF eld of chemical research concerned Triple-superphosphate is made by with the formation and properties of using phosphoric(V) acid in place of large assemblies of molecules held sulphuric acid; the product contains together by intramolecular forces 45–50% ‘available’ P O : (hydrogen bonds, van der Waals’ 2 5 forces, etc.). One feature of supramol- Ca10(PO4)6F2 + 14H3PO4 = ecular chemistry is that of self- 10Ca(H2PO4)2 + 2HF assembly (see self-organization), in superplasticity The ability of which the structure forms sponta- some metals and alloys to stretch neously as a consequence of the na- uniformly by several thousand per- ture of the molecules. The molecular cent at high temperatures, unlike units are sometimes known as syn- normal alloys, which fail after being thons. Another aspect is the study of stretched 100% or less. Since 1962, very large molecules able to be used when this property was discovered in in complex chemical reactions in a fashion similar to, for example, the an alloy of zinc and aluminium (22%), actions of the naturally occurring many alloys and ceramics have been haemoglobin and nucleic acid mol- shown to possess this property. For ecules. Such molecules have great po- superplasticity to occur, the metal tential in such areas as medicine, grain must be small and rounded and electronics, and optics. The *helicate the alloy must have a slow rate of de- and *texaphyrin molecules and *den- formation. drimers are typical examples of com- superradiant Denoting a *laser in pounds of interest in this Üeld. The which the efÜciency of the transition Üeld also includes host–guest chem- of stimulated emission is sufÜciently istry, which is concerned with mol- large for the radiation to be produced ecules speciÜcally designed to accept by a single pulse of electromagnetic other molecules. Examples include radiation without using mirrors. An *crown ethers, *cryptands, and *cal- example of a superradiant laser is the ixarenes. nitrogen laser based on the ultravio- surface-extended X-ray absorp- 3Π → 3Π let transition C u B g. tion Üne-structure spectroscopy sexafs supersaturated solution See sat- See . s urated. surface tension Symbol γ. The supersaturation 1. The state of property of a liquid that makes it be- the atmosphere in which the relative have as if its surface is enclosed in an humidity is over 100%. This occurs in elastic skin. The property results pure air where no condensation nu- from intermolecular forces: a mol- clei are available. Supersaturation is ecule in the interior of a liquid expe- usually prevented in the atmosphere riences interactions from other by the abundance of condensation molecules equally from all sides, whereas a molecule at the surface is nuclei (e.g. dust, sea salt, and smoke only affected by molecules below it particles). 2. The state of any vapour in the liquid. The surface tension is whose pressure exceeds that at deÜned as the force acting over the which condensation usually occurs surface per unit length of surface (at the prevailing temperature). perpendicular to the force. It is meas- supplementary units See si units. ured in newtons per metre. It can 515 synthon equally be deÜned as the energy re- reÛection, and an axis of rotation are quired to increase the surface area the symmetry elements. See also mo- isothermally by one square metre, lecular symmetry. i.e. it can be measured in joules per symmetry-adapted linear com- metre squared (which is equivalent binations See salc. to N m–1). The property of surface tension is syn See torsion angle. responsible for the formation of liq- synchrotron radiation Electro- uid drops, soap bubbles, and menis- magnetic radiation produced when cuses, as well as the rise of liquids in charged particles travel in a curved a capillary tube (capillarity), the ab- path. In a generator of synchrotron sorption of liquids by porous sub- radiation, electrons are accelerated in stances, and the ability of liquids to a synchrotron and injected into a wet a surface. storage ring where they circulate at a surfactant (surface active agent) A constant energy. Radiation can be substance, such as a *detergent, taken from the storage ring tangen- added to a liquid to increase its tially at various points around the spreading or wetting properties by ring. The advantage of this type of reducing its *surface tension. source is that it gives a narrow in- suspension A mixture in which tense beam that is tunable to a par- small solid or liquid particles are sus- ticular wavelength over a whole pended in a liquid or gas. range from infrared radiation to hard X-rays. Synchrotron radiation is used SYBYL line notation A type of for a variety of studies including crys- line notation, similar to *SMILES but tallography, X-ray scattering, and ion- with some additional features. In ization experiments. SLN, hydrogens are not omitted, so ethane is CH3CH3. Wild-card atoms synclinal See torsion angle. Ü and groups can be speci ed for syndiotactic See polymer. searching. syneresis The spontaneous separa- sylvite (sylvine) A mineral form of tion of the solid and liquid compo- *potassium chloride, KCl. nents of a gel on standing. moment of in- symmetric top See synperiplanar See torsion angle. ertia. s synthesis The formation of chemi- symmetry The property of an ob- cal compounds from more simple ject that enables it to undergo cer- compounds. tain manipulations, called symmetry operations, such that its new state is synthesis gas See haber process. indistinguishable from its original synthetic Describing a substance state. Examples include inversion that has been made artiÜcially; i.e. through a point, reÛection through a one that does not come from a nat- plane, and rotation about an axis. ural source. The geometrical feature of the object with respect to which the symmetry synthon A fragment obtained by operation is carried out is termed a disconnection in *retrosynthetic symmetry element. In the above ex- analysis. By analogy, units building amples, a point through which inver- up structures by self-assembly in sion can be performed, a plane of *supramolecular chemistry are some- Système International d’Unités 516

time known as supramolecular syn- the chemical bond between the atom thons. and the molecule of which it forms Système International d’Unités part. Thus, although the atom that has absorbed the neutron is an iso- See si units. tope of the original atom it is in a dif- Szilard–Chalmers effect An ef- ferent form chemically, enabling fect discovered by L. Szilard (1898– separation to take place. For exam- 1964) and T. A. Chalmers in 1934; it ple, if an aqueous solution of sodium has been used to separate radioactive chlorate (NaClO3) is subjected to products in a nuclear reaction involv- bombardment by slow neutrons, the ing the absorption of a neutron and Cl37 is converted to Cl38, with many the emission of gamma rays. If a ma- of the Cl38 atoms breaking from the terial absorbs a neutron and subse- chlorate and moving into the solu- quently emits a gamma ray, the tion in the form of chloride ions. emission of the gamma ray causes This is an example of a ‘hot atom’ re- the nucleus to recoil. Frequently, the action. The Cl38 can be precipitated recoil energy is sufÜcient to break out using silver nitrate.

s T

α η 2,4,5-T 2,4,5-trichlorophenoxyacetic j = j0exp[(1 – )f ] acid (2,4,5-trichlorophenoxyethanoic In the case of a negative overpoten- acid): a synthetic auxin formerly tial, the Ürst exponential can be ig- α η widely used as a herbicide and de- nored, which gives j = j0exp(– f ), so α η foliant. It is now banned in many that ln(–j) = lnj0 – f . Thus, in the countries as it tends to become con- Tafel plot the value of α can be ob- taminated with the toxic chemical tained from the slope and the value dioxin. of j0 can be obtained from the inter- η tabun A highly toxic colourless or cept at = 0. brown liquid, C5H11N2O2P; r.d. 1.09; Takayama test See haemochro- m.p. –50°C; b.p. 247.5°C. It is an mogen test. organophosphorus compound, ethyl N,N-dimethylphosphoramido- talc A white or pale-green mineral cyanidate. Tabun was discovered in form of magnesium silicate, 1936 and belongs to the G-series of Mg3Si4O10(OH)2, crystallizing in the *nerve agents (GA). It was used by triclinic system. It forms as a sec- Iraq in the Iran–Iraq war (1980–88). ondary mineral by alteration of magnesium-rich olivines, pyroxenes, and tactic polymer See polymer. amphiboles of ultrabasic rocks. It is tactosol A colloidal sol containing soaplike to touch and very soft, hav- nonspherical particles capable of ori- ing a hardness of 1 on the Mohs’ entating themselves. An example of a scale. Massive Üne-grained talc is tactosol is given by an aged vana- known as soapstone or steatite. Talc dium pentoxide sol, in which the in powdered form is used as a lubri- particles are rod-shaped. When a tac- cant, as a Üller in paper, paints, and tosol is placed in a magnetic Üeld its rubber, and in cosmetics, ceramics, particles arrange themselves along and French chalk. It occurs chieÛy in the lines of force of the Üeld. the USA, Russia, France, and Japan. Tafel plot The graph of the loga- tannic acid A yellowish complex rithm of the current density j against organic compound present in certain the overpotential η in electrochem- plants. It is used in dyeing as a mor- istry in the high overpotential limit. dant. This limit can occur in electrolysis ei- tannin One of a group of complex ther when the overpotential is large organic chemicals (see depsides) com- and positive, the electrode being the monly found in leaves, unripe fruits, anode, or when the overpotential is and the bark of trees. Their function large and negative, the electrode is uncertain though the unpleasant being the cathode. In the case of pos- taste may discourage grazing ani- itive overpotential, the second expo- mals. Some tannins have commercial nential in the *Butler–Volmer uses, notably in the production of equation can be ignored, as it is leather and ink. much smaller than the Ürst exponential, and thus tantalum Symbol Ta. A heavy blue- tar 518

grey metallic *transition element; tartrate A salt or ester of *tartaric a.n. 73; r.a.m. 180.948; r.d. 16.63; acid. m.p. 2996°C; b.p. 5427°C. It is found tartrazine A food additive (E102) with niobium in the ore columbite– that gives foods a yellow colour. Tar- tantalite (Fe,Mn)(Ta,Nb) O . It is ex- 2 6 trazine can cause a toxic response in tracted by dissolving in hydroÛuoric the immune system and is banned in acid, separating the tantalum and Û some countries. niobium uorides to give K2TaF7, and reduction of this with sodium. The TATP See triacetone triperoxide. element contains the stable isotope tautomerism A type of *iso- tantalum–181 and the long-lived merism in which the two isomers radioactive isotope tantalum–180 (tautomers) are in equilibrium. See > 7 (0.012%; half-life 10 years). There keto–enol tautomerism. are several other short-lived isotopes. krebs cycle The element is used in certain alloys TCA cycle See . and in electronic components. Tanta- technetium Symbol Tc. A radio- lum parts are also used in surgery be- active metallic *transition element; cause of the unreactive nature of the a.n. 43; m.p. 2172°C; b.p. 4877°C. The metal (e.g. in pins to join bones). element can be detected in certain Chemically, the metal forms a pas- stars and is present in the Üssion sive oxide layer in air. It forms com- products of uranium. It was Ürst plexes in the +2, +3, +4, and +5 made by Carlo Perrier and Emilio oxidation states. Tantalum was Segré (1905–89) by bombarding identiÜed in 1802 by Anders Ekeberg molybdenum with deuterons to give (1767–1813) and Ürst isolated in 1820 technetium–97. The most stable iso- by *Berzelius. tope is technetium–99 (half-life 2.6 × A 106 years); this is used to some extent • Information from the WebElements site in labelling for medical diagnosis. There are sixteen known isotopes. tar Any of various black semisolid Chemically, the metal has properties mixtures of hydrocarbons and free intermediate between manganese carbon, produced by destructive dis- and rhenium. tillation of *coal or by *petroleum A reÜning. • Information from the WebElements site retrosyn- target molecule See Û thetic analysis. Te on Tradename for a form of *polytetraÛuoroethene. t tar sand See oil sand. Teichmann test See haematin tartaric acid A crystalline natu- test. rally occurring carboxylic acid, tele-substitution A type of substi- (CHOH)2(COOH)2; r.d. 1.8; m.p. 171–174°C. It can be obtained from tution reaction in which the entering tartar (potassium hydrogen tartrate) group takes a position on an atom deposits from wine vats, and is used that is more than one atom away from the atom to which the leaving in baking powders and as a food- cine- stuffs additive. The compound is group is attached. See also substitution. optically active (see optical activity). The systematic name is 2,3- tellurides Binary compounds of tel- dihydroxybutanedioic acid. lurium with other more electroposi- 519 temperature tive elements. Compounds of tel- values 0 and 100 to them, respec- lurium with nonmetals are covalent tively, and divides the scale between (e.g. H2Te). Metal tellurides can be them into 100 degrees. This method made by direct combination of the el- is serviceable for many practical pur- ements and are ionic (containing poses (see temperature scales), but Te2–) or nonstoichiometric interstitial lacking a theoretical basis it is awk- compounds (e.g. Pd4Te, PdTe2). ward to use in many scientiÜc con- tellurium Symbol Te. A silvery met- texts. In the 19th century, Lord alloid element belonging to *group *Kelvin proposed a thermodynamic 16 (formerly VIB) of the periodic method to specify temperature, based on the measurement of the table; a.n. 52; r.a.m. 127.60; r.d. 6.24 Û (crystalline); m.p. 449.5°C; b.p. quantity of heat owing between 989.8°C. It occurs mainly as *tel- bodies at different temperatures. lurides in ores of gold, silver, copper, This concept relies on an absolute and nickel and it is obtained as a by- scale of temperature with an *ab- product in copper reÜning. There are solute zero of temperature, at which eight natural isotopes and nine radio- no body can give up heat. He also active isotopes. The element is used used Sadi Carnot’s concept of an ideal frictionless perfectly efÜcient in semiconductors and small carnot cycle amounts are added to certain steels. heat engine (see ). This Tellurium is also added in small Carnot engine takes in a quantity of quantities to lead. Its chemistry is heat q1 at a temperature T, and exhausts heat q2 at T2, so that T1/T2 = similar to that of sulphur. It was dis- Ü Ü covered by Franz Müller (1740–1825) q1/q2. If T2 has a value xed by de ni- in 1782. tion, a Carnot engine can be run be- A tween this Üxed temperature and any unknown temperature T , en- • Information from the WebElements site 1 abling T1 to be calculated by measur- temperature The property of a ing the values of q1 and q2. This body or region of space that deter- concept remains the basis for deÜn- mines whether or not there will be a ing thermodynamic temperature, net Ûow of heat into it or out of it quite independently of the nature of from a neighbouring body or region the working substance. The unit in and in which direction (if any) the which thermodynamic temperature heat will Ûow. If there is no heat Ûow is expressed is the *kelvin. In prac- the bodies or regions are said to be in tice thermodynamic temperatures thermodynamic equilibrium and at cannot be measured directly; they the same temperature. If there is a are usually inferred from measure- t Ûow of heat, the direction of the Ûow ments with a gas thermometer con- is from the body or region of higher taining a nearly ideal gas. This is temperature. Broadly, there are two possible because another aspect of methods of quantifying this prop- thermodynamic temperature is its re- erty. The empirical method is to take lationship to the *internal energy of two or more reproducible tempera- a given amount of substance. This ture-dependent events and assign can be shown most simply in the Üxed points on a scale of values to case of an ideal monatomic gas, in these events. For example, the Cel- which the internal energy per mole sius temperature scale uses the freez- (U) is equal to the total kinetic energy ing point and boiling point of water of translation of the atoms in one as the two Üxed points, assigns the mole of the gas (a monatomic gas has temperature-independent paramagnetism 520

no rotational or vibrational energy). Practical Temperature Scale (1968), According to *kinetic theory, the which is designed to conform as thermodynamic temperature of such closely as possible to thermodynamic a gas is given by T = 2U/3R, where R is temperature and is expressed in the the universal *gas constant. unit of thermodynamic temperature, Ü temperature-independent para- the *kelvin. The eleven xed points magnetism (TIP) Orbital paramag- of the scale are given in the table, Ü netism that is independent of with the instruments speci ed for in- temperature. In some substances TIP terpolating between them. Above the can make the molecules paramag- freezing point of gold, a radiation py- netic, although all the electrons in rometer is used, based on Planck’s the ground state are paired, if there law of radiation. The scale is ex- Ü are low-lying excited states to which pected to be re ned in the late 1980s. electrons can readily move. tempering The process of increas- temperature scales A number of ing the toughness of an alloy, such as empirical scales of *temperature steel, by heating it to a predeter- have been in use: the *Celsius scale mined temperature, maintaining it at is widely used for many purposes and this temperature for a predetermined in certain countries the *Fahrenheit time, and cooling it to room temper- scale is still used. These scales both ature at a predetermined rate. In rely on the use of Üxed points, such steel, the purpose of the process is to as the freezing point and the boiling heat the alloy to a temperature that point of water, and the division of will enable the excess carbide to pre- the fundamental interval between cipitate out of the supersaturated these two points into units of tem- solid solution of *martensite and perature (100 degrees in the case of then to cool the saturated solution the Celsius scale and 180 degrees in fast enough to prevent further pre- the Fahrenheit scale). cipitation or grain growth. For this However, for scientiÜc purposes reason steel is quenched rapidly by the scale in use is the International dipping into cold water.

T/K t/°C

triple point of equilibrium hydrogen 13.81 –259.34

temperature of equilibrium hydrogen when its vapour pressure is 25/76 standard atmosphere 17.042 –256.108 t b.p. of equilibrium hydrogen 20.28 –252.87 b.p. of neon 27.102 –246.048 triple point of oxygen 54.361 –218.789

b.p. of oxygen 90.188 –182.962 triple point of water 273.16 0.01 b.p. of water 373.15 100 f.p. of zinc 692.73 419.58 f.p. of silver 1235.08 961.93 f.p. of gold 1337.58 1064.43

Temperature scales 521 terpinene temporary hardness See hard- energy levels being called a multiplet. ness of water. The multiplicity of a multiplet is given by (2S + 1) since the number of tera- Symbol T. A preÜx used in the atomic energy levels a term splits metric system to denote one million into because of spin–orbit coupling is million times. For example, 1012 volts = 1 teravolt (TV). (2S + 1). This is the case because the total electronic angular momentum J terbium Symbol Tb. A silvery of an atom can have the (2S + 1) pos- metallic element belonging to the sible values: L + S, L + S –1, etc. Each *lanthanoids; a.n. 65; r.a.m. 158.92; level in a multiplet has the value of J r.d. 8.23 (20°C); m.p. 1356°C; b.p. written as a right subscript to the 3123°C. It occurs in apatite and xeno- term symbol. The degeneracy of a time, from which it is obtained by an level in a multiplet is 2J + 1. The ion-exchange process. There is only names given to multiplets with 2S +1 one natural isotope, terbium–159, = 1, 2, 3, 4 are singlet, doublet, which is stable. Seventeen artiÜcial triplet, quartet, respectively, to corre- isotopes have been identiÜed. It is spond to the number of atomic levels used as a dopant in semiconducting 3 in the multiplet. For example, P1 is devices. It was discovered by Carl the J = 1 level in a P triplet. Mosander (1797–1858) in 1843. A ternary compound A chemical compound containing three different • Information from the WebElements site elements. terephthalic acid (1,4-benzenedi- terpenes A group of unsaturated carboxylic acid) A colourless crys- hydrocarbons present in plants (see talline solid, C H (COOH) , m.p. 6 4 2 essential oil). Terpenes consist of 300°C. It is made by oxidizing p- isoprene units, CH :C(CH )CH:CH . xylene (1,4-dimethylebenzene) and 2 3 2 Monoterpenes have two units, used for making polyesters, such as C H , sesquiterpenes three units, Terylene. 10 16 C15H24, diterpenes four units, C20H32, term An electronic energy level in etc. Terpenoids, which are derivatives an atom. A term is characterized by a of terpenes, include abscisic acid and term symbol, which is given by a cap- gibberellin (plant growth substances) ital letter indicating the total orbital and the *carotenoid and *chloro- angular momentum L of the atom, phyll pigments. with a left superscript that gives the A value of 2S + 1, where S is the total • Information about IUPAC nomenclature spin angular momentum of the t atom. Analogously to the letters used terpinene A cyclic *terpene, for single electron angular momen- C10H16, used in perfumery and as a tum the letters S, P, D, F correspond Ûavouring. It exists in three isomeric to L = 0, 1, 2, 3 respectively. For ex- forms. α-Terpinene is an oily liquid ample, a term for which L = 1 and S = that smells of lemons, b.p. 182°C. It 1 has the term symbol 3P. occurs in the herbs cardamom, co- In the absence of *spin–orbit cou- riander and marjoram although it is pling the degeneracy of a term is usually extracted from the essential (2L + 1)(2S + 1). In the presence of oils of other plants. β-Terpinene, b.p. spin–orbit coupling a term splits up 174°C, is generally synthesized from into several closely spaced energy oil of savin. γ-Terpinene, b.p. 182°C, levels, with this set of closely spaced occurs in various plant oils, including tertiary alcohol 522

those of coriander, cumin, lemon, in cosmetics and skin-care prepara- and samphire. It can be made by the tions. The name comes from the action of an alcoholic sulphuric acid Latin name for nutmeg, Myristica fra- solution on pinene (the major com- grans. ponent of turpentine). tetraethyl lead See lead(iv) tertiary alcohol See alcohols. tetraethyl. tertiary amine See amines. tetragonal See crystal system. tervalent (trivalent) Having a va- tetrahedral angle The angle be- lency of three. tween the bonds in a *tetrahedral compound (approximately 109° for a Terylene Tradename for a type of regular tetrahedron). *polyester used in synthetic Übres. tetrahedral compound A com- tesla Symbol T. The SI unit of mag- pound in which four atoms or groups Û netic ux density equal to one weber situated at the corners of a tetrahe- Û of magnetic ux per square metre, dron are linked (by covalent or coor- –2 i.e. 1 T = 1 Wb m . It is named after dinate bonds) to an atom at the Nikola Tesla (1870–1943), Croatian- centre of the tetrahedron. See also born US electrical engineer. complex. tetrachloroethene A colourless tetrahedron A polyhedron with Û non ammable volatile liquid, four triangular faces. In a regular ° CCl2:CCl2; r.d. 1.6; m.p. –22 C; b.p. tetrahedron all four triangles are con- 121°C. It is used as a solvent. gruent equilateral triangles. It consti- tetrachloromethane (carbon tutes a regular triangular pyramid. tetrachloride) A colourless volatile tetrahydrate A crystalline hydrate liquid with a characteristic odour, containing four moles of water per virtually insoluble in water but misci- mole of compound. ble with many organic liquids, such as ethanol and benzene; r.d. 1.586; tetrahydrocannabinol (THC) The * m.p. –23°C; b.p. 76.54°C. It is made main cannabinoid found in the plant Cannabis sativa, and the com- by the chlorination of methane (pre- pound largely responsible for its psy- viously by chlorination of carbon choactive effects. There have been disulphide). The compound is a good reports that use of THC has been solvent for waxes, lacquers, and rub- linked to the onset of psychotic bers and the main industrial use is as episodes in both healthy users and, a solvent, but safer substances (e.g. t in particular, in schizophrenics. It 1,1,1-trichloroethane) are increasingly being used. Moist carbon tetra- CH chloride is partly decomposed to 3 phosgene and hydrogen chloride and this provides a further restriction on OH its use. H tetradecanoic acid (myristic acid) A saturated carboxylic acid CH3 H ° (CH2)12COOH; r.d. 0.86; m.p. 58.8 C; CH b.p. 250.5°C (100 mmHg). Its glyc- 3 O (CH ) CH CH 2 4 3 erides are found in nutmeg, palm oil, 3 and butter fat. Compounds are used Tetrahydrocannabinol 523 thermal equilibrium has been used as a possible treat- are easily reduced to the thallium(I) ment for a number of medical condi- state and are therefore strong oxidiz- tions. It is used illegally in the form ing agents. The element was discov- of *cannabis, which is a class C drug ered by Sir William *Crookes in in the UK. Tetrahydrocannabinol can 1861. be detected by the *Duquenois– A Levine test. • Information from the WebElements site tetrahydrofuran (THF) A colour- theobromine (3,7-dimethylxan- less volatile liquid, C H O; r.d. 0.89; 4 8 thine) An alkaloid, C H N O , struc- m.p. –65°C; b.p. 67°C. It is made by 7 8 4 2 turally related to caffeine. It is found the acid hydrolysis of polysaccha- in small quantities in tea and larger rides in oat husks, and is widely used as a solvent. quantities in the cacao tree, hence its presence in cocoa and chocolate. Like tetrahydroxomonoxodiboric(III) theophylline it is used in treating cer- acid See boric acid. tain respiratory conditions. Note that tetraoxophosphoric(V) acid See the compound contains no bromine. phosphoric(v) acid. The name comes from the name of the cacao tree, Theobroma cacao. See tetraoxosulphuric(VI) acid See also methylxanthines. sulphuric acid. theophylline (1,3-dimethylxan- tetravalent (quadrivalent) Having thine) An alkaloid, C H N O , a valency of four. 7 8 4 2 structurally related to caffein. It is texaphyrin A synthetic molecule found in small quantities in tea and similar to a porphyrin but containing is used medicinally for certain respi- Üve central nitrogen atoms rather ratory conditions. See also methylxan- than four, thus increasing the size of thines. the central ‘hole’ and enabling larger theory See laws, theories, and hy- cations, such as cadmium, to be potheses bound stably. See also supramolecu- . lar chemistry. thermal analysis A technique for thallium Symbol Tl. A greyish chemical analysis and the investiga- metallic element belonging to tion of the products formed by heat- *group 13 (formerly IIIB) of the peri- ing a substance. In differential odic table; a.n. 81; r.a.m. 204.39; r.d. thermal analysis (DTA) a sample is 11.85 (20°C); m.p. 303.5°C; b.p. heated, usually in an inert atmos- 1457±10°C. It occurs in zinc blende phere, and a plot of weight against t and some iron ores and is recovered temperature made. In differential in small quantities from lead and scanning calorimetry (DSC) heat is zinc concentrates. The naturally oc- added to or removed from a sample curring isotopes are thallium–203 electrically as the temperature is in- and thallium–205; eleven radioiso- creased, thus allowing the enthalpy topes have been identiÜed. It has few changes due to thermal decomposi- uses – experimental alloys for special tion to be studied. purposes and some minor uses in thermal capacity See heat capac- electronics. The sulphate has been ity. used as a rodenticide. Thallium(I) compounds resemble those of the al- thermal equilibrium See equilib- kali metals. Thallium(III) compounds rium. thermite 524

thermite A stoichiometric pow- known as the Ürst law of thermody- dered mixture of iron(III) oxide and namics. aluminium for the reaction: All natural processes conform to 2Al + Fe O → Al O + 2Fe this law, but not all processes con- 2 3 2 3 forming to it can occur in nature. The reaction is highly exothermic Most natural processes are irre- and the increase in temperature is versible, i.e. they will only proceed in Ü suf cient to melt the iron produced. one direction (see reversible It has been used for localized weld- process). The direction that a natural ing of steel objects (e.g. railway lines) process can take is the subject of the in the Thermit process. Thermite is second law of thermodynamics, also used in incendiary bombs. which can be stated in a variety of thermochemistry The branch of ways. R. Clausius (1822–88) stated the physical chemistry concerned with law in two ways: “heat cannot be heats of chemical reaction, heats of transferred from one body to a second body at a higher temperature formation of chemical compounds, without producing some other ef- etc. fect” and “the entropy of a closed sys- thermodynamics The study of the tem increases with time”. These laws that govern the conversion of statements introduce the thermody- energy from one form to another, namic concepts of *temperature (T) the direction in which heat will Ûow, and *entropy (S), both of which are and the availability of energy to do parameters determining the direc- work. It is based on the concept that tion in which an irreversible process in an isolated system anywhere in can go. The temperature of a body or the universe there is a measurable system determines whether heat will Û quantity of energy called the *inter- ow into it or out of it; its entropy is nal energy (U) of the system. This is a measure of the unavailability of its the total kinetic and potential energy energy to do work. Thus T and S de- of the atoms and molecules of the termine the relationship between Q Ü system of all kinds that can be trans- and W in the statement of the rst ferred directly as heat; it therefore law. This is usually presented by stating the second law in the form ∆U = excludes chemical and nuclear en- T∆S – W. ergy. The value of U can only be The second law is concerned with changed if the system ceases to be changes in entropy (∆S). The third isolated. In these circumstances U law of thermodynamics provides an t can change by the transfer of mass to absolute scale of values for entropy or from the system, the transfer of by stating that for changes involving heat (Q) to or from the system, or by only perfect crystalline solids at *ab- the work (W) being done on or by the solute zero, the change of the total system. For an adiabatic (Q = 0) sys- entropy is zero. This law enables ab- ∆ tem of constant mass, U = W. By solute values to be stated for en- convention, W is taken to be positive tropies. if work is done on the system and One other law is used in thermody- negative if work is done by the sys- namics. Because it is fundamental to, tem. For nonadiabatic systems of and assumed by, the other laws of constant mass, ∆U = Q + W. This thermodynamics it is usually known statement, which is equivalent to the as the zeroth law of thermodynamics. law of conservation of energy, is This states that if two bodies are each 525 thiazole in thermal equilibrium with a third ture by making use of some property body, then all three bodies are in of a substance that varies with tem- thermal equilibrium with each other. perature. For example, liquid-in-glass See also enthalpy; free energy. thermometers depend on the expan- A sion of a liquid, usually mercury or • A tutorial from the Division of Chemical alcohol coloured with dye. These con- Ü Education, Purdue University, Indiana sist of a liquid- lled glass bulb at- Ü • Thermodynamic properties from the tached to a partially lled capillary committee on Data for Science and tube. In the bimetallic thermometer Technology the unequal expansion of two dissimilar metals that have been bonded to- thermodynamic temperature gether into a narrow strip and coiled See temperature. is used to move a pointer round a thermoluminescence *Lumines- dial. The gas thermometer, which is cence produced in a solid when its more accurate than the liquid-in- temperature is raised. It arises when glass thermometer, measures the free electrons and holes, trapped in a variation in the pressure of a gas solid as a result of exposure to ioniz- kept at constant volume. The resis- ing radiation, unite and emit photons tance thermometer is based on the of light. The process is made use of change in resistance of conductors or in thermoluminescent dating, which semiconductors with temperature assumes that the number of elec- change. Platinum, nickel, and copper trons and holes trapped in a sample are the metals most commonly used of pottery is related to the length of in resistance thermometers. time that has elapsed since the pot- thermoplastic See plastics. Ü tery was red. By comparing the lu- plastics minescence produced by heating a thermosetting See . piece of pottery of unknown age thermostat A device that controls with the luminescence produced by the heating or cooling of a substance heating similar materials of known in order to maintain it at a constant age, a fairly accurate estimate of the temperature. It consists of a tempera- age of an object can be made. ture-sensing instrument connected to thermoluminescent dating See a switching device. When the tem- thermoluminescence. perature reaches a predetermined level the sensor switches the heating thermolysis (pyrolysis) The chemi- or cooling source on or off according cal decomposition of a substance by to a predetermined program. The t heat. It is an important process in sensing thermometer is often a chemical manufacture, such as the bimetallic strip that triggers a simple thermal *cracking of hydrocarbons electrical switch. Thermostats are in the petroleum industry. used for space-heating controls, in thermometer An instrument used water heaters and refrigerators, and for measuring the *temperature of a to maintain the environment of a sci- Ü substance. A number of techniques enti c experiment at a constant tem- and forms are used in thermometers perature. depending on such factors as the de- THF See tetrahydrofuran. gree of accuracy required and the thiamin(e) See vitamin b complex. range of temperatures to be measured, but they all measure tempera- thiazole A heterocyclic compound thienyl ring 526

containing a Üve-membered ring group). Thiols are analogues of alco- with sulphur and nitrogen hetero hols in which the oxygen atom is re- atoms, C3SNH3. A range of thiazole placed by a sulphur atom. They are dyes are manufactured containing named according to the parent hy- this ring system. drocarbon; e.g. ethane thiol (C2H5SH). A characteristic property is their S strong disagreeable odour. For example the odour of garlic is produced by ethane thiol. Unlike alcohols they are N acidic, reacting with alkalis and cer- Thiazole tain metals to form saltlike compounds. The older name, mercaptan, thiophene thienyl ring See . comes from their ability to react with thin-layer chromatography A (‘seize’) mercury. technique for the analysis of liquid thionyl chloride See sulphur mixtures using *chromatography. dichloride oxide. The stationary phase is a thin layer of an absorbing solid (e.g. alumina) pre- thionyl group The group =SO, as pared by spreading a slurry of the in *sulphur dichloride oxide. solid on a plate (usually glass) and thiophene A colourless liquid com- drying it in an oven. A spot of the ° ° pound, C4H4S; m.p. –38 C; b.p. 84 C. mixture to be analysed is placed near The compound is present in commer- one edge and the plate is stood up- cial benzene. The ring system is also right in a solvent. The solvent rises known as a thienyl ring. through the layer by capillary action carrying the components up the S plate at different rates (depending on the extent to which they are absorbed by the solid). After a given time, the plate is dried and the loca- Thiophene tion of spots noted. It is possible to identify constituents of the mixture thiosulphate A salt containing the 2– by the distance moved in a given ion S2O3 formally derived from time. The technique needs careful thiosulphuric acid. Thiosulphates control of the thickness of the layer readily decompose in acid solution to and of the temperature. See also rf give elemental sulphur and hydro- value – t . gensulphite (HSO3 ) ions. thiocyanate A salt or ester of thio- thiosulphuric acid An unstable

cyanic acid. acid, H2S2O3, formed by the reaction thiocyanic acid An unstable gas, of sulphur trioxide with hydrogen sulphuric acid HSCN. sulphide. See also . thio ethers See sulphides. thiourea A white crystalline solid, ° thiols (NH2)2CS; r.d. 1.4; m.p. 182 C. It is thiol group See . used as a Üxer in photography. thiols (mercaptans; thio alcohols) thixotropy See newtonian fluid. Organic compounds that contain the group –SH (called the thiol group, Thomson, Sir Joseph John mercapto group, or sulphydryl (1856–1940) British physicist, who be- 527 thymol came a professor at Cambridge Uni- threonine See amino acid. versity in 1884. He is best known for thulium Symbol Tm. A soft grey his work on cathode rays, which led metallic element belonging to the to his discovery of the electron in *lanthanoids; a.n. 69; r.a.m. 168.934; 1897. He went on to study the con- r.d. 9.321 (20°C); m.p. 1545°C; b.p. duction of electricity through gases, 1947°C. It occurs in apatite and xeno- and it is for this work that he was time. There is one natural isotope, awarded the Nobel Prize for physics thulium–169, and seventeen artiÜcial in 1906. His son, Sir George Paget isotopes have been produced. There Thomson (1892–1975), discovered are no uses for the element, which *electron diffraction, for which he was discovered by Per Cleve (1840– shared the 1937 Nobel Prize for 1905) in 1879. physics with Clinton J. Davisson A (1881–1958), who independently • Information from the WebElements site made the same discovery. kelvin, thymidine A nucleoside consisting Thomson, Sir William See of one thymine molecule linked to a lord . d-doxyribose sugar molecule. thoria See thorium. O thorium Symbol Th. A grey radioactive metallic element belonging to 3CH the *actinoids; a.n. 90; r.a.m. NH 232.038; r.d. 11.5–11.9 (17°C); m.p. ° ° 1740–1760 C; b.p. 4780–4800 C. It oc- N O curs in monazite sand in Brazil, India, and USA. The isotopes of thorium have mass numbers from 223 O to 234 inclusive; the most stable isotope, thorium–232, has a half-life of × 10 1.39 10 years. It has an oxidation CH OH state of (+4) and its chemistry resem- 2 bles that of the other actinoids. It can OH be used as a nuclear fuel for breeder Thymidine reactors as thorium–232 captures slow neutrons to breed uranium–233. thymine A *pyrimidine derivative Thorium dioxide (thoria, ThO2) is and one of the major component t used on gas mantles and in special bases of *nucleotides and the nucleic refractories. The element was discov- acid *DNA. ered by J. J. *Berzelius in 1829. A O • Information from the WebElements site 3CH thorium series See radioactive se- N ries. three-body problem See many- N O body problem . Thymine threnardite A mineral form of *sodium sulphate, Na2SO4. thymol A pungent-smelling colour- tie line 528

CH3 alloys (e.g. phosphor bronze, gun metal, solder, Babbitt metal, and pewter). Chemically it is reactive. It combines directly with chlorine and oxygen and displaces hydrogen from OH dilute acids. It also dissolves in alkalis to form *stannates. There are two se- C ries of compounds with tin in the +2 H 3CH CH3 and +4 oxidation states. Thymol A • Information from the WebElements

less crystalline compound, C10H14O; site ° m.p. 51 C. It occurs in various essen- tin(II) chloride A white solid, tial oils, particularly oil of thyme, SnCl , soluble in water and ethanol. and can be made by using iron(III) 2 It exists in the anhydrous form chloride to oxidize piperitone (itself (rhombic; r.d. 3.95; m.p. 246°C; extracted from eucalyptus oil). Its b.p. 652°C) and as a dihydrate, antiseptic properties are exploited in gargles and mouthwashes. SnCl2.2H2O (monoclinic; r.d. 2.71; m.p. 37.7°C). The compound is made tie line A horizontal line in a liq- by dissolving metallic tin in hy- uid–vapour phase diagram that is drochloric acid and is partially hy- perpendicular to an *isopleth. The drolysed in solution. starting point is the Ürst point at 2+ ˆ + + which the liquid can coexist with its Sn + H2O SnOH + H vapour as the pressure is reduced; Excess acid must be present to pre- the end point is the point beyond vent the precipitation of basic salts. which only vapour is present. In the presence of additional chloride – time-of-Ûight mass spectrom- ions the pyramidal ion [SnCl3] is eter See mass spectroscopy. formed; in the gas phase the SnCl2 molecule is bent. It is a reducing tin Symbol Sn. A silvery malleable agent in acid solutions and oxidizes metallic element belonging to slowly in air: *group 14 (formerly IVB) of the peri- 2+ → 4+ odic table; a.n. 50; r.a.m. 118.69; r.d. Sn Sn + 2e ° ° 7.28; m.p. 231.88 C; b.p. 2260 C. It is tin(IV) chloride A colourless fum- found as tin(IV) oxide in ores, such as ing liquid, SnCl , hydrolysed in cold t cassiterite, and is extracted by reduc- 4 water, decomposed by hot water, and tion with carbon. The metal (called soluble in ethers; r.d. 2.226; m.p. white tin) has a powdery nonmetallic –33°C; b.p. 114°C. Tin(IV) chloride is allotrope grey tin, into which it changes below 18°C. The formation a covalent compound, which may be of this allotrope is called tin plague; prepared directly from the elements. it can be reversed by heating to It dissolves sulphur, phosphorus, 100°C. The natural element has 21 bromine, and iodine, and there is evi- isotopes (the largest number of any dence for the presence of species element); Üve radioactive isotopes are such as SnCl2I2. In hydrochloric acid also known. The metal is used as a and in chloride solutions the coordi- thin protective coating for steel plate nation is extended from four to six 2– and is a constituent of a number of by the formation of the SnCl6 ion. 529 titanium(IV) chloride tincture A solution with alcohol as with a soluble tin(IV) salt or by the the solvent (e.g. tincture of iodine). action of heat on thiostannic acid, tin(IV) hydride (stannane) A H2SnS3. The golden-yellow form used highly reactive and volatile gas (b.p. for producing a gilded effect on wood ° is prepared by heating tin, sulphur, –53 ), SnH4, which decomposes on moderate heating (150°C). It is pre- and ammonium chloride. pared by the reduction of tin chlo- TIP See temperature-independent rides using lithium tetrahydrido- paramagnetism. aluminate(III) and is used in the titanium(iv) oxide synthesis of some organo-tin com- titania See . pounds. The compound has reducing titanic chloride See titanium(iv) properties. chloride. tin(IV) oxide (tin dioxide) A white titanic tetrachloride See tita- solid, SnO2, insoluble in water; tetra- nium(iv) chloride. hedral; r.d. 6.95; m.p. 1127°C; sublimes between 1800°C and 1900°C. titanium Symbol Ti. A white metal- Tin(IV) oxide is trimorphic: the com- lic *transition element; a.n. 22; r.a.m. mon form, which occurs naturally as 47.9; r.d. 4.5; m.p. 1660±10°C; b.p. the ore *cassiterite, has a rutile lat- 3287°C. The main sources are rutile tice but hexagonal and rhombic (TiO2) and, to a lesser extent, il- forms are also known. There are also menite (FeTiO3). The element also oc- two so-called dihydrates, SnO2.2H2O, curs in numerous other minerals. It known as α- and β-stannic acid. is obtained by heating the oxide with These are essentially tin hydroxides. carbon and chlorine to give TiCl4, Tin(IV) oxide is amphoteric, dissolv- which is reduced by the *Kroll ing in molten alkalis to form *stan- process. The main use is in a large nates; in the presence of sulphur, number of strong light corrosion- thiostannates are produced. resistant alloys for aircraft, ships, tin plague See tin. chemical plant, etc. The element forms a passive oxide coating in air. tin(II) sulphide A grey-black cubic At higher temperatures it reacts with or monoclinic solid, SnS, virtually in- oxygen, nitrogen, chlorine, and other soluble in water; r.d. 5.22; m.p. nonmetals. It dissolves in dilute ° ° 882 C; b.p. 1230 C. It has a layer acids. The main compounds are tita- structure similar to that of black nium(IV) salts and complexes; tita- phosphorus. Its heat of formation is nium(II) and titanium(III) compounds low and it can be made by heating t ° are also known. The element was the elements together. Above 265 C Ürst discovered by William Gregor it slowly decomposes (disproportion- (1761–1817) in 1789. ates) to tin(IV) sulphide and tin metal. The compound reacts with hy- A drochloric acid to give tin(II) chloride • Information from the WebElements site and hydrogen sulphide. titanium(IV) chloride (titanic chlo- tin(IV) sulphide (mosaic gold) A ride; titanium tetrachloride) A bronze or golden yellow crystalline colourless volatile liquid, TiCl4, made compound, SnS2, insoluble in water by strongly heating titanium(IV) and in ethanol; hexagonal; r.d. 4.5; oxide and carbon in a stream of dry decomposes at 600°C. It is prepared chorine gas. It fumes in moist air to by the reaction of hydrogen sulphide produce titanium oxychlorides. It is titanium dioxide 530

used to produce other titanium com- prized), and great hardness (8 on the pounds and pure titanium metal. Mohs’ scale). When heated, yellow or brownish topaz often becomes a titanium dioxide See titanium(iv) rose-pink colour. The main sources of oxide. topaz are Brazil, Russia, and the USA. titanium(IV) oxide (titania; tita- torr A unit of pressure, used in nium dioxide) A white oxide, TiO , 2 high-vacuum technology, deÜned as occurring naturally in various forms, 1 mmHg. 1 torr is equal to 133.322 particularly the mineral rutile. It is pascals. The unit is named after used as a white pigment and as a Evangelista Torricelli (1609–47). Üller for plastics, rubber, etc. torsion angle In a nonlinear chain titration A method of volumetric of atoms A–B–C–D, the angle be- analysis in which a volume of one tween the plane containing atoms reagent (the titrant) is added to a ABC and the plane containing BCD. known volume of another reagent The torsion angle can have any value slowly from a burette until an end from 0° to 180°. If the chain is indicator point is reached (see ). The viewed along the line BC, the torsion volume added before the end point is angle is positive if the bond AB reached is noted. If one of the solu- would have to be rotated in a clock- tions has a known concentration, wise sense (less than 180°) to eclipse that of the other can be calculated. (i.e. align with) the bond CD. If the TNT See trinitrotoluene. rotation of AB has to be in an anticlockwise sense, the torsion angle vitamin e tocopherol See . would be negative. Tollens reagent A reagent used in There is a terminology relating to testing for aldehydes. It is made by the steric arrangements of atoms adding sodium hydroxide to silver ni- based on the size of the torsion angle conformation trate to give silver(I) oxide, which is (see also ). The syn dissolved in aqueous ammonia (giv- arrangement is one in which the size + of the torsion angle is ±90°. The anti ing the complex ion [Ag(NH3)2] ). The sample is warmed with the reagent arrangement is one with a torsion in a test tube. Aldehydes reduce the synperiplanar complex Ag+ ion to metallic silver, forming a bright silver mirror on the 0° –30° +30° inside of the tube (hence the name silver-mirror test). Ketones give a neg- sp sp t ative result. It is named after Bern- –synclinal +synclinal hard Tollens (1841–1918). toluene See methylbenzene. –sc +sc –90° +90° topaz A variably coloured aluminium silicate mineral, –anticlinal +anticlinal Al2(SiO4)(OH,F)2, that forms orthorhombic crystals. It occurs chieÛy –ac +ac in acid igneous rocks, such as gran- ap ites and pegmatites. Topaz is valued –150° +150° as a gemstone because of its trans- antiperiplanar parency, variety of colours (the wine- yellow variety being most highly Torsion angle 531 transactinide elements angle between ±90° and 180°. An- trans See isomerism; torsion other distinction is between clinal angle. arrangements (value between 30° transactinide elements Elements and 150° or –30° and –150°) and peri- with an atomic number greater than planar arrangements (value between 103, i.e. elements above lawrencium 0 and ±30° or ±150° and 180°). Com- in the *periodic table. So far, el- bining the two gives four ranges of ements up to 116 have been de- torsion angle in an arrangement: tected. Because of the highly synperiplanar (sp) 0° to ±30°; radioactive and transient nature of synclinal (sc) 30° to 90° or –30° to these elements, there has been much –90°; dispute about priority of discovery anticlinal (ac) 90° to 150° or –90° to and, consequently, naming of the el- –150°; ements. The International Union of antiperiplanar (ap) ±150° to 180°. Pure and Applied Chemistry (IUPAC) The antiperiplanar conformation is introduced a set of systematic tempo- trans also called a conformation; the rary names based on afÜxes, as synperiplanar conformation corre- shown in the table. sponds to a cis conformation. The synclinal conformation is also called a gauche or skew conformation. Afﬁx Number Symbol total-radiation pyrometer See pyrometry. nil 0 n un 1 u tourmaline A group of minerals bi 2 b composed of complex cyclosilicates tri 3 t containing boron with the general quad 4 q 2+ pent 5 p formula NaR3 Al6B3Si6O27(H,F)4, 2+ hex 6 h where R = Fe , Mg, or (Al + Li). The sept 7 s crystals are trigonal, elongated, and oct 8 o variably coloured, the two ends of enn 9 e the crystals often having different colours. Tourmaline is used as a gemstone and because of its double re- All these element names end in fraction and piezoelectric properties -ium. So, for example, element 109 in is also used in polarizers and some this system is called un + nil + enn + pressure gauges. ium, i.e. unnilennium, and given the symbol u+n+e, i.e. Une. town gas Any manufactured gas One long-standing dispute was t supplied as a fuel gas to domestic about the element 104 (ruther- and industrial users. Examples in- fordium), which has also been called clude *coal gas, substitute natural kurchatovium (Ku). There have also gas (*SNG), and *natural gas itself. Most town gases are based on hydro- been disputes between IUPAC and gen or methane, although coal gas the American Chemical Union about also contains up to 8% carbon element names. monoxide. In 1994 IUPAC suggested the following list: essential el- trace element See mendelevium (Md, 101) ement . nobelium (No, 102) tracing (radioactive tracing) See la- lawrencium (Lr, 103) belling. dubnium (Db, 104) transamination 532

joliotium (Jl, 105) direct the incoming ligand into the rutherfordium (Rf, 106) trans position. The order of ligands bohrium (Bh, 107) in decreasing trans-directing power hahnium (Hn, 108) is: meitnerium (Mt, 109) – > > – > – > – > > CN NO2 I Br Cl NH3 The ACU favoured a different set of H2O. names: mendelevium (Md, 101) transfer coefÜcient Symbol α. A nobelium (No, 102) quantity used in the analysis of lawrencium (Lr, 103) processes in electrochemistry; it is rutherfordium (Rf, 104) the fraction of the potential differ- hahnium (Ha, 105) ence at the surface of an electrode seaborgium (Sg, 106) that assists charge difference in one nielsbohrium (Ns, 107) direction but discourages it in the hassium (Hs, 108) other direction. Its value lies between meitnerium (Mt, 109) 0 and 1, frequently being about ½. It A compromise list was adopted by is related to the slope of a graph of IUPAC in 1997 and is generally ac- the logarithm of the current against cepted: the potential. mendelevium (Md, 101) transformation The conversion of nobelium (No, 102) a compound into a particular prod- lawrencium (Lr, 103) uct, irrespective of the reagents or rutherfordium (Rf, 104) mechanism involved. dubnium (Db, 105) seaborgium (Sg, 106) transient species A short-lived in- bohrium (Bh, 107) termediate in a chemical reaction. hassium (Hs, 108) transition A change of a system meitnerium (Mt, 109) from one quantum state to another. Element 110 was named as darmstadtium in 2003 and element 111 transition elements A set of el- was named roentgenium in 2004. So ements in the *periodic table in far elements 112 (ununbium, Uub), which Ülling of electrons in an inner 113 (ununtrium, Uut), 114 (ununqua- d- or f-level occurs. With increasing dium, Uuq), 115 (ununpentium, proton number, electrons Üll Uup), and 116 (ununhexium, Uuh) atomic levels up to argon, which are not ofÜcially named. All these el- has the electron conÜguration ements are unstable and have very 1s22s22p63s23p6. In this shell, there t short half-lives. are 5 d-orbitals, which can each contain 2 electrons. However, at this transamination A biochemical re- point the subshell of lowest energy is action in amino acid metabolism in not the 3d but the 4s. The next two which an amine group is transferred elements, potassium and calcium, from an amino acid to a keto acid to have the conÜgurations [Ar]4s1 and form a new amino acid and keto [Ar]4s2 respectively. For the next el- acid. The coenzyme required for this ement, scandium, the 3d level is of reaction is pyridoxal phosphate. lower energy than the 4p level, and trans effect An effect in the substi- scandium has the conÜguration tution of inorganic square-planar [Ar]3d14s2. This Ülling of the inner d- complexes, in which certain ligands level continues up to zinc [Ar]3d104s2, in the original complex are able to giving the Ürst transition series. 533 transition state

There is a further series of this type erties that result from the presence in the next period of the table: be- of unÜlled d-orbitals in the element tween yttrium ([Kr]4d5s2) and cad- or (in the case of copper, silver, and mium ([Kr]4d105s2). This is the second gold) in the ions. The elements from transition series. In the next period 104 to 109 and the undiscovered el- of the table the situation is rather ements 110 and 111 make up a more complicated. Lanthanum has fourth transition series. The el- the conÜguration [Xe]5d16s2. The ements zinc, cadmium, and mercury level of lowest energy then becomes have Ülled d-orbitals both in the el- the 4f level and the next element, ements and in compounds, and are cerium, has the conÜguration usually regarded as nontransition el- [Xe]4f15d16s2. There are 7 of these f- ements forming group 12 of the peri- orbitals, each of which can contain odic table. 2 electrons, and Ülling of the f- The elements of the three main levels continues up to lutetium transition series are all typical metals ([Xe]4f145d16s2). Then the Ülling of the (in the nonchemical sense), i.e. most 5d levels continues from hafnium to are strong hard materials that are mercury. The series of 14 elements good conductors of heat and elec- from cerium to lutetium is a ‘series tricity and have high melting and within a series’ called an inner transi- boiling points. Chemically, their be- tion series. This one is the *lan- haviour depends on the existence of thanoid series. In the next period unÜlled d-orbitals. They exhibit vari- there is a similar inner transition se- able valency, have coloured com- ries, the *actinoid series, from tho- pounds, and form *coordination rium to lawrencium. Then Ülling of compounds. Many of their com- the d-level continues from element pounds are paramagnetic as a result 104 onwards. of the presence of unpaired elec- In fact, the classiÜcation of chemi- trons. Many of them are good cata- cal elements is valuable only in so far lysts. They are less reactive than the as it illustrates chemical behaviour, s- and p-block metals. and it is conventional to use the term transition point (transition tem- ‘transition elements’ in a more re- perature) 1. The temperature at stricted sense. The elements in the which one crystalline form of a sub- inner transition series from cerium stance changes to another form. (58) to lutetium (71) are called the 2. The temperature at which a sub- lanthanoids; those in the series from stance changes phase. 3. The tem- thorium (90) to lawrencium (103) are perature at which a substance the actinoids. These two series to- t becomes superconducting. 4. The gether make up the f-block in the pe- temperature at which some other riodic table. It is also common to change, such as a change of magnetic include scandium, yttrium, and lan- properties, takes place. thanum with the lanthanoids (because of chemical similarity) and to transition state (activated com- include actinium with the actinoids. plex) The association of atoms of Of the remaining transition el- highest energy formed during a ements, it is usual to speak of three chemical reaction. The transition main transition series: from titanium state can be regarded as a short-lived to copper; from zirconium to silver; intermediate that breaks down to and from hafnium to gold. All these give the products. For example, in a elements have similar chemical prop- SN2 substitution reaction, one atom transmission electron microscope 534

or group approaches the molecule as organic peroxide, C9H18O6, derived the other leaves. The transition state from acetone; r.d. 1.18; m.p. 91°C. It is an intermediate state in which is a highly explosive white crystalline both attacking and leaving groups substance, very sensitive to heat and are partly bound to the molecule, e.g. shock. It is favoured by some terror- B + RA → B—-R—-A → BR + A ist groups because it can easily be made from commonly available com- In the theory of reaction rates, the pounds. For example, it is probable reactants are assumed to be in equi- that it was used in the London bomb- librium with this activated complex, ings of 7 July 2005. which decomposes to the products. CH CH transmission electron micro- 3 3 scope See electron microscope. O transmutation The transforma- O O CH3 tion of one element into another by CH O O 3 bombardment of nuclei with parti- O cles. For example, plutonium is obtained by the neutron bombardment CH CH of uranium. 3 3 Triacetone triperoxide transport coefÜcients Quantities that characterize transport in a system. Examples of transport coefÜ- triacylglycerol See triglyceride. cients include electrical and thermal conductivity. One of the main pur- triatomic molecule A molecule poses of *nonequilibrium statistical formed from three atoms (e.g. H2O or mechanics is to calculate such coefÜ- CO2). Ü cients from rst principles. It is triazine See azine. difÜcult to calculate transport coefÜ- cients exactly for noninteracting systems and it is therefore necessary to use approximation techniques. A transport coefÜcient gives a measure for Ûow in a system. An inverse transport coefÜcient gives a measure Triazine of resistance to Ûow in a system. tribology The study of friction, lu- transport number Symbol t. The brication, and lubricants. t fraction of the total charge carried by a particular type of ion in the con- triboluminescence *Lumines- duction of electricity through elec- cence caused by friction; for exam- trolytes. ple, some crystalline substances emit light when they are crushed as a re- transuranic elements Elements sult of static electric charges gener- with an atomic number greater than ated by the friction. 92, i.e. elements above uranium in the *periodic table. Most of these el- tribromomethane (bromoform) A ements are unstable and have short colourless liquid *haloform, CHBr3; half-lives. See also transactinide el- r.d. 2.9; m.p. 8°C; b.p. 150°C. ements . tricarbon dioxide (carbon subox- triacetone triperoxide (TATP) An ide) A colourless gas, C3O2, with an 535 trimethylaluminium unpleasant odour; r.d. 1.114 (liquid at triclinic See crystal system. 0°C); m.p. –111.3°C; b.p. 7°C. It is the tridymite A mineral form of *sili- acid anhydride of malonic acid, from con(IV) oxide, SiO . which it can be prepared by dehydra- 2 tion using phosphorus(V) oxide. The triethanolamine See ethanol- molecule is linear (O:C:C:C:O). amine. tricarboxylic acid cycle See krebs triglyceride (triacylglycerol) An cycle. ester of glycerol (propane-1,2,3-triol) trichloroethanal (chloral) A liquid in which all three hydroxyl groups are esteriÜed with a fatty acid. aldehyde, CCl3CHO; r.d. 1.51; m.p. –57.5°C; b.p. 97.8°C. It is made by Triglycerides are the major con- chlorinating ethanal and used in stituent of fats and oils and provide a making DDT. See also 2,2,2- concentrated food energy store in liv- trichloroethanediol. ing organisms as well as cooking fats and oils, margarines, etc. Their physi- 2,2,2-trichloroethanediol (chloral cal and chemical properties depend hydrate) A colourless crystalline on the nature of their constituent solid, CCl3CH(OH)2; r.d. 1.91; m.p. fatty acids. In simple triglycerides all ° ° 57 C; b.p. 96.3 C. It is made by the three fatty acids are identical; in hydrolysis of trichloroethanal and is mixed triglycerides two or three dif- unusual in having two –OH groups ferent fatty acids are present. on the same carbon atom. Gem diols of this type are usually unstable; in trigonal bipyramid See illustra- this case the compound is stabilized tion at complex. by the presence of the three Cl trihydrate A crystalline hydrate atoms. It is used as a sedative. that contains three moles of water trichloroethene (trichlorethylene) per mole of compound. A colourless liquid, CCl2=CHCl, b.p. trihydric alcohol See triol. 87°C. It is toxic and nonÛammable, with a smell resembling that of chlo- 3,4,5-trihydroxybenzoic acid See roform (trichloromethane). It is gallic acid. widely used as a solvent in dry clean- triiodomethane (iodoform) A yel- ing and degreasing. It is also used to low volatile solid sweet-smelling extract oils from nuts and fruit, as *haloform, CHI ; r.d. 4.1; m.p. 115°C. Ü 3 an anaesthetic, and as a re extin- It is made by the haloform reaction. guisher. triiron tetroxide (ferrosoferric t trichloromethane (chloroform) A oxide) A black magnetic oxide, colourless volatile sweet-smelling liq- Fe3O4; r.d. 5.2. It is formed when iron uid *haloform, CHCl3; r.d. 1.48; m.p. ° ° is heated in steam and also occurs –63.5 C; b.p. 61.7 C. It can be made naturally as the mineral *magnetite. by chlorination of methane (followed The oxide dissolves in acids to give a by separation of the mixture of prod- mixture of iron(II) and iron(III) salts. ucts) or by the haloform reaction. It is an effective anaesthetic but can trimethylaluminium (aluminium cause liver damage and it has now trimethyl) A colourless liquid, been replaced by other halogenated Al(CH3)3, which ignites in air and re- hydrocarbons. Chloroform is used as acts with water to give aluminium a solvent and raw material for mak- hydroxide and methane, usually with ing other compounds. extreme vigour; r.d. 0.752; m.p. 0°C; 2,4,6-trinitrophenol 536

b.p. 130°C. Like other aluminium triphenylmethyl An unusual aro- alkyls it may be prepared by reacting matic compound that can exist as a Grignard reagent with aluminium an anion, cation or free radical, trichloride. Aluminium alkyls are (C6H5)3C. Treatment of triphenyl- used in the *Ziegler process for the carbinol in alcoholic solution with manufacture of high-density poly- mineral acids produces triphenyl- ethene (polythene). methyl salts, containing the cation (C H ) C+, which crystallize as or- 2,4,6-trinitrophenol See picric 6 5 3 ange-red solids. The action of sodium acid. on a solution of triphenylmethyl trinitrotoluene (TNT) A yellow chloride in ether produces the yellow – highly explosive crystalline solid, (C6H5)3C anion. The free radical ° CH3C6H2(NO2)3; r.d. 1.65; m.p. 82 C. It form is made by treating triphenyl- is made by nitrating toluene (methyl- methyl chloride with mercury or benzene), the systematic name being zinc in the absence of oxygen. 1-methyl-2,4,6-trinitrobenzene. triple bond See chemical bond. triol (trihydric alcohol) An *alcohol triple point The temperature and containing three hydroxyl groups per pressure at which the vapour, liquid, molecule. and solid phases of a substance are in triose A sugar molecule that con- equilibrium. For water the triple tains three carbon atoms. See mono- point occurs at 273.16 K and 611.2 Pa. saccharide. This value forms the basis of the deÜnition of the *kelvin and the trioxoboric(III) acid See boric thermodynamic *temperature scale. acid. triplet An atomic state in which trioxosulphuric(IV) acid See sul- two spin angular momenta of elec- phurous acid. trons combine together to give a trioxygen See ozone. total nonzero spin. A triplet state

ice line

solid triple point liquid

pressure/Pa t

611.2

273.16 temperature/K hoarfrost steam vapour line line Triple point 537 tungsten usually has a lower energy than a formed by ionization of a tritium *singlet state because of the effect of atom. See also hydron. correlations of spin on the Coulomb trivalent (tervalent) Having a va- interactions between electrons (as in lency of three. *Hund’s rules). This can lead to substantial energy differences between trona A mineral form of sodium triplet and singlet states. sesquicarbonate, consisting of a silane mixed hydrated sodium carbonate trisilane See . and sodium hydrogencarbonate, trisodium phosphate(V) (sodium Na2CO3.NaHCO3.2H2O. orthophosphate) A colourless crys- tropylium ion The positive ion talline compound, Na3PO4, soluble in + C7H7 , having a ring of seven carbon water and insoluble in ethanol. It is atoms. The ion is symmetrical and known both as the decahydrate (oc- has characteristic properties of *aro- tagonal; r.d. 2.54) and the dodeca- matic compounds. hydrate (trigonal; r.d. 1.62) The dodecahydrate loses water at about trypsin An enzyme that digests pro- 76°C and the decahydrate melts at teins (see protease). It is secreted in 100°C. Trisodium phosphate may be an inactive form (trypsinogen) by the prepared by boiling sodium carbon- pancreas into the duodenum. There, ate with the stoichiometric amount trypsinogen is acted on by an en- of phosphoric acid and subsequently zyme (enterokinase) produced in the adding sodium hydroxide to the dis- duodenum to yield trypsin. The ac- odium salt thus formed. It is useful as tive enzyme plays an important role an additive for high-pressure boiler in the digestion of proteins in the an- feed water (for removal of calcium terior portion of the small intestine. and magnesium as phosphates), in It also activates other proteases in emulsiÜers, as a water-softening the pancreatic juice. agent, and as a component in deter- trypsinogen See trypsin. gents and cleaning agents. Sodium phosphate labelled with the radio- tryptamine A naturally occurring active isotope 32P is used in the study alkaloid, C10H12N2, having an indole of the role of phosphate in biological ring system with a –CH2–CH2–NH2 side chain in the 3-position of the processes and is also used (intra- nitrogen-containing ring. Deriva- venously) in the treatment of poly- tives of tryptamines have hallucino- cythaemia. genic effects. An example in *psilo- tritiated compound See cybin. t labelling. tryptophan See amino acid. tritium Symbol T. An isotope of hy- tub See ring conformations. drogen with mass number 3; i.e. the nucleus contains 2 neutrons and 1 tuneable laser See dye laser. proton. It is radioactive (half-life 12.3 tungsten Symbol W. A white or years), undergoing beta decay to grey metallic *transition element helium–3. Tritium is used in *la- (formerly called wolfram); a.n. 74; belling. r.a.m. 183.85; r.d. 19.3; m.p. 3410°C; triton A nucleus of a tritium atom, b.p. 5660°C. It is found in a number consisting of a proton and two neu- of ores, including the oxides wol- + trons bound together; the ion T framite, (Fe,Mn)WO4, and scheelite, tungsten carbide 538

CaWO4. The ore is heated with con- being the most sought after. It usu- centrated sodium hydroxide solution ally occurs in veinlets and as masses to form a soluble tungstate. The and is formed by the action of sur- oxide WO3 is precipitated from this face waters on aluminium-rich rocks. by adding acid, and is reduced to the The Ünest specimens are obtained metal using hydrogen. It is used in from Iran. various alloys, especially high-speed twinning The growth of crystals in steels (for cutting tools) and in lamp Ü such a way that two distinct crystals laments. Tungsten forms a protec- form sharing a common plane of tive oxide in air and can be oxidized atoms. at high temperature. It does not dissolve in dilute acids. It forms com- twist See ring conformations. pounds in which the oxidation state Tyndall effect The scattering of ranges from +2 to +6. The metal was light as it passes through a medium Ü rst isolated by Juan d’Elhuyer and containing small particles. If a poly- Fausto d’Elhuyer (1755–1833) in chromatic beam of light is passed 1783. through a medium containing parti- A cles with diameters less than about • Information from the WebElements site one-twentieth of the wavlength of light, the scattered light appears tungsten carbide A black powder, blue. This accounts for the blue ap- WC, made by heating powdered pearance of tobacco smoke. At tungsten metal with lamp black at higher particle diameters, the scat- 1600°C. It is extremely hard (9.5 on tered light remains polychromatic. Mohs’ scale) and is used in dies and The effect is seen in suspensions and cutting tools. A ditungsten carbide, certain colloids. It is named after W C, also exists. 2 John Tyndall (1820–93). tunnel effect An effect in which type A and B metals A classiÜca- electrons are able to tunnel through tion of metal ions according to the a narrow potential barrier that would stability of their complexes for a constitute a forbidden region if the given ligand. Type A metals cations electrons were treated as classical include the ions of group 1 (Li+ to particles. That there is a Ünite proba- Cs+), the ions of group 2 (Be2+ to bility of an electron tunnelling from Ba2+), and ions of lighter transition one classically allowed region to an- metals in high oxidation states (e.g. other arises as a consequence of Co3+, Ti4+, Fe3+). The type B metal *quantum mechanics. The effect is cations are those of heavier transi- t made use of in the tunnel diode. tion metals in lower oxidation states turpentine An oily liquid extracted (e.g. Ag+, Cu+, Ni2+, Pd2+, Pt2+). Certain from pine resin. It contains pinene, ligands tend to form more stable C10H16, and other terpenes and is complexes with type A metals; others mainly used as a solvent. form more stable complexes with type B. For example, the tendency of turquoise A mineral consisting of a halide anions to complex with type A hydrated phosphate of aluminium metals is in the sequence and copper, CuAl6(PO4)4(OH)8.4H2O), that is prized as a semiprecious F– > Cl– > Br– > I–. stone. It crystallizes in the triclinic Their tendency to complex with type system and is generally blue in col- B metals is the opposite sequence. our, the ‘robin’s egg’ blue variety This led to a classiÜcation of ligands 539 tyrosine into type A ligands (e.g. F–) which tend type A metal See type a and b to complex with type A metals and metals. type B ligands (e.g. I–), which tend to type B metal See type a and b complex with type B metals. The metals. classiÜcation was introduced by Ahr- land in 1958. See also hsab principle. tyrosine See amino acid.

t U

ubiquinone Any of a group of re- techniques are also used industrially lated quinone-derived compounds to test for Ûaws in metals, to clean that serve as electron carriers in the surfaces, to test the thickness of *electron transport chain reactions parts, and to form colloids. of cellular respiration. Coenzyme Q ultraviolet radiation (UV) Electro- molecules have side chains of differ- magnetic radiation having wave- ent lengths in different types of or- lengths between that of violet light ganisms but function in similar ways. and long X-rays, i.e. between 400 ultracentrifuge A high-speed cen- nanometres and 4 nm. In the range trifuge used to measure the rate of 400–300 nm the radiation is known sedimentation of colloidal particles as the near ultraviolet. In the range or to separate macromolecules, such 300–200 nm it is known as the far ul- as proteins or nucleic acids, from so- traviolet. Below 200 nm it is known lutions. Ultracentrifuges are electri- as the extreme ultraviolet or the vac- cally driven and are capable of uum ultraviolet, as absorption by the speeds up to 60 000 rpm. oxygen in the air makes the use of ultrahigh frequency (UHF) A evacuated apparatus essential. The radio frequency in the range 3 × sun is a strong emitter of UV radia- 109– 0.3 × 109 Hz; i.e. having a wave- tion but only the near UV reaches length in the range 10 cm to 1 m. the surface of the earth as the *ozone layer of the atmosphere ab- ultramicroscope A form of micro- sorbs all wavelengths below 290 nm. scope that uses the Tyndall effect to Ultraviolet radiation is classiÜed in reveal the presence of particles that three ranges according to its effect cannot be seen with a normal optical on the skin. The ranges are: microscope. Colloidal particles, UV-A (320–400 nm); smoke particles, etc., are suspended UV-B (290–320 nm); in a liquid or gas in a cell with a UV-C (230–290 nm). black background and illuminated by The longest-wavelength range, UV-A, an intense cone of light that enters is not harmful in normal doses and is the cell from the side and has its used clinically in the treatment of Ü apex in the eld of view. The parti- certain skin complaints, such as pso- cles then produce diffraction-ring sys- riasis. It is also used to induce *vita- tems, appearing as bright specks on min D formation in patients that are the dark background. allergic to vitamin D preparations. ultrasonics The study and use of UV-B causes reddening of the skin pressure waves that have a frequency followed by pigmentation (tanning). in excess of 20 000 Hz and are there- Excessive exposure can cause severe fore inaudible to the human ear. blistering. UV-C, with the shortest Ultrasonics are used in medical diag- wavelengths, is particularly dam- nosis, particularly in conditions such aging. It is thought that short-wave- as pregnancy, in which X-rays could length ultraviolet radiation causes have a harmful effect. Ultrasonic skin cancer and that the risk of con- 541 universal indicator tracting this has been increased by classical relationship between cause the depletion of the ozone layer. and effect at the atomic level. Most UV radiation for practical use undetermined multipliers See la- is produced by various types of mer- grange multipliers. cury-vapour lamps. Ordinary glass absorbs UV radiation and therefore UNFO Urea nitrate–fuel oil. An ex- lenses and prisms for use in the UV plosive based in the fertilizer urea ni- are made from quartz. trate mixed with fuel oil, similar in its action to ammonium nitrate–fuel ultraviolet–visible spectroscopy oil (ANFO). It has been used in a (UV–visible spectroscopy) A tech- number of terrorist attacks, most no- nique for chemical analysis and the tably a car bombing in the basement determination of structure. It is of the World Trade Centre, New based on the principle that electronic York, on 26 February 1993. transitions in molecules occur in the visible and ultraviolet regions of the ungerade See gerade. electromagnetic spectrum, and that a uniaxial crystal A double-refract- given transition occurs at a character- ing crystal (see double refraction) istic wavelength. having only one optic axis. uncertainty principle (Heisenberg unimolecular reaction A chemi- uncertainty principle; principle of in- cal reaction or step involving only determinism) The principle that it is one molecule. An example is the denot possible to know with unlimited composition of dinitrogen tetroxide: accuracy both the position and mo- N O → 2NO mentum of a particle. This principle, 2 4 2 discovered in 1927 by Werner Molecules colliding with other mol- *Heisenberg, is usually stated in the ecules acquire sufÜcient activation Δ Δ ≥ π Δ energy to react, and the activated form: x px h/4 , where x is the uncertainty in the x-coordinate of the complex only involves the atoms of a Δ single molecule. particle, px is the uncertainty in the x-component of the particle’s unit A speciÜed measure of a physi- momentum, and h is the *Planck cal quantity, such as length, mass, constant. An explanation of the un- time, etc., speciÜed multiples of certainty is that in order to locate a which are used to express magni- particle exactly, an observer must be tudes of that physical quantity. For able to bounce off it a photon of radi- scientiÜc purposes previous systems ation; this act of location itself alters of units have now been replaced by the position of the particle in an un- *SI units. predictable way. To locate the position accurately, photons of short unit cell The group of particles u wavelength would have to be used. (atoms, ions, or molecules) in a crys- These would have associated large tal that is repeated in three dimen- momenta and cause a large effect on sions in the *crystal lattice. See also crystal system the position. On the other hand, using . long-wavelength photons would have univalent (monovalent) Having a less effect on the particle’s position, valency of one. but would be less accurate because of universal constants See funda- the longer wavelength. The principle mental constants. has had a profound effect on scienti- Üc thought as it appears to upset the universal indicator A mixture of unnil- 542

acid–base *indicators that changes the *actinoids; a.n. 92; r.a.m. 238.03; colour (e.g. red-yellow-orange-green- r.d. 19.05 (20°C); m.p. 1132±1°C; b.p. blue) over a range of pH. 3818°C. It occurs as *uraninite, from unnil- See transactinide elements. which the metal is extracted by an ion-exchange process. Three isotopes unsaturated 1. (of a compound) are found in nature: uranium–238 Having double or triple bonds in its (99.28%), uranium–235 (0.71%), and molecules. Unsaturated compounds uranium–234 (0.006%). As ura- can undergo addition reactions as well nium–235 undergoes nuclear Üssion as substitution. Compare saturated. with slow neutrons it is the fuel used 2. (of a solution) See saturated. in nuclear reactors and nuclear unstable equilibrium See equilib- weapons; uranium has therefore as- rium. sumed enormous technical and polit- ical importance since their invention. UPS Ultraviolet photoelectron spec- It was discovered by Martin Klaproth photoelectron spectroscopy. See (1747–1817) in 1789. troscopy. A UPVC Unplasticized PVC: a tough • Information from the WebElements site hardwearing form of PVC used for uranium(VI) Ûuoride (uranium window frames and similar applica- hexaÛuoride) A volatile white solid, tions. ° UF6; r.d. 4.68; m.p. 64.5 C. It is used uracil A *pyrimidine derivative and in the separation of uranium isotopes one of the major component bases of by gas diffusion. *nucleotides and the nucleic acid Û ura- *RNA. uranium hexa uoride See nium(vi) fluoride. O uranium–lead dating A group of methods of dating certain rocks that NH depends on the decay of the radioisotope uranium–238 to lead–206 (half- life 4.5 × 109 years) or the decay of N O uranium–235 to lead–207 (half-life H 7.1 × 108 years). One form of ura- Uracil nium–lead dating depends on measuring the ratio of the amount of uraninite A mineral form of ura- helium trapped in the rock to the nium(IV) oxide, containing minute amount of uranium present (since amounts of radium, thorium, polo- the decay 238U → 206Pb releases eight u nium, lead, and helium. When urani- alpha-particles). Another method of nite occurs in a massive form with a calculating the age of the rocks is to pitchy lustre it is known as pitch- measure the ratio of radiogenic lead blende, the chief ore of uranium. (206Pb, 207Pb, and 208Pb) present to Uraninite occurs in Saxony (Ger- nonradiogenic lead (204Pb). These many), Romania, Norway, the UK methods give reliable results for ages (Cornwall), E Africa (Democratic Re- of the order 107–109 years. public of Congo), USA, and Canada uranium(IV) oxide A black solid, (Great Bear Lake). ° UO2; r.d. 10.9; m.p. 3000 C. It occurs uranium Symbol U. A white radio- naturally as *uraninite and is used in active metallic element belonging to nuclear reactors. 543 UV–visible spectroscopy uranium series See radioactive he was awarded the 1939 Nobel Prize series. for physics. urea (carbamide) A white crystalline uric acid The end product of purine ° solid, CO(NH2)2; r.d. 1.3; m.p. 135 C. breakdown in most primates, birds, It is soluble in water but insoluble in terrestrial reptiles, and insects and certain organic solvents. Urea is the also (except in primates) the major major end product of nitrogen excre- form in which metabolic nitrogen is tion in mammals, being synthesized excreted. Being fairly insoluble, uric by the *urea cycle. Urea is synthe- acid can be expelled in solid form, sized industrially from ammonia and which conserves valuable water in carbon dioxide for use in *urea– arid environments. The accumula- Û formaldehyde resins and pharmaceu- tion of uric acid in the synovial uid ticals, as a source of nonprotein ni- of joints causes gout. trogen for ruminant livestock, and as O a nitrogen fertilizer. H N urea cycle (ornithine cycle) The se- HN ries of biochemical reactions that O converts ammonia to *urea during O N N the excretion of metabolic nitrogen. H H Urea formation occurs in mammals Uric acid and, to a lesser extent, in some other animals. The liver converts ammonia uridine A nucleoside consisting to the much less toxic urea, which is of one uracil molecule linked to a d- excreted in solution in urine. ribose sugar molecule. The derived urea–formaldehyde resins Syn- mucleotide uridine diphosphate thetic resins made by copolymerizing (UDP) is important in carbohydrate urea with formaldehyde (methanal). metabolism. They are used as adhesives or ther- O mosetting plastics. urea nitrate See unfo. NH urethane resins (polyurethanes) Synthetic resins containing the repeating group –NH–CO–O–. There are N O numerous types made by copolymerizing isocyanate esters with polyhy- OH O dric alcohols. They have a variety of uses in plastics, paints, and solid u foams. HO OH Urey, Harold Clayton (1894–1981) US physical chemist, who became a Uridine professor at the University of Califor- UV See ultraviolet radiation. nia in 1958. His best-known work was the discovery of *deuterium UV–visible spectroscopy See ul- (heavy hydrogen) in 1932, for which traviolet–visible spectroscopy. V

vacancy See crystal defect. canonical forms (see resonance). See also density-function theory; mo- vacuum A space in which there is a lecular-orbital theory. low pressure of gas, i.e. relatively few atoms or molecules. A perfect vac- valence electron An electron in uum would contain no atoms or mol- one of the outer shells of an atom ecules, but this is unobtainable as all that takes part in forming chemical the materials that surround such a bonds. space have a Ünite *vapour pressure. valency (valence) The combining In a soft (or low) vacuum the pres- power of an atom or radical, equal to sure is reduced to about 10–2 pascal, the number of hydrogen atoms that whereas a hard (or high) vacuum has the atom could combine with or dis- a pressure of 10–2–10–7 pascal. Below 10–7 pascal is known as an ultrahigh place in a chemical compound (hy- vacuum. See also vacuum pump. drogen has a valency of 1). It is equal to the ionic charge in ionic com- vacuum distillation Distillation pounds; for example, in Na2S, under reduced pressure. The depres- sodium has a valency of 1 (Na+) and sion in the boiling point of the sub- sulphur a valency of 2 (S2–). In cova- stance distilled means that the lent compounds it is equal to the temperature is lower, which may number of bonds formed; in CO2 oxy- prevent the substance from decom- gen has a valency of 2 and carbon posing. has a valency of 4. vacuum pump A pump used to re- valine See amino acid. duce the gas pressure in a container. diazepam The normal laboratory rotary oil-seal Valium See . pump can maintain a pressure of vanadium Symbol V. A silvery- 10–1 Pa. For pressures down to 10–7 Pa white metallic *transition element; a *diffusion pump is required. *Ion a.n. 23; r.a.m. 50.94; r.d. 5.96; m.p. pumps can achieve a pressure of 1890°C; b.p. 3380°C. It occurs in a 10–9 Pa and a *cryogenic pump com- number of complex ores, including bined with a diffusion pump can vanadinite (Pb5Cl(VO4)3) and carnotite reach 10–13 Pa. (K2(ClO2)2(VO4)2). The pure metal can valence See valency. be obtained by reducing the oxide energy bands with calcium. The element is used in valence band See . a large number of alloy steels. Chem- valence-bond theory A method ically, it reacts with nonmetals at of computational chemistry in which high temperatures but is not affected the electrons are considered as be- by hydrochloric acid or alkalis. It longing to deÜnite bonds consisting forms a range of complexes with oxi- of pairs of electrons associated with dation states from +2 to +5. Vana- pairs of atoms in the molecule. The dium was discovered in 1801 by actual state of the molecule can be Andrés del Rio (1764–1849), who al- regarded as the result of a set of lowed himself to be persuaded that 545 variational principle what he had discovered was an im- van’t Hoff’s isochore An equation pure form of chromium. The el- formulated by van’t Hoff for the vari- ement was rediscovered and named ation of equilibrium constant with by Nils Sefström (1787–1854) in 1880. temperature: ∆ 2 A (d loge K)/dT = H/RT , • Information from the WebElements site where K is the equilibrium constant, R is the gas constant, T is the thermo- vanadium(V) oxide (vanadium ∆ pentoxide) A crystalline compound, dynamic temperature, and H the enthalpy of the reaction. V2O5, used extensively as a catalyst in industrial gas-phase oxidation Van Urk’s reagent See p-dimethyl- processes. aminobenzaldehyde. vanadium pentoxide See vana- vapour density The density of a dium(v) oxide. gas or vapour relative to hydrogen, oxygen, or air. Taking hydrogen as van der Waals’ equation See equation of state the reference substance, the vapour . density is the ratio of the mass of a van der Waals’ force An attrac- particular volume of a gas to the tive force between atoms or mol- mass of an equal volume of hydrogen ecules, named after Johannes van der under identical conditions of pres- Waals (1837–1923). The force ac- sure and temperature. Taking the counts for the term a/V2 in van der density of hydrogen as 1, this ratio is Waals’ equation (see equation of equal to half the relative molecular state). These forces are much weaker mass of the gas. than those arising from valence vapour pressure The pressure ex- bonds and are inversely proportional erted by a vapour. All solids and liq- to the seventh power of the distance uids give off vapours, consisting of between the atoms or molecules. atoms or molecules of the substances They are the forces responsible for that have evaporated from the con- nonideal behaviour of gases and for densed forms. These atoms or mol- the lattice energy of molecular crys- ecules exert a vapour pressure. If the tals. There are three factors causing substance is in an enclosed space, the such forces: (1) dipole–dipole interac- vapour pressure will reach an equi- tion, i.e. electrostatic attractions librium value that depends only on between two molecules with per- the nature of the substance and the manent dipole moments; (2) dipole- temperature. This equilibrium value induced dipole interactions, in which occurs when there is a dynamic equi- the dipole of one molecule polarizes librium between the atoms or mol- a neighbouring molecule; (3) disper- ecules escaping from the liquid or sion forces arising because of small solid and those that strike the sur- instantaneous dipoles in atoms. face of the liquid or solid and return v to it. The vapour is then said to be a van’t Hoff, Jacobus See hoff. saturated vapour and the pressure it van’t Hoff factor Symbol i. A fac- exerts is the saturated vapour pres- tor appearing in equations for *col- sure. ligative properties, equal to the ratio variational principle In calcula- of the number of actual particles pre- tions in quantum mechanics the sent to the number of undissociated principle that if a trial *wave func- particles. tion is used to calculate the energy of Vaseline 546

a system, then the energy calculated tational transition that accompanies cannot be lower than the actual the vibrational transition. energy of the ground state of the Some features of vibrational spec- system. To use variational principles tra can be analysed by regarding the effectively in quantum mechanics vibrations as simple harmonic mo- a trial wave function contains para- tion, but a realistic account of mo- meters that can be varied, with lecular vibrations requires that these parameters being changed anharmonicity is taken into until the lowest possible energy is account. found. A diatomic molecule can only have petroleum jelly a vibrational–rotational spectrum if it Vaseline See . has a permanent dipole moment. A verdigris A green patina of basic polyatomic molecule can only have a copper salts formed on copper. The vibrational–rotational spectrum if composition of verdigris varies the normal modes of vibration cause depending on the atmospheric the molecule to have an oscillating conditions, but includes the basic dipole moment. carbonate CuCO3.Cu(OH)2, the basic vicinal (vic) Designating a molecule sulphate CuSO4.Cu(OH)2.H2O, and in which two atoms or groups are sometimes the basic chloride linked to adjacent atoms. For exam- CuCl2.Cu(OH)2. ple, 1,2-dichloroethane (CH2ClCH2Cl) vermiculite See clay minerals. is a vicinal (or vic) dihalide and can be named vic-dichloroethane. very high frequency (VHF) A radio frequency in the range 3 × Victor Meyer’s method A 108– 0.3 × 108 Hz, i.e. having a wave- method of measuring vapour density, length in the range 1–10 m. devised by Victor Meyer (1848–97). A weighed sample in a small tube is very low frequency (VLF) A radio dropped into a heated bulb with a × 4 × frequency in the range 3 10 – 0.3 long neck. The sample vaporizes and 4 10 Hz, i.e. having a wavelength in displaces air, which is collected over the range 10–100 km. water and the volume measured. The vibrational relaxation A process vapour density can then be calcu- in which a polyatomic molecule in lated. an excited vibrational state returns to villiaumite A mineral form of a lower vibrational state in the same sodium Ûuoride, NaF. electronic state by colliding with other molecules. vinegar A dilute solution of *ethanoic acid (up to 6%), used as a vibrational spectroscopy The Ûavouring and pickling medium. Nat- spectroscopic investigation of the vi- ural vinegar is made by the contin- v brational energy levels of molecules. ued fermentation of alcoholic In the infrared region of the electro- liquors, usually by Acetobacter species, magnetic spectrum vibrational tran- which oxidize ethanol to ethanoic sitions are accompanied by rotational acid. Vinegar is also made by diluting transitions. Infrared spectra of mol- synthetic ethanoic acid. ecules are series of bands, with each vinyl acetate ethenyl band being associated with a vibra- See ethanoate. tional transition and every line in that band being associated with a ro- vinylation The catalysed reaction 547 vitamin A between ethyne and a compound of electromagnetic radiations to with an active hydrogen atom, such which the human eye is sensitive. as alcohol, amine, or carboxylic acid. vitamin One of a number of or- Addition takes place across the triple ganic compounds required by living bond of ethyne to produce an eth- organisms in relatively small enyl (vinyl) compound, containing amounts to maintain normal health. the group CH2=CH–. There are some 14 generally recog- vinyl chloride See chloroethene. nized major vitamins: the water- soluble *vitamin B complex (contain- vinyl group The organic group ing 9) and *vitamin C and the fat- CH2:CH–. soluble *vitamin A, *vitamin D, virial coefÜcients See virial equa- *vitamin E, and *vitamin K. Most B tion. vitamins and vitamin C occur in plants, animals, and microorganisms; virial equation A gas law that at- they function typically as *coen- tempts to account for the behaviour zymes. Vitamins A, D, E, and K occur of real gases, as opposed to an ideal only in animals, especially verte- gas. It takes the form brates, and perform a variety of pV = RT + Bp + Cp2 + Dp3 + …, metabolic roles. Animals are unable to manufacture many vitamins them- where B, C, and D are known as virial selves and must have adequate coefÜcients. amounts in the diet. Foods may con- viscose process See rayon. tain vitamin precursors (called provi- tamins) that are chemically changed viscosity A measure of the resis- to the actual vitamin on entering the tance to Ûow that a Ûuid offers when body. Many vitamins are destroyed it is subjected to shear stress. For a by light and heat, e.g. during cooking. Newtonian Ûuid, the force, F, needed to maintain a velocity gradient, vitamin A (retinol) A fat-soluble dv/dx, between adjacent planes of a vitamin that cannot be synthesized Ûuid of area A is given by: F = by mammals and other vertebrates ηA(dv/dx), where η is a constant, and must be provided in the diet. called the coefÜcient of viscosity. In Green plants contain precursors of SI units it has the unit pascal second the vitamin, notably carotenes, that (in the c.g.s. system it was measured are converted to vitamin A in the in- in *poise). Non-Newtonian Ûuids, testinal wall and liver. The aldehyde retinal such as clays, do not conform to this derivative of vitamin A, , is a constituent of the visual pigment simple model. See also kinematic vis- rhodopsin. DeÜciency affects the cosity. eyes, causing night blindness, xe- visible spectrum The *spectrum rophthalmia, and eventually total v

Vitamin A vitamin B complex 548

3CH N NH2 S 2CH OH

CH N N 2 C H2 CH3

Vitamin B1

blindness. The role of vitamin A in zymes *FAD and FMN, which have other aspects of metabolism is less an important role in the metabolism clear. of all major nutrients as well as in A the oxidative phosphorylation reac- • Information about IUPAC nomenclature tions of the *electron transport Ü Û chain. De ciency of B2 causes in am- vitamin B complex A group of mation of the tongue and lips and water-soluble vitamins that charac- mouth sores. teristically serve as components of Vitamin B6 (pyridoxine) is widely *coenzymes. Plants and many mi- distributed in cereal grains, yeast, croorganisms can manufacture B liver, milk, etc. It is a constituent of a vitamins but dietary sources are es- coenzyme (pyridoxal phosphate) in- sential for most animals. Heat and volved in amino acid metabolism. light tend to destroy B vitamins. DeÜciency causes retarded growth, Vitamin B1 (thiamin(e)) is a precur- dermatitis, convulsions, and other sor of the coenzyme thiamine py- symptoms. rophosphate, which functions in Ü carbohydrate metabolism. De ciency CH3 leads to beriberi in humans and to polyneuritis in birds. Good sources OH include brewer’s yeast, wheatgerm, N beans, peas, and green vegetables. Vitamin B (riboÛavin) occurs in 2 HOH C green vegetables, yeast, liver, and 2 milk. It is a constituent of the coen- CH2OH Vitamin B O 6

Vitamin B12 (cyanocobalamin; 3CH N NH cobalamin) is manufactured only by microorganisms and natural sources are entirely of animal origin. Liver is N N O CH especially rich in it. One form of B12 v 3 functions as a coenzyme in a number CH OH 2 of reactions, including the oxidation of fatty acids and the synthesis of OH DNA. It also works in conjunction OH with *folic acid (another B vitamin) in the synthesis of the amino acid

2CH methionine and it is required for nor- OH mal production of red blood cells.

Vitamin B2 Vitamin B12 can only be absorbed 549 vitamin D from the gut in the presence of a gly- species must obtain it from their coprotein called intrinsic factor; lack diet. It is required for the mainte- Ü of this factor or de ciency of B12 re- nance of healthy connective tissue; sults in pernicious anaemia. deÜciency leads to scurvy. Vitamin C Other vitamins in the B complex is readily destroyed by heat and light. include *nicotinic acid, *pantothenic A acid, *biotin, and *lipoic acid. See also • Information about IUPAC nomenclature choline. A vitamin D A fat-soluble vitamin occurring in the form of two steroid de- • Information about IUPAC nomenclature rivatives: vitamin D2 (ergocalciferol, or calciferol), found in yeast; and OH vitamin D3 (cholecalciferol), which oc- CH 2 curs in animals. Vitamin D2 is formed OH O O from a steroid by the action of ultraviolet light and D3 is produced by the action of sunlight on a cholesterol derivative in the skin. Fish-liver oils OH are the major dietary source. The ac- OH tive form of vitamin D is manufac- Vitamin C tured in response to the secretion of parathyroid hormone, which occurs vitamin C (ascorbic acid) A colour- when blood calcium levels are low. It less crystalline water-soluble vitamin causes increased uptake of calcium found especially in citrus fruits and from the gut, which increases the green vegetables. Most organisms supply of calcium for bone synthesis. synthesize it from glucose but man Vitamin D deÜciency causes rickets and other primates and various other in growing animals and osteomalacia

Vitamin D vitamin E 550

CH3

CH3 CH3 CH3 CH3

CH 3CH O 3

CH3

Vitamin E

in mature animals. Both conditions pated between the points is one watt. are characterized by weak deformed It is named after Alessandro Volta. bones. Volta, Alessandro Giuseppe An- A tonio Anastasio (1745–1827) Ital- • Information about IUPAC nomenclature ian physicist. In 1774 he began vitamin E (tocopherol) A fat-soluble teaching in Como and in that year in- vitamin consisting of several closely vented the electrophorus. He moved related compounds, deÜciency of to Pavia University in 1778. In 1800 he made the *voltaic cell, thus pro- which leads to a range of disorders in Ü different species, including muscular viding the rst practical source of dystrophy, liver damage, and infertil- electric current. The SI unit of poten- ity. Good sources are cereal grains tial difference is named after him. and green vegetables. Vitamin E pre- voltaic cell (galvanic cell) A device vents the oxidation of unsaturated that produces an e.m.f. as a result of fatty acids in cell membranes, so chemical reactions that take place maintaining their structure. within it. These reactions occur at A the surfaces of two electrodes, each • Information about IUPAC nomenclature of which dips into an electrolyte. The Ürst voltaic cell, devised by Alessan- vitamin K A fat-soluble vitamin dro Volta, had electrodes of two dif- consisting of several related com- ferent metals dipping into brine. See pounds that act as coenzymes in the primary cell; secondary cell. synthesis of several proteins necessary for blood clotting. DeÜciency of voltaic pile An early form of battery, devised by Alessandro Volta, vitamin K, which leads to extensive Û bleeding, is rare because a form of consisting of a number of at * the vitamin is manufactured by in- voltaic cells joined in series. The liq- testinal bacteria. Green vegetables uid electrolyte was absorbed into paper or leather discs. and egg yolk are good sources. v voltameter (coulometer) 1. An vitreous Having a glasslike appear- electrolytic cell formerly used to ance or structure. measure quantity of electric charge. volt Symbol V. The SI unit of elec- The increase in mass (m) of the cath- tric potential, potential difference, or ode of the cell as a result of the depo- e.m.f. deÜned as the difference of position on it of a metal from a solution tential between two points on a con- of its salt enables the charge (Q) to be ductor carrying a constant current of determined from the relationship Q = one ampere when the power dissi- m/z, where z is the electrochemical 551 VX equivalent of the metal. 2. Any V-series See nerve agents. other type of electrolytic cell used for vulcanite (ebonite) A hard black in- measurement. sulating material made by the vul- volume Symbol V. The space occu- canization of rubber with a high pied by a body or mass of Ûuid. proportion of sulphur (up to 30%). volumetric analysis A method of vulcanization A process for hard- quantitative analysis using measure- ening rubber by heating it with sul- ment of volumes. For gases, the main phur or sulphur compounds. technique is in reacting or absorbing gases in graduated containers over VX A highly toxic colourless oily liq- mercury, and measuring the volume uid, C11H26NO2PS; r.d. 1.01; m.p. changes. For liquids, it involves –50°C; b.p. 298°C. It is an organo- *titrations. phosphorus compound and is probably one of the most toxic of all von Baeyer nomenclature A sys- *nerve agents. VX was discovered in tem of naming polycyclic compounds 1952 at the Porton Down research originally introduced by Adolf von station in Wiltshire. VX is the best Baeyer in 1900 and since extended. A known of the V-series of nerve agents (the V denotes very high persistence • Information about the IUPAC system in the environment). Other, less VSEPR theory Valence-shell elec- known, members of the series in- tron-pair repulsion theory; a theory clude VG and VM, which are both for predicting the shapes of mol- organophosphorus compounds re- ecules. See lone pair. lated to VX.

v W

Wacker process A process for the there are n+1 skeletal electron pairs, manufacture of ethanal by the air ox- then the structure is closo. If the for- idation of ethene. A mixture of air mula is and ethene is bubbled through a so- BnHn+4 lution containing palladium(II) chloride and copper(II) chloride. The Pd2+ and these are n+2 skeletal electron ions form a complex with the ethene pairs, the structure is nido. If the for- in which the ion is bound to the pi mula is electrons in the C=C bond. This de- BnHn+6 creases the electron density in the bond, making it susceptible to nu- and these are n+3 skeletal electron cleophilic attack by water molecules. pairs, the structure is arachno. The The complex formed breaks down to rules are named after the British ethanal and palladium metal. The chemist Kenneth Wade, who Ürst for- Cu2+ ions oxidize the palladium back mulated them in the early 1970s. 2+ + to Pd , being reduced to Cu ions in Wagenaar test See acetone– the process. The air present oxidizes chlor–haemin test. Cu+ back to Cu2+. Thus the copper(II) and palladium(II) ions effectively act Wagner-Meerwein rearrange- as catalysts in the process, which is ment A rearrangement in which an now the main source of ethanal and, alkyl group moves from a carbon by further oxidation, ethanoic acid. It atom to an adjacent carbon atom can also be applied to other alkenes. during a reaction. It often occurs to It is named after Alexander von stabilize a carbocation formed as an Wacker (1846–1922). intermediate during the reaction. Re- arrangements of this type have been Wade’s rules A set of rules for pre- extensively studied in terpene chem- dicting the structure of a cluster istry. compound based in the number of electrons in the framework counted WAHUHA A pulse sequence in *nu- in a particular way. The electrons clear magnetic resonance (NMR) used counted are known as skeletal elec- to reduce linewidth. The WAHUHA trons. The rules apply to polyhedra sequence (named after its inventors that have triangular faces (known as Waugh, Huber, and Haberlen) effects deltahedra). They were originally ap- an averaging procedure by twisting plied to the boron hydrides. Electron the magnetization vector in various pairs in bonding between two boron directions. atoms are counted as skeletal elec- Walden’s rule An empirical rule trons but the pairs in B–H units are suggested by P. Walden (1863–1957) ignored. However, if a boron atom is concerning ions in solutions, stating connected to two hydrogens (BH2), that the product of the molar con- Λ η the second bond is counted with the ductivity, m, and the viscosity, , is skeletal electrons. According to the approximately constant for the same 2– rules, if the formula is [BnHn] and ions in different solvents. Some 553 water gas

Ü justi cation for Walden’s rule is pro- which each H2O unit is tetrahedrally vided by the proportional relation- surrounded by four other H2O units. Λ ship between m and the diffusion Because of its angular shape the coefÜcient, D; as D is inversely pro- water molecule has a permanent di- Λ portional to the viscosity, m is in- pole moment and in addition it is versely proportional to η, which is in strongly hydrogen bonded and has a accordance with Walden’s rule. How- high dielectric constant. These prop- ever, different solvents hydrate the erties combine to make water a pow- same ions differently, so that both erful solvent for both polar and ionic the radius and the viscosity change compounds. Species in solution are when the solvent is changed. It is frequently strongly hydrated and this fact that limits the validity of the in fact ions frequently written as, for example, Cu2+ are essentially rule. 2+ [Cu(H2O)6] . Crystalline *hydrates warfarin 3-(alpha-acetonylbenzyl)- are also common for inorganic sub- 4-hydroxycoumarin: a synthetic anti- stances; polar organic compounds, coagulant used both therapeutically particularly those with O–H and N–H in clinical medicine and, in lethal bonds, also form hydrates. doses, as a rodenticide (see pesticide). Pure liquid water is very weakly dissociated into H O+ and OH– ions washing soda *Sodium carbonate 3 by self ionization: decahydrate, Na CO .10H O. 2 3 2 ˆ + – H2O H + OH water A colourless liquid, H2O; r.d. (see ionic product) and consequently 1.000 (4°C); m.p. 0.000°C; b.p. ° any species that increases the con- 100.000 C. In the gas phase water centration of the positive species, consists of single H2O molecules in H O+, is acidic and species increasing ° 3 which the H–O–H angle is 105 . The the concentration of the negative structure of liquid water is still con- species, OH–, are basic (see acid). The troversial; hydrogen bonding of the phenomena of ion transport in water type H2O…H–O–H imposes a high de- and the division of materials into hy- gree of structure and current models drophilic (water loving) and hy- supported by X-ray scattering studies drophobic (water hating) substances have short-range ordered regions, are central features of almost all bio- which are constantly disintegrating logical chemistry. A further property and re-forming. This ordering of the of water that is of fundamental im- liquid state is sufÜcient to make the portance to the whole planet is its density of water at about 0°C higher strong absorption in the infrared than that of the relatively open- range of the spectrum and its trans- structured ice; the maximum density parency to visible and near ultravio- occurs at 3.98°C. This accounts for let radiation. This allows solar the well-known phenomenon of ice radiation to reach the earth during Ûoating on water and the contraction hours of daylight but restricts rapid of water below ice, a fact of enor- heat loss at night. Thus atmospheric w mous biological signiÜcance for all water prevents violent diurnal oscil- aquatic organisms. lations in the earth’s ambient tem- Ice has nine distinct structural perature. modiÜcations of which ordinary ice, water gas A mixture of carbon or ice I, has an open structure built monoxide and hydrogen produced by of puckered six-membered rings in passing steam over hot carbon (coke): water glass 554

→ hardness of H2O(g) + C(s) CO(g) + H2(g) water softening See water The reaction is strongly endothermic . but the reaction can be used in con- watt Symbol W. The SI unit of junction with that for *producer gas power, deÜned as a power of one for making fuel gas. The main use of joule per second. In electrical con- water gas before World War II was in texts it is equal to the rate of energy producing hydrogen for the *Haber transformation by an electric current process. Here the above reaction was of one ampere Ûowing through a combined with the water-gas shift conductor the ends of which are reaction to increase the amount of maintained at a potential difference hydrogen: of one volt. The unit is named after ˆ James Watt (1736–1819). CO + H2O CO2 + H2 Most hydrogen for the Haber process wave equation A partial differen- is now made from natural gas by tial equation of the form: steam *reforming. ∇2u = (1/c2)∂2u/∂t2, water glass A viscous colloidal so- where lution of sodium silicates in water, ∇2 = ∂2/∂x2 + ∂2/∂y2 + ∂2/∂z2 used to make silica gel and as a size is the Laplace operator. It represents and preservative. the propagation of a wave, where u is water of crystallization Water the displacement and c the speed of present in crystalline compounds in propagation. See also schrödinger deÜnite proportions. Many crystalline equation. salts form hydrates containing 1, 2, wave function A function ψ(x,y,z) 3, or more moles of water per mole appearing in *Schrödinger’s equation of compound, and the water may be in wave mechanics. The wave func- held in the crystal in various ways. tion is a mathematical expression in- Thus, the water molecules may sim- volving the coordinates of a particle ply occupy lattice positions in the in space. If the Schrödinger equation crystal, or they may form bonds with can be solved for a particle in a given the anions or the cations present. In system (e.g. an electron in an atom) the pentahydrate of copper sulphate then, depending on the boundary (CuSO4.5H2O), for instance, each cop- conditions, the solution is a set of per ion is coordinated to four water allowed wave functions (eigen- molecules through the lone pairs on functions) of the particle, each cor- the oxygen to form the *complex responding to an allowed energy 2+ [Cu(H2O)4] . Each sulphate ion has level (eigenvalue). The physical one water molecule held by hydro- signiÜcance of the wave function is gen bonding. The difference between that the square of its absolute value, the two types of bonding is demon- |ψ|2, at a point is proportional to the strated by the fact that the pentahy- probability of Ünding the particle in a w drate converts to the monohydrate at small element of volume, dxdydz, at 100°C and only becomes anhydrous that point. For an electron in an above 250°C. Water of constitution is atom, this gives rise to the idea of an obsolete term for water combined atomic and molecular *orbitals. in a compound (as in a metal hydrox- wave mechanics A formulation of ide M(OH)2 regarded as a hydrated *quantum mechanics in which the oxide MO.H2O). dual wave–particle nature of such en- 555 Weissenberg technique tities as electrons is described by the thought of as de Broglie waves in *Schrödinger equation. Schrödinger *electron diffraction. put forward this formulation of wave vector A vector k associated quantum mechanics in 1926 and in with a *wave number k. In the case the same year showed that it was of free electrons the wave vector k is equivalent to matrix mechanics. Tak- related to the momentum p in ing into account the *de Broglie *quantum mechanics by p = បk, wavelength, Schrödinger postulated where ប is the rationalized *Planck a wave mechanics that bears the constant. In the case of *Bloch’s theo- same relation to Newtonian mechan- rem, the wave vector k can only have ics as physical optics does to geomet- certain values and can be thought of rical optics. as a quantum number associated wave number Symbol k. The num- with the translational symmetry of ber of cycles of a wave in unit length. the crystal. It is the reciprocal of the wavelength. wax Any of various solid or semi- wave packet A superposition of solid substances. There are two main waves with one predominant *wave types. Mineral waxes are mixtures of number k, but with several other hydrocarbons with high molecular wave numbers near k. Wave packets weights. ParafÜn wax, obtained from are useful for the analysis of scatter- *petroleum, is an example. Waxes ing in *quantum mechanics. Concen- secreted by plants or animals are trated packets of waves can be used mainly esters of fatty acids and usu- to describe localized particles of both ally have a protective function. matter and *photons. The Heisen- weak acid An *acid that is only berg *uncertainty principle can be partially dissociated in aqueous solu- derived from a wave-packet description. tion of entities in quantum mechanics. The motion of a wave packet is in weber Symbol Wb. The SI unit of accord with the motion of the corre- magnetic Ûux equal to the Ûux that, sponding classical particle, if the po- linking a circuit of one turn, pro- tential energy change across the duces in it an e.m.f. of one volt as it dimensions of the packet is very is reduced to zero at a uniform rate small. This proposition is known as in one second. It is named after Wil- Ehrenfest’s theorem, named after the helm Weber (1804–91). Dutch physicist Paul Ehrenfest Weissenberg technique A tech- (1880–1933), who proved it in 1927. nique used to overcome the problem wave–particle duality The con- of overlapping reÛections in the iden- cept that waves carrying energy may tiÜcation of the symmetry and the di- have a corpuscular aspect and that mensions of a unit cell in *X-ray particles may have a wave aspect; crystallography. In this technique, a which of the two models is the more screen is placed in front of the Ülm w appropriate will depend on the prop- allowing only one set of reÛections to erties the model is seeking to ex- be exposed. The Weissenberg tech- plain. For example, waves of nique produces distorted photo- electromagnetic radiation need to be graphs, but this can be overcome by visualized as particles, called *pho- having a coupling between the mo- tons, to explain the photoelectric ef- tions of the crystal and the Ülm. fect while electrons need to be Using the precession camera tech- Weston cell 556

nique undistorted photographs can and were subsequently extended to be obtained. polyatomic molecules. The Wigner– Witmer rules are also known as Weston cell (cadmium cell) A type correlation rules since they involve of primary *voltaic cell devised by the correlation between atomic elec- Edward Weston (1850–1936), which tronic states and molecular elec- is used as a standard; it produces a tronic states. They are useful in constant e.m.f. of 1.0186 volts at analysing the *electronic spectra of 20°C. The cell is usually made in an molecules. H-shaped glass vessel with a mercury anode covered with a paste of cad- Williamson’s synthesis Either of mium sulphate and mercury(I) sul- two methods of producing ethers, phate in one leg and a cadmium both named after the British chemist amalgam cathode covered with cad- Alexander Williamson (1824–1904). mium sulphate in the other leg. The 1. The dehydration of alcohols using electrolyte, which connects the two concentrated sulphuric acid. The electrodes by means of the bar of the overall reaction can be written H, is a saturated solution of cadmium → 2ROH H2O + ROR sulphate. In some cells sulphuric acid The method is used for making is added to prevent the hydrolysis of ethoxyethane (C2H5OC2H5) from mercury sulphate. ethanol by heating at 140°C with ex- white arsenic See arsenic(iii) cess of alcohol (excess acid at 170°C oxide. gives ethene). Although the steps in the reaction are all reversible, the white mica See muscovite. ether is distilled off so the reaction white spirit A liquid mixture of can proceed to completion. This is hydrocarbons obtained from petro- Williamson’s continuous process. In leum, used as a solvent for paint general, there are two possible mech- (‘turpentine substitute’). anisms for this synthesis. In the Ürst Wigner–Seitz cell A polyhedron in (favoured by primary alcohols), an a crystal that is bounded by planes alkylhydrogen sulphate is formed ˆ formed by perpendicular bisectors of ROH + H2SO4 ROSO3H + H2O bonds between lattice sites. The This reacts with another alcohol mol- Wigner–Seitz cell was used by Hun- ecule to give an oxonium ion garian-born US physicist Eugene → + ROH + ROSO3H ROHR Wigner (1902–95) and Frederick Seitz This loses a proton to give ROR. (1911–2008) in 1933 in the course of The second mechanism (favoured their analysis of the cohesion of met- by tertiary alcohols) is formation of a als. The concept has been used exten- carbonium ion sively in the theory of solids. ROH + H+ → H O + R+ Wigner–Witmer rules A set of re- 2 This is attacked by the lone pair on sults found by applying group theory the other alcohol molecule w that states which molecular electronic states can exist, starting from R+ + ROH → ROHR+ the electronic states of the isolated and the oxonium ion formed again atoms. The rules were stated for di- gives the product by loss of a proton. atomic molecules by Hungarian- The method can be used for mak- born US physicist Eugene Wigner ing symmetric ethers (i.e. having (1902–95) and E. E. Witmer in 1928 both R groups the same). It can suc- 557 Wood’s metal cessfully be used for mixed ethers witherite A mineral form of *bar- only when one alcohol is primary ium carbonate, BaCO3. and the other tertiary (otherwise a WLN See wiswesser line notation. mixture of the three possible products results). 2. A method of prepar- Wöhler, Friedrich (1800–82) Ger- ing ethers by reacting a haloalkane man physician and chemist, who be- with an alkoxide. The reaction, dis- came a professor of chemistry at covered in 1850, is a nucleophilic Göttingen. In 1828 he made his best- known discovery, the synthesis of substitution in which the negative urea (an organic compound) from alkoxide ion displaces a halide ion; ammonium cyanate (an inorganic for example: salt). This Ünally disproved the asser- RI + –OR′→ROR′ + I– tion that organic substances can be A mixture of the reagents is reÛuxed formed only in living things. Wöhler in ethanol. The method is particu- also isolated aluminium (1827), beryl- larly useful for preparing mixed lium (1828), and yttrium (1828). ethers, although a possible side reac- Wöhler’s synthesis A synthesis of tion under some conditions is an urea performed by Friedrich Wöhler elimination to give an alcohol and an in 1828. He discovered that urea alkene. (CO(NH2)2) was formed when a solution of ammonium isocyanate Wiswesser line notation (WLN) (NH NCO) was evaporated. At the An early *line notation for chemical 4 time it was believed that organic sub- structures. The symbols used are the stances such as urea could be made upper-case letters of the alphabet only by living organisms, and its pro- (A–Z), the numerals (0–9), with three duction from an inorganic com- other symbols: the ampersand (&), pound was a notable discovery. It is the hyphen (-), and the oblique stroke sometimes (erroneously) cited as end- (/), and a blank space. Atomic sym- ing the belief in vitalism. bols with one letter, such as B and F, wolfram See tungsten. are unchanged. Frequently occurring elements with more than one letter wolframite (iron manganese tung- and functional groups are also as- sten) A mineral consisting of a signed one letter; for example, G mixed iron–manganese tungstate, stands for chlorine, Q for hydroxyl, (FeMn)WO4, crystallizing in the monoclinic system; the principal ore and Z for NH2. The numerals indicate the number of carbon atoms in an of tungsten. It commonly occurs as blackish or brownish tabular crystal unbranched internally saturated Û alkyl chain. For example, CH is de- groups. It is found chie y in quartz 3 veins associated with granitic rocks. noted 1 and CH CH is denoted 2. To 3 2 China is the major producer of wol- establish the notation of a compound framite. the characters for the fragments are given in an established order. For ex- wood alcohol See methanol. w ample, the notation for C2H5OH is Wood’s metal A low-melting Q2. Rules for structures with (71°C) alloy of bismuth (50%), lead branched chains and fused rings are (25%), tin (12.5%), and cadmium also given. WLN provides a short and (12.5%). It is used for fusible links in unambiguous notation, which is suit- automatic sprinkler systems. The able for database searches. melting point can be changed by Woodward, Robert Burns 558

varying the composition. It is named structure and is particularly apparent after William Wood (1671–1730). with iron, copper, aluminium, etc., Woodward, Robert Burns whereas with lead and zinc it does (1917–79) US organic chemist who not occur as these metals are capable worked at Harvard. He is remem- of recrystallizing at room tempera- bered for his work in organic synthe- ture. sis, producing many organic wrought iron A highly reÜned compounds including quinine, cho- form of iron containing 1–3% of slag lesterol, cortisone, lysergic acid, (mostly iron silicate), which is evenly strychnine, chlorophyll, and vitamin distributed throughout the material B12. In 1965 he formulated the Wood- in threads and Übres so that the prod- ward–Hoffmann rules for certain uct has a Übrous structure quite dis- types of addition reactions. Wood- similar to that of crystalline cast iron. ward was awarded the 1965 Nobel Wrought iron rusts less readily than Prize for chemistry. other forms of metallic iron and it Woodward–Hoffmann rules welds and works more easily. It is Rules governing the formation of used for chains, hooks, tubes, etc. products during certain types of or- wurtzite structure A type of ionic ganic concerted reactions. The theory crystal structure in which the anions of such reactions was put forward in have a hexagonal close packed 1969 by Woodward and Roald Hoff- arrangement with the cations occu- mann (1937– ), and is concerned with the way that orbitals of the re- pying one type of tetrahedral hole. actants change continuously into or- Each type of ion has a coordination bitals of the products during reaction number of 4. Examples of this struc- and with conservation of orbital sym- ture are found in ZnS, ZnO, AlN, SiC, metry during this process. It is some- and NH4F. times known as frontier-orbital A theory. • An interactive version of the structure work function A quantity that Wurtz reaction A reaction to pre- determines the extent to which pare alkanes by reacting a thermionic or photoelectric emission haloalkane with sodium: will occur according to the Richard- 2RX + 2Na → 2NaX + RR son equation or Einstein’s photo- Û electric equation. It is sometimes The haloalkane is re uxed with expressed as a potential difference sodium in dry ether. The method is (symbol φ) in volts and sometimes as named after the French chemist the energy required to remove an Charles-Adolphe Wurtz (1817–84). electron (symbol W) in electronvolts The analogous reaction using a or joules. The former has been called haloalkane and a haloarene, for ex- the work function potential and the ample: latter the work function energy. → w C6H5Cl + CH3Cl + 2Na 2NaCl + work hardening An increase in C6H5CH3 the hardness of metals as a result of is called the Fittig reaction after the working them cold. It causes a per- German chemist Rudolph Fittig manent distortion of the crystal (1835–1910). X

° xanthates Salts or esters contain- C13H8O2; m.p. 174 C. The ring system ing the group –SCS(OR), where R is is present in xanthone dyes. an organic group. Cellulose xanthate xenobiotic Any substance foreign is an intermediate in the manufac- to living systems. Xenobiotics include ture of *rayon by the viscose process. drugs, pesticides, and carcinogens. xanthene A heterocyclic com- DetoxiÜcation of such substances oc- pound having three fused rings with curs mainly in the liver. one oxygen atom, C13H10O; m.p. 101–102°C b.p. 310–312°C. The ring xenon Symbol Xe. A colourless structure is present in a class of xan- odourless gas belonging to group 18 noble thene dyes. of the periodic table (see gases); a.n. 54; r.a.m. 131.30; d. 5.887 gdm–3; m.p. –111.9°C; b.p. –107.1°C. It is present in the atmosphere (0.00087%) from which it is extracted O by distillation of liquid air. There are nine natural isotopes with mass Xanthene numbers 124, 126, 128–132, 134, and 136. Seven radioactive isotopes are also known. The element is used in xanthine A purine base, C H N O , 5 4 4 2 Ûuorescent lamps and bubble cham- found in many organisms. See methylxanthines. bers. Liquid xenon in a supercritical state at high temperatures is used as O a solvent for infrared spectroscopy and for chemical reactions. The com- H + – Ü N pound Xe PtF6 was the rst noble- NH gas compound to be synthesized. Several other compounds of xenon are known, including XeF , XeF , O N N 2 4 H XeSiF6, XeO2F2, and XeO3. Recently, Xanthine compounds have been isolated that contain xenon–carbon bonds, such as [C H Xe][B(C H ) F] (pentaÛuoro- xanthone (dibenzo-4-pyrone) A 6 5 6 5 3 phenylxenon Ûuoroborate), which is colourless crystalline compound, stable under normal conditions. The O element was discovered in 1898 by ARamsey and Travers. • Information from the WebElements site XPS X-ray photoelectron spec- O troscopy. See photoelectron spec- Xanthone troscopy. X-ray crystallography 560

X-ray crystallography The use of X-rays Electromagnetic radiation of *X-ray diffraction to determine the shorter wavelength than ultraviolet structure of crystals or molecules. radiation produced by bombardment The technique involves directing a of atoms by high-quantum-energy beam of X-rays at a crystalline sam- particles. The range of wavelengths is ple and recording the diffracted X- 10–11 m to 10–9 m. Atoms of all the el- rays on a photographic plate. The ements emit a characteristic X-ray diffraction pattern consists of a pat- spectrum when they are bombarded tern of spots on the plate, and the by electrons. The X-ray photons are crystal structure can be worked out emitted when the incident electrons from the positions and intensities of knock an inner orbital electron out the diffraction spots. X-rays are dif- of an atom. When this happens an fracted by the electrons in the mol- outer electron falls into the inner ecules and if molecular crystals of a shell to replace it, losing potential compound are used, the electron energy (∆E) in doing so. The wave- density distribution in the molecule length λ of the emitted photon will can be determined. then be given by λ = ch/∆E, where c is X-ray diffraction The diffraction the speed of light and h is the Planck of X-rays by a crystal. The wave- constant. lengths of X-rays are comparable in X-rays can pass through many size to the distances between atoms forms of matter and they are there- in most crystals, and the repeated fore used medically and industrially pattern of the crystal lattice acts like to examine internal structures. X-rays a diffraction grating for X-rays. Thus, are produced for these purposes by a crystal of suitable type can be used an X-ray tube. to disperse X-rays in a spectrometer. X-ray spectrum See x-rays. X-ray diffraction is also the basis of X- ray crystallography. xylenes See dimethylbenzenes. X-ray Ûuorescence The emission xylenol (hydroxydimethylbenzene) of *X-rays from excited atoms pro- Any of six isomeric solid aromatic duced by the impact of high-energy compounds, C6H3(CH3)2OH. An im- electrons, other particles, or a pri- pure mixture of isomers is a liquid mary beam of other X-rays. The made from coal tar and employed as wavelengths of the Ûuorescent X-rays a solvent. The pure substances re- can be measured by an X-ray spec- semble *phenol in their reactions. trometer as a means of chemical They are used to make thermosetting analysis. X-ray Ûuorescence is used in polymer resins, and the chloro- such techniques as electron-probe derivative of 1,2,5-xylenol is an in- microanalysis. dustrial disinfectant. x Y

yeasts A group of unicellular fungi topes are known. It is used in certain of the class Hemiascomycetae and steels. The element was discovered phylum Ascomycota. They occur as by Jean de Marignac (1817–94) in single cells or as groups or chains of 1878. cells; yeasts reproduce asexually by A budding and sexually by producing • Information from the WebElements site ascospores. Yeasts of the genus Sac- charomyces ferment sugars and are yttrium Symbol Y. A silvery-grey used in the baking and brewing in- metallic element belonging to group dustries. 3 (formerly IIIA) of the periodic table; a.n. 39; r.a.m. 88.905; r.d. 4.469 ylide A chemical species derived (20°C); m.p. 1522°C; b.p. 3338°C. It from an onium ion by loss of a hy- * dron. For example, the phosphorus occurs in uranium ores and in lanthanoid ores, from which it can be ylide (C6H5)2P=CR2, derived from (C H ) PCHR + by loss of H+. extracted by an ion exchange 6 5 2 2 process. The natural isotope is yt- Ü yocto- Symbol y. A pre x used in trium–89, and there are 14 known ar- –24 the metric system to indicate 10 . tiÜcial isotopes. The metal is used in –24 For example, 10 second = 1 yocto- superconducting alloys and in alloys second (ys). for strong permanent magnets (in yotta- Symbol Y. A preÜx used in both cases, with cobalt). The oxide 24 the metric system to indicate 10 . (Y2O3) is used in colour-television For example, 1024 metres = 1 yotta- phosphors, neodymium-doped lasers, metre (Ym). and microwave components. Chemi- ytterbium Symbol Yb. A silvery cally it resembles the lanthanoids, forming ionic compounds containing metallic element belonging to the 3+ *lanthanoids; a.n. 70; r.a.m. 173.04; Y ions. The metal is stable in air ° r.d. 6.965 (20°C); m.p. 819°C; b.p. below 400 C. It was discovered in 1194°C. It occurs in gadolinite, mon- 1828 by Friedrich Wöhler. azite, and xenotime. There are seven A natural isotopes and ten artiÜcial iso- • Information from the WebElements site Z

Zeeman effect The splitting of the of an enyl complex, synthesized by lines in a spectrum when the source W.C. Zeise in 1827. of the spectrum is exposed to a mag- zeolite A natural or synthetic hy- netic Üeld. It was discovered in 1896 drated aluminosilicate with an open by Pieter Zeeman (1865–1943). In the three-dimensional crystal structure, normal Zeeman effect a single line is in which water molecules are held in split into three if the Üeld is perpen- cavities in the lattice. The water can dicular to the light path or two lines be driven off by heating and the zeo- if the Üeld is parallel to the light lite can then absorb other molecules path. This effect can be explained by of suitable size. Zeolites are used for classical electromagnetic principles in terms of the speeding up and slow- separating mixtures by selective ab- ing down of orbital electrons in the sorption – for this reason they are source as a result of the applied Üeld. often called molecular sieves. They The anomalous Zeeman effect is a are also used in sorption pumps for complicated splitting of the lines into vacuum systems and certain types several closely spaced lines, so called (e.g. Permutit) are used in ion- because it does not agree with classi- exchange (e.g. water-softening). cal predictions. This effect is ex- zepto- Symbol z. A preÜx used in plained by quantum mechanics in the metric system to indicate 10–21. terms of electron spin. For example, 10–21 second = 1 zepto- A second (zs). • Pieter Zeeman’s original paper zero order See order. Zeisel reaction A method of deter- zero-point energy The energy re- mining the number of methoxy maining in a substance at the *ab- (–OCH3) groups in an organic com- solute zero of temperature (0 K). This pound. The compound is heated wih is in accordance with quantum excess hydriodic acid, forming an al- theory, in which a particle oscillating cohol and iodomethane: with simple harmonic motion does → R–O–CH3 + HI ROH + CH3I not have a stationary state of zero ki- The iodomethane is distilled off and netic energy. Moreover, the *uncer- led into an alcoholic solution of sil- tainty principle does not allow such a ver nitrate, where it precipitates sil- particle to be at rest at exactly the ver iodide. This is Ültered and centrepoint of its oscillations. weighed, and the number of iodine zeroth law of thermodynamics atoms and hence methoxy groups See thermodynamics. can be calculated. The method was developed by S. Ziesel in 1886. zetta- Symbol Z. A preÜx used in the metric system to indicate 1021. Zeise’s salt A complex of platinum For example, 1021 metres = 1 zetta- and ethene, PtCl (CH CH ), in which 3 2 2 metre (Zm). the Pt coordinates to the pi bond of the ethene. It was the Ürst example Ziegler process An industrial 563 zinc oxide process for the manufacture of high- to give *zincates. Most of its com- density polyethene using catalysts of pounds contain the Zn2+ ion. titanium(IV) chloride (TiCl4) and alu- A minium alkyls (e.g. triethylalu- • Information from the WebElements site minium, Al(C2H5)3). The process was introduced in 1953 by the German zincate A salt formed in solution by chemist Karl Ziegler (1898–1973). It dissolving zinc or zinc oxide in alkali. 2– allowed the manufacture of poly- The formula is often written ZnO2 thene at lower temperatures (about although in aqueous solution the 60°C) and pressures (about 1 atm.) ions present are probably complex 2+ than used in the original process. ions in which the Zn is coordinated – 2– Moreover, the polyethene produced to OH ions. ZnO2 ions may exist in had more straight-chain molecules, molten sodium zincate, but most giving the product more rigidity and solid ‘zincates’ are mixed oxides. a higher melting point than the ear- zinc blende A mineral form of lier low-density polyethene. The reac- *zinc sulphide, ZnS, the principal ore tion involves the formation of a of zinc (see sphalerite). titanium alkyl in which the titanium zinc-blende structure See spha- can coordinate directly to the pi lerite structure. bond in ethene. In 1954 the process was developed zinc chloride A white crystalline further by the Italian chemist Giulio compound, ZnCl2. The anhydrous Natta (1903–79), who extended the salt, which is deliquescent, can be use of Ziegler’s catalysts (and similar made by the action of hydrogen chlo- catalysts) to other alkenes. In particu- ride gas on hot zinc; r.d. 2.9; m.p. lar he showed how to produce stereo- 283°C; b.p. 732°C. It has a relatively speciÜc polymers of propene. low melting point and sublimes easily, indicating that it is a molecular zinc Symbol Zn. A blue-white metal- compound rather than ionic. Various lic element; a.n. 30; r.a.m. 65.38; r.d. ° ° hydrates also exist. Zinc chloride is 7.1; m.p. 419.88 C; b.p. 907 C. It oc- used as a catalyst, dehydrating agent, curs in sphalerite (or zinc blende, and Ûux for hard solder. It was once ZnS), which is found associated with known as butter of zinc. the lead sulphide, and in smithsonite zinc chloride cell See dry cell. (ZnCO3). Ores are roasted to give the oxide and this is reduced with carbon zinc group The group of elements (coke) at high temperature, the zinc in the periodic table consisting of vapour being condensed. Alterna- zinc (Zn), cadmium (Cd), and mer- tively, the oxide is dissolved in sul- cury (Hg). See group 2 elements. phuric acid and the zinc obtained by electrolysis. There are Üve stable iso- zincite A mineral form of *zinc topes (mass numbers 64, 66, 67, 68, oxide, ZnO. and 70) and six radioactive isotopes zinc oxide A powder, white when are known. The metal is used in gal- cold and yellow when hot, ZnO; r.d. vanizing and in a number of alloys 5.606; m.p. 1975°C. It occurs natu- (brass, bronze, etc.). Chemically it is a rally as a reddish orange ore zincite, reactive metal, combining with oxy- and can also be made by oxidizing gen and other nonmetals and react- hot zinc in air. It is amphoteric, ing with dilute acids to release forming *zincates with bases. It is z hydrogen. It also dissolves in alkalis used as a pigment (Chinese white) zinc sulphate 564

and a mild antiseptic in zinc oint- (ZrSiO4; the main source) and in bad- ments. An archaic name is philoso- deleyite (ZnO2). Extraction is by chlo- pher’s wool. rination to give ZrCl4 which is puriÜed by solvent extraction and re- zinc sulphate A white crystalline duced with magnesium (Kroll water-soluble compound made by Ü heating zinc sulphide ore in air and process). There are ve natural iso- dissolving out and recrystallizing the topes (mass numbers 90, 91, 92, 94, sulphate. The common form is the and 96) and six radioactive isotopes are known. The element is used in heptahydrate, ZnSO4.7H2O; r.d. 1.9. This loses water above 30°C to give nuclear reactors (it is an effective the hexahydrate and more water is neutron absorber) and in certain al- ° loys. The metal forms a passive layer lost above 70 C to form the monohy- ° drate. The anhydrous salt forms at of oxide in air and burns at 500 C. 280°C and this decomposes above Most of its compounds are complexes 500°C. The compound, which was of zirconium(IV). Zirconium(IV) oxide formerly called white vitriol, is used (zirconia) is used as an electrolyte in fuel cells. The element was identiÜed as a mordant and as a styptic (to Ü check bleeding). in 1789 by Klaproth and was rst isolated by Berzelius in 1824. zinc sulphide A yellow-white A water-soluble solid, ZnS. It occurs • Information from the WebElements site naturally as *sphalerite (see also zinc blende) and wurtzite. The compound zirconium(IV) oxide See zirco- sublimes at 1180°C. It is used as a nium. pigment and phosphor. Z-isomer See e–z convention. zircon A naturally occurring silicate zone reÜning A technique used to of zirconium, ZrSiO4, used as a gem- reduce the level of impurities in cer- stone. The colour depends in small tain metals, alloys, semiconductors, amounts of other metals and may be and other materials. It is based on red, brown, yellow, or green. Red the observation that the solubility of gem-quality zircon is sometimes an impurity may be different in the called jacinth; gem-quality zircons liquid and solid phases of a material. with other colours are called jar- To take advantage of this observa- goons. There is also a naturally oc- tion, a narrow molten zone is moved curring colourless variety. Zircon along the length of a specimen of the gems can be given other colours, or material, with the result that the im- made colourless, by heat treatment. purities are segregated at one end of The colourless varieties (either nat- the bar and the pure material at the ural or treated) are sometimes called other. In general, if the impurities Matura diamonds (after Matura in Sri lower the melting point of the ma- Lanka). The name ‘zircon’ is often er- terial they are moved in the same di- roneously applied to a synthetic form rection as the molten zone moves, * of the oxide cubic zircona, which is and vice versa. used as a diamond substitute. zwitterion (ampholyte ion) An ion zirconia zirconium See . that has a positive and negative zirconium Symbol Zr. A grey-white charge on the same group of atoms. metallic *transition element; a.n. 40; Zwitterions can be formed from com- z r.a.m. 91.22; r.d. 6.49; m.p. 1852°C; pounds that contain both acid groups b.p. 4377°C. It is found in zircon and basic groups in their molecules. 565 zwitterion

For example, aminoethanoic acid Aminoethanoic acid, as a conse- (the amino acid glycine) has the for- quence, has some properties charac- mula H2N.CH2.COOH. However, teristic of ionic compounds; e.g. a under neutral conditions, it exists in high melting point and solubility in the different form of the zwitterion water. In acid solutions, the positive + – + H3N.CH2.COO , which can be re- ion H3NCH2COOH is formed. In garded as having been produced by basic solutions, the negative ion – an internal neutralization reaction H2NCH2COO predominates. The (transfer of a proton from the car- name comes from the German zwei, boxyl group to the amino group). two.

Appendices

569 Appendix 1

Appendix 1. The Greek alphabet

Letters Name Letters NameAA

A ␣ alpha ⌵␯ nu 〉␤ beta ⌶␰ xi ⌫␥ gamma ⌷␱ omicron ⌬␦ delta ⌸␲ pi ⌭⑀ epsilon ⌹␳ rho ⌮␨ zeta ⌺␴ sigma ⌯␩ eta ⌻␶ tau ⍜␪ theta ⌼␷ upsilon ⌱␫ iota ⌽␾ phi ⌲␬ kappa ⌾␹ chi ⌳␭ lambda ⌿␺ psi ⌴␮ mu ⍀␻ omega

Appendix 2. Fundamental constants

Constant Symbol Value in SI unitsAAAAAAAJJ acceleration of free fall g 9.806 65 m s–2 23 –1 Avogadro constant L, NA 6.022 141 79(30) × 10 mol –23 –1 Boltzmann constant k = R/NA 1.380 6504(24) × 10 J K –12 –1 electric constant ε0 8.854 187 817 × 10 F m electronic charge e 1.602 176 487(40) × 10–19 C –31 electronic rest mass me 9.109 382 15(45) × 10 kg Faraday constant F 9.648 3399(24) × 104 C mol–1 gas constant R 8.314 472(15) J K–1 mol–1 gravitational constant G 6.674 28(67) × 10–11 m3 kg–1 s–2 25 –3 Loschmidt’s constant NL 2.686 7774(47) × 10 m –7 –1 magnetic constant µ0 4π×10 H m –27 neutron rest mass mn 1.674 927 211(84) × 10 kg Planck constant h 6.626 068 96(33) × 10–34 J s –27 proton rest mass mp 1.672 621 637(83) × 10 kg speed of light c 2.997 924 58 × 108 m s–1 Stefan–Boltzmann constant σ 5.670 400(40) × 10–8 W m–2 K–4