Facts on File DICTIONARY of INORGANIC CHEMISTRY

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The Facts On File DICTIONARY of INORGANIC CHEMISTRY

Edited by John Daintith

® The Facts On File Dictionary of Inorganic Chemistry

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The Facts on File dictionary of inorganic chemistry / edited by John Daintith. p. cm. Includes bibliographical references. ISBN 0-8160-4926-2 (alk. paper). 1. Chemistry—Dictionaries. I. Title: Dictionary of inorganic chemistry. II. Daintith, John.

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This book is printed on acid-free paper CONTENTS

Preface vii

Entries A to Z 1

Appendixes I. The Periodic Table 244 II. The Chemical Elements 245 III. The Greek Alphabet 247 IV. Fundamental Constants 247 V. Webpages 248

Bibliography 248

PREFACE

This dictionary is one of a series covering the terminology and concepts used in important branches of science. The Facts on File Dictionary of Inorganic Chemistry has been designed as an additional source of information for students taking Advanced Placement (AP) Science courses in high schools. It will also be helpful to older students taking introductory college courses.

This volume covers inorganic chemistry and includes basic concepts in physical chemistry, classes of compound, reaction mechanisms, and many important named inorganic compounds. The entries on individual chemical elements are designed to give a basic survey of the chemistry of the element. The definitions are intended to be clear and informative and, where possible, we have illustrations of chemical structures. The book also has a selec- tion of short biographical entries for people who have made important contributions to the field. There are appendixes providing a list of all the chemical elements and a periodic table. A short list of useful webpages and a bibliography are also included.

The book will be a helpful additional source of information for anyone studying the AP Chemistry course, especially the section on Descriptive Chemistry. It will also be useful to students of metallurgy and other related fields.

ACKNOWLEDGMENTS

Contributors

John O. E. Clark B.Sc. Richard Rennie B.Sc., Ph.D. Tom Shields B.Sc.

vii

AAS See atomic absorption spec- sorb gases or liquids often have a porous troscopy. structure. The absorption of gases in solids is sometimes called sorption. Compare ad- absolute temperature Symbol: T A sorption. temperature defined by the relationship: T = θ + 273.15 absorption indicator (adsorption indica- where θ is the Celsius temperature. The ab- tor) An indicator used for titrations that solute scale of temperature was a funda- involve a precipitation reaction. The mental scale based on Charles’ law applied method depends upon the fact that at the to an ideal gas: equivalence point there is a change in the αθ V = V0(1 + ) nature of the ions absorbed by the precipi- θ where V is the volume at temperature , V0 tate particles. Fluorescein – a fluorescent the volume at 0, and α the thermal expan- compound – is commonly used. For exam- sivity of the gas. At low pressures, when ple, in the titration of sodium chloride so- real gases show ideal behavior, α has the lution with added silver nitrate, silver value 1/273.15. Therefore, at θ = –273.15 chloride is precipitated. Sodium ions and the volume of the gas theoretically be- chloride ions are absorbed in the precipi- comes zero. In practice, of course, sub- tate. At the end point, silver ions and ni- stances become solids at these trate ions are in slight excess and silver ions temperatures. Nevertheless, the extrapola- are then absorbed. If fluorescein is present, tion can be used to create a scale of tem- negative fluorescein ions absorb in prefer- perature on which –273.15 degrees Celsius ence to nitrate ions, producing a pink com- (°C) corresponds to zero (0°). This scale plex. was also known as the ideal-gas scale; on it temperature interval units were called de- absorption spectrum See spectrum. grees absolute (°A) or degrees Kelvin (°K), and were equal in size to the Celsius de- abundance 1. The relative amount of a gree. It can be shown that the absolute tem- given element among others; for example, perature scale is identical to the the abundance of oxygen in the Earth’s THERMODYNAMIC TEMPERATURE scale, on crust is approximately 50% by mass. which the temperature interval unit is the 2. The amount of a nuclide (stable or kelvin. radioactive) relative to other nuclides of the same element in a given sample. The absolute zero The zero value of ther- natural abundance is the abundance of a modynamic temperature; 0 kelvin or nuclide as it occurs in nature. For instance, –273.15 degrees Celsius. chlorine has two stable isotopes of masses 35 and 37. The abundance of 35Cl is absorption A process in which a gas is 75.5% and that of 37Cl is 24.5%. For some taken up by a liquid or solid, or in which a elements the abundance of a particular nu- liquid is taken up by a solid. In absorption, clide depends on the source. the substance absorbed goes into the bulk of the absorping material. Solids that ab- acac Abbreviation for the bidentate

1 accelerator

H 2 achiral Describing a molecule that does CH3 C CH 3 not exhibit optical activity. See chirality. C C acid A substance than contains hydro- O O gen and dissociates in solution to give hydrogen ions: HA ˆ H+ + A– More accurately, the hydrogen ion is sol- H vated (a hydroxonium ion): ˆ + – CH3 C CH3 HA + H2O H3O + A C C Strong acids are completely dissociated in _ water. Examples are sulfuric acid and tri- choloroethanoic acid. Weak acids are only O O partially dissociated. Most organic car- Acac: the bidentate acetylacetonato ligand boxylic acids are weak acids. In distinction formed from a diketone to an acid, a base is a compound that produces hydroxide ions in water. Bases are ei- acetyacetonato ligand, derived from acetyl- ther ionic hydroxides (e.g. NaOH) or acetone (CH3COCH2COCH3). compounds that form hydroxide ions in water. These may be metal oxides, for ex- accelerator A CATALYST added to in- ample: crease the rate at which a chemical reaction → + – Na2O + H2O 2Na + 2OH occurs. Ammonia, amines, and other nitrogenous compounds can also form OH– ions in acceptor The atom or group to which a water: pair of electrons is donated in a coordinate ˆ + – NH3 + H2O NH4 + OH bond. Pi-acceptors are compounds or As with acids, strong bases are completely groups that accept electrons into pi, p or d dissociated; weak bases are partially disso- orbitals. ciated. This idea of acids and bases is known as accumulator (secondary cell; storage bat- the Arrhenius theory (named for the tery) An electric cell or battery that can Swedish physical chemist Svante August be charged by passing an electric current Arrhenius (1859–1927). through it. Because the chemical reaction In 1923 the Arrhenius idea of acids and in the cell is reversible, current passed bases was extended by the British chemist through it in the opposite direction to Thomas Martin Lowry (1874–1936) and, which it supplies current will convert the independently, by the Danish physical reaction products back into their original chemist Johannes Nicolaus Brønsted forms. The most common example is the (1879–1947). In the Lowry–Brønsted lead-acid battery used in automobiles and theory an acid is a compound that can do- other vehicles powered by internal com- nate a proton and a base is a compound bustion engines. that can accept a proton. Proton donators are called Brønsted acids (or protic acids) acetate See ethanoate. and proton acceptors are called Brønsted bases. For example, in the reaction: ˆ – acetic acid See ethanoic acid. CH3COOH + H2O CH3COO + + H3O acetyacetonato See acac. the CH3COOH is the acid, donating a proton H+ to the water molecule. The water is acetylene See ethyne. the base because it accepts the proton. In + the reverse reaction, the H3O ion is the Acheson process See carbon. acid, donating a proton to the base

2 actinic radiation

– CH3COO . If two species are related by acidic hydrogen A hydrogen atom in a loss or gain or a proton they are described molecule that enters into a dissociation as conjugate. So, in this example, equilibrium when the molecule is dissolved – CH3COO is the conjugate base of the acid in a solvent. For example, in ethanoic acid CH3COOH and CH3COOH is the conju- (CH3COOH) the acidic hydrogen is the – gate acid of the base CH3COO . one on the carboxyl group, –COOH. In a reaction of an amine in water, for example: acidic oxide An oxide of a nonmetal ˆ + – R3N + H2O R3NH + OH that reacts with water to produce an acid The amine R3N accepts a proton from or with a base to produce a salt and water. water and is therefore acting as a base. For example, sulfur(VI) oxide (sulfur triox- + R3NH is its conjugate acid. Water donates ide) reacts with water to form sulfuric acid: → the proton to the R3N and, in this case, SO3 + H2O H2SO4 + water is acting as an acid (H3O is its con- and with sodium hydroxide to produce jugate base). Note that water can act as sodium sulfate and water: → both an acid and a base depending on the SO3 + NaOH Na2SO4 + H2O circumstances. It can accept a proton (from See also amphoteric; basic oxide. CH3COOH) and donate a proton (to R3N). Compounds of this type are de- acidic salt See acid salt. scribed as amphiprotic. One important aspect of the Lowry– acidimetry A volumetric analysis or Brønsted theory is that, because it involves acid-base titration in which a standard so- proton transfers, it does not necessarily lution of an acid is gradually added to the have to involve water. It is possible to de- unknown (base) solution containing an in- scribe reactions in nonaqueous solvents, dicator. In the converse procedure, alka- such as liquid ammonia, in terms of limetry, the standard solution is of a base acid–base reactions. and the unknown solution is acidic. A further generalization of the idea of acids and bases was the Lewis theory put acidity constant See dissociation con- forward, also in 1923, by the US physical stant. chemist Gilbert Newton Lewis (1875– 1946). In this, an acid (a Lewis acid) is a acid rain See pollution. compound that can accept a pair of electrons and a base (a Lewis base) is one that acid salt (acidic salt) A salt in which donates a pair of electrons. there is only partial replacement of the acidic hydrogen of an acid by metal or acid-base indicator An indicator that is other cations. For polybasic acids the for- either a weak base or a weak acid and mulae for such salts are of the type whose dissociated and undissociated forms NaHSO4 (sodium hydrogensulfate) and differ markedly in color. The color change Na3H(CO3)2.2H2O (sodium sesquicarbon- must occur within a narrow pH range. Ex- ate). For monobasic acids such as HF the amples are METHYL ORANGE and PHENOLPH- acid salts are of the form KHF2 (potassium THALEIN. hydrogen difluoride). Although monobasic acid salts were at one time formulated as acidic Having a tendency to release a normal salts plus excess acid (i.e. KF.HF), proton or to accept an electron pair from a it is preferable to treat them as hydrogen- donor. In aqueous solutions the pH is a bonded systems of the type K+(F–H–F)–. measure of the acidity, i.e. an acidic solution is one in which the concentration of actinic radiation Radiation that can + H3O exceeds that in pure water at the cause a chemical reaction; for example, ul- same temperature; i.e. the pH is lower than traviolet radiation is actinic. See also pho- 7. A pH of 7 is regarded as being neutral. tochemistry.

3 actinides actinides See actinoids. activated complex See transition state. actinium A soft, silvery-white, highly activated complex theory A theory of radioactive metallic element of group 3 chemical reactions in which the rate at (formerly IIIB) of the periodic table. It is which chemical reactions take place is re- usually considered to be the first member lated to the rate at which the transition of the ACTINOID series. It occurs in minute state (activated complex) is converted into quantities in uranium ores as a result of the products. Activated complex theory is natural radioactive decay of 235U. The sometimes known as TRANSITION STATE THEORY metal can be obtained by reducing AcF3 . It was put forward by Henry with lithium or it can be produced by bom- Eyring in 1935. barding radium with neutrons. It is used as a source of alpha particles and has also ...... been used to generate thermoelectric A B C power. The metal glows in the dark; it reacts with water to produce hydrogen.

Symbol: Ac; m.p. 1050±50°C; b.p. energy 3200±300°C; r.d. 10.06 (20°C); p.n. 89; most stable isotope 227Ac (half-life 21.77 AB ؉ C years); other isotopes have very short half- H lives. AB ؉ C actinoid contraction The decrease in the atomic or ionic radius that occurs in the ti di t Activation energy actinoids as the atomic number increases from actinium through nobelium. The in- activation energy Symbol: E The min- crease in atomic number in the actinoids is a imum energy that a particle, molecule, ion, associated with the filling of the inner 5f etc. must acquire before it can react; i.e. the subshell. It is similar to the LANTHANOID energy required to initiate a reaction re- CONTRACTION. gardless of whether the reaction is exothermic or endothermic. Activation energy is actinoids (actinides) A group of 15 often represented as an energy barrier that radioactive elements whose electronic con- must be overcome if a reaction is to take figurations display filling of the 5f level. As place. See Arrhenius equation. with the lanthanoids, the first member, ac- 1 2 tinium, has no f electrons (Ac [Rn]6d 7s ) activator See promoter. but other members also show deviations from the smooth trend of f-electron filling active mass See mass action. expected from simple considerations, e.g. 2 2 thorium Th [Rn]6d 7s , berkelium Bk activity 1. Symbol: a A corrective con- 8 1 2 [Rn]5f 6d 7s . The actinoids are all radio- centration or pressure factor introduced active and their chemistry is often ex- into equations that describe real solvated tremely difficult to study. The first eight, systems. Certain thermodynamic proper- actinium, thorium, protactinium, uranium, ties of a solvated substance are dependent neptunium, plutonium, americium, and on its concentration (e.g. its tendency to curium occur naturally, although with the react with other substances). Real sub- exception of thorium and uranium only in stances show departures from ideal behav- trace amounts. The others are generated by ior and thus require such correction artificial methods using high-energy bom- factors. bardment. See also transuranic elements. 2. Symbol: A The average number of atoms disintegrating per unit time in a radioactive activated charcoal See charcoal. substance.

4 alchemy activity coefficient Symbol: f A meas- solid surfaces take up layers of gas from the ure of the degree of deviation from ideality surrounding atmosphere. The adsorbed of a solvated substance, defined as: layer may be held by chemical bonds a = fc (chemisorption) or by weaker van der where a is the activity and c the concentra- Waals forces (physisorption). tion. For an ideal solute f = 1; for real sys- Compare absorption. tems f can be less or greater than unity. adsorption indicator See absorption acyclic Describing a compound that is indicator. not cyclic (i.e. a compound that does not contain a ring in its molecules). aerosol See sol. addition reaction A reaction in which AES See atomic emission spectroscopy. additional atoms or groups of atoms are introduced into an unsaturated organic com- affinity The extent to which one sub- pound, such as an alkene or ketone. A stance is attracted to or reacts with an- simple example is the addition of bromine other. across the double bond in ethene: → H2C:CH2 + Br2 BrH2CCH2Br afterdamp See firedamp. Addition reactions can be induced either by electrophiles, which are ions or agate A hard microcrystalline form of molecules that are electron deficient and the mineral chalcedony, which is a variety can therefore accept electrons, or by nucle- of quartz. Typically agate has greenish or ophiles, which are ions or molecules that brownish bands of coloration, and is used can donate electrons. for making jewelry and ornaments. Moss agate is not banded, but has mosslike pat- adduct See coordinate bond. terns resulting from the presence of iron and manganese oxides. Agate is also used adiabatic change A change during in instrument bearings, because of its resis- which no heat enters or leaves the system. tance to wear. In an adiabatic expansion of a gas, mechanical work is done by the gas as its vol- air The mixture of gases that surrounds ume (V) increases, its pressure (p) the Earth. At sea level the composition of decreases, and its temperature (T) falls. For dry air, by volume, is nitrogen 78.08%, an ideal gas undergoing a reversible adia- oxygen 20.95%, argon 0.93%, carbon batic change it can be shown that dioxide 0.03%, neon 0.0018%, helium γ pV = K1 0.0005%, krypton 0.0001%, and xenon γ 1–γ T p = K2 0.00001%. γ–1 and TV = K3 Air also contains a variable amount of γ where K1, K2, and K3 are constants and is water vapor, as well as particulate matter the ratio of the principal specific heat ca- (e.g. dust and pollen) and small amounts of pacities. Compare isothermal change. other gases. adsorbate A substance that is adsorbed air gas See producer gas. on a surface. See adsorption. alabaster A dense, translucent, fine- adsorbent The substance on whose sur- grained mineral form of dihydrate CALCIUM face adsorption takes place. See adsorp- SULFATE (CaSO4.2H2O). Due to its softness tion. it is often carved and polished to make ornaments or works of art. adsorption A process in which a layer of atoms or molecules of one substance alchemy An ancient pseudoscience that forms on the surface of a solid or liquid. All was the precursor of chemistry, dating

5 alcohol from early Christian times until the 17th follow from these facts. For example, century. It combined mysticism and exper- lithium reacts in a fairly controlled way imental techniques. Many ancient al- with water, sodium ignites, and potassium chemists searched for the philosopher’s explodes. There is also a general decrease stone – a substance that could transmute in the following: melting points, heats of base metals into gold and produce the sublimation, lattice energy of salts, hydra- elixir of life, a universal remedy for all ills. tion energy of M+, ease of decomposition of nitrates and carbonates, and heat of for- alcohol A type of organic compound of mation of the ‘-ide’ compounds (fluoride, the general formula ROH, where R is a hy- hydride, oxide, carbide, chloride). drocarbon group. Examples of simple alco- Lithium has the smallest ion and there- hols are methanol (CH3OH) and ETHANOL fore has the highest charge/size ratio and is (C2H5OH). polarizing with a tendency towards cova- By definition alcohols have one or more lent character in its bonding; the remaining –OH groups attached to a carbon atom elements form typical ionic compounds in that is not part of an aromatic ring. Thus, which ionization, M+X– is regarded as C6H5OH, in which the –OH group is at- complete. The slightly anomalous position tached to the ring, is a phenol whereas of lithium is illustrated by the similarity of phenylmethanol (C6H5CH2OH) is an alco- its chemistry to that of magnesium, in ac- hol. cordance with their diagonal relationship Common alcohols are used as solvents, in the periodic table. For example, lithium denaturing agents, chemical feedstocks, hydroxide is much less soluble than the hy- and in antifreeze preparations. Ethanol is droxides of the other group 1 elements; the intoxicating ingredient in alcoholic lithium perchlorate is soluble in several or- beverages. Propane-1,2,3,-triol (glycerol) is ganic solvents. Because of the higher lattice used to make polymers, cosmetic emol- energies associated with smaller ions lients, and sweeteners. lithium hydride and nitride are fairly stable compared to NaH, which decomposes at ° alkali A water-soluble strong base. 345 C. Na2N, K3N etc., are not obtained Strictly the term refers to the hydroxides of pure and decompose below room tempera- the alkali metals (group 1, formerly sub- ture. group IA) only, but in common usage it The oxides also display the trend in refers to any soluble base. Thus borax so- properties as lithium forms M2O with only lution may be described as mildly alkaline. traces of M2O2, sodium forms M2O2 and at high temperatures and pressures MO2, alkali metals (group 1 elements) A potassium, rubidium, and cesium form group of soft reactive metals, each repre- M2O2 if oxygen is restricted but MO2 if senting the start of a new period in the pe- burnt in air. Hydrolysis of the oxides or di- riodic table and having an electronic rect reaction of the metal with water leads configuration consisting of a rare-gas to the formation of the hydroxide ion. structure plus one outer electron. The al- Salts of the bases MOH are known for kali metals are lithium (Li), sodium (Na), all acids and these are generally white crys- potassium (K), rubidium (Rb), cesium (Cs), talline solids. The ions M+ are hydrated in and francium (Fr). They formerly were water and remain unchanged in most reac- classified in subgroup IA of the periodic tions of alkali metal salts. table. Because of the ease of formation of the The elements all easily form positive ions M+ there are very few complexes of + + ions M and consequently are highly reac- the type MLn apart from solvated species tive (particularly with any substrate that is of very low correlation times. oxidizing). As the group is descended there Francium is formed only by radioactive is a gradual decrease in ionization potential decay and in nuclear reactions; all the iso- and an increase in the size of the atoms; the topes of francium have short half-lives, the group shows several smooth trends that longest of which (223Fr) is 21 minutes. The

6 alloy few chemical studies that have been carried for barium the peroxide BaO2 in addition out on francium indicate that it has similar to BaO. The heavier oxides, CaO, SrO, and properties to those of the other alkali met- BaO, react with water to form hydroxides, als. M(OH)2; magnesium oxide reacts only at high temperatures and beryllium oxide not alkalimetry See acidimetry. at all. The metals Ca, Sr, and Ba all react readily with water to give the hydroxide: → 2+ – alkaline earth See alkaline-earth met- M + 2H2O M + 2OH + H2 als. In contrast, magnesium requires dilute acids in order to react (to the salt plus hy- alkaline-earth metals (group 2 el- drogen), and beryllium is resistant to acid ements) A group of moderately reactive attack. A similar trend is seen in the direct metals, harder and less volatile than the al- reaction of hydrogen: under mild condi- kali metals. They were formerly classified tions calcium, strontium, and barium give in subgroup IIA of the periodic table. The ionic hydrides, high pressures are required term alkaline earth strictly refers to the ox- to form magnesium hydride, and beryllium ides, but is often used loosely for the el- hydride can not be prepared by direct com- ements themselves. The electronic bination. configurations are all those of a rare-gas Because of its higher polarizing power, structure with an additional two electrons beryllium forms a range of complexes in in the outer s orbital. The elements are which the beryllium atom should be beryllium (Be), magnesium (Mg), calcium treated as an electron acceptor (i.e. the va- (Ca), strontium (Sr), barium (Ba), and ra- cant p orbitals are being used). Complexes dium (Ra). The group shows an increasing such as etherates, acetylethanoates, and the tendency to ionize to the divalent state tetrafluoride (BeF 2–) are formed, all of M2+. The first member, beryllium, has a 4 which are tetrahedral. In contrast Mg2+, much higher ionization potential than the Ca2+, Sr2+, and Ba2+ have poor acceptor others and the smallest atomic radius. properties and form only weak complexes, Thus it has a high charge/size ratio and even with donors such as ammonia or edta. consequently the bonding in beryllium compounds is largely covalent. The chem- All isotopes of radium are radioactive and radium was once widely used for istry of the heavier members of the group is 226 largely that of divalent ions. radiotherapy. The half-life of Ra 238 The group displays a typical trend to- (formed by decay of U) is 1600 years. ward metallic character as the group is descended. For example, beryllium allotropy The ability of certain elements hydroxide is amphoteric; magnesium hy- to exist in more than one physical form (al- droxide is almost insoluble in water and is lotrope). Carbon, sulfur, and phosphorus slightly basic; calcium hydroxide is spar- are the most common examples. Allotropy ingly soluble and distinctly basic; and is more common in groups 14, 15, and 16 strontium and barium hydroxides are in- of the periodic table than in other groups. creasingly soluble in water and strongly See also enantiotropy; monotropy. Com- basic. The group also displays a smooth pare polymorphism. trend in the solubilities of the sulfates (MgSO4 is soluble, CaSO4 sparingly solu- alloy A mixture of two or more metals BRONZE BRASS ble, and BaSO4 very insoluble). The trend (e.g. or ) or a metal with to increasing metallic character is also small amounts of nonmetals (e.g. STEEL). shown by the increase in thermal stabilities Alloys may be completely homogeneous of the carbonates and nitrates with increas- mixtures or may contain small particles of ing relative atomic mass. one PHASE in the other phase. Alloys are The elements all burn in air (beryllium stronger, harder, and often more corrosion must be finely powdered) to give the oxide resistant than their components, but ex- MO (covalent in the case of beryllium) and hibit reduced ductility and lower electrical

7 Alnico conductivity. Most metals encountered in are numerous minerals of aluminum; it is everyday life are actually alloys. the most common metallic element in the Earth’s crust (8.1% by mass) and the third Alnico (Trademark) Any of a group of in order of abundance. Commercially im- very hard brittle alloys used to make pow- portant minerals are bauxite (hydrated erful permanent magnets. They contain Al2O3), corundum (anhydrous Al2O3), nickel, aluminum, cobalt, and copper in cryolite (sodium hexafluroaluminate various proportions. Iron, titanium, and Na3AlF6), and clays and mica (aluminosil- niobium can also be present. They magne- icates). tize strongly when exposed to an exciting The metal is produced on a massive magnetic field and resist demagnitization scale by the Hall-Heroult method in which even when exposed to a reverse magnetiz- aluminum oxide, a nonelectrolyte, is dis- ing force. solved in molten cryolite and electrolyzed in a large cell. The bauxite contains iron 2+ alpha particle A He ion emitted with oxide and other impurities, which would high kinetic energy by a radioactive sub- contaminate the product, so the bauxite is stance undergoing alpha decay. Alpha par- dissolved in hot alkali, the impurities are ticles are emitted at high velocity, and are removed by filtration, and the pure alu- used to cause nuclear disintegration reac- minum oxide then precipitated by acidifi- tions. cation. In the cell, molten aluminum is tapped off from the base and carbon diox- alum A type of double salt. Alums are ide evolved at the graphite anodes, which double sulfates obtained by crystallizing are consumed in the process. The alu- mixtures in the correct proportions. They minum atom is much bigger than boron have the general formula: (the first member of group 13) and its ion- M SO .M′ (SO ) .24H O 2 4 2 4 3 2 ization potential is not particularly high. where M is a univalent metal or ion, and Consequently aluminum forms positive M′ is a trivalent metal. Thus, ALUMINUM Al3+ ions. However, aluminum also has POTASSIUM SULFATE (called potash alum, or nonmetallic chemical properties. Thus, it is simply alum) is amphoteric and also forms a number of K2SO4.Al2(SO4)3.24H2O and aluminum ammonium sulfate (called covalently bonded compounds. ammonium alum) is Unlike boron, aluminum does not form a vast range of hydrides – AlH3 and Al2H6 (NH4)2SO4.Al2(SO4)3.24H2O The name alum originally came from may exist at low pressures, and the only the presence of Al3+ as the trivalent ion, but stable hydride, (AlH3)n, must be prepared is now also applied to other double salts by reduction of aluminum trichloride. The – containing trivalent ions, thus, ion AlH4 is widely used in the form of chromium(III) potassium sulfate (chrome LiAlH4 as a powerful reducing agent. alum) is The reaction of aluminum metal with oxygen is very exothermic but at ordinary K2SO4.Cr2(SO4)3.24H2O temperatures an impervious film of the alumina See aluminum oxide. oxide protects the bulk metal from further attack. This oxide film also protects alu- aluminate See aluminum hydroxide. minum from oxidizing acids. There is only one oxide, Al2O3, but a variety of poly- aluminosilicate See silicates. morphs and hydrates are known. It is relatively inert and has a high melting point, aluminum A soft moderately reactive and for this reason is widely used as a fur- metal; the second element in group 13 (for- nace lining and for general refractory merly IIIA) of the periodic table. Alu- brick. Aluminum metal will react with al- minum has the electronic structure of neon kalis, releasing hydrogen to initially pro- – plus three additional outer electrons. There duce Al(OH)3, then Al(OH)4 . 8 aluminum oxide

Aluminum reacts readily with the halo- (CH3COO)2. It is prepared by dissolving gens; in the case of chlorine thin sheets of aluminum hydroxide in ethanoic acid and the metal will burst into flame. Aluminum is used extensively as a mordant in dyeing, fluoride has a high melting point (1290°C) as a size for paper and cardboard products, and is ionic. The other halides are dimers in and in tanning. The solution is hydrolyzed the vapor phase (two halogen bridges). and contains various complex aluminum- Aluminum also forms a sulfide (Al2S3), ni- hydroxyl species and colloidal aluminum tride (AlN), and carbide (Al4C), the latter hydroxide. two at extremely high temperatures. Because of aluminum’s ability to ex- aluminum fluoride (AlF3) A white pand its coordination number and ten- crystalline solid that is slightly soluble in dency towards covalence it forms a variety water but insoluble in most organic sol- 2– – of complexes such as AlF6 and AlCl4 . vents. Its primary use is as an additive to ° Symbol: Al; m.p. 660.37 C; b.p. the cryolite (Na3AlF6) electrolyte in the 2470°C; r.d. 2.698 (20°C); p.n. 13; r.a.m. production of aluminum. 26.981539. aluminum hydroxide (aluminate; aluminum acetate See aluminum Al(OH)3) A white powder prepared as a ethanoate. colorless gelatinous precipitate by adding ammonia solution or a small amount of aluminum bromide (AlBr3) A white sodium hydroxide solution to a solution of solid soluble in water and many organic an aluminum salt. It is an amphoteric hy- solvents. droxide and is used as a foaming agent in fire extinguishers and as a mordant in dyeing. Cl Its amphoteric nature causes it to dis- Cl Cl solve in excess sodium hydroxide solution to form the aluminate (tetrahydroxoalumi- Al Al nate(III) ion): – → – Cl Cl Al(OH)3 + OH Al(OH)4 + H2O Cl When precipitating from solution, aluminum hydroxide readily absorbs colored Aluminum chloride: the dimer Al2Cl6 matter from dyes to form lakes. aluminum chloride (AlCl3) A white co- aluminum nitrate (Al(NO3)3.9H2O) A valent solid that fumes in moist air and re- hydrated white crystalline solid prepared acts violently with water according to the by dissolving freshly prepared aluminum equation: hydroxide in nitric acid. It is used as a → AlCl3 + 3H2O Al(OH)3 + 3HCl mordant. It cannot be prepared by the ac- It is prepared by heating aluminum in tion of dilute nitric acid on aluminum be- dry chlorine or dry hydrogen chloride or cause the metal is rendered passive by a industrially by heating aluminum oxide thin surface layer of oxide. and carbon in the presence of chlorine. Vapor-density measurements show that its aluminum oxide (alumina; Al2O3)A structure is a dimer; it consists of Al2Cl6 white crystalline powder that is almost in- molecules in the vapor. Aluminum chloride soluble in water, occurring in two main is used as a catalyst in various organic reforms, one of which is weakly acidic, and actions, and in the cracking of petroleum. the other amphoteric. It occurs naturally as bauxite, corundum, and emery, and with aluminum ethanoate (aluminum acetate; minute amounts of chromium and cobalt Al(OOCCH3)3) A white solid soluble in as ruby and sapphire, respectively. It is water. It is usually obtained as the dibasic manufactured by heating aluminum hy- salt, basic aluminum ethanoate, Al(OH)- droxide. It is used in the extraction by elec-

9 aluminum potassium sulfate trolysis of aluminum, as an abrasive amatol A high explosive that consists of (corundum), in furnace linings (because of a mixture of ammonium nitrate and TNT its refractory properties), and as a catalyst (trinitrotoluene). (e.g. in the dehydration of alcohols). ambidentate ligand See isomerism. aluminum potassium sulfate (potash alum; Al2(SO4)3.K2SO4.24H2O) A white americium A highly toxic radioactive crystalline solid, soluble in water but insol- silvery element of the actinoid series of uble in alcohol, prepared by mixing solu- metals. A transuranic element, it is found tions of ammonium and aluminum sulfates naturally on Earth in trace amounts in ura- followed by crystallization. It is used as a nium ore. It can also be synthesized by 239 mordant for dyes, as a waterproofing bombarding Pu with neutrons. The agent, and as a tanning additive. metal can be obtained by reducing the trifluoride with barium. It reacts with oxy- 241 aluminum sulfate (Al (SO ) .18H O, gen, steam, and acids. Am has been used 2 4 3 2 in gamma-ray radiography and in smoke A12504) A white crystalline solid. Both the hydrated and anhydrous forms are soluble alarms. Symbol: Am; m.p. 1172°C; b.p. in water, but only the anhydrous form is 2607°C; r.d. 13.67 (20°C); p.n. 95; most soluble in ethanol, and to only a slight de- stable isotope 243Am (half-life 7.37 × 103 gree. It is used as a size for paper, a precip- years). itating agent in sewage and water treatment, a foaming agent in fire control, amethyst A purple form of the mineral and as a fireproofing agent. Its solutions quartz (silicon(IV) oxide, SiO2) used as a are acidic by hydrolysis, containing such semiprecious gemstone. The color comes 2+ species as Al(H2O)5(OH) . It is prepared from impurities such as oxides of iron. by dissolving Al(OH)3 in sulfuric acid. ammine A complex in which ammonia aluminum trimethyl See trimethylalu- molecules are coordinated to a metal ion; minum. 2+ e.g. [Cu(NH3)4] . amalgam An alloy of mercury with at ammonia (NH3) A colorless gas with a least one other metal. Amalgams may be characteristic pungent odor. On cooling liquid or solid. An amalgam of sodium and compression it forms a colorless liq- (Na/Hg) with water is used as a source of uid, which becomes a white solid on fur- NASCENT HYDROGEN: ther cooling. Ammonia is very soluble in → 2Na/Hg + 2H2O (l) 2NaOH(aq) + water (a saturated solution at 0°C contains H2(g) + Hg 36.9% of ammonia): the aqueous solution An amalgam of Ha/Ag/Sn was used in den- is alkaline and contains a proportion of tistry. free ammonia. Ammonia is also soluble in

H H H

N ˆ N ˆ N H H

H H HH

Ammonia: umbrella inversion of the molecule

10 ammonium phosphate ethanol. It occurs naturally to a small ex- to ammonia, carbon dioxide, and water. tent in the atmosphere, and is usually pro- The white solid sold commercially as am- duced in the laboratory by heating an monium carbonate is actually a double salt ammonium salt with a strong alkali. Am- of both ammonium hydrogencarbonate monia is synthesized industrially from hy- (NH4HCO3) and ammonium amino- drogen and atmospheric nitrogen by the methanoate (NH2CO2NH4). This salt is HABER PROCESS. manufactured from ammonium chloride The compound does not burn readily in and calcium carbonate. It decomposes on air, but ignites – giving a yellowish-brown exposure to damp air into ammonium hy- flame – in oxygen. It will react with atmos- drogencarbonate and ammonia, and it re- pheric oxygen in the presence of platinum acts with ammonia to give the true or a heavy metal catalyst – a reaction used ammonium carbonate. Commercial am- as the basis of the commercial manufacture monium carbonate is used in baking pow- of nitric acid, which involves the oxidation ders, smelling salts, in the dyeing and of ammonia to nitrogen monoxide and wool-scouring industries, and in cough then to nitrogen dioxide. Ammonia coordi- medicines. nates readily to form ammines and reacts with sodium or potassium to form inor- ammonium chloride (sal ammoniac; ganic amides and with acids to form am- NH4Cl) A white crystalline solid with a monium salts; for example, it reacts with characteristic saline taste. It is very soluble hydrogen chloride to form ammonium in water. Ammonium chloride can be man- chloride: ufactured by the action of ammonia on hy- → NH3(g) + HCl(g) NH4Cl(g) drochloric acid. It sublimes on heating Ammonia is used commercially in the according to the equilibrium reaction: ˆ manufacture of fertilizers, mainly ammo- NH4Cl(s) NH3(g) + HCl(g) nium nitrate, urea, and ammonium sulfate. Ammonium chloride is used in galva- It is also used to make explosives, resins, nizing, as a flux for soldering, as a mordant and dyes. As a liquefied gas it is used in the in dyeing and calico printing, and in the refrigeration industry. Liquid ammonia is manufacture of Leclanché and ‘dry’ cells. an excellent solvent for certain substances, which ionize in the solutions to give ionic ammonium hydroxide (ammonia solu- reactions similar to those occurring in tion; NH4OH) An alkali that is formed aqueous solutions. Ammonia is marketed when ammonia dissolves in water. It prob- as the liquid, compressed in cylinders (‘an- ably contains hydrated ammonia mol- + – hydrous ammonia’), or as aqueous solu- ecules as well as some NH4 and OH ions. tions of various strengths. See also It is a useful reagent and cleansing agent. ammonium hydroxide. + ammonium ion The ion NH4 , formed + ammoniacal Describing a solution in by coordination of NH3 to H . It has tetra- aqueous ammonia. hedral symmetry. ammonia-soda process See Solvay ammonium nitrate (NH4NO3) A col- process. orless crystalline solid that is very soluble in water and also soluble in ethanol. It is ammonia solution See ammonium hy- usually manufactured by the action of am- droxide. monia gas on nitric acid. It is used in fertilizers because of its high nitrogen content, ammonium alum See alum. and in the manufacture of explosives and rocket propellants. ammonium carbonate (sal volatile; (NH4)2CO3) A white solid that crystal- ammonium phosphate (triammonium lizes as plates or prisms. It is very soluble in phosphate(V); (NH4)3PO4) A colorless water and readily decomposes on heating crystalline salt made from ammonia and

11 ammonium sulfate phosphoric(V) acid, used as a fertilizer to ties. The term is most commonly applied to add both nitrogen and phosphorus to the the oxides and hydroxides of metals that soil. can form both cations and complex anions. For example, zinc oxide dissolves in acids ammonium sulfate ((NH4)2SO4) A col- to form zinc salts and also dissolves in al- 2– orless crystalline solid that is soluble in kalis to form zincates, [Zn(OH)4] . The water but not in ethanol. When heated amino acids are also considered to be am- carefully it gives ammonium hydrogensul- photeric because they contain both acidic fate, which on stronger heating yields ni- and basic groups. trogen, ammonia, sulfur(IV) oxide (sulfur dioxide), and water. Ammonium sulfate is amu See atomic mass unit. manufactured by the action of ammonia on sulfuric acid. It is an important ammo- analysis The process of determining the nium salt because of its widespread use as constituents or components of a sample. a fertilizer. Its only drawback as a fertilizer There are two broad major classes of is that it tends to leave an acidic residue in analysis, QUALITATIVE ANALYSIS – essentially the soil. answering the question ‘what is it?’ – and QUANTITATIVE ANALYSIS – answering the amorphous Describing a solid sub- question ‘how much of such and such a stance that has no ‘long-range’ regular component is present?’ There is a vast arrangement of atoms; i.e. is not crys- number of analytical methods that can be talline. Amorphous materials can consist applied, depending on the nature of the of minute particles that possess order over sample and the purpose of the analy- very short distances. Glasses are amor- sis. These include GRAVIMETRIC ANALYSIS, phous, because the atoms in the solid have VOLUMETRIC, and systematic qualitative a random arrangement. X-ray diffraction analysis (classical wet methods); and in- analysis has shown that many substances strumental methods, such as CHROMATOG- that were once described as amorphous are RAPHY, SPECTROSCOPY, NUCLEAR MAGNETIC in fact composed of very small crystals. For RESONANCE, POLAROGRAPHY, and fluores- example, charcoal, coke, and soot (all cence techniques. forms of carbon) are made up of small graphitelike crystals. ångstrom Symbol: Å A unit of length defined as 10–10 meter, formerly used to amount of substance Symbol: n A measure wavelengths of radiation, includ- measure of the number of elementary enti- ing those of visible light, and inter-molecu- ties present in a substance. It is measured in lar distances. The preferred SI unit for such MOLES. See also Avogadro constant. measurements is the nanometer. One ångstrom equals 0.1 nanometer. The ampere Symbol: A The SI base unit of ångstrom was named for Anders Jonas electric current, defined as the constant Ångstrom (1814–74), a Swedish physicist current that, maintained in two straight and astronomer. parallel infinite conductors of negligible circular cross section placed one meter anhydride A compound formed by re- apart in vacuum, would produce a force moving water from an acid or, less com- between the conductors of 2 × 10–7 newton monly, a base. Many nonmetal oxides are per meter. anhydrides of acids: for example CO2 is the anhydride of H2CO3 and SO3 is the anhy- amphiprotic See acid; solvent. dride of H2SO4. ampholyte ion See zwitterion. anhydrite See calcium sulfate. amphoteric Describing material that anhydrous Describing a substance that can display both acidic and basic proper- lacks moisture, or a salt lacking water of

12 antimony(III) chloride crystallization. For example, on strong anode is the terminal from which electrons heating, blue crystals of copper(II) sulfate flow out of the system. pentahydrate, CuSO4.5H2O, form white anhydrous copper(II) sulfate, CuSO4. anode sludge See electrolytic refining. anion A negatively charged ion, formed anodizing An industrial process for pro- by the addition of electrons to atoms or tecting aluminum with an oxide layer. The molecules. In electrolysis anions are at- aluminum object is made the anode in an tracted to the positive electrode or anode. electrolytic cell containing an oxidizing Compare cation. acid (e.g. sulfuric(VI) acid). The layer of Al2O3 formed is porous and can be colored anionic detergent See detergents. with certain dyes. anionic resin An ION-EXCHANGE ma- anthracite The highest grade of COAL, terial that can exchange anions, such as Cl– with a carbon content of between 92% and and OH–, for anions in the surrounding 98%. It burns with a hot blue flame, gives medium. Such resins are often produced by off little smoke and leaves hardly any ash. the addition of a quaternary ammonium + antibonding orbital See orbital. group (–N(CH3)3 ) or a phenolic group (–OH–) to a stable polyphenylethene resin. A typical exchange reaction is: anti-isomer See isomerism. + – ˆ resin–N(CH3)3 Cl + KOH resin–N(CH ) +OH– + KCl antimonic Designating an antimony(V) 3 3 compound. Anionic resins can be used to separate mixtures of halide ions. Such mixtures can antimonous Designating an antimony(III) be attached to the resin and recovered sep- compound. arately by ELUTION. antimony A toxic metalloid element of anisotropic Describing certain subgroup 15 (formerly VA) of the periodic stances that have one or more physical table. It exists in three allotropic forms; the properties, such as refractive index, that most stable is a hard, brittle, silvery-blue differ according to direction. Most crystals metal. Yellow and black antimony, the are anisotropic. other two allotropes, are unstable and nonmetallic. Antimony is found in many annealing A type of heat treatment ap- minerals, principally stibnite (Sb2S3), from plied to metals to change their physical which it is recovered by reduction with properties. The metal is heated to, and held iron or by first roasting it to yield the at, an appropriate temperature before oxide. It is also a poor conductor of heat being cooled at a suitable rate to produce and electricity, making it useful in the the desired grain structure. Annealing is manufacture of semiconductors. Antimony most commonly used to remove the compounds are used in pigments, flame re- stresses that have arisen during rolling, to tardants, medical treatments, ceramics, increase the softness of the metal, and to and glass. make it easier to machine. Objects made of Symbol: Sb; m.p. 630.74°C; b.p. glass can also be annealed to remove 1750°C; r.d. 6.691; p.n. 51; r.a.m. 112.74. strains. antimony(III) chloride (antimony anode In electrolysis, the electrode that trichloride; SbCl3) A white deliquescent is at a positive potential with respect to the solid, formerly known as butter of anti- cathode, and to which anions are therefore mony. It is prepared by direct combination attracted. In any electrical system, such as of antimony and chlorine. It is readily hy- a discharge tube or electronic device, the drolyzed by cold water to form a white pre-

13 antimony(III) chloride oxide cipitate of antimony(III) chloride oxide ments associated with electron spin are (antimonyl chloride, SbOCl): aligned in opposite directions. SbCl3 + H2O = SbOCl + 2HCl apatite A common, naturally occurring antimony(III) chloride oxide See anti- phosphate of calcium, Ca5(PO4)3(OH,F,Cl), mony(III) chloride. that occurs in several color varieties. Crys- tals are hexagonal and have a greasy luster. antimonyl chloride See antimony(III) Apatite occurs in rocks of igneous and chloride. metamorphic origin and is mined as a source of phosphorus for use in fertilizers. antimony(III) oxide (antimony trioxide; Gemstone quality crystals are known from Sb2O3) A white insoluble solid. It is an several locations. amphoteric oxide with a strong tendency to act as a base. It can be prepared by di- aprotic See solvent. rect oxidation by air, oxygen, or steam and is formed when antimony(III) chloride is aqua fortis An old name for nitric acid, hydrolyzed by excess boiling water. It is HNO3. used as a flame retardant in plastics and as an additive in paints to make them more aqua regia A mixture of concentrated opaque. nitric acid and three to four parts of concentrated hydrochloric acid. With the ex- antimony(V) oxide (antimony pentox- ception of silver, with which it forms an ide; Sb O ) A yellow solid. It is usually 2 5 insoluble chloride, it dissolves all metals, formed by the action of concentrated nitric including gold. The mixture contains chlo- acid on antimony or by the hydrolysis of rine and NOCl (nitrosyl chloride). The antimony(V) chloride. Although an acidic name means ‘royal water’. oxide, it is only slightly soluble in water. It is used as a flame retardant. aqueous Describing a solution in water. antimony pentoxide See antimony(V) aragonite An anhydrous mineral form oxide. of calcium carbonate, CaCO3, which oc- antimony trichloride See antimony(III) curs associated with limestone and in some chloride. metamorphic rocks. It is also the main ingredient of pearls. It is not as stable as cal- antimony trioxide See antimony(III) cite, into which it may change over time. oxide. Pure aragonite is colorless or white, but impurities such as strontium, zinc, or lead antioxidant A substance that inhibits may tint it various colors. oxidation. Antioxidants are added to such products as foods, paints, plastics, and argentic oxide See silver(II) oxide. rubber to delay their oxidation by atmospheric oxygen. Some work by forming argentous oxide See silver(I) oxide. CHELATES with metal ions, thus neutralizing the catalytic effect of the ions in the oxida- argon An inert colorless odorless tion process. Other types remove interme- monatomic element of the rare-gas group. diate oxygen FREE RADICALS. Naturally It forms 0.93% by volume of air, from occurring antioxidants can limit tissue or which it is obtained by fractional distilla- cell damage in the body. They include vita- tion. Argon is used to provide an inert at- mins C and E, and β-carotene. mosphere in electric and fluorescent lights, in aluminum welding, and in titanium and antiparallel spins Spins of two neigh- silicon extraction. The element forms no boring electrons in which the magnetic mo- known compounds.

14 arsenic(III) oxide

° 3– Symbol: Ar; m.p. –189.37 C; b.p. AsO3 . Copper arsenate(III) is used as an –185.86°C; r.d. 0.0001 784 (0°C); p.n. 18; insecticide. r.a.m. 39.95. arsenate(V) A salt of arsenic(V) acid, Arrhenius, Svante August (1859– made by reacting arsenic(III) oxide, As2O3, 1927) Swedish physical chemist who first with nitric acid. Arsenate(V) salts contain 3– postulated that the electrical conductivity the ion AsO4 . Disodiumhydrogenarsen- of electrolytes is due to the dissolved sub- ate(V) is used in printing calico. stance being dissociated into electrically charged particles (ions). He put forward arsenic A toxic metalloid element of this idea in 1883 and developed it in 1887. group 15 (formerly VB) of the periodic Arrhenius also made a major contribution table. It exists in three allotropic forms; the to the theory of chemical reactions in 1889 most stable is a brittle gray metal. Yellow when he suggested that a molecule can take and black arsenic, the other allotropes, are part in a chemical reaction only if its en- not metallic. Arsenic is found native and in ergy is higher than a certain value. This several ores including mispickel or ar- gave rise to the Arrhenius equation relating senopyrite (FeSAs), realgar (As4S4), and or- the rate of a chemical reaction to the ab- piment (As2S3). The ores are roasted to solute temperature. He also performed cal- produce arsenic(III) oxide, which is then culations that led him to the idea of the reduced with carbon or hydrogen to re- greenhouse effect. Arrhenius won the 1903 cover the element. Arsenic reacts with hot Nobel Prize for chemistry for his theory of acids and molten sodium hydroxide but is electrolytes. unaffected by water, acids, or alkalis at normal temperatures. It is used in semicon- Arrhenius equation An equation, pro- ductors, alloys, lasers, and fireworks. Ar- posed by Svante Arrhenius in 1889, that re- senic and its compounds are poisonous and lates the rate constant of a chemical therefore also find use in insecticides, ro- reaction to the temperature at which the re- dentricides, and herbicides. action is taking place: Symbol: As; m.p. 817°C (gray) at 3 ° k = Aexp(–Ea/RT) MPa pressure; sublimes at 616 C (gray); where A is a constant for the given reac- r.d. 5.78 (gray at 20°C); p.n. 33; r.a.m. tion, k the rate constant, T the thermody- 74.92159. namic temperature in kelvins, R the gas constant, and Ea the activation energy of arsenic(III) chloride (arsenious chloride; the reaction. AsCl3) A poisonous oily liquid. It fumes Reactions proceed at different rates at in moist air due to hydrolysis with water different temperatures, i.e. the magnitude vapor: of the rate constant is temperature depend- AsCl3 + 3H2O = As2O3 + 6HCl ent. The Arrhenius equation is often writ- Arsenic(III) chloride is covalent and exten in a logarithmic form, i.e. hibits nonmetallic properties. logek = logeA – Ea/RT This equation enables the ACTIVATION arsenic hydride See arsine. ENERGY for a reaction to be determined. The equation can also be applied to prob- arsenic(III) oxide (white arsenic; arse- lems dealing with diffusion, viscosity, elec- nious oxide; As2O3) A colorless crys- trolytic conduction, etc. talline solid that is very poisonous (0.1 g would be a lethal dose). Analysis of the Arrhenius theory See acid; base. solid and vapor states suggests a dimerized structure of As4O6. An amphoteric oxide, arsenate(III) (arsenite) A salt of the hy- arsenic(III) oxide is sparingly soluble in pothetical arsenic(III) acid, formed by re- water, producing an acidic solution. It is acting arsenic(III) oxide, A2O3, with formed when arsenic is burned in air or alkalis. Arsenate(III) salts contain the ion oxygen. It is used as an insecticide, herbi-

15 arsenic(V) oxide cide, and defoliant, and in the manufacture and curium, elements 93 through 96, are of glass and ceramics having a milky iri- only found in very minute amounts in na- descence. ture and thus are also usually produced artificially. arsenic(V) oxide (arsenic oxide; As2O5) A white amorphous deliquescent solid. It is asbestos Any of several fibrous varieties an acidic oxide prepared by dissolving ar- of various rock-forming silicate minerals, senic(III) oxide in hot concentrated nitric such as the amphiboles and chrysotile. As- acid, followed by crystallization then heat- bestos has many uses that employ its prop- ing to 210°C. erties of exceptional heat-resistance, chemical inertness, and electrical resis- arsenide A compound of arsenic and an- tance. It is, for example, spun and woven other metal. For example, with iron arsenic into fabric that is used to make fireproof forms iron(III) arsenide, FeAs2, while with clothing and brake linings, uses for which gallium arsenic forms gallium arsenide, there are currently no adequate substitutes. GaAs. Gallium arsenide is an important However, prolonged exposure to asbestos semiconductor. dust may cause asbestosis – a serious, progressive disease that eventually leads to res- arsenious chloride See arsenic(III) piratory failure. Mesothelioma, a chloride. malignant cancer of the membrane enclosing the lung, may also result. arsenious oxide See arsenic(III) oxide. aspirator An apparatus for sucking a arsenite See arsenate(III). gas or liquid from a vessel or body cavity. arsine (arsenic hydride; AsH3) A highly associated liquids See association. poisonous colorless gas with an unpleasant smell. It is produced by reacting mineral association The combination of mol- acids with arsenides or by reducing arsenic ecules of a substance with those of another compounds with nascent hydrogen. Arsine to form more complex species. An example decomposes to arsenic and hydrogen at is a mixture of water and ethanol (which 230°C. This phenomenon is put to use in are termed associated liquids), the mol- Marsh’s test for arsenic, in which arsine ecules of which combine via hydrogen generated from a sample is fed through a bonding. glass tube. Here the arsine, if present, decomposes to leave a brown deposit of ar- astatine A radioactive halogen element senic, which can be distinguished from of group 17 (formerly VIIA) of the periodic antimony by the fact that antimony will table. It occurs in minute quantities in ura- not dissolve in NaOC1. Arsine is used to nium ores as the result of radioactive make n-type semiconductors doped with decay, and can be artifically created by trace amounts of arsenic. bombarding 200Bi with alpha particles. Many short-lived radioisotopes are artificial radioactivity Radioactivity known, all alpha-particle emitters. Due to induced by bombarding stable nuclei with its rapid decay it is used as a radioactive high-energy particles. For example: tracer in medicine. 27 1 → 24 4 ° 13Al + 0n 11Na + 2He Symbol: At; m.p. 302 C (est.); b.p. represents the bombardment of aluminum 337°C (est.); p.n. 85; most stable isotope with neutrons to produce an isotope of 210At (half-life 8.1 hours). sodium and helium nuclei (alpha particles). All transuranic elements above curium, Aston, Francis William (1877–1945) atomic number 96, are artificially radio- British chemist and physicist. Aston’s main active because they do not occur in nature. contribution to science was the develop- Even neptunium, plutonium, americium, ment of the mass spectrograph. This en-

16 atom abled atomic masses to be determined and indications of high pressure; in particular, demonstrated the existence of isotopes. those of high-pressure industrial processes. From experiments soon after World War I Aston was able to explain Prout’s hypoth- atom The smallest part of an element esis and the exceptions to it. He discovered that can exist as a stable entity. Atoms con- that neon consists of two isotopes neon-20 sist of a small, dense, positively charged and neon-22, with the former being ten nucleus, made up of neutrons and protons, times more common. This gives an average with electrons in a cloud around this nu- of 20.2 for the weight of a large collection cleus. The chemical reactions of an element of neon atoms. The atomic weights (rela- are determined by the number of its elec- tive atomic masses) of other elements were trons, which is equal to the number of pro- explained in a similar way. Between the tons in its nucleus. All atoms of a given mid-1920s and mid-1930s he was able to element have the same number of protons, determine atomic weights more accurately an identifying quantity known as the ele- using a new mass spectrograph. He found ment’s PROTON NUMBER. A given element small discrepancies from the whole num- may also have two or more isotopes, which bers postulated by Prout due to the binding differ in the number of neutrons in their energy of nuclei. He was awarded the 1922 nuclei. Nobel Prize for chemistry. The electrons surrounding the nucleus are grouped into energy levels traditionally asymmetric atom See isomerism; opti- called SHELLS – i.e. main orbits around the cal activity. nucleus. Within these main orbits there may be subshells corresponding to atomic atmolysis The separation of gases by ORBITALS. using their different rates of diffusion An electron in an atom is specified by four through a porous membrane or partition. quantum numbers: 1. The principal quantum number (n), atmosphere A unit of atmospheric pres- which specifies the main energy levels. sure, equal to 101.325 kilopascals in SI The principal quantum number is a pos- units. It is used in chemistry only for rough itive integer which can have values 1, 2,

Quantum Integer values Quantum number property

Principal, n 1,2,3, ... 1 23

Orbital, l 0 to n –1 0 0 1 10 2

Magnetic, m –l, ..., 0, ..., + l 00–10+1 0 –10+1–2 –10+1+2

Atom: quantum numbers

17 atomic absorption spectroscopy

1s 2s 2p

nitrogen atom (ground state)

1s 2s 2p 3s

magnesium atom (ground state)

1s 2s 2p

3s 3p 4s 3d

scandium atom (ground state)

1s 2s 2p

carbon atom (ground state)

1s 2s 2p

beryllium atom (ground state)

1s 2s 2p

beryllium atom (excited state)

Atom: examples of box-and-arrow diagrams

3, etc. The corresponding shells are de- 4. The spin quantum number (ms), which noted by letters K, L, M, etc., the K shell (n specifies the intrinsic angular momen- = 1) being the one nearest to the nucleus. tum of the electron. It can have values The maximum number of electrons in a +½ and –½. given shell is 2n2. According to the exclusion principle 2. The orbital quantum number (l), which enunciated by Pauli in 1925 (the Pauli ex- specifies the angular momentum of the clusion principle), no two electrons in an electron and thus the shape of the or- atom can have the same set of four quan- bital. For a given value of n, 1 can have tum numbers. Each electron’s unique set of possible integer values of n–1, n–2, … 2, four quantum numbers therefore specifies 1, 0. For instance, the M shell (n = 3) has its QUANTUM STATE and helps to explain the three subshells with different values of l electronic structure of atoms. See also Bohr (0, 1, and 2). Subshells with angular mo- theory. mentum 0, 1, 2, and 3 are designated by letters s, p, d, and f. atomic absorption spectroscopy (AAS) 3. The magnetic quantum number (m) A method of chemical analysis in which a which determines the orientation of sample is vaporized and an absorption the electron orbital in a magnetic field. spectrum is taken of the vapor. The el- This number can have integer values ements present are identified by their char- –l, –(l – 1) … 0 … + (l – l), +l. acteristic absorption lines.

18 auric chloride atomic emission spectroscopy (AES) units denoting 10–18. For example, 1 at- A technique of chemical analysis that in- tometer (am) = 10–18 meter (m). volves vaporizing a sample of material, with atoms in their excited states emitting Aufbau principle A principle that gov- electromagnetic radiation at particular fre- erns the order in which the atomic orbitals quencies characteristic of that type of are filled in elements of successive proton atom. number; i.e. in order of increasing energy. The name is from the German aufbauen, atomic force microscope (AFM) An meaning ‘to build up’. The order is as fol- instrument used to investigate surfaces. A lows: small probe consisting of a very small chip 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p6, 5s2, of diamond is held just above a surface of 4d10, 5p6, 6s2, 4f14, 5d10, 6p6, 7s2, 5f14, a sample by a spring-loaded cantilever. As 6d10 the probe is slowly moved over the surface with the superscript indicating the maxi- the force between the surface and the tip is mum number of electrons for each level. measured and recorded and the probe is Note that degenerate orbitals are occu- automatically raised and lowered to keep pied singly before spin pairing occurs. this force constant. Scanning the surface in Note also the unexpected position of the d this way enables a contor map of the sur- levels, the filling of which give rise to the face to be generated with the help of a com- first, second, and third transition series, puter. An atomic force microscope closely and the unusual position of the f levels, the resembles a SCANNING TUNNELLING MICRO- filling of which give rise to the lanthanoids SCOPE (STM) in some ways, although it uses and actinoids. forces rather than electrical signals to investigate the surface. Like an STM, it can Auger effect An effect in which an ex- resolve individual molecules. Unlike an cited ion decays by emission of an electron STM, it can be used to investigate noncon- rather than by a photon. For example, if a ducting materials, a feature that is useful in substance is bombarded by high-energy investigating biological samples. electrons or gamma rays, an electron from an inner shell may be ejected. The result is atomic heat See Dulong and Petit’s law. a positive ion in an excited state. This ion will decay to its ground state as an outer atomicity The number of atoms per electron falls to an inner shell. The energy molecule of an element. Helium (He), for released may result in the emission of a example, has an atomicity of one, nitrogen photon in the x-ray region of the electro- (N2) two, and ozone (O3) three. magnetic spectrum (x-ray fluorescence). Alternatively, the energy may be released atomic mass unit (amu) Symbol: u A in the form of a second electron ejected unit of mass used to indicate the relative from the atom resulting in a doubly atomic mass of atoms and molecules, equal charged ion. The emitted electron, known to 1/12 of the mass of an atom of carbon- as an Auger electron, has a characteristic 12. It is equal to 1.660540 × 10–27 kilo- energy corresponding to the difference in gram. In biochemistry it is sometimes energy levels in the ion. The Auger effect is known as the dalton. a form of autoionization. It is named for French physicist Pierre Auger (1899– atomic number See proton number. 1994), who discovered it in 1925. atomic orbital See orbital. Auger electron See Auger effect. atomic weight See relative atomic mass auric Designating a compound of (r.a.m.). gold(III). atto- Symbol: a A prefix used with SI auric chloride See gold(III) chloride.

19 aurous aurous Designating a compound of that molecules of hydrogen and oxygen are gold(I). diatomic and that the formula of water is H2O. He was also able to give an explana- autocatalysis See catalyst. tion of Gay-Lussac’s law. Avogadro’s work made little impact in his lifetime and autoclave An apparatus consisting of an was revived later by Stanislao CANNIZ- airtight container whose contents are ZARO. heated by high-pressure steam; the apparatus may also have a mechanism by which Avogadro constant (Avogadro’s num- to agitate the contents. Autoclaves are used ber) Symbol: NA The number of particles to react substances under pressure at high in one MOLE of a substance. Its value is temperature, to sterilize objects and sub- 6.02242 × 1023 mol–1. stances, and to carry out industrial processes. Avogadro’s law (Avogadro’s hypothesis) The principle that equal volumes of all autoionization The spontaneous ion- gases at the same temperature and pressure ization of excited atoms, ions, or mol- contain equal numbers of molecules. It is ecules, as in the AUGER EFFECT. often called Avogadro’s hypothesis because it was first proposed by the Italian average atomic mass See relative chemist and physicist Amedeo Avogadro atomic mass. Count of Quaregna and Cer- (1776–1856) in 1811. It is strictly true only reto for ideal gases.

Avogadro, Lorenzo Romano Amedeo Avogadro’s number See Avogadro Carlo, Count of Quaregna and Cer- constant. reto (1776–1856) Italian scientist. Avo- gadro is mainly remembered for a paper he azide An inorganic compound contain- – wrote in 1811 in which he put forward ing the ion N3 , or an organic compound what is now known as AVOGADRO’S LAW. having the general formula RN3. Heavy Avogadro was able to use this idea to show metal azides are highly explosive.

20 B

back e.m.f. An e.m.f. that opposes the eral equation for spectral lines in 1885. See normal flow of electric charge in a circuit also Bohr theory; spectral series. or circuit element. In some electrolytic cells a back e.m.f. is caused by the layer of hy- banana bond (bent bond) A multicen- drogen bubbles that builds up on the cath- ter bond of the type present in BORON HY- ode as hydrogen ions pick up electrons and DRIDES. The term is used in a quite separate form gas molecules (i.e. as a result of PO- sense for the bonds in certain strained-ring LARIZATION of the electrode). organic compounds. baking powder A mixture of sodium band spectrum A SPECTRUM that ap- hydrogencarbonate (sodium bicarbonate, pears as a number of bands of emitted or baking soda) and a weakly acidic sub- absorbed radiation. Band spectra are char- stance, such as tartaric acid or potassium acteristic of molecules. Often each band hydrogentartrate (cream of tartar). The ad- can be resolved into a number of closely dition of moisture or heat causes a reaction spaced lines. The different bands corre- that produces bubbles of carbon dioxide spond to changes of electron orbit in the gas, which make dough or cake mixture molecules. The closely spaced lines in each band, seen under higher resolution, are the rise. It is used as a yeast substitute in bak- result of different vibrational states of the ing certain types of bread. molecule. baking soda See sodium hydrogencar- Barff process A process formerly used bonate. for protecting iron from corrosion by heating it in steam to form a layer of tri-iron ball mill A device commonly used in the tetroxide (Fe O ). It is also known as the chemical industry for grinding solid ma- 3 4 Bower–Barff process and is now only of terial. Ball mills usually have slowly rotat- historical interest. ing steel-lined drums containing steel balls. The material is crushed by the tumbling ac- barites See barium sulfate. tion of the balls in the drum. Compare hammer mill. barium A dense, low-melting reactive metal; the fifth member of group 2 (for- Balmer series A series of lines in the merly IIA) of the periodic table and a typi- spectrum of radiation emitted by excited cal alkaline-earth element. The electronic hydrogen atoms. The lines correspond to configuration is that of xenon with two ad- the atomic electrons falling into the second ditional outer 6s electrons. Barium is of lowest energy level, emitting energy as ra- low abundance; it is found in mineral form λ diation. The wavelengths ( ) of the radia- as witherite (BaCO3) and barytes (BaSO4). tion in the Balmer series are given by: The metal is obtained by the electrolysis of 1/λ = R(1/22 – 1/n2) the fused chloride using a cooled cathode where n is an integer and R is the Rydberg which is slowly withdrawn from the melt. constant. The series is named for J. J. Because of its low melting point barium is Balmer (1825–98), who discovered a gen- readily purified by vacuum distillation.

21 barium bicarbonate

Barium metal is used as a ‘getter’, i.e., a tory barium chloride solution is used as a compound added to a vacuum system to test for sulfates, with which it gives a white remove the last traces of oxygen. It is also precipitate. a constituent of certain bearing alloys. Barium has a low ionization potential barium hydrogencarbonate (barium bi- and a large radius. It is therefore strongly carbonate; Ba(HCO3)2) A compound electropositive and its properties, and that occurs only in aqueous solution. It is those of its compounds, are very similar to formed by the action of cold water con- those of the other alkaline-earth elements taining carbon dioxide on barium carbon- calcium and strontium. Notable differ- ate. ˆ ences in the chemistry of barium from the BaCO3 + CO2 + H2O Ba(HCO3)2 rest of the group are: 1. The much higher stability of the carbon- barium hydroxide (baryta; Ba(OH)2) ate. A white solid usually obtained as the oc- 2. The formation of a peroxide below tahydrate, Ba(OH)2.8H2O. It can be made 800°C. Barium peroxide decomposes on by adding water to barium oxide. Barium strong heating to give oxygen and bar- hydroxide is the most soluble of the group ium oxide: 2 hydroxides. It is used in volumetric ˆ BaO2 BaO + O analysis for the estimation of weak acids Barium is also notable for the very low using phenolphthalein as an indicator. solubility of the sulfate. Barium compounds give a characteristic green color to barium oxide (BaO) A white powder flames, a phenomenon that is used in qual- prepared by heating barium in oxygen or itative analysis. Barium salts are all highly by heating barium carbonate. It has been toxic with the exception of the most insol- used in the manufacture of additives for lu- uble materials. Barium sulfate is used as a bricating oils, and in pigments. radiopaque material in making x-ray photographs of the gastric and intestinal barium peroxide (BaO2) A dense off- tract. white powder that can be prepared by care- Symbol: Ba; m.p. 729°C; b.p. 1640°C; fully heating barium oxide in oxygen. The r.d. 3.594 (20°C); p.n. 56; r.a.m. 137.327. reaction, which is reversible, was the basis of the now obsolete BRIN PROCESS for ob- barium bicarbonate See barium hydro- taining oxygen. Barium peroxide is used gencarbonate. for bleaching straw and silk and is also used in the laboratory preparation of hy- barium carbonate (BaCO3) A white in- drogen peroxide. soluble salt that occurs naturally as the mineral witherite. It can be precipitated by barium sulfate (BaSO4) A white solid adding an alkali carbonate to a solution of that occurs naturally as the mineral barites, a barium salt. On heating it decomposes re- also known as heavy spar. Barium sulfate is versibly with the formation of the oxide very insoluble in water and can be pre- and carbon dioxide: pared easily as a precipitate by adding sul- ˆ BaCO3 BaO + CO2 furic acid to barium chloride. It is an The compound is highly toxic and is used important industrial chemical. Under the as a rat poison. name of blanc fixe it is used as a pigment extender in surface coating compositions. barium chloride (BaCl2) A white solid It is also used in the glass and rubber in- that can be prepared by dissolving barium dustries. Because barium compounds are carbonate in hydrochloric acid and crystal- opaque to x-rays barium sulfate is used in lizing out the dihydrate (BaCl2.2H2O). medicine for taking radiographs of the di- Barium chloride is used as the electrolyte in gestive system. the extraction of barium, as a rat poison, and in the leather industry. In the labora- Bartlett, Neil (1932– ) British–

22 basic salt

American chemist. Bartlett was studying are examples of organic bases. See also metal fluorides and found that the com- acid; Lewis acid. pound platinum hexafluoride (PtF6) has an extremely high electron affinity, and reacts base-catalyzed reaction A reaction with molecular oxygen to form the novel catalyzed by bases; i.e. by hydroxide ions. + – compound O2 PtF6 . This was the first example of a compound containing the oxy- base metal A common metal such as gen cation. At the time it was an iron, lead, or copper, as distinguished from unquestioned assumption of chemistry that a precious metal such as gold, silver, or the rare gases – helium, neon, argon, kryp- platinum. ton, and xenon – were completely inert. Bartlett knew that the ionization potential base unit A unit within a system of of xenon was not too much greater than measurement from which other units may the ionization potential of the oxygen mol- be derived by combining it with one or ecule and was able to produce xenon hexa- more other base units. For example the fluoroplatinate (XePtF6) by direct reaction joule, a derived unit, can be defined in – the first compound of a rare gas. Once terms of base units as one metre (m) the first compound had been detected squared, kilogram (kg) per second (s) xenon was soon shown to form other com- squared, or m2kgs–2. With the current ex- pounds, such as xenon fluoride (XeF4) and ception of the kilogram, base SI UNITS are oxyfluoride (XeOF4). Krypton and radon defined in terms of reproducible physical were also found to form compounds al- phenomena. though the lighter rare gases have so far re- mained inactive. basic Behaving as a base. Any solution in which the concentration of OH– ions is baryta See barium hydroxide. greater than that in pure water at the same temperature is described as basic; i.e. the basalt A dark-colored basic igneous pH of a basic solution is greater than 7. rock derived from solidified volcanic lava. It consists mainly of fine crystals of pyrox- basic aluminum ethanoate See alu- ine and plagioclase feldspar. minum ethanoate. base In the Arrhenius theory, a com- basic oxide An oxide of a metal that re- pound that releases hydroxide ions, OH–, acts with water to form a base, or with an in aqueous solution. Basic solutions have a acid to form a salt and water. Calcium pH greater than 7. In the Lowry-Brønsted oxide is a typical example. It reacts with theory of ACIDS and bases a base is a sub- water to form calcium hydroxide: → stance that tends to accept a proton. Thus CaO + H2O Ca(OH)2 OH– is basic because it accepts H+ to form and with hydrochloric acid to produce cal- water, but H2O is also a base (although cium chloride and water: → somewhat weaker) because it can accept a CaO + 2HCl CaCl2 + H2O + further proton to form H3O . In this treat- Compare acidic oxide. ment the ions of classical mineral acids 2– – such as SO4 and NO3 are weak conju- basic oxygen process See Bessemer gate bases of their respective conjugate process. acids; i.e. H2SO4 and HNO3, respectively. Ammonia is also a base. In solution it do- basic salt A compound intermediate be- nates an electron pair to a proton to form tween a normal salt and a hydroxide or an ammonium ion: oxide. The term is often restricted to ˆ + – NH3 + H2O NH4 + OH hydroxyhalides (such as Pb(OH)Cl, Here the ammonium ion is the conjugate Mg2(OH)3Cl2.4H2O, and Zn(OH)F) and acid. Other nitrogenous compounds, such hydroxy-oxy salts (for example, 2PbCO3.- as trimethylamine ((CH3)3N) and pyridine, Pb(OH)2, Cu(OH)2.CuCl2, Cu(OH)2.- 23 basic slag

CuCO3). The hydroxyhalides are essen- passing through a material to the length of tially closely packed assemblies of OH– the path of the radiation through the ma- with metal or halide ions in the octahedral terial. It has the form log I/0I = E c l, where holes. The hydroxy-oxy salts usually have I0 is the incident intensity of light, I is the more complex structures than those im- intensity of light after it has passed through plied by the formulae. a sample of the material of length l, c is the concentration of absorbing species in the basic slag See slag. material and E is a constant known as the absorption coefficient. The law is named battery A number of electric cells work- for the German mathematician Johann ing together. Many dry ‘batteries’ used in Heinrich Lambert (1728–77) and the Ger- radios, flashlights, etc., are in fact single man astronomer Wilhelm Beer (1797– cells. If a number of identical cells are con- 1850). The physical significance of the nected in series, the total e.m.f. of the bat- Beer–Lambert law is that the intensity of tery is the sum of the e.m.f.s of the electromagnetic radiation passing through individual cells. If the cells are in parallel, a sample of material decreases exponen- the e.m.f. of the battery is the same as that tially with the thickness and concentration of one cell, but the current drawn from of the sample (for a given wavelength of each is less (the total current is split among electromagnetic radiation). If deviations the cells). from the Beer–Lambert law occur this is due to such phenomena as dissociation or bauxite A mineral hydrated form of alu- the formation of complexes. minum hydroxide; the principal ore of aluminum. Belousov–Zhabotinskii reaction (B–Z reaction) An oscillating chemical reaction b.c.c. See body-centered cubic crystal. in which there are periodic oscillations in the color of a mixture of sulfuric acid, Beckmann thermometer A type of potassium bromate, cerium (or iron) sul- mercury thermometer designed to measure fate, and propanedioic acid. The period of small differences in temperature rather oscillation is about one minute. The color than scale degrees. Beckmann thermome- changes are caused by repeated oxidations ters have a larger bulb than common ther- and reductions of cerium (or iron) ions. mometers and a stem with a small internal The reaction was first observed by the diameter, so that a range of 5°C covers Russian chemist B. P. Belousov in the case about 30 centimeters in the stem. The mer- of cerium and modified to iron by A. M. cury bulb is connected to the stem in such Zhabotinskii in 1963. The mechanism of a way that the bulk of the mercury can be the B–Z reaction is highly complicated and separated from the stem once a particular involves a large number of individual steps. 5° range has been attained. The thermometer can thus be set for any particular range. bench dilute acid See dilute. It is named for the German chemist Ernst Beckmann (1853–1923). beneficiation The process of separating an ore into a useful component and waste becquerel Symbol: Bq The SI unit of ac- material (known as gangue). The process is tivity equal to the activity of a radioactive sometimes described as ore dressing. substance that has one spontaneous nuclear change per second; 1 Bq = 1 s–1. It is bent bond See banana bond. named for the discoverer of radioactivity, the French physicist Antoine Henri Be- bentonite A type of clay that is used as querel (1852–1908). an absorbent in making paper and as a precipitating agent to remove proteins from Beer–Lambert law A law that relates beer and wine. The gelatinous suspension the intensity of electromagnetic radiation it forms with water is also used to bind to-

24 beryllium gether the sand used for making castings. first element in group 2 (formerly IIA) of Chemically bentonite is an aluminosilicate the periodic table. It has the electronic con- of variable composition. figuration of helium with two additional outer 2s electrons. bent sandwich compound See sand- Beryllium occurs in a number of miner- wich compound. als such as beryllonite (NaBePO4), chrysoberyl (Be(AlO2)2), bertrandite Bergius, Friedrich Karl Rudolph (4BeO.2SiO2), and beryl (3BeO.Al2O3.- (1884–1949) German industrial chemist. 6SiO2). The element accounts for only Bergius worked with Hermann Nernst at 0.0006% by mass of the Earth’s crust. The Berlin and Fritz Haber at Karlsruhe, where metal is obtained by conversion of the ore he became interested in high-pressure to the sulfate at high temperature and pres- chemical reactions. He is noted for his de- sure with concentrated sulfuric acid, then velopment of the BERGIUS PROCESS. He to the chloride, followed by electrolysis of shared the Nobel Prize for chemistry with the fused chloride. Alternatively, it is possi- Carl Bosch in 1931. ble to treat the ore with hydrogen fluoride followed by electrolysis of the fused fluo- Bergius process A process formerly ride. The metal has a much lower general used for making hydrocarbon fuels from reactivity than other elements in group 2. It coal. A powdered mixture of coal, heavy is used as an antioxidant and hardener in oil, and a catalyst was heated with hydro- copper, steel, bronze, and nickel alloys. gen at high pressure. The process was used Beryllium has the highest ionization po- by Germany as a source of motor fuel in tential of group 2 and the smallest atomic World War II. It was developed by radius. Consequently it is less electroposi- Friedrich Bergius in 1912. tive and more polarizing than other members of the group. Thus, Be2+ ions do not berkelium A silvery radioactive exist as such in either solids or solutions, transuranic element of the actinoid series and there is partial covalent character in of metals, not found naturally on Earth. the bonds, even with the most electronega- The first samples were prepared by bom- tive elements. The metal reacts directly barding 291Am with alpha particles. Sev- with oxygen, nitrogen, sulfur, and the eral radioisotopes have since been halogens at various elevated temperatures, synthesized. The metal reacts with oxygen, to form the oxide BeO, nitride Be3N2, sul- steam, and acids. It tends to collect in bone fide BeS, and halides BeX2, all of which are and thus may one day find use in medical covalent. Beryllium does not react directly research. with hydrogen but a polymeric hydride ° Symbol: Bk; m.p. 1050 C; b.p. un- (BeH2)n can be prepared by reduction of ° known; r.d. 14.79 (20 C); p.n. 97; most (CH3)2Be using lithium tetrahydroalumi- 247 stable isotope Bk (half-life 1400 years). nate(III) (LiAlH4). Beryllium is amphoteric forming beryl- 2– beryl A mineral, 3BeO.Al2O3.6SiO2, late species, such as [Be(OH)4] and 3+ used as a source of beryllium. Crystals are [Be(OH)]3 . The hydroxide is only weakly hexagonal. There are several color varieties basic. The element does not form a true including the gemstones emerald, which is carbonate; the basic beryllium carbonate, colored green by traces of chromium BeCO3.Be(OH)2 is formed when sodium oxide, and aquamarine, which has a blue- carbonate is added to solutions of beryl- green color. lium compounds. Beryllium hydride, chloride, and di- beryllate See beryllium; beryllium hy- methylberyllium form polymeric bridged droxide. species but, whereas the bridging in the chloride is via an electron pair on chlorine beryllium A light metallic element, sim- atoms and can be regarded as an electron- ilar to aluminum but somewhat harder; the pair donor bond, the bonding in the hy-

25 beryllium bicarbonate dride and in the methyl compound involves by the thermal decomposition of beryllium two-electron three-center bonds similar to hydroxide or carbonate. Beryllium oxide is those found in the BORON HYDRIDES. COM- insoluble in water but it shows basic prop- PLEXES are quite common with beryllium; erties by dissolving in acids to form beryl- 2– some examples include [BeCl4] , (R2O)2- lium salts: + → 2+ BeCl2, and [Be(NH3)4]Cl2. Beryllium and BeO + 2H Be + H2O its compounds are very toxic and may However, beryllium oxide also resem- cause serious respiratory diseases if in- bles acidic oxides by reacting with alkalis haled. to form beryllates: ± ° – → 2– Symbol: Be; m.p. 1278 5 C; b.p. BeO + 2OH + H2O Be(OH)4 2970°C (660 kPa); r.d. 1.85 (20°C); p.n. 4; It is thus an amphoteric oxide. Beryl- r.a.m. 9.012182. lium oxide is used in the production of refractory materials, high-output transistors, beryllium bicarbonate See beryllium and printed circuits. Its chemical properties hydrogencarbonate. of beryllium oxide are similar to those of aluminum oxide. beryllium bronze See bronze. beryllium sulfate (BeSO4) An insoluble beryllium carbonate (BeCO3) An un- salt prepared by the reaction of beryllium stable solid prepared by prolonged treat- oxide with concentrated sulfuric acid. On ment of a suspension of beryllium heating, it breaks down to give the oxide. hydroxide with carbon dioxide. The resulting solution is evaporated and filtered in an Berzelius, Jöns Jacob (1779–1848) atmosphere of carbon dioxide. Swedish chemist who had a major influence on the development of modern chem- beryllium chloride (BeCl2) A white istry. He introduced the symbols now used solid obtained by passing chlorine over a to denote chemical elements and coined a heated mixture of beryllium oxide and car- number of words still used in chemistry, bon. The compound is not ionic: it is a such as ‘catalysis’, ‘halogen’, ‘isomerism’, poor conductor in the fused state and in the and ‘protein’. He also determined relative solid form consists of a covalent polymeric atomic masses (atomic weights) accurately, structure. It is readily soluble in organic thereby helping establish Dalton’s atomic solvents but is hydrolyzed in the presence theory. Berzelius was also involved in the of water to give the hydroxide. The anhy- discovery of several elements: cerium, sili- drous salt is used as a catalyst. con, selenium, and thorium, and the introduction of some experimental apparatus, beryllium hydrogencarbonate (beryl- such as rubber tubes and filter paper, into lium bicarbonate; Be(HCO3)2) A com- chemistry. pound formed in solution by the action of carbon dioxide on a suspension of the car- Bessemer process A process for mak- bonate, to which it reverts on heating: ing steel from PIG IRON. A vertical cylindri- ˆ BeCO3 + CO2 + H2O Be(HCO3)2 cal steel vessel (converter) is used, lined with a refractory material. Air is blown beryllium hydroxide (Be(OH)2)A over and through the molten iron to oxi- white solid that can be precipitated from dize and thereby remove impurities such as beryllium-containing solutions by an al- carbon, silicon, sulfur, and phosphorus by kali. Beryllium hydroxide, like aluminum converting them to slag. For instance, irons hydroxide, is amphoteric. In an excess of containing large amounts of phosphorus alkali, beryllium hydroxide dissolves to are treated in a converter lined with a basic 2– give a beryllate, Be(OH)4 . material, so that a phosphate slag is formed. The required amount of carbon is beryllium oxide (BeO) A white solid then added to the iron to produce the deformed by heating beryllium in oxygen or sired type of steel. The process was in-

26 bismuth vented in 1856 by the British engineer, Sir compounds and reactions occurring in liv- Henry Bessemer (1813–98). In the basic ing organisms. oxygen process, a modern version of the Bessemer process, the air is replaced by a bioinorganic chemistry The study of mixture of oxygen and steam in order to biological molecules that contain metal minimize the amount of nitrogen that is ab- atoms or ions. Many enzymes have active sorbed by the steel. metal atoms and bioinorganic compounds are important in other roles such as protein beta particle An electron or positron folding, oxygen transport, and electron emitted by a radioactive substance as it de- transfer. Two important examples of cays. bioinorganic compounds are hemoglobin (containing iron) and chlorophyll (contain- Bethe, Hans Albrecht (1906– ) Ger- ing magnesium). man physicist who initiated the subject of crystal field theory in 1929 when he analysed the splitting of atomic energy lev- N N els by surrounding ligands in terms of group theory. This work led to an understanding of the optical, magnetic, and spectroscopic properties of transition-metal and rare-earth complexes. Bethe’s main in- Bipyridine: 2,2′-bipyridine terest was nuclear physics. In particular, he explained the energy of stars in terms of bipy Abbreviation for the bidentate lig- nuclear reactions that fuse hydrogen into and 2,2′-bipyridine. helium. Bethe won the 1967 Nobel Prize for physics for this work. bipyridine See bipy. bi- Prefix used formerly in naming acid Birkeland–Eyde process An industrial salts. The prefix indicates the presence of process for fixing nitrogen (as nitrogen hydrogen; for instance, sodium bisulfate monoxide) by passing air through an elec- (NaHSO4) is sodium hydrogensulfate, etc. tric arc: In organic chemistry it is sometimes used → N2 + O2 2NO with the meaning ‘two’; e.g. biphenyl. The process was invented by two Norwe- gian chemists, Kristian Birkeland 1867– bicarbonate See hydrogencarbonate. 1913) and Samuel Eyde (1866–1940), who introduced it in 1903 to exploit the cheap bimolecular Describing a reaction or sources of hydroelectricity then available step that involves two molecules, ions, etc. in Scandinavia. See also nitrogen fixation. A common example of this type of reaction is the decomposition of hydrogen iodide: bismuth A brittle pinkish metallic el- → 2HI H2 + I2 ement belonging to group 15 (formerly This takes place between two molecules VA) of the periodic table. It occurs native and is therefore a bimolecular reaction. and in the ores bismuthinite (Bi2S3) and Other common examples are: bismite (Bi2O3). The element does not react → – H2O2 + I2 OH + HIO with oxygen or water under normal tem- – + → OH + H H2O. peratures. It can be dissolved by concentrated nitric acid. It is the most diamagnetic binary compound A chemical com- of the metals and has less thermal conduc- pound formed from two elements; e.g. tivity than any metal except mercury. Bis- Fe2O3 or NaCl. muth is widely used in alloys, especially low-melting alloys, which find use in heat- biochemistry The study of chemical activated sprinkler systems. The element

27 bismuth(III) carbonate dioxide has the unusual property of expanding bismuthyl chloride See bismuth(III) when it solidifies, making it a desirable chloride. component of alloys used to make detailed castings. Compounds of bismuth are used bismuthyl compound A compound in cosmetics and medicines. It is also used containing the (BiO)+ ion or BiO grouping. as a catalyst in the textile industry. Symbol: Bi; m.p. 271.35°C; b.p. bismuthyl ion See bismuth(III) carbon- 1560±5°C; r.d. 9.747 (20°C); p.n. 83; ate dioxide. r.a.m. 208.980 37. bismuthyl nitrate See bismuth(III) ni- bismuth(III) carbonate dioxide (bis- trate oxide. muthyl carbonate; Bi2O2CO3) A white solid prepared by mixing solutions of bis- bistability The ability of a chemically muth nitrate and ammonium carbonate. It reacting system to occur in two steady contains the (BiO)+ ion (sometimes known states. For an OSCILLATING REACTION to as the bismuthyl ion). occur it is necessary for there to be bistability. The two steady states are not states bismuth(III) chloride (bismuth tri- of thermodynamic equilibrium There is a chloride; bismuth(III) chloride oxide; sudden jump from one of the bistable states to the other when a certain critical concen- BiCl3) A white deliquescent solid. It can be prepared by direct combination of bis- tration of one of the reactants occurs. See also oscillating reaction. muth and chlorine. Bismuth(III) chloride dissolves in excess dilute hydrochloric acid bisulfate See hydrogensulfate. to form a clear liquid, but if diluted it produces a white precipitate of bismuth(III) bisulfite See hydrogensulfite. chloride oxide (bismuthyl chloride, BiOCl): bittern The liquid that is left after BiCl + H O ˆ BiOCl + 2HCl 3 2 sodium chloride has been crystallized from This reaction is often discussed in sea water. It is a source of iodine, bromine, chemistry as a typical example of a re- and some magnesium salts. versible reaction. It is also a confirmatory test for bismuth in analysis. Bismuth(V) bitumen A mixture of solid or semisolid chloride does not exist. hydrocarbons obtained naturally or from coal, oil, etc. bismuth(III) chloride oxide See bismuth(III) chloride. bituminous coal A type of second- grade COAL, containing more than 65% bismuth(III) nitrate oxide (bismuthyl ni- carbon but also quantities of gas, coal tar, trate; bismuth subnitrate; BiONO3)A and water. It is the most abundant type of white insoluble solid precipitated when coal. Domestic use of coal is practically bismuth(III) nitrate is diluted and contains nonexistent in the U.S.A. and Canada the (BiO)+ ion. Bismuth(III) nitrate oxide is today. used in pharmaceutical preparations. bivalent See divalent. bismuth subnitrate See bismuth(III) nitrate oxide. Black, Joseph (1728–99) Scottish chemist and physicist, one of the first to use bismuth trichloride See bismuth(III) quantitative methods in developing mod- chloride. ern chemistry. He discovered carbon dioxide (which he called ‘fixed air’) when he bismuthyl carbonate See bismuth(III) investigated the chemical reactions of lime- carbonate dioxide. stone in a quantitative way, reporting the

28 Bohr magneton work in 1756. The idea of latent heat oc- blende A sulfide ore of a metal; e.g. zinc curred to him in 1757 and in subsequent blende (ZnS2). years he determined the latent heat in the formation of ice and steam experimentally. Bloch, Felix (1905–83) Swiss-born He was careful to distinguish between the American physicist who invented the tech- concepts of heat and temperature, a dis- nique of nuclear magnetic resonance tinction that led him to the concept of spe- (NMR) in 1946 (as did Edward Purcell, in- cific heat. dependently). This led to the extensive use of NMR in investigating the structue of blackdamp See firedamp. complex molecules. Bloch and Purcell shared the 1952 Nobel Prize for physics for blanc fixe See barium sulfate. their work in this field. Bloch also worked on the quantum theory of solids. blast furnace A tall furnace for SMELT- ING iron from various iron ores, especially blue vitriol Copper(II) sulfate pentahy- hematite (Fe2)3) and magnetite (Fe3O4). A drate (CuSO4.5H2O). mixture of the ore with coke and a flux is fed into the top of the furnace and heated body-centered cubic crystal (b.c.c.) A by pre-heated air blown into the bottom of crystal structure in which the unit cell has the furnace, where temperatures are much an atom, ion, or molecule at each corner of higher. The flux is often calcium oxide a cube and also at the center of the cube. In (from limestone). As it descends the fur- this type of structure the coordination nace the ore is first reduced to iron(II) number is 8. It is less close-packed than the oxide (FeO) and then to molten iron, both face-centered cubic structure. The alkali by the action of carbon monoxide (CO) metals form crystals with body-centered obtained by burning coke in air. Molten cubic structures. See also cubic crystal. PIG IRON is then run off from the bottom of the furnace. Bohr, Niels Hendrik David (1885– 1962) Danish physicist. Niels Bohr was re- bleach Any substance used to remove sponsible for a key development in our un- color from materials such as cloth and derstanding of atomic structure when he paper. All bleaches are oxidizing agents, showed (1913) how the structure of the and include chlorine, sodium chlorate(I) atom could be explained by imposing solution (NaClO, sodium hypochlorite), ‘quantum conditions’ on the orbits of elec- hydrogen peroxide, and sulfur(IV) oxide trons, thus allowing only certain orbits. (SO2, sulfur dioxide). Sunlight also has a This theory accounted for details of the hy- bleaching effect. drogen spectrum. Bohr also contributed to nuclear physics, particularly the theory of bleaching powder A white solid that nuclear fission. He was awarded the 1922 can be regarded as a mixture of calcium Nobel Prize for physics for his work on chlorate(I) (calcium hypochlorite atomic theory. (Ca(ClO)2)), calcium chloride, and calcium hydroxide. It is prepared commercially by bohrium A synthetic radioactive el- passing chlorine through a tilted cylinder ement first detected by bombarding a bis- down which is passed calcium hydroxide. muth target with chromium nuclei. Only a Bleaching powder has been used for small number of atoms have ever been pro- bleaching paper pulp and fabrics and for duced. sterilizing water. Its activity arises from the Symbol: Bh; p.n. 107; most stable iso- formation, in the presence of air containing tope 262Bh (half life 0.1s). carbon dioxide, of the oxidizing agent chloric(I) acid (hypochlorous acid, HClO): Bohr magneton A unit of magnetic mo- → Ca(ClO)2.Ca(OH)2.CaCl2 + 2CO2 ment that is convenient at the atomic level. µ µ 2CaCO3 + CaCl2 + 2HClO It is denoted by B and given by B = 29 Bohr theory

π eh/(4 me), where e is the charge of an elec- is less successful for larger atoms. Different tron, h is the PLANCK CONSTANT, and me is values of n1 and n2 correspond to different the rest mass of an electron. The Bohr mag- spectral series, with lines given by the ex- neton has a value of 9.274 × 10–24 A m2. pression: λ 2 2 1/ = R(1/n1 – 1/n2 ) Bohr theory A theory introduced by R is the Rydberg constant. Its experi- Niels Bohr (1911) to explain the spectrum mental value is 1.09678 × 107 m–1. The 4 πε 2 2 of atomic hydrogen. The model he used value from Bohr theory (me /8 1 ch ) is was that of a nucleus with charge +e, or- 1.097 00 × 107 m–1. See also atom. bited by an electron with charge –e, moving in a circle of radius r. If v is the velocity boiling The process by which a liquid is of the electron, the centripetal force, mv2/r converted into a gas or vapor by heating at is equal to the force of electrostatic attrac- its boiling point; i.e. the temperature at 2 πε 2 tion, e /4 0r . Using this, it can be shown which the vapor pressure of a liquid is that the total energy of the electron (kinetic equal to atmospheric pressure. This tem- 2 πε and potential) is –e /8 0r. perature is always the same for a particular If the electron is considered to have pure liquid at a given pressure (for refer- wave properties, there must be a whole ence purposes usually taken as standard number of wavelengths around the orbit, pressure). otherwise the wave would be a progressive wave. For this to occur Boltzmann constant Symbol: k The nλ = 2πr constant 1.38054 J K–1, equal to the gas where n is an integer, 1, 2, 3, 4, … The constant (R) divided by the Avogadro con- λ wavelength, , is h/mv, where h is the stant (NA). It is named for the Austrian Planck constant and mv the momentum. physicist Ludwig Edward Boltzmann Thus for a given orbit: (1844–1906). nh/2π = mvr This means that orbits are possible only Boltzmann formula A fundamental re- when the angular momentum (mvr) is an sult in statistical mechanics stating that the integral number of units of h/2π. Angular entropy S of a system is related to the num- momentum is thus quantized. In fact, Bohr ber W of distinguishable ways in which the in his theory did not use the wave behavior system can be realised by the equation: S = of the electron to derive this relationship. k lnW, where k is the Boltzmann constant. He assumed from the beginning that angu- This formula is a quantitative expression lar momentum was quantized in this way. of the idea that the entropy of a system is a Using the above expressions it can be measure of its disorder. It was discovered shown that the electron energy is given by: by the Austrian physicist Ludwig Boltz- 4 ε 2 2 2 E = –me /8 0 n h mann in the late 19th century in the course Different values of n (1, 2, 3, etc.) cor- of his investigations into the foundations respond to different orbits with different of statistical mechanics. energies; n is the principal quantum number. In making a transition from an orbit n1 bomb calorimeter A sealed insulated to another orbit n2 the energy difference container, used for measuring energy re- ∆W is given by: leased during combustion of substances ∆ W = W1 – W2 (e.g. foods and fuels). A known amount of 4 2 2 ε 2 2 = me (1/n2 – 1/n1 )/8 0 h the substance is ignited inside the calorime- This is equal to hv where v is the fre- ter in an atmosphere of pure oxygen, and quency of radiation emitted or absorbed. undergoes complete combustion at con- Since vλ = c, then stant volume. The resultant rise in temper- λ 4 2 2 ε 2 3 1/ = me (1/n1 – 1/n2 )/8 0 ch ature is related to the energy released by The theory is in good agreement with ex- the reaction. Such energy values (calorific periment in predicting the wavelengths of values) are often quoted in joules per kilo- lines in the hydrogen spectrum, although it gram (J kg–1).

30 boride

BORAX-BEAD COLORS (H = hot; C = cold) Metal Oxidizing flame Reducing flame chromium green H+C green H+C cobalt blue H+C blue H+C copper green H, blue C often opaque iron brown-red H, yellow C green H+C manganese violet H+C colourless H+C nickel red–brown C gray–black C

bond See chemical bond. borax-bead test A preliminary test in qualitative inorganic analysis that can be a bond dissociation energy See bond en- guide to the presence of certain metals. A ergy. bead is formed by heating a little disodium tetraborate decahydrate (borax) on a loop bond energy The energy involved in in a platinum wire. A minute sample of the forming a chemical bond. For ammonia, compound to be tested is introduced into for instance, the energy of the N–H bond is the bead and the color observed in both the one third of the energy involved for the oxidizing and reducing areas of a Bunsen- process burner flame. The color is also noted when → NH3 N + 3H the bead is cold. It is thus one third of the heat of atomization. This is also known as the mean bond boric acid (boracic acid; orthoboric acid; energy. trioxoboric(III) acid; H3BO3) A white The bond dissociation energy is meas- crystalline solid soluble in water; in solu- ured in the opposite direction. It is the en- tion it is a very weak acid. Boric acid is ergy required to break a particular bond in used as a mild antiseptic eye lotion and was a compound, e.g.: formerly used as a food preservative. It is → NH3 NH2 + H used in glazes for enameled objects and is a More formally, it is common to specify the constituent of borosilicate glass. bond enthalpy. Trioxoboric(III) acid is the full systematic name for the solid acid and its dilute bond enthalpy See bond energy. solutions; in more concentrated solutions polymerization occurs to give polydioxo- bonding orbital See orbital. boric(III) acid. bond length The length of a chemical boric anhydride See boron oxide. bond, i.e. the distance between the centers of the nuclei of two atoms joined by a boric(III) oxide See boron oxide. chemical bond. Bond lengths may be measured by electron or x-ray diffraction. boride A compound of boron, especially one with a more electropositive element. boracic acid See boric acid. Borides have a wide range of stoichiometries, from M4B through to MB6, and can borane See boron hydride. exist in close-packed arrays, chains, and two-dimensional nets. Due to their high borate See boron. melting points and unreactivity with nonoxidizing acids, metal borides are used borax See disodium tetraborate decahy- in refractory materials. Borides are also drate. good abrasives.

31 Born, Max

Born, Max (1882–1970) German that the motion of atomic nuclei is very physicist who was one of the founders of much slower than the motion of electrons quantum mechanics in the 1920s. In par- and that it is a good approximation to take ticular, he put forward the ‘Born interpre- the nuclei to be in fixed positions when dis- tation’ for the wave-function of an electron cussing electron transitions. The German in terms of probability in 1926. Born also physicist Max Born and the American made major contributions to the theory of physicist Julius Robert Oppenheimer first crystals and to the quantum theory of mol- used this approximation in 1927. ecules. He was awarded a share of the 1954 Nobel Prize in physics (together with borohydride ions See boron hydride. Walther Bothe) for his work on quantum mechanics. boron A hard, rather brittle metalloid element of group 3 (formerly IIIA) of the Born–Haber cycle A cycle used in cal- periodic table. It exists in two forms: as an culating the lattice energies of solids. The amorphous yellow-brown powder and as a steps involved are: black metallic crystal. It has the electronic Atomization of sodium: structure 1s22s22p1. Boron is of low abun- Na(s) → Na(g) ∆H 1 dance (0.0003%) but occurs in very con- Ionization of sodium: centrated forms in its natural minerals, Na(g) → Na+(g) ∆H 2 which include borax (Na B O .10H O), Atomization of chlorine: 2 4 7 2 → ∆ ulexite (NaCaB5O9.H2O), howlite Cl2(g) 2Cl(g) H3 Ionization of chlorine: (Ca2B5SiO9(OH)5), and colemanite – → – ∆ (Ca2B6O11). The element is obtained by Cl(g) + e Cl (g) H4 Formation of solid: conversion to boric acid followed by de- + – → ∆ hydration to B O then reduction with Na (g) + Cl (g) NaCl(s) H5 2 3 This last step involves the lattice energy magnesium. High-purity boron for semi- ∆ conductor applications is obtained by con- H5. The sum of all these enthalpies is equal to the heat of the reaction: version to BORON TRICHLORIDE, which can ½ → Na(s) + Cl2(g) NaCl(s) be purified by distillation, then reduction See also Hess’s law. using hydrogen. Only small quantities of elemental boron are needed commercially; Born–Oppenheimer approximation the vast majority of boron is used in the An approximation used in the quantum form of disodium tetraborate decahydrate theory of molecules in which it is assumed (borax) or boric acid.

_ _ _ O O O O B B B _ O O B _ O O O

B O B

_ O O

_ 3 n _ B O ion 3 6 (BO ) ion 2 n

Borate: examples of cyclic and linear borate ions

32 boron hydride

OH the halides are obtained via the BF3 route from boron oxide: → B B2O3 + 3CaF2 + 3H2SO4 3CaSO4 + 3H2O + 2BF3 O O → BF3 + AlCl3 AlF3 + BCl3 The halides are all covalent molecules, BHO O BOHwhich are all planar and trigonal in shape. Boron halides are industrially important as catalysts or promoters in a variety of or- O O ganic reactions. The decomposition of B boron halides in atmospheres of hydrogen at elevated temperatures is also used to deposit traces of pure boron in semiconduc- OH tors. Boron forms a range of compounds 2– The ion [B4O5(OH)4] present in borax with elements that are less electronegative than itself, called BORIDES. Natural boron Because boron has a small atom and consists of two isotopes, 10B (18.83%), has a relatively high ionization potential its used in steel alloys for control rods in nu- compounds are predominantly covalent; clear reactors, and 11B (81.17%). These the ion B3+ does not exist. Boron does not percentages are sufficiently high for their react directly with hydrogen but the hy- detection by splitting of infrared absorp- drolysis of magnesium boride does pro- tion or by nuclear magnetic resonance duce a range of BORON HYDRIDES. The spectroscopy. Both borax and boric acid are used as species BH3 is only a short-lived reaction intermediate. mild antiseptics and are not generally re- Finely divided boron burns in oxygen garded as toxic; boron hydrides are, how- above 600°C to give BORON OXIDE, B O , ever, highly toxic. 2 3 ° ° an acidic oxide which will dissolve slowly Symbol: B; m.p. 2300 C; b.p. 2658 C; r.d. 2.34 (20°C); p.n. 5; r.a.m. 10.811. in water to give boric acid (H3BO3) and rapidly in alkalis to give borates such as Na B O . A number of polymeric species 2 4 7 H H H with B–B and B–O links are known, e.g. a lower oxide (BO) , and a polymeric acid x B B (HBO2)n. Although the parent acid is weak, many salts containing borate anions are known but their stoichiometry gives lit- H H H tle indication of their structures, many of which are cyclic or linear polymers. These Boron hydride: the bonding in diborane polymers contain both BO3 planar groups and BO4 tetrahedra. Boric acid and the bo- boron hydride (borane) Any of a num- rates give a range of glassy substances on ber of binary compounds of boron and hy- heating; these contain cross-linked B–O–B drogen. A mixture of boron hydrides can chains and nets. In a procedure known as be obtained by the action of acid on mag- the BORAX-BEAD TEST, these materials in the nesium boride (MgB2); others can be made molten state react with metal ions to form by controlled PYROLYSIS of diborane borates, which on cooling give characteris- (B2H6), which is the simplest member of tic colors to the glass. this class of compounds. Many boranes Boron reacts with nitrogen on strong have formulae of the type BnHn + 4 and ° heating (1000 C) to give BORON NITRIDE. BnHn + 6, where n is an integer. Examples Elemental boron reacts directly with fluo- are B5H9, B6H10, B4H10, and B5H11. Typi- rine and chlorine but for practical purposes cally, the boron hydrides are volatile,

33 boron nitride highly reactive compounds that oxidize in boron oxide (boric anhydride; boric(III) air, some explosively. oxide; diboron trioxide; B2O3) A glassy The boron hydrides are examples of hygroscopic solid that eventually forms ELECTRON-DEFICIENT COMPOUNDS, in which boric acid. It has amphoteric properties. there are not enough electrons to form classical electron-pair bonds. For example, diboron tribromide (BBr3) A colorless borane (B2H6) has a formular resembling liquid. See boron; boron trichloride. that of ethane (C2H6), but its molecular structure and bonding are quite different. boron trichloride (BCl3) A fuming liq- The two boron atoms are not linked di- uid made by passing dry chlorine over rectly but rather are joined through two heated boron. It is rapidly hydrolysed by hydrogen bridges. These B-H-B links can- water: → not be explained in terms of two localized BCl3 + 3H2O 3HCl + H3BO3 bonds. Instead, the bonding involves the As there are only three pairs of shared idea of a MULTICENTER BOND in which three electrons in the outer shell of the boron atoms are bound by two electrons. These atom, boron halides form very stable addi- are known as 3c,2e bonds, i.e. having three tion compounds with ammonia by the ac- centers and two electrons, in contrast to ceptance of a lone electron pair in a conventional electron-pair bonds, which coordinate bond to complete a shared are 2c,2e bonds. In more complex boron octet. See also boron. hydrides it is possible for three boron atoms to be at the vertices of a triangle boron trifluoride (BF3) A colorless with three orbitals overlapping at the cen- fuming gas made by heating a mixture of ter, the resulting molecular orbital contain- boron oxide, calcium fluoride, and concen- ing two electrons. trated sulfuric acid. See boron; boron In addition to the neutral boron hy- trichloride. drides there are numbers of negative borohydride ions. Typically these have borosilicates Complex compounds sim- 2– 2– formulae of the type BnHn , e.g. B6H6 . ilar to silicates, but containing BO3 and There are various types of boron hydride BO4 units in addition to the SiO4 units. and borohydride ion. Those with a closed Certain crystalline borosilicate minerals polyhedron of boron atoms are designated such as danburite, CaB2Si2O8, are known. closo compounds. Ones in which there is In addition, borosilicate glasses can be an incomplete polyhedron (by removal of made by using boron oxide in addition to one vertex) are called nido compounds silicon(IV) oxide. These glasses tend to be (from the Greek word for ‘nest’). Those tougher and more heat resistant than stan- with a more open structure (removal of dard silicate glass. Pyrex, for example, is two or more vertices) are arachno com- commonly used in laboratory glassware pounds (from the Greek ‘spider’). and in cookware. boron nitride (BN) A compound Bosch, Carl (1874–1940) German in- formed by heating boron in nitrogen in the dustrial chemist. Bosch joined the large vicinity of 1000°C. The material has an ex- German dyestuffs company, Badische tremely high melting point and is thermally Anilin und Soda Fabrik (BASF), in 1899. very stable but there is sufficient bond po- Following Fritz Haber’s successful small- larity in the B–N links to permit slow hy- scale ammonia synthesis in 1909, Bosch drolysis by water to give ammonia. It has began to develop a high-pressure ammonia two crystalline forms. One is a slippery plant at Oppau for BASF. The plant was white solid with a layer structure similar to opened in 1912 – a successful application that of graphite, i.e. hexagonal rings of al- of the HABER PROCESS on a large scale. ternating B and N atoms. The other is a ‘di- Bosch also introduced the use of the water- amond-like’ form of B–N that some claim gas shift reaction as a source of hydrogen to be as hard or even harder than diamond. for the process:

34 brass

CO + H2O = CO2 + H2 Brackett series See hydrogen atom After World War I the large-scale am- spectrum. monia fertilizer industry was established and the high-pressure technique was ex- Bragg, Sir William Henry (1862– tended by BASF to the synthesis of 1942) and Bragg, Sir William Lawrence methanol from carbon monoxide and hy- (1890–1971) British physicists. The Braggs drogen in 1923. Bosch shared the Nobel (father and son) realized very soon after the Prize for chemistry with Friedrich Bergius discovery of x-ray diffraction in 1912 that in 1931. this phenomenon could be used to determine the structure of crystals. Lawrence Bosch process The reaction between Bragg formulated the Bragg equation in carbon monoxide and steam over a hot cat- 1912. This initiated the subject of x-ray alyst: crystallography. One of their early results → CO + H2O CO2 + H2 was to show that sodium chloride consists It has been used as a source of hydrogen for of sodium ions and chloride ions rather the Haber process. The process was devel- than NaCl molecules. Lawrence Bragg sub- oped by the German chemist Carl Bosch sequently studied silicates and encouraged (1874–1940). research on complex molecules of biological interest. The Braggs shared the 1915 bowl classifier A device that separates Nobel Prize for physics. solid particles in a mixture of solids and liquid into fractions according to particle Bragg equation An equation used to size. Feed enters the center of a shallow deduce the crystal structure of a material bowl, which contains revolving blades. using data obtained from x-rays directed at The coarse solids collect on the bottom, its surface. The conditions under which a fine solids at the edge. crystal will reflect a beam of x-rays with maximum intensity is: Boyle, Robert (1627–91) English scien- nλ = 2dsinθ tist who is generally regarded as one of the where θ is the angle of incidence and re- founders of modern chemistry. He estab- flection (Bragg angle) that the x-rays make lished the law known as BOYLE’S LAW relat- with the crystal planes, n is a small integer, ing the pressure and volume of gases λ experimentally and gave an explicit state- is the wavelength of the x-rays, and d is ment of it in 1662. In 1661 he published a the distance between the crystal planes. book entitled The Sceptical Chymist in The equation was discovered by Lawrence which he put forward the idea that all mat- Bragg in 1912. ter is corpuscular in nature, with the cor- puscules having different sizes and shapes. branched chain See chain. This idea enabled him to distinguish between compounds and mixtures. Boyle was brass Any of a group of copper–zinc al- also able to distinguish between acids and loys containing up to 50% of zinc. The bases by using vegetable dyes as indicators. color of brass changes from red-gold to golden to silvery-white with increasing zinc Boyle’s law At a constant temperature, content. Brasses are easy to work and resist the pressure of a fixed mass of a gas is in- corrosion well. Brasses with up to 35% versely proportional to its volume: i.e. zinc can be worked cold and are specially pV = K suited for rolling into sheets, drawing into Here K is a constant whose value depends wire, and making into tubes. Brasses with on the temperature and on the nature of 35–46% zinc are harder and stronger but the gas. The law holds strictly only for less ductile; they require hot working (e.g. ideal gases. Real gases follow Boyle’s law forging). The properties of brass can be im- at low pressures and high temperatures. proved by the addition of other elements; See gas laws. lead improves its ability to be machined,

35 bremsstrahlung while aluminum and tin increase its corro- fumigants. Bromine compounds are also sion resistance. See also bronze. used as flame retardants and refrigerants. The electropositive elements form elec- bremsstrahlung See x-radiation. trovalent bromides and the nonmetals form fully covalent bromides. Like chlo- brine A concentrated solution of sodium rine, bromine forms oxides, Br2O and chloride or calcium chloride. The term is BrO2, both of which are unstable. The re- also applied to naturally occurring solu- lated oxo-acid anions hypobromite (BrO–) – tions of other salts. and bromate (BrO3 ) are formed by the reaction of bromine with cold aqueous alkali Brin process An obsolete process for and hot aqueous alkali respectively, but the producing oxygen by first heating barium bromine analogs of chlorite and perchlo- oxide in air to produce the peroxide rate are not known. (BaO2); then heating the peroxide at higher Bromine and the interhalogens, chemi- temperatures to release oxygen. cal compounds formed between two hao- gens, are highly toxic. Liquid bromine and bromic(I) acid (hypobromous acid; bromine solutions are also very corrosive HBrO) A pale yellow liquid made by re- and goggles and gloves should always be acting mercury(II) oxide with BROMINE worn when handling such compounds. WATER. Bromic(I) acid is a weak acid in Symbol: Br; m.p. –7.25°C; b.p. aqueous solution but is a good oxidizing 58.78°C; r.d. 3.12 (20°C); p.n. 35; r.a.m. agent with strong bleaching power. 79.904. bromic(V) acid (HBrO3) A colorless bromine trifluoride (BrF3) A colorless liquid made by the addition of dilute sulfu- fuming liquid made by direct combination ric acid to barium bromate. It is a strong of fluorine and bromine. It is a very reac- acid in aqueous solution. tive compound, its reactions being similar to those of its component halogens. bromide See halide. bromine water A yellow solution of bromination See halogenation. bromine in water, which contains bromic(I) acid (hypobromous acid, HBrO). bromine A deep red, moderately reac- It is a weak acid and a strong oxidizing tive element belonging to the halogens; i.e. agent, which decomposes to give a mixture – – group 17 (formerly VIIA) of the periodic of bromide (Br ) and bromate(V) (BrO3 ) table. Bromine is a volatile liquid at room ions. temperature (mercury being the only other element with this property). It occurs in Brønsted acid See acid. small amounts in seawater, salt lakes, and salt deposits but is much less abundant bronze Any of a group of copper-tin al- than chlorine. Bromine reacts with most loys usually containing 0.5–15% of tin. metals but generally with less vigor than They are generally harder, stronger in com- chlorine. It has a lower oxidizing power pression, and more corrosion resistant than chlorine and consequently can be re- than brass. Zinc is often added, as in gun- leased from solutions of bromides by reac- metal (2–4% zinc), to increase strength tion with chlorine gas. The laboratory and corrosion-resistance; bronze coins method is the more convenient oxidation often contain more zinc (2.5%) than tin of bromides using manganese dioxide. In- (0.5%). The presence of lead improves its dustrially most bromine is produced by machining qualities. displacement with chlorine from naturally Some copper-rich alloys containing no occurring brines. Bromine and its com- tin are also called bronzes. Aluminum pounds are used in pharmaceuticals, pho- bronzes, for example, with up to 10% alu- tography, chemical synthesis, and minum, are strong, resistant to corrosion

36 Bunsen, Robert Wilhelm and wear, and can be worked cold or hot; This procedure also produces less stable silicon bronzes, with 1–5% silicon, have carbon clusters, such as C70. Buckminster- high corrosion-resistance; beryllium fullerene can be produced more conve- bronzes, with about 2% beryllium, are niently using an electric arc between very hard and strong. See also brass. graphite electrodes in an inert gas. The allotrope is soluble in benzene, from which it brown coal See lignite. can be crystallized to give yellow crystals. This solid form is known as fullerite. brown-ring test A qualitative test used The discovery of buckminsterfullerene for the detection of an ionic nitrate. A led to a considerable amount of research freshly prepared solution of iron(II) sulfate into its properties and compounds. Partic- is mixed with the sample and concentrated ular interest has been shown in trapping sulfuric acid is introduced slowly to the metal ions inside the carbon cage to form bottom of the tube using a dropping enclosure compounds. The term fullerene pipette, so that two layers are formed. A also applies to derivatives of buckminster- brown ring formed at the interface where fullerene and to similar clusters (e.g. C70). the liquids meet indicates the presence of Carbon structures similar to that in C60 can nitrate. The brown color is caused by the also form small tubes, known as bucky presence of [Fe(NO)]SO4, which breaks tubes. down on shaking. bucky ball See buckminsterfullerene.

bucky tube See buckminsterfullerene.

buffer A solution in which the pH remains reasonably constant when acids or alkalis are added to it; i.e. it acts as a buffer against (small) changes in pH. Buffer solutions generally contain a weak acid and one of its salts derived from a strong base; e.g. a solution of ethanoic acid and sodium ethanoate. If an acid is added, the H+ reacts with the ethanoate ion (from dissociated sodium ethanoate) to form undissociated ethanoic acid; if a base is added the OH– reacts with the ethanoic acid to form water Buckminsterfullerene and the ethanoate ion. The effectiveness of the buffering action is determined by the buckminsterfullerene (fullerene) An concentrations of the acid–anion pair: + – allotrope of carbon discovered in soot in K = [H ][CH3COO ]/[CH3COOH] 1985, that contains clusters of 60 carbon where K is the dissociation constant. atoms bound in a highly symmetric poly- Phosphate, oxalate, tartrate, borate, hedral structure. The C60 polyhedron has a and carbonate systems can also be used for combination of pentagonal and hexagonal buffer solutions. faces similar to the panels on a soccer ball. The molecule was named after the Ameri- bumping Violent boiling of a liquid can architect Richard Buckminster Fuller caused when bubbles form at a pressure (1895–1983) because its structure resem- above atmospheric pressure. bles the geodesic dome, which was invented by Fuller. The C60 polyhedra are Bunsen, Robert Wilhelm (1811–99) informally called bucky balls. The original German chemist. Along with Gustav Kir- method of making the allotrope was to fire choff, Bunsen pioneered the use of spec- a high-power laser at a graphite target. troscopy to identify chemical elements.

37 Bunsen burner

This work was started in 1859 and soon the negative electrode consists of zinc led them to the discovery of rubidium and plates in sulfuric acid solution. cesium. The Bunsen burner is named for Robert Bunsen, although it was probably burette A piece of apparatus used for developed by Bunsen’s technician Peter the addition of variable volumes of liquid Desdega. in a controlled and measurable way. The burette is a long cylindrical graduated tube Bunsen burner A gas burner consisting of uniform bore fitted with a stopcock and of a vertical metal tube with an adjustable a small-bore exit jet, enabling a drop of liq- air-inlet hole at the bottom. Gas is admit- uid at a time to be added to a reaction ves- ted into the bottom of the tube and the sel. Burettes are widely used for titrations in volumetric analysis. Standard burettes gas–air mixture is burnt at the top. With 3 too little air the flame is yellow and sooty. permit volume measurement to 0.005 cm and have a total capacity of 50 cm3; a vari- Correctly adjusted, the burner gives a ety of smaller microburette is available. flame with a pale blue inner cone of in- Similar devices are used to introduce meas- completely burnt gas, and an almost invis- ured volumes of gas at regulated pressure ible outer flame where the gas is fully in the investigation of gas reactions. oxidized and reaches a temperature of ° about 1500 C. The inner region is the re- by-product A substance obtained dur- ducing part of the flame and the outer re- ing the manufacture of a main chemical gion the oxidizing part. The Bunsen burner product. For example, calcium chloride is a is named for Robert Bunsen, who helped by-product of the Solvay process for mak- make it popular, although it was probably ing sodium carbonate. Some metals are ob- invented by Michael Faraday and im- tained as by-products in processes to proved by Bunsen’s technician Peter Des- extract other metals. Cadmium, for in- dega. See also borax-bead test. stance, is a by-product in the extraction of zinc. Bunsen cell A type of primary cell in which the positive electrode is formed by B–Z reaction See Belousov–Zhabotin- carbon plates in a nitric acid solution and skii reaction.

38 C

cadmium A transition metal, an element (formerly IIA) of the periodic table. The in group 12 (formerly IIB) of the periodic electronic configuration is that of argon table, obtained as a by-product during the with an additional pair of 4s electrons. extraction of zinc. It is used to protect Calcium is widely distributed in the other metals from corrosion, as a neutron Earth’s crust and is the third most abun- absorber in nuclear reactors, in alkali bat- dant element. Large deposits occur as teries, and in certain pigments. It is highly chalk or marble, CaCO3; gypsum, CaSO4.- toxic. 2H2O; anhydrite, CaSO4; fluorspar, CaF; ° Symbol: Cd; m.p. 320.95 C; b.p. and apatite, CaF2.Ca3(PO4)3. However 765°C; r.d. 8.65 (20°C); p.n. 48; r.a.m. sufficiently large quantities of calcium 112.411. chloride are available as waste from the Solvay process to satisfy industrial require- cadmium cell See Weston cadmium ments for the metal, which is produced by cell. electrolysis of the fused salt. Large quantities of lime, Ca(OH)2, and quicklime, caesium See cesium. CaO, are produced by decomposition of the carbonate for use in both building and cage compounds See clathrate. agriculture. Several calcium minerals are mined as a source of other substances. calamine 1. A mineral consisting of hy- Thus, limestone is a cheap source of carbon drated zinc silicate (2ZnO.SiO2.H2O). It is dioxide, gypsum and anhydrite are used in also known as hemimorphite. the manufacture of sulfuric acid, rock 2. Zinc carbonate (ZnCO3), which occurs phosphate is a source of phosphoric acid, naturally as a mineral also known as smith- and fluorspar for a range of fluorochemi- sonite. The carbonate is used medicinally cals. in suspension as a soothing lotion for sun- Calcium has a low ionization potential burn and skin complaints. Formerly, the and a relatively large atomic radius. It is natural mineral was used but now medici- therefore a very electropositive element. nal calamine is made by precipitating a The metal is very reactive and the com- basic zinc carbonate from a solution of a pounds contain the divalent ion Ca2+. Cal- zinc salt. It is usually colored pink by the cium forms the oxide (CaO), a white ionic addition of small quantities of iron(III) solid, on burning in air, but for practical oxide. purposes the oxide is best prepared by heating the carbonate, which decomposes calcination The formation of a calcium at about 800°C. Both the oxide and the carbonate deposit from hard water. metal itself react with water to give the basic hydroxide (Ca(OH)2). On heating calcite A mineral form of calcium car- with nitrogen, sulfur, or the halogens, cal- bonate occurring in limestone, chalk, and cium reacts to form the nitride (Ca3N2), marble. sulfide (CaS), or the halides (CaX2). Cal- cium also reacts directly with hydrogen to calcium A moderately soft, low-melting give the hydride CaH2 and borides, ar- reactive metal; the third element in group 2 senides, carbides, and silicides can be pre-

39 calcium acetylide pared in a similar way. Both the carbonate bonate are slowly formed when water is and sulfate are insoluble. Calcium salts im- added: → part a characteristic brick-red color to CaCN2 + 3H2O CaCO3 + 2NH3 flames which is an aid to qualitative analy- Other uses include the defoliation of cot- sis. At ordinary temperatures calcium has ton plants and the production of the face-centered cubic structure with a melamine. transition at 450°C to the close-packed hexagonal structure. calcium dicarbide (calcium acetylide; cal- Symbol: Ca; m.p. 839°C; b.p. 1484°C; cium carbide; CaC2) A colorless solid r.d. 1.55 (20°C); p.n. 20; r.a.m. 40.0878. when pure. In countries where electricity is cheap, calcium dicarbide is produced on a calcium acetylide See calcium dicar- large scale by heating calcium oxide with ° bide. coke at a temperature in excess of 2000 C in an electric-arc furnace. Water is then calcium bicarbonate See calcium hy- added to give ethyne (acetylene, C2H2), an drogencarbonate. important industrial organic chemical: → CaC2 + 2H2O C2H2 + Ca(OH)2 calcium carbide See calcium dicarbide. The structure of calcium dicarbide is interesting because the carbon is present as 2– carbide ions (C2 ). See ethyne. calcium carbonate (CaCO3) A white solid that occurs naturally in two crystalline forms: calcite and aragonite. These calcium fluoride (CaF2) A white crys- minerals make up the bulk of such rocks as talline compound found naturally as fluo- marble, limestone, and chalk. Calcium car- rite (flourspar). bonate also occurs in the mineral dolomite calcium-fluoride structure (fluorite (CaCO .MgCO ). It is sparingly soluble in 3 3 structure) A form of crystal structure in water but dissolves in rainwater containing which each calcium ion is surrounded by carbon dioxide to form calcium hydrogen- eight fluorine ions arranged at the corners carbonate, which causes temporary hard- of a cube and each fluorine ion is sur- ness of water. Calcium carbonate is a basic rounded tetrahedrally by four calcium raw material in the Solvay process and is ions. used for making glass, mortar, and cement. calcium hydrogencarbonate (calcium calcium chloride (CaCl2) A white solid bicarbonate, Ca(HCO3)2) A solid formed that occurs in a number of hydrated forms when water containing carbon dioxide dis- and is readily available as a by-product of solves calcium carbonate: the Solvay process. It is readily soluble in → CaCO3 + H2O + CO2 Ca(HCO3)2 water and the solution, known as brine, is Calcium hydrogencarbonate is a cause of used in refrigerating plants. Other applica- temporary hard water. The solid is un- tions that depend on its water-absorbing known at room temperature. See hardness. property and the low freezing point of the aqueous solution include the suppression calcium hydroxide (slaked lime; caustic of dust on roads and in mines and the melt- lime; Ca(OH)2) A white solid that dis- ing of snow. Calcium chloride is also used solves sparingly in water to give the alkali as the electrolyte in the production of cal- known as limewater. Calcium hydroxide is cium. manufactured by adding water to the oxide (lime or quicklime), a process known as calcium cyanamide (CaCN2) A solid slaking, which evolves much heat. If just prepared by heating calcium dicarbide to a sufficient water is added so that the oxide temperature in excess of 800°C in an turns to a fine powder, the product is atmosphere of nitrogen. It is used as a fer- slaked lime. If more water is added, a thick tilizer because ammonia and calcium car- suspension called milk of lime is formed.

40 calomel electrode

Calcium hydroxide has many uses. As a component of the slag produced in smelt- base, it is used to neutralize acid soil and in ing iron and other metals in a blast furnace. industrial processes such as the Solvay process. It is also used in the manufacture calcium stearate See calcium octade- of mortar, whitewash, and bleaching pow- canoate. der and for the softening of temporary hard water. calcium sulfate (anhydrite; CaSO4)A white solid that occurs abundantly as the calcium nitrate (Ca(NO3)2) A deliques- mineral anhydrite and as the dihydrate cent salt that is very soluble in water. It is (CaSO4.2H2O), known as gypsum or al- usually crystallized as the tetrahydrate abaster. When heated, gypsum loses water Ca(NO3)2.4H2O. When the hydrate is to form the hemihydrate (2CaSO4.H2O), heated, the anhydrous salt is first produced which is plaster of Paris. If the water is re- and this subsequently decomposes to give placed, gypsum reforms and sets as a solid. calcium oxide, nitrogen dioxide, and oxy- Plaster of Paris is therefore used for taking gen. Calcium nitrate is used as a nitroge- casts and for setting broken limbs. Calcium nous fertilizer. sulfate is sparingly soluble in water and is a cause of permanent hardness in water. It calcium octadecanoate (calcium is used in ceramics, paint, and in paper stearate; Ca(CH3(CH2)16COO)2) An in- making. soluble salt of octadecanoic acid. It is formed as ‘scum’ when soap, containing Calgon (Trademark) A substance often the soluble salt sodium octadecanoate, is added to detergents to remove unwanted mixed with hard water containing calcium chemicals that have dissolved in water and ions. See detergents. would otherwise react with soap to form a scum. Calgon consists of complicated calcium oxide (quicklime; CaO) A sodium polyphosphate molecules, which white solid formed by heating calcium in absorb dissolved calcium and magnesium oxygen or, more widely, by the thermal de- ions. The metal ions become trapped composition of calcium carbonate. On a within the Calgon molecules. large scale, limestone (calcium carbonate) is heated in a tall tower called a lime kiln to caliche An impure commercial form of a temperature of 550°C. The reversible re- sodium nitrate. action: ˆ CaCO3 CaO + CO2 californium A silvery radioactive trans- proceeds in a forward direction as the car- uranic element of the actinoid series of bon dioxide is carried away by the upward metals, not found naturally on Earth. Sev- current through the kiln. Calcium oxide is eral radioisotopes have been synthesized, used in extractive metallurgy to produce a including californium-252, which is used slag with the impurities in metal ores; it is as an intense source of neutrons in certain also used as a drying agent and it is an in- types of portable detector and in the treat- termediate for the production of calcium ment of cancer. hydroxide. Symbol: Cf; m.p. 900°C; p.n. 98; most stable isotope 251Cf (half-life 900 years). calcium phosphate (Ca3(PO4)2)A solid that occurs naturally in the mineral calixarene See host–guest chemistry. apatite (CaF2.Ca3(PO4)3) and in rock phosphate. It is the chief constituent of animal calomel See mercury(I) chloride. bones and is used extensively in fertilizers. calomel electrode A half cell having a calcium silicate (Ca2SiO4) A white in- mercury electrode coated with mercury(I) soluble crystalline compound found in var- chloride (called calomel), in an electrolyte ious cements and minerals. It is also a consisting of potassium chloride and (satu-

41 calorie rated) mercury(I) chloride solution. Its organic reaction known as Cannizzaro’s standard electrode potential against the hy- reaction in 1853. drogen electrode is accurately known (–0.2415 V at 25°C) and it is a convenient canonical form See resonance. secondary standard. carbide A compound of carbon with a calorie Symbol: cal A unit of energy ap- more electropositive element. The carbides proximately equal to 4.2 joules. It was for- of the elements are classified into: merly defined as the energy needed to raise 1. Ionic carbides, which contain the car- the temperature of one gram of water by bide ion C4–. An example is aluminum one degree Celsius. Because the specific carbide, Al4C3. Compounds of this type thermal capacity of water changes with react with water to give methane (they temperature, this definition is not precise. were formerly also called methanides). The mean or thermochemical calorie The dicarbides are also ionic carbon compounds but contain the dicarbide (calTH) is defined as 4.184 joules. The in- ion –C:C–. The best-known example is ternational table calorie (calIT) is defined as 4.1868 joules. Formerly the mean calorie calcium dicarbide, CaC2, also known as was defined as one hundredth of the heat calcium carbide, or simply carbide. needed to raise one gram of water from Compounds of this type give ethyne 0°C to 100°C, and the 15°C calorie as the (acetylene) with water. They were for- heat needed to raise it from 14.5°C to merly called acetylides or ethynides. Ionic carbides are formed with very elec- 15.5°C. tropositive metals. They are crystalline. 2. Covalent carbides, which have giant calorific value See bomb calorimeter. molecular structures, as in SILICON CAR- BIDE (SiC) and boron carbide (B C ). calorimeter A device or apparatus for 4 3 These are hard solids with high melting measuring thermal properties such as spe- points. cific heat capacity, calorific value, etc. See 3. Interstitial carbides, which are INTERSTI- bomb calorimeter. TIAL COMPOUNDS of carbon with transition metals. Titanium carbide (TiC) is an calx A metal oxide obtained by heating example. These compounds are all hard an ore to high temperatures in air. solids with high melting points and metallic properties. Some carbides (e.g. candela Symbol: cd The SI base unit of nickel carbide Ni3C) have properties in- luminous intensity, equal to that, in a given termediate between those of interstitial direction, of a source emitting monochro- and ionic carbides. matic radiation having a frequency of 540 terahertz and a radiant intensity of 1/683 carbon The first element of group 14 watt. (formerly IVA) of the periodic table. Car- bon is a universal constituent of living mat- Cannizzaro, Stanislao (1826–1910) ter and the principal deposits of carbon Italian chemist. Cannizzaro was responsi- compounds, i.e. chalk and limestone, from ble for reviving interest in the ideas of which we obtain carbonates, and coal, oil, Amedeo AVOGADRO in a pamphlet pub- and gas fields, from which we obtain fossil lished in 1858 and in an address to the fuels, are derived from once living organ- Chemical Congress at Karlsruhe, Ger- isms. Carbon also occurs in the mineral many. His pamphlet clarified the concepts dolomite. The element forms only 0.032% of atomic and molecular weights and also by mass of the Earth’s crust. Minute quan- showed how the molecular weight (now tities of elemental carbon also occur as the termed relative molecular mass) of a com- allotropes graphite and diamond. A third pound could be determined by measuring allotrope, BUCKMINSTERFULLERENE (C60), its vapor density. He also discovered the was discovered in the mid-1980s.

42 carbon

The industrial demand for graphite is tained by heating the metal oxide with car- such that it is manufactured in large quan- bon or heating the metal with a hydrocar- tities using the Acheson process in which bon. There is a bewilderingly wide range of coke and small amounts of asphalt or clay metal carbides, both saltlike with elec- are raised to high temperatures. Large tropositive elements (for example, CaC2) quantities of impure carbon are also con- and covalent with the metalloids (for ex- sumed in the reductive extraction of met- ample, SiC), and there are also many inter- als. In addition to the demand for diamond stitial carbides formed with metals such as as a gemstone there is a large industrial de- Cr, Mn, Fe, Co, and Ni. mand for small low-grade diamonds for Compounds with C–N bonds form a use in drilling and grinding machinery. significant branch of the inorganic chem- Studies show that graphite and diamond istry of carbon: these comprise hydrogen ° can be interconverted at 3000 C under ex- cyanide (HCN) and the cyanides, cyanic tremely high pressures, but that commer- acid (HNCO) and the cyanates, and thio- cial exploitation of this process would not cyanic acid (HNCS) and the thiocyanates. be viable. Naturally occurring carbon has the iso- Carbon burns in oxygen to form carbon topic composition 12C (98.89%), 13C dioxide and carbon monoxide. Carbon (1.11%), and radioactive 14C (minute dioxide is soluble in water, forming the traces of which are produced in the upper weakly acidic carbonic acid, which is the atmosphere as the result of the capture of parent acid of the metal carbonates. In con- 14 trast CO is barely soluble in water but will high-energy neutrons by N atoms, which react with alkali to give the methanoate each then release a proton to decay into 14 14 (formate) ion: C). Carbon-14 ( C) is used for radiocar- – → – bon dating because it is taken up in minute CO + OH HCO2 Carbon will react readily with sulfur at quantities by organisms when they are alive and decays over the comparatively red heat to form carbon disulphide, CS2, but it does not react directly with nitrogen. long half-life of 5780 years after their Cyanogen, (CN)2, must be prepared by death. heating covalent metal cyanides such as Symbol: C; m.p. 3550°C; b.p. 4830°C ° CuCN. Carbon will also react directly with (sublimes); C60 sublimes at 530 C; r.d. many metals at elevated temperatures to 3.51 (diamond), 2.26 (graphite), 1.65 (C60) give CARBIDES. Carbides can also be ob- (all at 20°C); p.n. 6; r.a.m. 12.011.

Graphite

Diamond

Carbon

43 carbonate carbonate Any salt of carbonic acid (i.e. are used, for instance, to strengthen POLY- 2– containing the ion CO3 ). Many alkali MERS. They are made by heating stretched and alkaline-earth carbonates are impor- textile fibers and have an orientated crystal tant industrially. structure. carbonation 1. The solution of carbon carbonic acid (H2CO3)A DIBASIC ACID dioxide in a liquid under pressure, as in formed in small amounts in solution when carbonated soft drinks. carbon dioxide dissolves in water: ˆ 2. The addition of carbon dioxide to com- CO2 + H2O H2CO2 pounds. It forms two series of salts: hydrogencar- – 2– bonates (HCO3 ) and carbonates (CO3 ). carbon black A finely divided form of The pure acid cannot be isolated. carbon produced by the incomplete combustion of such hydrocarbon fuels as nat- carbonize (carburize) To convert an or- ural gas or oil. It is used as a black pigment ganic compound into carbon by incom- in inks, paints, and plastics, and as a filler plete oxidation at high temperature. for rubber in tire manufacture. carbon monoxide (CO) A colorless carbon dioxide (CO2) A colorless flammable toxic gas formed by the incom- odorless nonflammable gas formed when plete combustion of carbon or carbon-con- carbon burns in excess oxygen. It is also taining compounds. Industrially, it is produced by respiration. Carbon dioxide is produced by the oxidation of carbon or present in the atmosphere (0.03% by vol- methane, or by the reaction used to pro- ume) and is converted in plants to carbo- duce WATER GAS. It is a powerful reducing hydrates by photosynthesis. In the agent and is used as such in metallurgy. laboratory it is made by the action of dilute Carbon monoxide is neutral and only acid on metal carbonates. Industrially, it is sparingly soluble in water. It forms metal obtained as a by-product in certain carbonyls with transition metals. Its toxic- processes, such as fermentation and the ity derives from the fact that hemoglobin manufacture of calcium oxide. The main (in the blood) has a far stronger affinity for uses of carbon dioxide are as a refrigerant carbon monoxide than for oxygen, which in its solid state, called dry ice, and in fire is essential for cell metabolism. extinguishers and carbonated drinks. In- creased levels of carbon dioxide in the at- carbonyl A complex in which carbon mosphere from the combustion of fossil monoxide ligands are coordinated to a fuels are thought to contribute to the metal atom. A common example is greenhouse effect. tetracarbonyl nickel(0), Ni(CO)4. Carbon dioxide is the anhydride of the weak acid carbonic acid, which is formed carbonyl chloride (phosgene; COCl2) in water: A colorless, toxic gas with a choking smell, → CO2 + H2O H2CO3 made by reacting carbon monoxide and chlorine in the presence of light or a cata- carbon disulfide (CS2) A colorless poi- lyst. It is used as a chlorinating agent and sonous flammable liquid made from to make certain plastics and insecticides; it methane (derived from natural gas) and was formerly employed as a war gas. sulfur. It is a solvent for waxes, oils, and rubber, and is used in the manufacture of carbonyl group The group =C=O. It viscose rayon. The pure compound is vir- occurs in organic compounds, and in car- tually odorless, but CS2 usually has a re- bonyl complexes of transition metals. volting smell because of the presence of other sulfur compounds. carborundum See silicon carbide. carbon fibers Fibers of graphite, which carboxylate ion The ion –COO–, pro-

44 catalyst duced by ionization of a carboxyl group. In the working substance. If heat is absorbed a carboxylate ion the negative charge is at temperature T1 and given out at T2, then generally delocalized over the O–C–O the efficiency according to Carnot’s princi- grouping and the two C–O bonds have the ple is (T1 – T2)/T1. See Carnot cycle. same length, intermediate between that of a double C=O and a single C–O. Carnot’s theorem See Carnot’s principle. carboxyl group The organic group –CO.OH, present in carboxylic acids. Caro’s acid See peroxosulfuric(VI) acid. carboxylic acid A type of organic com- carrier gas The gas used to carry the pound containing the CARBOXYL GROUP. sample in gas chromatography. Simple carboxylic acids have the general formula RCOOH. Many carboxylic acids case hardening Processes for increasing occur naturally in plants and (in the form the hardness of the surface or ‘case’ of the of esters) in fats and oils, hence the alter- steel used to make such components as native name fatty acids. gears and crankshafts. The oldest method is carburizing, in which the carbon content carburizing See case hardening. of the surface layer is increased by heating the component in a carbon-rich environ- carcinogen An agent that causes or pro- ment. Nitriding involves the diffusion of motes the development of cancer in ani- nitrogen into the surface layer of the steel, mals, such as tobacco smoke or ionizing thus forming intensely hard NITRIDE parti- radiation. cles in the structure. A combination of both carburizing and nitriding is sometimes em- carnallite A mineral chloride of potas- ployed. sium and magnesium, KCl.MgCl2.6H2O. It is used as a source of potassium salts for cast iron Any of various alloys of iron fertilizers. and carbon made by remelting the crude iron produced in a blast furnace. The car- Carnot cycle The idealized reversible bon content is usually between 2.4 and cycle of four operations occurring in a per- 4.0% and may be present as iron carbide fect heat engine. These are the successive (FeC) or as graphite. In the former case the adiabatic compression, isothermal expan- product is known as white cast iron, and in sion, adiabatic expansion, and isothermal the latter as gray cast iron. Other metals compression of the working substance. may be added to improve the properties of The cycle returns to its initial pressure, vol- the alloy. Additional elements such as ume, and temperature, and transfers en- phosphorus, sulfur, and manganese are ergy to or from mechanical work. The also present as impurities. Cast iron is efficiency of the Carnot cycle is the maxi- cheap and has an extensive range of possi- mum attainable in a heat engine. It was ble properties. It is used on a very large published in 1824 by the French physicist scale. Nicolas L. S. Carnot (1796–1832). See Carnot’s principle. catalyst A substance that alters the rate of a chemical reaction without itself being Carnot’s principle (Carnot theorem) changed chemically in the reaction. The The efficiency of any heat engine cannot be catalyst can, however, undergo physical greater than that of a reversible heat engine change; for example, large lumps of cata- operating over the same temperature lyst can, without loss in mass, be converted range. Carnot’s principle follows directly into a powder. Small amounts of catalyst from the second law of thermodynamics, are often sufficient to alter the rate of reac- and means that all reversible heat engines tion considerably. A positive catalyst in- have the same efficiency, independent of creases the rate of a reaction and a negative

45 catalytic converter catalyst reduces it. Homogeneous catalysts tracted to the negatively charged electrode are those that act in the same phase as the or cathode. Compare anion. reactants (i.e. in gaseous and liquid systems). For example, nitrogen(II) oxide gas cationic detergent See detergents. will catalyze the reaction between sulfur(IV) oxide and oxygen in the gaseous cationic resin An ION-EXCHANGE ma- phase. Heterogeneous catalysts act in a dif- terial that can exchange cations, such as H+ ferent phase from the reactants. For exam- and Na+, for cations in the surrounding ple, finely divided nickel (a solid) will medium. Such materials are often pro- catalyze the hydrogenation of oil (liquid). duced by adding a sulfonic acid group – + The function of a catalyst is to provide (–SO3 H ) to a stable resin. A typical ex- a new pathway for the reaction, along change reaction is: – + ˆ which the rate-determining step has a resin–SO3 H + NaCl – + lower activation energy than in the uncat- resin–SO3 Na + HCl alyzed reaction. A catalyst does not change Cationic resins have been used to great the products in an equilibrium reaction effect to separate mixtures of cations of and their concentration remains identical similar size having the same charge. During to the concentration of the products in an this procedure, the mixtures are attached uncatalyzed reaction; i.e. the position of to the resins, after which progressive ELU- the equilibrium remains unchanged. The TION is used to recover the cations in order catalyst simply changes the rate at which of decreasing ionic radius. Promethium equilibrium is attained. was first isolated using this technique. In autocatalysis, one of the products of the reaction itself acts as a catalyst. In this caustic lime See calcium hydroxide. type of reaction the reaction rate increases with time up to a maximum value and fi- caustic potash See potassium hydrox- nally slows down. See also promoter. ide. catalytic converter A device fitted to caustic soda See sodium hydroxide. the exhaust system of gasoline-fueled vehicles to remove pollutant gases from the ex- Cavendish, Henry (1731–1810) Eng- haust. It typically consists of a structure lish chemist and physicist. Cavendish per- honeycombed to provide maximum area formed pioneering work on gases. He and coated with platinum, palladium, and showed that ordinary air is a mixture of rhodium catalysts. Such devices can con- oxygen and nitrogen, with nitrogen being vert carbon monoxide to carbon dioxide, four times more common. In 1781 he also oxides of nitrogen to nitrogen, and un- demonstrated that when hydrogen and burned fuel to carbon dioxide and water. oxygen are mixed in the proportions of 2:1 and the resulting mixture exploded (using catenation The formation of chains of an electric spark) then water is produced, atoms in molecules. thus showing that water is a compound and not an element. He also used a spark to cathode In electrolysis, the electrode combine nitrogen with oxygen and dis- that is at a negative potential with respect solved the resulting gas in water, thus gen- to the anode, and to which CATIONS are at- erating nitric acid. Cavendish also did tracted. In any electrical system, such as a important work in physics, particularly in discharge tube or electronic device, the electricity and gravitation. cathode is the terminal at which electrons enter the system. Compare anode. celestine (celestite) A naturally occurring mineral sulfate of strontium, SrSO4, cation A positively charged ion, formed found in sedimentary rocks. It is a source by removal of electrons from atoms or of strontium and is also used to make spe- molecules. In electrolysis, cations are at- cial glasses.

46 cerium celestite See celestine. limestone (CaCO3) with clay and grinding the result. When mixed with water, reac- cell A system having two ELECTRODES in tions occur with the water (hence the name a conducting liquid or paste ELECTROLYTE. hydraulic cement) and a hard solid alumi- An electrolytic cell is used to produce a nosilicate is formed. chemical reaction by means of a current passing through the electrolyte (i.e. by cementite (Fe3C) A constituent of cer- ELECTROLYSIS). Conversely, a voltaic (or tain cast irons and steels. The presence of galvanic) cell produces an e.m.f. by means cementite increases the hardness of the of chemical reactions that occur at each alloy. electrode. Electrons are transferred to or from the electrodes, giving each a net centi- Symbol: c A prefix used with SI charge. units denoting 10–2. For example, 1 cen- There is a convention for writing cell re- timeter (cm) = 10–2 meter (m). actions in voltaic cells. The DANIELL CELL, for instance, consists of a zinc electrode in centigrade scale See Celsius scale. a solution of Zn2+ ions connected, via a porous partition, to a solution of Cu2+ ions centrifugal pump A device commonly in which a copper electrode is placed. The used for transporting fluids around a reactions at the electrodes are chemical plant. Centrifugal pumps usually Zn → Zn2+ + 2e have 6–12 curved vanes that rotate inside a i.e. oxidation of the zinc to zinc(II), and fixed circular casing. As the blades rotate, Cu2+ + 2e → Cu the fluid is impelled out of the pump along i.e. reduction of copper(II) to copper. A cell a discharge pipe. Centrifugal pumps do not reaction of this type is written: produce high pressures but they have the Zn|Zn2+(aq)|Cu2+(aq)|Cu advantages of being relatively cheap due to The e.m.f. is the potential of the electrode their simple design, have no valves, and on the right minus the potential on the left. work at high speeds. In addition they are Copper is positive in this case and the not damaged if a blockage develops. Com- e.m.f. of the cell is stated as +1.10 volts. See pare displacement pump. also accumulator. centrifuge An apparatus for rotating a Celsius scale (centigrade scale) A tem- container at high speeds, used to increase perature scale in which the temperature of the rate of sedimentation of suspensions or pure melting ice at standard pressure is the separation of two immiscible liquids. fixed at 0° and the temperature of pure See also ultracentrifuge. boiling water at standard pressure is fixed at 100°. The temperature interval unit on ceramics Useful high-melting inorganic the Celsius scale is the degree Celsius (°C), materials. Ceramics include silicates and which is equal in magnitude to the kelvin. aluminosilicates, refractory metal oxides, Before 1948 the Celsius scale was known and metal nitrides, borides, etc. Pottery as the centigrade scale. Celsius’s original and porcelain are examples of ceramics. scale had 0° as the water steam fixed point Various types of glass are also sometimes and 100° as the ice/water fixed point. See included. also temperature scale; Fahrenheit scale. cerium A ductile, malleable, gray el- cement A generic term for chemical ement of the lanthanoid series of metals. It preparations typically used as binding occurs in association with other lan- agents to bond disparate particles together thanoids in many minerals, including mon- into a strong, solid mass, e.g. concrete. azite and bastnaesite. The metal reacts Such cements commonly consist of a pow- rapidly with hot water, tarnishes in air, and dered mixture of calcium silicates and alu- will ignite under friction. It is used as a cat- minates, which is made by heating alyst in several alloys (especially for lighter

47 cermet flints), in tracer bullets and in gas mantles, now almost been abandoned in favor of SI and in compound form in carbon-arc units. searchlights, etc., and in the glass industry. Symbol: Ce; m.p. 799°C; b.p. 3426°C; chain When two or more atoms form r.d. 6.7 (hexagonal structure, 25°C); p.n. bonds with each other in a molecule, a 58; r.a.m. 140.15. chain of atoms results. This chain may be a straight chain, in which each atom is added cermet A synthetic composite material to the end of the chain, or it may be a made by combining a ceramic and a sin- branched chain, in which the main chain of tered metal. Cermets have better tempera- atoms has one or more smaller side chains ture and corrosion resistance than straight branching off it. ceramics. For example, a chromium–alumina cermet is used to make blades for gas- chain reaction A self-sustaining chemi- turbine engines. cal reaction consisting of a series of steps, each of which is initiated by the one before cerussite A naturally occurring form of it. An example is the reaction between hy- lead(II) carbonate (PbCO3) that is an im- drogen and chlorine: → portant lead ore. It forms orthorhombic Cl2 2Cl• → crystals and is often found together with H2 + Cl• HCl + H• → galena (PbS). H• + Cl2 HCl + Cl• → 2H• H2 → cesium (caesium) A soft, silvery, highly 2Cl• Cl2 reactive, low-melting element of the alkali- The first stage, chain initiation, is the dis- metal group, the sixth element of group 1 sociation of chlorine molecules into atoms; (formerly IA) of the periodic table. It is this is followed by two chain propagation found in several silicate minerals, including reactions. Two molecules of hydrogen pollucite (CsAlSi2O6). The metal oxidizes chloride are produced and the ejected chlo- in air and reacts violently with water. Ce- rine atom is ready to react with more hy- sium is used in photocells, as a catalyst, drogen. The final steps, chain termination, and in the cesium atomic clock. The radio- stop the reaction. active isotopes 134Cs (half life 2.065 years) Induced nuclear fission reactions also and 137Cs (half life 30.3 years) are pro- depend on chain reactions; the fission reac- duced in nuclear reactors and are poten- tion is maintained by the two or three neu- tially dangerous atmospheric pollutants. trons set free in each fission. Cesium-137 is also used as a radiation source in cancer therapy, and as a medical chalcogens See group 16 elements. tracer. Symbol: Cs; m.p. 28.4°C; b.p. 678.4°C; chalk A soft, natural, fine-grained form ° r.d. 1.873 (20 C); p.n. 55; r.a.m. 132.91. of calcium carbonate (CaCO3) formed from the skeletal remains of ancient marine cesium-chloride structure A form of organisms. So-called blackboard chalk is crystal structure that consists of alternate made from a different compound, calcium layers of cesium ions and chloride ions sulfate (CaSO4). with the center of the lattice occupied by a cesium ion in contact with eight chloride chamber process See lead-chamber ions (i.e. four chloride ions in the plane process. above and four in the plane below). chaotic reaction A chemical reaction in c.g.s. system An early metric system of which the concentrations of the reactants units based on the centimeter, the gram, show chaotic behavior, i.e. the evolution of and the second as the fundamental me- the reaction may become unpredictable. chanical units. Much early scientific work An example is the BELOUSOV–ZHABOTINSKII was performed using this system, but it has REACTION. Reactions of this type ususally

48 chemical combination, laws of involve a large number of complex inter- H H linked steps. See also bistability; oscillating 2 2 N N reaction. H2C CH charcoal An amorphous form of carbon 2+ Cu made by heating wood or other organic H C CH material in the absence of air. Activated 2 charcoal is charcoal heated to drive off ab- N N sorbed gas. It is used for absorbing gases H2 H2 and for removing impurities from liquids.

Charles’ law (Gay-Lussac’s law) For a Chelate given mass of gas at constant pressure, the volume increases by a constant fraction of metal ions in fertilizer preparations, while the volume at 0°C for each Celsius degree chelating agents, or compounds capable of rise in temperature. The constant fraction forming chelates, may be used to trap (α) has almost the same value, about heavy metals and thus render them harm- 1/273, for all gases and Charles’ law can less in the case of metal poisonings. thus be written in the form ‘Chelate’ comes from the Greek word α θ V = V0(1 + v ) meaning ‘claw’. Ligands forming chelates where V is the volume at temperature θ°C are classified according to the number of ° α and V0 the volume at 0 C. The constant v sites at which they can coordinate: bi- is the thermal expansivity of the gas. For an dentate, tridentate, etc. See also sequestra- ideal gas its value is 1/273.15. tion. A similar relationship exists for the pressure of a gas heated at constant vol- chemical bond A force holding atoms ume: together in a molecule or in a crystal. α θ p = p0(1 + p ) Chemical bonds are formed by transfer or α Here, p is the pressure coefficient. For an sharing of electrons (see electrovalent ideal gas bond; covalent bond) and typically have α α –1 p = v energies of about 1000 kJ mol . Weaker although they differ slightly for real gases. interactions, such as HYDROGEN BONDS and It follows from Charles’ law that for a gas VAN DER WAALS FORCES, are not regarded as heated at constant pressure, chemical bonds. See also coordinate bond; V/T = K metallic bond. where T is the thermodynamic temperature and K is a constant. Similarly, at constant chemical combination, laws of A volume, p/T is a constant. group of chemical laws, developed during Charles’ law is named for the French the late 18th and early 19th centuries, chemist and physicist J. A. C. Charles, who which arose from the recognition of the im- discovered it by means of experiments portance of quantitative (as opposed to begun in 1787. The law was published in qualitative) study of chemical reactions. 1802, however, by J. Gay-Lussac, another The laws are: French chemist and physicist, is sometimes 1. The law of conservation of mass (mat- known as Gay-Lussac’s law. ter); 2. The law of constant (definite) propor- chelate A metal coordination COMPLEX tions; in which one molecule or ion forms a coor- 3. The law of multiple proportions; dinate bond at two or more points to the 4. The law of equivalent (reciprocal) pro- same metal ion, thus becoming a ligand. portions. The resulting complex contains rings of These laws played a significant part in Dal- atoms that include the metal atom. ton’s development of his atomic theory in Chelates may be used to deliver essential 1808.

49 chemical dating chemical dating A method of using A more complex equation represents the chemical analysis to find the age of an ar- reaction between disodium tetraborate chaeological specimen in which composi- decahydrate (borax) and aqueous hy- tional changes have taken place over time. drochloric acid to give boric acid and For example, the determination of the sodium chloride: → amount of fluorine in bone that has been Na2B4O7 + 2HCl + 5H2O 4H3BO3 + buried gives an indication of the time the 2NaCl bone has been underground because phosphate in the bone is gradually replaced by chemical equilbrium See equilibrium. fluoride ions from groundwater. Another dating technique depends on the fact that, chemical formula See formula. in living organisms, amino acids are opti- µ cally active. After death a slow RACEMIZA- chemical potential Symbol: . For the TION reaction occurs and a mixture of L- ith component of a mixture the chemical µ and D-isomers forms. The age of bones can potential i is defined by the partial deriva- be accurately determined by measuring the tive of the Gibbs free energy G of the sys- relative amounts of optically active versus tem with respect to the amount ni of the optically inactive acids present. component, when the temperature, pressure, and amounts of other components µ ∂ ∂ chemical engineering The branch of are constant, i.e. i= G/ ni. If the chemi- engineering concerned with the design and cal potentials of components are equal maintenance of a chemical plant and its then the components are in equilibrium. Also, in a one-component system with two ability to withstand extremes of tempera- phases it is necessary for the chemical po- ture, pressure, corrosion, and wear. It en- tentials to be equal in the two phases for ables laboratory processes producing there to be equilibrium. grams of material to be converted into a large-scale plant processes yielding tonnes chemical reaction A process in which of material. Chemical engineers plan large- one or more elements or chemical com- scale chemical processes by linking to- pounds (the reactants) react to produce a gether the appropriate unit processes and different substance or substances (the by studying such parameters as heat and products). mass transfer, separations, and distillations. chemical shift See nuclear magnetic resonance. chemical equation A method of representing a chemical reaction using chemical chemical symbol A letter or pair of let- FORMULAS. The formulas of the reactants ters or, in the case of unnamed elements, a are given on the left-hand side of the equa- triplet of letters that stand for a chemical tion, with the formulas of the products element, as used for example in equations given on the right. The two halves are sep- and chemical formulas. Unlike abbrevia- arated by a directional arrow or arrows, or tions, symbols do not take periods, are an equals sign. A number preceding a for- written exactly the same in all languages mula, called a stoichiometric coefficient, and applications, and are designed to be indicates the number of molecules of that universally understood to represent spe- substance involved in the reaction. The cific elements, units, quantities, etc. equation must balance – that is, the number of atoms of any one element must be chemiluminescence The emission of the same on both sides of the equation. A light during a chemical reaction. simple example is the equation for the reaction between hydrogen and oxygen to chemisorption See adsorption. form water: → 2H2 + O2 2H2O Chile saltpeter See sodium nitrate. 50 chlorine

China clay (kaolin) A white powdery contact with organic material it is danger- clay obtained from the natural decomposi- ously explosive. tion of granites. It is used as a filler in The hydrate (HClO4.H2O) of chlo- paints and paper-making, in the pottery in- ric(VII) acid is a white crystalline solid at dustries, and in pharmaceuticals. See kaoli- room temperature and has an ionic lattice + – nite. structure of the form (H3O) (ClO4) . chiral Having the property of CHIRALITY. chloric acid See chloric(V) acid. chiral center See isomerism. chloride See halide. chirality The property of existing in left- chlorination 1. Treatment with chlo- and right-handed forms; i.e. mirror images rine; for instance, the use of chlorine to dis- that are not superimposable. In chemistry infect water. the term is applied to the existence of opti- 2. See halogenation. cal isomers. ‘Chiral’ comes from the Greek word ‘cheir’, meaning hand. See iso- chlorine A green reactive gaseous el- merism; optical activity. ement belonging to the halogens; i.e. group 17 (formerly VIIA) of the periodic table, of chlorate A salt of chloric(V) acid. which it is the second element, occurring in seawater, salt lakes, and underground de- chloric(I) acid (hypochlorous acid; posits of the mineral halite as NaCl. It ac- HClO) A colorless liquid produced when counts for about 0.055% of the Earth’s chlorine dissolves in water. It is a bleach crust. and gives chlorine water its disinfectant Chlorine is strongly oxidizing and can properties. To increase the yield of acid, be liberated from its salts only by strong the chlorine water can be shaken with a oxidizing agents, such as manganese(IV) small amount of mercury(II) chloride. The oxide, potassium permanganate(VII), or Cl–O bond is broken more easily than the potassium dichromate; sulfuric acid is not O–H bond in aqueous solution; the acid is sufficiently oxidizing to release chlorine consequently a poor proton donor and from chlorides. Industrially, chlorine is hence a weak acid. prepared by the electrolysis of brine or magnesium chloride, although in some chloric(III) acid (chlorous acid; HClO2) processes chlorine is recovered by the high- A pale yellow liquid produced by reacting temperature oxidation of waste hydrochlo- chlorine dioxide with water. It is a weak ric acid. Chlorine is used in large acid and oxidizing agent. quantities, both as the element to produce chlorinated organic solvents, for the pro- chloric(V) acid (chloric acid; HClO3)A duction of polyvinyl chloride (PVC), the colorless liquid with a pungent odor, major thermoplastic in use today, and in formed by the action of dilute sulfuric acid the form of hypochlorites for bleaching. on barium chlorate. It is a strong acid and Chlorine reacts directly and often vig- has bleaching properties. Chloric(V) acid is orously with many elements; it reacts ex- also a strong oxidizing agent; in concen- plosively with hydrogen in sunlight to form trated solution it will ignite organic sub- hydrogen chloride, HCl, and combines stances, such as paper and sugar. with the electropositive elements to form metal chlorides. The metals of main groups chloric(VII) acid (perchloric acid; 1 and 2 form electrovalent chlorides but an HClO4) A colorless liquid that fumes increase in the metal charge/size ratio leads strongly in moist air. It is made by vacuum to the chlorides becoming increasingly co- distillation of a mixture of potassium per- valent. For example, CsCl is totally elec- chlorate and concentrated sulfuric acid. It trovalent, AlCl3 has a layer lattice, and is a strong acid and oxidizing agent, and in TiCl4 is essentially covalent. The elec- 51 chlorine dioxide tronegative elements form volatile molecu- concentrated sulfuric acid on potassium lar chlorides characterized by the single co- chlorate. It is a powerful oxidizing agent, valent bond to chlorine. With the and its explosive properties in the presence 2+ + 2+ exception of Pb , Ag , and Hg2 , the elec- of a reducing agent were used to make one trovalent chlorides are soluble in water, of the first combustible matches. It is dissolving to give the hydrated metal ion widely used in the purification of water and the chloride ion Cl–. Chlorides of met- and as a bleach in the flour and wood-pulp als other than the most electropositive are industry. On an industrial scale an aqueous hydrolyzed if aqueous solutions are evapo- solution of chlorine dioxide is made by rated; for example, passing nitrogen dioxide up a tower → ZnCl2 + H2O Zn(OH)Cl + HCl packed with a fused mixture of aluminum → FeCl3 + 3H2O Fe(OH)3 + 3HCl oxide and clay, down which a solution of Chlorine forms four main oxides, sodium chlorate flows. dichlorine oxide, Cl2O; chlorine dioxide, ClO2; dichlorine hexoxide, Cl2O6; and chlorine monoxide See dichlorine dichlorine heptoxide, Cl2O7, all of which oxide. are highly reactive and explosive. Chlorine dioxide finds commercial application as an chlorine(I) oxide See dichlorine oxide. active oxidizing agent, but because of its explosive nature it is usually diluted by air chlorite A a salt of chloric(III) acid or other gases. The chloride ion is able to (chlorous acid). function as a ligand with a large variety of – metal ions forming such species as [FeCl4] , chloroplatinic(IV) acid (platinic chlo- 3– + [CuCl4] , and [Co(NH3)4Cl2] . The for- ride; H2PtCl6) A reddish compound pre- mation of anionic chloro-complexes is ap- pared by dissolving platinum in aqua regia. plied to the separation of metals by When crystallized from the resulting solu- ion-exchange methods. tion, crystals of the hexahydrate Because of the hydrolysis of many metal (H2PtCl6.6H2O) are obtained. The crystals chlorides when solutions are evaporated, are needle-shaped and deliquesce on expo- special techniques must be used to prepare sure to moist air. Chloroplatinic acid is a anhydrous chlorides. These are: relatively strong acid, giving rise to the 1. Reaction of dry chlorine with the hot family of chloroplatinates. metal; 2. For metals having lower valences, reac- chlorous acid See chloric(III) acid. tion of dry hydrogen chloride with the hot metal; chromate Any oxygen-containing deriv- 3. Reaction of dry hydrogen chloride on ative of chromium, most particularly the the hydrated chloride. chromate(VI) species. The solubility of inorganic metal chlorides is such that they are not an environmental chromatography A technique used to problem unless the metal ion itself is toxic. separate or analyze complex mixtures. A However, chlorine and hydrogen chloride number of related techniques exist; all de- are both highly toxic. Thus chlorides that pend on two phases: a mobile phase, which hydrolyze to release HCl should also be re- may be a liquid or a gas, and a stationary garded as toxic. They should not be han- phase, which is either a solid or a liquid dled without gloves and precautions held by a solid. The sample to be separated should be taken against inhalation. or analyzed is carried by the mobile phase Symbol: Cl; m.p. –100.38°C; b.p. through the stationary phase. Different –33.97°C; d. 3.214 kg m–3 (0°C); p.n. 17; components of the mixture are absorbed or r.a.m. 35.4527. dissolved to different extents by the stationary phase, and consequently move chlorine dioxide (ClO2) An orange gas along at different rates. In this way the formed in the laboratory by the action of components are separated. There are many

52 chromium(III) oxide different forms of chromatography de- chromium A hard, silver-gray transi- pending on the techniques used and the nation metal of group 6 (formerly VIB) of the ture of the partition process between periodic table. It occurs naturally as mobile and stationary phases. The main CHROMITE (FeCr2O4). The ore is first con- classification is into column chromatogra- verted into sodium dichromate(VI) and phy and planar chromatography. then reduced with carbon to chromium(III) A simple example of column chro- oxide, after which it is finally reduced to matography is in the separation of liquid metallic chromium with aluminum. mixtures. A vertical column is packed with Chromium is used to make strong alloy an absorbent material, such as alumina steels and stainless steel, and for decorative (aluminum oxide) or silica gel, represent- electroplated coatings. It resists corrosion ing the stationary phase. The sample is in- at normal temperatures. It reacts slowly troduced into the top of the column and with dilute hydrochloric and sulfuric acids washed down it (the mobile phase) using a to give hydrogen and blue chromium(II) solvent. This process is known as ELUTION. compounds, which quickly oxidize in air to If the components are colored, visible green chromium(III) ions. The bright col- bands appear down the column as the sam- ors of many chromium compounds make ple separates out. The components are sep- them useful as pigments. The oxidation 2– arated as they emerge from the bottom of states are +6 in CHROMATES (CrO4 ) and 2– the column. In this particular example of dichromates (Cr2O7 ), +3 (the most sta- chromatography the partition process is ble), and +2. In acidic solutions the yellow adsorption on the particles of alumina or chromate(VI) ion changes to the orange silica gel. Column chromatography can dichromate(VI) ion. Dichromates are also be applied to mixtures of gases, in strong oxidizing agents and are used as which case it is known as GAS CHROMATOG- such in the laboratory. For example they RAPHY. are used as a test for sulfur(IV) oxide (sul- Planar chromatography works in simi- fur dioxide) and to oxidize alcohols. See lar fashion, except that the stationary potassium dichromate. phase is a flat sheet of absorbent material Symbol: Cr; m.p. 1860±20°C; b.p. such as paper. Components of the mixture 2672°C; r.d. 7.19 (20°C); p.n. 24; r.a.m. are held back by the stationary phase either 51.9961. by adsorption (e.g. on the surface of alumina) or because they dissolve in it (e.g. in chromium dioxide See chromium(IV) the moisture within paper). See paper chro- oxide. matography; thin-layer chromatography. chromium(II) oxide (chromous oxide; chrome alum See alum. CrO) A black powder prepared by the oxidation of chromium amalgam with di- chrome iron ore See chromite. lute nitric acid. At high temperatures (around 1000°C) chromium(II) oxide is re- chromic anhydride See chromium(VI) duced by hydrogen. oxide. chromium(III) oxide (chromic oxide; chromic oxide See chromium(III) oxide. chromium sesquioxide; Cr2O3) A green powder that is almost insoluble in water. It chromite (chrome iron ore; iron has the same crystal structure as iron(III) chromium oxide; FeCr2O4) A mineral oxide and aluminum(III) oxide. Chro- that consists of mixed oxides of chromium mium(III) oxide is prepared by gently heat- and iron, the principal ore of chromium. It ing chromium(III) hydroxide or by heating occurs as brownish black masses with a ammonium dichromate. Alternative prepa- metallic luster in rocks of igneous origin. rations include the heating of a mixture of Large deposits are found in Zimbabwe and ammonium chloride and potassium the western United States. dichromate or the decomposition of

53 chromium(IV) oxide chromyl chloride by passing it through a cinnabar A red mineral form of mer- red-hot tube. Chromium(III) oxide is used cury(II) sulfide (HgS), associated with as a pigment in the paint and glass indus- areas of volcanic activity. It is the principal tries. ore of mercury. chromium(IV) oxide (chromium diox- cis-isomer See isomerism. ide; CrO2) A black solid prepared by heating chromium(III) hydroxide in oxy- cis-trans isomerism See isomerism. gen at a temperature of 300–350°C. Chromium(IV) oxide is very unstable. Clark cell A type of cell formerly used as a standard source of e.m.f. It consists of chromium(VI) oxide (chromium triox- a mercury cathode coated with mercury sulfate, and a zinc anode. The electrolyte is ide; chromic anhydride; CrO3) A red crystalline solid formed when concentrated zinc sulfate solution. The e.m.f. produced ° sulfuric acid is added to a cold saturated is 1.4345 volts at 15 C. The Clark cell has solution of potassium dichromate. The been superseded as a standard by the WE- long prismatic needle-shaped crystals that STON CADMIUM CELL. are produced are extremely deliquescent. Chromium(VI) oxide is readily soluble in clathrate (cage compound; enclosure water, forming a solution that contains compound) A substance in which small several of the polychromic acids i.e. acids ‘guest’ molecules are trapped within the lattice of a crystalline ‘host’ compound. containing more than one atom of Clathrates are formed when suitable host chromium. On heating it decomposes to compounds are crystallized in the presence give chromium(III) oxide. Chromium(VI) of molecules of the appropriate size. Al- oxide is used as an oxidizing reagent. though the term ‘clathrate compound’ is often used, clathrates are not true com- chromium sesquioxide See chro- pounds; no chemical bonds are formed, mium(III) oxide. and the guest molecules interact by weak van der Waals forces. The clathrate is See chromium(VI) chromium trioxide maintained by the cagelike lattice of the oxide. host. The host lattice must be broken down, for example by heating or dissolu- chromophore A group of atoms in a tion, in order to release the guest. In ZEO- molecule that is responsible for the color of LITES, in comparison, the holes in the host the compound. lattice are large enough to permit entrance or emergence of the guest without breaking chromous oxide See chromium(II) bonds in the host lattice. Water (ice), for oxide. instance, forms a clathrate with xenon. chromyl chloride (CrO2Cl2) A dark Clausius, Rudolf Julius Emmanuel red covalent liquid prepared either by dis- (1822–88) German physicist. Clausius is tilling the vapors evolved when a dry mix- best remembered as one of the founders of ture of potassium dichromate and sodium thermodynamics. In papers published in chloride is treated with concentrated sulfu- the 1850s he stated the second law of ther- ric acid or by the action of concentrated modynamics and he coined the word en- sulfuric acid on chromium(VI) oxide dis- tropy in 1865. Clausius made other solved in concentrated hydrochloric acid. contributions to thermodynamics and also Chromyl chloride is violently hydrolyzed to the development of the kinetic theory of by water, and with solutions of alkalis it gases. In electrochemistry he pioneered the undergoes immediate hydrolysis to pro- idea of dissociation of substances into ions duce chromate ions. It is used as a potent in solution. He also studied the dielectric oxidizing agent. properties of materials.

54 cobalt(III) oxide clays Naturally occurring aluminosili- coal A brown or black sedimentary de- cates that form pastes and gels with water. posit that consists mainly of carbon, used as a fuel and as a source of organic chemi- cleavage The splitting of a crystal along cals. It is the fossilized remains of decayed planes of atoms, to form smooth surfaces. plants and algae that chiefly grew in the Carboniferous and Permian periods and close packing The arrangement of par- were then buried and subjected to high ticles (usually atoms) in crystalline solids in pressures underground. The various types which the UNIT CELL has an atom, ion, or of coal, including ANTHRACITE, BITUMINOUS molecule at each corner and also at the cen- COAL, and LIGNITE, are classified according ter of each face of a cube. Each particle to their increasing carbon content. thus has 12 nearest neighbors: six in the same layer (or plane) as itself and three cobalt A lustrous, silvery-blue, hard fer- each in the layer above and below for a CO- romagnetic transition metal, the first el- ORDINATION NUMBER of 12. This arrange- ement of group 9 (formerly subgroup ment provides the most economical use of VIIIB) of the periodic table. It occurs in as- space (74% efficiency). The two principal sociation with nickel, copper, and arsenic types of close packing are FACE-CENTERED in such minerals as cobaltite [(Co,Fe)AsS], skutterudite [(Co,Ni)As ], and erythrite CUBIC CRYSTAL and HEXAGONAL CLOSE PACK- 3 (Co (AsO ) .8H O). It is used in alloys for ING. 3 4 2 2 magnets, high-temperature cutting tools, cluster A three-dimensional structure and electrical heating elements. Cobalt consisting of atoms, with the number of compounds are used in catalysts and some paints. It is an essential dietary mineral, atoms ranging between a few dozen and a being a component of vitamin B12. few thousand. The atoms in clusters can ei- Symbol: Co; m.p. 1495°C; b.p. ther be metal or nonmetal atoms. It is 2870°C; r.d. 8.9 (20°C); p.n. 27; r.a.m. found that when clusters are made (for ex- 58.93320. ample, by sputtering the surface of a solid), clusters with certain magic numbers of cobaltic oxide See cobalt(III) oxide. atoms occur far more abundantly than for other numbers of atoms. These numbers, cobaltous oxide See cobalt(II) oxide. which occur for several monovalent metallic elements including sodium, silver, and cobalt(II) oxide (cobaltous oxide; gold, are 8, 20, 40, 58, 92 and correspond CoO) A green powder prepared by the to the electron shells being filled in an ex- action of heat on cobalt(II) hydroxide in ternal potential. It is predicted that clusters the absence of air. Alternatively it may be of atoms or cluster compounds in which prepared by the thermal decomposition of the number of valence electrons is a magic cobalt(II) sulfate, nitrate, or carbonate, number should be stable. For example, also in the absence of air. Cobalt(II) oxide Al12C, which has a total of 40 valence elec- is a basic oxide, reacting with acids to give trons, is predicted to be a stable cluster solutions of cobalt(II) salts. It is stable in compound. air up to temperatures of around 600°C, after which it absorbs oxygen to form tri- cluster compound A type of compound cobalt tetroxide (Co3O4). Cobalt(II) oxide in which a cluster of metal atoms are joined can be reduced to cobalt by heating in a by metal–metal bonds. Cluster compounds stream of carbon monoxide or hydrogen. It are formed by certain transition elements, is used in the pottery industry and in the such as molybdenum and tungsten. production of vitreous enamels. coagulation The irreversible associa- cobalt(III) oxide (cobaltic oxide; Co2O3) tion of particles such as colloids into clus- A dark gray powder formed by the thermal ters. See flocculation. decomposition of either cobalt(II) nitrate

55 coherent units or carbonate in air. If heated in air, 2. The elevation of boiling point. cobalt(III) oxide undergoes further oxida- 3. The lowering of freezing point. tion to give tricobalt tetroxide, Co3O4. 4. Osmotic pressure. Colligative properties are all based coherent units A system or subset of upon empirical observation. The explana- UNITS (e.g. SI units) in which the derived tion of these closely related phenomena de- units are obtained by multiplying or divid- pends on intermolecular forces and the ing together base units, with no numerical kinetic behavior of the particles, which is factor involved. qualitatively similar to those used in deriv- ing the kinetic theory of gases. coinage metals A group of malleable metals forming group 11 (formerly sub- collimator An arrangement for produc- group IB) of the periodic table. They are ing a parallel beam of radiation for use in a copper (Cu), silver (Ag), and gold (Au). spectrometer or other instrument. A sys- These metals all have an outer s1 electronic tem of lenses and slits is utilized. configuration but they differ from the alkali metals, which also have an outer s1 colloid A heterogeneous system in configuration, in having inner d-electrons. which the interfaces between phases, The coinage metals also have much higher though not visibly apparent, are important IONIZATION POTENTIALS than the alkali met- factors in determining the system’s proper- als and high positive standard ELECTRODE ties. The three important attributes of col- POTENTIALS, and are therefore much more loids are: difficult to oxidize. A further significant 1. They contain particles, commonly made difference from the alkali metals is the va- up of large numbers of molecules, form- riety of oxidation states observed for the ing the distinctive unit or dispersed coinage metals. Thus copper in aqueous phase. chemistry is the familiar (hydrated) blue di- 2. The particles are distributed in a contin- valent Cu2+ ion but colorless copper(I) uous medium (the continuous phase). compounds can be prepared with groups 3. There is a stabilizing agent, which has an such as CN– that stabilize low valences, affinity for both the particle and the e.g. CuCN. A few compounds of medium; in many cases the stabilizer is a copper(III) have also been prepared. The polar group. common form of silver is Ag(I), e.g. Particles in the dispersed phase typically –6 –4 AgNO3, with a few Ag(II) compounds sta- have diameters in the range 10 –10 mil- ble as solids only. The most common oxi- limeter. Milk, rubber, and water-based dation state of gold is Au(III), although the paints are typical examples of colloids. See cyanide ion again stabilizes Au(I) com- also emulsion; gel; sol. 3– pounds, e.g., [Au(CN)4] . The metals all form a large number of coordination com- colorimetric analysis Quantitative pounds, again unlike the alkali metals, and analysis in which the concentration of a are generally described with the other el- colored solute is measured by the intensity ements of the appropriate transition series. of the color. The test solution can be compared against standard solutions. coke A dense substance obtained from the carbonization of coal. It is used as a columbium A former name for nio- fuel and as a reducing agent. bium. It is still sometimes used in metallurgy and mineralogy. colligative properties A group of prop- Symbol: Cb. erties of solutions that depends on the number of particles present, rather than column chromatography See chro- the nature of the particles. Colligative matography; gas chromatography. properties include: 1. The lowering of vapor pressure. combustion A reaction with oxygen

56 complex with the production of heat and light. The type of compound in which molecules or combustion of solids and liquids occurs ions form COORDINATE BONDS with a metal when they release flammable vapor, which atom or ion. The coordinating species reacts with oxygen in the gas phase. Com- (called ligands) have lone pairs of elec- bustion reactions usually involve a com- trons, which they can donate to the metal plex sequence of free-radical chain atom or ion. They are molecules such as reactions. The light is produced by excited ammonia or water, or negative ions such as atoms, molecules, or ions. In highly lumi- Cl– or CN–. The resulting complex may be nous flames it comes from small incandes- neutral or it may be a complex ion. For ex- cent particles of carbon. ample: 2+ → 2+ Sometimes the term is also applied to Cu + 4NH3 [Cu(NH3)4] 3+ – → 3– slow reactions with oxygen, and also to re- Fe + 6CN [Fe(CN)6] 2+ – → 4– actions with other gases (for example, cer- Fe + 6CN [Fe(CN)6] tain metals ‘burn’ in chlorine). The formation of such coordination complexes is typical of transition metals. common salt See sodium chloride. Often the complexes contain unpaired electrons and are paramagnetic and col- complex (coordination compound) A ored. See also chelate.

X X

M X X M X X

X X

square-planar X octahedral

X X

X X M M X X X

X X

tetrahedral trigonal-bipyramid

Complex: typical shapes of inorganic complexes

57 complex ion complex ion See complex. method in which the electrical conductance of the reaction mixture is continuously component One of the separate chemi- measured throughout the addition of the cal substances in a mixture in which no titrant and well beyond the equivalence chemical reactions are taking place. For ex- point. The operation is carried out in a ample, a mixture of ice and water has one conductance cell, which is part of a resis- component; a mixture of nitrogen and oxy- tance bridge circuit. The method depends gen has two components. When chemical on the fact that ions have different ionic reactions occur between the substances in a mobilities, H+ and OH– having particularly mixture, the number of components is de- high values. The method is used in place of fined as the number of chemical substances traditional end-point determination by in- present minus the number of equilibrium dicators, and is especially useful for weak reactions taking place. Thus, the system: acid–strong base and strong acid–weak ˆ N2 + 3H2 2NH3 is a two-component base titrations for which color-change system. See also phase rule. titrations are unreliable. compound A chemical combination of configuration 1. (electron configura- atoms of different elements to form a sub- tion) The arrangement of electrons about stance in which the ratio of combining the nucleus of an atom. Configurations are atoms remains fixed and is specific to that represented by symbols, which contain: substance. The constituent atoms cannot 1. An integer, which is the value of the be separated by physical means; a chemical principal quantum number (shell num- reaction is required for the compounds to ber). be formed or to be changed. The existence 2. A lower-case letter representing the of a compound does not necessarily imply value of the orbital quantum number (l), that it is stable. Many compounds have i.e. lifetimes of less than a second. Compare s means l = 0, p means l = 1, mixture. d means l = 2, f means l = 3. 3. A numerical superscript giving the num- concentrated Denoting a solution in ber of electrons in that particular set; for which the amount of solute in the solvent is 2 3 5 relatively high. The term is always relative; example, 1s , 2p , 3d . for example, whereas concentrated sulfuric The ground state electronic configuration (i.e. the most stable or lowest energy acid may contain 96% H2SO4, concentrated potassium chlorate may contain as state) may then be represented as follows, for example, He, 1s2; N, 1s22s22p5. How- little as 10% KClO3. Compare dilute. ever, element configurations are commonly concentration The amount of sub- abbreviated by using an inert gas to repre- stance in a solution per unit quantity of sol- sent the ‘core’, e.g. Zr has the configura- 2 2 vent. Molar ‘concentration’ (symbol c) is tion [Kr]4d 5s . the amount of substance per cubic meter. It 2. The arrangement of atoms or groups in replaces molarity, a term formerly used to a molecule. See also atom. describe the amount of substance per cubic decimeter (liter). DENSITY or mass concen- conjugate acid See acid; base. tration (symbol ρ) is kilograms of solute per cubic meter. Molality (symbol m) or conjugate base See acid; base. molal concentration is the amount of substance per kilogram of solute. conservation of energy, law of Enun- ciated by Helmholtz in 1847, this law condensation The conversion of a gas states that in all processes occurring in an or vapor into a liquid or solid by cooling. isolated system the energy of the system remains constant. The law of course permits conductiometric titration A titration energy to be converted from one form to

58 copper another (including mass, since energy and gions by hot solids, liquids, and pressur- mass are equivalent). ized gases. See also spectrum. conservation of mass (matter), law converter See Bessemer process. of Formulated by Lavoisier in 1774, this law states that matter cannot be created or coordinate bond (dative bond; dipolar destroyed. Thus in a chemical reaction the bond; semipolar bond) A COVALENT total mass of the products equals the total bond in which the bonding pair is visual- mass of the reactants, with ‘total mass’ in- ized as arising from the donation of a LONE cluding any solids, liquids, and gases – such PAIR from one species to another species, as air – that participate in the reaction. which behaves as an electron acceptor. The definition includes such examples as the constantan A copper-nickel (cupron- ‘donation’ of the lone pair of the ammonia + + ickel) alloy containing 45% nickel. It has a molecule to H (an acceptor) to form NH4 2+ 2+ high electrical resistivity and very low tem- or to Cu to form [Cu(NH3)4] . perature coefficient of resistance and is The donor groups are known as Lewis therefore used in thermocouples and resis- bases and the acceptors are either hydro- tors. gen ions or LEWIS ACIDS. Simple combina- → tions, such as H3N BF3, are known as constant composition, law of See adducts. See also complex. constant proportions; law of. coordination compound See complex. constant (definite) proportions, law of 1. The number (constant composition, law of) The prin- coordination number of atoms, molecules, or ions, surrounding ciple that the proportion of each element in any particular atom, ion, or molecule in a a compound is fixed or constant. It follows crystal. that the composition of a pure chemical 2. The number of coordinate bonds formed compound is independent of its method of to a central atom or ion in a complex. preparation. The law was formulated by Proust in 1779 after the analysis of a large copper A malleable, ductile, golden-red number of compounds. transition metal, the first element of group 11 (formerly subgroup IB) of the periodic contact process An industrial process table. It occurs naturally principally in sul- for the manufacture of sulfuric acid. Sul- fides such as chalcopyrite (CuFeS ), chal- fur(IV) oxide and air are passed over a 2 cocite (Cu2S), and bornite (Cu5FeS4). It is heated catalyst (vanadium(V) oxide or extracted by roasting the ore in a con- platinum) and sulfur(VI) oxide is pro- trolled air supply and purified by electroly- duced: sis of copper(II) sulfate solution using → 2SO2 + O2 2SO3 impure copper as the anode and pure cop- The sulfur(VI) oxide is dissolved in sulfuric per as the cathode. Copper is used exten- acid: sively in electrical conductors and in such → SO3 + H2SO4 H2S2O7 alloys as brass and bronze. It is also an ex- The resulting OLEUM is then diluted to give cellent roofing material. sulfuric acid: Copper(I) compounds are white (except → H2S2O7 + H2O 2H2SO4 the oxide, which is red), and copper(II) compounds are blue in solution. Copper is continuous phase See colloid. unreactive to dilute acids with the exception of nitric acid. Copper(I) compounds continuous spectrum A spectrum com- are unstable in solution and decompose to posed of a continuous range of emitted or copper and copper(II) ions. Both copper(I) absorbed radiation. Continuous spectra and copper(II) ions form complexes, cop- are produced in the infrared and visible re- per(II) ions being identified by the dark

59 copper(II) carbonate

2+ blue complex [Cu(NH3)4] formed with oxygen. The white anhydrous salt is pre- excess ammonia solution. pared by adding a solution of dinitrogen ° Symbol: Cu; m.p. 1083.5 C; b.p. pentoxide (N2O5) in nitric acid to crystals 2567°C; r.d. 8.96 (20°C); p.n. 29; r.a.m. of the trihydrate. 63.546. copper(I) oxide (cuprous oxide; CuO) copper(II) carbonate (CuCO3) A green An insoluble, red, covalent solid powder crystalline compound that occurs in min- prepared by the heating of copper with eral form as the basic salt in azurite and copper(II) oxide or the reduction of an al- malachite. See also verdigris. kaline solution of copper(II) sulfate. Cop- per(I) oxide is easily reduced by hydrogen copper(I) chloride (cuprous chloride; when heated; it is oxidized to copper(II) CuCl) A white solid, insoluble in water, oxide when heated in air. Copper(I) oxide prepared by heating copper(II) chloride in undergoes DISPROPORTIONATION in acid so- concentrated hydrochloric acid with excess lutions, producing copper(II) ions and cop- copper turnings. When the solution is col- per. The oxide dissolves in concentrated orless, it is poured into air-free water (or hydrochloric acid due to the formation of – water containing sulfur(IV) oxide) and a the complex ion [CuCl2] . It is used in the white precipitate of copper(I) chloride is glass and electronics industries. obtained. On exposure to air this precipitate turns green due to the formation of copper(II) oxide (cupric oxide; CuO) A basic copper(II) chloride. Copper(I) chlo- black solid prepared by the action of heat ride is essentially covalent in structure. It on copper(II) nitrate, hydroxide, or car- absorbs carbon monoxide gas and is used bonate. It is a basic oxide and reacts with in the rubber industry and in organic chem- dilute acids to form solutions of copper(II) istry. salts. Copper(II) oxide can be reduced to copper by heating in a stream of hydrogen copper(II) chloride (cupric chloride; or carbon monoxide. It can also be reduced CuCl2) A compound prepared by dis- by mixing with carbon and heating the solving excess copper(II) oxide or mixture. Copper(II) oxide is stable up to its copper(II) carbonate in dilute hydrochloric melting point, after which it decomposes to acid. On crystallization, emerald green give oxygen, copper(I) oxide, and eventu- crystals of the dihydrate (CuCl2.2H2O) are ally copper. obtained. The anhydrous chloride may be prepared as a brown solid by heating cop- copper(II) sulfate (cupric sulfate; CuSO4) per in excess chlorine. Alternatively the di- A compound prepared as the hydrate by hydrate may be dehydrated using the action of dilute sulfuric acid on cop- concentrated sulfuric acid. Dilute solutions per(II) oxide or copper(II) carbonate. On of copper(II) chloride are blue, concen- crystallization, blue triclinic crystals of the trated solutions are green, and solutions in pentahydrate (blue vitriol, CuSO4.5H2O) the presence of excess hydrochloric acid are formed. Industrially copper(II) sulfate are yellow. is prepared by passing air through a hot mixture of dilute sulfuric acid and scrap copper(II) nitrate (cupric nitrate; copper. The solution formed is recycled Cu(NO3)2) A compound prepared by until the concentration of the copper(II) dissolving either excess copper(II) oxide or sulfate is sufficient. Copper(II) sulfate is copper(II) carbonate in dilute nitric acid. readily soluble in water. The monohydrate ° On crystallization, deep blue crystals of the (CuSO4.H2O) is formed at 100 C and the ° trihydrate (Cu(NO3)2.3H2O) are obtained. anhydrous salt at 250 C. Anhydrous cop- The crystals are prismatic in shape and are per(II) sulfate is white; it is extremely hy- extremely deliquescent. On heating, cop- groscopic and turns blue on absorption of per(II) nitrate decomposes to give water. It decomposes on heating to give copper(II) oxide, nitrogen(IV) oxide, and copper(II) oxide and sulfur(VI) oxide.

60 covalent radius

Copper(II) sulfate is used as a wood molecules. He wrote several books, includ- preservative, as a fungicide in Bordeaux ing Valence, the first edition of which was mixture, and in the dyeing and electroplat- published in 1952. Coulson also con- ing industries. tributed to the theoretical understanding of electrons in solids. coral A form of calcium carbonate that is secreted by various marine animals (such coupling A chemical reaction in which as Anthozoa) for support and living space. two groups or molecules join together. corrosion Reaction of a metal with an covalent bond A bond formed by the acid, oxygen, or other compound with de- sharing of an electron pair between two struction of the surface of the metal. Rust- atoms. The covalent bond is convention- ing is a common form of corrosion. ally represented as a horizontal line between chemical symbols, thus H–Cl corundum (Al2O3) A naturally occur- indicates that between the hydrogen atom ring mineral form of aluminum oxide that and the chlorine atom there is an electron sometimes contains small amounts of iron pair formed by electrons of opposite spin, and silicon(IV) oxide. It is found in rocks implying that the binding forces are of igneous and metamorphic origin, and in strongly localized between the two atoms. placer deposits. RUBY and SAPPHIRE are im- Molecules are combinations of atoms pure crystalline forms. It is extremely hard bound together by covalent bonds; cova- (only second to diamond) and is used in lent bonding energies are of the order various polishes, abrasives, and grinding 103kJmol–1. wheels. See also emery. Modern bonding theory treats the electron pairing in terms of the interaction of coulomb Symbol: C The SI derived unit electron (atomic) ORBITALS and describes of electric charge, equal to the charge the covalent bond in terms of both ‘bond- transported by a steady electric current of ing’ and ‘anti-bonding’ molecular orbitals. one ampere flowing for one second. In base See also coordinate bond; electrovalent units, 1 C = 1 A s. bond; polar bond. coulombmeter (coulometer; voltameter) A device for determining electric charge or covalent carbide See carbide. electric current using electrolysis. The mass m of material released in time t is meas- covalent crystal A crystal in which the ured, after which this value is used to cal- atoms are covalently bonded. They are culate the charge (Q) and the current (I) sometimes referred to as giant lattices or from the electrochemical equivalent (z) of MACROMOLECULAR CRYSTALS. The best the element, using the formula Q = m/z or known completely covalent crystal is dia- I = m/zt. mond. coulometer See coulombmeter. covalent radius The radius an atom is assumed to have when involved in a cova- Coulson, Charles Alfred (1910–74) lent bond. For diatomic molecules com- British theoretical chemist. Coulson was prised of the same element (e.g. Cl2) this is very influential in promoting the use of simply half the measured internuclear dis- quantum mechanics in chemistry, particu- tance. For molecules comprised of different larly the use of molecular orbital theory. atoms substitutional methods are used. For He used this theory to describe conjugated example, the internuclear distance of systems such as benzene and to justify the bromine fluoride (BrF) is about 180 pi- concept of partial valence. Coulson was cometers (pm). Therefore using 71 pm as also able to use molecular orbital theory to the covalent radius of fluorine (from F2) we predict the properties of many organic get 109 pm as the covalent radius of

61 cream of tartar bromine. This is close to the accepted value cryohydrate See eutectic. of 114 pm. cryolite See sodium hexafluoroalumi- cream of tartar See potassium hydro- nate. gentartrate. cryoscopic constant See depression of critical point The conditions of temper- freezing point. ature and pressure under which a liquid being heated in a closed vessel becomes in- crystal A solid substance that has a def- distinguishable from its gas or vapor inite geometric shape. A crystal has fixed phase. At temperatures below the CRITICAL angles between its faces, which have distinct edges. The crystal will sparkle if the TEMPERATURE (Tc) the substance can be liquefied by applying pressure; at tempera- faces are able to reflect light. The fixed angles are caused by the regular arrange- tures above Tc this is not possible. For each substance there is one critical point; for ex- ments of the atoms, ions, or molecules of ample, for carbon dioxide it is at 31.1°C the substance in the crystal. The faces and and 7397 kilopascals. their angles bear a definite relationship to the arrangement of these particles. If bro- critical pressure The lowest pressure ken, a large crystal will form smaller crys- needed to bring about liquefaction of a gas tals. at its CRITICAL TEMPERATURE. crystal-field theory A theory of the critical temperature The temperature properties of metal complexes. Originally it was introduced by Hans Bethe in 1929 to (Tc) below which a gas can be liquefied by account for the properties of transition el- applying pressure and above which no ements in ionic crystals. In the theory, the amount of pressure is sufficient to bring ligands surrounding the metal atom or ion about liquefaction. Some gases have criti- are thought of as negative charges, and the cal temperatures above room temperature effect of these charges on the energies of (e.g. carbon dioxide 31.1°C and chlorine, the d orbitals of the central metal ion is 144°C) and have thus been known in the considered. Crystal-field theory was suc- liquid state for many years. Liquefaction cessful in describing the spectroscopic, proved much more difficult for those gases ° magnetic, and other properties of com- (e.g. oxygen, –118 C, and nitrogen, plexes. It has been superseded by ligand- ° –146 C) that have very low critical tem- field theory, in which the overlap of the d peratures. orbitals on the metal ion with orbitals on the ligands is taken into account. critical volume The volume of one mole of a substance at its CRITICAL POINT. crystal habit The shape of a crystal. The habit depends on the way in which the crossed-beam reaction A chemical re- crystal has grown; i.e. the relative rates of action performed with two molecular development of different faces and their beams intersecting at an angle. It is possi- proportions. ble to regard one of the beams as the incident (projectile) beam and the other as the crystalline Describing a substance that target beam. It is possible to control both has a regular internal arrangement of the incident and target beams of molecules atoms, ions, or molecules, even though it and a considerable amount of information may not appear visually as geometrically can be obtained about the mechanism and regular crystals. For instance, lead and the kinetics of the chemical reaction. rest of the metals are crystalline because they are composed of regular accumula- cross linkage An atom or short chain tions of tiny invisible crystals. Compare joining two longer chains in a polymer. amorphous.

62 Curie, Marie crystallite A small rudimentary crystal. tered cubic crystal; face-centered cubic The term is often used in mineralogy to de- crystal; crystal system. See illustration scribe specimens that contain accumula- overleaf. tions of many minute crystals of undetermined chemical composition and cupellation A technique used to sepa- crystal structure. rate relatively chemically unreactive metals such as silver, gold, platinum, and palla- crystallization The process of forming dium from impurities by the use of an eas- crystals. When a substance cools from the ily oxidizable metal such as lead. The gaseous or liquid state to the solid state, impure metal is heated in a furnace and a crystallization occurs. Crystals will also blast of hot air is directed upon it. The lead form from a solution saturated with a or other oxidizable metal forms an oxide, solute. leaving the relatively unreactive metal behind. crystallography The study of the formation, structure, and properties of crys- cupric chloride See copper(II) chloride. tals. See also x-ray crystallography. cupric nitrate See copper(II) nitrate. crystalloid A substance that is not a colloid and which will therefore not pass cupric oxide See copper(II) oxide. through a semipermeable membrane. See colloid; semipermeable membrane. cupric sulfate See copper(II) sulfate. crystal structure The particular repeat- cuprite A red or reddish-black mineral ing arrangement of atoms, molecules, or form of copper(I) oxide (Cu2O). It is a ions, in a crystal. ‘Structure’ refers to the principal ore of copper. internal arrangement of particles, not the external appearance. cupronickel A series of durable copper- nickel alloys containing up to 45% nickel. crystal system A classification of crys- Those containing 20% and 30% nickel are tals based on the shapes of their UNIT CELL. very malleable, can be worked cold or hot, If the unit cell is a parallelopiped with and are very corrosion-resistant. They are lengths a, b, and c and the angles between used, for example, in condenser tubes in these edges are α (between b and c), β (be- power stations. Cupronickel with 25% tween a and c), and γ (between a and b), nickel is widely used in coinage. See also then crystals can be classified as follows: Constantan. cubic: a = b = c; α = β = γ = 90° tetragonal: a = b ≠ c; α = β = γ = 90° cuprous chloride See copper(I) chlo- orthorhombic: a ≠ b ≠ c; α = β = γ = 90° ride. hexagonal: a = b ≠ c; α = β = 90°; γ = 120° trigonal: a = b ≠ c; α = β = γ≠90° cuprous oxide See copper(I) oxide. monoclinic: a ≠ b ≠ c; α = γ = 90°≠β triclinic: a ≠ b ≠ c; α≠β≠γ curie Symbol: Ci A unit of radioactivity, The orthorhombic system is also called the equivalent to the amount of a given radio- rhombic system. active substance that produces 3.7 × 1010 disintegrations per second, approximately cubic close packing See face-centered equal to the number of disintegrations pro- cubic crystal. duced by one gram of radium-226 per second. In SI units it is equal to 37 giga cubic crystal Denoting a crystal in becquerels (gBg). which the UNIT CELL is a cube. In a simple cubic crystal the particles are arranged at Curie, Marie (1867– 1934) Polish-born the corners of a cube. See also body-cen- French chemist and physicist. Marie Curie

63 curium

face-centered

body-centered simple cubic

Cubic crystal: types of unit cell was, together with her husband Pierre, a sult of bombarding a plutonium isotope pioneer in the study of radioactivity. One with helium nuclei. It can accumulate in of her first discoveries was that thorium is bone marrow and destroy the body’s ca- radioactive. By conducting a very thorough pacity to produce red blood cells. Curium- analysis of a sample of the uranium min- 244 and curium-242 have been used in eral pitchblende the Curies discovered the thermoelectric power generators. radioactive elements polonium and radium Symbol: Cm; m.p. 1340±40°C; b.p. ≅ in 1898. They shared the 1903 Nobel Prize 3550°C; r.d. 13.3 (20°C); p.n. 96; most for physics with Henri Becquerel for their stable isotope 247Cm (half-life 1.56 × 107 work on radioactivity. By performing ex- years). periments with different compounds of uranium under various physical conditions Curl, Robert Floyd Jnr (1933– ) Marie Curie established that only the American chemist. Curl’s initial work was amount of uranium determined the ra- on small clusters of atoms of semiconduc- dioactivity. This established that radioac- tors, such as germanium and silicon. In tivity is an atomic property of uranium. 1984, under the influence of Harry KROTO, Marie Curie was awarded the 1911 Nobel he became interested in the possibility of Prize for chemistry for her discovery of producing long-chain carbon molecules, polonium and radium. and persuaded his colleague Richard SMAL- LEY to deploy the resources of his labora- curium A highly toxic radioactive sil- tory towards this end. Although they very element of the actinoid series of met- expected on theoretical grounds to dis- als. A transuranic element, it is found cover linear chain clusters with up to 33 naturally on Earth only in very trace carbon atoms, they in fact came across an amounts in uranium ore. Its discovery in unexpected molecule with 60 carbon 1944 was based on its synthesis as the re- atoms and with a cage-like structure. The

64 cyclization discovery of this new allotrope of carbon, cyanoferrate A compound containing 4– later named BUCKMINSTERFULLERENE, the ion [Fe(CN)6] (the hexacyanofer- opened up a new branch of materials sci- 3– rate(II) ion) or the ion [Fe(CN)6] (the ence. Curl shared the 1996 Nobel Prize for hexacyanoferrate(III) ion). These ions are chemistry with Smalley and Kroto. usually encountered as their potassium salts. Potassium hexacyanoferrate(II) cyanamide process An industrial process for fixing nitrogen by heating cal- (K4Fe(CN)6, potassium ferrocyanide) is a cium dicarbide in air. yellow crystalline compound. Potassium → CaC2 + N2 CaCN2 hexacyanoferrate(III) (K3Fe(CN)6, potas- The product, CALCIUM CYANAMIDE, hy- sium ferricyanide) is an orange crystalline drolyzes to ammonia and can be used as a compound. Its solution gives a deep blue fertilizer. The process can be expensive if precipitate with iron(II) ions, and is used as cheap electrical energy is not available to a test for iron(II) (ferrous) compounds. produce CALCIUM DICARBIDE. See also nitro- Prussian blue is a blue pigment containing gen fixation. hexacyanoferrate ions. cyanide A salt of hydrogen cyanide, containing the cyanide ion (CN–). cyanogen (C2N2) A highly toxic flam- Cyanides are extremely poisonous due to mable gas prepared by heating covalent their ability to form coordinate bonds with metal hydrides such as mercury(II) the iron in red blood cells. See hydrocyanic cyanide. See also pseudohalogens. acid. cyclic compound A compound con- cyanide process A technique used for taining a ring of atoms. Although the ma- the extraction of gold from its ores. After jority of the cyclic compounds are organic, crushing the gold ore to a fine powder it is examples are found in inorganic com- agitated with a very dilute solution of pounds such as the silicates, silicones, and POTASSIUM CYANIDE. The gold is dissolved by the cyanide to form potassium di- polyphosphates. cyanoaurate(I) (potassium aurocyanide, cyclization A reaction in which a KAu(CN)2). This complex is reduced with zinc, filtered, and then melted to obtain straight-chain compound is converted into pure gold. a cyclic compound.

65 D

dalton See atomic mass unit. sures of each individual constituent gas, with the partial pressure of each gas being Dalton, John (1766–1844) English the pressure that it would exert if it alone chemist. Dalton is best remembered as lay- occupied the same volume. Dalton’s law is ing the foundations for modern atomic strictly true only for ideal gases. theory. One of his first experiments was to determine the density of water as a func- Daniell cell A type of primary cell in- tion of temperature. In the late 18th and vented by British chemist John Daniell early 19th centuries Dalton investigated (1790–1845) in 1836. It consists of two properties of gases. This led to the state- electrodes in different electrolytes sepa- ment of Dalton’s law of partial pressures in rated by a porous pot. The positive elec- 1801. Dalton first stated his atomic theory trode is copper immersed in copper(II) in 1803. He gave a definitive exposition of sulfate solution and the negative zinc–mer- his views in the book A New System of cury amalgam electrode is in either dilute Chemical Philosophy, the first volume of sulfuric acid or a zinc sulfate solution. The which was published in 1808, with subse- porous pot prevents mixing of the elec- quent volumes being published in 1810 trolytes, but allows ions to pass. With sul- and 1827. furic acid the e.m.f. is about 1.08 volts; with zinc sulfate it is about 1.10 volts. Dalton’s atomic theory A theory ex- At the copper electrode copper ions in plaining the formation of compounds by solution gain electrons from the metal and elements, first published by John Dalton in are deposited as copper atoms: 1803. It was the first modern attempt to Cu2+ + 2e– → Cu describe chemical behavior in terms of The copper electrode thus gains a posi- atoms. The theory was based on certain tive charge. At the zinc electrode, zinc postulates: atoms from the electrode lose electrons and 1. All elements are composed of small, in- dissolve into solution as zinc ions, leaving divisible particles, which Dalton called a net negative charge on the electrode: atoms. Zn → 2e– + Zn2+ 2. All atoms of the same element are identical and have the same properties. darmstadtium A radioactive metallic 3. Atoms can neither be created nor de- element that does not occur naturally on stroyed. the Earth. It is made either by bombarding 4. Atoms combine to form ‘compound a lead target with nickel nuclei or by bom- atoms’ (i.e. molecules) in simple ratios. barding a plutonium target with sulfur nu- Dalton also suggested symbols for the clei. There are several isotopes; the most elements. The theory was used to explain stable is 269Ds, with a half-life of about the law of CONSERVATION OF MASS and the 1.7 × 10–4 s. The chemical properties of laws of CHEMICAL COMBINATION. darmstadtium should be similar to those of platinum. Dalton’s law (of partial pressures) The Symbol Ds; p.n. 110. pressure of a mixture of gases in a particular volume is the sum of the partial pres- dative bond See coordinate bond.

66 deci-

Davy, Sir Humphry (1778–1829) Eng- physicist. Debye made a number of impor- lish chemist. Davy is best known for the tant contributions to molecular structure discovery of sodium and potassium and for and the theory of solids. Debye is perhaps inventing a safety lamp used in mines. He best known for his work with Erich HÜCKEL began his researches in electrochemistry in in 1923 on electrolytes. In his later years he the early 19th century, soon after the in- was concerned with scattering of light and vention of the electric cell. In 1807 he dis- sound by liquids and with polymers. He covered sodium and potassium by won the 1936 Nobel Prize for chemistry electrolysis of fused soda and potash. In for his work on molecular structure and di- 1810 he showed that chlorine, which he pole moments. named, is an element and did not contain oxygen, as had been thought. This helped Debye–Hückel theory A theory of destroy the theory that all acids contain weak electrolytes. It assumes that the elec- oxygen and prompted him to suggest that trolytes are fully dissociated and nonideal all acids contain hydrogen. behavior arises from electrostatic interactions. It is accurate only for very dilute so- Davy lamp See safety lamp. lutions. d-block elements The TRANSITION EL- Debye–Scherrer method A method EMENTS of the first, second, and third long used in X-RAY DIFFRACTION in which a crys- periods of the periodic table, i.e. scandium tal in powder form is exposed to a beam of to zinc, yttrium to cadmium, and lan- monochromatic x-rays. Because the crystal thanum to mercury. They are so called be- is in powder form all possible orientations cause in general they have inner d-levels of the crystal are presented to the x-ray with configurations of the type (n – 1)dxns2 beam. This has the result that the diffracted where x = 1–10. x-rays form cones concentric with the original beam. The Debye–Scherrer method is deactivation A process in which the re- particularly useful for determining the lat- activity of a substance is lessened, or even tice type of a crystal and the dimensions of totally removed . Usually this deactivation its unit cell. The method was first devel- is unwanted, as in the poisoning of cata- oped by Peter Debye and Paul Scherrer. lysts. deca- Symbol: da A prefix used with SI de Broglie wave A wave associated units, denoting 10. For example, 1 de- with a particle, such as an electron or pro- cameter (dam) = 10 meters (m). ton. In 1924, Louis de Broglie (1892– 1987) suggested that, since electromag- decahydrate A crystalline solid contain- netic waves can be described as particles ing ten molecules of water of crystalliza- (photons), particles of matter could also tion per molecule of compound. have wave properties. The wavelength (λ) has the same relationship to momentum decant To pour off the clear liquid (p) as in electromagnetic radiation: above a sediment or heavier, immiscible λ = h/p liquid. where h is the Planck constant. See also quantum theory. decay 1. The spontaneous breakdown of a radioactive nuclide. See half-life; radioac- debye Symbol: D A unit of electric DI- tivity. POLE MOMENT equal in SI units to 3.33564 2. The transition of excited atoms, ions, × 10–30 coulomb meter. It is used in ex- molecules, etc., to the ground state. pressing the dipole moments of molecules. deci- Symbol: d A prefix used with SI Debye, Peter Joseph William (1884– units, denoting 10–1. For example, 1 1966) Dutch-born American chemist and decimeter (dm) = 10–1 meter (m).

67 decompostion decomposition A chemical reaction in degrees Kelvin See absolute tempera- which a compound is broken down into ture. compounds with simpler molecules or into elements. degrees of freedom 1. The independent ways in which particles can take up energy. decrepitation A crackling sound heard In a monatomic gas, such as helium or when certain crystalline solids are heated, argon, the atoms have three translational usually as a result of loss of water of crys- degrees of freedom, corresponding to their tallization. motion in space in three mutually perpendicular directions (i.e. along x, y, and z co- defect An irregularity in the ordered ordinates). The mean energy per atom for arrangement of particles in a crystal lattice. each degree of freedom is kT/2, where k is There are two main kinds of defects in the Boltzmann constant and T the thermo- crystals: point defects and dislocations. dynamic temperature; the mean energy per Point defects occur at single lattice points. atom is thus 3kT/2. There are three types. A vacancy, also A diatomic gas, in addition to three known as a Schottky defect, is a missing translational degrees, also has two rota- atom; i.e. a vacant lattice point. An inter- tional degrees of freedom located about the stitial is an atom that is in a position that is two axes perpendicular to the bond, and not a normal lattice point. If an atom one vibrational degree of freedom located moves off its normal lattice point to an in- along the axis of the bond. The rotational terstitial position the result (vacancy plus degrees also each contribute kT/2 to the av- interstitial) is called a Frenkel defect. All erage energy. The vibrational degree con- solids above absolute zero have a number tributes kT (kT/2 for kinetic energy and of point defects, the concentration of kT/2 for potential energy). Thus, the aver- which depends on temperature. Point de- age energy per molecule for a diatomic fects can also be produced by strain or by molecule is 3kT/2 (translation) + kT (rota- irradiation. tion) + kT (vibration) = 7kT/2. Dislocations (or line defects) are also Linear triatomic molecules also have produced by strain in solids. These defects two significant rotational degrees of free- are irregularities extending over a number dom; nonlinear molecules have three. For of lattice points along a line of atoms. nonlinear polyatomic molecules, the number of vibrational degrees of freedom is 3N definite proportions, law of See con- – 6, where N is the number of atoms in the stant proportions. molecule. The molar energy of a gas is the average degassing The removal of adsorbed, energy per molecule multiplied by the Avo- dissolved, or absorbed gases from liquids gadro constant. For a monatomic gas, for or solids. example, it is 3RT/2. 2. The independent physical quantities degenerate Describing different quan- (e.g. pressure, temperature, etc.) that de- tum states that have the same energy. For fine the state of a given system. See phase instance, the five d orbitals in a transition- rule. metal atom all have the same energy but different values of the magnetic quantum dehydration 1. Removal of water from number m. Differences in energy occur if a a substance. magnetic field is applied or if the arrange- 2. Removal of the elements of water (i.e. ment of ligands around the atom is not hydrogen and oxygen in a 2:1 ratio) to symmetrical. The degeneracy is then said form a new compound. to be ‘lifted’. deionization A method of removing degrees absolute See absolute tempera- ions from a solution using ion exchange. ture. The term is commonly applied to the pu-

68 depression of freezing point rification of tap water; deionized water has 1960s and has been used extensively in superseded distilled water in many chemi- chemistry and solid state physics since that cal applications. See ion exchange. time. deliquescent Describing a solid HYGRO- depolarizer A substance used in a SCOPIC compound that absorbs so much voltaic cell to prevent polarization. For ex- water from the atmosphere that a solution ample, hydrogen bubbles forming on the eventually forms. electrode can be removed by an oxidizing agent such as manganese(IV) oxide delocalization Diffusion of one or more (MnO2). of a molecule’s bonding electrons over two or more of the bonds between the mole- depression of freezing point A COL- cule’s atoms. LIGATIVE PROPERTY of solutions in which the freezing point of a given solvent is low- delocalized bond (nonlocalized bond) ered by the presence of a solute. The A type of molecular bonding in which the amount of the reduction is proportional to electrons forming the delocalized bond are the molal concentration (m) of the solute. no longer regarded as remaining between The depression depends only on this con- two atoms but instead are spread over sev- centration and is independent of solute eral atoms or even the whole molecule. composition. The formula is: ∆ The electron density of the delocalized t = KfCM, bond is spread by means of a delocalized where ∆t is the lowering of the tempera- molecular ORBITAL and may be regarded as ture, Kf is the proportionality constant a series of pi bonds extending over several (also known as the freezing point constant atoms, for example the C–O pi bonds in or the cryoscopic constant), and Cm is the the carbonate ion. See also metallic bond; molal concentration. Note that the unit of –1 resonance. Kf is the kelvin kilogram mole (K kg mol–1). The formula can be applied to the delta brass See delta metal. measurement of relative molecular mass with considerable precision. A known delta metal (delta brass) A strong alloy mass of pure solvent is slowly frozen, with of copper and zinc, containing also a little stirring, in a suitable cold bath and the iron. Its main use is for making cartridge freezing temperature measured using a cases. Beckmann thermometer. A known mass of solute of known molecular mass is intro- dendritic growth Growth of crystals in duced, the solvent thawed out, and the a branching (‘treelike’) habit. cooling process and measurement repeated. The addition is repeated several denitrification See nitrogen cycle. times and an average value of Kf for the ∆ solvent obtained by plotting t against Cm. density (mass density) Symbol: ρ The The whole process is then repeated using mass per unit volume of a given substance. the unknown solute and its relative mo- It is typically measured in grams per cubic lecular mass determined using the value of –3 decimeter (g m ), which is equivalent to Kf previously obtained. grams per liter (g L–1), or in kilograms per The effect is applied to more precise cubic meter (kg m–3). See also relative den- measurement of relative molecular mass by sity. using a pair of Dewar flasks (pure solvent and solution) and measuring ∆t by means density functional theory A method of of thermocouples. The theoretical explana- calculating the electronic structure of mol- tion for the effect is similar to that for low- ecules using the electron density. The ering of vapor pressure. The freezing point theory was developed by Walter Kohn of the solvent is that point at which the (1923– ) and his colleagues in the mid- curve representing the vapor pressure

69 derivative above the liquid phase intersects the curve wholly or partially decomposes in the ab- representing the vapor pressure above the sence of air to produce volatile products, frozen solvent. The addition of solute de- which are subsequently condensed. The de- presses the former curve but as the solid structive distillation of coal was the phase that separates is always pure solvent process for manufacturing coal gas and (i.e. above the eutectic point), there is no coal tar. Formerly, methanol was made by attendant depression of the latter curve. the destructive distillation of wood. Consequently the point of intersection is depressed, resulting in a lowering of the detergents A group of substances that freezing point. See also lowering of vapor improve the cleansing action of solvents, pressure. Compare elevation of boiling particularly water. The majority of deter- point. gents, including soap, have the same basic structure. Their molecules have a nonpolar derivative A compound obtained by re- hydrocarbon chain (tail) that lacks an action from another compound. The term affinity for water molecules and is there- is most often used in organic chemistry of fore said to be hydrophobic (water-fearing compounds that have the same general or repelling). Attached to this tail is a small structure as the parent compound. polar group (head) that has an affinity for water molecules and is said to be hy- derived unit A measurement unit de- drophilic (water-loving or attracting). fined in terms of the BASE UNITS of a system Detergents reduce the surface tension of of measurement, and not directly from a water and thus improve its wetting power standard value of the quantity it measures. by anchoring their hydrophilic heads in the For example, the newton is a unit of force water with their hydrophobic tails pro- defined in base SI UNITS as a kilogram meter truding above it. Consequently the water second–2 (kg m s–2). surface is broken up, enabling the water to spread over the material to be cleaned and desalination Any of various techniques penetrate between the material and dirt to for removing the salts (mainly sodium which it has been exposed. With the assis- chloride) from seawater to make it fit for tance of agitation, the dirt can then be drinking, irrigation, use in water-cooled floated off. At the same time the hy- engines, and for making steam for steam drophobic tails of the detergent molecules turbines. There are various methods. dissolve in grease and oils. The protruding Those based on distillation depend on a hydrophilic heads of the molecules repel cheap source of heat energy, of which solar each other, causing the grease or oil to roll energy is the most promising, especially in up and form tiny drops, which float off hot climates. Flash evaporation (evapora- into the water to form an emulsion. tion under reduced pressure) and freezing Unlike soaps synthetic detergents, to make pure ice are other methods, as are which are often derived from petrochemi- electrodialysis, ion exchange, reverse os- cals, do not form insoluble scums with mosis, and the use of molecular sieves. hard water. Synthetic detergents are of three types. Anionic detergents form ions desiccation Removal of moisture from consisting of a hydrocarbon chain to which a substance. is attached either a sulfonate group, – – –SO2–O , or a sulfate group, –O–SO2–O . desiccator A laboratory apparatus for The corresponding metal salts are soluble drying solids or for keeping solids free of in water. Cationic detergents have organic + moisture. It is a container in which is kept positive ions of the type RNH3 , in which a hygroscopic material (e.g. calcium chlo- R has a long hydrocarbon chain. Nonionic ride or silica gel). detergents are complex chemical compounds called ethoxylates. They owe their destructive distillation The process of detergent properties to the presence of a heating an organic substance such that it number of oxygen atoms in one part of the

70 diamond molecule, which are capable of forming hy- dextrorotatory See optical activity. drogen bonds with the surface water molecules, thus reducing the surface tension of D-form See optical activity. the water. diagonal relationship There is a gen- deuterated compound A compound in eral trend in the periodic table for elec- which one or more 1H atoms have been re- tronegativity to increase from left to right placed by deuterium (2H) atoms. along a period and to decrease down a group. Thus a move of ‘one across and one deuteride A compound of deuterium down’, i.e. a diagonal move in the table, with other elements; i.e. a hydride in which gives rise to effects that tend to cancel each the deuterium isotope is present rather other. There is a similar general trend and than 1H. See hydride. combined effect for size, which decreases along a period but increases down a group. deuterium (heavy hydrogen) Symbol: D, These diagonal relationships give rise to 2H A naturally occurring, stable isotope similarities in chemical properties, which of hydrogen in which the nucleus contains are particularly noticeable for the follow- one proton and one neutron. The atomic ing diagonal pairs. Li–Mg; Be–Al; B–Si. mass is thus approximately twice that of Li–Mg: 1. both have carbonates that give 1 H. Chemically it behaves almost identi- CO2 on heating; 2. both burn in air to give cally to hydrogen, forming analogous com- the normal oxide only; 3. both form a ni- pounds, although reactions of deuterium tride; 4. both form hydrated chlorides that compounds are often slower than those of hydrolyze slowly. Be–Al: 1. both give hy- corresponding 1H compounds. This fact is drogen with alkalis; 2. both give water-in- made use of in KINETICS where the rate of a soluble hydroxides that dissolve in alkali; reaction may depend on transfer of a hy- 3. both form complex ions of the type – drogen atom (i.e. a kinetic isotope effect). MCl3 ; 4. both have covalently bridged chlorides. B–Si: 1. both form acidic oxides deuterium oxide (heavy water; (D2O) A of a giant covalent-molecule type with high naturally occurring form of water in which melting points; 2. both form low-stability deuterium isotopes (2H) have replaced the hydrides that ignite in air; 3. both form far more common isotope of hydrogen readily hydrolyzable chlorides that fume in (1H) in the water molecule. Deuterium air; 4. both have amorphous and crys- oxide represents only about 0.003% by talline forms and both form glasses with mass of the water on Earth, from which it basic oxides, such as Na2O. can be separated by either electrolysis or fractional distillation. The chief use of deu- diamagnetism See magnetism. terium oxide is to slow down fast neutrons in nuclear fission reactors designed to use diamond An allotrope of carbon and unenriched uranium (238U) as fuel. the hardest naturally occurring substance known. It is used for jewelry and, industri- deuteron The nucleus of the deuterium ally, for cutting and drilling equipment. In atom, 2H+ or D+. It is used to bombard diamond, each carbon atom is surrounded other nuclei in nuclear accelerators. by four equally spaced carbon atoms arranged tetrahedrally. The carbon atoms Dewar flask (vacuum flask) A double- form a three-dimensional network with walled container of thin glass with the each carbon-carbon bond equal to 0.154 space between the walls evacuated and nm and set at an angle of 109.5° with its sealed to stop conduction and convection neighbors. Millions of atoms are cova- of energy through it. The glass is often sil- lently bonded in diamonds to form a giant vered to reduce radiation. molecular structure, the great strength of which results from the strong covalent dextro-form See optical activity. bonds between carbon atoms. Diamonds

71 diatomaceous earth can be formed synthetically from graphite are a distance d apart in a medium with a under extreme temperature and pressure in permittivity ε this means that the force F the presence of a catalyst. Although small, between the charges is given by F = πε 2 such diamonds are of adequate size for (1/4 )(Q1Q2/d ). many industrial uses. Natural diamonds The dielectric constant (relative permittiv- ε ε ε ε are thought to form deep in the Earth’s ity) r of the medium is given by r = / 0, ε crust and are often associated with ancient where 0 is the permittivity of free space. volcanic vents when discovered. Major de- The dielectric constant of air is very slightly posits occur in Africa, Australia, Brazil, greater than 1, while that of water is about Canada, and Russia. See also carbon. 80. The value of the dielectric constant of a medium has important physical and chem- diatomaceous earth See diatomite. ical consequences, particularly for ions in solutions. diatomic Describing a molecule that consists of two atoms. Hydrogen (H2), diffusion Movement of a gas, liquid, or oxygen (O2), nitrogen (N2), and the halo- solid as a result of the random thermal mo- gens are examples of elements that form tion of its particles (atoms or molecules). A idatomic molecules. Sodium chloride drop of ink in water, for example, will (NaCl) is another example of a diatomic slowly spread throughout the liquid. Diffu- molecule. sion in solids takes place very slowly at normal temperatures. See also Graham’s diatomite (diatomaceous earth; kiesel- law. guhr) A whitish powdered mineral consisting mainly of SILICON(IV) OXIDE derived dihydrate A crystalline compound hav- from the shells of diatoms. It is used to ing two molecules of water of crystalliza- make fireproof cements and as an ab- tion per molecule of compound. sorbent in the manufacture of dynamite. diiodine hexachloride (iodine trichlo- dibasic acid An acid that has two active ride; I2Cl6) A yellow crystalline solid protons, such as sulfuric acid. Dibasic made by reacting excess chlorine with io- acids can give rise to two series of salts. For dine. It is a strong oxidizing agent. At 70°C example, sulfuric acid (H2SO4) forms it dissociates into iodine monochloride and 2– sulfates (SO4 ) and hydrogensulfates chlorine. – (HSO4 ). dilead(II) lead(IV) oxide (red lead; diboron trioxide See boron oxide. Pb3O4) A powder made by heating lead(II) oxide or lead(II) carbonate hydrox- dicarbide See carbide. ide at 400°C. It is black when hot and red or orange when cold. On strong heating it dichlorine oxide (chlorine monoxide; decomposes to lead(II) oxide and oxygen. chlorine(I) oxide; Cl2O) An orange gas Dilead(II) lead (IV) oxide is used as a pig- made by passing chlorine over mercury(II) ment and in glass making. It is not stoi- oxide. It is a strong oxidizing agent and chiometric and tends to have less oxygen dissolves in water to give chloric(I) acid. than denoted by its formula. dichromate(VI) See chromium; potas- diluent A solvent that is added to reduce sium dichromate. the strength of a solution. dielectric constant (relative permittiv- dilute Denoting a solution in which the ity) A quantity that characterizes how a amount of solute is low relative the amount medium reduces the electric field strength of solvent. The term is always relative and associated with a distribution of electric includes dilution at trace level as well as the charges. If two point charges Q1 and Q2 common term bench dilute acid, which 72 disodium hydrogen phosphate(V) usually means a 2M solution. Compare dipolar bond See coordinate bond. concentrated. dipole–dipole interactions The inter- dimensionless units Units defined in action of two molecules resulting from terms of the ratio of two comparable quan- their DIPOLE MOMENTS. The strength of this tities expressed in like units, which there- interaction depends on the size of the di- fore reduce to one. For example, the radian pole moments of the molecules and their and steradian, both SI derived units, are di- distance apart. The dipole moments that mensionless because they are expressed as can interact include permanent dipole mo- ratios of meter per meter and square meter ments in molecules such as water, induced per square meter, respectively. See SI units. dipole moments caused by the presence of polar molecules in close proximity, thus dimer A compound (or molecule) giving rise to induced dipole–dipole inter- formed by combination or association of actions, and the charge separation associ- two molecules of a monomer. For instance, ated with VAN DER WAALS FORCES. aluminum chloride (AlCl3) is a dimer µ (Al2Cl6) in the vapor phase. dipole moment Symbols: , p A quantitative measure of polarity in either a bond dimorphism See polymorphism. (bond moment) or a molecule as a whole (molecular dipole moment). The unit is the × –30 dinitrogen oxide (nitrous oxide; N2O) A debye (equivalent to 3.335 64 10 colorless gas with a faintly sweet odor and coulomb meter). Molecules such as HF, taste. It is appreciably soluble in water but H2O, and NH3, possess dipole moments; more soluble in ethanol. It is prepared CCl4, N2, and PF5 do not. commercially by carefully heating ammo- The molecular dipole moment can be nium nitrate: estimated by vector addition of individual → NH4NO3(s) N2O(g) + 2H2O(g) bond moments if the bond angles are Dinitrogen oxide is fairly easily decom- known. The possession of a dipole moment posed on heating to temperatures above permits direct interaction with electric 520°C, giving nitrogen and oxygen. The fields or interaction with the electric com- gas is used as a mild anesthetic in medicine ponent of radiation. and dentistry, being marketed in small steel cylinders. It is sometimes called laughing Dirac, Paul Adrien Maurice (1902– gas because it induces a feeling of elation. 84) English physicist. Dirac was one of the It is also used as an aerosol propellent. founders of quantum mechanics. In a remarkable series of papers in the second half dinitrogen tetroxide (N2O4) A color- of the 1920s he formulated quantum me- less gas that becomes a pale yellow liquid chanics in a general way that incorporated below 21°C and solidifies below –11°C. the matrix mechanics of Werner HEISEN- On heating, the gas dissociates to nitrogen BERG and the wave mechanics of Erwin dioxide molecules: SCHRÖDINGER as special cases. He shared → N2O4(g) 2NO2(g) the 1933 Nobel Prize for physics with This dissociation is complete at 140°C. Schrödinger. Liquid dinitrogen tetroxide has good solvent properties and is used as a nitrating dislocation See defect. agent. disodium hydrogen orthophosphate diphosphane (diphosphine; P2H4)A See disodium hydrogen phosphate(V). yellow liquid that can be condensed out from PHOSPHINE in a freezing mixture. It ig- disodium hydrogen phosphate(V) (di- nites spontaneously in air. sodium hydrogen orthophosphate; Na2- HPO4) A white solid prepared by diphosphine See diphosphane. titrating phosphoric acid with sodium hy-

73 disodium oxide droxide solution using phenolphthalein as common example is the displacement of the indicator. On evaporation the solution hydrogen from acids by metals: → yields efflorescent monoclinic crystals of Zn + 2HCl ZnCl2 + H2 the dodecahydrate, Na2HPO4.12H2O. The See also double decomposition. effloresced salt contains 7H2O. Disodium hydrogen phosphate is used in the textile disproportionation A chemical reac- industry. tion in which there is simultaneous oxidation and reduction of the same compound. disodium oxide See sodium monoxide. An example is the reaction of copper(I) chloride to copper and copper(II) chloride: → disodium tetraborate decahydrate 2CuCl Cu + CuCl2 in which there is simultaneous oxidation (borax; Na2B4O7.10H2O) A white crystalline solid, sparingly soluble in cold (to Cu(II)) and reduction (to Cu(0)). water but readily soluble in hot water. It Another example is the reaction of occurs naturally as salt deposits in dry lake chlorine with water: → + – – beds, especially in California. It is an im- Cl2 + H2O 2H + Cl + ClO – portant industrial material, being used in in which there is reduction to Cl and oxi- – the manufacture of enamels and heat-resis- dation to ClO . tant glass, as a paper glaze, and as a source of borium compounds. It is also used in dissociation Breakdown of a molecule laundry products and as a mild antiseptic. into two molecules, atoms, radicals, or ions. Often the reaction is reversible, as in In solution hydrolysis occurs: the ionic dissociation of weak acids in B O 2– + 7H O = 2OH– + 4H BO 4 7 2 3 3 water: See also borax-bead test. ˆ + – HCN + H2O H3O + CN See also dissociation constant. dispersed phase See colloid. dissociation constant Symbol: K The dispersing agent A compound used to EQUILIBRIUM CONSTANT of a reversible dis- help produce EMULSIONS or dispersions of sociation reaction. For example, the disso- IMMISCIBLE liquids, such as water and oil. ciation constant of a reaction: AB ˆ A + B dispersion force See van der Waals is given by: force. K = [A][B]/[AB] where the brackets denote concentration displacement pump A commonly used (activity). device for transporting liquids and gases Often the degree of dissociation is used around chemical plants. It works on the – the fraction (α) of the original compound principle of the bicycle pump: a piston that has dissociated at equilibrium. For an raises the pressure of the fluid and, when it original amount of AB of n moles in a vol- is high enough, a valve opens and the fluid ume V, the dissociation constant is given is discharged through an outlet pipe. As the by: piston moves back the pressure falls and K = α2n/(1 – α)V the cycle continues. Displacement pumps Note that this expression is for dissociation can be used to generate very high pressures into two molecules. (e.g. in the synthesis of ammonia) but be- Acid dissociation constants (or acidity cause of the system of valves, they are more constants, symbol: Ka) are dissociation expensive than other types of pump. Com- constants for the dissociation into ions in pare centrifugal pump. solution: ˆ + – HA + H2O H3O + A displacement reaction A chemical re- If the concentration of water is taken as action in which an atom or group displaces unity, the acidity constant is given by: + – another atom or group from a molecule. A Ka = [H3O ][A ]/[HA] 74 double decompostion

The acidity constant is a measure of the central member, when placed in order of strength of the acid. Base dissociation con- increasing relative atomic mass, has a rela- stants (symbol Kb) are similarly defined. tive atomic mass approximately equal to The expression: the average of the outer two. Other chemi- K = α2n/(1 – α)V cal and physical properties of the central applied to an acid is known as Ostwald’s member also lie between those of the first dilution law. In particular if α is small (a and last members of the triad. German weak acid) then K = α2n/V, or α = C√V, chemist Johann Döbereiner (1780–1849) where C is a constant. The degree of disso- noted this relationship in 1817; each triad ciation is then proportional to the square is now recognized as comprised of consec- root of the dilution. utive members of a group of the periodic table; e.g. calcium, strontium, and barium; distillation The process of boiling a liq- chlorine, bromine, and iodine. uid and condensing the vapor. Distillation is used to purify liquids or to separate com- dolomite (pearl spar) A typically white, ponents of a liquid mixture. See also frac- pink, or colorless mineral, CaCO3.- tional distillation; steam distillation; MgCO3, used as an ore of magnesium and vacuum distillation. for the lining of open-hearth steel furnaces and Bessemer converters. distilled water Water that has been purified by distillation in order to separate it donor 1. The atom, ion, or molecule from dissolved solids or other substances. that provides the pair of electrons in form- Laboratory-grade water is usually distilled ing a covalent bond. several times. 2. The impurity atoms used in doping semiconductors. disulfur dichloride (sulfur monochloride; S Cl ) A red fuming liquid with a 2 2 doping The incorporation of impurities strong smell. It is prepared by passing chlo- within the crystal lattice of a solid so as to rine over molten sulfur and is used to alter its physical properties. For instance, harden (vulcanize) rubber. silicon, when doped with boron, becomes dithionate A salt of dithionic acid. semiconducting. Dithionates are reducing agents. double bond A covalent bond between dithionic acid (hyposulfuric acid; two atoms that includes two pairs of electrons, one pair being the single bond equiv- H2S2O6) A strong sulfur oxyacid that decomposes slowly on standing or heating. alent (the sigma bond) and the other forming an additional bond, the pi bond (π dithionite (sulfinate) A salt of dithio- bond). It is conventionally represented by nous acid. Dithionites are powerful reduc- two lines, for example H2C=O. See multi- ing agents. ple bond; orbital. dithionous acid (sulfinic acid; hyposul- double decomposition (metathesis) A chemical technique for making an insolu- furous acid; H2S2O4) An unstable sulfur oxyacid that is known only in solution. ble salt by mixing two soluble salts. The ions from the two reactants ‘change part- divalent (bivalent) Having a valence of ners’ to form a new soluble compound and two. an insoluble compound, which is precipitated. For example, mixing solutions of dl-form See optical activity. potassium iodide (KI) and lead(II) nitrate (Pb(NO3)2) forms a solution of potassium Döbereiner’s triads Groups of three nitrate (KNO3) and a yellow precipitate of chemically similar elements in which the lead(II) iodide (PbI2). 75 double salt double salt When equivalent quantities town near Moscow, in 1967 by bombard- of certain salts are mixed in aqueous solu- ing americium ions with neon ions. It was tion and the solution evaporated, a salt almost simultaneously made in the U.S.A. having two different anions or cations may by bombarding californium nuclei with ni- form, e.g. FeSO4.(NH4)2SO4.6H2O. In trogen nuclei. aqueous solution the salt behaves as a mix- Symbol: Db; m.p., b.p., and r.d. un- ture of the two individuals. These salts are known, p.n. 105; most stable isotope called double salts to distinguish them 262Db (half-life 34 s). from complex salts, which yield complex ions in solution. See also alum. Dulong and Petit’s law The law that states that the molar heat capacity of a Downs process An electrolytic process solid element is approximately equal to for making sodium and chlorine from 3R, where R is the GAS CONSTANT (25 J K–1 fused sodium chloride. The process takes mol–1). The law applies only to elements place in a Downs cell, which has a centrally with simple crystal structures at normal located graphite anode surrounded by a temperatures. At lower temperatures the cylindrical steel cathode. A coaxial steel molar heat capacity falls with decreasing grid keeps the electrodes and their prod- temperature. Molar heat capacity was for- ucts separate. Chlorine gas is collected via merly called atomic heat – the product of a cone-shaped dome above the anode, the relative atomic mass of a substance and while molten sodium floats on top of the its specific heat capacity. denser electrolyte to be siphoned off via a The law was derived by French collecting pipe. chemists Pierre Dulong (1785–1838) and Alexis Petit (1791–1820) in 1819. Dow process An industrial method whereby magnesium is extracted from seawater via the precipitation of magnesium Duralumin (Trademark) A strong light- hydroxide (Mg(OH) ) when calcium hy- weight aluminum alloy containing 3–4% 2 copper, with small amounts of magnesium, droxide (Ca(OH)2) is added, followed by SOLVATION of the precipitated hydroxide manganese, and sometimes silicon. It is by hydrochloric acid (HCl). widely used in aircraft bodies and similar applications requiring strength combined dry cell A voltaic cell in which the elec- with lightness. trolyte is typically in the form of a jelly or paste and the metal container holding the dysprosium A very soft malleable sil- electrolyte forms the negative electrode. very-white element of the lanthanoid series Dry cells are extensively used for flash- of metals. It occurs in association with lights, toys, and other portable applica- other lanthanoids. It is used in lasers and tions. compact disks, and as a neutron absorber in nuclear reactors; it is also a constituent dry ice See carbon dioxide. of certain magnetic alloys. Symbol: Dy; m.p. 1412°C; b.p. 2562°C; dubnium A radioactive synthetic trans- r.d. 8.55 (20°C); p.n. 66; most common actinide element first made at Dubna, a isotope 164 Dy; r.a.m. 162.50.

76 E

ebullioscopic constant See elevation of heat given out at the end of the process. See boiling point. also Carnot’s principle; Carnot cycle. ebullition The boiling or bubbling of a efflorescence The process in which a liquid. crystalline hydrated solid loses water of crystallization to the air. A powdery de- Edison cell See nickel–iron accumula- posit is gradually formed on the surface of tor. the compound. edta Abbreviation for ethylenedi- Eigen, Manfred (1927– ) German aminetetraacetic acid, a tetrabasic car- chemist. Eigen was a pioneer of the study boxylic acid. It is important in inorganic of very fast chemical reactions. Starting in chemistry because the carboxylate ion can 1954, he studied such reactions by relax- act as a ligand in forming complexes. Edta ation techniques in which very quick is a hexadentate ligand, able to coordinate changes in the pressure, temperature, and to a metal ion at the four COO– groups and electric field were applied to the system and the two nitrogen atoms. It is able to form the consequences studied using spec- octahedral complexes with certain metal troscopy. He shared the 1967 Nobel Prize ions. for chemistry with Ronald NORRISH and George PORTER for this work. Subsequently effervescence The evolution of gas in Eigen used relaxation techniques to study the form of bubbles in a liquid. very fast biochemical reactions. efficiency Symbol: η A measure used for eigenfunction A function that is a processes of energy transfer; the ratio of solution of an EIGENVALUE equation. An ex- the useful energy produced by a system or ample of an eigenfunction is the WAVE- device to the energy input. For a reversible FUNCTION for a quantum mechanical heat engine, the idealized maximum effi- system described by the SCHRÖDINGER ciency is given by EQUATION. η = (T1 – T2)/T1 where T1 is the temperature of the heat as eigenvalue One of the set of possible sourced and T2 is the temperature of the values of numbers that make up the solu-

H2 H2 H2 H2 HOOC C C C C COOH

N N

HOOC C C COOH H2 H2

Edta

77 einsteinium tions of an eigenvalue equation. An eigen- metals above hydrogen displace hydrogen value equation has the general form: Ôψ = from acids: → Eψ, where Ô is an operator which operates Zn + 2HCl ZnCl2 + H2 on the EIGENFUNCTION ψ and E is a number The series is based on ELECTRODE POTEN- which multiplies the same eigenfunction. TIALS, which measure the tendency of el- The solution of an eigenvalue equation ements to form positive ions. The series is consists in finding the sets of eigenfunc- one of increasing electrode potential for tions ψ and eigenvalues E that satisfy the half cells of the type Mn+|M. Thus, copper eigenvalue equation. The fundamental (EŠ for Cu2+|Cu = + 0.34 V) is lower than equations of quantum mechanics such as zinc (EŠ for Zn2+|Zn = –0.76V). The hy- the SCHRÖDINGER EQUATION are eigenvalue drogen half cell by definition has the value Š equations. In the case of the Schrödinger E = 0. equation the eigenfunctions are the WAVE- FUNCTIONS for the quantum states of the electrochemistry The study of the for- system and the eigenvalues are the values mation and behavior of ions in solutions. It of the quantized energy levels associated includes electrolysis and the generation of with these quantum states. electricity by chemical reactions in cells. einsteinium A radioactive transuranic electrochromatography See electro- element of the actinoid series, not found phoresis. naturally on Earth and originally discovered in the radioactive fallout of the first electrode Any part of an electrical device or system that emits or collects elec- large hydrogen bomb. It can be produced trons or other charge carriers. An electrode in milligram quantities by bombarding may also be used to deflect charged parti- 239Pu with neutrons to give 253Es (half-life cles by the action of the electrostatic field 20.47 days). Several other short-lived iso- that it produces. See also half cell. topes have also been synthesized. Symbol: Es; m.p. 860 ± 30°C; b.p. and electrodeposition (electroplating) The r.d. unknown; p.n. 99; most stable isotope 254 process of depositing a layer of solid Es (half-life 276 days). (metal) on an electrode by means of electrolysis. Positive ions in solution gain elec- electrochemical equivalent Symbol: trons at the cathode and are deposited z The mass of an element released from a as atoms. Copper, for instance, can be solution of its ions when a current of one deposited on a metal cathode from an acid- ampere flows for one second during elec- ified copper sulfate solution. Electro- trolysis (i.e. by one coulomb of electricity). deposition allows metals to be plated with other, more decorative or corrosion- electrochemical series (activity series; resistant metals, e.g. chromium on steel. electromotive series) A series giving the activities of metals for reactions that in- electrode potential (reduction potential) volve ions in solution. In decreasing order Symbol: E A measure of the tendency of of activity, the series for the most commer- an element to form ions in solution. For ex- cially important metals is ample, a metal in a solution containing M+ K, Ba, Ca, Na, Mg, Al, Mn, ions may dissolve in the solution as M+ Zn, Cr, Fe, Cd, Co, Ni, Sn, ions; the metal then has an excess of elec- Pb, H, Cu, Hg, Ag, Pt, Au trons and the solution an excess of positive Any member of the series will displace ions ions. Thus, the metal becomes negative of a lower member from solution. For ex- with respect to the solution. Alternatively, ample, zinc metal will displace Cu2+ ions: the positive ions may gain electrons from Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) the metal and be deposited as metal atoms. Zinc has a greater tendency than copper In this case, the metal becomes positively to form positive ions in solution. Similarly, charged with respect to the solution. In ei-

78 electrolytic cell ther case, a potential difference is devel- deionized water in the center compartment oped between solid and solution, and an of the cell. In practice, an array of alternat- equilibrium state will be reached at which ing membranes is used. Electrodialysis is further reaction is prevented. The equilib- used to make drinking water in areas rium value of this potential difference where the available water supply is brack- would give an indication of the tendency to ish. See also desalination. form aqueous ions. It is not, however, possible to measure electrolysis The production of chemical this value for an isolated half cell, because change by passing electric charge through any measurement requires a circuit, which certain conducting liquids known as elec- sets up another half cell in the solution. To trolytes. Electrolytes may be solutions or overcome this limitation, electrode poten- molten salts, etc. The current is conducted tials are defined by comparison with a HY- by the migration of ions – positive ones DROGEN ELECTRODE (a type of half cell), (cations) to the cathode (negative elec- which is connected to the half cell under trode), and negative ones (anions) to the investigation by a salt bridge. The e.m.f. of anode (positive electrode). Reactions take the full cell can then be measured. place at the electrodes via the transfer of In referring to a given half cell the more electrons to or from them. reduced form is written on the right for a In the electrolysis of water (containing a half-cell reaction. For the half cell Cu2+|Cu, small amount of acid to make it conduct the half-cell reaction is a reduction: adequately) hydrogen gas is given off at the Cu2+(aq) + 2e– → Cu cathode and oxygen is evolved at the The cell formed in comparison with a hy- anode. At the cathode the reaction is: drogen electrode is: H+ + e– → H + 2+ → Pt(s)H2(g)|H (aq)|Cu (aq)|Cu 2H H2(g) The e.m.f. of this cell is +0.34 volt (V) At the anode: measured under standard conditions. OH– → e– + OH → Thus, the standard electrode potential 2OH H2O + O Š → (symbol: E ) is +0.34 V for the half cell 2O O2(g) Cu2+|Cu. The standard conditions are 1.0 In certain cases the electrode material molar solutions of all ionic species, stan- may dissolve. For instance, in the electrol- dard pressure, and a temperature of 298 K. ysis of copper(II) sulfate solution with cop- Half cells can also be formed by a solu- per electrodes, copper atoms of the anode tion of two different ions (e.g. Fe2+ and dissolve as copper ions: Fe3+). In such cases, a platinum electrode, a Cu → 2e– + Cu2+ half cell employing a platinum plate, is See Faraday’s laws. used under standard conditions. electrolyte A liquid containing positive electrodialysis A method of removing and negative ions that conducts electricity ions from water by selective flow through by the flow of those charges. Electrolytes membranes under the influence of an elec- can be solutions of acids or metal salts tric field. In the simplest arrangement a cell (‘ionic compounds’), usually in water. Al- is divided into three compartments by two ternatively they may be molten ionic com- semipermeable membranes, one permeable pounds, which also allow ions to move to positive ions and the other permeable to freely. Liquid metals in which conduction negative ions. Electrodes are placed in the is by free electrons rather than ions are not outer compartments of the cell – the posi- classified as electrolytes. See also electroly- tive electrode next to the membrane that sis. allows negative ions to pass, and the negative electrode next to the membrane that electrolytic Relating to the behavior or allows positive ions to pass. In this reactions of ions in solution. arrangement, positive and negative ions pass through the membranes, leaving electrolytic cell See cell; electrolysis.

79 electrolytic corrosion

ELECTROMAGNETIC SPECTRUM (note: the figures are only approximate) Radiation Wavelength (m) Frequency (Hz) gamma radiation –10–10 1019– x-rays 10–12 –10–9 1017 –1020 ultraviolet radiation 10–9 –10–7 1015 –1017 visible radiation 10–7 –10–6 1014 –1015 infrared radiation 10–6 –10–4 1012 –1014 microwaves 10–4 –1 109 –1013 radio waves 1 – –109

electrolytic corrosion Corrosion by est potential difference that can be supplied electrochemical reaction, e.g. rusting. by a source of electrical energy. The unit is the volt (V). electrolytic refining A method of purifying metals by electrolysis. Copper is puri- electromotive series See electrochemi- fied by making the impure metal the anode cal series. in an electrolytic cell containing an acidified copper sulfate electrolyte. The cathode electron An elementary particle of nega- is a thin strip of pure copper. The copper at tive charge (–1.602 192 × 10–19 coulomb) the anode dissolves as Cu2+ ions and pure and rest mass 9.109 558 × 10–31 kilogram. copper is deposited on the cathode. In this Electrons are present in all atoms in shells particular process, gold and silver are ob- around the nucleus. tained as by-products, deposited as an anode sludge on the bottom of the cell. electron affinity Symbol: A The energy released when an atom (or molecule or electromagnetic radiation Energy waves group) gains an electron in the gas phase to propagated by electric and magnetic fields form a negative ion. It is thus the energy of: that oscillate at right angles to one another A + e– → A– as they travel through space. Electromag- A positive value of A (often in electron- netic radiation forms a whole electromag- volts) indicates that heat is given out. Often netic spectrum, depending on frequency, the molar enthalpy is given for this process ranging from high-frequency radio waves of electron attachment (∆H). Here the to low-frequency gamma rays. units are joules per mole (J mol–1), and, by Electromagnetic radiation can be the usual convention, a negative value indi- thought of as waves (electromagnetic cates that energy is released. waves) or as streams of photons. The frequency (v) and wavelength (λ) are related electron configuration See configura- by tion. λv = c where c is the speed of light. The energy electron-deficient compounds Com- carried depends on the frequency. pounds in which the number of electrons available for bonding is insufficient for the electromagnetic spectrum See electro- bonds to consist of conventional two-elec- magnetic radiation. tron covalent bonds. In diborane, B2H6, for example, each boron atom has two ter- electromagnetic waves See electro- minal hydrogen atoms bound by conven- magnetic radiation. tional electron-pair bonds and in addition two hydrogen atoms bridging the boron electromotive force (e.m.f.) The great- atoms (B–H–B). In each bridge there are

80 electronvolt only two electrons for the bonding orbital. transitions occur. This has the effect that See also boron hydride; multicenter bond. there are spectral bands associated with changes in vibrational motion, with these electron diffraction A technique used bands having fine structure due to changes to help determine the structure of sub- in rotational motion. Because electronic stances, principally the shapes of molecules transitions are associated with changes in in the gaseous phase. A beam of electrons vibrational motion the corresponding spec- directed through a gas at low pressure pro- tra are sometimes called vibrational spec- duces a series of concentric rings on a pho- tra. In contrast to the cases of rotational tographic plate. The dimensions of these and vibrational spectroscopy, all molecules rings are related to the interatomic dis- can give rise to electronic spectra. The elec- tances in the molecules. See also x-ray dif- tronic spectra of molecules are used to ob- fraction. tain information about energy levels in molecules, interatomic distances, dissocia- electron donor See reduction. tion energies of molecules, and force constants of chemical bonds. electronegative Describing an atom or molecule that attracts electrons, forming electronic transition The demotion or negative ions. Examples of electronegative promotion of an electron between elec- elements include the halogens (fluorine, tronic energy levels in an atom or molecule. chlorine etc.), which readily form negative ions, e.g. F–, Cl–, etc. See also electronega- electron pair Two electrons in one or- tivity. bital with opposing spins (spin paired), such as the electrons in a covalent bond or electronegativity A measure of the ten- lone pair. dency of an atom in a molecule to attract electrons to itself. Elements to the right- electron spin See spin. hand side of the periodic table are strongly electronegative (values from 2.5 to 4); electron spin resonance (ESR) A tech- those on the left-hand side have low elec- nique similar to that used in NUCLEAR MAG- tronegativities (0.8–1.5) and are sometimes NETIC RESONANCE, but applied to unpaired called ELECTROPOSITIVE elements. Different electrons in a molecule rather than to the electronegativities of atoms in the same nuclei. It is a powerful means by which to molecule give rise to polar bonds and study free radicals and transition-metal sometimes to polar molecules. complexes. As the concept of electronegativity is not precisely defined it cannot be precisely electron-transfer reaction A chemical measured and several electronegativity reaction that involves the transfer, addi- scales exist. Although the actual values be- tion, or removal of electrons. Many elec- tween these scales differ, they are in good tron-transfer reactions involve complexes relative agreement. See also electron affin- of transition metals. The rates of such re- ity; ionization potential. actions vary enormously and can be explained in terms of the way in which electronic energy level See energy molecules of the solvent that start off sol- level. vating the reactants rearrange so as to solvate the products. electronic spectra of molecules The spectra associated with transitions between electronvolt Symbol: eV A unit of en- the electronic states of molecules. These ergy equal in SI derived units to 1.602192 transitions correspond to the visible or ul- × 10–19 joule. It is defined as the energy re- traviolet regions of the electromagnetic quired to move an electron charge across a spectrum. There are changes in vibrational potential difference of one volt. It is often and rotational energy when electronic used to measure the kinetic energies of ele-

81 electrophoresis mentary particles or ions, or the ionization tifically created alloy of gold and silver potentials of molecules. (containing up to 45% silver) that resembles pure gold in appearance. electrophoresis The application of an 2. A NICKEL-SILVER containing 52% copper, electric field between two electrodes lo- 26% nickel, and 22% zinc. cated at either side of a solution in order to cause charged particles of a colloid to move element A substance that cannot be through the solution. The technique is used chemically decomposed into more simple to separate and identify colloidal sub- substances. The atoms of an element all stances such as carbohydrates, proteins, have the same number of protons (given by and nucleic acids. Various experimental the element’s proton number) and thus the arrangements are used. One simple tech- same number of electrons, which deter- nique uses a strip of adsorbent paper mines the element’s chemical activity. soaked in a buffer solution with electrodes At present there are 114 reported chem- placed at two points on the paper. This ical elements, although research is continu- technique is sometimes called electrochro- ing to synthesize new ones. The elements matography. In gel electrophoresis, used to from hydrogen (proton number 1) to ura- separate DNA fragments, the solution is nium (92) all occur naturally on Earth, replaced by a layer of gel. with the exception of technetium (43), which is produced artificially by particle electroplating The process of coating a bombardment. Neptunium (93), pluto- solid surface with a layer of metal by nium (94), americium (95) and curium (96) means of electrolysis (i.e. by electrodeposi- also occur naturally in very small quanti- tion). ties in uranium ores. Technetium and elements with proton numbers higher than electropositive Describing an atom or 84 (polonium) are radioactive. Radioactive molecule that tends to lose electrons, form- isotopes also exist for other elements, ei- ing positive ions. Examples of electroposi- ther naturally in small amounts or synthet- tive elements include the alkali metals ically via particle bombardment. The (lithium, sodium, etc.), which readily form elements with proton number higher than positive ions, e.g. Li+, Na+, etc. See also 92 are the transuranic elements. The electronegative. transuranics are all synthesized. Thus, neptunium and plutonium can be made by electrovalent bond (ionic bond) A neutron bombardment of uranium nuclei binding force between the ions in com- while the other transuranics are made by pounds in which the ions are formed by high-energy collision processes between complete transfer of electrons from one el- nuclei. The higher proton number elements ement to another element or radical. For have been detected only in extremely example, Na + Cl becomes Na+ + Cl–. The minute quantities – in some cases, only a electrovalent bond arises from the excess few atoms have been produced. of the net attractive force between the ions There has been some controversy about of opposite charge over the net repulsive the naming of the higher synthetic elements force between ions of like charge. The mag- because of disputes about their discovery. nitude of electrovalent interactions is of the A long-standing problem concerned el- order 102–103 kJ mol–1 and electrovalent ement-104 (rutherfordium) which was for- compounds are generally solids with rigid merly also known by its Russian name of lattices of closely packed ions. The kurchatovium. More recent confusion has strengths of electrovalent bonds vary with been caused by differences between names the reciprocal of the inter-ionic distances suggested by the International Union of and are discussed in terms of lattice ener- Pure and Applied Chemistry (IUPAC) and gies. the names suggested by the American Chemical Union (ACU). electrum 1. A naturally occurring or ar- The original IUPAC names (1994) were:

82 empirical formula

mendelevium (Md, 101) rectly proportional to the number of solute nobelium (No, 102) molecules introduced rather than to any lawrencium (Lr, 103) specific aspect of the solute composition. dubnium (Db, 104) The proportionality constant, kB, is called joliotium (Jl, 105) the boiling-point elevation constant or rutherfordium (Rf, 106) sometimes the ebullioscopic constant. The bohrium (Bh, 107) relationship is ∆ hahnium (Hn, 108) t = kBCm ∆ meitnerium (Mt, 109) where t is the rise in boiling point and Cm The ACU names were: is the molal concentration; the units of kB mendelevium (Md, 101) are kelvins kilograms moles–1 (K kg mol–1). nobelium (No, 102) The property permits the measurement of lawrencium (Lr, 103) the relative molecular mass of involatile rutherfordium (Rf, 104) solutes. Compare depression of freezing hahnium (Ha, 105) point. seaborgium (Sg, 106) nielsbohrium (Ns, 107) Elinvar (Trademark) A steel alloy of hassium (Hs, 108) chromium, iron, and nickel containing meitnerium (Mt, 109) some tungsten and manganese. It is used A compromise list of names was adopted for making hairsprings for clocks and by IUPAC in 1997: watches because its elasticity does not vary mendelevium (Md, 101) with temperature. nobelium (No, 102) lawrencium (Lr, 103) elixir of life See alchemy. rutherfordium (Rf, 104) dubnium (Db, 105) elution The removal of an adsorbed seaborgium (Sg, 106) substance in a chromatography column or bohrium (Bh, 107) ion-exchange column using a solvent (elu- hassium (Hs, 108) ent), giving a solution called the eluate. The meitnerium (Mt, 109) CHROMATOGRAPHY column can selectively These are the names used in this dictionary. adsorb one or more components from the Dubnium is named after Dubna in Russia, mixture. To ensure efficient recovery of where much work has been done on syn- these components graded elution is used. thetic elements. Hassium is the Latin name The eluent is changed in a regular manner for the German state of Hesse, where the starting with a nonpolar solvent and grad- German synthetic element research group ually replacing it by a more polar one. This is located. will wash the strongly polar components In addition to the above elements, five from the column. other elements have been reported with proton numbers 110, 111, 112, 114, and emery A mineral of corundum (alu- 116. Element 110 has been named darm- minum oxide, A12O3) containing some stadtium (Ds, 110). So far the balance of magnetitie (Fe3O4), hematite (Fe2O3), or these elements have not been named and spinel (MgAl2O4). It is extremely hard and have their temporary systematic IUPAC is used as an abrasive and polishing ma- names (see unun-). terial. These are: unununium (Uuu, 111), unun- bium (Uub, 112), ununquadium (Uuq, e.m.f. See electromotive force. 114) and ununhexium (Uuh, 116). emission spectrum See spectrum. elevation of boiling point A colligative property of solutions in which the boil- empirical formula The formula of a ing point of a solution is raised relative to compound showing the simplest ratio of that of the pure solvent. The elevation is di- the atoms present. The empirical formula

83 emulsion is the formula obtained by experimental unit: the joule (J). It is convenient to divide analysis of a compound. It can be related to energy into kinetic energy (energy of mo- a molecular formula only if the relative tion) and potential energy (‘stored’ en- molecular mass is known. For example ergy). Names are given to many different P2O5 is the empirical formula of phospho- forms of energy depending on their source, rus(V) oxide although its molecular for- i.e. chemical, electrical, nuclear, etc.; the mula is P4O10. Compare molecular only real difference lies in the system under formula; structural formula. discussion. For example, chemical energy is the kinetic and potential energies of elec- emulsion A colloid in which a liquid trons in a chemical compound. phase (small droplets with a diameter range 10–5–10–7 centimeter) is dispersed or energy level One of the discrete energies suspended in a liquid medium. Emulsions that an atom or molecule, for instance, can are classed as lyophobic (solvent-repelling have according to quantum theory. Thus in and generally unstable) or lyophilic (sol- an ATOM there are certain definite shells vent-attracting and generally stable). and orbitals that the electrons can be in, corresponding to definite electronic energy en See ethylenediamine. levels of the atom. Similarly, a vibrating or rotating molecule can have discrete vibra- enantiomer (enantiomorph) A com- tional and rotational energy levels. pound whose structure is not superimposable on its mirror image: one of any pair of energy profile A diagram that traces the optical isomers. See also isomerism; optical changes in the energy of a system during activity. the course of a reaction. Energy profiles are obtained by plotting the potential energy enantiomorph See enantiomer. of the reacting particles against the reaction coordinate, i.e. the pathway for which enantiotropy The existence of different the energy is a minimum. To obtain the re- stable allotropes of an element at different action coordinate the energy of the total in- temperatures; sulfur, for example, exhibits teracting system is plotted against position enantiotropy. The phase diagram for an for the molecules. enantiotropic element has a point at which all the allotropes can coexist in a stable enthalpy Symbol: H The sum of the in- equilibrium. At temperatures above or ternal energy (U) and the product of pres- below this point, one of the allotropes will sure (p) and volume (V) of a system: be more stable than the other(s). See also H = U + pV allotropy; monotropy. In a chemical reaction carried out at constant pressure, the change in enthalpy enclosure compound See clathrate. measured is the internal energy change plus the work done by the volume change: endothermic Describing a process in ∆H = ∆U + p∆V which heat is absorbed (i.e. heat flows In SI units enthalpy is measured in joules from outside the system, or the tempera- (J) or kilojoules (kJ). ture falls). The dissolving of a salt in water, for instance, is often an endothermic entropy Symbol: S In any system that process. Compare exothermic. undergoes a reversible change, the change of entropy is defined as the heat Q ab- end point See equivalence point; volu- sorbed divided by the thermodynamic tem- metric analysis. perature T: ∆S = ∆Q/T energy Symbol: W A property of a sys- A given system is said to have a certain en- tem; a measure of its capacity to do work. tropy, although absolute entropies are sel- Energy and work have the same SI derived dom used: it is rather the change in entropy

84 equivalent weight that is important, particularly as the en- changed by changing the conditions (e.g. tropy of a system measures the availability temperature or pressure). of energy to do work. In any real (i.e. irreversible) change in a equilibrium constant Symbol: Kc, closed system entropy always increases. Al- Kp In a chemical equilibrium of the type though the total energy of the system has xA + yB ˆ zC + wD not changed, according to the first law of The expression: THERMODYNAMICS, the available energy is [A]x[B]y/[C]z[D]w less as a consequence of the second law of where the square brackets indicate concen- thermodynamics. trations, is a constant (Kc) when the system The concept of entropy has been is at equilibrium. Kc is the equilibrium con- widened to take in the general idea of dis- stant of the given reaction; its units depend order – the higher the entropy, the more on the stoichiometry of the reaction. For disordered the system. For instance, a gas reactions, pressures are often used in- chemical reaction involving polymeriza- stead of concentration. The equilibrium n tion may well have a decrease in entropy constant is then Kp, where Kp = Kc . Here n because there is a change to a more ordered is the number of moles of product minus system. The ‘thermal’ definition of entropy the number of moles of reactant; for in- is actually a special case of this idea of dis- stance, in ˆ order, because it explains how transferred 3H2 + N2 2NH3 energy is distributed among particles of n is 2 – (1 + 3) = –2. matter. equipartition of energy The principle Epsom salt See magnesium sulfate. that the total energy of a molecule is, on average, equally distributed among the equation See chemical equation. available DEGREES OF FREEDOM. It is only approximately true in most cases. equation of state An equation that in- terrelates the pressure, temperature, and equivalence point The point in a titra- volume of a system, such as a gas. The tion at which the reactants have been equation for an ideal gas and the VAN DER added in equivalent proportions, so that WAALS EQUATION are examples. See gas there is no excess of either. It differs laws. slightly from the end point, which is the observed point of complete reaction, be- equilibrium In a reversible chemical re- cause of the effect of the indicator, errors, action: etc. A + B ˆ C + D The reactants are forming the products: equivalent proportions, law of (law of A + B → C + D reciprocal proportions) When two chem- which also react to give the original reac- ical elements both form compounds with a tants: third element, a compound of the first two C + D → A + B elements will contain each in the same rel- The concentrations of A, B, C, and D ative proportions in which they exist in change with time until a state is reached at compounds with the third element. For ex- which both reactions are taking place at ample, the mass ratio of carbon to hydro- the same rate. The concentrations (or pres- gen in methane (CH4) is 12:4; the mass sures) of the components are then constant ratio of oxygen to hydrogen in water – the system is said to be in a state of chem- (H2O) is 16:2. In carbon monoxide (CO), ical equilibrium. Note that the equilibrium the ratio of carbon to oxygen is 12:16. is a dynamic one; the reactions still take place but at equal rates. The relative pro- equivalent weight A measure of ‘com- portions of the components determine the bining power’ formerly used in calcula- ‘position’ of the equilibrium, which may be tions for chemical reactions. The

85 erbium equivalent weight of an element is the num- Apart from its use in drinks, alcohol is ber of grams that could combine with or used as a solvent and to form ethanal displace one gram of hydrogen (or 8 grams (acetaldehyde). Formerly, the main source of oxygen or 35.5 grams of chlorine). It is was by fermentation of molasses, but now the relative atomic mass (atomic weight) catalytic hydration of ethene is used to divided by the valence. For a compound manufacture industrial ethanol. the equivalent weight depends on the reaction considered. An acid, for instance, in ethene (ethylene; C2H4) A gaseous acid-base reactions has an equivalent alkene. Ethene is not normally present in weight equal to its molecular weight di- the gaseous fraction of crude oil but can be vided by the number of acidic hydrogen obtained from heavier fractions by cat- atoms. alytic cracking. This is the principal industrial source. The compound is important as erbium A soft, malleable, silvery el- a starting material in the organic-chemicals ement of the lanthanoid series of metals. It industry (e.g. in the manufacture of occurs in association with other lan- ethanol) and as the starting material for the thanoids in such minerals as apatite, production of polyethene. See also Zeise’s gadolinite, monazite, and bastnaesite. It salt. does not oxidize as easily as other lanthanoids. It is used to make vanadium al- ethoxylate See detergents. loys more workable, to make pink pigments for paints and ceramics, and as a ethyl alcohol See ethanol. signal amplifier in fiber-optic cables. Symbol: Er; m.p. 1529°C; b.p. 2863°C; ethylene See ethene. r.d. 9.066 (25°C); p.n. 68; most common isotope 166Er; r.a.m. 167.26. ethylenediamine An organic compound, H2NCH2CH2NH2. It is important Erlenmeyer flask A conical glass labo- in inorganic chemistry because it functions ratory flask with a narrow neck and wide as a bidentantate ligand, coordinating to a bottom. metal ion by the lone pairs on the two nitrogen atoms. In the names of complexes it ESR See electron spin resonance. is given the abbreviation en.

– ethanoate (acetate; CH3COO ) A salt ethylenediaminetetraacetic acid See or ester of ETHANOIC ACID (acetic acid). edta. ethanoic acid (acetic acid; CH3COOH) ethyne (acetylene; C2H2) An unstable A colorless viscous liquid or glassy solid or- colorless, gaseous, organic compound. ganic acid with a pungent odor. It is the Traditionally ethyne was made by the ac- acid in vinegar. Below 16.6°C it solidifies tion of water on calcium dicarbide, and the to a glassy solid known as glacial ethanoic gas has found use in oxy-acetylene welding acid. It is made by the oxidation of ethanol torches, since its combustion with oxygen or butane, or by the bacterial fermentation produces a flame of very high temperature. of beer or wine, and is used as a food It is also important in the organic chemi- preservative and for making polymers. cals industry for the production of vinyl compounds. ethanol (ethyl alcohol; alcohol; C2H5OH) A colorless volatile liquid alcohol. Ethanol ethynide See carbide. occurs in intoxicating drinks, as the result of the anaerobic fermentation of simple eudiometer An apparatus for the volu- sugars: metric analysis of gases. It often consists of → C6H12O6 2C2H5OH + 2CO2 a sealable graduated glass tube housing 86 Eyring, Henry wires that are used to spark a reaction be- atom, molecule, etc. is elevated from its tween gas mixtures. ground to an excited state. europium A silvery-white element of excitation energy The energy required the lanthanoid series of metals. It occurs in to change an atom, molecule, etc. from one association with other lanthanoids in such quantum state to a state with a higher en- minerals as monazite, bastnaesite, gadolin- ergy. The excitation energy (sometimes ite, and samarskite. It is highly reactive and called excitation potential) is the difference will ignite in air when heated above 150°C. between two energy levels of the system. Its main use is in a mixture of europium and yttrium oxides widely employed as the excitation potential See excitation en- red phosphor in television screens. The ergy. metal is used in some superconducting alloys. excited state The state of an atom, mol- Symbol: Eu; m.p. 822°C; b.p. 1597°C; ecule, or other system when it has an en- r.d. 5.23 (25°C); p.n. 63; most common ergy greater than that of its GROUND STATE. isotope 153Eu; r.a.m. 151.965. exclusion principle See atom. eutectic A mixture of two substances in such proportions that no other mixture of exothermic Denoting a chemical reac- the same substances has a lower freezing tion in which heat is evolved (i.e. heat point. If a solution containing the propor- flows from the system or the temperature tions of the eutectic is cooled no solid is de- rises). Combustion is an example of an posited until the freezing point of the exothermic process. Compare endother- eutectic is reached, when two solid phases mic. are simultaneously deposited in the same proportions as the mixture; these two solid explosive A substance or mixture that phases form the eutectic. If one of the sub- can rapidly decompose upon detonation, stances is water the eutectic can also be re- producing large amounts of heat and gases. ferred to as a cryohydrate. The three most important classes of explosives are: evaporation 1. A change of state from 1. Propellants, which burn steadily, and liquid to gas (or vapor). Evaporation can are used as rocket fuels, take place at any temperature, although 2. Initiators, which are very sensitive and the rate increases with temperature. Evap- are used in small amounts to detonate oration occurs because some molecules in less sensitive explosives, and the liquid, if they are near the surface and 3. High explosives, which need an initiator moving in the right direction, have enough but are very powerful. energy to escape into the gas phase. Be- cause these are the molecules with higher Eyring, Henry (1901–81) American kinetic energies, the liquid is cooled as a re- chemist. Eyring was a prolific chemist sult. whose main contribution was to the theory 2. A change from solid to vapor, especially of chemical reactions. In the late 1920s and occurring at high temperatures close to the early 1930s he calculated a number of po- melting point of the solid. Thin films of tential energy surfaces in terms of quantum metal can be evaporated onto a surface in mechanics. He then expressed the temper- this way. ature dependence of the rate of chemical reactions in terms of the parameters of po- exa- Symbol: E A prefix used with SI tential energy surfaces. Eyring gave an ac- units, denoting 1018. count of this work in the book The Theory of Rate Processes (1941) which he wrote excitation The process by which an with Samuel Glasstone and Keith Laidler.

87 F

face-centered cubic crystal (f.c.c.) Fajan’s third rule states that covalent char- (cubic close packing) A close-packed crys- acter is greater for cations with a nonrare tal structure made up of layers in which gas configuration than for those with a each atom, ion, or molecule is surrounded complete octet, charge and size being ap- by six others arranged hexagonally. The proximately equal. For example, Cu+ is layers are packed one on top of the other, more polarizing than Na+ (ions of approx- with the second layer fitting into the re- imately the same size) because the d elec- cesses left in the first layer, and the third trons around Cu+ do not shield the nucleus layer fitting into yet a different set of holes as effectively as the complete octet in Na+. formed by the second layer. If the layers are designated A, B, C, the packing is AB- farad Symbol: F The SI derived unit of CABC. This is in contrast to HEXAGONAL capacitance. When the plates of a capacitor CLOSE PACKED CRYSTAL, in which the layers are charged by one coulomb and there is a are arranged ABABAB. Copper and alu- potential difference of one volt between minum have face-centered cubic crystal them, the capacitor is said to have a capac- structures. See close packing. itance of one farad. One farad = 1 coulomb volt–1 (CV–1). fac-isomer See isomerism. Faraday, Michael (1791–1867) Eng- Fahrenheit scale A temperature scale in lish physicist and chemist. Faraday was re- which the temperature of pure melting ice sponsible for a remarkably large number of at standard pressure is fixed at 32° and the important discoveries in chemistry and temperature of pure boiling water at stan- physics. His early work was mostly de- dard pressure is fixed at 212°. The scale is voted to chemistry. In 1820 he prepared not used for scientific purposes. To convert chlorides of carbon. In 1835 he discovered from degrees Fahrenheit (°F) to degrees benzene. In the 1830s Faraday discovered Celsius (°C) the formula what are now known as FARADAY’S LAWS of °F = 1.8(°C) + 32 electrolysis. He also introduced the terms is used. See also Celsius scale; temperature anode, cathode, cation, anion, electrode, scale. and electrolyte. Faraday made fundamental contributions to the study of electricity Fajan’s rules Rules that deal with varia- and magnetism. He constructed the first tions in the degree of covalent character in electric motor and discovered electromag- ionic compounds in terms of polarization netic induction. He also developed the con- effects. Decrease in size and increase in cept of electric and magnetic fields. His charge for cations is regarded as making investigations of matter in magnetic fields them more polarizing, while for anions an led to the discovery of diamagnetism and increase in both charge and size makes paramagnetism. In 1845 he discovered them more polarizable. Thus covalent Faraday rotation, i.e. the rotation of the character is said to increase with increasing plane of polarization when polarized light polarizing power of the cation and/or in- passes through matter in a magnetic field. creasing polarizability of the anion. These were originally stated as rules one and two. faraday Symbol: F A non-SI unit of elec-

88 ferrocene tric charge equal to the charge required to chemical reactions, that take place on discharge one mole of a singly-charged ion. timescales of about a femtosecond (10–15s). In SI units one faraday is equal to Femtochemistry became possible with the 96.48670 kilocoulombs (kC). See Fara- construction of lasers that could be pulsed day’s laws. in femtoseconds. Femtochemistry can be regarded as very high speed ‘photography’ Faraday constant See Faraday’s laws. of chemical reactions. It has enabled entities that exist only for a short time, such as Faraday’s laws (of electrolysis) Two activated complexes, to be studied. The de- laws resulting from the work of Michael tailed mechanisms of many chemical reac- Faraday on electrolysis: tions have been investigated in this way, 1. The amount of chemical change pro- notably by the Egyptian-born American duced is proportional to the electric chemist Ahmed Zewail. charge passed. 2. The amount of chemical change pro- fermi A non-SI unit of length equal to duced by a given charge depends on the one femtometer (10–15 meter) in SI units. It ion concerned. was formerly used in atomic and nuclear More strictly it is proportional to the physics. relative ionic mass of the ion, and the charge on it. Thus the charge Q needed to fermium A highly radioactive trans- deposit m grams of an ion of relative ionic uranic element of the actinoid series, not mass M carrying a charge Z is given by: found naturally on Earth. It was discov- Q = FmZ/M ered in the fallout from the first hydrogen F, the Faraday constant, has a value of one bomb. It can be produced in very small faraday, i.e. 9.648670 × 104 coulombs. quantities by bombarding 239Pu with neutrons to give 253Fm (half-life 3 days). Sev- f-block elements The block of elements eral other short-lived isotopes have also in the periodic table that have two outer s been synthesized. electrons and an incomplete penultimate Symbol: Fm; m.p., b.p. and r.d. un- subshell of f electrons. The f block consists known; p.n. 100; most stable isotope of the lanthanoids (cerium to lutetium) and 257Fm (half-life 100.5 days). the actinoids (thorium to lawrencium). ferric chloride See iron(III) chloride. f.c.c. See face-centered cubic crystal. ferric oxide See iron(III) oxide. feldspar (felspar) A member of a large group of very abundant crystalline igneous ferric sulfate See iron(III) sulfate. rocks, mainly silicates of aluminum with calcium, potassium, and sodium. The al- ferricyanide See cyanoferrate. kali feldspars, e.g. orthoclase, microcline, have mainly potassium with a little sodium ferrite A nonconducting, magnetic, and no calcium; the plagioclase feldspars, metal oxide ceramic having the general for- e.g. albite, anorthite, labradorite, vary mula MO.Fe2O3, where M is a divalent from sodium-only to calcium-only, with transition metal such as cobalt, nickel, or little or no potassium. Feldspars are used in zinc. Ferrites are used to make powerful making ceramics, enamels, and glazes. magnets for use in high-frequency electronic devices such as radar sets. femto- Symbol: f A prefix used with SI –15 units denoting 10 . For example, 1 fem- ferrocene (Fe(C5H5)2) An orange crys- tometer (fm) = 10–15 meter (m). talline solid. It is an example of a sandwich compound, in which an iron(II) ion is co- femtochemistry The study of atomic ordinated to two cyclopentadienyl ions. and molecular processes, particularly The bonding involves overlap of d orbitals

89 ferrocyanide on the iron with the pi electrons in the cy- plastics, paints, and resins. Slate powder, clopentadienyl rings. The compound melts glass fiber, mica, and cotton are all used as at 173°C, is soluble in organic solvents, fillers. and can undergo substitution reactions on the rings. It can be oxidized to the blue filter See filtration. + cation (C5H5)2Fe . The systematic name is di-π-cyclopentadienyl iron(II). filter pump A type of vacuum pump in which a jet of water forced through a noz- ferrocyanide See cyanoferrate. zle carries air molecules out of the system. Filter pumps cannot produce pressures ferromagnetism See magnetism. below the vapor pressure of water. They are used in the laboratory for vacuum fil- ferrosoferric oxide See triiron tetrox- tration, distillation, and similar techniques ide. requiring a low-grade vacuum. ferrous chloride See iron(II) chloride. filtrate See filtration. ferrous oxide See iron(II) oxide. filtration The process of removing suspended particles from a fluid by passing or ferrous sulfate See iron(II) sulfate. forcing the fluid through a porous material (the filter). The fluid that passes through fertilizer A substance added to soil to in- the filter is the filtrate. In laboratory filtra- crease its fertility. Artificial fertilizers gen- tion, filter paper or sintered glass is com- erally consist of a mixture of chemicals monly used. designed to chiefly supply three main nu- trients: nitrogen (N), phosphorus (P), and fine structure Closely spaced lines seen potassium (K), and are consequently at high resolution in a spectral line or band. known as PKN fertilizers. Typical chemi- Fine structure may be caused by vibration cals used include ammonium nitrate, am- of the molecules or by electron spin. Hy- monium phosphate, ammonium sulfate, perfine structure, seen at very high resolu- and potassium carbonate. tion, is caused by the atomic nucleus affecting the possible energy levels of the Fick’s law A law describing the diffu- atom. sion that occurs when solutions of different concentrations come into contact, with fireclay A refractory clay containing molecules moving from regions of higher high proportions of alumina (aluminum concentration to regions of lower concen- oxide) and silica (silicon(IV) oxide), used tration. Fick’s law states that the rate of for making firebricks and furnace linings. diffusion dn/dt, called the diffusive flux and denoted J, across an area A is given by: firedamp Methane and other com- dn/dt = J = –DA∂c/∂x, where D is a con- bustible gases occurring in coal mines. Ex- stant called the diffusion constant, ∂c/∂x is plosions or fires can occur if quantities of the concentration gradient of the solute these gases accumulate. The deoxygenated, and dn/dt is the amount of solute crossing suffocating air left after an explosion or the area A per unit time. D is constant for fire, consisting chiefly of carbon dioxide a specific solute and solvent at a specific and nitrogen, is called blackdamp. After- temperature. Fick’s law was formulated by damp is the poisonous carbon monoxide the German physiologist Adolf Eugen Fick formed. (1829–1901) in 1855. first-order reaction A reaction in filler A solid material used to modify the which the rate of reaction is proportional physical properties or reduce the cost of to the concentration of one of the reacting synthetic compounds, such as rubbers, substances. The concentration of this re-

90 flocculent acting substance is raised to the power one; FLAME TEST COLORS i.e. rate = k[A]. For example, the decompo- Element Flame color sition of hydrogen peroxide is a first-order barium green reaction, calcium brick red rate = k[H2O2] lithium crimson Similarly the rate of decay of radioactive potassium pale lilac material is a first-order reaction, sodium yellow rate = k[radioactive material] strontium red For a first-order reaction, the time for a definite fraction of the reactant to be consumed is independent of the original con- flame test A preliminary test in qualita- centration. The units of k, the RATE tive analysis in which a small sample of a CONSTANT, are s–1. chemical is introduced into a nonluminous Bunsen-burner flame on a clean platinum Fischer, Ernst Otto (1918–94) German wire. The flame vaporizes part of the sam- inorganic chemist. Fischer is noted for his ple and excites some of the atoms, which work on inorganic complexes. In 1951 two emit light at wavelengths characteristic of chemists, T. Kealy and P. Pauson, were at- the metallic elements in the sample. Thus tempting to join two five-carbon (cyclo- the observation of certain colors in the pentadiene) rings together and discovered flame indicates the presence of these elements. The same principles are applied in a compound C5H5FeC5H5, which they proposed had an iron atom joined to a the modern instrumental method of spectrographic analysis. carbon atom on each ring. Fischer, on reflection, considered such a structure inade- flare stack A special chimney in an oil quate for he failed to see how it could refinery, rig, or other chemical plant at the provide sufficient stability with its carbon– top of which unwanted gases are burnt. iron–carbon bonds. The British chemist Geoffrey WILKINSON suggested a more flash photolysis A technique for inves- novel structure in which the iron atom was tigating free radicals in gases. The gas is sandwiched between two parallel rings and held at low pressure in a long glass or thus formed bonds with the electrons in the quartz tube, and an absorption spectrum rings, rather than with individual carbon taken using a beam of light passing down atoms. Compounds of this type are called the tube. The gas can be subjected to a very SANDWICH COMPOUNDS. By careful x-ray brief intense flash of light from a lamp out- analysis Fischer confirmed the proposed side the tube, producing free radicals, structure of FERROCENE, as the compound which are identified by their spectra. Meas- was called, and for this work shared the urements of the intensity of spectral lines Nobel Prize for chemistry with Wilkinson with time can be made using an oscillo- in 1973. scope, and the kinetics of very fast reactions can thus be investigated. fixation of nitrogen See nitrogen fixation. flash point The lowest temperature at which vapor given off by a flammable liq- fixing, photographic A stage in the de- uid will be sufficient to momentarily sus- velopment of exposed photographic film tain a flame in the presence of a spark. or paper in which unexposed silver halides in the emulsion are dissolved away. The flocculation The combining of the par- fixing bath typically contains a solution of ticles of a finely divided precipitate, such as ammonium or sodium thiosulfate(IV), and a colloid, into flocculent clumps that sink is employed after the initial development and are easier to filter off. See coagulation. (chemical reduction) of the latent silver image. flocculent See flocculation.

91 flotation flotation See froth flotation. ture of potassium fluoride and hydrogen fluoride electrolytes, using copper or steel fluid A state of matter that is not a solid, apparatus. Its preparation by chemical i.e. a liquid or a gas. All fluids can flow, methods is impossible. and the resistance they pose to such flow is Fluorine compounds are used in the called VISCOSITY. steel industry, glass and enamels, uranium processing, aluminum production, and in a fluidization The suspension of a finely- range of fine organic chemicals. Fluorine is divided solid, e.g. a catalyst, in an upward- the most reactive and most electronegative flowing liquid or gas. This suspension element known; in fact it reacts directly mimics many properties of liquids, includ- with almost all the elements, including the ing allowing the solids to ‘float’ in it and rare gas xenon. As the most powerful oxi- achieve a uniform temperature with the dizing agent known, it has the pronounced fluid medium. Fluidized beds so prepared characteristic of bringing out the higher are used industrially to carry out catalytic oxidation states when combined with reactions and to minimize the production other elements. Ionic fluorides contain the of pollutants from coal-fired furnaces. small F– ion, which is very similar in reactivity to the O2– ion. With the more elec- fluorescein A fluorescent dye used as an tronegative elements, fluorine forms an absorption indicator and water flow extensive range of compounds, e.g. SF , marker. 6 NF3, and PF5, in which the bonding is essentially covalent. The absorption of energy fluorescence Fluorine reacts explosively with hydro- by atoms, molecules, etc., followed by im- gen, even in the dark, to form hydrogen mediate emission of electromagnetic radia- fluoride, HF, which polymerizes as a result tion as the particles make transitions to of hydrogen bonding and has a boiling lower energy states. Compare phosphores- point very much higher than that of HCl cence. (HF b.p. 19°C; HCl b.p. –85°C). Unlike the other halogens, fluorine does not form fluoridation The introduction of small higher oxides or oxyacids; oxygen difluo- quantities of fluoride compounds, e.g. sodium fluoride (NaF), into the water sup- ride in fact reacts with water to give hy- ply as a public-health measure to reduce drogen fluoride. the incidence of tooth decay. Fluorine exhibits a strong electron withdrawing effect on adjacent bonds, thus fluoride See halide. CF3COOH is a strong acid whereas CH3COOH is not. Although the coordina- fluorination See halogenation. tion number seldom exceeds one there are cases in which fluorine bridging occurs, fluorine A slightly greenish-yellow, e.g. (SbF5)n. The element is sufficiently re- highly reactive, poisonous, gaseous el- active to combine directly with the rare gas ° ement belonging to the halogens; i.e. group xenon at 400 C to form XeF2 and XeF4. At 17 (formerly VIIA) of the periodic table, of 600°C and high pressure it will even form which it is the first element. It occurs no- XeF6. tably as fluorite (CaF2) and cryolite Fluorine and hydrogen fluoride are ex- (Na3AlF3) but traces of fluorine are also tremely dangerous and should be used only widely distributed with other minerals. It is in purpose-built apparatus; gloves and face slightly more abundant than chlorine, ac- shields should be used when working with counting for about 0.065% of the Earth’s hydrofluoric acid, and accidental exposure crust. Fluorine’s high reactivity delayed its should be treated as a hospital emergency. isolation until 1886, and means it is never Symbol: F; m.p. –219.62°C; b.p. found as a free element. Fluorine is now –188.14°C; d. 1.696 kg m–3 (0°C); p.n. 9; prepared by electrolysis of a molten mix- r.a.m. 18.99840.32.

92 free radical fluorite (fluorspar) See calcium fluoride. (fractionating column) containing an inert material (e.g. glass beads) attached to the fluorite structure See calcium-fluoride vessel containing the mixture. There is a structure. decreasing temperature gradient from the bottom to the top of the column. Vapor fluorspar See calcium fluoride. rises in the column until it reaches a part at which it condenses back to liquid, and runs flux 1. A substance used to keep metal back down the column. A steady state is surfaces free of oxide in soldering. See sol- reached at which the more volatile compo- der. nents are concentrated near the top of the 2. A substance used in smelting metals to column and the less volatile components react with silicates and other impurities are near the bottom. It is possible to draw and form a low-melting slag. off fractions at points on the column. On an industrial scale, large towers containing fluxional molecule A molecule in perforated trays are used for extracting which the constituent atoms change their fractions in petroleum refining. relative positions so quickly at room temperature that the normal concept of struc- fractionating column See fractional ture is inadequate; i.e. no specific structure distillation. exists for longer than about 10–2 second and the relative positions become indistin- ° fractionation See fractional distillation. guishable. For example ClF3 at –60 C has a distinct ‘T’ shape but at room tempera- francium A radioactive element of the ture the fluorine atoms are visualized as alkali-metal group, the seventh element of moving rapidly over the surface of the group 1 (formerly IA) of the periodic table. chlorine atom in a state of exchange and It is found on Earth only as a short-lived are effectively identical. product of radioactive decay, occurring in uranium ores in minute quantities. At least formula (chemical formula) A represen- 22 isotopes are known. tation of a chemical compound using sym- ° ° bols for the atoms and subscript numbers Symbol: Fr; m.p. 27 C; b.p. 677 C; r.d. ° 223 to show the numbers of atoms present. See 2.4 ; p.n. 87; most stable isotope Fr empirical formula; general formula; mo- (half-life 21.8 minutes). lecular formula; structural formula. Frasch process See sulfur. fraction See fractional distillation. free electron An electron that can move fractional crystallization Crystalliza- from one atom or molecule to another tion of one component from a mixture in under the influence of an applied electric solution. When two or more substances are field. Movement of free electrons in a con- present in a liquid (or in solution), on grad- ductor constitutes an electric current. Free ual cooling to a lower temperature one electrons are also involved in conducting substance will preferentially form crystals, heat and in METALLIC BONDS. leaving the other substance in the liquid (or dissolved) state. Fractional crystallization free energy A measure of the ability of a can thus be used to purify or separate sub- system to do useful work. See Gibbs func- stances. tion; Helmholtz function. fractional distillation (fractionation) free radical An atom or group of atoms Distillation in which a mixture of liquids is with a single unpaired electron. Free radi- separated into components (fractions) de- cals are produced by breaking a covalent pending on their boiling points. A common bond; for example: → technique is to use a long vertical column CH3Cl CH3• + Cl• 93 freezing

They are often formed in light-induced bubbles of air, which adhere to it preferen- reactions and as a result of chemical de- tially. The froth is then skimmed off. Vari- composition at high temperatures. Free ous additives are also used to modify the radicals are extremely reactive and can be surface properties of the mineral particles stabilized and isolated only under special (e.g. to increase the adherence of air bub- conditions. bles). freezing The process by which a liquid is fuel cell A type of cell in which fuel is converted into a solid by cooling; the re- converted directly into electricity. In one verse of melting. form, hydrogen gas and oxygen gas are fed to the surfaces of two separately compart- freezing mixtures Two or more sub- mentalized porous nickel electrodes im- stances mixed together to produce a tem- mersed in a potassium hydroxide solution. perature below the freezing point of water The oxygen reacts to form hydroxyl (OH–) (0°C). A mixture of sodium chloride and ions, which it releases into the solution, ice in water, for example, can yield a tem- leaving a positive charge on one electrode. perature as low as –20°C. The hydrogen reacts with the OH– ions in the solution to form water, giving up elec- freezing point The temperature at trons to leave a negative charge on the which a liquid is in equilibrium with its other electrode. Large fuel cells can gener- solid phase at standard pressure and below ate tens of amperes. Usually the e.m.f. is which the liquid freezes or solidifies. This about 0.9 volt and the efficiency around temperature is always the same for a par- 60%. ticular pure liquid and is numerically equal to the melting point of the solid. fullerene See buckminsterfullerene. freezing point constant See depression fullerite See buckminsterfullerene. of freezing point. fuller’s earth A natural clay, the pre- French chalk See talc. dominate mineral in bentonite, used as an absorbent and decolorizing agent, and as a Frenkel defect see defect. drilling mud and industrial catalyst. frontier orbital theory A theory of the fuming sulfuric acid See oleum. chemical and spectroscopic properties of molecules that emphasizes the frontier or- fundamental units The BASE UNITS of a bitals, i.e. the highest occupied molecular measurement system, especially those that orbital (HOMO) and the lowest unoccu- deal with force and motion. In most sys- pied molecular orbital (LUMO). The tems these are the units for length, mass, theory was developed by the Japanese and time, plus at least one electrical unit. chemist Kenichi Fukui (1919–98) in the The fundamental units in the SI are consid- 1950s. Frontier orbital theory has some- ered to be the meter, the kilogram, the sec- times been used in conjunction with the ond, and the ampere. WOODWARD–HOFFMANN RULES. fused Describing a solid that has been froth flotation (flotation) An industrial melted and solidified into a single mass. technique for separating the required parts Fused silica, for example, is produced by of ores from unwanted gangue. The min- melting sand. eral is pulverized and mixed with water, to which a frothing agent is added. The mix- fused ring See ring. ture is aerated, after which particles of one constituent are carried to the surface by fusion Melting.

94 G

gadolinium A ductile malleable silvery is coated with zinc by dipping it in a bath element of the lanthanoid series of metals. of molten zinc, or by electrodeposition. It occurs in association with other lan- The zinc is preferentially attacked by car- thanoids in such minerals as gaolinite, bonic acid (H2CO3), thus protecting the bastnaesite, monazite, and xenotime. It is steel by forming zinc carbonates. also a product of nuclear fission and can be found in association with uranium ores. gamma radiation A form of ELECTRO- Gadolinium is used in alloys and magnets, MAGNETIC RADIATION emitted by changes in and in electronic components designed to the nuclei of atoms. Gamma rays have very withstand very high temperatures. Two high frequencies and thus short wave- isotopes, gadolinium-155 and gadolinium- length. Gamma radiation shows particle 157, are the best neutron absorbers known properties much more often than wave and thus find use in the nuclear industry. properties. The radiant energy (W) of a Symbol: Gd; m.p. 1313°C; b.p. gamma photon, given by 3266°C; r.d. 7.9 (25°C); p.n. 64; most W = hv common isotope 158Gd; r.a.m. 157.25. where h is the Planck constant and v the velocity, can be very high. A gamma photon galena (lead glance) A mineral form of of 1024 hertz (Hz) has an energy of 6.6 × lead(II) sulfide (PbS), usually occurring as 10–10 joule (J). grayish-blue cubes or octahedrons. It is the principal ore of lead. gamma rays Streams of gamma radiation. gallium A soft silvery low-melting metallic element belonging to group 13 gangue The waste rock or other unde- (formerly IIIA) of the periodic table. It is sirable material occurring in an ore. found in minute quantities in several ores, including sphalerite (ZnS), bauxite gas The state of matter in which forces (Al2O3.H2O), and kaolinite (Al2(OH)4- of attraction between the particles of a sub- S12O5. Gallium is used in low-melting al- stance are small. The particles have free- loys, high-temperature thermometers, and dom of movement and gases therefore have as a doping impurity in semiconductors. no fixed shape or volume. The atoms and Gallium arsenide is a semiconductor used molecules of a gas are in a continual state in light-emitting diodes and in microwave of motion and are continually colliding apparatus. with each other and with the walls of any Symbol: Ga; m.p. 29.78°C; b.p. containing vessel. These collisions create 2403°C; r.d. 5.907 (solid at 20°C), 6.114 the pressure of a gas. (liquid); p.n. 31; most common isotope 696a; r.a.m. 69.723. gas chromatography A technique widely used for the separation and analysis galvanic cell See cell. of mixtures. Gas chromatography employs a column packed with either a solid sta- galvanizing The process by which steel tionary phase (gas-solid chromatography

95 gas constant or GSC) or a solid coated with a non- gas. The main gas laws are BOYLE’S LAW volatile liquid (gas-liquid chromatography and CHARLES’ LAW. The laws are not or GLC). The whole column is placed in a obeyed exactly by any real gas, but many thermostatically controlled heating jacket. common gases obey them under certain A volatile sample is introduced into the col- conditions, particularly at high tempera- umn using a syringe, and an unreactive car- tures and low pressures. A gas that would rier gas, such as nitrogen, passed through obey the laws over all pressures (p) and it. The components of the sample will be temperatures (T) is a perfect or ideal gas. carried along in this mobile phase. How- Boyle’s and Charles’ laws can be com- ever, some of the components will cling bined into a gas equation of state for ideal more readily to the stationary phase than gases: others, either because they become at- pVm = RT tached to the solid surface or because they where Vm is the molar volume and R is the dissolve in the liquid. The time taken for molar gas constant. For n moles of gas, different components to pass through the then column is characteristic and can be used to pV = nRT identify them. The emergent sample is All real gases deviate to some extent passed through a detector, which registers from the gas laws, which are applicable the presence of the different components in only to idealized systems of particles of the carrier gas. negligible volume with no intermolecular Two types of detector are in common forces. There are several modified equa- use: the katharometer, which measures tions of state that give a better description changes in thermal conductivity, and the of the behavior of real gases, the best flame-ionization detector, which turns the known being the VAN DER WAALS EQUATION. volatile components into ions and registers the change in electrical conductivity. gas-liquid chromatography See gas chromatography. gas constant (universal gas constant) Symbol: R The universal constant 8.31451 gas-solid chromatography See gas J mol–1 K–1 appearing in the equation of chromatography. state for an ideal gas. See gas laws. gauss Symbol: G The unit of magnetic gaseous diffusion separation A tech- flux density in the c.g.s. system. It is equal nique for the separation of gases that relies to 10–4 tesla (T) in SI units. on their slightly different atomic masses. For example, the isotopes of uranium (235U Gay-Lussac, Joseph-Louis (1778–1850) and 238U) can be separated by first prepar- French chemist and physicist. Gay-Lussac ing gaseous uranium hexafluoride (UF6) is best remembered for his work on gases. from uranium ore. If this is then made to In 1802 he formulated the law sometimes pass through a series of fine pores, the mol- called Charles’ law (after the work of ecules containing the lighter isotope of ura- Jacques Charles in 1787) or Gay-Lussac’s nium will pass through more quickly. As law that, if the pressure remains constant, the gases pass through successive ‘di- gases expand equally for a given increase in aphragms’, the proportion of lighter mol- temperature. In 1805 he found that two ecules increases. A separation of over 99% volumes of hydrogen combine with one is thus attainable. Gaseous diffusion sepa- volume of oxygen to produce water. In ration has also been applied to the separa- 1808 he formulated what is known as Gay- tion of the isotopes of hydrogen. Lussac’s law of combining volumes, which states that gases combine in simple propor- gas equation See gas laws. tions by volume, with the volumes of the products being related to the volume of the gas laws Laws relating the temperature, reactants. He was an early investigator of pressure, and volume of a fixed mass of boron, iodine, and the cyanides. He also in-

96 Gibbs function vented the Gay-Lussac tower for recover- table. It is found in sulfide ores such as ar- ing nitrogen oxides produced in the manu- gyrodite (4Ag2S.GeS2) and in zinc ores and facture of sulphuric acid. coal. Most germanium is recovered during zinc or copper refining as a by-product. Gay-Lussac’s law 1. Gases react in vol- Germanium was extensively used in early umes that are in simple ratios to each other semiconductor devices but has now been and to their products, if they are gases, as- largely superseded by silicon. It is used as suming that all volumes are measured at an alloying agent and catalyst, and in phos- the same temperature and pressure. phors and infrared equipment. 2. See Charles’ law. Symbol: Ge; m.p. 937.45°C; b.p. 2830°C; r.d. 5.323 (20°C); p.n. 32; most gel A lyophilic colloid that is normally common isotope 74Ge; r.a.m. 72.61. stable but may be induced to coagulate partially under certain conditions (e.g. germanium(IV) oxide (GeO2) A com- lower temperatures). It is these conditions pound made by strongly heating germa- that produce a pseudo-solid or easily de- nium in air or by hydrolyzing formable jellylike mass called a gel, in germanium(IV) chloride. It occurs in two which intertwining particles enclose the forms. One is slightly soluble in water whole dispersing medium. Gels may be fur- forming an acidic solution; the other is in- ther subdivided into elastic gels (e.g. soluble in water. Germanium oxide dis- 2– gelatin) and rigid gels (e.g. silica gel). solves in alkalis to form the anion GeO3 . gel electrophoresis see electrophoresis. German silver See nickel-silver. gel filtration See molecular sieve. Gibbs, Josiah Willard (1839–1903) American mathematician and physicist. gem positions Positions in a molecule Between 1873 and 1878 Gibbs founded on the same atom. For example, 1,1- the subject of chemical thermodynamics in dichloroethane (CH3CHCl2) is a gem di- a series of lengthy papers. In one of these halide. papers entitled On the Equilibrium of Het- erogeneous Substances, he put forward the general formula A representation of phase rule concerning physical systems the chemical formula common to a group with different phases. His work on chemi- of compounds. For example, ROH (where cal thermodynamics also included the con- R is a hydrocarbon group) is the general cepts of the Gibbs free energy and chemical formula for an alcohol. See also empirical potential. In addition, he contributed to formula; molecular formula; structural for- work on the thermodynamics of surfaces. mula. Gibbs was one of the pioneers of statistical mechanics. He gave a classic exposition of geochemistry The study of the chem- the subject in the book Elementary Princi- istry of the Earth. This includes the chemi- ples of Statistical Mechanics (1902). cal composition of the Earth, the abundance of the elements and their iso- Gibbs free energy See Gibbs function. topes, the atomic structure of minerals, and chemical processes that occur at the Gibbs function (Gibbs free energy) high temperatures and pressures inside the Symbol: G A thermodynamic function de- Earth. fined by G = H – TS geometrical isomerism See isomerism. where H is the enthalpy, T the thermodynamic temperature, S the entropy, and G germanium A hard, brittle, grayish- the energy absorbed or liberated. It is use- white metalloid element belonging to ful for specifying the conditions of chemi- group 14 (formerly IVA) of the periodic cal equilibrium for reactions for constant

97 giga- temperature and pressure (G is a mini- chloride; AuCl3) A compound prepared mum). It is named for U.S. physicist and by dissolving gold in aqua regia. The bright mathematician Josiah Willard Gibbs yellow crystals (chloroauric acid, HAuCl4) (1839–1903), who proposed it in 1877. produced on evaporation are heated to See also free energy. form dark red crystals of gold(III) chloride. The chloride decomposes easily (at 175°C) giga- Symbol: G A prefix used with SI to give gold(I) chloride (AuCl) and chlo- units, denoting 109. For example, 1 giga- rine; at higher temperatures it decomposes hertz (GHz) = 109 hertz (Hz). further to give gold and chlorine. Gold(III) chloride is used in photography. It exists as glass A hard transparent material made a dimer, Au2Cl6. by heating calcium oxide (lime, CaO), sodium carbonate (Na2CO3), and sand (sil- Goldschmidt, Victor Moritz (1888– icon(IV) oxide, SiO2). This produces a cal- 1947) Swiss-born Norwegian chemist. cium silicate (Ca2SiO4) – the normal type Goldschmidt is frequently described as the of glass, called soda glass. Special types of founder of geochemistry. He pioneered the glass can be obtained by incorporating application of chemical thermodynamics boron oxide (B2O3) in the glass (borosili- to processes in geology. After the develop- cate glass, used for laboratory apparatus) ment of x-ray crystallography by the or by including metals other than calcium, Braggs, he and his colleagues worked out e.g. lead or barium. the structure of over 200 crystals. This led Glass is an amorphous substance, in the him to produce tables of atomic and ionic sense that there is no long-range ordering radii. Goldschmidt also devoted a lot of at- of the atoms on a lattice. It can be regarded tention to the problem of determining the as a supercooled liquid, which has not crys- cosmic abundance of the chemical el- tallized. Solids with similar noncrystalline ements. He summarized his work in the structures, e.g. obsidian, are also called book Geochemistry (1954), which was glasses. published posthumously.

Glauber’s salt See sodium sulfate. Goldschmidt process A process for extracting certain metals from their oxides by GLC Gas-liquid chromatography. See reduction with aluminum, named for Ger- gas chromatography. man chemist Hans Goldschmidt (1861– 1923), who discovered it. See thermite. gold A ductile malleable lustrous yellow transition metal, the third element of group gold trichloride See gold(III) chloride. 11 (formerly subgroup IB) of the periodic table. Gold is largely unreactive and is Graham’s law (of diffusion) The prin- therefore found native, often in veins in ig- ciple that gases diffuse at a rate that is in- neous rock or in placer deposits. Native versely proportional to the square root of gold almost always contains 2–20% silver. their density. It was discovered in 1829 by Gold is used in jewelry, often alloyed with the Scottish chemist Thomas Graham copper, and in electronics and colored (1805–69). Light molecules, in other glass. The purity of gold is traditionally words, diffuse faster than heavy molecules. stated in karats (or carats), with 24 karats The principle is used in the separation of indicating pure gold. Thus, a ring stated to isotopes. be 18 karat gold would consist of an alloy of 18 parts gold and six parts other metals. grain A crystal in a metal that has been Symbol: Au; m.p. 1064.43°C; b.p. prevented from attaining its regular geo- 2807°C; r.d. 19.320 (20°C); p.n. 79; most metrical form. common isotope 197Au; r.a.m. 196.96654. gram (gramme) Symbol: g A metric unit gold(III) chloride (gold trichloride; auric of mass defined as 10–3 kilogram. It was

98 group 13 elements the fundamental unit of mass in the c.g.s. ample, the hydrogen atom in the ground system. state has its single electron in the 1s orbital. If energy is provided (e.g. by electromag- granulation A process for enlarging netic radiation, electron impact, high tem- particles to improve the flow properties of perature, etc.) it is possible to form solid reactants and products in industrial hydrogen atoms in an EXCITED STATE (for chemical processes. The larger a particle, example, with the electron in the 2s or- and the freer from fine materials in a solid, bital). the more easily it will flow. Dry granulation produces pellets from dry materials, group In the periodic table, a series of which are crushed into the desired size. chemically similar elements that have simi- Wet granulation involves the addition of a lar electronic configurations. A group is liquid to the material, and the resulting thus a column of the periodic table. For ex- paste is extruded and dried before cutting ample, the alkali metals, all of which have into the required size. outer s1 configurations, belong to group 1. See also periodic table. graphite An allotrope of CARBON. The atoms are arranged in layers as a series of group 0 elements See rare gases. flat, hexagonal rings. Graphite is a good conductor of heat and electricity. The lay- group 1 elements See alkali metals. ers cleave easily, making graphite useful as a solid lubricant. group 2 elements See alkaline-earth metals. gravimetric analysis A method of quantitative analysis in which the final an- group 3-12 elements See transition el- alytical measurement is made by determi- ements. nation of mass. There are many variations in the method but in essence they all con- group 13 elements A group of elements sist of: in the periodic table consisting of the el- 1. taking a sample whose mass has been ac- ements boron (B), aluminum (Al), gallium curately determined into solution; (Ga), indium (In), and thallium (Tl). These 2. precipitating a known compound by a elements were formerly classified as group quantitative reaction; III elements and belonged to the IIIA sub- 3. performing digestion and coagulation group. The IIIB subgroup consisted of the procedures; elements scandium (Sc), yttrium (Y), lan- 4. filtering and washing; thanum (La), and actinium (Ac). 5. drying and determining mass as a pure The group 13 elements all have three compound. outer electrons (s2p1) with no partly filled Filtration is a key element in the inner shells. The group is the first group of method and a variety of special filter pa- elements in the p-block. As with all the pers and sinter-glass filters are available. groups there is an increase in metallic character as the group is descended. gray Symbol: Gy The SI derived unit of The boron atom is both small and has a absorbed energy dose per unit mass result- high ionization potential, so bonds to ing from the passage of ionizing radiation boron are largely covalent with only small through living tissue. One gray is an energy degrees of polarization. In fact boron is absorption of one joule per kilogram of classed as a metalloid. It has volatile hy- mass. drides and a weakly acidic oxide. As the group is descended the ionization poten- gray cast iron See cast iron. tials tend to decrease and the atomic radii increase, leading to increasingly polar in- ground state The state of lowest energy teractions and the formation of distinct of an atom (or ion, molecule, etc.). For ex- M3+ ions. The increase in metallic charac-

99 group 14 elements ter is clearly illustrated by the hydroxides: group 15 elements A group of elements boric acid B(OH)3 is acidic; aluminum and in the periodic table consisting of the el- gallium hydroxides are amphoteric, dis- ements nitrogen (N), phosphorus (P), ar- solving in acids to give Al3+ and Ga3+ and senic (As), antimony (Sb), and bismuth in bases to give aluminates and gallates; the (Bi). These elements were formerly classi- hydroxides of indium and thallium are dis- fied as group V elements and belonged to tinctly basic. As the elements get heavier the VA subgroup. The VB subgroup con- the bond energies with other elements be- sisted of the elements vanadium (V), nio- come generally smaller. The monovalent bium (Nb), and tantalum (Ta), which are state (removal of the p electron only) be- transition elements. comes progressively stable. For example The group 15 elements all have elec- gallium(I) is known in the gas phase and in tronic configurations corresponding to coordination complexes (sometimes as outer s2p3 electrons with no vacancies in ‘GaCl2’). Monovalent indium halides are inner levels. The ionization potentials are known, for example InX, and thallium(I) is high and the lighter members of the group, distinctly more stable than thallium(III). N and P, are distinctly electronegative and nonmetallic in character. Arsenic and anti- group 14 elements A group of elements mony are metalloids and bismuth is weakly in the periodic table consisting of the el- metallic (Bi O dissolves in acids to give ements carbon (C), silicon (Si), germanium 2 3 Bi(OH)3). (Ge), tin (Sn), and lead (Pb). These el- Nitrogen is dissimilar from the other ements were formerly classified as group members of the group in that it forms sta- IV elements and belonged to the IVA sub- ble multiple bonds, it is limited to coordi- group. The IVB subgroup consisted of the nation number 4, e.g. NH +, it is elements titanium (Ti), zirconium (Zr), and 4 sufficiently electronegative to form hydro- hafnium (Hf), which are transition metals. gen bonds and the nitride ion N3–, and in The group 14 elements all have elec- that its oxides are irregular with the rest of tronic structures with four outer electrons the group. (s2p2) and no partly filled inner shells. The Group members display a remarkable group shows the common trend toward metallic character with the heavier el- number of allotropes, showing the trend ements; thus carbon is a typical nonmetal, from nonmetallic forms through to metal- silicon and germanium are metalloids, and lic forms. Thus nitrogen has only the di- tin and lead are characteristically metallic. atomic form; phosphorus has a highly As observed with other groups, the first reactive form P1 (brown), and a tetrahedral member of the group is quite different from form P4 (white), forms based on broken the rest. The carbon atom is smaller and tetrahedra Pn (red and violet), and a hexag- has a higher ionization potential, both onal layer-type lattice (black). Arsenic and favouring predominance of covalence, but antimony have the As4 and Sb4 forms, additional factors are: which are less stable than the layer-type 1. The widespread nature of extensive lattice form; bismuth has the layer-lattice catenation in carbon compounds. form only. 2. The possibility of strong overlap of p orbitals or double bonds group 16 elements (chalcogens) A 3. The unavailability of d orbitals in car- group of elements of the periodic table con- bon compounds. sisting of the elements oxygen (O), sulfur Significant differences are: (S), selenium (Se), tellurium (Te), and polo- 1. Both oxides of carbon are gaseous, CO nium (Po). These elements were formerly and CO2; the other elements form solid classified as group VI elements and be- oxides, e.g. SiO2, Pb3O4. longed to the VIA subgroup. The VIB sub- 2. The heavier elements readily expand group consisted of the elements chromium 2– their coordination number, e.g. SiF6 (Cr), molybdenum (Mo), and tungsten 2– and SnCl6 . (W), which are transition elements. 100 gyromagnetic ratio

The electronic configurations of the which are transition elements. See halo- group 6 elements are all s2p4 with no va- gens. cant inner orbitals. These configurations are all just two electrons short of a rare-gas group 18 elements See rare gases. structure. Consequently they have high electron affinities and are almost entirely group theory A branch of mathematics nonmetallic in character. used to analyze symmetry in a systematic The group shows the normal property way. A group consists of a set of elements of a trend toward metallic character as it is A, B, C, etc. and a rule for forming the descended. Selenium, tellurium, and polo- product of these elements, such that certain nium have ‘metallic’ allotropes, and polo- conditions are satisfied: nium has generally metalloid-type 1. Every product AB of two elements A and properties where it has been possible to B is also an element of the set. study these (Po is very rare). All the el- 2. Any three elements A, B, C, must satify ements combine with a large number of A(BC) = (AB)C. other elements, both metallic and non- 3. There is an element of the set, denoted I, metallic, but in contrast to compounds of which is the identity element, i.e. for all the halogens they are more generally insol- A in the set AI = IA =A. 4. For each element A of the set there is an uble in water, and even when soluble do –1 not ionize readily. inverse, denoted A , which also belongs to the set which satisfies the relation Oxygen behaves like the first members AA–1 = A–1 = A = I. of other groups in displaying great differ- In the case of symmetry operations the ences from the rest of its group. For exam- product is the successive performance of ple, oxygen forms hydrogen bonds these operations. If a group has a finite whereas sulfur and the others do not; oxy- number of elements it is said to be a dis- gen cannot expand its outer shell and pos- crete group. The point groups, which arise itive oxidation states are uncommon (OF2, + – from the rotations and reflections of bodies O2F2, O2 PtF6 ). Oxygen is paramagnetic. such as isolated molecules are discrete Excluding oxygen, group 16 shows similar groups. trends with increasing size to those in group 15. For example, with increasing GSC Gas-solid chromatography. See gas size there is decreasing stability of H2X, chromatography. greater tendency to form complexes such 2– as SeBr6 , decreasing stability of high for- gunmetal See bronze. mal positive oxidation states, and marginal metallic properties (e.g. Po forms a hy- gunpowder A finely powdered mixture droxide). of sulfur, charcoal, and potassium nitrate, used as an explosive. group 17 elements A group of elements in the periodic table consisting of the el- gypsum See calcium sulfate. ements fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Formerly gyromagnetic ratio The ratio of the they belonged to group VII and were clas- magnetic moment of a system to the angu- sified in the VIIA subgroup. The VIIB sub- lar momentum of that system. The gyro- group consisted of the elements manganese magnetic ratio is a useful quantity in the (Mn), technetium (Te), and rhenium (Re), theory of electrons and atomic nuclei.

101 H

Haber, Fritz (1868–1934) German iron with small amounts of potassium and physical chemist. Haber is noted for his aluminum oxides present. The yield is discovery of the industrial process for syn- about 15%. Liquid ammonia is condensed thesizing ammonia from nitrogen and hy- out of the gas equilibrium mixture at drogen. The need at the time was for –50°C. nitrogen compounds for use as fertilizers – most plants cannot utilize free nitrogen habit See crystal habit. from the air, and need ‘fixed’ nitrogen. The main source was deposits of nitrate salts in hafnium A bright silvery transition Chile, but these would have a limited life. metal, the third element of group 4 (for- Haber, in an attempt to solve this problem, merly subgroup IVB) of the periodic table. began investigating the reaction: It is found in zirconium ores. Hafnium is ˆ N2 + 3H2 2NH3 difficult to work and can burn in air. It is Under normal conditions the yield is very used in control rods for nuclear reactors low. Haber (1907–09) showed that practi- and in certain specialized alloys and ce- cal yields could be achieved at high tem- ramics. ° peratures (250 C) and pressures (250 Symbol: Hf; m.p. 2230°C; b.p. 5197°C; atmospheres) using a catalyst (iron is the r.d. 13.31 (20°C); p.n. 72; most common catalyst now used). The process was devel- isotope 180Hf; r.a.m. 178.49. oped industrially by Carl BOSCH around 1913 and is still the main method for the hahnium Symbol: Ha or Hn A name fixation of nitrogen. Haber received the formerly suggested for element-105, now Nobel Prize for chemistry for this work in known as dubnium (Db), and also for el- 1918. ement-108, now known as hassium (Hs). Haber process An important industrial See element. process for the manufacture of ammonia, which is used for fertilizers and for making half cell An electrode in contact with a nitric acid. The reaction is the equilibrium: solution of ions. In general there will be an ˆ e.m.f. set up between electrode and solu- N2 + 3H2 3NH3 The nitrogen used is obtained by fractional tion by transfer of electrons to or from the distillation of liquid air and the hydrogen electrode. The e.m.f. of a half cell cannot by the oxidation of hydrocarbons (from be measured directly, since setting up a cir- natural gas). The nitrogen and hydrogen cuit results in the formation of another half are purified and mixed in the correct pro- cell. See electrode potential. portions. The equilibrium amount of ammonia is favoured by low temperatures, half-life (half-life period; Symbol: t½) The but in practice the reaction rate would be time taken for half the nuclei of a sample of too slow to be economic. An optimum tem- a radioactive nuclide to decay. The half-life perature of about 450°C is therefore used, of a nuclide is a measure of its stability. along with a catalyst. High pressure also (Stable nuclei can be thought of as having favors the reaction and a pressure of about infinitely long half-lives.) If N0 is the origi- 250 atmospheres is used. The catalyst is nal number of nuclei, the number remain-

102 hammer mill ing at the end of one half-life is N0/2, at the shells that are one electron short of a rare- end of two half-lives is N0/4, etc. gas configuration. Because of this, the halogens are characterized by high electron half sandwich compound See sand- affinities and high electronegativities, fluo- wich compound. rine being the most electronegative element known. The high electronegativities of the halide A compound containing a halo- halogens favor the formation of both gen. The inorganic halides of electroposi- uninegative ions, X–, particularly com- tive elements contain ions of the type bined with electropositive elements (e.g. + – + – Na Cl (sodium chloride) or K Br (potas- NaCl, KBr, CsI, CaF2), and the single co- sium bromide). Transition metal halides valent bond –X with elements of moderate often have some covalent bonding. Non- to high electronegativity (e.g. HCl, SiF4, metal halides are covalent compounds, SF4, BrCH3, Cl2O). The halogens all form which are usually volatile. Examples are diatomic molecules, X2, which are charac- tetrachloromethane (CCl4) and silicon terized by their high reactivities with a tetrachloride (SiCl4). Halides are named as wide range of other elements and com- bromides, chlorides, fluorides, or iodides. pounds. As a group, the elements increase in both size and polarizability as the proton halite (rock salt) A naturally occurring number increases and there is an attendant mineral form of sodium chloride, NaCl. It decrease in electronegativity and reactivity. forms colorless or white crystals when This means that there is no chemistry of pure, but is often colored by impurities. positive species apart from a few cationic iodine compounds. The heavier halogens halogenation A reaction in which a Cl, Br, and I all form oxo-species with for- HALOGEN atom is introduced into a gener- mal positive oxidation numbers +1 and +5, ally organic molecule. Halogenations are chlorine and iodine also forming +3 and +7 specified as chlorinations, brominations, species (e.g. HOCl, HBrO3, I2O5, HIO4). fluorinations, etc., according to the el- The elements decrease in oxidizing > > > ement involved. There are several methods. power in the order F2 Cl2 Br2 I2 and the 1. Direct reaction with the element using ions X– may be arranged in order of in- high temperature or ultraviolet radia- creasing reducing power F–

103 hardness hardness (of water) Hard water is water involved must be in their normal equilib- that will not readily form a lather with rium physical states under these conditions soap owing to the presence of dissolved (e.g. carbon as graphite, water as the liq- calcium, iron, and magnesium compounds. uid, etc.). Note that the measured enthalpy Such compounds can react with soap to change will not usually be the standard produce insoluble salts, which collect as change. In addition, it is common to spec- solid scum. The effectiveness of the cleans- ify the entity involved. For instance ∆ Š ing solution is thus reduced. Hardness of Hf (H2O) is the standard molar en- water is of two types, temporary hardness thalpy of formation for one mole of H2O and permanent hardness. Only temporary species. hardness can be removed by boiling the water. heat engine (thermodynamic engine) A Hardness of water is usually expressed device for converting heat energy into by assuming that all the hardness is due to work. Heat engines operate by transferring dissolved calcium carbonate, which is pre- energy from a high-temperature source to a sent as ions. It can be estimated by titration low-temperature sink. The theoretical op- with a standard soap solution or with edta. eration of heat engines is useful in the See also permanent hardness; detergents; theory of thermodynamics. See Carnot temporary hardness; water softening. cycle. hard vacuum See vacuum. heat exchangers Devices that enable the heat from a hot fluid to be transferred hard water See hardness. to a cool fluid without allowing the two fluids to come into contact. The normal hassium A transactinide element that is arrangement is for one of the fluids to flow formed artificially. in a coiled tube through a jacket containing Symbol: Hs; p.n. 108; most stable iso- the second fluid. Both the cooling and heat- tope 265Hs (half-life 2 × 10–3s). ing effect may be of benefit in conserving the energy used in a chemical plant and in hausmannite See manganese. controlling a process. heat Energy transferred as a result of a heat of atomization The energy re- temperature difference. The term is often quired in dissociating one mole of a sub- loosely used to mean internal energy (i.e. stance into atoms. See heat. the total kinetic and potential energy of the particles). It is common in chemistry to de- heat of combustion The energy liber- fine such quantities as heat of combustion, ated when one mole of a substance burns in heat of neutralization, etc. These are in fact excess oxygen. See heat. molar enthalpies for the change, given the ∆ Š symbol HM . The superscript symbol de- heat of crystallization The energy lib- notes standard conditions, while the sub- erated when one mole of a substance crys- script M indicates that the enthalpy change tallizes from a saturated solution of that is for one mole. The unit is usually the kilo- substance. joule per mole (kJ mol–1). By convention, ∆H is negative for an exothermic reaction. heat of dissociation The energy re- Molar enthalpy changes stated for chemi- quired to dissociate one mole of a sub- cal reactions are changes for standard con- stance into its constituent elements. ditions, which are defined as 298 K (25°C) and 101325 Pa (1 atmosphere). Thus, the heat of formation The energy change standard molar enthalpy of reaction is the when one mole of a substance is formed enthalpy change for reaction of substances from its elements. See heat. under these conditions producing reactants under the same conditions. The substances heat of neutralization The energy lib-

104 helium erated when one mole of an acid or base is Werner Heisenberg, who discovered it in neutralized. 1927. heat of reaction The energy change Heitler, Walter (1904–81) German- when molar amounts of given substances born physicist. Heitler is most famous for a react completely. See heat. classic paper he wrote with Friz London in 1927 in which they showed that the chem- heat of solution The energy change ical bond in the hydrogen molecule could when one mole of a substance is dissolved be described by quantum mechanics. in a given solvent to infinite dilution (in Heitler extended this work to more com- practice, to form a dilute solution). plicated molecules and was a pioneer of the use of group theory in quantum mechanics. heavy hydrogen See deuterium. He wrote a classic book entitled The Quantum Theory of Radiation, the third heavy-metal pollution See pollution. edition of which was published in 1954. heavy water See deuterium oxide. helicate See supramolecular chemistry. hecto- Symbol: h A prefix used with SI helium An inert colorless odorless units, denoting 102. For example, 1 hec- monatomic gas, the first member of the tometer (hm) = 102 meters (m). rare gases; i.e. group 18 (formerly VIIIA or 0) of the periodic table. Helium has the Heisenberg, Werner Karl (1901–76) electronic configuration 1s2 and consists of German physicist. Heisenberg was one of a nucleus of two protons and two neutrons the founders of quantum mechanics. In (equivalent to an alpha particle) with two 1925, along with Max BORN and Pascual extra-nuclear electrons. It has an extremely Jordan, he formulated matrix mechanics. high ionization potential and is completely In 1926 he studied the spectrum of mo- resistant to chemical attack of any sort. lecular hydrogen and predicted the exis- Helium is the second most abundant el- tence of ortho- and para-hydrogen from ement in the universe, the primary process the splitting of the spectral lines. His most in the Sun and other stars being the nuclear famous contribution is the HEISENBERG UN- fusion of hydrogen to give helium. Never- CERTAINTY PRINCIPLE, put forward in 1927. theless the gas accounts for only 5.2 × 10–4% of the Earth’s atmosphere although Heisenberg uncertainty principle The some natural gas deposits contain up to impossibility of making simultaneous 7%. measurements of both the position and the Helium is recovered commercially by momentum of a subatomic particle (e.g. an the fractional distillation of natural gas electron) with unlimited accuracy. The un- and also forms part of ammonia plant tail certainty arises because, in order to detect gas if natural gas is used as a feedstock. Its the particle, radiation has to be ‘bounced’ applications are in fields in which inertness off it, and this process itself disrupts the is required and where cheaper alternatives, particle’s position. Heisenberg’s uncer- such as nitrogen, would prove too reactive. tainty principle is not a consequence of Examples include high-temperature metal- ‘experimental error’. It represents a funda- lurgy, powder technology, and use as a mental limit to objective scientific observa- coolant in nuclear reactors. Helium is also tion, and arises from the wave–particle favoured over nitrogen for diluting oxygen duality of particles and radiation. In one in breathing mixtures designed for deep- direction, the uncertainty in position sea diving due to its lower solubility in ∆x and momentum ∆p are related by blood and as a pressurizer for liquefied gas ∆x∆p ∼ h/4π, where h is the Planck con- fuels in rockets (due to its total inertness). stant. It is named for the German physicist It is also used as an ideal gas for balloons

105 Helmholtz free energy

(no fire risk) and for low-temperature hemimorphite See calamine. physics research. Helium is unusual in that it is the only henry Symbol: H The SI derived unit of known substance for which there is no inductance, equal to the inductance of a triple point (i.e., no combination of pres- closed circuit that has a magnetic flux of sure and temperature at which all three one weber per ampere of current in the cir- phases can co-exist in equilibrium). This is cuit. 1 H = 1 Wb A–1. It is named for U.S. because the interatomic forces, which nor- physicist Joseph Henry (1797–1878). mally participate in the formation of solids, are so weak in helium that they are Henry’s law The concentration (C) of a of the same order as the zero-point energy. gas in solution is proportional to the par- At 2.186 K helium undergoes a transition tial pressure (p) of that gas in equilibrium from liquid helium I to liquid helium II, the with the solution, i.e. p = kC, where k is a latter being a true liquid but exhibiting su- proportionality constant. The relationship perconductivity and an immeasurably low is similar in form to that for Raoult’s law, viscosity (SUPERFLUIDITY). This low viscos- which deals with ideal solutions. ity allows the liquid to spread in layers a A consequence of Henry’s law is that few atoms thick, described by some as an the ‘volume solubility’of a gas is indepen- action like ‘flowing uphill’. dent of pressure. It was discovered in 1801 Helium has two naturally occurring iso- by British chemist William Henry (1774– topes, of which helium-4 (4He) is the most 1836). common. Helium-3 (3He) is formed in nu- heptahydrate A crystalline hydrated clear reactions and by the decay of tritium. compound containing one molecule of This isotope also undergoes a phase change compound per seven molecules of water of at temperatures close to absolute zero. crystallization. Symbol: He; m.p. 0.95 K (under pressure); b.p. 4.216 K; r.d. 0.178; p.n. 2; most heptavalent (septivalent) Having a va- common isotope 4He; r.a.m. 4.002602. lence of seven.

Helmholtz free energy See Helmholtz hertz Symbol: Hz The SI derived unit of function. frequency, defined as one cycle per second (s–1). Note that the hertz is used for regu- Helmholtz function (Helmholtz free en- larly repeated processes, such as vibrations ergy) Symbol: F A thermodynamic func- or wave motions. It is named for the Ger- tion defined by man physicist Heinrich Hertz (1857–94). F = U – TS where U is the internal energy, T the ther- Herzberg, Gerhard (1904–99) German- modynamic temperature, and S the en- born Canadian spectroscopist. Herzberg tropy. It is a measure of the ability of a investigated and interpreted the spectra of system to do useful work in an isothermal many molecules. He wrote a series of clas- process. See also free energy. sic, definitive books on atomic and molecular spectra between the 1930s and the hematite A reddish-brown, gray, or 1970s. Herzberg was awarded the Nobel black mineral form of iron(III) oxide Prize for chemistry in 1971. (Fe2O3), found in igneous, metamorphic, and sedimentary rocks. It is the principal Hess’s law (law of constant heat summa- ore of iron and is also used as a pigment. tion) A derivative of the first law of thermodynamics. It states that the total heat hemihydrate A crystalline compound change for a given chemical reaction in- having one molecule of water of crystal- volving alternative series of steps is inde- lization per two molecules of compound pendent of the route taken. It is named for (e.g. 2CaSO4.H2O). the Swiss-born Russian chemist Germain 106 Hund's rule

Hess (1802–50), who stated it in 1840. occurs in association with other lanthanoids in such minerals as euxenite, heterogeneous Relating to more than gadolinite, monazite, samarskite, and one phase. A heterogeneous mixture, for xenotime. It has a few applications in elec- instance, contains two or more distinct tronics due to the highly magnetic nature phases. of its compounds. Symbol: Ho; m.p. 1474°C; b.p. heterogeneous catalyst See catalyst. 2695°C; r.d. 8.795 (25°C); p.n. 67; most common isotope 165Ho; r.a.m. 164.93032. heterolysis See heterolytic fission. HOMO See frontier orbital theory. heterolytic fission (heterolysis) The breaking of a covalent bond so that both homogeneous Relating to a single electrons of the bond remain with one frag- phase. A homogeneous mixture, for in- ment. A positive ion and a negative ion are stance, consists of only one phase. produced: RX → R+ + X– homogeneous catalyst See catalyst. Compare homolytic fission. homolysis See homolytic fission. hexacyanoferrate See cyanoferrate. homolytic fission (homolysis) The hexagonal close-packed crystal A breaking of a covalent bond so that one close-packed crystal structure in which lay- electron from the bond is left on each frag- ers of close-packed atoms, ions, or mol- ment. Two free radicals result: ecules are stacked in an ABABAB RR′→R• + R′• arrangement. In this arrangement, the B Compare heterolytic fission. layer fits into the recesses left in the A layer, with the third layer then fitting in the hornblende Any of a group of dark-col- recesses in the second layer that are directly ored rock-forming minerals consisting of above the atoms, etc. in the first layer. Zinc complex silicates of calcium, sodium, alu- and magnesium have hexagonal close- minum, iron, and magnesium. It is a major packed structures. See close packing. Com- component of granite and other igneous pare face-centered cubic crystal. and metamorphic rocks. hexagonal crystal See crystal system. host–guest chemistry A branch of supramolecular chemistry in which a mo- Hinshelwood, Sir Cyril Norman lecular structure acts as a ‘host’ to hold a (1897–1967) British chemist. Hinshel- ‘guest’ ion or molecule. The guest may be wood was a leading figure in the study of coordinated to the host or may be trapped the kinetics and mechanisms of chemical by its structure. See also supramolecular reactions. He wrote a classic book on this chemistry. topic entitled The Kinetics of Chemical Change in Gaseous Systems (1926). In HPLC High-performance liquid chro- 1956 he shared the Nobel Prize for chem- matography; a sensitive analytical tech- istry with Nikolay SEMENOV for his work nique, similar to gas-liquid chroma- on the mechanisms of chain reactions. Hin- tography but using a liquid carrier. The shelwood worked extensively on chemical carrier is specifically choosen for the par- reactions in bacteria, summing up his re- ticular substance to be detected. search in the book The Chemical Kinetics of the Bacterial Cell (1954). Humphreys series See hydrogen atom spectrum. holmium A soft malleable silvery element of the lanthanoid series of metals. It Hund’s rule A rule that states that the

107 hybrid orbital electronic configuration in degenerate or- sition metals absorb hydrogen to form in- bitals will have the minimum number of terstitial hydrides. paired electrons. It is named for the Ger- man physicist Friedrich Hund (1896– hydrobromic acid (HBr) A colorless 1993), who stated it in 1925. liquid produced by dissolving HYDROGEN BROMIDE in water. It shows the typical hybrid orbital See orbital. properties of a strong acid and is a strong reducing agent. A convenient way of pro- hydrate A compound coordinated with ducing hydrobromic acid is to bubble hy- water molecules. When water is bound up drogen sulfide through bromine water. in a compound it is known as the WATER OF Although it is not as strong as hydrochloric CRYSTALLIZATION. acid it dissociates extensively in water and is a good proton donor. hydration The solvation of species such as ions in water. hydrocarbon Any compound containing only the elements carbon and hydro- hydraulic cement See cement. gen. Methane and ethane are examples. hydrazine (N2H4) A colorless liquid hydrochloric acid (HCl) A colorless that can be prepared by the oxidation of fuming liquid made by dissolving HYDRO- ammonia with sodium chlorate(I) or by the GEN CHLORIDE to water: gas phase reaction of ammonia with chlo- → + HCl(g) + H2O1(l). H3O (aq) + rine. Hydrazine is a weak base, forming Cl–(aq) salts (e.g. N H .HCl) with strong acids. It 2 4 Dissociation into ions is extensive and is a powerful reducing agent, reducing salts hydrochloric acid shows the typical prop- of the noble metals to their respective met- erties of a strong acid. It reacts with car- als. Anhydrous hydrazine ignites sponta- bonates to give carbon dioxide and yields neously in oxygen and reacts violently with hydrogen when reacted with all but the oxidizing agents. The aqueous solution, most unreactive metals. Hydrochloric acid hydrazine hydrate, has been used as a fuel is used in the manufacture of dyes, drugs, for jet engines and for rockets. and photographic materials. It is also used hydrazine hydrate See hydrazine. to pickle metals, i.e. clean their surfaces prior to electroplating. It donates protons with ease and is the strongest of the hydro- hydrazoic acid (hydrogen azide; HN3) A colorless liquid with a nauseating smell. It halic acids. The concentrated acid is oxi- is highly poisonous and explodes in the dized to chlorine by such agents as presence of oxygen and oxidizing agents. It potassium manganate(VII) and man- can be made by distilling a mixture of ganese(IV) oxide. sodium azide (NaN3) and a dilute acid. It is usually used as an aqueous solution. The hydrocyanic acid (prussic acid; HCN) salts of hydrazoic acid (azides), especially A highly poisonous weak acid formed when hydrogen cyanide gas dissolves in lead azide (Pb(N3)2), are used in detonators because they explode when given a me- water. Its salts are cyanides. Hydrogen chanical shock. cyanide is used in making acrylic plastics. hydride A compound of hydrogen. Ionic hydrofluoric acid (HF) A colorless liq- hydrides are formed with highly elec- uid produced by dissolving HYDROGEN FLU- tropositive elements and contain the H– ion ORIDE in water. It is a weak acid, but will (hydride ion). Nonmetals form covalent dissolve most silicates and hence can be hydrides, as in methane (CH4) or silane used to etch glass. As the interatomic dis- (SiH4). The boron hydrides are electron- tance in HF is relatively small, the H–F deficient covalent compounds. Many tran- BOND ENERGY is very high and hydrogen

108 hydrogen fluoride is not a good proton donor. It The hydrogen atom consists of a proton does, however, form hydrogen bonds. (positive charge) with one extranuclear electron (s1). Hydrogen is thus the simplest hydrogen A colorless odorless gaseous of all atoms, giving it a unique position element traditionally placed in group 1 among the elements. The chemistry of hy- (formerly IA) of the periodic table. Al- drogen depends on one of three processes: though it has some similarities to both the 1. Loss of the electron to form H+. alkali metals (group 1) and the halogens 2. Gain of an electron to form H–. (group 17), hydrogen is now not normally 3. Sharing of electrons by covalent bond classified in any particular periodic group. formation, as in H2 or HCl. It is the most abundant element in the uni- The hydrogen atom with the 1s electron verse and the ninth most abundant element removed would have an extremely small in the Earth’s crust and atmosphere (by ionic radius and the positive hydrogen ion mass). It occurs in nature principally as a in fact occurs only in association with + + constituent of water, compounds formed other species, as in H NH3 or H FH. The + by organisms, and hydrocarbon deposits ion commonly written H in solution is ac- (coal, oil, and natural gas); traces of mo- tually the solvated proton (hydroxonium + lecular hydrogen are found in some natural or hydronium) H3O , which is formed by gases and in the upper atmosphere. ionization of acids, e.g.: → + – The gas may be prepared in the labora- H2O + HCl H3O + Cl It is believed that the lifetime of any one tory by the reaction of dilute hydrochloric + acid with a metal that lies above hydrogen H3O ion is extremely short, as the protons in the electromotive series, magnesium and appear to undergo very rapid exchange between water molecules. zinc being commonly used: Hydrogen also forms a number of com- Zn + 2HCl → H + ZnCl 2 2 pounds in which it is regarded as gaining Reactions of the amphoteric metals zinc an electron and becoming H– (the hydride and aluminum with dilute aqueous alkali, ion). It can form ionic HYDRIDES only with to form zincates or aluminates, are also the most electropositive elements (i.e., convenient means of obtaining laboratory those in groups 1 and 2). The ionic nature hydrogen. Although electrolysis of dilute of these compounds is indicated by the fact mineral acids may be used as well, care that their melts are good conductors and must be taken to avoid mixing the hydro- that hydrogen is liberated at the anode dur- gen released at the cathode with the oxygen ing their electrolysis. These hydrides are released at the anode, because such mix- prepared by heating the element in a tures are potentially explosive. Industri- stream of hydrogen: ally, hydrogen is obtained as a by-product → H2 + 2M 2MH of the electrolytic cells used in the produc- Examples of hydrides with covalent tion of sodium hydroxide, i.e. by reacting bonds are CH4, NH3, and HCl. These com- the Na/Hg amalgam (produced when pounds are generally low-boiling. General sodium liberated from the brine solution methods of preparation are: alloys with the mercury cathode) with 1. The hydrolysis of the appropriate ‘-ide’ water, or by the water-gas route in which compound, e.g. silicides give silane, ni- steam is decomposed by hot coke. trides give ammonia, sulfides give H2S. The main use of hydrogen is as a chem- 2. Reduction of a chloride; for example: → ical feedstock for the manufacture of am- SiCl4 + LiAlH4 SiH4 + LiCl + AlCl3 monia and a range of organic compounds. A third kind of hydride is that of metal- Small-scale uses include reducing atmos- lic hydrides formed between hydrogen and pheres for metallurgy, hydrogenation of many transition metals. Palladium in par- edible oils, and pharmaceutical manufac- ticular is renowned for its ability to absorb turing. Hydrogen also has significant po- hydrogen as PdHx, where x takes values up tential as a fuel, especially using fuel cell to 1.8. Titanium and zirconium behave technology. similarly, but the exact nature of the com-

109 hydrogenation pounds formed and the type of bond in- hydrogenation The reaction of a com- volved remains uncertain. There are pound with hydrogen. An example is the changes in the magnetic properties of the hydrogenation of nitrogen to form ammo- metal, indicating some electron interac- nia in the Haber process. In organic chem- tion, and in the lattice dimensions of the istry, hydrogenation refers to the addition compound, but discrete phases of the type of hydrogen to multiple bonds, usually MH or MH2 have not been isolated. These with the aid of a catalyst. Unsaturated nat- compounds are sometimes referred to as ural liquid vegetable oils can be hydro- the interstitial hydrides. genated to form saturated semisolid fats – Atomic nuclei possess the property of a reaction used in making types of mar- ‘spin’ and for diatomic molecules there ex- garine and cooking oils. Consumption of ists the possibility of having the spins of ad- such hydrogenated oils is now thought to jacent nuclei aligned (ortho) or opposed be associated with increased risk of cardio- (para). Because of the small mass of hydro- vascular disease. See Bergius process. gen, these forms are more important in hydrogen molecules than in other diatomic hydrogen atom spectrum The spec- molecules. The two forms are in equilib- trum of the hydrogen atom is characterized rium, with parahydrogen dominant at low by several series of sharp spectral lines de- temperatures. However the equilibrium scribed by simple laws. The general law for ratio rises to 3 parts orthohydrogen to 1 these series of lines is: λ 2 2 part parahydrogen at room temperatures. 1/ = R (1/n1 – 1/n2 ), Although chemically identical, the melting where λ is the wavelength associated with point and boiling point of the para form a spectral line, R is the RYDBERG CONSTANT ° ≥ are both about 0.1 C lower than the 3:1 and n1 and n2 are integers, with n2 n1. equilibrium mixture. The first of these series to be discovered Natural hydrogen in molecular or com- was the Balmer series in which n1 = 2, n2 = bined forms contains about one part in 3,4,5,… This series is in the visible region 2000 of DEUTERIUM, symbol D, an isotope and was discovered by the Swiss mathe- of hydrogen that contains one proton and matician and physicist Johann Jakob one neutron in its nucleus. The artificially Balmer (1825–98) in 1885. The series in created radioactive isotope TRITIUM, sym- which n1 = 1 is the Lyman series which lies bol T, has one proton and two neutrons. in the ultraviolet region. This series was Although the effect of isotopes on chemical discovered by Theodore Lyman (1874– properties is normally small, in the case of 1954). The Lyman series is a conspicuous hydrogen the difference in mass number feature of the spectrum of the Sun. leads to a lowering of some reaction rates, In the Paschen series (n1 = 3), the Brack- a phenomenon known as the ‘isotope ef- ett series (n1 = 4), the Pfund series (n1 = 5) fect’. and the Humphreys series (n1 = 6) the spec- Hydrogen also exhibits two less com- tral lines occur in the infrared region. mon forms of bonding. Boron hydrides The explanation for these regular series form a wide variety of compounds in lies in the existence of discrete, quantized which the hydrogen acts as a bridging energy levels. In 1913 Niels Bohr was able species involving ‘three-center two-elec- to derive the formula for these series in tron’ bonds. Such species are said to be terms of the ad hoc quantum assumptions ‘electron deficient’ because they do not of the BOHR THEORY. In the mid-1920s the have sufficient electrons for conventional formula was derived in a deductive way two-electron covalent bonds. The second, from quantum mechanics. In the wave me- less common, form is that of the coordi- chanics formulation of quantum mechan- nated hydrides in which the H– ion acts as ics it is possible to derive the formula a ligand bound to a transition metal atom. because the Schrödinger equation can be Symbol: H; m.p. 14.01 K; b.p. 20.28 K; solved exactly for the hydrogen atom. r.d. 0.089 88; p.n. 1; most common isotope 1H; r.a.m. 1.0079. hydrogen azide See hydrazoic acid.

110 hydrogen molecule ion hydrogen bond (H-bonding) An inter- hydrogen cyanide See hydrocyanic molecular bond between molecules in acid. which hydrogen is bound to a strongly electronegative element. Bond polarization hydrogen electrode (standard hydrogen by the electronegative element X leads to a half cell) A type of half cell based on hy- δ δ positive charge on hydrogen X ––H +; this drogen, and assigned zero electrode poten- hydrogen can then interact directly with tial, so that other elements may be electronegative elements of adjacent mol- compared with it. The hydrogen is bubbled ecules. The hydrogen bond is represented over a platinum electrode, coated in ‘plat- as a dotted line: inum black’, in a 1 M acid solution. Hy- δ δ δ δ X – – H + ...... X – – H + … drogen is adsorbed on the platinum black, The length of a hydrogen bond is charac- which has a high surface area, enabling the teristically 0.15–0.2 nm. Hydrogen bond- equilibrium ing may lead to the formation of dimers H(g) ˆ H+(aq) + e– (for example, in carboxylic acids) and is to be set up. The platinum is inert and has used to explain the anomalously high boil- no tendency to form platinum ions in solu- ing points of H2O and HF. tion. See also electrode potential. hydrogen bromide (HBr) A colorless hydrogen fluoride (HF) A colorless liq- sharp-smelling gas that is very soluble in uid produced by the reaction of concen- water, giving HYDROBROMIC ACID. It is pro- trated sulfuric acid with calcium fluoride: duced by direct combination of hydrogen → CaF2(s) + H2SO4(aq) CaSO4(aq) + and bromine in the presence of a platinum 2HF(l) catalyst or by the reaction of phosphorus It produces toxic corrosive fumes and dis- tribromide with water. Hydrogen bromide solves readily in water to give HYDROFLUO- is rather inactive chemically. It will not RIC ACID. conduct electricity in the liquid state, indi- Hydrogen fluoride is atypical of the hy- cating that it is a molecular compound. drogen halides inasmuch as the individual H–F units are associated into much larger hydrogencarbonate (bicarbonate) A units, forming zigzag chains and rings. salt containing the ion –HCO . 3 This effect is caused by hydrogen bonds hydrogen chloride (HCl) A colorless that form between the hydrogen and the gas that has a strong irritating odor and highly electronegative fluoride ions. Hy- fumes strongly in moist air. It is prepared drogen fluoride is used extensively as a cat- by the action of concentrated sulfuric acid alyst in the petroleum industry. on sodium chloride. The gas is made industrially by burning a stream of hydrogen hydrogen ion A positively charged hy- + in chlorine. It is not particularly reactive drogen atom, H , i.e. a proton. Hydrogen but will form dense white clouds of ammo- ions are produced by all acids in water, in nium chloride when mixed with ammonia. which they are hydrated to hydroxonium + It is very soluble in water and ionizes al- (hydronium) ions, H3O . See acid; pH. most completely to give HYDROCHLORIC + ACID. It will also dissolve in many non- hydrogen molecule ion (H2 ) The sim- aqueous solvents, including toluene, but plest type of molecule. It consists of two will not ionize, indicating that the resulting hydrogen nuclei and one electron. In the solution will show no acidic properties. BORN–OPPENHEIMER APPROXIMATION in Hydrogen chloride is used in the manufac- which the nuclei are regarded as being ture of organic chlorine compounds, such fixed the SCHRÖDINGER EQUATION for the as polyvinyl chloride (PVC). hydrogen molecule ion can be solved ex- Unlike the other hydrogen halides, hy- actly. This enables ideas and approxima- drogen chloride will not dissociate on heat- tion techniques concerned with chemical ing, indicating a strong H–Cl bond. bonding to be tested quantitatively.

111 hydrogen peroxide hydrogen peroxide (H2O2) A colorless hydrohalic Describing acids formed by syrupy liquid, usually used in solution in hydrogen halides, e.g. HF, HCI, HBr, water. Although it is stable when pure, on when dissolved in water. contact with bases such as manganese(IV) oxide it gives off oxygen, the hydroiodic acid See iodine. manganese(IV) oxide acting as a catalyst: → 2H2O2 2H2O + O2 hydrolysis A reaction between a com- Hydrogen peroxide can act as an oxi- pound and water, e.g.: dizing agent, converting iron(II) ions to Salts of weak acids → iron(III) ions, or as a reducing agent with Na2CO3 + 2H2O 2NaOH + H2CO3 potassium manganate(VII). It is used as a and certain inorganic halides → bleach and mild antiseptic, and as an oxi- SiCl4 + 4H2O Si(OH)4 + 4HCl dant in rocket fuel. The strength of H2O2 solutions is usually given as volume hydronium See acid; hydrogen ion. strength, i.e. the volume (in cubic decime- ters) of oxygen released at STP by the de- hydrophilic See lyophilic. composition of one cubic decimeter of the solution. hydrophobic See lyophobic.

– hydrosol A colloid in aqueous solution. hydrogensulfate (bisulfate; HSO4 )An acidic salt of sulfuric acid (H SO ), in 2 4 A compound containing the which only one of the acid’s hydrogen hydroxide ion OH– or the group –OH. atoms has been replaced by a metal. An example is sodium hydrogensulfate, hydroxonium ion See acid; hydrogen NaHSO . 4 ion. hydrogen sulfide (sulfuretted hydrogen; hydroxyl group A group (–OH) con- H S) A colorless very poisonous gas with 2 taining hydrogen and oxygen, characteris- an odor of bad eggs. Hydrogen sulfide is tic of alcohols and some hydroxides. It prepared by reacting hydrochloric acid should not be confused with the hydroxide with iron(II) sulfide. Its presence can be de- ion (OH–). termined by mixing with lead nitrate solution, with which H2S gives a black hygroscopic Describing a substance precipitate. Its aqueous solution is weakly that absorbs moisture from the atmos- acidic. Hydrogen sulfide reduces iron(III) phere. See also deliquescent. chloride to iron(II) chloride, forming hydrochloric acid and a yellow precipitate of hyperfine structure See fine structure. sulfur. Hydrogen sulfide precipitates metals from insoluble sulfides in acid solution, hypo See sodium thiosulfate(IV). and is used in qualitative analysis. It burns with a blue flame in oxygen to form sul- hypobromous acid See bromic(I) acid. fur(IV) oxide and water. Natural gas contains some hydrogen sulfide, which is hypochlorous acid See chloric(I) acid. removed before supply to the consumer. hypophosphorous acid See phosphinic – hydrogensulfite (bisulfite; HSO3 )An acid. acidic salt of sulfurous acid (H2SO3), in which only one of the acid’s hydrogen hyposulfuric acid See dithionic acid. atoms has been replaced by a metal. An example is sodium hydrogensulfite, hyposulfurous acid See dithionous NaHSO3. acid.

112 I

Iceland spar A pure transparent nearly ogous to the transition pH in acid–base flawless mineral form of calcite (calcium systems. carbonate, CaCO3), noted for its property Complexometric titrations require indi- of birefringence (double refraction). It cators that complex with metal ions and comes from Eskifjördhur in Iceland and change color between the free state and the has been used in optics. complex state. See also absorption indicator. icosahedron A polyhedron bounded by 20 triangular faces. The icosahedron is one indium A soft malleable ductile silvery of the main structural units of boron and metallic element belonging to group 13 its compounds. Icosohedral symmetry also (formerly IIIA) of the periodic table. A rare occurs in certain CLUSTER COMPOUNDS. element, it is found in minute quantities, primarily in zinc ores. It is used in alloys, in ideal gas (perfect gas) See gas laws; ki- several electronic devices, and in electro- netic theory. plating. Symbol: In; m.p. 155.17°C; b.p. 2080°C; r.d. 7.31 (25°C); p.n. 49; most ideal-gas scale See absolute tempera- 115 ture. common isotope In; r.a.m. 114.818. See rare gases. ideal solution See Raoult’s law. inert gases infrared (IR) Electromagnetic radiation immiscible Describing two or more liq- with longer wavelengths than visible radia- uids that will not mix, such as oil and tion. The wavelength range is approxi- water. After being shaken together and left mately 0.7 µm to 1 mm. Many materials to stand they form separate layers. transparent to visible light are opaque to infrared, including glass. Rock salt, quartz, A compound that reversibly indicator germanium, or polyethene prisms and changes color depending on the pH of the lenses are suitable for use with infrared. In- solution in which it is dissolved. The visual frared radiation is produced by movement observation of this change is therefore a of charges on the molecular scale; i.e. by guide to the pH of the solution and it fol- vibrational or rotational motion of mol- lows that careful choice of indicators per- ecules. Infrared spectroscopy is of particu- mits a wide range of end points to be lar importance in organic chemistry and detected in acid–base titrations. absorption spectra are used extensively in Redox titrations require either specific identifying compounds. Certain bonds be- indicators, which detect one of the compo- tween pairs of atoms (C–C, C=C, C=O, nents of the reaction (e.g. starch for iodine, etc.) have characteristic vibrational fre- potassium thiocyanate for Fe3+), or true quencies, which correspond to bands in the redox indicators in which the transition infrared spectrum. Infrared spectra are potential of the indicator between oxidized thus used in finding the structures of new and reduced forms is important. The tran- organic compounds by indicating the pres- sition potential of a redox indicator is anal- ence of certain groups. They are also used

113 inhibitor to ‘fingerprint’ and thus identify known silicon layers in talc and pyrophyllite re- compounds. At shorter wavelengths, in- spectively. See also lamellar compound. frared absorption corresponds to transitions between rotational energy levels, and interhalogen See bromine. can be used to find the dimensions of molecules (by their moment of inertia). intermediate 1. A compound that requires further chemical treatment to pro- inhibitor A substance that slows down duce a finished industrial product such as a the rate of reaction. Hydrogen sulfide, hy- dye or pharmaceutical chemical. drogen cyanide, mercury salts, and arsenic 2. A transient chemical entity in a complex compounds readily inhibit heterogeneous reaction. See also precursor. catalysts by adsorption. For example, arsenic compounds inhibit platinum catalysts intermediate bonding Describing a in the oxidation of sulfur(IV) oxide to sul- form of covalent bond that also has an fur(VI) oxide. Inhibitors must not be con- ionic or electrovalent character. See polar fused with negative catalysts; inhibitors do bond. not change the pathway of a reaction. intermolecular forces Forces of attrac- inorganic chemistry The branch of tion between molecules rather than forces chemistry concerned with elements other within the molecule (chemical bonding). If than carbon and with the preparation, these intermolecular forces are weak the properties, and reactions of their com- material will be gaseous and as their pounds. Certain simple carbon compounds strength progressively increases materials are treated in inorganic chemistry, includ- become progressively liquids and solids. ing the oxides, carbon disulfide, the The intermolecular forces are divided into halides, hydrogen cyanide, and salts, such H-bonding forces and van der Waals as the cyanides, cyanates, carbonates, and forces, and the major component is the hydrogencarbonates. electrostatic interaction of dipoles. See hydrogen bond; van der Waals force. insoluble Describing a compound that has a very low solubility (in a specified sol- internal energy Symbol: U The energy vent). of a system that is the total of the kinetic and potential energies of its constituent instrumentation The measurement of particles (e.g. atoms and molecules). If the the conditions and the control of processes temperature of a substance is raised, by within a chemical plant. The instruments transferring energy to it, the internal en- can be classified into three groups: those ergy increases (the particles move faster). for current information using mercury Similarly, work done on or by a system re- thermometers, weighing scales, and pres- sults in an increase or decrease in the inter- sure gauges; those for recording viscosity, nal energy. The relationship between heat, fluid flow, pressure, and temperature; and work, and internal energy is given by the those instruments that control and main- first law of thermodynamics. Sometimes tain the desired conditions including pH the internal energy of a system is loosely and the flow of materials. spoken of as ‘heat’ or ‘heat energy’. Strictly, this is incorrect; heat is the trans- intercallation compound A com- fer of energy as a result of a temperature pound that has a structure based on layers difference. and in which there are layers of a different character interleaved in the basic structural internal resistance Resistance of a units. For example, the micas phlogopite source of electricity. In the case of a cell, (KMg3(OH)2Si3AlO10) and muscovite when a current is supplied, the potential (KAl2(OH)2Si3AlO10) are formed by inter- difference (p.d.) between the terminals is leaving K+ ions replacing a quarter of the lower than the e.m.f. The difference (i.e.

114 iodine e.m.f. – p.d.) is proportional to the current used to determine the presence of small supplied. The internal resistance (r) is given amounts of manganese in steel. by: r = (E – V)/I iodide See halide. where E is the e.m.f., V the potential difference between the terminals, and I the iodine A dark-violet volatile solid el- current. ement belonging to the halogens i.e. group 17 (formerly VIIA) of the periodic table, of interstitial See defect. which it is the fourth element. It occurs in seawater and is concentrated by various interstitial carbide See carbide. marine organisms in the form of iodides. Significant deposits also occur in the form interstitial compound A crystalline of iodates, i.e. salts containing iodine compound in which atoms of a nonmetal oxyanions. The element is conveniently (e.g. carbon, hydrogen, or boron) occupy prepared by the oxidation of iodides in interstitial positions in the crystal lattice of acid solution (using MnO2). Industrial a metal (usually a transition metal). Inter- methods similarly use oxidation of iodides stitial compounds are often nonstoichio- or reduction of iodates to iodides by sul- metric. Their physical properties are often fur(IV) oxide (sulfur dioxide) followed by similar to those of metals; e.g. they have a oxidation, depending on the source of the metallic luster and are electrical conduc- raw materials. Iodine and its compounds tors. are used in chemical synthesis, photography, pharmaceuticals, and dyestuffs manufacture. Trace amounts are essential in the interstitial hydrides See hydrogen. diet to ensure proper production of thyroid hormones. iodate See iodine. Iodine has the lowest electronegativity of the stable halogens and consequently is iodic acid See iodic(V) acid. the least reactive. It combines only slowly with hydrogen to form hydroiodic acid, iodic(V) acid (iodic acid; HIO ) A col- 3 HI. Iodine also combines directly with orless deliquescent crystalline solid pro- many electropositive elements, but does so duced by the reaction of concentrated much more slowly than does bromine or nitric acid with iodine. Iodic(V) acid is a chlorine. Because of the larger size of the strong oxidizing agent. It will liberate io- iodine ion and the consequent low lattice dine from solutions containing iodide ions energies, the iodides are generally more and it reacts vigorously with organic mate- soluble than related bromides or chlorides. rials, often producing flames. It dissociates As with the other halides, iodides of Ag(I), extensively in water and hence is a strong Cu(I), Hg(I), and Pb(II) are insoluble unless acid. complexing ions are present. Iodine also forms a range of covalent iodic(VII) acid (periodic acid; H5IO6) A iodides with the metalloids and nonmetal- white crystalline solid made by low-tem- lic elements, but these are generally less perature electrolysis of concentrated thermodynamically stable and are more iodic(V) acid. It exists in a number of readily hydrolyzed than chlorine or forms, the most common of which is bromine analogs. paraiodic(VII) acid. Iodic(VII) acid is a Four oxides are known, of which io- powerful oxidizing agent. It is a weak acid, dine(V) oxide, I2O5, is the most important. which – by cautious heating under vacuum The other oxides, I2O4, I4O9, and I2O7 are – can be converted to dimesoiodic(VII) much less stable and of uncertain structure. acid (H4I2O9), and metaiodic(VII) acid Like chlorine (but not bromine) iodine – – (HIO4). All three will oxidize manganese forms oxo-species based on IO , IO2 , – – to manganate(VII), reactions that can be IO3 , and IO4 . The chemistry of the other 115 iodine monochloride oxo-species in solution is complex. Ele- occur. This has the effect that electrical mental iodine reacts with alkalis in a simi- conductivities that are found experimen- lar way to bromine and chlorine. tally are lower than the expectations of the The ionization potential of iodine is suf- DEBYE–HÜCKEL THEORY. Ion association is a ficiently low for it to form a number of dynamic phenomenon in which there is compounds in which it is electropositive. It rapid exchange between the ions in the forms I+ cations, for example, by reaction electrolyte solution. A theory to take ion of solid silver nitrate with iodine solution. association into account, and hence im- Iodine also exhibits the interesting prop- prove on the Debye–Hückel theory, was erty of forming solutions that are violet proposed by Neils Bjerrum (1879–1958) in colored in nondonor type solvents, but 1926. Bjerrum found that ion association dark brown solutions in donor solvents, becomes more significant as the dielectric such as ethanol. Even though iodine solu- constant of the solvent becomes lower. tions were once common as antiseptic agents, the element is classified as toxic and ion exchange A process that takes place care should be taken to avoid eye intru- in certain insoluble materials, which con- sions or excessive skin contact. tain ions capable of exchanging with ions ° ° Symbol: I; m.p. 113.5 C; b.p. 184 C; having the same charge in the surrounding ° r.d. 4.93 (20 C); p.n. 53; most common ZEOLITES 127 medium. , the first ion exchange isotope I; r.a.m. 126.90447. materials, were used for water softening. These have largely been replaced by syn- (ICl) A dark red iodine monochloride thetic resins made of an inert backbone liquid made by passing chlorine over io- material, such as polyphenylethene, to dine. It has properties similar to those of its which ionic groups are weakly attached. If constituent halogens. Iodine monochloride the ions exchanged are positive, the resin is is used as a nonaqueous solvent and as an a CATIONIC RESIN. An ANIONIC RESIN ex- iodating agent in organic reactions. changes negative ions. When all available ions on the resin have been exchanged (e.g. iodine(V) oxide (iodine pentoxide; sodium ions replaced by calcium ions from I2O5) A white crystalline solid made by heating iodic(V) acid to a temperature of the medium) the material can usually be re- 200°C. It is a very strong oxidizing agent. generated by passing a concentrated solu- Iodine oxide is the acid anhydride of tion containing the original resin ion (e.g. iodic(V) acid, which is reformed when sodium chloride) through it. In the exam- water is added to the oxide. Its main use is ple cited, the calcium ions will be replaced in titration work, measuring traces of car- by sodium ions. Ion-exchange techniques bon monoxide in the air. are used for a vast range of purification and analytical purposes. iodine pentoxide See iodine(V) oxide. For example, solutions with ions that can be exchanged for OH– and H+ can be iodine trichloride See diiodine hexa- estimated by titrating the resulting solution chloride. with an acid or a base. ion An atom or molecule that has a neg- ionic bond See electrovalent bond. ative or positive charge as a result of losing or gaining one or more electrons. See elec- ionic carbide See carbide. trolysis; ionization. ionic crystal A crystal composed of ions ion association The association of pairs of two or more elements. The positive and of ions in electrolyte solutions occurring negative ions are arranged in definite pat- because of electrostatic attraction between terns and are held together by electrostatic the ions. Ion association means that com- attraction. Common examples are sodium plete dissociation of electrolytes does not chloride and cesium chloride.

116 iridium ionic product The product of ionic con- photons with sufficient energy to break up centrations in a solution. In the case of molecules or detach electrons from atoms: pure water the equilibrium constant is: A → A+ + e–. Negative ions can be formed + – KW = [H ][OH ] by capture of electrons by atoms or mol- as a result of a small amount of self-ioniza- ecules: A + e– → A–. tion. ionization energy See ionization poten- ionic radius A measure of the effective tial. radius of an ion in a compound. For an isolated ion, the concept is not very meaning- ionization potential (IP; Symbol: I) The ful, since the ion is a nucleus surrounded by energy required to remove an electron an ‘electron cloud’. Values of ionic radii from an atom (or molecule or group) in the can be assigned, however, based on the dis- gas phase, i.e. the energy required for the tances between ions in crystals. process: Different methods exist for determining M → M+ + e– ionic radii and often different values are It gives a measure of the ability of metals to quoted for the same ion. The two main form positive ions. The second ionization methods are those of Goldschmidt and of potential is the energy required to remove Pauling. The Goldschmidt radii are deter- two electrons and form a doubly charged mined by substituting data from one com- ion: pound to another, to produce a set of ionic M → M2+ + e– radii for different ions. The Pauling radii Ionization potentials stated in this way are assigned by a more theoretical treat- are positive; often they are given in elec- ment for apportioning the distance be- tronvolts. Ionization energy is the energy tween an anion and a cation. required to ionize one mole of the substance, and is usually stated in kilojoules ionic strength For an ionic solution a per mole (kJ mol–1). quantity can be introduced that empha- In chemistry, the terms ‘second’, ‘third’, sizes the charges of the ions present: etc., ionization potentials are usually used ½Σ 2 I = imiz i for the formation of doubly, triply, etc., where m is the molality and z the ionic charged ions. However, in spectroscopy charge. The summation is continued over and physics, they are often used with a dif- all the different ions in the solution, i. ferent meaning. The second ionization potential is the energy to remove the second ionization The process of producing least strongly bound electron in forming a ions. There are several ways in which ions singly charge ion. For lithium (ls22s1) it may be formed from atoms or molecules. would refer to removal of a 1s electron to In certain chemical reactions ionization oc- produce an excited ion with the configura- curs by transfer of electrons; for example, tion 1s12s1. Note also that ionization po- sodium atoms and chlorine atoms react to tentials are now stated as energies. form sodium chloride, which consists of sodium ions (Na+) and chloride ions (Cl–). ion pair A positive ion and a negative Certain molecules can ionize in solution; ion in close proximity in solution, held by acids, for example, form hydrogen ions as the attractive force between their charges. in the reaction See electrolysis. → + 2– H2SO4 2H + SO4 The ‘driving force’ for ionization in a IP See ionization potential. solution is solvation of the ions by molecules of the solvent. H+, for example, is IR See infrared. solvated as a hydroxonium (hydronium) + ion, H3O . iridium A hard brittle white transition Ions can also be produced by ionizing metal, the third element of group 9 (for- radiation; i.e. by the impact of particles or merly part of subgroup VIIIB) of the peri-

117 iron odic table. Iridium is found native and in give a green precipitate with sodium hy- association with platinum, to which it is droxide solution, whereas iron(III) ions chemically similar. Unusually high concen- give a brown precipitate. The concentra- trations of iridium are also found world- tion of iron(II) ions can be estimated in wide in clay bands associated with the acid solution by titration with standard boundary marking the end of the Creta- potassium manganate(VII) solution. ceous Period 65 million years ago. This Iron(III) ions must first be reduced to iridium is thought to have been deposited iron(II) ions with sulfur(IV) oxide (sulfur by the explosive impact of an ancient as- dioxide). teroid, which in turn may account for the Symbol: Fe; m.p. 1535°C; b.p. 2750°C; subsequent mass extinctions of many or- r.d. 7.874 (20°C); p.n. 26; most common ganisms, including the dinosaurs. isotope 56Fe; r.a.m. 55.845. Iridium is highly resistant to corrosion making it desirable for alloys for high-pre- iron(II) chloride (ferrous chloride; cision instruments and bearings. It is also FeCl2) A compound prepared by passing used in electrical contacts, in pen points, in dry hydrogen chloride gas over heated spark plugs, and in jewelry. iron. White feathery anhydrous crystals are Symbol: Ir; m.p. 2410°C; b.p. 4130°C; produced. Hydrated iron(II) chloride ° r.d. 22.56 (17 C); p.n. 77; most common (FeCl2.6H2O) is prepared by reacting ex- isotope 193Ir; r.a.m. 192.217. cess iron with dilute or concentrated hydrochloric acid. Green crystals of the iron A malleable ductile medium-hard hexahydrate are obtained on crystalliza- silvery-gray ferromagnetic transition el- tion from solution. Iron(II) chloride is ement, the first element of group 8 (for- readily soluble in water, producing an merly part of subgroup VIIIB) of the acidic solution due to salt hydrolysis. periodic table. Iron occurs in many ores, especially hematite (Fe2O3), magnetite iron(III) chloride (ferric chloride; (Fe3O4), siderite (FeCO3), and pyrite FeCl3) A compound prepared in the an- (FeS2). It is extracted in blast furnaces hydrous state as dark red crystals by pass- using coke, limestone, and hot air. The ing dry chlorine over heated iron. The coke and oxygen in the air form carbon product sublimes and is collected in a monoxide, which then reduces the iron in cooled receiver as brownish-black irides- the ore to the metal. The limestone recent scales. The hydrated salt moves acidic impurities and forms a layer (FeCl3.6H2O) is prepared by adding excess of slag above the molten iron at the base of iron(III) oxide to concentrated hydrochlo- the furnace. Carbon STEEL is formed by re- ric acid. On crystallization yellow-brown ducing the carbon content to between 0.1 crystals of the hexahydrate are formed. and 1.5%. Iron(III) chloride is very soluble in water Iron is used to form structural supports and undergoes salt hydrolysis. At tempera- for a wide variety of applications, includ- tures below 400°C the anhydrous salt ex- ing bridges, and as a catalyst in the Haber ists as a dimer, Fe2Cl6. process for ammonia production. Iron is also an essential element in the diet, be- iron chromium oxide See chromite. cause it is required to make hemoglobin, a component of red blood cells which ab- iron(II) oxide (ferrous oxide; FeO) A sorbs oxygen in the lungs and transports it black powder formed by the careful reduc- to the tissues. tion of iron(III) oxide using either carbon Iron corrodes in air and moisture to hy- monoxide or hydrogen. It can also be pre- drated iron(III) oxide (rust). pared by heating iron(II) oxalate in the ab- The most stable oxidation state is +3, sence of air. It is only really stable at high compounds of which are yellow or brown, temperatures and disproportionates slowly but +2 (green) and +6 (easily reduced) on cooling to give iron(III) oxide and iron. states also exist. Solutions of iron(II) ions Iron(II) oxide can be reduced by heating in

118 isomerism a stream of hydrogen. When exposed to Iron(II) sulfate crystals are isomor- air, it is oxidized to iron(III) oxide. It is a phous with the sulfates of zinc, magne- basic oxide, dissolving readily in dilute sium, nickel, and cobalt. acids to form iron(II) salt solutions. If heated to a high temperature in an inert at- iron(III) sulfate (ferric sulfate; Fe2(SO4)3) mosphere, iron(II) oxide disproportionates A compound prepared in the hydrated to give iron and triiron tetroxide (Fe3O4). state by the oxidation of iron(II) sulfate dissolved in dilute sulfuric acid using an iron(III) oxide (ferric oxide; Fe2O3)A oxidizing agent, such as hydrogen peroxide rusty-brown solid prepared by the action or concentrated nitric acid. On crystalliza- of heat on iron(III) hydroxide or iron(II) tion, the solution deposits a white mass of sulfate. It occurs in nature as the mineral the nonahydrate (Fe2(SO4)3.9H2O). The HEMATITE. Industrially it is obtained by anhydrous salt can be prepared by gently roasting iron pyrites. Iron(III) oxide dis- heating the hydrated salt. Iron(III) sulfate solves in dilute acids to produce solutions decomposes on heating to give iron(III) of iron(III) salts. It is stable at red heat, de- oxide and sulfur(VI) oxide. It forms alums composes around 1300°C to give triiron with the sulfates of the alkali metals. tetroxide (Fe O ), and can be reduced to 3 4 See reversible change. iron by hydrogen at 1000°C. Iron(III) irreversible change oxide is not ionic in character but has a irreversible reaction A reaction in structure similar to that of aluminum which conversion to products is complete; oxide. i.e. there is little or no back reaction. iron(II) sulfate (ferrous sulfate; green vit- isobars 1. Two or more nuclides that riol; FeSO .7H O) A compound that oc- 4 2 have the same NUCLEON NUMBERS but dif- curs in nature as the mineral melanterite ferent proton numbers. Actinium-89 and (or copperas). It is made industrially from thorium-90 are isobars. iron pyrites. In the laboratory iron(II) sul- 2. Lines on a chart or graph joining points fate is prepared by dissolving excess iron in of equal pressure. dilute sulfuric acid. On crystallization, green crystals of the heptahydrate are ob- isoelectronic Describing compounds tained. Careful heating of the hydrated salt that have the same number of electrons. yields anhydrous iron(II) sulfate; on fur- For example, carbon monoxide (CO) and ther heating the sulfate decomposes to give nitrogen (N2) are isoelectronic. iron(III) oxide, sulfur(IV) oxide, and sulfur(VI) oxide. The hydrated crystals oxi- isomer See isomerism. dize easily on exposure to air owing to the formation of basic iron(III) sulfate. A isomerism The existence of two or more freshly prepared solution of iron(II) sulfate chemical compounds with the same mo- absorbs nitrogen monoxide (BROWN-RING lecular formulae but different structural TEST). formulae or different spatial arrangements

Y X

Y M X Y M Y

X X

cis-isomer trans-isomer

Isomerism: isomers of a square-planar complex

119 isomorphism of atoms. The different forms are known as and the same groups differ only in the isomers. arrangement of the groups in space. There In structural isomerism the structural are various types of stereoisomerism. formulae of the compounds differ as to Cis-trans (or syn-anti) isomerism occurs molecular structure or to the position of when there is restricted rotation about a functional groups. For example, the com- bond between two atoms (e.g. a double pound C4H10 may be butane (with a bond or a bond in a ring). Groups attached straight chain of carbon atoms) or 2- to each atom may be on the same side of methyl propane (CH3CH(CH3)CH3, with the bond (the cis- or syn-isomer) or oppo- a branched chain). A particular case of this site sides (the trans- or anti-isomer). Cis- occurs in metal complexes when a ligand trans isomerism occurs in square-planar may coordinate to a metal ion in two ways. complexes of the type MX2Y2, where M is For example, NO2 can coordinate through a metal ion and X and Y are different lig- N (the nitro ligand) or through O (the ni- ands. If the X ligands are adjacent the iso- trido ligand). Such ligands are said to be mer is a cis-isomer; if they are opposite, it ambidentate. Complexes that differ only in is a trans-isomer. This type of isomerism the way in which the ligand coordinates are can also occur in octahedral complexes of said to show linkage isomerism. the type MX2Y4. Octahedral complexes of Stereoisomerism occurs when two com- the type MX3Y3 show a different type of pounds with the same molecular formulae stereoisomerism. If the three X ligands are

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

Y Y

Y X fac-isomer mer-isomer

Isomerism: isomers of octahedral complexes

120 isotopic weight in a plane that includes the M ion (with the characterized by its proton number). Iso- three Y ligands in a plane at right angles), topes of the same element have very similar then the structure is called the mer-isomer properties because they have the same elec- (‘mer’ stands for meridional). If the three X tron configuration, but differ slightly in (and Y) ligands are all on a face of the oc- their physical properties. An unstable iso- tahedron, the structure is the fac-isomer tope is termed a radioisotope or radio- (‘fac’ stands for facial). Cis-trans isomerism active isotope. was formerly called geometrical isomerism. Isotopes of elements are useful in chem- Optical isomerism occurs when the istry for studies of the mechanisms of compound has no plane of symmetry and chemical reactions. A standard technique is can exist in left- and right-handed forms to label one of the atoms in a molecule by that are mirror images of each other. See using an isotope of a component element, also optical activity. which is known as a tracer. It is then possible to follow the way in which this atom isomorphism The existence of com- behaves throughout the course of the reac- pounds with the same crystal structure. tion. In labeling, radioisotopes are detected by counters; stable isotopes can also be isomorphism, law of See Mitscher- used, and detected by a mass spectrum. lich’s law. Isotopes are also used in kinetic studies. For example, if the bond between two isotherm A line on a chart or graph join- atoms X–Y is broken in the rate-determining points of equal temperature. See also ing step, and Y is replaced by a heavier iso- isothermal change. tope of the element, Y*, the reaction rate will be slightly lower with the Y* present. isothermal change A process that takes This kinetic isotope effect is particularly place at a constant temperature. Through- noticeable with hydrogen and deuterium out an isothermal process, the system is in compounds, because of the large relative thermal equilibrium with its surroundings. difference in mass. For example, a cylinder of gas in contact with a constant-temperature bath may be isotope separation The separation of compressed slowly by a piston. The work different isotopes of a chemical element, done appears as energy, which flows into making use of the some differences in the the bath to keep the gas at the same tem- physical properties of these isotopes. For perature. Isothermal changes are con- small-scale isotope separation a MASS SPEC- trasted with adiabatic changes, in which no TROMETER is frequently used. For large- energy enters or leaves the system, and the scale isotope separation the methods used temperature changes. In practice no include diffusion in the gas phase (used for process is perfectly isothermal and none is uranium from the gas uranium hexafluo- perfectly adiabatic, although some can ap- ride), distillation (which was used to pro- proximate in behavior to one of these ideals. duce heavy water), electrolysis, the use of centrifuges, and the use of lasers, with ex- isotones Two or more nuclides that citation of one isotope followed by its sep- have the same neutron numbers but differ- aration using an electromagnetic field. ent proton numbers. isotopic mass (isotopic weight) The isotonic Describing solutions having the mass number of a given isotope of an el- same osmotic pressure. ement. isotopes Two or more species of the isotopic number The difference be- same element differing in their mass num- tween the number of neutrons in an atom bers because of differing numbers of neu- and the number of protons. trons in their nuclei. The nuclei must have the same number of protons (an element is isotopic weight See isotopic mass.

121 J

Jahn–Teller effect A distortion of mol- joliotium Symbol: Jl A name formerly ecules that occurs so as to prevent the mol- suggested for element-105, now known as ecule having degenerate valence orbitals. dubnium (Db). See element. The Jahn–Teller effect is observed in metal complexes. Some complexes that would be joule Symbol: J The SI derived unit of all expected to be regular octahedra are dis- forms of energy and work, equal to the en- torted octahedra because of the effect. It ergy required or work done when the point was predicted theoretically by H. A. Jahn of application of a force of one newton and Edward Teller in 1937. moves another object one meter in the direction of the force. 1 J = 1 N m. It is jeweler’s rouge Iron(III) oxide (Fe2O3), named for the British physicist James Joule used as a mild abrasive. (1818–89).

122 K

kainite A mineral form of hydrated crys- SI base unit of mass, equal to the mass of talline magnesium sulfate (MgSO4) con- the international prototype of the kilo- taining also some potassium chloride. It is gram, a cylinder consisting of a plat- used as a fertilizer and source of potassium inum–iridium alloy kept in a vault at Sèvres compounds. in France. The kilogram is the only SI base unit that to date has yet to be defined in kaolin See China clay. terms of physical constants. kaolinite A hydrated aluminosilicate kilogramme See kilogram. mineral, Al2(OH)4Si2O5, the major constituent of CHINA CLAY. It is formed by the kilowatt-hour Symbol: kWh An m.k.s. weathering of FELDSPARS from granites. system unit of electrical energy often used to to measure electrical power consump- katal Symbol: kat The SI derived unit of tion, equal to the energy transferred or catalytic activity, equal to the amount of work done by one kilowatt of power in one substance in moles that can be catalyzed hour. It has a value of 3.6 × 106 joules in SI per second. 1 kat = l mol s–1. units. kelvin Symbol: K The SI base unit of kinetic energy See energy. thermodynamic temperature. It is defined as the fraction 1/273.16 of the thermody- kinetic isotope effect See isotopes. namic temperature of the triple point of water. Zero kelvin (0 K) is absolute zero. kinetics A branch of physical chemistry An interval of one kelvin is the same as an concerned with the study of rates of chem- interval of one degree Celsius on the Cel- ical reactions and the effect of physical sius scale. It is named for British physicist conditions that influence the rate of reac- Baron William Thompson Kelvin (1824– tion, e.g. temperature, light, concentration, 1907), who devised the ABSOLUTE TEMPERA- etc. The measurement of these rates under TURE scale that now bears his name in different conditions gives information on 1848. the mechanism of the reaction, i.e. on the sequence of processes by which reactants kieselguhr See diatomite. are converted into products. killed spirits Zinc(II) chloride solution kinetic theory A theory explaining when used as a flux for solder. physical properties in terms of the motion of particles. The kinetic theory of gases as- kilo- Symbol: k A prefix used with SI sumes that the molecules or atoms of a gas units denoting 103. For example, 1 kilome- are in continuous random motion and the ter (km) = 103 meters (m). pressure (p) exerted on the walls of a containing vessel arises from the bombard- kilocalorie See calorie. ment by these fast moving particles. When the temperature is raised the speeds in- kilogram (kilogramme) Symbol: kg The crease; so consequently does the pressure.

123 Kipp’s apparatus

If more particles are introduced or the vol- ride with magnesium. Titanium can be ob- ume is reduced there are more particles to tained in this way: → bombard unit area of the walls and the TiCl4 + 2Mg 2MgCl2 + Ti pressure also increases. When a particle It is named for its inventor, Luxembourg collides with the wall it experiences a rate metallurgist William J. Kroll (1889–1973), of change of momentum, which is propor- who developed it in 1932. tional to the force exerted. For a large number of particles this provides a steady Kroto, Sir Harold Walter (1939– ) force per unit area (or pressure) on the wall. British chemist. Along with his Sussex Uni- Following certain additional assump- versity colleague David Walton, Kroto had tions, the kinetic theory leads to an expres- a long-standing interest in molecules con- sion for the pressure exerted by an ideal taining carbon chains linked by alternate gas. The assumptions are: triple and single bonds. Such chains with 1. The particles behave as if they are hard five, seven, and nine carbon atoms had smooth perfectly elastic points. been identified by radioastronomers in 2. They do not exert any appreciable force space. In 1984 Kroto heard that the Amer- on each other except during collisions. ican chemist Richard SMALLEY had devel- 3. The volume occupied by the particles oped new techniques involving laser themselves is a negligible fraction of the bombardment for the production of clus- volume of the gas. ters of atoms. In 1985 he visited Smalley in 4. The duration of each collision is negligi- Houston and persuaded him to direct his ble compared with the time between col- laser beam at a graphite target. Clusters of lisions. carbon atoms were indeed produced but, By considering the change in momen- more interesting than the small chains tum on impact with the walls it can be he was looking for, Kroto found a mass- shown that spectrum signal for a molecule of exactly p = ρc2/3 60 carbon atoms. The first suggestion was where ρ is the density of the gas and c is the that it had a sandwichlike graphite struc- root-mean-square speed of the molecules. ture. However, such a planar fragment The mean-square speed of the molecules is would have reactive carbon atoms at the proportional to the absolute temperature: edges, whereas C60 appeared to be stable. 2 Nmc = RT Kroto called C60 BUCKMINSTERFULLERENE. See also degrees of freedom. He shared the 1996 Nobel Prize for chemistry with Smalley and with Robert CURL. Kipp’s apparatus An apparatus for the production of a gas from the reaction of a krypton A colorless odorless mon- liquid on a solid. It consists of three globes, atomic gas, the fourth member of the rare- the upper globe being connected via a wide gases; i.e. group 18 (formerly VIIIA or 0) of tube to the lower globe. The upper globe is the periodic table. It occurs in minute the liquid reservoir. The middle globe con- quantities (0.001% by volume) in air, from tains the solid and also has a tap at which which it is recovered by fractional distilla- the gas may be drawn off. When the gas is tion. Krypton is used in fluorescent lights, drawn off the liquid rises from the lower lasers, and pneumatic heart valves. It is globe to enter the middle globe and reacts known to form unstable compounds with with the solid, thereby releasing more gas. fluorine. When turned off the gas released forces the Symbol: Kr; m.p. –156.55°C; b.p. liquid back down into the lower globe and –152.3°C; mass density 3.749 (0°C) kg up into the reservoir, thus stopping the re- m–3; p.n. 36; most common isotope 84Kr; action. It is named for Dutch chemist r.a.m. 83.80. Petrus Kipp (1808–84). kurchatovium Symbol: Ku A name for- Kroll process A method of obtaining merly used for element-104 now known as certain metals by reducing the metal chlo- rutherfordium (Rf). See element.

124 L

label See isotopes. ing Langmuir in 1916 using the kinetic theory of gases. labile Describing a complex having ligands that can easily be replaced by more lanthanide contraction The decrease strongly bonded ligands. in atomic and ionic radii in the series of lanthanoid elements from lanthanum to lake A pigment made by combining an lutetium as the atomic number increases. organic dyestuff with an inorganic com- This contraction occurs because the 4f pound (e.g. aluminum oxide). electrons in this series of elements are rather ineffective in shielding the outer lamellar compound A compound with electrons from the attraction of a positively a crystal structure composed of thin plates charged nucleus. In the case of the triposi- or layers. Silicates form many compounds tive ions the radius of the lanthanum ion is with distinct layers. Typical examples are 1.17Å and the radius of the lutetium ion is talc (Mg3(OH)2Si4O10) and pyrophyllite 1.00Å. A similar effect, the ACTINOID CON- (Al (OH) Si O ). See also intercallation 2 2 4 10 TRACTION, occurs for 5f electrons. compound. lanthanides See lanthanoids. lamp black A black pigment; a finely divided form of carbon formed by incom- lanthanoids (lanthanides; lanthanons; plete combustion of an organic compound. rare earths) A group of 15 silvery, reactive metals whose electronic configurations Langmuir, Irving (1881–1957) Ameri- display back-filling of the 4f level. There is can chemist. In 1919 Langmuir extended a maximum of 14 electrons in an f-orbital. the model of electrons in atoms proposed The element lanthanum itself has no f-elec- by Gilbert LEWIS to try to account for the 1 2 chemical valence of all atoms. In the 1920s trons (La [Xe]5d 6s ) and is thus strictly he investigated the chemistry of surfaces not a lanthanoid, but it is included by con- very extensively. He won the 1932 Nobel vention, thus giving a closely related series Prize for chemistry for his work on chemi- of 15 elements. Excluding lanthanum, the x 2 cal reactions at surfaces. elements all have 4f 6s configurations, although gadolinium and lutecium have an 1 Langmuir isotherm An equation used additional 5d electron. to describe the adsorption of a gas onto a The characteristic oxidation state is 3+ plane surface at a fixed temperature. It can M and the great similarity in the size of be written in the form: the ions leads to a very close similarity of θ = bp/(1 + bp), chemical properties and hence to great dif- where θ is the fraction of the surface cov- ficulties of separation using conventional ered by the gas, p is the gas pressure and b methods. In addition, cerium can assume a is a constant known as the adsorption co- Ce4+ state and ytterbium a Yb2+ state. efficient, which is the equilibrium constant Chromatographic and solvent-extraction for the adsorption process. This equation methods have been specially developed for was derived by the American chemist Irv- the lanthanoids.

125 lanthanons lanthanons See lanthanoids. heat absorbed when a substance changes from a solid to a liquid. lanthanum A soft ductile malleable silvery highly reactive metallic element of laterite A red fine-grained type of clay group 3 (formerly IIIB) of the periodic formed in tropical climates by the weather- table. It is usually considered to be the first ing of igneous rocks. Its color comes from member of the LANTHANOID series. It is the presence of iron(III) hydroxide. found associated with other lanthanoids in many minerals, including monazite and lattice A regular three-dimensional bastnaesite. Lanthanum is used in several arrangement of points. A lattice is used to alloys, especially those for lighter flints, be- describe the positions of the particles cause it sparks or ignites readily when (atoms, ions, or molecules) in a crystalline scratched. It is also used as a catalyst, and solid. The lattice structure can be exam- in making optical glass to which it imparts ined by x-ray diffraction techniques. enhanced refractive properties. Symbol: La; m.p. 921°C; b.p. 3457°C; lattice energy The energy released when r.d. 6.145 (25°C); p.n. 57; most stable iso- ions of opposite charge are brought to- tope 139La; r.a.m. 138.9055. gether from infinity to form one mole of a given crystal. The lattice energy is a meas- lapis lazuli See lazurite. ure of the stability of a solid ionic substance, with respect to ions in the gas. See laser An acronym for Light Amplifica- also Born–Haber cycle. tion by Stimulated Emission of Radiation. The vibrations of the A laser device produces high-intensity lattice vibrations atoms or ions in a crystal lattice about their monochromatic coherent beams of light. In equilibrium positions. Such vibrations the laser process the molecules of a sample occur even at the absolute zero tempera- (such as ruby doped with Cr3+ ions) are ture because of zero point energy. At low promoted to an excited state. Because the temperatures the vibrations are well de- sample is in a cavity between two reflective scribed by the simple harmonic motion of surfaces, when a molecule emits sponta- the atoms. As the temperature is increased neously, the photon so generated ricochets the anharmonicity of the vibrations be- backward and forward. In this way other comes more pronounced. molecules are stimulated to emit photons of the same energy. If one of the reflective laughing gas See dinitrogen oxide. surfaces is partially transmitting this radiation can be tapped. Lavoisier, Antoine Laurent (1743– 94) French chemist. Lavoisier is frequently laser spectroscopy Spectroscopy that referred to as the founder of modern chem- uses lasers. The special features of lasers, istry. Perhaps his most significant contri- notably their ability to produce beams of bution was to peform careful quantitative coherent monochromatic radiation, have experiments that disproved the PHLOGIS- several important advantages over other TON THEORY of combustion. This led him spectroscopic techniques. For example, to establish that oxygen is one of the gases spectroscopy that makes use of the RAMAN present in air. He also noticed the presence EFFECT has greatly benefitted from the ap- of an inert gas in air, which was subse- plication of lasers because there are major quently named nitrogen. He summarized experimental problems with other sources his work in the influential book Elemen- of light. tary Treatise on Chemistry, which stated the law of mass conservation in chemical latent heat The heat evolved or ab- reactions. Lavoisier, who had been a tax sorbed when a substance changes its phys- farmer, was executed in 1794, in the after- ical state, e.g. the latent heat of fusion is the math of the French Revolution.

126 lead law of conservation of energy See precious gemstones. Lazurite was the orig- conservation of energy. inal source of the pigment ultramarine. law of conservation of mass See con- LCAO (linear combination of atomic servation of mass. orbitals) See orbital. law of constant composition See con- leaching The washing out of a soluble stant proportions. material from an insoluble solid by use of a solvent. This process is often carried out in law of constant heat summation See batch tanks or by dispersing the crushed Hess’s law. solid in a liquid. law of constant proportions See con- lead A dense dull gray soft metallic toxic stant proportions. element; the fifth member of group 14 (formerly IVA) of the periodic table. It is the law of definite proportions See con- end product of most radioactive decay se- stant proportions. ries. It occurs in small quantities in a wide variety of minerals but only a few are eco- law of equivalent proportions See nomically important. The most important equivalent proportions. of these is galena (PbS), found in Australia, Mexico, the USA, and Canada. Other eco- law of isomorphism See Mitscherlich’s nomically important minerals are anglesite law. (PbSO4), litharge (PbO), and cerussite (PbCO3). law of mass action See mass action, Galena is often associated with zinc ores and the smelting operations of the lead law of. and zinc industries are thus closely integrated. Lead ore from galena is first con- law of octaves See Newlands’ law. centrated by froth flotation and the concentrate then roasted and reduced as law of reciprocal proportions See follows: equivalent proportions. → 2PbS + 3O2 2PbO + 2SO2 PbS + 2O → PbSO A radioactive synthetic 2 4 lawrencium 2PbO + 2C → 2Pb + 2CO transuranic element of the actinoid series → 2PbO + PbS 3Pb + SO2 of elements, first made in 1961 by bom- → PbSO4 + 2C Pb + 2CO + SO2 barding californium-252 targets with Additionally, various silver sulfides are boron nuclei. It can also be made by bom- often found in small quantities with barding berkelium-249 targets with oxy- galena, meaning that silver is often recov- gen-18 nuclei. Several very short-lived ered in economic quantities with crude isotopes have been synthesized. lead. Symbol: Lr; p.n. 103; m.p., b.p., and The outer s2p2 configurations of both r.d. unknown; most stable isotope 262Lr tin and lead give these metals similar prop- (half-life 261 minutes). erties. There is, however, a much greater predominance of the divalent state with laws of chemical combination See lead. Both lead oxides are amphoteric, chemical combination; laws of. lead(II) oxide (PbO) leading to plumbites and lead(IV) oxide (PbO2) to plumbates on lazurite An uncommon typically azure dissolution in alkalis. Lead also forms blue mineral consisting of a silicate of mixed oxides Pb2O3 (a yellow solid, better sodium, calcium, and aluminum with some written as Pb(II)Pb(IV)O3) and Pb3O4 (a sulfur. It and its parent rock, lapis lazuli, red powder, better written as Pb2(II)- are widely used for ornaments and as semi- Pb(IV)O4). Both lead and tin have low 127 lead(II) acetate melting points and neither is attacked by The electrolyte contains hydrogen ions + 2– dilute acids; they differ however in their re- (H ) and sulfate ions (SO4 ). During dis- action with concentrated nitric acid, with charge, H+ ions react with the lead(IV) lead reacting as follows: oxide to give lead(II) oxide and water → + – → 3Pb + 8HNO3 3Pb(NO3)2 + 2NO + PbO2 + 2H + 2e PbO + H2O 4H2O This reaction takes electrons from the Tin, on the other hand, gives hydrated lead(IV) oxide plates, causing the positive Sn(IV) oxide. Concentrated hydrochloric charge. A further reaction follows, which acid will not attack lead. yields soft lead sulfate: 2– + → Lead, like tin, forms halides, PbX2, with PbO + SO4 + 2H PbSO4 + H2O + – all the halogens. Lead(IV) chloride (PbCl4) 2e is the only known lead(IV) halide, whereas Electrons are thus released to the negative all halides of SnX4 are known. The great electrode, producing the negative charge. stability of the Pb(II) state leads to Pb(IV) During charging the reactions are reversed: – → 2– compounds being powerful oxidizing PbSO4 + 2e Pb + SO4 → + 2– agents. PbSO4 + 2H2O PbO2 + 4H + SO4 Lead is used in lead–acid storage bat- + 2e– teries, alloys, radiation shielding, and water and sound proofing. It is also used in lead(II) carbonate (PbCO3) A white the petrochemical, munitions, paint, and poisonous powder that occurs naturally as glass industries, although to a far lesser ex- the mineral CERUSSITE. It forms rhombic tent than previously, due to the toxic na- crystals and can be precipitated by reacting ture of the metal and its compounds. a cold aqueous solution of a soluble Symbol: Pb; m.p. 327.5°C; b.p. lead(II) salt (e.g. lead(II) nitrate) with am- 1830°C; r.d. 11.35 (20°C); p.n. 82; most monium carbonate. It decomposes to common isotope 208Pb; r.a.m. 207.2. lead(II) oxide and carbon dioxide above 315°C. lead(II) acetate See lead(II) ethanoate. lead(II) carbonate hydroxide (white lead–acid battery A type of battery lead; 2PbCO3.Pb(OH)2) The most im- used in vehicles. It has two sets of plates: portant basic lead carbonate. It is manu- spongy lead plates connected in series to factured electrolytically using high-purity the negative terminal, and lead(IV) oxide lead anodes. It was once widely used as a plates connected to the positive terminal. pigment in white and colored paints and in The material of the electrodes is held in a ceramics. Because the compound is poiso- hard lead-alloy grid. The plates are inter- nous, its use for these purposes has been leaved. The electrolyte is dilute sulfuric largely discontinued. acid. The e.m.f. of each cell when fully lead-chamber process A process for- charged is about 2.2 volts (V). This value merly used to manufacture sulfuric acid by falls to a steady 2 V when current is drawn; oxidizing sulfur(IV) oxide with nitrogen a typical battery has six cells, giving an monoxide. The reaction, which was car- overall voltage of about 12 volts. As the ried out in expensive large lead chambers, battery begins to run down, the e.m.f. falls has now been replaced by the CONTACT further. During discharge the electrolyte PROCESS. becomes more dilute and its relative density falls. To recharge the battery, current is lead dioxide See lead(IV) oxide. passed through it in the opposite direction to the direction of current supply. This lead(II) ethanoate (lead(II) acetate; process reverses the cell reactions and in- Pb(CH3CO2)2) A poisonous compound creases the relative density of the elec- usually occurring as the hydrate trolyte, which should be about 1.26–1.29 Pb(CH3CO2)2.3H2O, forming monoclinic for a fully charged battery. crystals. At 100°C it loses ethanoic acid

128 Le Chatelier’s principle and water, forming a basic lead(II) ble sulfide. Lead(II) sulfide has the capacity ethanoate. It is extremely soluble in water to rectify alternating current. – 50 g per 100 g of water at 25°C. Lead(II) ethanoate can be obtained as an anhydrous lead tetraethyl (tetraethyl lead; salt. Its chief asset is that it is one of the few Pb(CH2CH3)4) A poisonous liquid that is common lead(II) salts that are soluble in insoluble in water but soluble in organic water. It has been used to make varnishes solvents. It is manufactured by the reaction and enamels. of an alloy of sodium and lead with chloroethane (CH2ClCH3). The product is lead(IV) hydride See plumbane. obtained by steam distillation. Lead tetraethyl was once used as an additive in lead monoxide See lead(II) oxide. internal-combustion engine fuel to increase its octane number and thus prevent preig- lead(II) oxide (lead monoxide; PbO) A nition (knocking). Lead-free compounds yellow crystalline poisonous powder are now prescribed or encouraged for this formed by roasting molten lead in air. purpose in many countries. Litharge, the most common form, is obtained when lead(II) oxide is heated above Leblanc process An obsolete process its melting point. If prepared below its for the manufacture of sodium carbonate. melting point, another form, called massi- Sodium chloride is converted to sodium cot, is obtained. Litharge is used in the rub- sulfate by heating with sulfuric acid. This ber industry, in the manufacture of paints sulfate is then roasted in a rotary furnace and varnishes, and in the manufacture of where it is first reduced to the sulfide using lead glazes for pottery. carbon, and then immediately converted to the carbonate by the action of limestone. lead(IV) oxide (lead dioxide; PbO2)A Sodium carbonate solution is obtained by poisonous dark-brown solid forming leaching the product with water and this hexagonal crystals. On heating to 310°C it solution is subsequently dried and calcined decomposes into lead(II) oxide and oxy- to obtain the solid. The process was in- gen. It can be prepared electrolytically or vented by French chemist Nicolas LeBlanc by reacting lead(II) oxide with potassium (1742–1806) in 1783. The Solvay process chlorate. Lead(IV) oxide is an oxidizing is now used instead. agent and has been used in the manufacture of matches. Le Chatelier’s principle If a system is at equilibrium and a change is made in the lead(II) sulfate (PbSO4) A poisonous conditions, the equilibrium adjusts so as to white crystalline solid that occurs naturally oppose the change. The principle was first as the mineral anglesite. It is almost insolu- stated by French chemist Henri Le Chate- ble in water and can be precipitated by re- lier (1850–1936) in 1888 and can be ap- acting an aqueous solution containing plied to the effect of temperature and sulfate ions with a solution of a soluble pressure on chemical reactions. A good ex- lead(II) salt (e.g. lead(II) ethanoate). It ample is the Haber process for synthesis of forms basic lead(II) sulfates when shaken ammonia: ˆ together with lead(II) hydroxide and N2 + 3H2 2NH3 water. Due to their toxicity, basic lead(II) The ‘forward’ reaction → sulfates have been largely discontinued in N2 + 3H2 2NH3 their former use as pigments. is exothermic. Thus, reducing the temperature displaces the equilibrium toward pro- lead(II) sulfide (PbS) A black solid that duction of NH3 (because this tends to occurs naturally as the mineral galena. It increase temperature). Increasing the pres- can be prepared as a precipitate by reacting sure also favors formation of NH3, because a solution of a soluble lead(II) salt with hy- this leads to a reduction in the total num- drogen sulfide or with a solution of a solu- ber of molecules (and hence pressure).

129 Leclanché cell

Leclanché cell A primary voltaic cell levorotatory See optical activity. consisting, in its ‘wet’ form, of a carbon- rod anode and a zinc cathode, with a Lewis, Gilbert Newton (1875–1946) 10–20% solution of ammonium chloride American physical chemist. Lewis pro- as electrolyte. Manganese(IV) oxide mixed posed the idea that valence and bonding of with crushed carbon in a porous bag or pot atoms is associated with electrons. He first surrounding the anode acts as a depolariz- put forward this idea in 1916, introducing ing agent. The dry form (dry cell) is widely the concept of a covalent bond in which the used for flashlight batteries, transistor ra- bonding between two atoms occurs be- dios, etc. It has a mixture of ammonium cause the atoms share a pair of electrons, chloride, zinc chloride, flour, and gum with each atom supplying one electron. He forming an electrolyte paste. Sometimes also suggested that the outer electrons in the dry cell is arranged in layers to form a atoms form octets. He developed his ideas rectangular battery, which has a longer life more fully in the book Valence and the than the cylinder type. Structure of Atoms and Molecules (1923). Lewis also performed important work in LEED (low-energy electron diffraction) chemical thermodynamics, with his book A method of electron diffraction that is with Merle Randall entitled Thermody- used to investigate surfaces. A surface is namics and the Free Energy of Chemical bombarded with a beam of low-energy Substances (1923) being very influential. electrons, with the diffracted electrons hit- ting a fluorescent screen. The electron Lewis acid A substance that can accept beam has a low energy so that it interacts an electron pair to form a coordinate bond; with the surface rather than the bulk of the a Lewis base is an electron-pair donor. In material. The technique gives a great deal this model, the neutralization reaction is of useful information about surfaces and seen as the acquisition of a stable octet by processes occurring at surfaces. the acid, for example: → Cl3B + :NH3 Cl3B:NH3 Lennard-Jones, Sir John Edward Metal ions in coordination compounds (1894–1954) British chemist. Lennard- are also electron-pair acceptors and there- Jones was a theoretical chemist who did a fore Lewis acids. The definition includes great deal of work on intermolecular forces. the ‘traditional’ Brønsted acids since H+ is Starting in the late 1920s he helped develop an electron acceptor, but in common usage and promote molecular orbital theory. In the terms Lewis acid and Lewis base are re- particular, in 1929 he showed that the served for systems without acidic hydrogen theory readily explains the paramagnetism atoms. of molecular oxygen. He subsequently showed how molecular orbital theory can Lewis base See Lewis acid. also explain the directionability of chemical bonds. Lewis formula See Lewis structure.

Lennard-Jones potential An expres- Lewis octet theory See octet. sion used to represent the potential energy V of intermolecular interactions as a func- Lewis structure (Lewis formula) A tion of the intermolecular distance r. V can two-dimensional depiction, by means of be written in the form V = –A/r6 + B/r12, chemical element and electron dot sym- where A and B are constants. The Lennard- bols, of the possible structure of a molecule Jones potential has been used extensively or ion, showing each atom in relation to its to study intermolecular interactions. It was neighbors, the bonds that hold the atoms proposed by the British theoretical chemist together, and the lone pairs in each atom’s Sir John Lennard-Jones in 1924. outer shell. levo-form See optical activity. L-form See optical activity.

130 liquid crystal

Liebig condenser A simple type of lab- Linde process A method of liquefying oratory condenser. It consists of a straight gases by compression followed by expan- glass tube, in which the vapor is con- sion through a nozzle. The process is used densed, with a surrounding glass jacket for producing liquid oxygen and nitrogen through which cooling water flows. It is by liquefying air and then fractionating it. named for German chemist Justus von Liebig (1803–73). line defect See defect. ligand See complex; chelate. line spectrum A SPECTRUM composed of a number of discrete lines corresponding to ligand-field theory A theory that de- single wavelengths of emitted or absorbed scribes the properties of complexes. Lig- radiation. Line spectra are produced by and-field theory is an extension of CRYSTAL atoms or simple (monatomic) ions in gases. FIELD THEORY in which covalent bonding Each line corresponds to a change in elec- between the central atom or ion and the tron orbit, with emission or absorption of ligands is taken into account. radiation. light (visible radiation) A form of elec- linkage isomerism See isomerism. tromagnetic radiation able to be detected by the human eye. Its wavelength range is Lipscomb, William Nunn (1919– ) between about 400 nanometers (nm) (far American inorganic chemist. Lipscomb is red) and about 700 nm (far violet). The noted for his work on boranes – hydrides boundaries are not precise, because indi- of boron first investigated by Alfred STOCK viduals vary in their ability to detect ex- in the early part of the 20th century. Using treme wavelengths; this ability also low-temperature x-ray diffraction analysis, declines with age. Lipscomb tackled the problem of investigating the notoriously unstable boranes, lignite (brown coal) The poorest grade producing evidence of some remarkable of coal, containing up to 60% carbon and structures, totally original and completely having a high moisture content. unsuspected by earlier chemists. The basic concept of a multi-center bond was derived lime See calcium hydroxide; calcium from a structure for diborane proposed by oxide. LONGUET-HIGGINS. Lipscomb was awarded the Nobel Prize for chemistry in 1976. limestone A natural form of CALCIUM CARBONATE. It is used in making calcium liquid The state of matter in which the compounds, carbon dioxide, and cement. particles of a substance are loosely bound by intermolecular forces. The weakness of limewater A solution of calcium hy- these forces permits movement of the par- droxide in water. If carbon dioxide is bub- ticles and consequently liquids can change bled through lime water a milky precipitate their shape within a fixed volume. The liq- of calcium carbonate forms. Prolonged uid state lacks the order of the solid state. bubbling of carbon dioxide turns the solu- Thus, amorphous materials, such as glass, tion clear again as a result of the formation in which the particles are disordered and of soluble calcium hydrogencarbonate can move relative to each other, can be (Ca(HCO3)2). classed as liquids. limonite (bog iron ore) A geological liquid air A pale blue liquid. It is a mix- field term for a group of yellowish-brown ture of mainly liquid oxygen (boiling at to dark-colored minerals consisting mainly –183°C) and liquid nitrogen (boiling at of iron(III) hydroxide and hydrous oxides –196°C). of iron. Limonite is a major iron ore. It is also used as a pigment. liquid crystal A substance that can flow

131 liter like a viscous liquid but has a considerable its bonds. Lithium also has the highest ion- amount of molecular order. There are ization potential of the alkali metals. three types of liquid crystal. In a smectic The element reacts with hydrogen to liquid crystal there are layers of aligned form lithium hydride (LiH) a white high- molecules, with the long axes of the mol- melting solid that releases hydrogen at the ecules being perpendicular to the layers. In anode during electrolysis (confirming the a cholesteric liquid crystal there are also ionic nature Li+H–). Lithium reacts with layers of aligned molecules, but with the oxygen to give Li2O (sodium gives the per- axes of the molecules being parallel to the oxide) and with nitrogen to form Li3N on planes of the layers. In a nematic liquid fairly gentle warming. The metal itself re- crystal the molecules are all aligned in the acts only slowly with water, giving the hy- same direction but not arranged in layers. droxide (LiOH) but lithium oxide reacts much more vigorously to give again the hy- liter Symbol: L or l A metric unit of vol- droxide; the nitride is hydrolyzed to am- ume equal in SI units to one cubic decime- monia. With halogens the metal reacts to ter (dm3) or 10–3 meter3, or in c.g.s. units to form halides (LiX). 1000 cm3. One milliliter (mL or ml) is thus Apart from the fluoride the halides are the same volume as one cubic centimeter readily soluble both in water and in oxy- (cm3). However, the liter is not recom- gen-containing organic solvents. In this mended for precise measurements since it property lithium partly resembles magne- was formerly defined as the volume of one sium, which has a similar charge/size ratio. Compared to the other carbonates of kilogram of pure water at 4°C and stan- group 1, lithium carbonate (Li CO ) is dard pressure. By this older definition, one 2 3 3 thermally unstable, decomposing to Li2O liter was the same as 1000.028 cm . + and CO2. This is because the small Li ion leads to particularly high lattice energies, litharge See lead(II) oxide. favouring the formation of Li2O. Lithium compounds impart a characteristic purple lithia See lithium oxide. color to flames. Lithium is used to make alloys for air- lithia water See lithium hydrogencar- craft parts and other components where bonate. lightness is an important quality. It is also used to make lithium batteries and lithium lithium A light silvery moderately reac- grease, and to scavenge oxygen in metal- tive alkali metal; the first element of group lurgy. Certain lithium salts are used to treat 1 (formerly IA) of the periodic table. It oc- depression. Lithium is also used to certain curs in a number of complex silicates, such glasses and ceramic glazes. as spodumene, lepidolite, and petalite, and Symbol: Li; m.p. 180.54°C; b.p. a mixed phosphate, tryphilite. It is a rare 1347°C; r.d. 0.534 (20°C); p.n. 3; most element, accounting for 0.0065% of the common isotope 7Li; r.a.m. 6.941. Earth’s crust, and the lightest of the metals. Lithium ores are treated with concentrated lithium aluminum hydride See lithium sulfuric acid and lithium sulfate subse- tetrahydroaluminate(III). quently separated by crystallization. The element can be obtained by conversion to lithium carbonate (Li2CO3) A white the chloride and electrolysis of the fused solid obtained by the addition of excess chloride. sodium carbonate solution to a solution of Lithium has the electronic configura- a lithium salt. It is soluble in the presence tion 1s22s1 and is entirely monovalent in its of excess carbon dioxide, forming lithium chemistry. The lithium ion is, however, hydrogencarbonate. If heated (to 780°C) in much smaller than the ions of the other al- a stream of hydrogen it undergoes thermal kali metals; consequently it is polarizing decomposition to give lithium oxide and and a certain degree of covalence occurs in carbon dioxide. Lithium carbonate forms

132 localized bond monoclinic crystals and differs from the closer resemblance to the group 2 hydrox- other group 1 carbonates in being only ides than to the group 1 hydroxides. It is sparingly soluble in water. It has a close re- used in lubricants and batteries, and to ab- semblance to the carbonates of magnesium sorb carbon dioxide. and calcium – an example of a diagonal relationship in the periodic table. Lithium lithium oxide (lithia; Li2O) A white carbonate is used in the preparation of solid produced by burning metallic lithium other lithium compounds and in the treat- in air above its melting point. It can also be ment of depression. produced by the thermal decomposition of lithium carbonate or hydroxide. Lithium lithium chloride (LiCl) A white solid oxide reacts slowly with water to form a produced by dissolving lithium carbonate solution of lithium hydroxide; the reaction or oxide in dilute hydrochloric acid and is exothermic. The compound has a cal- crystallizing the product. Below 19°C the cium-fluoride structure. Lithium oxide is dihydrate (LiCl.2H2O) is obtained; at used in lubricants, glass, ceramics, and 19°C the chloride loses one molecule of welding and brazing fluxes. water. It becomes anhydrous at 93.5°C. The compound forms cubic crystals; the lithium sulfate (Li2SO4) A white solid anhydrous salt is isomorphous with prepared by the addition of excess lithium sodium chloride. Lithium chloride is prob- oxide or carbonate to a solution of sulfuric ably the most deliquescent substance acid. It is readily soluble in water from known and forms hydrates with 1, 2, and 3 which it crystallizes as the monohydrate. In molecules of water. It is used as a flux in contrast to the other group 1 sulfates it welding aluminum. does not form alums and it is not isomorphous with these other sulfates. lithium hydride (LiH) A white crystalline solid produced by direct combina- lithium tetrahydroaluminate(III) (lith- tion of the elements at a temperature above ium aluminum hydride; LiAlH4) A white 500°C. Lithium hydride has a cubic struc- solid produced by the action of lithium hy- ture and is more stable than the other dride on aluminum chloride, the hydride group 1 hydrides. Electrolysis of the fused being in excess. Lithium tetrahydroalumi- salt yields hydrogen at the anode. Lithium nate reacts violently with water, releasing hydride reacts violently with water to give hydrogen. It is a powerful reducing agent, lithium hydroxide and hydrogen; the reac- much used in organic chemistry. In inor- tion is highly exothermic. The compound ganic chemistry it is used in the preparation is used as a reducing agent and in the of hydrides. preparation of other hydrides. litmus A natural pigment that changes lithium hydrogencarbonate (LiHCO3) color when in contact with acids and alka- A compound known only in solution, pro- lis; above a pH of 8.3 it is blue and below duced by the reaction between carbon a pH of 4.5 it is red. Thus it gives a rough dioxide, water, and lithium carbonate. indication of the acidity or basicity of a so- When a solution of the hydrogencarbonate lution; because of the rather broad range is heated, carbon dioxide and lithium car- over which it changes it is not used for pre- bonate are produced. Solutions of the hy- cise work. Litmus is used both in solution drogencarbonate are sold and used in and as litmus paper. medicine under the name of lithia water. lixiviation The process of separating lithium hydroxide (LiOH) A white soluble components from a mixture by solid made industrially as the monohydrate washing them out with water. (LiOH.H2O) by reacting calcium hydroxide with a lithium ore or with a salt made localized bond A molecular bond in from the ore. Lithium hydroxide has a which the electrons contributing to the

133 lodestone bond remain between the two atoms con- Charles COULSON on conjugated molecules. cerned, i.e. the bonding orbital is localized. Longuet-Higgins also applied statistical The majority of bonds are of this type. mechanics to chemical problems, particu- Compare delocalized bond. larly mixtures and polymer solutions. lodestone A naturally occurring mag- Lotka–Volterra mechanism A possi- netic oxide of iron, a form of MAGNETITE ble mechanism for OSCILLATING REACTIONS. (Fe3O4). A piece of lodestone is a natural A reactant R is converted into a product P. magnet and lodestones were used as mag- The chemical reaction is in a steady state netic compasses in ancient times. See mag- but is not in chemical equilibrium. There netite. are three steps in this mechanism: R + X → 2X, lone pair A pair of valence electrons X + Y → 2Y, having opposite spin that are located to- Y → P. gether on one atom, i.e. are not shared as Autocatalysis is involved in the first two in a covalent bond. Lone pairs occupy sim- steps of this process. It appears that oscil- ilar positions in space to bond pairs and lating chemical reactions have mechanisms account for the shapes of molecules. A that are different from the Lotka–Volterra molecule with a lone pair can donate this mechanism. This type of mechanism does pair to an electron acceptor, such as H+ or occur in certain types of complex system a metal ion, to form coordinate bonds. See such as predator–prey relationships in biol- complex. ogy. It was in the biological context that the mechanism was investigated by the Ital- long period See period. ian mathematician Vito Volterra (1860– 1940). Longuet-Higgins, Hugh Christopher (1923– ) British theoretical chemist. One lowering of vapor pressure A colliga- of his first contributions was to explain the tive property of solutions in which the properties of boron hydrides in terms of vapor pressure of a solvent is lowered as a bridged structures. In the second half of the solute is introduced. When both solvent 1940s he performed calculations with and solute are volatile the effect of increas-

C N

H H H H H H 109.5° 107°

H H Methane Ammonia

O S

F H 104.5° H F Water Sulfur tetrafluoride

Lone pair

134 lyophobic ing the solute concentration is to lower the ciation with other lanthanoids, especially partial vapor pressure of each component. in the mineral monazite. Lutetium is the When the solute is a solid of negligible rarest of the naturally occurring elements. vapor pressure the lowering of the vapor It is used as a catlyst. pressure of the solution is directly propor- Symbol: Lu; m.p. 1663°C; b.p. 3395°C; tional to the number of species introduced r.d. 9.84 (25°C); p.n. 71; most common rather than to their nature and the propor- isotope 175Lu; r.a.m. 174.967. tionality constant is regarded as a general solvent property. Thus the introduction of lux Symbol: lx The SI derived unit of il- the same number of moles of any solute lumination, equal to the illumination pro- causes the same lowering of vapor pres- duced by a luminous flux of one lumen sure, if dissociation does not occur. If the falling on a surface of one square meter. 1 solute dissociates into two species on dis- lx = 1 lm m–2. solution the effect is doubled. The kinetic model for the lowering of vapor pressure Lyman series See hydrogen atom spec- treats the solute molecules as occupying trum. part of the surface of the liquid phase and thereby restricting the escape of solvent lyophilic Solvent attracting. When the molecules. The effect can be used in the solvent is water, the word hydrophilic is measurement of relative molecular masses, often used. The terms are applied to: particularly for large molecules, such as 1. Ions or groups on a molecule. In aque- polymers. See also depression of freezing ous or other polar solutions ions or point; Raoult’s law. polar groups are lyophilic. For example, the –COO– group on a soap is the Lowry–Brønsted theory See acid; base. lyophilic (hydrophilic) part of the molecule. lumen Symbol: lm The SI derived unit of 2. The disperse phase in colloids. In luminous flux, equal to the luminous flux emitted in a solid angle of one steradian lyophilic colloids the dispersed particles (Sr) by a point source of one candela (cd), have an affinity for the solvent, and the radiating equally in all directions. 1 lm = 1 colloids are generally stable. Compare cd sr. lyophobic. luminescence The emission of radiation lyophobic Solvent repelling. When the from a substance in which the particles solvent is water, the word hydrophobic is have absorbed energy and gone into used. The terms are applied to: excited states. They then return to lower 1. Ions or groups on a molecule. In aque- energy states with the emission of electro- ous or other polar solvents, the lyopho- magnetic radiation. If the luminescence bic group is nonpolar. For example, the persists after the source of excitation is re- hydrocarbon group on a soap molecule moved it is called phosphorescence: if not, is the lyophobic (hydrophobic) part. it is called fluorescence. 2. The disperse phase in colloids. In lyophobic colloids the dispersed parti- LUMO See frontier orbital theory. cles are not solvated and the colloid is easily solvated. Gold and sulfur sols are lutetium A silvery element of the lan- examples. thanoid series of metals. It occurs in asso- Compare lyophilic.

135 M

macromolecular crystal A crystal rectly with oxygen, nitrogen, and sulfur on composed of atoms joined together by co- heating, to form the oxide, MgO, the ni- valent bonds, which form giant three-di- tride, Mg3N2, and the sulfide MgS, all of mensional or two-dimensional networks. which are hydrolyzed by water to give the Diamond and silicates are examples of hydroxide. It burns in air with an intense macromolecular crystals. white flame. Magnesium also reacts directly with the macromolecule A large molecule; e.g. a halogens to form halides. These show a natural or synthetic polymer. slight deviation in the general behavior of halides by dissolving normally in water Madelung constant A constant, de- and undergoing a significant amount of hy- noted α, which appears in calculations of drolysis only when the solutions are evap- the cohesive energy of ionic crystals when orated: → the electrostatic interactions between ions MgX2 + 2H2O Mg(OH)2 + 2HX in a lattice are summed. The Madelung Magnesium hydroxide is considerably constant is dimensionless and is a charac- less soluble than the hydroxides of the teristic of the specific crystal structure. It is heavier elements in group 2 and is a weaker named for E. Madelung who introduced it base. Magnesium forms a soluble sulfate, in 1918. chlorate, and nitrate. Like the heavier alkaline earths, it also forms an insoluble car- magnesia See magnesium oxide. bonate, but MgCO3 is the least thermally stable of the group. Unlike calcium, mag- magnesite See magnesium carbonate. nesium does not form a carbide. Magnesium metal is industrially impor- magnesium A light ductile malleable sil- tant as a major component, along with alu- ver-white alkaline-earth metal; the second minium and zinc, of lightweight alloys. element of group 2 (formerly group IIA) of The alloy surfaces develop an impervious the periodic table. It has the electronic con- oxide film which protects them from pro- figuration of neon with two additional gressive deterioration. The metal is also outer 3s electrons. The element accounts used in flash bulbs, fireworks, and flares. for 2.09% of the Earth’s crust and is eighth Because of its ionic nature magnesium in order of abundance. It occurs in a wide forms very few coordination compounds. variety of minerals such as brucite Donor–acceptor species in aqueous solu- (Mg(OH)2), carnallite (KCl.MgCl2.6H2O), tions are short-lived, but magnesium bro- epsomite (MgSO4.7H2O), magnesite mide and iodide have sufficient acceptor (MgCO3), and dolomite (CaCO3.MgCO3); properties to dissolve in donor solvents and also as magnesium chloride (MgCl2) in such as alcohols. Magnesium in trace sea water. The metal is obtained by several amounts is essential for living things, being routes, depending on the mineral used, all required by animals for proper metabolism eventually leading to the chloride which is and by green plants as a component of then fused and subjected to electrolysis. chlorophyll. The element has a fairly low ionization The element crystallizes in the close- potential and is electropositive. It reacts di- packed hexagonal structure.

136 magnetic moment

Symbol: Mg; m.p. 649°C; b.p. 1090°C; droxide is used as an antacid, in sugar re- r.d. 1.738 (20°C); p.n. 12; most common fining, and in uranium processing. isotope 24Mg; r.a.m. 24.3050. magnesium oxide (magnesia; MgO) A magnesium bicarbonate See magne- white solid that occurs naturally as the sium hydrogencarbonate. mineral periclase. It can be prepared by heating magnesium in oxygen or by ther- magnesium carbonate (MgCO3)A mal decomposition of magnesium hydrox- white solid that occurs naturally as the ide, carbonate, or nitrate. Magnesium mineral magnesite and in association with oxide is weakly basic because of the attrac- calcium carbonate in dolomite. Magne- tion of the oxide ions for protons from sium carbonate is sparingly soluble in water molecules: 2– → – water but reacts with dilute acids to give O + H2O 2OH the salt, carbon dioxide, and water. It thus Magnesium oxide melts at 2800°C, mak- finds use in medicine as a mild antacid. ing it useful as a refractory lining for fur- Magnesium carbonate also decomposes naces. It is also used as an antacid and to easily on heating to give magnesium oxide, make reflective coatings for optics and which is an important refractory material. windshields. magnesium chloride (MgCl2) A solid magnesium peroxide (MgO2) A white that exists in a variety of hydrated forms, insoluble solid prepared by reacting most commonly as the hexahydrate, sodium or barium peroxide with a concen- MgCl2.6H2O. When heated, this hydrated trated solution of a magnesium salt. Mag- form is hydrolyzed by its water of crystal- nesium peroxide is used as a bleach for lization, with the evolution of hydrogen dyestuffs and silk. chloride and the formation of the oxide: → MgCl2 + H2O MgO + 2HCl magnesium sulfate (MgSO4) A solid The anhydrous chloride must therefore be that occurs naturally in many salt deposits prepared by evaporating an aqueous solu- in combination with other minerals. One tion in an atmosphere of hydrogen chlo- hydrated form, MgSO4.7H2O, is known as ride. It is used in the preparation of cotton Epsom salt or epsomite. Epsom salt is fabrics and artificial leather, as a laxative, made on a commercial scale by reacting and to fireproof wood. Magnesium metal magnesium carbonate with sulfuric acid. is produced from the fused chloride by Magnesium sulfate is used as a purgative electrolysis. drug, as a treatment for indigestion, and as an antidote for barium and barbiturate magnesium hydrogencarbonate (mag- poisoning. It is also used in the dyeing and nesium bicarbonate; Mg(HCO3)2) The sizing of textiles, in tanning, in synthetic solid hydrogencarbonate is unknown at fiber production, and as a source of mag- room temperature. The compound is pro- nesium in fertilizers. duced in solution when water containing carbon dioxide dissolves magnesium car- magnetic constant See magnetic per- bonate: meability. → MgCO3 + H2O + CO2 Mg(HCO3)2 It is a cause of temporary hardness in magnetic moment The ratio of the water. torque exerted on a magnet, a loop that carries an electrical current, or a moving magnesium hydroxide (Mg(OH)2)A electrical charge in a magnetic field to the white solid, occurring naturally as the min- field strength of that magnetic field. An eral brucite, that dissolves sparingly in electron has an orbital magnetic moment water to give an alkaline solution. It can be because of the magnetic field generated by prepared by adding an alkali to a soluble its orbital motion around its nucleus and a magnesium compound. Magnesium hy- spin magnetic moment because of its quan-

137 magnetic permeability tum mechanical spin. Atomic nuclei can other extraterrestrial bodies have the same also have magnetic moments because of cause. their spin. The existence of electron and Substances may be classified on the nuclear magnetic moments is made use of basis of how samples interact with fields. in various branches of spectroscopy and in Different types of magnetic behavior result magnetochemistry. from the type of atom. Diamagnetism, which is common to all substances, is due magnetic permeability symbol µ. A to the orbital motion of electrons. Para- quantity that characterizes the response of magnetism is due to electron spin, and is a medium to a magnetic field. The perme- thus a property of materials containing un- µ ability of free space, which is denoted 0 paired electrons. It is particularly impor- and is also called the magnetic constant, is tant in transition-metal chemistry, in a constant that appears in Ampère’s law re- which the complexes often contain un- lating an electric current to the magnetic paired electrons. Magnetic measurements field produced by the current. It has the can give information about the bonding in value 4π×10–7 Hm–1 in S.I. units. The these complexes. Ferromagnetism, the magnetic permeability of a medium µ re- strongest effect, also involves electron spin lates the magnetic flux density B to the and the alignment of magnetic moments in magnetic field strength H by the equation: domains. B = µH. The relative magnetic permeabil- µ µ µ µ ity, r, of a medium is defined by = r 0. magnetite A black mineral form of mixed iron(II)–iron(III) oxide, Fe3O4, a magnetic quantum number See atom. major type of iron ore. It is also used as a flux in making ceramics. See also lode- magnetic separation A method of sep- stone; triiron tetroxide. arating crushed mineral mixtures using the magnetic properties that a component may magnetochemistry The branch of possess, such as ferromagnetism or dia- chemistry concerned with the magnetic magnetism. properties of molecules. Magnetochem- istry is used extensively in the study of magnetic susceptibility Symbol χ. A transition-metal complexes because meas- dimensionless quantity, related to the MAG- uring their MAGNETIC SUSCEPTIBILITY en- NETIC PERMEABILITY of a medium, that char- ables information about chemical bonding acterizes the magnetic nature of that in the complex to be obtained using LIGAND medium. The relation between the mag- FIELD THEORY. netic susceptibility and the relative mag- µ χ µ –1 µ netic permeability r is given by = r . If magneton Symbol . A unit for measur- the material is diamagnetic χ has a small ing magnetic moments at the atomic and µ negative value. If the material is paramag- subatomic scales. The Bohr magneton B is χ µ π netic has a small positive value. If the ma- given by B = eh/4 me, where e and m are terial is ferromagnetic χ has a large positive the charge and mass of an electron respec- value. tively, and h is the Planck constant. The Bohr magneton is the smallest unit the or- magnetism The study of the nature and bital magnetic moment of an electron in an µ cause of magnetic force fields, and how dif- atom can have. The nuclear magneton N ferent substances are affected by them. is given by replacing the mass of the elec- Magnetic fields are produced by moving tron by the mass of the proton and is there- charge – both on a large scale (as with cur- fore about 1840 times smaller than the rent in a wire coil, forming an electromag- Bohr magneton. net), or on the small scale of the moving charges in atoms. It is generally assumed malachite A green mineral consisting of that the Earth’s magnetism and that of copper(II) carbonate and hydroxide

138 manganite

(CuCO3.Cu(OH)2). It is used as an ore of ganese(II) oxide rapidly oxidizes. The com- copper, as a pigment, and in jewelry. pound has a crystal lattice similar to that of sodium chloride. manganate(VI) A salt containing the 2– ion MnO4 . manganese(III) oxide (manganic oxide; manganese sesquioxide; Mn2O3) A black manganate(VII) (permanganate) A salt powder obtained by igniting manga- – containing the ion MnO4 . Manga- nese(IV) oxide or a manganese(II) salt in nate(VII) salts are purple and strong oxi- air at 800°C. The powder reacts slowly dizing agents. In basic solutions the dark with cold dilute acids to form man- green manganate(VI) ion is formed. ganese(III) salts. Manganese(III) oxide occurs in nature as braunite manganese A transition metal, the first (3Mn2O3.MnSiO3) and as the monohy- element of group 7 (formerly VIIB) of the drate mineral manganite (Mn2O3.H2O). periodic table. It occurs naturally as ox- The manganese(III) oxide crystal lattice ides, e.g. pyrolusite (MnO2) and hausman- contains Mn3+ and O2– ions. With concen- nite (trimanganese tetraoxide, Mn3O4). trated alkalis, it undergoes disproportiona- Nodules found at various locations on the tion to give manganese(II) and ocean floor are about 25% manganese. manganese(IV) ions. The metal is recovered from its oxides after roasting by reduction with aluminum, manganese(IV) oxide (manganese diox- carbon, or magnesium, followed by elec- ide; MnO ) A black powder prepared by trolysis. Managanese is moderately elec- 2 the action of heat on manganese(II) nitrate. tropositive and combines with the non- A hydrated form occurs naturally as the metals, excluding hydrogen, when heated. mineral pyrolusite. Manganese(IV) oxide is Its main use is in alloy steels made by insoluble in water. It is a powerful oxidiz- adding pyrolusite to iron ore in an electric ing agent: it reacts with hot concentrated furnace. It decomposes cold water and di- hydrochloric acid to produce chlorine and lute acids to give hydrogen. Manganese ex- with warm sulfuric acid to give oxygen. hibits all possible positive oxidation states, with +7, +4, and (the most stable) +2 being The dihydrate (MnO2.2H2O) is formed the most common. Manganese(II) salts are when potassium manganate(VII) is re- pale pink. With alkali their solutions pre- duced in alkaline solution. It is used as a cipitate manganese(II) hydroxide, which catalyst in the laboratory preparation of rapidly oxidizes in air to brown man- chlorine, as a depolarizer in electric dry ganese(III) oxide. cells, and in the glass industry. At 500– ° Symbol: Mn; m.p. 1244°C; b.p. 600 C, manganese(IV) oxide decomposes 1962°C; r.d. 7.44 (20%C); p.n. 25; most to give manganese(III) oxide (Mn2O3) and common isotope 55Mn; r.a.m. 54.93805. trimanganese tetroxide (Mn3O4). It has good electrical conductivity. manganese dioxide See manganese(IV) oxide. manganese sesquioxide See manganese(III) oxide. manganese(II) oxide (manganous oxide; MnO) A green powder prepared by heat- manganic oxide See manganese(III) ing manganese(II) carbonate or oxalate in oxide. the absence of air. Alternatively it may be prepared by heating the higher manganese manganin An alloy of copper, man- oxides in a stream of hydrogen. Man- ganese, and nickel. It has a high electrical ganese(II) oxide is a basic oxide and is al- resistivity over a wide range of tempera- most insoluble in water. At high tures and is used to make resistors. temperatures it is reduced by hydrogen to manganese. On exposure to air, man- manganite See manganese(III) oxide.

139 manganous oxide manganous oxide See manganese(II) of any of the reactants will give a measure oxide. of the rate of reaction. For the reaction A + B → products, the manometer A device for measuring law of mass action states that pressure. A simple type is a U-shaped glass rate = k[A][B] tube containing mercury or other liquid. where [A] and [B] represent the concentra- The pressure difference between the arms tion of the reactants in mol dm–3 and k is a of the tube is indicated by the difference in constant dependent on the reaction. The heights of the liquid. active mass can equal the concentration in mol dm–3 only if there is no interaction or marble A dense metamorphic rock com- interference between the reacting mol- posed of recrystallized limestone or ecules. Many systems exhibit such interac- dolomite. It is used in construction and as tions and interference and consequently a decorative stone. the concentration has to be multiplied by an activity coefficient in order to obtain the Marcus, Rudolph Arthur (1923– ) effective active mass. See activity coeffi- Canadian–American chemist. In the 1950s cient. Marcus began to work on electron-transfer reactions. The addition, removal, and mass concentration See concentration. transfer of electrons is the driving force behind many basic chemical processes includ- mass density See concentration. ing photosynthesis, respiration, and the production of solar energy. Such reactions mass-energy equation The equation E are, in principle, very simple, involving the = mc2, where E is the total energy (rest- movement of an electron from one ion to mass energy + kinetic energy + potential form another ion. The rates of such reac- energy) of a mass m, c being the speed of tions can, however, vary widely. Marcus light. The equation is a consequence of Ein- was able to explain electron-transfer reac- stein’s special theory of relativity; mass is a tion rates in terms of the way in which the form of energy and energy also has mass. solvent molecules, initially configured to Conversion of rest-mass energy into kinetic solvate the reactant, reorganize to solvate energy (and thus heat) is the source of the products. He was awarded the 1992 power in radioactive substances and the Nobel Prize for chemistry for his work in basis of nuclear-power generation. this field. massicot See lead(II) oxide. Marsh’s test See arsine. mass number See nucleon number. mass Symbol: m A measure of the quantity of matter in an object. Mass is deter- mass spectrograph See mass spectrom- mined in two ways: the inertial mass of an eter. object determines its tendency to resist change in motion; the gravitational mass mass spectrometer An instrument for determines its gravitational attraction for producing ions in a gas and analyzing them other masses. The SI base unit of mass is according to their charge/mass ratio. The the kilogram. earliest experiments by J. J. Thomson (1856–1940) used a stream of positive ions mass action, law of At constant tem- from a discharge tube, which were de- perature, the rate of a chemical reaction is flected by parallel electric and magnetic directly proportional to the active mass of fields at right angles to the beam. Each type the reactants, the active mass being taken of ion formed a parabolic trace on a pho- as the concentration. In general, the rate of tographic plate (a mass spectrograph). reaction decreases steadily as the reaction In modern instruments, as originated by proceeds; a measure of the concentration Francis Aston (1877–1945), the ions are

140 meitnerium produced by ionizing the gas with elec- cause it was the first formulation of quan- trons. The positive ions are accelerated out tum mechanics. In 1926 the Austrian of this ion source into a high-vacuum re- physicist Erwin Schrödinger and others gion. Here, the stream of ions is deflected showed that matric mechanics and WAVE and focused by a combination of electric MECHANICS are equivalent, and so give the and magnetic fields, which can be varied so same answers to physical and chemical that different types of ion fall on a detector. problems. However, matrix mechanics is In this way, the ions can be analyzed ac- used less frequently to solve problems than cording to their mass, giving a mass spec- is wave mechanics. trum of the material. Mass spectrometers are used for accurate measurements of rel- matte A mixture of iron and copper sul- ative atomic mass and for analysis of iso- fides obtained at an intermediate stage in tope abundance. They can also be used to smelting copper ores. identify compounds and analyze mixtures. The relative proportions of different types maxwell Symbol: Mx A unit of mag- of ions in a mixture is used to find the netic flux used in the c.g.s. system. It is structure of new compounds. The charac- equal to 10–8 Wb in SI units. teristic spectrum can also identify compounds by comparison with standard Maxwell–Boltzmann distribution The spectra. statistical distribution of the speeds of molecules in a gas as a function of both the ab- mass spectrum See mass spectrometer. solute temperature and the mass of the molecules. It was derived by the British matrix 1. A continuous solid phase in physicist James Clerk Maxwell (1831–79) which particles of a different solid phase and the Austrian physicist Ludwig Boltz- are embedded. mann (1844–1906) in the second half of 2. (plural matrices) A rectangular array of the 19th century using statistical methods. quantities which are frequently real or complex numbers arranged in rows or mean bond energy See bond energy. columns. A matrix is usually indicated by enclosing the array in square brackets. Ma- mechanism A step-by-step description trices are used extensively in mathematics of the events taking place in a chemical re- and its physical applications. If the sizes of action. It is a theoretical framework ac- the matrices are right it is possible to add counting for the fate of bonding electrons and multiply matrices together. An impor- and illustrates which bonds are broken and tant feature of the multiplication of two which are formed. For example, in the matrices A and B is that, in general, chlorination of methane to give chloro- AB≠BA; i.e. matrix multiplication is non- methane: commutative. step 1 Cl:Cl → 2Cl• matrix mechanics A formulation of step 2 → quantum mechanics in which the funda- Cl• + CH4 HCl + CH3• mental equations are stated in terms of ma- step 3 → trices. This formulation was stated by the CH3• + Cl:Cl CH3Cl + Cl• German physicist Werner Heisenberg in 1925 as a set of mathematical rules for mega- Symbol: M A prefix used with SI physical quantities of interest. The German units, denoting 106. For example, 1 mega- physicist Max Born recognized that the hertz (MHz) = 106 hertz (Hz). rules for quantum mechanics corresponded to matrix algebra. Born, Heisenberg, and meitnerium A radioactive synthetic another German physicist Pascual Jordan transactinide metallic element created by developed matrix mechanics further. Ma- bombarding bismuth-209 nuclei with iron- trix mechanics is historically important be- 58 nuclei in a particle accelerator. Only a

141 melting few short-lived atoms of the element have dium, and germanium, which were discov- ever been created. ered subsequently and found to have the Symbol: Mt; m.p., b.p., and r.d. un- properties Mendeleev had predicted. known; p.n. 109; only known isotope 266Mt (half-life about 3.4 × 10–3s). mendelevium A synthetic silver-white radioactive transuranic element of the acti- melting (fusion) The process by which a noid series, first created by bombarding solid is converted into a liquid by heat or einsteinium-253 isotope with alpha parti- pressure. cles. Several short-lived isotopes have been synthesized. melting point The temperature at Symbol: Md; m.p. 1021°C; b.p. which a solid is in equilibrium with its liq- 3074°C; r.d. unknown, p.n. 101; most sta- uid phase at standard pressure and above ble isotope 258Md (half-life 57 minutes). which the solid melts. This temperature is always the same for a particular pure solid. mercuric chloride See mercury(II) chlo- Ionically bonded solids generally have ride. much higher melting points than those in which the forces are covalent or intermole- mercuric oxide See mercury(II) oxide. cular. mercuric sulfide See mercury(II) sul- membrane A thin pliable sheet of tissue fide. or other material acting as a boundary. The membrane may be either natural (as in mercurous chloride See mercury(I) cells, skin, etc.) or synthetic modifications chloride. of natural materials (cellulose derivatives or rubbers). In many physicochemical mercurous oxide See mercury(I) oxide. studies membranes are supported on porous materials, such as porcelain, to pro- mercurous sulfide See mercury(I) sul- vide mechanical strength. Membranes are fide. generally permeable to some degree. Membranes can be prepared to permit mercury A heavy tin-silver white transi- the passage of other molecules and micro- tion metal, the third element of group 12 molecular material. Because of permeabil- (fromerly IIB) of the periodic table. It is the ity effects, concentration diffferences at a only metal that is a liquid at room temper- membrane give rise to a whole range of ature (20–22°C). It occurs naturally as the membrane-equilibrium studies, of which mineral cinnabar (HgS, mercury(II) sul- osmosis, dialysis, and ultrafiltration are ex- fide); small drops of metallic mercury also amples. See also semipermeable mem- occur native in cinnabar and in some vol- brane. canic rocks. Elemental mercury vapor is very poisonous as are many mercury com- Mendeleev, Dmitri Ivanovich (1834– pounds. Mercury is used in thermometers 1907) Russian chemist. Mendeleev is re- and barometers, special AMALGAMS, scien- membered for developing the periodic tific apparatus, electrical switches, mer- table of chemical elements in a classic cury-vapor lamps, and in mercury cells. paper published in 1869 entitled On the Mercury compounds are used as fungi- Relation of the Properties to the Atomic cides,timber preservatives, and detonators. Weights of Elements. Other scientists such Symbol: Hg; m.p. –38.87°C; b.p. as Julius Lothar Meyer and John Newlands 356.58°C; r.d. 13.546 (20°C); p.n. 80; had similar ideas at about the same time most common isotope 202Hg; r.a.m. but Mendeleev developed his ideas much 200.59. See also zinc group. more fully, including the predictions of the existence and properties of hitherto un- mercury cell A voltaic or electrolytic known elements such as gallium, scan- cell in which one or both of the electrodes

142 metal carbonyl consists of mercury or an amalgam. Amal- ference in color is simply due to particle gam electrodes are used in the DANIELL CELL size. If the oxide is strongly heated it de- and the WESTON CADMIUM CELL. Flowing composes to give mercury and oxygen. mercury electrodes are also used in certain electrolytic cells. In the production of chlo- mercury(I) sulfide (mercurous sulfide; rine, sodium chloride solution is elec- Hg2S) The brownish-black precipitate trolyzed in a cell with carbon anodes and a originally formed when mercury is treated flowing mercury cathode. At the cathode, with cold concentrated sulfuric acid over sodium metal is formed, which forms an an extended period is thought to be mer- amalgam with the mercury. See also po- cury(I) sulfide. Alternatively it may be pre- larography. pared by the action of hydrogen sulfide or an alkaline sulfide on a mercury(I) salt so- mercury(I) chloride (mercurous chloride; lution. As soon as the mercury(I) sulfide is calomel; Hg2Cl2) A white precipitate formed it disproportionates to give mer- prepared by adding dilute hydrochloric cury(II) sulfide and mercury. acid to a mercury(I) salt solution or by sub- liming mercury(II) chloride with mercury. mercury(II) sulfide (mercuric sulfide; Mercury(I) chloride is sparingly soluble in HgS) A compound that occurs in nature water and is blackened by both ammonia as the minerals CINNABAR (a red solid) and gas, which it absorbs, and by alkalis. It is metacinnabarite (a black solid). It is pre- used in the calomel electrode and was for- pared as a black precipitate by the action of merly used as a purgative. hydrogen sulfide on a soluble mercury(II) salt solution. On heating, mercury(II) sul- mercury(II) chloride (mercuric chloride; fide sublimes and becomes red. It is insolu- HgCl2) A colorless crystalline compound ble in dilute hydrochloric and nitric acids prepared by direct combination of mercury but will dissolve in concentrated nitric acid with cold dry chlorine. Mercury(II) chlo- and aqua regia. Mercury(II) sulfide is used ride is soluble in water; it also dissolves in as the pigment vermilion. concentrated hydrochloric acid because of 2– the formation of complex ions, HgCl4 mer-isomer See isomerism. – and HgCl3 . On heating, mercury(II) chloride sublimes forming a white translucent meso-form See optical activity. mass. It is extremely poisonous, but in dilute solution (1:1000) it is used as an anti- mesomerism See resonance. septic. It is also used as a fungicide. meta- Certain acids regarded as formed mercury(I) oxide (mercurous oxide; from an anhydride and water are named Hg2O) The black precipitate formed on meta acids to distinguish them from the addition of sodium hydroxide solution to a more hydrated ortho acids. For example, solution of mercury(I) nitrate is thought by H2SiO3 (SiO2 + H2O) is metasilicic acid; some to be mercury(I) oxide; others, who H4SiO4 (SiO2 + 2H2O) is orthosilicic acid. doubt its existence, think that the black- See also para-. ness of the precipitate is due to some free mercury. X-ray examination of this black metacinnabarite See mercury(II) sul- compound has shown it to be an intimate fide. mixture of mercury(II) oxide and mercury. metaiodic(VII) acid See iodic(VII) mercury(II) oxide (mercuric oxide; acid. HgO) A poisonous compound formed as a yellow powder by the addition of metal See metals. sodium(I) hydroxide to a solution of mercury(II) nitrate or as a red solid by heating metal carbonyl A coordination com- mercury at 350°C for a long time. The dif- pound formed between a metal and CAR-

143 metallic bond

BONYL GROUPS. Transition metals form acids but also forms stannates with alkalis. many such compounds. Its oxide is amphoteric. Tin also has metallic (white tin) and nonmetallic (gray tin) al- metallic bond A bond formed between lotropes. atoms of a metallic element in its zero oxidation state and in an array of similar metallurgy The study of metals, espe- atoms. The outer electrons of each atom cially methods of extracting metals from are regarded as contributing to a free elec- their ores and the formation and properties tron ‘gas’, which occupies the whole crys- of alloys. tal of the metal. It is the attraction of the positive atomic cores for the negative elec- metals Any of a class of chemical el- tron gas that provides the strength of the ements with certain characteristic proper- metallic bond. ties. In everyday usage, metals are elements Quantum mechanical treatment of the (and alloys) such as iron, aluminum, and electron gas restricts its energy to a series of copper, which are typically lustrous mal- ‘bands’. It is the behavior of electrons in leable solids and good conductors of heat these bands that gives rise to semiconduc- and electricity. Such properties do not al- tors. See also delocalization. ways apply because some metals, e.g. bismuth, are poor conductors, mercury is a metallic crystal A crystal formed by liquid at room temperature, etc. metal atoms in the solid state. Each atom In chemistry, metals are distinguished contributes its valence (outer) electrons to by their chemical properties into two main a free electron ‘gas’. These electrons are groups. Reactive metals, such as the alkali free to migrate through the solid while the metals and alkaline-earth metals, are elec- remaining ions are arranged in a lattice. tropositive elements. They are high in the The ability of electrons to move through electromotive series and tend to form com- the lattice accounts for the electrical and pounds by losing electrons to give positive thermal conductivity of metals. ions. They have basic oxides and hydroxides. This typical metallic behavior de- metallocene A sandwich compound in creases across the periodic table and which a metal atom or ion is coordinated increases down a group in the table. to two cyclopentadienyl ions. Ferrocene The other group of metals is the transi- (Fe(C5H5)2) is the commonest example. tion elements, which are less reactive, have variable valences, and tend to form com- metalloid Any of a class of chemical el- plexes. In the solid and liquid states metals ements that are intermediate in properties have METALLIC BONDS, formed by positive between METALS and NONMETALS. Exam- ions with free electrons. See also metalloid; ples are germanium, arsenic, and tellurium, nonmetal. all of which are semiconductors. There is, in fact, no clear-cut distinction metaphosphoric acid See phosphoric(V) between metals and nonmetals. In the peri- acid. odic table, there is a change from metallic to nonmetallic properties across the table, metastable species An excited state of and an increase in metallic properties an atom, ion, or molecule, that has a rela- down a group. Consequently there are el- tively long lifetime before reverting to the ements (boron, silicon, germaium, arsenic, ground state. Metastable species are inter- antimony, and telluvium) that form a diag- mediates in some reactions. onal running to the right down the table near its right edge and which exhibit these metastable state A condition of a sys- intermediate properties. Other elements tem or object in which it appears to be in are also sometimes considered to be metal- stable equilibrium but, if disturbed, can loids, according to their chemical proper- settle into a lower energy state. For exam- ties. Tin, for instance, forms salts with ple, supercooled water is liquid below 0°C

144 mirror image

(at standard pressure). When a small crys- characteristic three-layered structure. The tal of ice or dust (for example) is intro- three main types are biotite, lepidolite, and duced, rapid freezing occurs. muscovite, which differ in their content of other elements (such as potassium, magne- metathesis See double decomposition. sium, and iron). Mica flakes are used as electrical insulators, dielectrics, and small meter (metre) Symbol: m The SI base heat-proof windows. unit of length, defined as the distance traveled by light in vacuum in 1/299 792 458 micro- Symbol: µ A prefix used with SI second. This definition was adopted in units, denoting 10–6. For example, 1 mi- 1983 to replace the 1960 definition of a crometer (µm) = 10–6 meter (m). length equal to 1 650 763.73 wavelengths in vacuum corresponding to the transition micron Symbol: µ A m.k.s.a. unit of between the levels 2p10 and 5d5 of the length equal to 10–6 meter in SI units. krypton-86 atom. microwaves A form of electromagnetic methane The simplest hydrocarbon, radiation, ranging in wavelength from CH4. It is the main component of natural about 1 mm (where it merges with in- gas. frared) to about 120 mm (bordering on radio waves). Microwaves are produced by methanide See carbide. various electronic devices including the klyston; they are often carried over short methyl orange An acid–base indicator distances in tubes of rectangular section that is red in solutions below a pH of 3.1 called waveguides. and yellow above a pH of 4.4. Because the Spectra in the microwave region can transition range is clearly on the acid side, give information on the rotational energy methyl orange is suitable for the titration levels of certain molecules. See also electro- of an acid with a moderately weak base, magnetic radiation. such as sodium carbonate. milk of lime See calcium hydroxide. methyl red An acid–base indicator that is red in solutions below a pH of 4.4 and milli- Symbol: m A prefix used with SI yellow above a pH of 6.0. It is often used units, denoting 10–3. For example, 1 mil- for the same types of titration as methyl or- limeter (mm) = 10–3 meter (m). ange but the transition range of methyl red is nearer neutral (pH 7) than that of methyl millimeter of mercury See mmHg. orange. The two molecules are structurally similar. mineral A naturally occurring inorganic compound, having a characteristic crys- metre See meter. talline structure and definite chemical composition; a mineral’s physical properties metric system A decimal system of are also more or less constant. In contrast, units based on the meter or centimeter as rocks are composed of mixtures of individ- the base unit of length, the gram or kilo- ual minerals. See rock. gram as the base unit of mass, and the second as the base unit of time. mineral acid An inorganic acid, especially an acid used commercially in large metric ton See tonne. quantities. Examples are hydrochloric, nitric, and sulfuric acids. mho See siemens. mirror image A shape that is identical mica A member of an important group to another except that its structure is re- of aluminosilicate minerals that have a versed as if viewed in a mirror. If an object

145 misch metal is not symmetrical it cannot be superim- m.k.s.a. system A coherent metric sys- posed on its mirror image. For example, tem of units for mechanics and electromag- the left hand is the mirror image of the netics based on the meter, the kilogram, the right hand. second, and the ampere. It superseded the m.k.s. system and was in turn superseded misch metal A pyrophoric alloy of by the SI in 1960. cerium and other lanthanoids, used in lighter flints. m.k.s. system A metric system of units for mechanics based on the meter, the kilo- miscible Denoting combinations of sub- gram, and the second. It superseded the stances that, when mixed, give rise to only c.g.s. system and was in turn superseded by one phase; i.e. substances that dissolve in the m.k.s.a. system in 1946. each other. See solid solution; solution. mmHg (millimeter of mercury) A c.g.s. Mitscherlich’s law (law of isomor- unit of pressure defined as the pressure that phism) The law stating that substances will support a column of mercury one mil- that crystallize in isomorphous forms (i.e. limeter high under specified conditions. It have identical crystalline forms and form is equal to 133.322 4 Pa in SI units. It con- mixed crystals) have similar chemical com- tinues to be used in health care. positions. The law can be used to indicate the formulae of compounds. For instance, mobile phase See chromatography. the fact that chromium(III) oxide is isomorphous with Fe O and Al O implies molal concentration See concentra- 2 3 2 3 tion. that its formula is Cr2O3. molality See concentration. mixed indicator A mixture of two or more indicators so as to decrease the pH molar 1. Denoting a physical quantity range, heighten the color, etc. over or at divided by the amount of substance. In al- which the change occurs. most all cases the amount of substance will be in moles. For example, volume (V) di- See oxygen. mixed oxides vided by the number of moles (n) is molar volume Vm = V/n. mixture Two or more substances form- 2. A molar solution contains one mole of ing a system in which there is no chemical solute per cubic decimeter of solvent. bonding between the two. In homogeneous mixtures (e.g. solutions or mixtures of molar heat capacity See Dulong and gases) the molecules of the substances are Petit’s law. mixed, and there is only one PHASE. In heterogeneous mixtures (e.g. certain alloys) molarity See concentration. different phases can be distinguished. Mix- tures differ from chemical COMPOUNDS in mole Symbol: mol The SI base unit of that: amount of substance, defined as the 1. The chemical properties of the compo- amount of substance that contains as many nents of a mixture are the same as those elementary entities as there are atoms in of the pure substances. 0.012 kilogram of 12C. The elementary en- 2. The mixture can be separated by physi- tities may be atoms, molecules, ions, elec- cal means (e.g. distillation or crystalliza- trons, photons, etc., and they must be tion) or mechanically. specified. The amount of substance is pro- 3. The proportions of the components can portional to the number of entities, the vary. Some mixtures (e.g. certain solu- constant of proportionality being the AVO- tions) can vary in proportions only be- GADRO CONSTANT. One mole contains tween definite limits. 6.022 192 × 1023 entities. One mole of an

146 mole fraction element with relative atomic mass A has a pores of constant dimensions in their mo- mass of A grams (this mass was formerly lecular structure. When a sample is passed called one gram-atom of the element). through a column packed with granules of this material, some of the molecules enter molecular beam A beam of molecules these pores and become trapped. The re- (or atoms or ions). Molecular beams are mainder of the mixture passes through the used in spectroscopy and in the study of in- interstices in the column. The trapped mol- termolecular forces, chemical reactions, ecules can be recovered by heating. Mo- and surfaces. lecular-sieve chromatography is widely used in chemistry and biochemistry labora- molecular crystal A crystal in which tories. A modified form of molecular sieve molecules, as opposed to atoms, occupy is used in gel filtration. The sieve is a con- lattice points. Examples include iodine and tinuous gel made from a polysaccharide. In solid carbon dioxide (dry ice). Because the this case, molecules larger than the largest forces holding the molecules together are pore size are totally excluded from the col- weak, molecular crystals have low melting umn. points. When the molecules are small, the crystal structure approximates to a close- molecular spectrum The absorption or packed arrangement. See close packing. emission spectrum that is characteristic of a molecule. Molecular spectra are usually molecular dipole moment See dipole band spectra. moment. molecular symmetry The set of sym- molecular formula The formula of a metry operations (rotations, reflections, compound showing the number and types etc.) that can be applied to a molecule. This of the atoms present in a molecule, but not set forms the point group of the molecule. the arrangement of the atoms. For exam- Molecular symmetry is analyzed systemat- ple, H2O2 represents the molecular for- ically using GROUP THEORY. mula of hydrogen peroxide but not its empirical formula (HO), or the arrange- molecular weight See relative molecu- ment of its atoms (HOOH). The molecular lar mass. formula can be determined only if the molecular mass is known. Compare empirical molecule A particle formed by the com- formula; general formula; structural for- bination of atoms in a whole-number ratio. mula. A molecule of an element (combining atoms are the same, e.g. O2) or of a com- molecularity The total number of react- pound (different combining atoms, e.g. ing molecules in the individual steps of a HCl) retains the properties of that element chemical reaction. Thus, a unimolecular or compound. Thus, any quantity of a step has molecularity 1, a bimolecular step compound is a collection of many identical 2, etc. Molecularity is always an integer, molecules. Molecular sizes are characteris- whereas the order of a reaction need not tically 10–10 to 10–9 m. necessarily be so. The molecularity of a re- Many molecules of natural products are action gives no information about the so large that they are regarded as giant mechanism by which it takes place. molecules (MACROMOLECULES); they may contain thousands of atoms and have com- molecular orbital See orbital. plex structural formulae that require very advanced techniques to identify. See also molecular sieve A substance through formula; relative molecular mass. which molecules of a limited range of sizes can pass, enabling volatile mixtures to be mole fraction The number of moles of a separated. Zeolites and other metal alu- given component in a mixture divided by minum silicates can be manufactured with the total number of moles present of all the

147 molybdenum components. The mole fraction of compo- allotrope always has a lower vapor pres- nent A is sure than the other(s) at all temperatures; nA/(nA + nB + nC + …) this is the stable allotrope. See also al- where nA is the number of moles of A, etc. lotropy; enantiotropy. molybdenum A hard brittle silver- monovalent (univalent) Having a va- white transition metal, the second element lence of one. of group 6 (formerly VIB) of the periodic table. It occurs naturally in the minerals mordant An inorganic compound used molybdenite (MoS2) and wulfenite to fix dye in cloth. The mordant (e.g. alu- (PbMoO4). Molybdenite, the main ore, is minum hydroxide or chromium salts) is converted to the oxide by roasting, which precipitated in the fibers of the cloth, and is then reduced to the metal with hydrogen. the dye then absorbs in the particles. Molybdenum is used in alloy steels, incandescent bulbs, and catalysts. In trace Moseley’s law Lines in the x-ray spec- amounts it is also an essential dietary el- tra of elements have frequencies that de- ement for animals. The compound ammo- pend on the proton number of the element. nium molybdate, dissolved in nitric acid, is used as a test for phosphates(V). Purified For a set of elements, a graph of the square root of the frequency of x-ray emission molybdenum(IV) sulfide (MoS2) is used in lubricants to enhance viscosity. against proton number is a straight line Symbol: Mo; m.p. 2620°C; b.p. (for spectral lines corresponding to the 4610°C; r.d. 10.22 (20°C); p.n. 42; most same transition). common isotope 98Mo; r.a.m. 95.94. Mössbauer effect The emission of monatomic Denoting a molecule, radi- gamma rays with a very narrow spread of cal, or ion consisting of only one atom. For wavelengths from a solid. Usually, when example, helium is a monatomic gas and gamma rays are emitted by a radioactive H• is a monatomic radical. nucleus they have a broad spread of wavelengths due to the recoil of the emitting nu- Mond process See nickel. clei. However, if the emitting nuclei are embedded in a crystal the whole lattice re- monobasic acid An acid that has only coils, meaning that there is very little effect one active proton, such as hydrochloric on the wavelength of the emitted gamma acid. Compare dibasic acid. rays. In Mössbauer spectroscopy, a source of monoclinic crystal See crystal system. gamma rays is mounted on a moving support close to a sample. Detectors monitor A salt that has a single monohydrate gamma rays scattered by the sample. The molecule of water of crystallization, such wavelength of the gamma rays from the as sodium carbonate monohydrate, source can be changed by varying the speed Na CO .H O. See water of crystallization. 2 3 2 at which the source is moved toward the monomer The molecule, group, or com- sample (the Doppler effect). The spectrum pound from which a dimer, trimer, or poly- is a plot of detector response against speed mer is formed. of source. A sharp decrease in response at a particular speed indicates resonance ab- monotropy The existence of a single al- sorption of gamma ray by a nuclide in the lotrope of an element that is always more sample. The wavelength (energy) at which stable than the other allotrope(s) regardless this occurs depends on the energy levels in of temperature; phosphorus, for example, the sample nucleus, but these are affected exhibits monotropy. The phase diagram to some extent by the surrounding elec- for a monotropic element shows that one trons (i.e. there are chemical shifts).

148 muriatic acid mother liquor The solution remaining to an increase in electron density above and after the formation of crystals. below the internuclear axis. Such bonds are called pi bonds. If the sigma bond axis Mulliken, Robert Sanderson (1896– is taken as the z-axis (i.e. the internuclear 1986) American chemist and physicist. axis) then overlap of orbitals along the x- Mulliken devoted most of his career to un- axis gives rise to a pi bond in the xz plane. derstanding the electronic structure of mol- Similarly orbitals on the y-axis form a pi ecules and molecular spectra in terms of bond in the yz plane. quantum mechanics. He was awarded the 1966 Nobel Prize for chemistry for his multiple proportions, law of Pro- ‘fundamental work concerning chemical bonds and the electronic structure of mol- posed by John Dalton in 1804, the law ecules by the molecular-orbital method’. states that when two elements A and B combine to form more than one com- multicenter bond A two-electron bond pound, the masses of B that combine with formed by the overlap of orbitals from a fixed mass of A are in small, whole-num- more than two atoms (usually 3). The ber ratios. For example, in dinitrogen bridging in diborane, B2H6, is believed to oxide, N2O, nitrogen monoxide, NO, and 3 take place by overlap of an sp hybrid or- dinitrogen tetroxide, N2O4, the amounts of bital from each boron atom with the 1s or- nitrogen combined with a fixed mass of bital on the hydrogen atom. This oxygen are in the ratio 4:2:1. multicenter bond is called a three-center two-electron bond. The molecule is elec- multiple-range indicator See universal tron-deficient. See also boron hydride; indicator. electron-deficient compounds. Mumetal (Trademark) A magnetic multidecker sandwich compound See alloy containing about 75% nickel, the re- sandwich compound. mainder being iron, copper, and chromium and sometimes molybdenum. It is easily multidentate ligand A ligand that pos- sesses at least two sites at which it can co- magnetized and demagnetized, has a high ordinate. permeability, and is used in transformer cores and electromechanical equipment. multiple bond A bond between two atoms involving more than one pair of elec- muriate An obsolete name for a chlo- trons, e.g. a double bond or a triple bond. ride; e.g. muriate of potash (KCl), muri- This additional bonding arises from over- atric acid (HCl). lap of atomic ORBITALS that are perpendicular to the internuclear axis and gives rise muriatic acid See muriate.

149 N

nano- Symbol: n A prefix used with SI negative catalyst See catalyst. units, denoting 10–9. For example, 1 nanometer (nm) = 10–9 meter (m). neodymium A soft toxic silvery yellow element belonging to the lanthanoid series nanotechnology The technology of de- of metals. It occurs in association with vices at the nanometer scale. In such small other lanthanoids in such minerals as al- devices the quantum mechanical nature of lanite, bastnaesite, cerite, and monazite. electrons has to be taken into account fully. Neodymium is used in various alloys, as a Instruments such as the ATOMIC FORCE catalyst, in compound form in carbon-arc MICROSCOPE which can identify and manip- searchlights, etc., and in the glass industry. ulate individual atoms are very useful in Symbol: Nd; m.p. 1021°C; b.p. nanotechnology. 3068°C; r.d. 7.0 (20°C); p.n. 60; most 140 nanotubes Tubular structures with di- common isotope Nd;r.a.m. 144.24. ameters of a few nanometers. Examples of nanotubes are the carbon structures neon An inert colorless odorless known as bucky tubes, which have a struc- monatomic gas the second member of the ture similar to that of BUCKMINSTER- rare gases; i.e. group 18 (formerly VIIIA or FULLERENE. Interest has been shown in 0) of the periodic table. It has a very high nanotubes as possible microscopic probes ionization potential and forms no com- in experiments, as semiconductor materi- pounds. It occurs in minute quantities als, and as a component of composite ma- (0.0018% by volume) in air and is ob- terials. tained from liquid air by fractional distillation. It is used in neon signs and lamps nascent hydrogen A particularly reac- (emitting a characteristic orange-red light), tive form of hydrogen, which is believed to electrical equipment, Geiger counters, and exist briefly between its generation (e.g. by gas lasers. the action of dilute acid on magnesium) Symbol: Ne; m.p. –248.67°C; b.p. and its appearance as bubbles of normal –246.05°C; mass density 0.9 kg m–3 (0°C); hydrogen gas. It is thought that part of the p.n. 10; most common isotope 20Ne; r.a.m. free energy of the production reaction re- 20.18. mains with the hydrogen molecules for a short time. Nascent hydrogen may be used neptunium A toxic radioactive silvery to produce the hydrides of phosphorus, ar- element of the actinoid series of metals that senic, and antimony, which are not readily formed from ordinary hydrogen. was the first transuranic element to be synthesized (1940). Found on Earth only in natron A naturally occurring mineral minute quantities in uranium ores, it is also form of hydrated SODIUM CARBONATE obtained as a by-product from uranium (Na2CO3.10H2O), found on the beds of fuel elements bombarded by neutrons in dried-out soda lakes. nuclear reactors. It can be converted to plutonium-238, which can be used as a nu- natural abundance See abundance. clear power source.

150 nickel

Symbol: Np; m.p. 640°C; b.p. 3902°C; because there is an interaction between the r.d. 20.25 (20°C); p.n. 93; most stable iso- magnetic moments of the neutrons and the tope 237Np (half-life 2.14 × 106 years). magnetic moments of atoms in the sample.

Nernst, Hermann Walther (1864– neutron number Symbol: N The num- 1941) German physical chemist. Nernst is ber of neutrons in the nucleus of an atom; best remembered for his contributions to i.e. the nucleon number (A) minus the pro- electrochemistry and for discovering the ton number (Z). third law of thermodynamics. His work on electrochemistry included the concept of Newlands’ law (law of octaves) The the solubility product and the use of buffer observation that, when the elements are solutions. In 1906 he stated a theorem con- arranged in order of increasing relative cerning the entropy of crystals at absolute atomic mass, there is a similarity between zero which, in slightly different form, be- members that are eight elements apart (in- came known as the third law of thermody- clusive). For example, lithium has similari- namics. He also studied photochemistry ties to sodium, beryllium to magnesium, and wrote an influential book entitled and so on. Newlands discovered this rela- Theoretical Chemistry (1893). He was tionship in 1863 before the announcement awarded the 1920 Nobel Prize for chem- of Mendeléev’s famous periodic law istry. (1869). It is now recognized that New- lands’ law arises from there being eight neutralization The stoichiometric reac- members in each short period. It is named tion of an acid and a base in volumetric for the British chemist John Newlands analysis. The neutralization point or end (1837–98). point is detected with indicators. newton Symbol: N The SI derived unit neutral oxide An oxide that forms nei- of force, equal to the force needed to accel- ther an acid nor a base on reaction with erate one kilogram by one meter second–2. water. Examples include carbon monoxide 1 N = 1 kg m s–2. (CO), dinitrogen oxide (N2O), and water (H2O). Compare acidic oxide; amphoteric; Nichrome (Trademark) Any of a group basic oxide. of nickel–chromium–iron alloys, containing 60–80% nickel and about 16% neutron diffraction A method of struc- chromium; small amounts of other el- ture determination used for solids, liquids, ements, such as carbon or silicon, may be and gases that makes use of the quantum added. They can withstand very high tem- mechanical wave nature of neutrons. Ther- peratures and their high electrical resistiv- mal neutrons with average kinetic energies ity makes them suitable for use in heating of about 0.025 eV have a wavelength of elements. about 0.1 nanometer, making them suitable for investigating the structure of mat- nickel A ductile malleable relaatively ter at the atomic level. Neutron diffraction hard corrosion-resistant transition metal, is particularly useful for identifying the po- the first member of group 10 (formerly sitions of hydrogen atoms. These are diffi- part of subgroup VIIIB) of the periodic cult to establish using x-rays since x-rays table. Nickel occurs in nature, most impor- interact with electrons, and hence are scat- tantly, as the sulfides millerite (NiS) and tered weakly by hydrogen atoms, which pentlandite (Ni,Fe)9S8. It is extracted by have only one electron. Protons scatter the Mond process, which involves roasting neutrons strongly, meaning that the posi- the ore to obtain the oxide. The oxide is tions of protons can readily be determined then reduced using carbon monoxide, fol- using neutron scattering. Neutron scatter- lowed by the formation and subsequent de- ing has also been used extensively in inves- composition of volatile NICKEL CARBONYL tigations of the magnetic ordering in solids Ni(CO)4. Nickel is used as a catalyst, in 151 nickel carbonyl coinage alloys, and in stainless steel. Its group of alloys containing nickel, copper, compounds in its main oxidation state (+2) and zinc (but no silver) in various propor- are usually green. tions. They are white or silvery in color, Symbol: Ni; m.p. 1453°C; b.p. 2732°C; can be highly polished, and have good cor- r.d. 8.902 (25°C); p.n. 28; most common rosion-resistance. They are used, for exam- isotope 58Ni; r.a.m. 58.6934. ple, as base metals in silver plating, chromium plating, and enameling. nickel carbonyl (tetracarbonyl nickel(0); Ni(CO)4) A colorless liquid with a highly nielsbohrium Symbol: Ns A name for- toxic vapor. It is a typical example of a merly suggested for element-107, now transition metal complex, prepared by known as bohrium (Bh). See element. passing carbon monoxide over finely divided nickel at a temperature of approxi- NIFE cell (Nife cell) See nickel–iron ac- ° mately 60 C. The product is purified by cumulator. fractional distillation. Nickel carbonyl decomposes on heating to give pure nickel. It niobium A soft silvery malleable ductile is used as a catalyst and in the preparation transition metal, the second member of of nickel by the Mond process. group 5 (formerly VB) of the periodic table. It occurs naturally chiefly in the min- nickelic oxide See nickel(III) oxide. erals columbite ((Fe,Mn) (Nb,Ta)2 O6) and tantalite ((Fe,Mn)(Ta,Nb)2O6). It is used in nickel–iron accumulator (Edison cell; welding, special steels for high temperature Nife or NIFE cell) A type of secondary environments, and in superconductor al- cell in which the electrodes are formed by loys. It was formerly known as colum- steel grids. The positive electrode is im- bium. pregnated with a nickel–nickel hydride Symbol: Nb; m.p. 2468°C; b.p. mixture and the negative electrode is im- 4742°C; r.d. 8.570 (20°C); p.n. 41; most pregnated with iron oxide. Potassium hy- common isotope 93Nb; r.a.m. 92.90638. droxide solution forms the electrolyte. A nickel–iron cell is lighter and more durable See potassium nitrate. than a lead accumulator, and it can work niter with higher currents. Its e.m.f. is approximately 1.3 volts. nitrate A salt of nitric acid. nickelous oxide See nickel(II) oxide. nitration A reaction introducing the nitro (–NO2) group into a compound. Ni- nickel(II) oxide (nickelous oxide; NiO) tration is usually carried out using a mix- A pale green powder formed by heating ture of concentrated nitric and sulfuric nickel(II) hydroxide, nitrate, or carbonate acids, although the precise conditions dif- in the absence of air. It is a basic oxide, dis- fer from compound to compound. The at- + solving in dilute acids to give green solu- tacking species is NO2 (the nitryl ion). tions of nickel(II) salts. Nickel(II) oxide may be reduced to the metal by carbon, nitric acid (HNO3) A colorless fuming carbon monoxide, or hydrogen. corrosive liquid that is a strong acid. Nitric acid can be made in a laboratory by the dis- nickel(III) oxide (nickelic oxide; Ni2O3) tillation of a mixture of an alkali metal ni- A black powder formed by the action of trate and concentrated sulfuric acid. heat on nickel(II) oxide in air. It can also be Commercially it is prepared by the cat- prepared by igniting nickel(II) nitrate or alytic oxidation of ammonia and is sup- carbonate. It exists also as the dihydrate, plied as concentrated nitric acid, which Ni2O3.2H2O. contains 68% of the acid and is often colored yellow by dissolved oxides of nitro- nickel-silver (German silver) Any of a gen.

152 nitrogen cycle

Nitric acid is a strong oxidizing agent. and other highly electropositive elements. Most metals are converted to their nitrates Consequently it is used to provide an inert with the evolution of oxides of nitrogen atmosphere for many metallurgical (the composition of the mixture of the ox- processes and electrical devices. The direct ides depends on the temperature and on the combination of nitrogen and hydrogen oc- concentration of the nitric acid used). curs at elevated temperatures and pressures Some nonmetals (e.g. sulfur and phospho- (400–600°C, 100 megapascals) and is the rus) react to produce oxyacids. basis of the industrially important Haber process for the manufacture of ammonia. nitric oxide See nitrogen monoxide. Liquid nitrogen is used in cryogenics. Nitrogen forms a number of oxides: nitride A compound of nitrogen with a 1. Dinitrogen oxide (nitrous oxide, N2O) – more electropositive element such as mag- a neutral oxide. nesium or calcium. 2. Nitrogen monoxide (nitric oxide, NO) – a neutral oxide. nitriding See case hardening. 3. Nitrogen(III) oxide (nitrogen sesquioxide, N2O3) – an unstable oxide that is nitrification The action of nitrifying the anhydride of nitrous acid. bacteria in converting ammonia and ni- 4. Nitrogen dioxide (NO2), which gives a trites to nitrates. It is an important stage in mixture of nitrous and nitric acids in the nitrogen cycle. See also nitrogen cycle; water. nitrogen fixation. 5. Dinitrogen tetroxide (N2O4), which is in equilibrium with nitrogen dioxide. nitrite A salt of nitrous acid. 6. Dinitrogen pentoxide (N2O5) – the anhydride of nitric acid. nitrogen A colorless odorless gaseous 7. Nitrogen trioxide (NO3) – an unstable element, the first member of group 15 (for- white solid formed by the reaction of merly VA) of the periodic table. It is a very dinitrogen pentoxide and ozone (O3). electronegative element existing in the un- Nitrogen also forms a number of binary combined state as gaseous diatomic N2 halides such as NF3 and NCl3, and halogen molecules. The nitrogen atom has the elec- azides such as ClN3. Except for NF3 these 2 3 tronic configuration [He]2s 2p . It is typi- are all very explosive, and even NF3, which cally nonmetallic and its bonding is is reputedly stable, has been known to ex- primarily by polarized covalent bonds. plode. With electropositive elements the nitride Transition metals form nitrides that are ion N3– may be formed. different from the ionic or covalent ni- Nitrogen accounts for about 78% of trides, the stoichiometries are, for example, the atmosphere (by volume) and it also oc- ZrN, W2N, Mn4N. They are often called curs as sodium nitrate in various mineral ‘interstitial’ nitrides. They are very hard deposits. It is separated for industrial use and their formation is the basis of nitriding by the fractional distillation of liquid air. – surface hardening by exposing hot metal Pure nitrogen is prepared by thermal de- to ammonia gas (see case hardening). composition of azides: Nitrogen has two isotopes; 14N, the → 15 2NaN3 2Na + 3N2 common isotope, and N (natural abun- The ‘active nitrogen’ gas obtained by dance 0.366%). The latter is used as a passing an electric discharge through nitro- marker in mass spectrometric studies. gen is a mixture of ordinary nitrogen mol- Symbol: N; m.p. –209.86°C; b.p. ecules and excited single nitrogen atoms. –195.8°C; mass density 1.2506 kg m–3 As a diatomic molecule nitrogen is ef- (0°C); p.n. 7; r.a.m. 14. fectively triple-bonded and has a high dissociation energy (940 kJ mol–1). It is nitrogen cycle The circulation of nitro- therefore relatively inert at room tempera- gen and its compounds in the environment, ture and reacts readily only with lithium one of the major natural cycles of an el-

153 nitrogen dioxide ement. Nitrogen occurs mainly as a di- monoxide can be released by heating. Ni- atomic gas in the atmosphere and as nitrogen monoxide can be prepared in the trates in the soil. Denitrification of these lab by the action of nitric acid on copper nitrates converts them to nitrogen. Plants turnings and purified by using a solution of also take up nitrates and are eaten by ani- iron(II) ions to absorb the product. Com- mals; the plants and animals convert them mercially, nitrogen monoxide is prepared to proteins, which after the death of the or- by the catalytic oxidation of ammonia or ganisms are broken down to yield ammo- by the direct union of nitrogen and oxygen nia, which NITRIFICATION converts back to in an electric arc. Nitrogen monoxide is the nitrates. NITROGEN FIXATION by bacteria or most heat-stable of the oxides of nitrogen, lightning also gives rise to nitrates. only decomposing above 1000°C. At ordinary temperatures it combines immediately nitrogen dioxide (NO2) A brown gas with oxygen to give nitrogen dioxide: produced by the dissociation of dinitrogen → 2NO(g) + O2(g) 2NO2(g) tetroxide N2O4, with which it is in equilibrium, the dissociation being complete at nitrogen oxides (NO ) Any of the vari- ° x 140 C. Further heating causes dissociation ous compounds formed between nitrogen to colorless nitrogen monoxide and oxy- and oxygen. See dinitrogen oxide; nitrogen: gen; nitrogen dioxide; nitrogen monoxide. 2NO2(g) = 2NO(g) + O2(g) Nitrogen dioxide can also be made by nitrous acid (HNO ) A weak acid the action of heat on metal nitrates, al- 2 known only in solution or as a gas, ob- though not the nitrates of the alkali metals tained by acidifying a solution of a nitrite. or some of the alkaline-earth metals. It readily decomposes on warming or shaking to nitrogen monoxide and nitric acid. It nitrogen fixation (fixation of nitrogen) is used extensively in the dyestuffs indus- A reaction in which atmospheric nitrogen try. Nitrous acid and the nitrites are nor- is converted into nitrogen compounds. Ni- trogen fixation occurs naturally as part of mally reducing agents but in certain the life cycle processes of certain bacteria circumstances they can behave as oxidizing present in the soil and in the roots of legu- agents, e.g. with sulfur dioxide and hydro- minous plants. Small amounts of nitrogen sulfide. gen(II) oxide (NO) are also formed in thunderstorms by direct combination of nitrous oxide See dinitrogen oxide. the elements during lightning discharges. Fixation of nitrogen is important because nitryl ion (nitronium ion) The positive + nitrogen is an essential element for life, and ion NO2 . See nitration. because vast amounts of nitrogenous fertilizers are used for agriculture. Former NMR See nuclear magnetic resonance. processes for fixing nitrogen were the nobelium A synthetic radioactive Birkeland-Eyde process (reacting N2 and transuranic element of the actinoid series, O2 in an electric arc – the equivalent of a lightning discharge in nature), and the created by bombarding a curium isotope cyanamide process (heating calcium dicar- with the nuclei of a carbon isotope. Several bide with nitrogen to give CaCN2). The very short-lived isotopes have been synthe- major method employed today is the sized. Haber process for making ammonia. Symbol: No; m.p. 863°C; b.p., r.d. unknown; p.n. 102; most stable isotope nitrogen monoxide (nitric oxide; NO) 259No (half-life 58 minutes). A colorless gas that is insoluble in water but dissolves in a solution containing noble gases See rare gases. iron(II) ions owing to the formation of the complex ion (FeNO)2+: the nitrogen nonionic detergents See detergents.

154 nuclear magnetic resonance nonlocalized bond See delocalized symbol N, e.g. 0.2N, N/10, etc. Because bond. there is not a clear definition of equivalent weight suitable for all reactions, a solution nonmetal Any of a class of chemical el- may have one value of normality for one ements. Nonmetals lie right of the diagonal reaction and another value in a different reformed by the METALLOIDS in the periodic action. Because of this many workers pre- table. They are electronegative elements fer the molar solution notation. with a tendency to form covalent compounds or negative ions. They have acidic Norrish, Ronald George Wreyford oxides and hydroxides. In the solid state, (1897–1978) British physical chemist. nonmetals are either covalent volatile crys- Norrish made important contributions to tals or macromolecular (giant-covalent) the study of fast chemical reactions, partic- crystals. See also metals. ularly those initiated by light. Between 1949 and 1965 he developed the tech- nonpolar compound A compound niques of flash photolysis and kinetic spec- that has molecules with no permanent di- troscopy with George PORTER to study fast pole moment. Examples of nonpolar com- chemical reactions. Norrish and Porter pounds are hydrogen and carbon dioxide. shared the 1967 Nobel Prize for chemistry with Manfred EIGEN for this work. In his nonpolar solvent See solvent. later years Norrish studied chain reactions and the kinetics of polymerization. nonstoichiometric compound A chemical compound whose molecules contain NOx See nitrogen oxides. fractional numbers of atoms. For example, titanium(IV) oxide in the form of the min- NTP See STP. eral rutile has the chemical formula TiO1.8. nuclear magnetic resonance (NMR) A normality The number of gram equiva- method of investigating nuclear spin. In an lents per cubic decimeter of a given solu- external magnetic field the nucleus of an tion. atom can have certain quantized energy states, corresponding to certain orienta- normal modes of vibration The basic tions of the spin magnetic moment. Hydro- vibrations of a polyatomic molecule. All vi- gen nuclei, for instance, can have two brational motion of a polyatomic molecule energy states, and transitions between the is a superposition of the normal modes of two occur by absorption of radiofrequency vibration of the molecule. If N is the num- radiation. In chemistry, this is the basis of ber of atoms in a molecule the number of a spectroscopic technique for investigating modes of vibration is 3N–5 for a linear the structure of molecules. Radiofrequency molecule and 3N–6 for a nonlinear mol- radiation is fed to a sample and the mag- ecule. Each of these vibrational modes has netic field is changed slowly. Absorption of a characteristic frequency, although it is the radiation is detected when the differ- possible for some of them to be degenerate. ence between the nuclear levels corre- For example, in a linear triatomic molecule sponds to absorption of a quantum of there are four normal modes of vibration radiation. This difference depends slightly since 3N–5 = 4 for N = 3. These vibrational on the electrons around the nucleus – i.e. modes are: (a) symmetric stretching the position of the atom in the molecule. (breathing) vibrations; (b) antisymmetric Thus a different absorption frequency is stretching vibrations; and (c) two bending seen for each type of hydrogen atom. In vibrations, which are degenerate. ethanol, for example, there are three frequencies, corresponding to hydrogen normal solution A solution that con- atoms on the CH3, CH2, and OH groups. tains one gram equivalent weight per liter The intensity of absorption also depends of solution. Values are designated by the on the number of hydrogen atoms (3:2:1).

155 nucleon number

NMR spectroscopy is a powerful method of 92 protons and 146 protons (mass 4 × of finding the structures of complex com- 10–25 kg, radius 9.54 × 10–15 m). Only cer- pounds. tain combinations of protons and neutrons form stable nuclei. Others undergo sponta- nucleon number (mass number) Symbol: neous decay. A The number of nucleons (protons plus A nucleus is depicted by a symbol indi- neutrons) in an atomic nucleus. cating nucleon number (mass number), proton number (atomic number), and el- 2 3 nucleus The compact positively charged ement name. For example, 1 1 Na repre- center of an atom, made up of one or more sents a nucleus of sodium having 11 nucleons (protons and neutrons) around protons and mass 23, hence there are (23 – which is a cloud of one or more electrons. 11) = 12 neutrons. The density of nuclei is about 1015 kg m–3. The number of protons in the nucleus de- nuclide A nuclear species with a given fines the element, being its proton number number of protons and neutrons; for ex- (also known as its atomic number). The ample, 23Na, 24Na, and 24Mg are all differ- nucleon number, or atomic mass number, ent nuclides. Thus: is the sum of the protons and neutrons. The 23Na has 11 protons and 12 neutrons simplest nucleus is that of a hydrogen 24Na has 11 protons and 13 neutrons atom, 1H, being simply one proton (mass 24Mg has 12 protons and 12 neutrons 1.67 × 10–27 kg). The most massive nucleus The term is applied to the nucleus and that occurs in any quantity on Earth is 238U often also to the atom in which it is found.

156 O

occlusion 1. The process in which small netic field strength. It is equal in SI units to amounts of one substance are trapped in 103/4π A m–1. the crystals of another; for example, pockets of liquid occluded during crystallization ohm Symbol: Ω The SI derived unit of from a solution. electrical resistance, equal to a resistance 2. Absorption of a gas by a solid; for ex- that passes a current of one ampere when ample, the occlusion of hydrogen by palla- there is an electric potential difference of dium. one volt across it. 1 Ω = 1 V A–1. ocher A clay or rock containing signifi- oil of vitriol Sulfuric(VI) acid. cant amounts of iron(III) oxide (Fe2O3), used as a yellow, orange, or brown pig- oleum (disulfuric(VI) acid; fuming sulfuric ment. acid; pyrosulfuric acid; H2S2O7) A colorless fuming liquid formed by dissolving sul- octahedral complex See complex. fur(VI) oxide in concentrated sulfuric acid. It gives sulfuric acid on dilution with octahydrate A crystalline compound water. See contact process. containing eight molecules of water of crystallization per molecule of compound. olivine A greenish rock-forming silicate mineral ((Mg,Fe)2SiO4) containing iron octavalent Having a valence of eight. and magnesium. Clear, green, gemstone quality olivine is called peridot. octaves, law of See Newlands’ law. onium ion An ion formed by addition octet A stable shell of eight electrons in of a proton (H+) to a previously neutral + an atom. The completion of the octet gives molecule. The hydronium ion (H3O ) and + rise to particular stability, which is the ammonium ion (NH4 ) are examples. basis of the Lewis octet theory: 1. The rare gases have complete octets and Onsager, Lars (1903–76) Norwegian- are essentially chemically inert. born American chemist. Onsager made 2. The bonding in small covalent molecules several important contributions to theoret- is frequently achieved by the central ical chemistry and physics. In 1926 he im- atom completing its octet by sharing proved on the Debye-Hückel theory of electrons with surrounding atoms, e.g. electrolytes by taking the Brownian motion CH4, H2O. of ions into account. He subsequently in- 3. The ions formed by electropositive and vestigated the dielectric constants of matter electronegative elements are generally that contains polar molecules. In 1931 he those with a complete octet, e.g. Na+, published fundamental work on nonequi- Ca2+, O2–, Cl–. librium thermodynamics. He was awarded the 1968 Nobel Prize for chemistry for this oersted Symbol: Oe A c.g.s. unit of mag- work.

157 oolite COOH COOH

C C CH3 OH CH3 H

H OH

(R)-lactic acid (S)-lactic acid

Optical activity: two forms of lactic acid oolite Any sedimentary rock consisting is passed through them. Optical activity of an aggregate of small spherical masses, can be observed in crystals, gases, liquids, most typically composed of calcium car- and solutions. The amount of rotation de- bonate (CaCO3), in a fine matrix. pends on the concentration of the active compound. opal A naturally occurring hydrated Optical activity is caused by the interac- noncrystalline form of silica (SIO2.nH2O), tion of the varying electric field of the light valued as gemstones. Various layers within with the electrons in the molecule. It occurs precious opal can reflect and refract light when the molecules are asymmetric – i.e. to give a play of spectral colors. Varieties they have no plane of symmetry. Such mol- include black, fire, and milky white opal. ecules have a mirror image that cannot be superimposed on the original molecule. In open-hearth process An obsolete organic compounds this usually means that method of making steel by heating pig iron they contain a carbon atom attached to in an open-hearth furnace by burning a four different groups, forming a chiral cen- mixture of producer gas and air. Oxygen is ter. The two mirror-image forms of an injected through pipes to oxidize impuri- asymmetric molecule are optical isomers. ties and burn off carbon, and lime is used One isomer will rotate the polarized light to form a slag. The basic oxygen process in one sense and the other by the same has replaced it. amount in the opposite sense. Such isomers are described as dextrorotatory (symbol d optical activity The ability of certain or (+)) or levorotatory (symbol l or (–)), ac- compounds to rotate the plane of polariza- cording to whether they rotate the plane to tion of plane-polarized light when the light the ‘right’ or ‘left’ respectively (rotation to

N N

X X

X M N N M X

N N

Optical activity: two forms of an octahedral complex. The bidentate ligands are represented by a curved line

158 orbital the left is clockwise to an observer viewing is R-. If descending priority is anticlockwise the light coming toward the observer). A it is S-. mixture of the two isomers in equal The existence of a carbon atom bound amounts does not show optical activity. to four different groups is not the strict Such a mixture is sometimes called the (±) condition for optical activity. The essential or dl-form, a racemate, or a racemic mix- point is that the molecule should be asym- ture metric. Inorganic octahedral complexes, Optical isomers have identical physical for example, can show optical isomerism properties (apart from optical activity) and (see illustration). It is also possible for a cannot be separated by fractional crystal- molecule to contain asymmetric carbon lization or distillation. Their general chem- atoms and still have a plane of symmetry. ical behavior is also the same, although One structure of tartaric acid has two parts they do differ in reactions involving other of the molecule that are mirror images, optical isomers. Many naturally occurring thus having a plane of symmetry. This substances are optically active (only one (called the meso-form) is not optically ac- optical isomer exists naturally) and bio- tive. See also resolution. chemical reactions occur only with the natural isomer. For instance, the natural form optical isomers See isomerism; optical of glucose is d-glucose and living organ- activity. isms cannot metabolize the l-form. The terms ‘dextrorotatory’ and ‘levoro- optical rotary dispersion (ORD) The tatory’ refer to the effect on polarized light. phenomenon in which the amount of rotation of plane-polarized light by an optically A more common method of distinguishing active substance depends on the wave- two optical isomers is by their D-form length of the light. Plots of rotation against (dextro-form) or L-form (levo-form). This wavelength can be used to give informa- convention refers to the absolute structure tion about the molecular structure of opti- of the isomer according to specific rules. cally active compounds. Sugars are related to a particular configuration of glyceraldehyde (2,3-dihydrox- optical rotation Rotation of the plane ypropanal). For alpha amino acids the of polarization of plane-polarized light by ‘corn rule’ is used: the structure of the acid an optically active substance. RC(NH2)(COOH)H is drawn with H at the top; viewed from the top the groups orbital A region around an atomic nu- spell CORN in a clockwise direction for all cleus in which there is a high probability of D-amino acids (i.e. the clockwise order is finding an electron. The modern picture of –COOH,R,NH2). The opposite is true for the atom, according to quantum mechan- L-amino acids. Note that this convention ics, does not have electrons moving in fixed does not refer to optical activity: D-alanine elliptical orbits. Instead, there is a finite is dextrorotatory but D-cystine is levorota- probability that the electron will be found tory. in any small region at all possible distances An alternative is the R/S system for from the nucleus. In the hydrogen atom the showing configuration. There is an order probability is low near the nucleus, in- of priority of attached groups based on the creases to a maximum, and falls off to in- proton number of the attached atom: finity. It is useful to think of a region in I, Br, Cl, SO3H, OCOCH3, OCH3, OH, space around the nucleus – in the case of NO2, NH2, COOCH3, CONH2, COCH3, hydrogen the region within a sphere – CHO, CH2OH, C6H5, C2H5, CH3, H within which there is a high chance of find- Hydrogen has the lowest priority. The ing the electron. Each of these regions, chiral carbon is viewed such that the group called an atomic orbital, corresponds to a of lowest priority is hidden behind it. If the subshell and can ‘contain’ either a single other three groups are in descending prior- electron or two electrons with opposite ity in a clockwise direction, the compound spins. Another way of visualizing an or-

159 z z z

y y y

x x x

dxy orbital dz2 orbital s orbital z z z

y y y

x x x

px orbital py orbital pz orbital

Orbital: atomic orbitals

ATOMIC ORBITALS HYBRID ORBITALS SHAPE

+ one s one p two sp3 linear

one s two p three sp2 planar

one s three p four sp3 tetrahedral

Orbital: hybrid orbitals

160 orbital

s orbitals sigma

px orbitals sigma

C z z

pz orbitals pi

Orbital: molecular orbitals bital is as a cloud of electron charge (the symmetrical about an axis between the nu- cloud being the average distribution of the clei, it is a sigma orbital. This can occur, charge with time). for instance, by overlap of two s orbitals, Similarly, in molecules the electrons as in the hydrogen atom, or two p orbitals move in the combined field of the nuclei with their lobes along the axis. However, and can be assigned to molecular orbitals. two p orbitals overlapping at right angles In considering bonding between atoms it is to the axis form a different type of molecu- useful to treat molecular orbitals as formed lar orbital – a pi orbital – with regions by overlap of atomic orbitals. above and below the axis. Pi orbitals are It is possible to calculate the shapes and also formed by overlap of d orbitals. Each energies of atomic and molecular orbitals molecular orbital can contain a pair of by QUANTUM THEORY. The shapes of atomic electrons, forming a sigma bond or pi orbitals depend on their orbital angular bond. A double bond, for example the momentum. For each shell there is, at a bond in ethene, is a combination of a sigma maximum, one s orbital, three p orbitals, bond and a pi bond. five d orbitals, and seven f orbitals. The s Hybrid orbitals are atomic orbitals orbitals are spherical, the p orbitals each formed by combinations of s, p, and d have two lobes, the d orbitals have more atomic orbitals, and are useful in describ- complex shapes, typically with four lobes, ing the bonding in compounds. There are and the f orbitals are more complex yet, various types. In carbon, for instance, the often with eight lobes. electron configuration is 1s22s22p2. Car- Molecular orbitals are formed by over- bon, in its outer (valence) shell, has one lap of atomic orbitals, and again there are filled s orbital, two filled p orbitals, and different types. If the orbital is completely one ‘empty’ p orbital. These four orbitals

161 ORD may hybridize (sp3 hybridization) to act as ore A mineral source of a chemical ele- four equal orbitals arranged tetrahedrally, ment. each with one electron. In methane, each hybrid orbital overlaps with a hydrogen s ore dressing See beneficiation. orbital to form a sigma bond. Alterna- tively, the s and two of the p orbitals may organic chemistry The chemistry of hybridize (sp2 hybridization) and act as compounds of carbon. Originally the term three orbitals in a plane at 120°. The re- organic chemical referred to chemical com- maining p orbital is at right angles to the pounds present in living matter, but now it plane, and can form pi bonds. Finally, sp covers any carbon compound with the ex- hybridization may occur, giving two or- ception of certain simple ones, such as the bitals in a line. More complex types of hy- carbon oxides, carbonates, cyanides, and bridization, involving d orbitals, explain cyanates. These are generally studied in in- the geometries of inorganic complexes. For organic chemistry. The vast numbers of example, square-planar complexes can be synthetic and natural organic compounds formed by sp2d hybridization; octahedral exist because of the ability of carbon to complexes can involve sp3d2 hybridization. form chains of atoms (catenation). Other The combination of two atomic orbitals elements are involved in organic com- in fact produces two molecular orbitals. pounds; principally hydrogen and oxygen One – the bonding orbital – has a concen- but also nitrogen, halogens, sulfur, and tration of electron density between the nu- phosphorus. clei, and thus tends to hold the atoms together. The other – the antibonding or- organometallic compound An orbital – has slightly higher energy and tends ganic compound containing a carbon– to repel the atoms. If both atomic orbitals metal bond. Tetraethyl lead, (C2H5)4Pb, is are filled, the two molecular orbitals are a well-known example. also filled and cancel each other out – there is no net bonding effect. If each atomic or- ortho- 1. Designating the form of a di- bital has one electron, the pair occupies the atomic molecule in which both nuclei have lower energy bonding orbital, producing a the same spin direction; e.g. orthohydro- net attraction. gen, orthodeuterium. 2. Certain acids, regarded as formed from ORD See optical rotary dispersion. an anhydride and water, were named ortho acids to distinguish them from the less hy- order The sum of the indices of the con- drated meta acids. For example, H4SiO4 centration terms in the expression that de- (from SiO2 + 2H2O) is orthosilicic acid; termines the rate of a chemical reaction. H2SiO3 (SiO2 + H2O) is metasilicic acid. For example, in the expression See also meta-; para-. rate = k[A]x[B]y x is called the order with respect to A, y the orthoboric acid See boric acid. order with respect to B, and (x + y) the order overall. The values of x and y are not orthohydrogen See hydrogen. necessarily equal to the coefficients of A and B in the molecular equation. Order is orthophosphates See phosphoric(V) an experimentally determined quantity de- acid. rived without reference to any equation or mechanism. Fractional orders do occur. orthophosphoric acid See phosphoric(V) For example, in the reaction acid. → CH3CHO CH4 + CO 1.5 the rate is proportional to [CH3CHO] orthophosphorous acid See phospho- i.e. it is of order 1.5. nic acid.

162 osmotic pressure orthorhombic crystal See crystal sys- closed system is necessary for liquid–vapor tem. equilibrium, the development of an OS- MOTIC PRESSURE on the solution side is nec- osmiridium A very hard naturally oc- essary for equilibrium at the membrane. curring alloy consisting chiefly of osmium and iridium, used to make pen nibs. osmotic pressure Symbol: π The pressure that must be exerted on a solution to osmium A hard bluish white transition prevent the passage of solvent molecules element, the third element of group 8 (for- into it when the solvent and solution are merly part of subgroup VIIIB) of the peri- separated by a semipermeable membrane. odic table. It is found associated with The osmotic pressure is therefore the pres- platinum. Osmium is the densest of all met- sure required to maintain equilibrium be- als. It has a characteristic chlorinelike smell tween the passage of solvent molecules resulting from the production of os- through the membrane in either direction mium(VIII) oxide (osmium tetroxide, and thus prevent the process of osmosis OsO4) when heated in air. The metal is from proceeding. The osmotic pressure can used in catalysts and in alloys for pen nibs, be measured by placing the solution, con- pivots, and electrical contacts. tained in a small perforated thimble cov- Symbol: Os; m.p. 3045°C; b.p. 5027°C; ered by a semipermeable membrane and r.d. 22.59 (20°C); p.n. 76; most common fitted with a length of glass tubing, in a isotope 192Os; r.a.m. 190.23. beaker of the pure solvent. Solvent molecules pass through the membrane, dilut- osmium(IV) oxide (osmium tetroxide; ing the solution and thereby increasing the OsO4) A volatile yellow crystalline solid volume on the solution side and forcing the with chlorinelike penetrating poisonous solution to rise up the glass tubing. The odor, used as an oxidizing agent and, in process continues until the pressure ex- aqueous solution, as a catalyst for organic erted by the solvent molecules on the mem- reactions. brane is balanced by the hydrostatic pressure of the solution in the tubing. A osmosis Systems in which a solvent is sample of the solution is then removed and separated from a solution by a semiperme- its concentration determined. Osmosis is a able membrane approach equilibrium by colligative property; therefore the method solvent molecules on the solvent side of the can be applied to the determination of rel- membrane migrating through it to the so- ative molecular masses, particularly for lution side; this process is called osmosis large molecules, such as proteins, but it is and always leads to dilution of the solu- restricted by the difficulty of preparing tion. The phenomenon is quantified by good semipermeable membranes. measurement of the osmotic pressure. The Because the osmotic pressure is a col- process of osmosis is of fundamental im- ligative property it is directly proportional portance in transport and control mecha- to the molar concentration of the solute if nisms in biological systems; for example, the temperature remains constant; thus π is plant growth and general cell function. proportional to the concentration n/V, Osmosis is a colligative property and its where n is the number of moles of solute theoretical treatment is similar to that for and V the solvent volume. The osmotic the lowering of vapor pressure. The mem- pressure is also proportional to the ab- brane can be regarded as equivalent to the solute temperature. Combining these two liquid-vapor interface, i.e. one that permits proportionalities gives πV = nCT, which free movement of solvent molecules but re- has the same form as the gas equation, PV stricts the movement of solute molecules. = nRT, and experimental values of C are The solute molecules occupy a certain area similar to those for R, the universal gas at the interface and therefore inhibit sol- constant. This gives considerable support vent egress from the solution. Just as the to the kinetic theory of colligative proper- development of a vapor pressure in a ties.

163 Ostwald, Friedrich Wilhelm

– 2+ → Ostwald, Friedrich Wilhelm (1854– 4CN + 2Cu C2N2 + 2CuCN 1932) Russian-born German physical where Cu2+ is the oxidizing agent and CN– chemist. Ostwald was one of the main fig- is oxidized. ures involved in establishing physical The oxidation state of an atom is indi- chemistry as a distinct subject. In 1888 he cated by the number of electrons lost or ef- put forward the OSTWALD DILUTION LAW for fectively lost by the neutral atom, i.e. its the degree of dissociation of an electrolyte. oxidation number. The oxidation number His own research was mainly concerned of a negative ion is negative. The process of with catalysts. This led to the discovery of oxidation is the converse of reduction. See the OSTWALD PROCESS for converting am- also redox. monia to nitric acid by passing air and ammonia over a platinum catalyst. Ostwald oxidation number See oxidation. won the 1909 Nobel Prize for chemistry for his work on catalysis. He also wrote an oxidation–reduction See redox. influential two-volume textbook entitled Textbook of General Chemistry (1885, oxidation state See oxidation. 1887). oxide A compound of OXYGEN and an- Ostwald’s dilution law See dissocia- other element. Oxides can be made by dition. rect combination of the elements or by heating a hydroxide, carbonate, or other ox See oxalic acid. salt. See also acidic oxide; basic oxide; neutral oxide; peroxide. oxalato See oxalic acid. oxidizing acid An acid that acts as an oxidizing agent, e.g. sulfuric acid or nitric O O acid. Because it is oxidizing, nitric acid can CC dissolve metals below hydrogen in the electrochemical series: _ _ → O O 2HNO3 + M MO + 2NO2 + H2O → MO + 2HNO3 H2O + M(NO3)2 Oxalic acid: the oxalato ligand oxidizing agent See oxidation. oxalic acid A dibasic organic carboxylic acid, CH2(COOH)2. It is of interest in in- oxyacid An ACID in which the replace- organic chemistry because the carboxylate able hydrogen atom is part of a hydroxyl ion functions as a bidentate ligand (the ox- group, including inorganic acids such as alato ligand) in forming certain complexes. phosphoric(V) acid and sulfuric acid. In the names of complexes it is given the symbol ox. oxygen A colorless odorless diatomic gas, the first member of group 16 (formerly oxidant See oxidation. VI) of the periodic table. It has the electronic configuration [He]2s22p4 and its oxidation An atom, an ion, or a mol- chemistry involves the acquisition of elec- ecule is said to undergo oxidation or to be trons to form either the di-negative ion O2– oxidized when it loses electrons. The or two covalent bonds. In each case the process may be effected chemically, i.e. by oxygen atom attains the configuration of reaction with an oxidizing agent (oxidant), the rare gas neon. or electrolytically, in which case oxidation Oxygen is the most plentiful element in occurs at the anode. For example, the Earth’s crust, accounting for over 49% → + – 2Na + Cl2 2Na + 2Cl by mass. It accounts for 23% of the volume where chlorine is the oxidizing agent and of the atmosphere and is a constituent of sodium is oxidized, and the majority of minerals and rocks (e.g. sil-

164 oxygen icates, SiO2, carbonates, CaCO3, alumi- stronger acids. The higher the oxidation nosilicates, Al2SiO5) as well as being the state of the central atom the stronger the major constituent of the sea. Oxygen is an acid (e.g. HOCl is weaker than HClO2, essential element for almost all living which is weaker than HClO4). Nonmetal things. It is also used in solid propellants oxides may be gaseous (SO2, CO2), liquid for rockets, in steel making and welding, in (N2O4), or solid (P2O5). The weakly non- medical environments and breathing appa- metallic elements such as silicon may re- ratus, and in the manufacture of various quire bases for dissolution and in some chemicals. Elemental oxygen has two al- cases the oxides have extended polymeric lotropes: the diatomic molecule O2 and the structures, e.g., SiO2. In between the acidic less stable molecule OZONE (trioxygen O3), and basic oxides are those that are ampho- which is formed by passing an electric dis- teric, i.e. they behave acidically to strong charge through oxygen gas. There are sev- bases and basically toward strong acids. eral convenient laboratory routes by which This behavior is associated with elements to produce oxygen: that are physically metallic but are chemi- 1. Heat on unstable oxides. cally only weakly cationic, e.g., ZnO gives → 2HgO 2Hg + O2 2– 2– Zn with acids and zincates, Zn(OH)4 , 2. Decomposition of peroxides. with bases. There is also a small class of → 2H2O2 2H2O + O2 mixed oxides that can be regarded as salts 3. Heat on ‘-ate’ compounds, such as chlo- between a ‘basic’ and an ‘acidic’ oxide of rates or manganate(VII). → the same metal: for example, 2KClO3 KCl + 3O2 FeO + Fe O → Fe(FeO ) = Fe O 2KMnO → K MnO + MnO + O 2 3 2 2 3 4 4 2 4 2 2 2PbO + PbO → Pb O Industrially oxygen is obtained by the 2 3 4 The neutral oxides are essentially insol- fractional distillation of liquid air. uble nonmetal oxides such as N O, NO, Oxygen reacts with elements from all 2 CO, and F O, although extreme conditions groups of the periodic table, including 2 can lead to some degree of acidic behavior xenon in group 18 (e.g. XeO , XeO , 3 4 in the case of CO, e.g., XeOF , XeO 2–). 2 2 CO + OH– → HCOO– The nature of the oxides has long been Oxygen is very limited in its catenation a convenient guide to metallic and non- but an interesting range of O–O species is metallic character. Thus, metals typically 2– formed by the following: peroxide, O2 ; combine with oxygen to form basic oxides, – which if soluble react with water to form superoxide, O2 ; oxygen gas, O2; oxygenyl + bases via the OH– ion. The more elec- cation, O2 . The O–O bond gets both pro- tropositive the metal, the more vigorous gressively shorter and stronger in the se- the reaction. Most of the group 1 and ries. The peroxides are formed with Na, group 2 metals react vigorously when Ca, Sr, and Ba and are formally related to 2– hydrogen peroxide (prepared by the action forming oxides, O , (e.g., Na2O, CaO); 2– of acids on ionic peroxides). The superox- peroxides, O2 , (e.g., Na2O2, BaO2); and – ides of K, Rb, and Cs are prepared by burn- superoxides, O2 , (e.g., RbO2, CsO2). Ox- ides of metallic elements are solids and x- ing the metal in air (the less electropositive ray studies show that discrete O2– ions do metals Na, Mg, and Zn form less stable su- + exist in the solid state even though they do peroxides). Oxygenyl cation, O2 , is not exist in solution: formed by the reaction of PtF6 (which has 2– → – O + H2O 2OH a high electron affinity) with oxygen at The nonmetals typically form acidic ox- room temperature. Other oxygenyl cation ides such as SO2, ClO2, and NO2. When species can be made with group 15 fluo- + – soluble in water they generally dissolve to rides, e.g. O2 AsF6 . form acids. The less electronegative ele- Oxygen occurs in three natural isotopic ments among the nonmetals generally form forms, 16O (99.76%), 17O (0.0374%), and 18 weaker acids, e.g. B(OH)3 and H2CO3. O (0.2039%); the rarer isotopes are used The more electronegative elements form in detailed studies of the behavior of oxy-

165 ozone gen-containing groups during reactions the stratosphere it forms a layer of (tracer studies). ozonized air that acts to screen the Earth Symbol: O; m.p. –218.4°C; b.p. from harmful short-wave ultraviolet radia- –182.962°C; d. 1.429 kg m–3 (0°C); p.n. 8; tion. In recent decades there has been con- r.a.m. 15.9994. siderable evidence to suggest that this layer is being depleted by fluorocarbons and ozone (trioxygen; O3) A poisonous blue other compounds produced by industry. diamagnetic allotrope of oxygen made nat- Consequently most industrialized nations urally by high-energy ultraviolet radiation have agreed to severely restrict their pro- in the stratosphere and to a lesser extent by duction and use of these chemicals. photochemical reactions involving pollu- Ozone is a strong oxidizing agent. Its tants such as NOx. Ozone can also be made concentration can be determined by react- artificially by passing oxygen through a ing it with iodide ions and titrating the silent electric discharge. Ozone is unstable iodine formed with standard sodium thio- and decomposes to oxygen on warming. In sulfate(VI).

166 P

paint A suspension of powdered pig- moisture and some components will have a ment in a liquid vehicle, used for decorative greater tendency than others to dissolve in and protective surface coatings. In oil- this moisture than in the mobile phase. In based paints, the vehicle contains solvents addition, some components will cling to and a drying oil or synthetic resin. In the surface of the paper. Therefore, as the water-based or emulsion paints, the vehicle solvent moves through the paper, certain is mostly water with an emulsified resin. components will be left behind and separated from each other. palladium A soft silvery white ductile When the solvent has almost reached transition metal, the second element of the top of the paper, the paper is removed group 10 (formerly part of subgroup and quickly dried. The paper is developed VIIIB) of the periodic table. It occurs native to locate the positions of colorless fractions in association with platinum and also in by spraying with a suitable chemical, e.g. nickel, copper, gold, and silver ores, from ninhydrin, or by exposure to ultraviolet ra- which it is obtained as a by-product. Al- diation. The components are identified by though it will dissolve in acid, it will react comparing the distance they have traveled with oxygen only at high temperatures, up the paper with standard solutions that making it resistant to corrosion in most en- have been run simultaneously, or by com- vironments. Consequently it finds use in puting an RF value. A simplified version of dentistry, surgical instruments, electrical paper chromatography uses a piece of filter relays, and jewelry (white gold is an alloy paper. The sample is spotted at the center of gold and palladium). It is also used as a of the paper and solvent passed through it. catalyst in hydrogenation processes. Hy- Separation of the components of the mix- drogen is purified by diffusing it through a ture again takes place as the mobile phase hot palladium barrier. spreads out on the paper. Symbol: Pd; m.p. 1552°C; b.p. 3140°C; Paper chromatography is an applica- r.d. 12.02 (20°C); p.n. 46; most common tion of the partition law. isotope 106Pd; r.a.m. 106.42. paper chromatography A technique widely used for the analysis of mixtures. Paper CHROMATOGRAPHY usually employs a specially produced paper as the station- developing ary phase. A base line is marked in pencil chamber near the bottom of the paper and a small sample of the mixture is spotted onto it paper strip using a capillary tube. The paper is then placed vertically in a suitable solvent, which rises up to the base line and beyond mobile phase by capillary action. The components within the sample mixture dissolve in this spotted samples mobile phase and are carried up the paper. However, the paper holds a quantity of Paper chromatography

167 para- para- Designating the form of a di- passive Describing a metal that is unre- atomic molecule in which both nuclei have active because of the formation of a thin opposite spin directions; e.g. parahydro- protective layer. Iron, for example, does gen, paradeuterium. not dissolve in concentrated nitric(V) acid See also meta-; ortho-. because of the formation of a thin oxide layer. parahydrogen See hydrogen. Pauli, Wolfgang (1900–58) Austrian- paramagnetism See magnetism. born Swiss theoretical physicist. Pauli is best known for his enunciation of the Pauli partial ionic character The electrons exclusion principle in 1925. This enabled of a covalent bond between atoms or the electronic structure of atoms to be un- groups with different electronegativities derstood, particularly how the shell struc- will be polarized toward the more elec- ture of atoms, and hence the periodic table tronegative constituent; the magnitude of of the elements, comes about. Pauli won this effect can be measured by the ionic the 1945 Nobel Prize for physics for this character of the bond. When the effect is work. Pauli made many other important small the bond is referred to simply as a contributions to physics including his pre- polar bond and is adequately treated using diction of the neutrino in beta decay, the dipole moments; as the effect grows incorporation of spin into quantum me- stronger the theoretical treatment requires chanics, and the explanation of paramag- other contributions to ionic character. Ex- netism in metals. He also wrote several amples of partial ionic character (from nu- classic books and reviews on quantum me- clear quadrupole resonance) are H–I, 21%; chanics. H–Cl, 40%; Li–Br, 90%. Pauli exclusion principle See atom. partial pressure See Dalton’s law. Pauling, Linus Carl (1901–94) Ameri- partition coefficient If a solute dis- can chemist. Pauling was one of the great- solves in two nonmiscible liquids, the par- est scientists of the 20th century. His early tition coefficient is the ratio of the work was on determining the structure of concentration in one liquid to the concen- complex minerals by x-ray diffraction. In tration in the other liquid. 1928–29 this led to Pauling’s rules govern- ing the structure of complex minerals. pascal Symbol: Pa The SI derived unit of Pauling was also a major pioneer in the ap- pressure, equal to a pressure of one newton plication of quantum mechanics to chemi- per square meter (1 Pa = 1 N m–2). cal bonding. In 1931 he published a classic paper entitled The Nature of the Chemical Paschen–Back effect See Zeeman ef- Bond that explained how a chemical bond fect. is formed from a pair of electrons. Pauling also introduced the concept of hybridiza- Paschen series A series of lines in the in- tion to explain the chemical bonding of the frared spectrum emitted by excited hydrocarbon atom. He also considered partially gen atoms. The lines correspond to the ionic bonds and put together his ideas atomic electrons falling into the third low- about chemical bonding in his great book est energy level and emitting energy as ra- The Nature of the Chemical Bond, the first diation. The wavelength (λ) of the edition of which was published in 1939. In radiation in the Paschen series is given by the mid-1930s Pauling turned his attention 1/λ = R(1/32 – 1/n2) where n is an integer to molecules of biological interest and was and R is the Rydberg constant. The series is one of the founders of molecular biology. named for German physicist Louis Paschen Together with Robert Corey, he showed (1865–1947), who discovered it in 1908. that many proteins have helical shapes. He See also spectral series. also worked on sickle-cell anemia. With E.

168 periodic table

Bright Wilson he co-authored the book In- composition is 12/16 = 75% carbon and (4 troduction to Quantum Mechanics (1935) × 1)/16 = 25% hydrogen. and wrote the influential books General Chemistry (1948) and College Chemistry perchloric acid See chloric(VII) acid. (1950). Pauling was awarded the 1954 Nobel Prize for chemistry. In the 1950s he perfect gas (ideal gas) See gas laws; ki- became concerned with nuclear weapons, netic theory. particularly their testing in the atmosphere. This led to him being awarded the 1962 peridot See olivine. Nobel Prize for peace. period One of the horizontal rows in the p-block elements The elements of the conventional periodic table. Each period main groups 13 (B to Tl), 14 (C to Pb), 15 represents the elements arising from pro- (N to Bi), 16 (O to Po), 17 (F to At), and 18 gressive filling of the outer shell (i.e. the ad- (He to Rn). They are so called because they dition of one extra electron for each new have outer electronic configurations that element), the elements being arranged in have occupied p levels. order of ascending proton number. In a strict sense hydrogen and helium represent pearl A lustrous spherical accretion of one period but convention refers to the el- nacre, comprised chiefly of calcium car- ements lithium to neon (8 elements) as the bonate (CaCO ), that builds up on a parti- first short period (n = 2), and the elements 3 sodium to argon (8 elements) as the second cle of foreign matter inside the shells of short period (n = 3). With entry to the n = oysters and some other mollusks. 4 level there is filling of the 4s orbital, then back-filling of the 3d orbitals, before the pearl spar See dolomite. 4p are filled. Thus this set is based on the filling of a total of 18 electrons (potassium peat A brown or black material formed to krypton) and is called a long period. The by the partial decomposition of plant re- next set, rubidium to xenon, is similarly a mains in marshy ground, the first stage in long period. See also Aufbau principle. the formation of coal. It is used to make charcoal and compost. When dried, it can periodic acid See iodic(VII) acid. also be burned as a fuel. periodic law The law upon which the PEC See photoelectrochemical cell. modern periodic table is based. Enunciated in 1869 by Mendeléev, this law stated that pentahydrate A crystalline solid con- the properties of the elements are a peri- taining five molecules of water of crystal- odic function of their atomic masses: if lization per molecule of compound. arranged in order of increasing atomic mass then elements having similar proper- pentavalent (quinquevalent) Having a ties occur at fixed intervals. Certain excep- valence of five. tions or gaps in the table lead to the view that the nuclear charge is a more charac- percentage composition A way of ex- teristic function, thus the modern state- pressing the composition of a chemical ment of the periodic law is that the physical compound in terms of the percentage (by and chemical properties of elements are a mass) of each of the elements that make it periodic function of their proton number. up. It is calculated by dividing the mass of each element (taking into account the num- periodic table A table of the elements ber of atoms present) by the relative mole- arranged in order of increasing proton cular mass of the whole molecule. For number to show similarities in chemical example, methane (CH4) has a relative behavior between elements. Horizontal molecular mass of 16 and its percentage rows of elements are called periods. Across

169 permanent gas a period there is a general trend from peroxide 1. An oxide containing the metallic to nonmetallic behavior. Vertical –O–O– ion. columns of related elements are called 2. A compound containing the –O–O– groups. Down a group there is an increase group. in atomic size and in electropositive (metallic) behavior. peroxosulfuric(VI) acid (Caro’s acid; Originally the periodic table was H2SO5) A white crystalline solid formed arranged in eight groups with the alkali by reacting hydrogen peroxide with sulfu- metals as group I, the halogens as group ric acid. It is a powerful oxidizing agent. VII, and the rare gases as group 0. The transition elements were placed in a block perturbation theory An approxima- in the middle of the table. Groups were tion technique in which a system is divided split into two subgroups. For example, into two parts, with one part that can be group I contained the main-group ele- solved exactly and a second part that ments, Li, Na, K, Rb, Cs, in subgroup IA makes the system too complicated to be and the subgroup IB elements, Cu, Ag, Au. solved exactly. For perturbation theory to The system was confusing because there be applicable it is necessary that the second was a difference in usage for subgroups part is a small correction to the first part. and the current form of the table has 18 The effects of the small corrections are ex- groups (see Appendix). pressed as an infinite series, called a per- See also group; period. turbation series, which modifies the results that could be calculated exactly. A classic permanent gas A gas that cannot be liq- example of perturbation theory is the mod- uefied by pressure alone – that is, a gas ification of the orbits of planets around the above its critical temperature. See critical Sun due to their gravitational interactions temperature. with other planets. In quantum mechanics an application of perturbation theory is the permanent hardness A type of water calculation of the effects of small electric or hardness caused by the presence of dis- magnetic fields on atomic energy levels. solved calcium, iron, and magnesium sulfates or chlorides. This form of hardness peta- Symbol: P A prefix used with SI cannot be removed by boiling. Compare units, denoting 1015. temporary hardness. See also water softening. petrochemicals Organic chemicals obtained from crude oil or natural gas. permanganate See manganate(VII). petrol See petroleum. Permutit (Trademark) A substance used to remove unwanted chemicals that have petroleum (crude oil) A mixture of hy- dissolved in water. It is a zeolite consisting drocarbons formed originally in shallow of a complex chemical compound, sodium basins from decayed, chiefly marine ani- aluminum silicate. When hard water is mals and plants, found today beneath the passed over this material, calcium and ground trapped between layers of sedimen- magnesium ions exchange with sodium tary rock. It is obtained by drilling. Differ- ions in the Permutit. This is a good exam- ent oilfields produce petroleum with ple of ION EXCHANGE. Once all the available differing compositions. The mixture is sep- sodium ions have been used up, the Permu- arated into fractions by fractional distilla- tit can be regenerated by washing it with a tion in a vertical column. The main saturated solution of sodium chloride. The fractions are: excess sodium ions then exchange with the Diesel oil (gas oil) in the boiling range ° calcium and magnesium ions in the Permu- 220–350 C, consisting mainly of C13–C25 tit. hydrocarbons. It is used in diesel engines.

170 phosphine

Kerosene (paraffin) in the range The lines show the conditions under which ° 160–250 C, consisting mainly of C11 and more than one phase can coexist at equi- C12 hydrocarbons. It is a fuel both for do- librium. For one-component systems (e.g. mestic heating and jet engines. water) the point at which all three phases Gasoline (petrol) in the range 40–180°C, can coexist at equilibrium is called the consisting mainly of C5–C10 hydrocarbons. triple point and is the point on the graph at It is used as motor fuel for gasoline engines which the pressure–temperature curves in- and as a raw material for making other tersect. chemicals. Refinery gas, consisting of C1–C4 gaseous phase equilibrium A state in which the hydrocarbons such as propane. proportions of the various phases in a In addition, lubricating oils and paraf- chemical system are fixed. When two or fin wax are obtained from the residue. The more phases are present at fixed tempera- black material left is composed of bitumen ture and pressure a dynamic condition will and asphalt. be established in which individual particles will leave one phase and enter another; at pewter A tarnish-resistant alloy of tin equilibrium this will be balanced by an and lead, today usually containing equal number of particles making the re- 80–90% tin. Modern pewter is hardened verse change, leaving the total composition by antimony and copper, which in the best unchanged. grades completely replaces the lead content. It is less dense than old pewter, and phase rule In a system at equilibrium having a bright or a soft satiny finish, is the number of phases (P), number of com- often used for decorative and ceremonial ponents (C), and number of degrees of free- purposes. dom (F) are related by the formula: P + F = C + 2 pH The logarithm to base 10 of the reciprocal of the hydrogen-ion concentration phenolphthalein An acid–base indica- of a solution. In pure water at 25°C the tor that is colorless in acid solutions and concentration of hydrogen ions is 1.00 × becomes red if the pH rises above the tran- 10–7 mol l–1, thus the pH equals 7 at neu- sition range of 8–9.6. It is used as the indi- trality. An increase in acidity increases the cator in titrations for which the end point value of [H+], decreasing the value of the lies clearly on the basic side (pH > 7), e.g. pH below 7. An increase in the concentra- oxalic acid or potassium hydrogentartrate tion of hydroxide ion [OH–] proportion- against caustic soda. ately decreases [H+], therefore increasing the value of the pH above 7 in basic solu- philosopher’s stone See alchemy. tions. Potential of hydrogen or pH values can be obtained approximately by using in- phlogiston theory A former theory of dicators. More precise measurements use combustion (disproved in the 18th century electrode systems. by Lavoisier). Flammable compounds were supposed to contain a substance (phlogis- phase One of the physically separable ton), which was presumed to be released parts of a chemical system. For example, a when they burned, leaving ash. mixture of ice (solid phase) and water (liquid phase) consists of two phases. A system phosgene See carbonyl chloride. consisting of only one phase is said to be homogeneous. A system consisting of more phosphates See phosphoric(V) acid. than one phase is said to be heterogeneous. phosphide A compound of phosphorus phase diagram A graphical representa- with a more electropositive element. tion of the state in which a substance will occur at a given pressure and temperature. phosphine (phosphorus(III) hydride;

171 phosphinic acid

PH3) A very poisonous colorless gas that cited atoms have relatively long lifetimes is slightly soluble in water. It has a charac- before they make transitions to lower en- teristic fishy smell. It can be made by react- ergy states. However, there is no defined ing water and calcium phosphide or by the time distinguishing phosphorescence from action of yellow phosphorus on a concen- fluorescence. trated alkali. Phosphine usually ignites 2. In general usage the term is applied to spontaneously in air because of contamina- the emission of ‘cold light’ – light produced tion with diphosphane. It decomposes into without a high temperature. The name its elements if heated to 450°C in the ab- comes from the fact that white phosphorus sence of oxygen and it burns in oxygen or glows in the dark as a result of a chemical air to yield phosphorus oxides. It reacts reaction with oxygen. The light comes with solutions of metal salts to precipitate from excited atoms produced directly in phosphides. Like its nitrogen analog am- the reaction – not from the heat produced. monia it forms salts, called phosphonium It is thus an example of chemilumines- salts. It also forms complex addition com- cence. There are also a number of pounds with metal ions. biochemical examples termed biolumines- cence; for example, phosphorescence is phosphinic acid (hypophosphorous acid; sometimes seen in the sea emanating from H3PO2) A white crystalline solid. It is a – marine organisms, or on rotting wood em- monobasic acid forming the anion H2PO2 anating from certain fungi (known as ‘fox in water. The sodium salt, and hence the fire’). acid, can be prepared by heating white (or yellow) phosphorus with sodium hydrox- phosphoric(V) acid (orthophosphoric ide solution. The free acid and its salts are acid; H PO ) A white solid that can be powerful reducing agents. 3 4 made by reacting phosphorus(V) oxide with water or by heating yellow phos- phosphonic acid (phosphorous acid; or- phorus with nitric acid. The naturally thophosphorous acid; H PO ) A color- 3 3 occurring phosphates (orthophosphates, less deliquescent solid that can be prepared M PO ) are salts of phosphoric(V) acid. by the action of water on phosphorus(III) 3 4 Aqueous solutions of phosphoric(V) acid oxide or phosphorus(III) chloride. It is a – have a sharp taste and have been used in dibasic acid producing the anions H2PO3 2– the manufacture of soft drinks. At about and HPO3 in water. The acid and its salts ° are weak reducing agents. On warming, 220 C phosphoric(V) acid is converted to phosphonic acid decomposes to phosphine pyrophosphoric acid (H4P2O7). Pure py- and phosphoric(V) acid. rophosphoric acid in the form of colorless crystals is made by warming solid phos- + phoric(V) acid with phosphorus(III) chlor- phosphonium ion The ion PH4 , derived from phosphine. ide oxide. Metaphosphoric acid is obtained as a polymer, (HPO3)n, by heating phos- ° phosphonium salts See phosphine. phoric(V) acid to 320 C. Both meta- and pyrophosphoric acids change to phos- phosphor A substance that shows lumi- phoric(V) acid in aqueous solution; the nescence or phosphorescence. change takes place faster at higher temperatures. Phosphoric(V) acid is tribasic, phosphorescence 1. The absorption of forming three series of salts with the anions – 2– 3– energy by atoms followed by emission of H2PO4 , HPO4 , and PO4 . These series electromagnetic radiation. Phosphores- of salts are acidic, neutral, and alkaline re- cence is a type of luminescence, and is dis- spectively. With the exception of lithium tinguished from fluorescence by the fact phophate the alkali metal phosphates are that the emitted radiation continues for soluble in water, but other metal phos- some time after the source of excitation has phates are insoluble. Phosphates are useful been removed. In phosphorescence the ex- in water softening and as fertilizers.

172 phosphorus phosphorous acid See phosphonic acid. Phosphorus has a sufficiently high ionization potential for the chemistry of its phosphorus A reactive solid nonmetal- compounds to be almost entirely covalent. lic element, the second element in group 15 Phosphorus compounds are formally P(III) (formerly VA) of the periodic table. It has and P(V) compounds. The element forms a 2 3 the electronic configuration [Ne]3s 3p hydride, PH3 (phosphine), which is analo- and is therefore formally similar to nitro- gous to ammonia, but it is not formed by gen. It is, however, very much more reac- direct combination. Phosphorus(V) hy- tive than nitrogen and is never found in drides are not known. nature in the uncombined state. Phospho- When phosphorus is burned in air the rus is widespread throughout the world; product is the highly hygroscopic white economic sources are rock phosphate (con- solid P4O10 (commonly called phosphorus sisting largely of calcium phosphate, pentoxide, P2O5); this is the P(V) oxide. In Ca3(PO4)2) and the apatites, variously a limited supply of air the P(III) oxide, occurring as both fluoroapatite (3Ca - 3 P4O6 (called phosphorus(III) oxide) is ob- (PO4)2.CaF2) and as chloroapatite (3Ca3- tained, contaminated with unreacted phos- (PO4)2CaCl2). Guano formed from the phorus. The two oxides both react with skeletal phosphate of fish in sea-bird drop- water to form acids: pings is also an important source of P O + 6H O → phosphorus. The largest amounts of phos- 4 6 2 4H3PO3 phosphonic acid phorus compounds produced are used in → P4O10 + 2H2O fertilizers and detergents, with smaller 4HPO metaphosphoric acid amounts being employed in safety matches, 3 P O + 6H O → pesticides, special alloys and glasses, food 4 10 2 4H PO phosphoric acid. and drinks, and fireworks. Phosphorus is 3 4 There are also many polyphosphoric an essential constituent of living tissue, acids and salts with different ring sizes and bones, and nucleic acids, and plays a very different chain lengths. Pyrophosphoric important part in metabolic processes and acid is obtained by melting phosphoric muscle action. Phosphorus is also required acid; further dehydration above 200°C for the development of vigorous root sys- leads to metaphosphoric acids: tems in green plants. → 2H3PO4 H4P2O7 + H2O The element is obtained industrially by → the reduction of rock phosphate using sand H4P2O7 2HPO3 + H2O (SiO ) and coke (C) in an electric furnace. Polyphosphates with molecular masses 2 from 1000 to 10 000 are industrially im- The phosphorus is distilled off as P4 molecules and collected under water as white portant in detergent formulations, while phosphorus (defined below): polyphosphates with lower mass numbers → are used to complex metal ions (sequestra- 2Ca3(PO4)2 + 6SiO2 + 10C P4 + tion). 6CaSiO3 + 10CO There are three common allotropes of Phosphorus forms the P(III) halides, phosphorus and several other modifica- PX3, with all the halogens, and P(V) tions of these, some of which have indefi- halides, PX5, with X = F, Cl, Br. The P(III) nite structures. The common forms and halides are formed by direct combination, their methods of preparation are: but because of hazards in the reaction of 1. white (or yellow) phosphorus recovered phosphorus and fluorine, PF3 is prepared by distillation of phosphorus as shown from PCl3. above; soluble in some organic solvents The P(III) halide molecules are all pyra- 2. red phosphorus obtained by the action midal (as is PH3) and are readily hy- of heat on white phosphorus; insoluble drolyzed. The P(V) halides are also readily in organic solvents hydrolyzed by water in two stages giving 3. black phosphorus obtained by the ac- first the oxyhalide, then phosphoric acid: → tion of heat on white phosphorus under PX5 + H2O POX3 + 2HX → high pressures using an Hg catalyst. POX3 + 3H2O H3PO4 + 3HX 173 phosphorus(III) bromide

In the gas phase the trihalides are pyra- limed solid formed by the action of chlo- midal and the pentahalides are trigonal rine on phosphorus(III) chloride. It is hy- bipyramids. However, in tetrachloro- drolyzed by water to phosphoric(V) acid methane solution phosphorus pentachlo- and hydrogen chloride. Its main use is as a ride is a dimer with two chlorine bridges, chlorinating agent in organic chemistry to exhibiting the ready expansion of the coor- replace a hydroxyl group with a chlorine dination number to six. In the solid state, atom. + – (PCl5) is [PCl4] [PCl6] (the first tetrahedral, the second octahedral) obtained by phosphorus(III) chloride oxide (phos- transfer of a chloride ion. phorus trichloride oxide; phosphorus oxy- Phosphorus interacts directly with sev- chloride; phosphoryl chloride, POCl3)A eral metals and metalloids to give: colorless liquid that can be obtained by re- 1. Ionic phosphides such as Na3P, Ca3P2, acting phosphorus(III) chloride with oxy- Sr3P2. gen or by distilling phosphorus(III) 2. Molecular phosphides such as P4S3 and chloride with potassium chlorate. The re- P4S5. actions of phosphorus(III) chloride oxide 3. Metallic phosphides such as Fe2P. are similar to those of phosphorus(III) Symbol: P; m.p. 44.1°C (white) 410°C chloride. Water hydrolysis yields phos- (red under pressure); b.p. 280.5°C; r.d. phoric(V) acid. Phosphorus(III) chloride 1.82 (white) 2.2 (red) 2.69 (black) (all at oxide forms complexes with many metal 20°C); p.n. 15; most common isotope 31P; ions. r.a.m. 30.973762. phosphorus(III) hydride See phos- phosphorus(III) bromide (phosphorus phine. tribromide; PBr3) A colorless liquid made by reacting phosphorus with bro- phosphorus(III) oxide (phosphorus tri- mine. It is readily hydrolyzed by water to oxide; P2O3) A white waxy solid with a phosphonic acid and hydrogen bromide. characteristic smell of garlic; it usually ex- Phosphorus(III) bromide is important in ists as tetrahedral P4O6 molecules in which organic chemistry, being used to replace a the phosphorus atoms are linked to each hydroxyl group with a bromine atom. other through oxygen bridges. It is soluble in organic solvents, and is made by burning phosphorus(V) bromide (phosphorus phosphorus in a limited supply of air. pentabromide; PBr5) A yellow crystalline Phosphorus(III) oxide oxidizes in air to solid that sublimes easily. It can be made phosphorus(V) oxide at room temperature by the reaction of bromine and phospho- and inflames above 70°C. It dissolves rus(III) bromide. Phosphorus(V) bromide slowly in cold water or dilute alkalis to give is readily hydrolyzed by water to phos- phosphonic acid or one of its salts. Phos- phoric(V) acid and hydrogen bromide. Its phorus(III) oxide reacts violently with hot main use is in organic chemistry to replace water to form phosphine and phos- a hydroxyl group with a bromine atom. phoric(V) acid. phosphorus(III) chloride (phosphorus phosphorus(V) oxide (phosphorus pen- trichloride; PCl3) A colorless liquid toxide; P2O5) A highly deliquescent formed from the reaction of phosphorus white powder that is soluble in organic sol- with chlorine. It is rapidly hydrolyzed by vents. It usually exists as P4O10 molecules. water to phosphonic acid and hydrogen Phosphorus(V) oxide can be prepared by chloride. Phosphorus(III) chloride is used burning phosphorus in an abundant supply in organic chemistry to replace a hydroxyl of oxygen. It readily combines with water group with a chlorine atom. to form phosphoric(V) acid and is therefore used as a drying agent for gases. It is a phosphorus(V) chloride (phosphorus useful dehydrating agent because it is able pentachloride; PCl5) A white easily sub- to remove the elements of water from cer- 174 photoemission tain oxyacids and other compounds con- istry dealing with reactions induced by taining oxygen and hydrogen; for example, light or ultraviolet radiation. it reacts with nitric acid (HNO3) to give nitrogen pentoxide (N2O5) on heating. photoelectric effect The emission of electrons from a solid (or liquid) surface phosphorus oxychloride See phos- when it is irradiated with electromagnetic phorus(III) chloride oxide. radiation. For most materials the photoelectric effect occurs with ultraviolet radia- phosphorus pentabromide See phos- tion or radiation of shorter wavelength; phorus(V) bromide. some show the effect with visible radiation. In the photoelectric effect, the number phosphorus pentachloride See phos- of electrons emitted depends on the inten- phorus(V) chloride. sity of the radiation and not on its frequency. The kinetic energy of the electrons phosphorus pentoxide See phospho- that are ejected depends on the frequency rus(V) oxide. of the radiation. This was explained, by Einstein, by the idea that electromagnetic phosphorus tribromide See phospho- radiation consists of streams of photons. rus(III) bromide. The photon energy is hv, where h is the Planck constant and v the frequency of the phosphorus trichloride See phospho- radiation. To remove an electron from the rus(III) chloride. solid a certain minimum energy must be supplied, known as the work function, φ. phosphorus trichloride oxide See Thus, there is a certain minimum threshold phosphorus(III) chloride oxide. frequency v0 for radiation to eject elec- θ trons: hv0 = . If the frequency is higher phosphorus trioxide See phosphorus(III) than this threshold the electrons are oxide. ejected. The maximum kinetic energy (W) of the electrons is given by Einstein’s equa- phosphoryl chloride See phosphorus(III) tion: chloride oxide. W = hv – φ The photoelectric effect also occurs phot A unit of illumination in the c.g.s. with gases. See photoionization. system defined as an illumination of one lumen per square centimeter. In SI units it photoelectrochemical cell (PEC) A is equal to 104 lux. type of voltaic cell in which one of the electrodes is a light-sensitive semiconductor photochemical reaction A reaction (such as gallium arsenide). A current is gen- brought about by light; examples include erated by light falling on the semiconduc- the bleaching of colored material, the re- tor, causing electrons to pass between the duction of silver halides, and the photosyn- electrode and the electrolyte and produce thesis of carbohydrates. Chemical changes chemical reactions in the cell. occur only when the reacting atoms or molecules absorb photons of the appropri- photoelectron An electron ejected from ate energy. The amount of substance that a solid, liquid, or gas by the PHOTOELECTRIC reacts is proportional to the quantity of en- EFFECT or by PHOTOIONIZATION. ergy absorbed. For example, in the reaction between hydrogen and chlorine, it is photoelectron spectroscopy See pho- not the concentrations of hydrogen or toionization. chlorine that dictate the rate of reaction but the intensity of the radiation. photoemission The emission of photoelectrons by the photoelectric effect or by photochemistry The branch of chem- photoionization.

175 photoionization photoionization The ionization of metal ion is bound to a pi-electron system, atoms or molecules by electromagnetic ra- as in FERROCENE. diation. Photons absorbed by an atom may have sufficient photon energy to free an pig iron The crude iron produced from electron from its attraction by the nucleus. a blast furnace, containing carbon, silicon, The process is and other impurities. The molten iron is M + hv → M+ + e– run out of the furnace into channels called where h is the Planck constant and v is the ‘sows’, which branch out into a number of frequency of the radiation. offshoots called ‘pigs’, in which the metal is As in the photoelectric effect, the radia- allowed to cool. tion must have a certain minimum threshold frequency. The energy of the pigment An insoluble, particulate color- photoelectrons ejected is given by W = hv – ing material as used in PAINT. I, where W is the kinetic energy and I is the ionization potential of the atom or mol- pi orbital See orbital. ecule. Analysis of the energies of the emitted electrons gives information on the pipette A device used to transfer a ionization potentials of the substance – a known volume of solution from one con- technique known as photoelectron spec- tainer to another; in general, several sam- troscopy. ples of equal volume are transferred for individual analysis from one stock solu- photolysis A chemical reaction that is tion. Pipettes are of two types, bulb produced by electromagnetic radiation pipettes, which transfer a known and fixed (light or ultraviolet radiation). Many pho- volume, and graduated pipettes, which can tolytic reactions involve the formation of transfer variable volumes. free radicals. pitchblende (uraninite) A highly ra- photon energy See photoelectric effect. dioactive black mineral consisting mostly of uranium(VI) oxide, UO3 along with ura- physical change A change to a sub- nium radioactive decay series elements stance that does not alter its chemical such as thorium and radium. It is the chief properties. Physical changes (e.g. melting, ore of uranium and radium. boiling, and dissolving) are comparatively easy to reverse. pK The logarithm to the base 10 of the reciprocal of an acid’s dissociation con- physical chemistry The branch of stant (Ka): chemistry concerned with the physical log10(1/Ka) properties of compounds and how these depend on the chemical bonds involved. planar chromatography See chromatography; thin-layer chromatography. physisorption See adsorption. Planck constant Symbol: h A funda- piano-stool compound See sandwich mental constant; the ratio of the energy compound. (W) carried by a photon to its frequency (v). A basic relationship in the quantum pi bond See orbital. theory of radiation is W = hv. The value of h is 6.626 196 × 10–34 J s. The Planck con- pico- Symbol: p A prefix used with SI stant appears in many relationships in units, denoting 10–12. For example, 1 pico- which some observable measurement is farad (pF) = 10–12 farad (F). quantized (i.e. can take only specific discrete values rather than any of a range of pi complex A complex in which the values). It is named for German physicist

176 plutonium

Max Karl Planck (1858–1947), who esti- concentrated hydrochloric acid to form a mated its value in 1900. complex acid. plane polarization A type of polariza- platinum(IV) chloride (PtCl4) A red- tion of electromagnetic radiation in which dish-brown hygroscopic solid prepared by the vibrations take place entirely in one the action of heat on chloroplatinic acid. plane. Crystals of the pentahydrate, PtCl4.5H2O, are formed; anhydrous platinum(IV) chlor- plane-polarized See polarization. ide is prepared by treating the hydrated crystals with concentrated sulfuric acid. plasma A very hot mixture of ions and The chloride dissolves in water to produce electrons, as in an electrical discharge. a strongly acidic solution. Platinum(IV) Sometimes, a plasma is described as a chloride forms a series of hydrates having fourth state of matter. 1, 2, 4, and 5 molecules of water; the tetrahydrate is the most stable. The plaster of Paris See calcium sulfate. strongly acidic solution produced on dissolving it in water probably contains platinum A silvery-white malleable duc- H2[PtCl4(OH)2]. tile transition metal the third element of group 10 (formerly part of subgroup platinum–iridium An alloy of platinum VIIIB) of the periodic table. It occurs native containing up to 30% iridium. Hardness or in association with other platinum met- and resistance to chemical attack increase als in Australia, Canada, Russia, and South as the iridium content is increased. It is Africa. It is also found in certain copper used in jewelry, electrical contacts, and hy- podermic needles. The international proto- and nickel ores, from which it is recovered type of the kilogram is a platinum–iridium during refining. It is resistant to oxidation alloy. and is not attacked by acids (except aqua regia) or alkalis. Platinum is used as a cata- platinum metals The second and third lyst for ammonia oxidation (to make nitric transition series metals ruthenium (Ru), acid) and in catalytic converters. It is also osmium (Os), rhodium (Rh), iridium (Ir), used in thermocouples and surgical instru- palladium (Pd), and platinum (Pt), some- ments, and to make jewelry. times classed together due to their related Symbol: Pt; m.p. 1772°C; b.p. 3830 ± ° ° properties and similar compounds. All 100 C; r.d. 21.45 (20 C); p.n. 78; most the metals, for example, are hard and 195 common isotope Pt; r.a.m. 195.08. corrosion-resistant. platinum black A finely divided black plumbane (lead(IV) hydride; PbH4)A form of platinum produced, as a coating, colorless unstable gas that can be obtained by evaporating platinum onto a surface in by the action of acids on a mixture of mag- an inert atmosphere. Platinum-black coat- nesium and lead pellets. ings are used as pure absorbent electrode coatings in experiments on electric cells, plumbic Designating a lead(IV) com- and as catalysts. They are also used, like pound. carbon-black coatings, to improve the ability of a surface to absorb radiation. plumbous Designating a lead(II) compound. platinum(II) chloride (PtCl2) A gray- brown powder prepared by the partial de- plutonium A radioactive silvery ele- composition of platinum(IV) chloride. It ment of the actinoid series of metals. It is a may also be prepared by passing chlorine transuranic element found on Earth only in over heated platinum. Platinum(II) chlor- minute quantities in uranium ores, but is ide is insoluble in water but dissolves in readily obtained, as 239Pu, by neutron

177 point defect bombardment of natural uranium. The electron cloud is deformed, i.e. polarized. readily fissionable 239Pu is a major nuclear In ions, an increase in size or negative fuel and nuclear explosive. Plutonium charge leads to an increase in polarizabil- emits high levels of alpha particles and is ity. The concept is of particular use in the thus highly toxic; the particles accumulate treatment of covalent contributions to pre- in bone where they can cause leukemia and dominantly ionic bonds embodied in Fa- radiation poisoning. jans’ Rules. Thus ions, such as I–, Se2–, Symbol: Pu; m.p. 641°C; b.p. 3232°C; Te2–, are especially prone to covalent char- r.d. 19.84 (25°C); p.n. 94; most stable iso- acter, particularly in combination with tope 244Pu (half-life 8.2 × 107 years). ions of small size and high positive charge (i.e. high ionic potential). point defect See defect. polarization 1. The restriction of the vi- point group See molecular symmetry. brations in a transverse wave so that the vibration occurs in a single plane. poison 1. A substance that destroys Electromagnetic radiation, for instance, is catalyst activity. a transverse wave motion. It can be 2. Any substance that endangers biological thought of as an oscillating electric field activity, whether by physical or chemical and an oscillating magnetic field, both at means. right angles to the direction of propagation and at right angles to each other. Usually, polar Describing a compound with mol- the electric vector is considered since it is ecules that have a permanent dipole mo- the electric field that interacts with charged ment. Hydrogen chloride and water are particles of matter and causes the effects. In examples of polar compounds. ‘normal’ unpolarized radiation, the electric field oscillates in all possible directions per- polar bond A covalent bond in which pendicular to the wave direction. On re- the bonding electrons are not shared flection or on transmission through certain equally between the two atoms. A bond be- substances (e.g. Polaroid) the field is con- tween two atoms of different electronega- fined to a single plane. The radiation is tivity is said to be polarized in the direction of the more electronegative atom, i.e. the then said to be plane-polarized. If the tip of electrons are drawn preferentially toward the electric vector describes a circular helix the atom. This leads to a small separation as the wave propagates, the light is said to of charge and the development of a bond be circularly polarized. DIPOLE MOMENT as in, for example, hydro- 2. See polarizability. gen fluoride, represented as H→F or as 3. The reduction of current in a voltaic cell, Hδ+–Fδ– (F is more electronegative). caused by the build-up of products of the The charge separation is much smaller chemical reaction. Commonly, the cause is than in ionic compounds; molecules in the build-up of a layer of bubbles (e.g. of which bonds are strongly polar are said to hydrogen). This reduces the effective area display partial ionic character. The effect of the electrode, causing an increase in the of the electronegative element can be trans- cell’s internal resistance. It can also pro- mitted beyond adjacent atoms. See also induce a back e.m.f. Often a substance such termolecular forces. as manganese(IV) oxide (a depolarizer) is added to prevent hydrogen build-up. polarimeter (polariscope) An instrument for measuring optical activity. See polar molecule A molecule in which the optical activity. individual polar bonds are not perfectly symmetrically arranged and are therefore polariscope See polarimeter. not ‘in balance’. Thus the charge separation in the bonds gives rise to an overall polarizability The ease with which an charge separation in the molecule as, for

178 polonium example, in water. Such molecules possess overall increase in the temperature of the a dipole moment. Earth due to its greenhouse effect. Car exhausts also emit carbon monoxide and, if polarogram See polarography. leaded gasoline (petrol) is used, lead. At- mospheric CO has not yet reached danger- polarography An analytical method in ous levels, but vegetation near main roads which current is measured as a function of has in the past been found to contain a high potential. A special type of cell is used in proportion of lead, with levels sufficiently which there is a small, easily polarizable high in urban areas to cause concern about cathode (the dropping-mercury electrode, the effects on children. Leaded gasoline has consisting of a thin tube through which thus been banned in many countries. Pho- mercury is slowly dripped into the solu- tochemical smog, caused by the action of tion) and a large nonpolarizable anode sunlight on volatile hydrocarbons and ni- (reference cell). The analytical reaction trogen oxides from car exhausts, is a prob- takes place at the cathode and is essentially lem in several countries. Catalytic a reduction of the cations, which are dis- converters have helped reduce harmful charged according to the order of their emissions from vehicle exhausts. electrode potential values. The data is ex- Water pollutants include those that are pressed in the form of a polarogram, which biodegradable, such as sewage effluent, is a plot of current against applied voltage. which cause no permanent harm if ade- As the applied potential is increased a point quately treated and dispersed, as well as is reached at which the ion is discharged. those that are nonbiodegradable, such as There is a step-wise increase in current, certain chlorinated hydrocarbon pesticides which levels off because of polarization ef- (e.g. DDT) and heavy metals, such as lead, fects. The potential at half the step height copper, and zinc in some industrial efflu- (called the half-wave potential) is used to ents, which cause heavy-metal pollution. identify the ion. Most elements can be de- When these pollutants accumulate in the termined by polarography. The optimum environment they can become very concen- concentrations are in the range trated in food chains. The pesticides DDT, 10–2–10–4M; modified techniques allow aldrin, and dieldrin are now banned in determinations in the parts per million many countries. Water supplies can be- range. come polluted by leaching of nitrates, pesticides, or animal wastes from agricultural polar solvent See solvent. land. The discharge of waste heat can cause thermal pollution of the environment, pollution Any damaging or unpleasant which can be reduced by the use of cooling change in the environment that results towers. In lakes, rivers, and the sea, from the physical, chemical, or biological spillage from tankers and the discharge of side-effects of human industrial or social inadequately treated sewage effluent are activities. Pollution can affect the atmos- the main problems. phere, rivers, lakes, seas, and soil. Other forms of pollution are noise from Air pollution is caused by the domestic aircraft, traffic, and industry and radioac- and industrial burning of carbonaceous tivity from improper disposal of radioac- fuels, by industrial processes, and by gases tive waste. in car exhausts. Among recent problems are industrial emissions of sulfur(IV) polonium A strongly radioactive rare oxide, a cause of acid rain, and emissions metallic or metalloid element belonging to of chlorofluorocarbons (CFCs), previously group 16 (formerly VIA) of the periodic widely used in refrigeration, aerosols, etc., table. It occurs in very minute quantities in and linked to the depletion of ozone in the uranium ores. Over 30 radioisotopes are stratosphere. Carbon dioxide, produced by known, nearly all alpha-particle emitters. burning fossil fuels, is slowly building up in Polonium is a volatile metal and evapo- the atmosphere, which could result in an rates with time. It is also extremely haz-

179 polyatomic ardous; a quantity of polonium quickly temperature as a result of different packing reaches a temperature of a few hundred de- arrangements of the particles. There is a grees Celsius because of alpha emission. transition temperature between forms and For this reason it has been used as a light- usually a marked change in density. weight heat source in space satellites. Symbol: Po; m.p. 254°C; b.p. 962°C; polyvalent Describing an atom or r.d. 9.32 (20°C); p.n. 84; stablest isotope group that has a valence of more than one. 209Po (half-life 102 years). porcelain A ceramic traditionally made polyatomic Describing a molecule (or from feldspar, kaolin (China clay), marble, ion or radical) that consists of several and quartz. atoms (three or more). Examples are calcium carbonate (CaCO3) and methane Porter, George, Baron (1920–2001) (CH4). British physical chemist. In collaboration with Ronald NORRISH, Porter developed the polybasic Describing an acid that has technique of flash photolysis for investigat- two or more replaceable hydrogen atoms. ing fast chemical reactions, starting in the For example, phosphorus(V) acid, H3PO4, late 1940s. He shared the 1967 Nobel Prize is tribasic. for chemistry with Norrish and Manfred EIGEN. polychromic acids See chromium(IV) oxide. positive catalyst See catalyst. polycrystalline Describing a substance potash See potassium carbonate; potas- composed of very many minute interlock- sium hydroxide. ing crystals that have solidified together. potash alum See alum; aluminum polycyclic Describing a compound that potassium sulfate. has two or more rings in its molecules. potassamide See potassium monoxide. polydioxoboric(III) acid See boric acid. potassium A light soft silvery reactive polymer A compound in which there alkali metal; the third element of group 1 are very large molecules made up of re- (formerly IA) of the periodic table. It has peating molecular units called MONOMERS. the argon electronic configuration of argon Polymers do not usually have a definite rel- plus an outer 4s1 electron. The element ative molecular mass, because there are gives a distinct lilac color to flames which, variations in the lengths of different chains. however, is easily masked by the intense They may be natural substances (e.g. cellu- yellow coloration resulting from any trace lose, starch, silicates, or proteins) or syn- impurity of sodium. Consequently the thetic materials (e.g. nylon or silicone). The flames must be viewed through blue cobalt two major classes of synthetic polymers are glass. The ionization potential of potas- thermosetting and thermoplastic. The for- sium is low and its chemistry is largely the mer are infusible, and heat may only make chemistry of the K+ ion. Potassium ac- them harder, whereas the latter soften on counts for 2.4% of the lithosphere and oc- heating. curs in large salt deposits such as carnallite (KCl.MgCl2.6H2O) and schönite (K2SO4), polymorphism The ability of certain and sylvite (KC1). Large amounts also chemical substances to exist in more than occur in mineral forms that are not of one physical form. For elements this is much use for recovery of potassium, e.g., called ALLOTROPY. The existence of two orthoclase, K2Al2Si6O10. The commercial forms is called dimorphism. Crystalline usage of potassium is much less than that structures of compounds can vary with of sodium; industrially the element is ob-

180 potassium chromate tained by electrolysis of the fused hydrox- potassium carbonate (pearl ash; potash; ide. Potassium hydroxide is in turn ob- K2CO3) A white deliquescent solid pre- tained by electrolysis of carnallite solutions pared by thermal decomposition of po- (the magnesium is initially precipitated as tassium hydrogencarbonate (KHCO3). Mg(OH)2), and both hydrogen and chlo- Potassium carbonate is very soluble in rine are recovered as by-products. water, its solutions being strongly alkaline The chemistry of potassium and its due to salt hydrolysis. It is used in the lab- compounds is very similar to that of the oratory as a drying agent and industrially other group 1 elements. Particular ways in in the manufacture of soft soap, hard glass, which the chemistry of potassium differs and in dyeing and wool finishing proce- from that of sodium can be summarized as dures. Potassium carbonate crystallizes out ° follows: between 10–25 C as K2CO3.3H2O; it de- ° 1. Burning in air produces the superoxide hydrates at 100 C to K2CO3.H2O and at 130°C to K CO . KO2. 2 3 2. Because of the larger size of the K+ ion and hence lower lattice energies, potas- potassium chlorate (KClO3) A white sium salts are often more soluble than soluble crystalline solid prepared by the corresponding sodium salts. electrolysis of a concentrated solution of 3. Potassium salts are usually less heavily potassium chloride. Industrially, it is pre- hydrated than corresponding sodium pared by the fractional crystallization of a salts. solution containing sodium chlorate (V) and potassium chloride. When heated just The most abundant isotope of potas- above its melting point of 356°C, potassium is 39K (93.1%), with 41K also a stable sium perchlorate is formed or it can be isotope (6.8%). The naturally occurring made by subjecting a hot solution of potas- radioactive isotope 40K (0.11%) has a half- sium chloride to electrolysis. When heated life of 1.28 × 109 years. further, potassium chlorate decomposes to Symbol: K; m.p. 63.65°C; b.p. 774°C; yield oxygen and potassium chloride. It is a r.d. 0.862 (20°C); p.n. 19; r.a.m. 39.0983. powerful oxidizing agent and is used in explosives, matches, weedkillers, and fire- potassium–argon dating A technique works, and as a disinfectant. It oxidizes for dating rocks. It depends on the ra- 40 iodide ions to iodine when in an acidic dioactive decay of the radioisotope K to medium. 40Ar (half-life 1.27 × 1010 years) and is based on the assumption that argon has potassium chloride (KCl) A white or been trapped in the rock since the time that colorless ionic solid prepared by neutraliz- the rock cooled (i.e. since it formed). If the ing hydrochloric acid with potassium hy- 40 40 ratio Ar/ K is measured it is possible to droxide solution. Potassium chloride estimate the age of the rock. See also ra- occurs naturally as the minerals sylvite and dioactive dating. carnallite, which both occur in deposits left by evaporated shallow seas. It is more sol- potassium bicarbonate See potassium uble than sodium chloride in hot water but hydrogencarbonate. less soluble in cold water. On evaporation of an aqueous solution, colorless cubic potassium bromide (KBr) A white or crystals similar to those of sodium chloride colorless cubic crystalline solid that is ex- are produced. Potassium chloride is used as tremely soluble in water. It is formed by the a fertilizer, in the manufacture of potas- action of bromine on hot potassium hy- sium hydroxide, and in photography. droxide solution or by the neutralization of the carbonate with hydrobromic acid. It is potassium chromate (K2CrO4)A used extensively in the manufacture of bright yellow toxic solid prepared by photographic plates, films, and papers and adding potassium hydroxide solution to a was once used as a sedative in medicine. solution of potassium dichromate. It is ex-

181 potassium cyanide tremely soluble in water. Addition of an bon dioxide through a saturated solution acid to an aqueous solution of potassium of potassium carbonate. Potassium hydro- chromate converts the chromate ions into gencarbonate is more soluble in water than dichromate ions. The salt is used as an in- sodium hydrogencarbonate. When heated, dicator in silver nitrate titrations and in it undergoes thermal decomposition to give pigments and enamels. The crystals are iso- the carbonate, water, and carbon dioxide. morphous with potassium sulfate. The salt forms monoclinic crystals. Its solutions are strongly alkaline as a result of potassium cyanide (KCN) A white salt hydrolysis. ionic solid that is very soluble in water and extremely poisonous. It smells of bitter al- potassium hydrogentartrate (cream of – + monds. It is made industrially by combin- tartar; HOOC(CHOH)2COO K ) A white ing potassium hydroxide with hydrogen crystalline solid, an acid salt of tartaric acid cyanide. Potassium cyanide is used to re- that occurs in grape juice. It is used as the cover gold and silver, in electroplating, and acid ingredient of BAKING POWDER. in fumigants. It is also used as a reducing agent and in chemical analysis. Aqueous potassium hydroxide (caustic potash; solutions of potassium cyanide are strongly lye; KOH) A highly corrosive white solid hydrolyzed, the solutions being alkaline. manufactured by the electrolysis of potas- On standing, these solutions slowly evolve sium chloride solution in a mercury cell. It hydrocyanic acid. can also be prepared by heating either potassium carbonate or potassium sulfate potassium dichromate (K2Cr2O7)An with slaked lime. In the laboratory, it can orange-red crystalline toxic solid prepared be made by reacting potassium, potassium by adding potassium chloride solution to a monoxide, or potassium superoxide with concentrated solution of sodium dichro- water. Potassium hydroxide resembles mate and crystallizing out or by acidifying sodium hydroxide but is more soluble in a solution of potassium chromate and water and alcohol. It is preferred over evaporating the same. It is less soluble than sodium hydroxide for the absorption of sodium dichromate in cold water but more carbon dioxide and sulfur(IV) oxide, due soluble in hot water. Potassium dichro- to its greater solubility. Potassium hydrox- mate is used as an oxidizing agent in volu- ide is used as an electrolyte in Ni–Cd, alka- metric analysis and organic chemistry, and line (Zn–Mn), mercury (Zn–Hg), and also in the manufacture of dyes, glass, silver (Zn–Ag) storage batteries and in the glues, and ceramics. It also finds use in fire- production of soft soaps. It forms crys- works, photography, and lithography. The talline hydrates with 1, 1.5, and 2 mol- crystals are triclinic, anhydrous, nondeli- ecules of water. quescent, and also a fire risk due to their strong oxidative capacity. Addition of an potassium iodate (KIO3) A white solid alkali to a solution of the dichromate yields formed either by adding iodine to a hot the chromate. concentrated solution of potassium hydroxide or by the electrolysis of potassium potassium hydride (KH) A white or iodide solution. No hydrates are known. It light gray ionic solid prepared by passing is a source of iodide and iodic acid. When hydrogen over heated potassium, the metal treated with a dilute acid and a reducing being suspended in an inert medium. It is agent, the iodate ions are reduced to io- an excellent reducing agent but also a fire dine. It is used in chemical analysis, and as hazard because it reacts with water to re- a food additive. lease hydrogen. potassium iodide (KI) A white ionic potassium hydrogencarbonate (potas- solid readily soluble in water. It is prepared sium bicarbonate; KHCO3) A white by dissolving iodine in hot concentrated crystalline solid prepared by passing car- potassium hydroxide solution. Both the io-

182 potential energy curve dide and iodate are formed but the latter is acids to produce solutions of nitrous acid. removed by fractional crystallization. Potassium nitrite is used in organic chem- Potassium iodide has a sodium chloride istry. cubic lattice. In solution with iodine it forms potassium triiodide PI3. Potassium potassium permanganate See potas- iodide is used in medicine, particularly in sium manganate(VII). the treatment of goiter resulting from iodine deficiency. It is also added to table salt potassium sulfate (K2SO4) A white to help prevent goiter, and is used in pho- solid prepared by the neutralization of ei- tography. Dilute acidified solutions of ther potassium hydroxide or potassium potassium iodide can act as reducing carbonate with dilute sulfuric acid. It oc- agents, manganate(VII) ions being reduced curs naturally as schönite in deposits at to manganese(II) ions, copper(II) ions to Stassfurt, Germany. Potassium sulfate is copper(I) ions, and iodate ions to iodine. soluble in water, forming a neutral solution. It is used as a fertilizer and in the potassium manganate(VII) (potassium chemical industry in the preparation of permanganate; KMnO4) A purple solid alums. It is also used in cement and in soluble in water. It is prepared by oxidizing glass-making. The anhydrous salt crystal- potassium manganate(VI) with chlorine. lizes in the rhombic form. Potassium permanganate is used in volumetric analysis as an oxidizing agent, as a potassium sulfide (K2S) A yellowish- bactericide, and as a disinfectant. In aque- brown deliquescent solid prepared by satu- ous solution its behavior as an oxidizing rating an aqueous solution of potassium agent depends on the pH of the solution. hydroxide with hydrogen sulfide and then adding an equal volume of potassium hy- potassium monoxide (K2O) An ionic droxide. Industrially it is produced by solid that is white when cold and yellow heating potassium sulfate and carbon at when hot. It is prepared by heating potas- high temperature. The sulfide crystallizes sium with potassium nitrate. Potassium out from aqueous solution as K2S.5H2O. monoxide dissolves violently in water to Its aqueous solutions undergo hydrolysis, form potassium hydroxide solution. The the solution being strongly alkaline. Potas- hydrate K2O.3H2O is known. Potassium sium sulfide dust is a fire risk. monoxide dissolves in liquid ammonia with the formation of potassium hydroxide potassium superoxide (KO2) A yellow and potassamide (KNH2). paramagnetic solid prepared by burning potassium in excess oxygen. When treated potassium nitrate (saltpeter; niter or with cold water or dilute mineral acids, hy- nitre; KNO3) A white solid, soluble in drogen peroxide is produced. If heated water, formed by fractional crystallization strongly, it yields oxygen and potassium of sodium nitrate and potassium chloride monoxide. Potassium superoxide is a pow- solutions. It occurs naturally as niter or erful oxidizing agent. nitre (saltpeter) in rocks in Hungary, Spain, India, South Africa, and Brazil. potassium thiocyanate (KSCN) A col- When heated it decomposes to give the ni- orless hygroscopic solid. Its solution is trite and oxygen. Unlike sodium nitrate it is used in a test for iron(III) compounds, with nondeliquescent. Potassium nitrate is a which it turns a blood-red color. strong oxidizing agent and is used in gunpowder, fireworks, fertilizers, and in the potential energy See energy. laboratory preparation of nitric acid. potential energy curve A curve of the potassium nitrite (KNO2) A creamy potential energy of electrons in a diatomic deliquescent solid that is readily soluble in molecule in which the potential energy of water. It reacts with cold dilute mineral an electronic state is plotted vertically and

183 potentiometric titration the interatomic distance is plotted horizon- from equilibrium. He called such systems tally, with the minimum of the curve being ‘dissipative structures’. Prigogine has been the average internuclear distance. particularly interested in applying his ideas to biological and social systems. He won potentiometric titration A titration in the 1977 Nobel Prize for chemistry for his which an electrode is used in the reaction investigations of non-equilibrium systems. mixture. The end point can be found by monitoring the electric potential during the primary cell A voltaic cell in which the titration. chemical reaction that produces the e.m.f. is not reversible. Compare accumulator. praseodymium A soft ductile malleable silvery-yellow element of the lanthanoid primary standard A substance that can series of metals. It occurs in association be used directly for the preparation of stan- with other lanthanoids in minerals such as dard solutions without reference to some bastnaesite and monazite. Praseodymium other concentration standard. Primary is used in several alloys, as a catalyst, in standards should be easy to purify, dry, ca- carbon arc lamp electrodes, in misch metal, pable of preservation in a pure state, unaf- in enamel, and in yellow glass for eye pro- fected by air or CO2, of a high molecular tection. mass (to reduce the significance of mass de- Symbol: Pr; m.p. 931°C; b.p. 3512°C; termination errors), stoichiometric, and r.d. 6.773 (20°C); p.n. 59; most common readily soluble. Any likely impurities isotope 141Pr; r.a.m. 140.91. should be easily identifiable. precipitate A suspension of small solid principal quantum number See atom. particles formed in a liquid as a result of a chemical reaction. producer gas (air gas) A combustible mixture of carbon monoxide (25–30%), precursor A substance from which an- nitrogen (50–55%), and hydrogen (10– other substance is formed in a chemical re- 15%), prepared by passing air with a little action. steam in it through a thick layer of white- hot coke in a furnace or ‘producer’. Pro- pressure Symbol: p The pressure on a ducer gas is used as a fuel while still hot to surface due to forces from another surface prevent heat loss for such purposes as in- or from a fluid acting at 90° to unit area of dustrial heating, the firing of retorts, and in the surface: glass furnaces. Compare water gas. pressure = force/area The SI derived unit by which pressure is product See chemical reaction. measured is the PASCAL (Pa). promethium A soft silvery radioactive Priestley, Joseph (1733–1804) English element of the lanthanoid series of metals. chemist and minister. Priestley was one of It occurs naturally in trace amounts in the the most eminent chemists of the 18th cen- mineral pitchblende as the result of ra- tury. In 1774 he discovered oxygen, but be- dioactive decay of such elements as ura- cause he believed in the phlogiston theory nium, thorium, and plutonium. It can also of combustion he called it ‘dephlogisti- be produced artificially by the fission of cated air’. He also discovered and isolated uranium. Promethium-147 is used in some a number of other compounds. Priestley miniature nuclear-powered batteries for was also interested in electricity and optics. use on spacecraft. Symbol: Pm; m.p. 1168°C; b.p. 2730°C Prigogine, Ilya (1917– ) Russian- (approx.); r.d. 7.22 (20°C); p.n. 61; most born Belgian chemist. Prigogine is best stable isotope 145Pm (half-life 18 years). known for extending thermodynamics and statistical mechanics to systems that are far promoter (activator) A substance that

184 pyrosulfuric acid improves the efficiency of a CATALYST. It stance in the presence of a large volume of does not itself catalyze the reaction but as- water, the concentration of water remains sists the catalytic activity. For example, approximately constant. The rate of reac- alumina or molybdenum promotes the tion is thus found experimentally to be pro- catalytic activity of finely divided iron in portional to the concentration of the the Haber process. The manner in which a substance, even though it also depends on promoter functions is not fully understood; the amount of water present. Such a reac- no one theory explains all the examples. tion is described as ‘bimolecular of the first order’. protactinium A toxic radioactive element of the actinoid series of metals. It oc- pseudohalogens A small group of sim- curs in minute quantities in uranium ores ple inorganic compounds with symmetrical such as autunite, tobernite, and pitch- molecules that resemble the halogens in blende as a radioactive decay product of some reactions and compounds. For exam- actinium. It is also synthesized by bom- ple, cyanogen (C2N2) has compounds anal- barding thorium nuclei with neutrons. ogous to halogen compounds (e.g. HCN, ° ° Symbol: Pa; m.p. 1840 C; b.p. 4000 C KCN, CH3CN, etc.). Thiocyanogen (approx.); r.d. 15.4 (calc.); p.n. 91; most (S2C2N2), is another example. stable isotope 231Pa (half-life 32 500 years). p-type semiconductor See semiconductor. protic acid See acid. Pyrex (Trademark) A particularly proton An elementary particle with a strong, heat resistant, chemically inert positive charge (+1.602 192 × 10–19 C) and borosilicate glass commonly used for labo- rest mass 1.672 614 × 10–27 kg. Protons ratory glassware. are nucleons, found in all nuclides. pyrites A mineral sulfide of a metal; e.g. proton number (atomic number) Symbol: iron pyrites, FeS2. Z The number of protons in the nucleus of an atom. In a neutral atom it is also pyrolusite See manganese(IV) oxide. equal to the number of electrons orbiting the nucleus. The proton number deter- pyrolysis The decomposition of chemi- mines the chemical properties of an ele- cal compounds by subjecting them to very ment because the element’s electron high temperature. structure, which determines chemical bonding, depends on electrostatic attrac- pyrometer An instrument used in the tion to the positively charged nucleus. An chemical industry to measure high temper- element’s proton number determines its ature, e.g. in reactor vessels. position in the periodic table. pyrophoric 1. Describing a compound Prussian blue See cyanoferrate. that ignites spontaneously in air. 2. Describing a metal or alloy (misch prussic acid See hydrocyanic acid. metal) that sparks when struck. Lighter flints are made of pyrophoric alloy. pseudo-first order Describing a reaction that appears to exhibit first-order ki- pyrophosphoric acid See phos- netics under special conditions, even phoric(V) acid. though the ‘true’ order is greater than one. For example, in the hydrolysis of a sub- pyrosulfuric acid See oleum.

185 Q

quadrivalent (tetravalent) Having a va- teger that specifies the value of a quantized lence of four. physical quantity (e.g. energy, angular momentum, etc.). See atom; Bohr theory; spin. qualitative analysis Analysis carried out with the purpose of identifying the quantum states States of an atom, elec- components of a sample. Classical meth- tron, particle, etc., specified by a unique set ods involved simple preliminary tests fol- of quantum numbers. lowed by a carefully devised scheme of systematic tests and procedures. Modern quantum theory A mathematical the- methods include the use of such techniques ory originally introduced by Max Planck as infrared spectroscopy and emission (1900) to explain the radiation emitted spectrography. Compare quantitative from hot bodies. Quantum theory is based analysis. on the idea that energy (or certain other physical quantities) can be changed only in quantitative analysis Analysis carried certain discrete amounts for a given sys- out with the purpose of determining the tem. Other early applications were the ex- concentration of one or more of the known planations of the photoelectric effect and components of a sample. Classical wet the Bohr theory of the atom. methods include volumetric and gravimet- Quantum mechanics is a system of me- ric analysis. A wide range of more modern chanics that developed from quantum the- instrumental techniques are also used, in- ory and is used to explain the behavior of cluding polarography and various types of atoms, molecules, etc. In one form it is chromatography and spectroscopy. Com- based on de Broglie’s idea that particles can pare qualitative analysis. have wavelike properties – this branch of quantized Describing a physical quan- quantum mechanics is called wave me- tity that can take only certain discrete val- chanics. See orbital. ues, and not a continuous range of values. quartz See silicon(IV) oxide. quantum (plural quanta) A definite amount of energy released or absorbed in a quasicrystal A type of crystal structure process. Energy often behaves as if it were in which there is long-range order but in ‘quantized’ in this way. The quantum of which the symmetry of the repeating unit is electromagnetic radiation is the photon. not one allowed in normal crystals. There are examples of solids, such as the com- quantum electrodynamics The use of pound AlMn, that have icosahedral sym- quantum mechanics to describe how parti- metry and a long-range repeating order in cles and electromagnetic radiation interact. the stucture. These are known as qua- sicrystals. quantum mechanics See quantum theory. quicklime See calcium oxide. quantum number An integer or half in- quinquevalent See pentavalent.

186 R

racemate See optical activity. dioactive element is transformed as it decays. racemic mixture See optical activity. radioactive isotope See radioisotope. racemization The conversion of an optical isomer into a racemic mixture, which radioactivity The disintegration or is not optically active. decay of certain unstable nuclides with emission of radiation. The emission of rad An m.k.s. unit formerly used to mea- ALPHA PARTICLES, BETA PARTICLES, and sure absorbed dose of ionizing radiation, GAMMA RAYS are the three most important defined as being equivalent to an absorp- forms of radiation that occur during decay. tion of 10–2 joule of energy in one kilogram of material. In SI units it is equal to 10–2 radiocarbon dating See carbon. gray. radiochemistry The chemistry of ra- radian Symbol: rad The SI derived unit dioactive isotopes of elements. Radio- of plane angle, equal to that subtended at chemistry involves such topics as the the center of a circle by an arc whose length preparation of radioactive compounds, the separation of isotopes by chemical reac- is equal to the circle’s radius. One complete tions, the use of radioactive labels in stud- revolution (360°) is 2π radian. ies of mechanisms, and experiments on the chemical reactions and compounds of ra- radiation In general, the emission of en- dioactive elements. ergy from a source, either as waves (light, sound, etc.) or as moving particles (beta radioisotope (radioactive isotope) A ra- rays or alpha rays). dioactive isotope of an element. Tritium, for instance, is a radioisotope of hydrogen. radical A group of atoms in a molecule. Radioisotopes are extensively used in re- See also free radical. search as souces of radiation and as tracers in studies of chemical reactions. Thus, if an radioactive Describing an element or atom in a compound is replaced by a ra- nuclide that exhibits natural radioactivity. dioactive nuclide of the element (a label) it is possible to follow the course of the radioactive dating (radiometric dating) chemical reaction. Radioisotopes are also A technique for dating archaeological spec- used in medicine for diagnosis and treat- imens, rocks, etc., by measuring the extent ment. to which some radionuclide has decayed to give a product. See carbon; potassium– radiolysis Chemical decomposition pro- argon dating; rubidium–strontium dating; duced by high-energy radiation, i.e. x-rays, uranium–lead dating. gamma rays, or particles. radioactive decay series The series of radiometric dating See radioactive dat- definite isotopes into which a given ra- ing.

187 radio waves radio waves A form of electromagnetic called Raman scattering, in contrast to radiation with wavelengths ranging from normal (Rayleigh) scattering. The Raman approximately 10–1 to 103 meters. See also effect was first observed by the Indian electromagnetic radiation. physicists Sir Chandrasekhara Venkata Raman (1888–1970) and his colleague radium A soft white radioactive lumi- Sir Kariamanikkam Srinivasa Krishnan in nescent metallic element of the alkaline- 1928. The effect has been used extensively earth group, the sixth element of group 2 in Raman spectroscopy for the deter- (formerly IIA) of the periodic table. Ra- mination of molecular structure, particu- dium is found in uranium ores, especially larly since the advent of the laser. In pitchblende and carnotite, from which it is some cases it is possible to draw conclu- ultimately obtained via electrolysis using a sions about molecular structure by observ- mercury electrode. It was formerly used in ing whether certain lines are present or luminous paints and radiotherapy, and absent. continues to be used as a radiation source in research laboratories. It has several Ramsay, Sir William (1852–1916) short-lived radioisotopes and one long- British chemist, famous for his discovery of lived isotope, radium-226 (half-life 1602 the rare gases. After a talk by Lord years). Rayleigh in 1892 on some discrepancies in Symbol: Ra; m.p. 700°C; b.p. 1140°C; the density of nitrogen, Ramsay predicted r.d. 5 (approx. 20°C); p.n. 88; r.a.m. that these discrepancies were due to a hith- 226.0254. erto undiscovered gas. After careful experiments he was able to isolate a new element radon A colorless odorless monatomic which Rayleigh and Ramsay named argon. radioactive gas, the sixth member of the Ramsay was also able to identify helium, in rare gases; i.e. group 18 (formerly VIIIA) of collaboration with the spectroscopic obser- the periodic table. It occurs naturally as a vations of Sir William Crookes. (Helium product of the uranium radioactive decay had previously been discovered in the Sun.) series. It has 19 short-lived radioisotopes; These discoveries led Ramsay to speculate the most stable, radon-222, is a decay in his book The Gases of the Atmosphere product of radium-226 and itself disinte- (1896) that there is a whole group of inert grates into an isotope of polonium with a elements in the periodic table. Ramsay and half-life of 3.82 days. 222Rn is sometimes his colleagues found the missing elements used in radiotherapy. Radon accumulates between 1894 and 1901. in uranium mines and can also seep into the basements of buildings constructed Raney nickel A catalytic form of nickel over rocks or sands containing uranium or produced by treating a nickel–aluminum radium ores, where it may pose a health alloy with sodium hydroxide. The alu- risk. minum dissolves (as aluminate) in the Symbol: Rn; m.p. –71°C; b.p. –61.8°C; sodium hydroxide, leaving the Raney mass density 9.73 kg m–3 (0°C); p.n. 86. nickel as a spongy mass, which is pyrophoric when dry. It is used especially for raffinate The liquid remaining after the catalyzing hydrogenation reactions. solvent extraction of a dissolved substance. See solvent extraction. Raoult’s law A relationship between the pressure exerted by the vapor of a solu- r.a.m. See relative atomic mass. tion and the presence of a solute. It states that the partial vapor pressure of a solvent Raman effect A change in the fre- above a solution (p) is proportional to the quency of electromagnetic radiation, such mole fraction of the solvent in the solution as light, that occurs when a photon of ra- (X) and that the proportionality constant is diation undergoes an inelastic collision the vapor pressure of pure solvent, (p0), at with a molecule. This type of scattering is the given temperature: i.e. p = p0X. It was 188 rate of reaction discovered by François Raoult (1830– blood. Liquid helium is extensively used in 1901), a French chemist. Solutions that low-temperature work, and radon is used obey Raoult’s law are said to be ideal solu- therapeutically as a source of alpha-parti- tions. Because of solvation forces this be- cles. havior is rare and in general Raoult’s law The gases helium and argon are formed holds only for dilute solutions. by radioactive decay in various minerals, for example argon by radioactivity decay rare earths See lanthanoids. of 40K. This mechanism can be applied to age-determination of minerals, a technique rare gases (noble gases; inert gases; group known as potassium-argon dating. Radon 0 elements; group 18 elements) A group is generally obtained as 222Rn from the of monatomic low-boiling gases belonging decay of radium. Its half-life is only about to group 18 of the periodic table and occu- 3.8 days. pying a position between the highly electronegative group 17 elements and the rate constant (velocity constant; specific highly electropositive group 1 elements. reaction rate) Symbol: k The constant of They are helium (He), neon (Ne), argon proportionality in the rate expression for a (Ar), krypton (Kr), xenon (Xe), and radon chemical reaction. For example, in a reac- (Rn). Their completely filled s and p levels tion A + B → C, the rate may be propor- gives a closed-shell structure that is associ- tional to the concentration of A multiplied ated with a general lack of chemical reac- by that of B; i.e. tivity and very high ionization energies. It rate = k[A][B] is the acquisition of a stable rare-gas con- where k is the rate constant for this partic- figuration that is associated with determin- ular reaction. The constant is independent ing the valence of many covalent and ionic of the concentrations of the reactants but compounds. depends on temperature; consequently the Prior to 1962 the rare gases were fre- temperature at which k is recorded must be quently called inert gases because chemical stated. The units of k vary depending on compounds with them were not known the number of terms in the rate expression, (there were a few clathrates and supposed but are easily determined remembering ‘hydrates’), but the realization that the ion- that rate has the unit s–1. ization potential of xenon was sufficiently low to be accessible to chemical reaction rate-determining step (limiting step) led to the preparation of several fluorides, The slowest step in a multistep reaction. oxides, oxyfluorides, and a hexafluoro- Many chemical reactions are made up of a platinate of xenon. Several unstable kryp- number of steps in which the one with the ton and radon compounds have also been lowest rate is the one that determines the synthesized. rate of the overall process. Argon forms about 1% of the atmos- The overall rate of a reaction cannot ex- phere but the other gases are present in air ceed the rate of the slowest step. For exam- in only minute traces. They are obtained by ple, the first step in the reaction between fractional distillation of liquid air or of nat- acidified potassium iodide solution and hy- ural gas. Considerable amounts of argon drogen peroxide is the rate-determining are produced as a by-product from ammo- step: – → – nia-plant tail gas. Applications for the rare H2O2 + I H2O + OI (slow) gases depend heavily on their inertness; for H+ + OI– → HOI (fast) + – → example, welding (Ar), filling light bulbs HOI + H + I I2 + H2O (fast) (Ar), gas-stirring in high temperature metallurgy (Ar), powder technology (Ar), dis- rate of reaction A measure of the charge tubes (Ne), and chemical research amount of reactant consumed in a chemi- as inert carriers (He, Ar). Neon has been cal reaction in unit time. It is thus a mea- proposed as an oxygen diluent for deep-sea sure of the number of effective collisions diving because of its lower solubility in between reactant molecules. The rate at

189 rationalized units which a reaction proceeds can be measured reciprocal proportions, law of See by the rate the reactants disappear or by equivalent proportions. the rate at which the products are formed. The principal factors affecting the rate of recrystallization The repeated crystal- reaction are temperature, pressure, concen- lization of a compound in order to ensure tration of reactants, light, and the action of purity of the sample or crystals of better a catalyst. The units usually used to mea- form. sure the rate of a reaction are mol dm–3 s–1. See also mass action. red lead See dilead(II) lead(IV) oxide. redox (oxidation–reduction) Relating to rationalized units A system of units in the process of oxidation and reduction, which the equations defining the system which are intimately connected in that dur- have a logical form related to the shape of ing oxidation by chemical agents the oxi- the system. SI units form a rationalized sys- dizing agent itself becomes reduced, and tem of units. For example, SI formulae con- vice versa. Thus an oxidation process is al- cerned with circular symmetry contain a ways accompanied by a reduction process. π factor of 2 ; those concerned with radial In electrochemical processes this is equally π symmetry contain a factor of 4 . true, oxidation taking place at the anode and reduction at the cathode. These sys- raw material A substance from which tems are often called redox systems, partic- other substances are made. In the chemical ularly when the interest centers on both industry it may be simple (such as nitrogen compounds. from air, used to make ammonia) or com- Oxidizing and reducing power is indi- plex (such as coal and petroleum, used to cated quantitatively by the redox potential make a wide range of products). or standard electrode potential, EŠ. Redox potentials are normally expressed reactant See chemical reaction. as reduction potentials. They are obtained by electrochemical measurements and the + reaction See chemical reaction. values are referred to the H /H2 couple for which EŠ is set equal to zero. Thus in- reaction coordinate See energy profile. creasingly negative potentials indicate increasing ease of oxidation or difficulty of reagent A compound that reacts with reduction. Thus in a redox reaction the half another. The term is usually used for com- reaction with the most positive value of Š mon laboratory chemicals, e.g. sodium hy- E is the reduction half and the half reac- Š droxide, hydrochloric acid, etc. used for tion with the least value of E (or most highly negative) becomes the oxidation experiment and analysis. half. See also electrode potential; oxidation; reduction. realgar The mineral form of arsenic(II) sulfide As S , one of the chief ores of ars- 4 4 redox potential See redox. enic. It is bright red in color and was formerly used as a pigment, in tanning, and in redox systems See redox. fireworks. Its use for these purposes has been largely discontinued due to its toxic- red phosphorus See phosphorus. ity. reducing agent See reduction. real gas See gas laws. reduction The gain of electrons by such rearrangement A reaction in which the species as atoms, molecules, or ions. It groups of a compound rearrange them- often involves the loss of oxygen from a selves to form a different compound. compound, or addition of hydrogen. Re-

190 resonance duction can be effected chemically, i.e. by does not have to be used only for com- the use of reducing agents (electron pounds that have discrete molecules; for donors), or electrically, in which case the ionic compounds (e.g. NaCl) and giant- reduction process occurs at the cathode. molecular structures (e.g. BN) the formula For example, unit is used. 2Fe3+ + Cu → 2Fe2+ + Cu2+ where Cu is the reducing agent and Fe3+ is relaxation The process by which an ex- reduced, and cited species loses energy and falls to a 2+ → 2H2O + SO2 + 2Cu lower energy level (such as the ground + 2– + 4H + SO4 + 2Cu state). 2+ where SO2 is the reducing agent and Cu is reduced. rem An m.k.s. unit formerly used for Tin(II) chloride, sulfur(IV) oxide, and measuring equivalent ionizing radiation the hydrogensulfite ion are common reduc- doses absorbed by living tissue. One rem is ing agents. See also redox. equivalent to a dosage of one roentgen of x-rays or gamma rays absorbed by an av- reduction potential See electrode po- erage adult male. In SI units it is equal to tential. 10–2 sievert. refining The process of removing impu- resin A yellowish insoluble organic com- rities from a substance or of extracting a pound exuded by trees as a viscous liquid substance from a mixture. that gradually hardens on exposure to air to form a brittle amorphous solid. Syn- refractory Describing a compound (e.g. thetic resins are artificial polymers used in an inorganic oxide) that has a very high making adhesives, insulators, and paints. melting point. Refractory materials are used as furnace linings. resolution (of racemates) The separation of a racemate into the two optical iso- relative atomic mass (r.a.m.; average mers. This cannot be done by normal atomic mass) Symbol: Ar The ratio of the methods, such as crystallization or distilla- average mass per atom of the naturally oc- tion, because the isomers have identical curring isotopes or, if given in parentheses, physical properties. The main methods in- the most stable isotope of an element to volve mechanical, chemical, or biochemi- 1/12 of the mass of an atom of nuclide 12C. cal techniques. It was formerly called atomic weight. resonance (mesomerism) The behavior relative density The ratio of the density of many molecules or ions cannot be ade- of a given substance to the density of some quately explained by a single structure reference substance. The relative densities using simple single and double bonds. The of liquids are usually measured with refer- bonding electrons of the molecule or ion ence to the density of water at 4°C. Rela- have instead a different distribution, one in tive densities are also specified for gases; which the actual bonds can be regarded as usually with respect to air at STP. The tem- a hybrid of two or more conventional perature of the substance is either stated or forms called resonance forms or canonical is understood to be 20°C. Relative density forms. The result is a resonance hybrid. For was formerly called specific gravity. example, three equivalent LEWIS STRUC- TURES can be written for sulfur(VI) oxide, relative molecular mass Symbol: depending on which of the three oxygen Mr The ratio of the average mass per atoms is double bonded to the sulfur atom. molecule of the naturally occurring form of However, experiments reveal that all the an element or compound to 1/12 of the bonds in sulfur(VI) oxide are of the same mass of an atom of nuclide 12C. This ratio length and strength. This means that the was formerly called molecular weight. It bonds to the oxygen atoms must be neither

191 resonance forms double nor single, but a hybrid of the pos- gas and the surroundings to maintain a sible Lewis structures. constant temperature. In practice, all real processes are irre- resonance forms See resonance. versible changes in which there is not an equilibrium throughout the change. In an resonance ionization spectroscopy irreversible change, the system can still be (RIS) A type of spectroscopy that detects returned to its original state, but not specific types of atoms using lasers. The through the same series of stages. For a laser ionizes the atoms of interest. The fre- closed system, there is always an entropy quency of the laser is chosen so that only increase involved in an irreversible change. the atoms of interest in a sample are excited by the laser. This method is very se- reversible reaction A chemical reaction lective because ionization occurs only for that can proceed in either direction. For ex- those atoms whose energy levels fit in with ample, the reaction: the frequency of the laser light. This selec- ˆ N2 + 3H2 2NH3 tivity has led to many practical uses for this is reversible. In general, there will be an technique. equilibrium mixture of reactants and products. retort A piece of laboratory apparatus consisting of a glass bulb with a long nar- rhenium A dense silvery rare transition row neck. In industrial chemistry, various metal, the third element of group 7 (for- metallic vessels in which distillations or re- merly VIIB) of the periodic table. It usually actions take place are called retorts. occurs naturally with molybdenum and platinum ores, and is usually extracted reverbatory furnace A furnace for from flue dust in molybdenum smelters. smelting metals. It has a curved roof so that The metal is chemically similar to man- heat is reflected downward, the fuel being ganese. It has the second highest melting in one part of the furnace and the ore in the point of all the elements, making it espe- other. cially useful for alloys for electrical con- reverse osmosis A process by which tacts, thermocouples, and other water containing a salt is separated via a high-temperature environments. It is also semipermeable membrane into pure water used as a catalyst. ° ° and salt water streams by subjecting the in- Symbol: Re; m.p. 3180 C; b.p. 5630 C; ° coming water to pressures significantly r.d. 21.02 (20 C); p.n. 75; most common 187 greater than its OSMOTIC PRESSURE. This ac- isotope Re; r.a.m. 186.207. tion causes pure water to be forced through the membrane, leaving the salt behind. See rheology The study of the ways in osmosis. which matter can flow. This topic is of particular interest in the study of polymers. reversible change A change in the pressure, volume, or other properties of a sys- rhodium A rare silvery hard transition tem, in which the system remains at metal, the second element of group 9 (for- equilibrium throughout the change. Such merly part of subgroup VIIIB) of the peri- processes could be reversed; i.e. returned to odic table. It is difficult to work and highly the original starting position through the resistant to corrosion. Rhodium occurs na- same series of stages. They are never real- tive in the natural alloy osmiridium but is ized in practice. An isothermal reversible most often obtained from copper and compression of a gas, for example, would nickel ore refinery residues. It is used in have to be carried out infinitely slowly and protective finishes, high-temperature al- involve no friction, etc. Ideal energy trans- loys, and as a catalyst. Rhodium com- fer would have to take place between the pounds are characteristically pink.

192 Russell–Saunders coupling

Symbol: Rh; m.p. 1966°C; b.p. 3730°C; rotational spectroscopy The spectro- r.d. 12.41 (20°C); p.n. 45; most common scopic study of the rotational motion of isotope 103Rh; r.a.m. 102.90550. molecules. Rotational spectroscopy gives information about interatomic distances. rhombic crystal See crystal system. The transitions between different rotational energy levels in molecules corre- ring A closed loop of atoms in a mol- spond to the microwave and far infrared ecule, as in the crown-shaped allotropes of regions of the electromagnetic spectrum. It sulfur (S8) and selenium (Se8). A fused ring is possible for there to be transitions be- is one joined to another ring in such a way tween rotational energy levels in pure rota- that they share two atoms. tional spectra only if the molecule has a permanent dipole moment. In the near in- RIS See resonance ionization spectros- frared region rotational transitions are su- copy. perimposed on vibrational transitions, resulting in a vibrational–rotational spec- rock A definable part of the Earth’s trum. This type of spectrum is considerably crust made up of a mixture of minerals, more complicated than a purely rotational some of which usually predominate in spectrum. abundance over other, accessory minerals. Rock may be hard (for example, granite), rubidium A soft light silvery highly re- soft (chalk), consolidated (clay), or loose active alkali metal; the fourth element of (sand). See also mineral. group 1 (formerly IA) of the periodic table. It is found in small amounts in several com- rock salt (halite) A transparent natu- plex silicate minerals, including lepidolite, and in some brines. Naturally occurring rally occurring mineral form of sodium rubidium comprises two isotopes, one of chloride. which, 87Rb, is radioactive (half-life 5 × 1010 years). Rubidium is used in vacuum roentgen Symbol: R A c.g.s. unit of ra- tubes, photocells, and in making special diation exposure, formerly used for x-rays glass. Rubidium–strontium dating relies on and gamma rays, defined in terms of the the decay rate of 87Rb. ionizing effect of one electrostatic unit of Symbol: Rb; m.p. 39.05°C; b.p. 688°C; electricity on a cubic centimeter of air. In SI r.d. 1.532 (20°C); p.n. 37; most common × –4 units one roentgen is equal to 2.58 10 isotope 85Rb; r.a.m. 85.4678. coulomb per kilogram of dry air. rubidium–strontium dating A tech- Rose’s metal A fusible alloy containing nique for dating rocks. It depends on the 50% bismuth, 25–28% lead, and the bal- radioactive decay of the radioisotope 87Rb ance in tin. Its low melting point (about to 87Sr (half-life 4.7 × 1010 years). If the ° 100 C) leads to its use in fire-protection ratio 87Sr/87Rb is measured and compared devices. to the implied original quantity of 87Rb, it is possible to estimate the age of the rock. rotary dryers Devices commonly used See also radioactive dating. in the chemical industry for the drying, mixing, and sintering of solids. They con- ruby A rare and highly valued gemstone sist essentially of a heated rotating inclined variety of the mineral corundum (alu- cylinder, which is longer in length than in minum oxide, Al2O3) colored red by diameter. Gases flow through the cylinder chromium impurities. Rubies can also be in either a countercurrent or co-current di- made synthetically. They are used in jew- rection to regulate the flow of solids, which elry, as bearings in watches, and in some are fed into the end of the cylinder. Rotary lasers. dryers can be applied to both batch and continuous processes. Russell–Saunders coupling The type

193 rusting of coupling of electronic angular momen- ent types of radiation alpha rays, beta rays, tum vectors that describes light atoms. and gamma rays. He showed that radioac- Russell–Saunders coupling is applicable if tivity is characterized by a half-life. Fol- the energies of electrostatic Coulomb re- lowing a suggestion of Rutherford, Hans pulsion are considerably greater than the Geiger and Ernest Marsden performed an energies of SPIN–ORBIT COUPLING. The con- experiment involving the scattering of cept of Russell–Saunders coupling was pio- alpha particles which Rutherford inter- neered by the American astronomer Henry preted in 1911 as showing that an atom Norris Russell (1877–1957) and American consists of a nucleus at the center sur- physicist Frederick Albert Saunders rounded by electrons. In 1919 he showed (1875–1963) in 1925. that artificial disintegration of the nucleus could be induced by bombarding it with rusting The corrosion of iron in air to alpha particles. Rutherford also pioneered form an oxide. Both oxygen and moisture the use of radioactivity in dating rocks. He must be present for rusting to occur. The won the 1908 Nobel Prize for chemistry. process is electrolytic, involving electrochemical reactions in which different areas rutherfordium A radioactive synthetic of the wet iron serve as anode and cathode: transactinide metallic element, the first → 2+ – Fe(s) Fe (aq) + 2e transactinide. Atoms of rutherfordium are – → – 2H2Ol + O2(aq) + 4e 4OH (aq) produced by bombarding 249Cf with 12C or Iron(II) hydroxide precipitates and is by bombarding 248Cm with 18O. Only a oxidized to the red hydrated iron(III) oxide very few atoms have ever been created. Fe O .H O. Rusting is greatly speeded up 2 3 2 Symbol: Rf; m.p. 2100°C (est.); b.p. by impurities in the iron and electrolytes in 5200°C (est.); r.d. 23 (est.); p.n. 104; most the water. stable isotope 261Rf (half-life 65 s). ruthenium A hard brittle silver-white rutile A naturally occurring reddish- transition metal, the second element of brown to black form of titanium(IV) group 8 (formerly part of subgroup VIIIB) oxide, TiO . It is an ore of titanium and is of the periodic table. It occurs naturally 2 with platinum in the natural alloy osmirid- also used in making ceramics and to coat ium and in some sulfide ores. Its relatively welding rods. Clear varieties are used as a high melting point and hardness makes it semiprecious gemstone. useful in hard heat-resistant alloys, such as those used for electrical contacts. It is also Rydberg constant A constant that oc- used as a catalyst, and as a pigment in dec- curs in formula for the frequencies of spec- orative glasses and enamels. Ruthenium is tral lines in atomic spectra. For the × also used in jewelry alloyed with palla- hydrogen atom it has the value 1.0968 7 –1 dium. 10 m . The value of the Rydberg constant Symbol: Ru; m.p. 2310°C; b.p. can be calculated from the BOHR THEORY of 3900°C; r.d. 12.37 (20°C); p.n. 44; most the hydrogen atom and from quantum me- common isotope 102Ru; r.a.m. 101.07. chanics. These calculations showed that: Μ 2 4 3 3 R = 0 me c /8h Μ Rutherford, Ernest, Lord Rutherford where 0 is the magnetic constant, m and (1871–1937) New Zealand-born physicist. e are the mass and charge respectively of an Ernest Rutherford is generally regarded as electron, h is the Planck constant and c is one of the greatest experimental physicists the speed of light in a vacuum. The Ryd- ever. He conducted fundamental research berg constant is named for the Swedish on radioactivity and nuclear physics. In physicist Johannes Robert Rydberg (1854– 1899–1900 he found that there are three 1919) for his work on atomic spectra. See types of radioactivity. He called the differ- also hydrogen atom spectrum.

194 S

sacrificial protection A method of pro- salt bridge An electrical contact be- tection against electrolytic corrosion (espe- tween two half cells, used to prevent mix- cially rusting). In protecting steel pipelines, ing. A glass U-tube filled with potassium for instance, zinc or magnesium rods are chloride in agar is often used. buried in the ground at points along the line and connected to the pipeline. Rusting saltcake An impure form of SODIUM SUL- of iron is an electrochemical process in FATE, formerly produced industrially as a which Fe2+ ions dissolve in the water in by-product of the LEBLANC PROCESS. contact with the surface. A more electropositive element, such as zinc, protects salt hydrate A metallic salt in which the against this because Zn2+ ions dissolve in metal ions are surrounded by a fixed num- preference. In other words, the zinc rod is ber of water molecules. The water mol- ‘sacrificed’ for the steel pipeline. The same ecules are bound to the metal ions by effect occurs with galvanized iron. If the ion–dipole interactions. In the solid state zinc coating is scratched, the exposed iron the water molecules form part of the crys- does not rust until all the zinc has dissolved tal structure. It is thus a clathrate. away. (Note that a tin coating, being less Salt hydrates that contain several mol- electropositive than iron, has the opposite ecules of water for each metal ion can lose effect.) them progressively (effloresce) if the pressure of water vapor is kept below the dis- safety lamp (Davy lamp) A type of oil sociation pressure of the system; eventually lamp for use in coal mines designed so that an anhydrous salt is formed. Alternatively, it will not ignite any methane (firedamp) if the vapor pressure is continuously raised present and cause an explosion. The flame the system will add molecules of water is surrounded by a fine metal gauze which (deliquesce) until a saturated solution dissipates the heat from the flame but never forms. reaches the ignition temperature of methane. If methane is present it burns in- salting out The addition of an ionic salt side the gauze; the lamp can be used as a to a solution in order to precipitate a solute test for pockets of methane. (or cause it to be evolved as a gas). The dissolved substance is more soluble in pure sal ammoniac See ammonium chloride. water than in the ionic solution. Certain colloids may also be precipitated in this saline Containing a salt, especially an al- way. kali-metal halide such as sodium chloride. saltpeter See potassium nitrate. salt A compound with an acidic and a basic radical, or a compound formed by sal volatile See ammonium carbonate. total or partial replacement of the hydrogen in an acid by a metal; the product of samarium A hard brittle silver-gray ele- neutralizing an acid with a base. In general ment of the lanthanoid series of metals. It terms a salt is a material that has identifi- occurs in association with other lan- able cationic and anionic components. thanoids in such minerals as allanite, bast-

195 sand naesite, gadolinite, monazeite, and other colors by impurities. Sapphires can samarskite. Samarium is alloyed with also be made synthetically. They are used cobalt to produce powerful permanent in jewelry and certain kinds of lasers. magnets. It is also used in alloys for nuclear reactor parts, as a catalyst in organic reac- saturated solution A solution that con- tions, and as an ingredient in optical glass. tains the maximum equilibrium amount of Symbol: Sm; m.p. 1077°C; b.p. solute at a given temperature. A solution is 1791°C; r.d. 7.52 (20°C); p.n. 62; most saturated if it is in equilibrium with its common isotope 152Sm; r.a.m. 150.36. solute. If a saturated solution of a solid is cooled slowly, the solid may stay tem- sand A mineral form of silicon(IV) oxide porarily in solution; i.e. the solution may (silica, SiO2) consisting chiefly of separate contain more than the equilibrium amount grains of quartz. It results from erosion of of solute. Such solutions are said to be su- quartz-containing rocks; the grains may be persaturated solutions. rounded by the tumbling action of water or wind. Sand is used in concrete and mortar saturated vapor A vapor that is in equi- and in making glass and other ceramics. librium with its solid or liquid phase. A saturated vapor is at the maximum pressure sandstone A sedimentary rock consist- (the saturated vapor pressure) at a given ing of consolidated sand grains cemented temperature. If the temperature of a satu- together with calcium carbonate, clay, or rated vapor is lowered, the vapor con- oxides of iron. It is used mainly as a build- denses. Under certain circumstances, the ing stone. substance may stay temporarily in the vapor phase; i.e. the vapor contains more sandwich compound A type of than the equilibrium concentration of the organometallic compound formed between substance. The vapor is then said to be a transition metal ions and aromatic com- supersaturated vapor. pounds, in which the metal ion is ‘sandwiched’ between two rings. Four, five, six, s-block elements The elements of the seven, and eight-membered rings are first two groups of the periodic table; i.e. known to form complexes with a number group 1 (H, Li, Na, K, Rb, Cs, Fr) and of elements including V, Cr, Mn, Co, Ni, group 2 (Be, Mg, Ca, Sr, Ba, Ra). They are and Fe. The bonding in such compounds is so called because their outer shells have the not between the metal ion and individual electronic configurations ns1 or ns2. The s- atoms on the ring; rather it involves bond- block excludes those with inner (n – 1)d ing of d electrons of the ion with the pi- levels occupied (i.e. it excludes transition electron system of the ring as a whole. elements, which also have s2 and occasion- FERROCENE is the original example, having ally s1 configurations). two parallel cyclopentadienyl rings sand- wiching the iron ion. There are a number scandium A light soft reactive silvery of related types of compound. Thus, half transition metal, the first member of group sandwich compounds (sometimes known 3 (formerly IIIB) of the periodic table. It is as piano-stool compounds) have only one found in minute amounts in over 800 min- ring bound to the central ion. Bent sand- erals, especially thortveitite ((Sc,Y)2Si2O7) wich compounds have two rings that are and minerals often associated with lan- not parallel, and the ion is attached to thanoids. Scandium is used in high-inten- other ligands. Multidecker sandwich com- sity lights, in electronic devices, and in pounds have more than two parallel rings. titanium carbide alloys. Scandium has sev- See illustration. eral radioactive isotopes, some of which are used as tracers in medicine. sapphire A valuable gemstone variety of Symbol: Sc; m.p. 1541°C; b.p. 2831°C; the mineral corundum (aluminum oxide, r.d. 2.989 (0°C); p.n. 21; most common 45 Al2O3) colored blue, green, pink, violet, or isotope Sc; r.a.m. 44.955910. 196 scanning tunneling microscope

Fe Mo

CO CO

C O

parallel sandwich half sandwich (piano stool)

Ni H

Ti H

bent sandwich multidecker sandwich Sandwich compound scanning tunneling microscope (STM) curring depending on both the distance be- A type of microscope that makes use of tween the surface and the tip and the elec- quantum-mechanical tunneling to investi- tron density at the surface. The tunneling gate the atomic structure of a sample of produces an electric current, which is kept material. In this technique a sharp con- constant by moving the tip up or down as ducting tip is brought near a surface. This it is moved over the surface; a computer causes electrons to tunnel from the surface converts the movements into an image. to the tip, with the probability of this oc- Single atoms can be observed using STM.

197 Scheele, Karl Wilhelm

Scheele, Karl Wilhelm (1742–86) seaborgium A radioactive synthetic Swedish chemist. Scheele was one of the transactinide metallic element created by greatest chemists of the 18th century. He bombarding 249Cf with 18O nuclei using a discovered a large number of elements and cyclotron. Six isotopes are known. compounds including oxygen (indepen- Symbol: Sg; m.p., b.p. and r.d. un- 266 dently of Joseph PRIESTLEY), nitrogen, chlo- known; p.n. 106; most stable isotope Sg rine (although he did not recognize it as an (half-life 27.3s). element), citric acid, glycerol, hydrogen cyanide, hydrogen fluoride, hydrogen sul- second Symbol: s The base unit of time fide, lactic acid, and uric acid. He also dis- in all metric systems, including the SI. In covered that light affects certain silver the SI it is defined as the duration of salts. He wrote a book entitled A Chemical 9 192 631 770 periods of a particular Treatise on Air and Fire (1777) in which he wavelength of radiation corresponding to a described his experiments. transition between two hyperfine levels in the ground state of the cesium-133 atom, schönite See potassium sulfate. an unvarying physical constant. Earlier metric systems defined the second as the Schottky defect See defect. fraction 1/86400 of a mean solar day. Ir- regularities in the Earth’s rotation, how- Schrödinger, Erwin (1887–1961) Aus- ever, make the mean solar day less than trian physicist. Schrödinger developed the constant. branch of quantum mechanics known as wave mechanics. In this work Schrödinger secondary cell See accumulator. formulated what is known as the Schrö- second-order reaction A reaction in dinger equation. He solved his equation for which the rate of reaction is proportional simple systems such as the hydrogen atom. to the product of the concentrations of two He also showed how wave mechanics and of the reactants or to the square of the con- the matrix mechanics of Werner HEISEN- centration of one of the reactants; i.e. BERG are equivalent. Schrödinger shared rate = k[A][B] the 1933 Nobel Prize for physics with Paul or DIRAC for his work on quantum mechanics. rate =k[A]2 Schrödinger wrote a book entitled What is For example, the hydrolysis by dilute Life? (1944) which was influential in stim- alkali of an ester is a second-order reaction: ulating interest in molecular biology. rate = k[ester][alkali] The rate constant for a second-order re- Schrödinger equation The fundamen- action has the units mol–1 dm3 s–1. Unlike a tal equation in the WAVE MECHANICS for- first-order reaction, the time for a definite mulation of quantum mechanics. For fraction of the reactants to be consumed is calculating energy levels of electrons in dependent on the original concentrations. atoms, molecules, and solids, the time-independent Schrödinger equation is used. sedimentation The settling of a suspen- This equation can be written: sion, either under gravity or in a centrifuge. ∇2ψ + 8π2m(E – U)ψ/h2 = 0 The speed of sedimentation can be used to where ∇2 is an operator, U the potential estimate the average size of the particles. energy, h the Plank constant, and ψ the This technique is used with an ultracen- wavefunction. E is the energy of the sys- trifuge to find the relative molecular tem. The operator ∇2 is: masses of macromolecules. ∂/∂x2 + ∂/∂y2 + ∂/∂z2 seed A small crystal added to a gas or scrubber The part of a chemical plant liquid to assist solidification or precipita- that removes impurities from a gas by pass- tion of a substance from a solution. The ing it through a liquid. seed, usually a crystal of the substance to

198 shell be formed, enables particles to pack into tor can be formed by introducing small predetermined positions so that a larger amounts of an impurity to an ultrapure crystal can form. material. If these impurities can withdraw electrons from the occupied energy level, selenium A metalloid or nonmetallic el- leaving ‘holes’ that permit conduction, the ement; the third member of group 16 (for- result is known as a p-type semiconductor. merly VIA) of the periodic table. It exists in Alternatively, these impurities may donate several allotropic forms. It occurs in electrons that enter the unoccupied energy minute quantities in sulfide ores and is also level, thus forming an n-type semiconduc- recovered from anode sludges formed dur- tor. Semiconductors are used extensively ing electrolytic refining. The common gray by the electronics industry in such devices metallic allotrope is both a semiconductor as integrated circuits. and very light-sensitive, and thus finds use in photocells, solar cells, and xerography. semipermeable membrane A mem- It is also used in some glasses and enamels, brane that, when separating a solution and in trace amounts is an essential dietary from a pure solvent, permits the solvent mineral. The red allotrope is unstable and molecules to pass through it but does not reverts to the gray form under normal con- allow the transfer of solute molecules. Syn- ditions. thetic semipermeable membranes are gen- Symbol: Se; m.p. 217°C (gray); b.p. erally supported on a porous material, 684.9°C (gray); r.d. 4.79 (gray); p.n. 34; such as unglazed porcelain or fine wire most common isotope 80Se; r.a.m. 78.96. screens, and are commonly formed of cellulose or related materials. They are used in self-organization The order that can osmotic studies, gas separations, reverse arise in a system that is far from thermody- osmosis water systems for homes and bev- namic equilibrium. The existence of self- erage industries, and in medical applica- organization does not contradict the tions. second law of thermodynamics since self- Equilibrium is reached at a semiperme- organization occurs in systems that are able membrane if the chemical potentials open and through which there is an energy on both sides become identical; migration flow. Self-organization can be analyzed in of solvent molecules towards the solution terms of chaos theory and nonequilibrium is an attempt by the system to reach equi- statistical mechanics and thermodynamics. librium. The pressure required to halt this migration is the OSMOTIC PRESSURE. Semenov, Nikolay Nikolaevich (1896– 1986) Russian physical chemist. Semenov semipolar bond See coordinate bond. studied chemical chain reactions in the 1920s. He showed how such reactions septivalent See heptavalent. could lead to combustion and violent ex- plosions when branching occurs in the sequestration The formation of a com- chain. He gave an account of his work in plex with an ion in solution, so that the ion the influential book Chemical Kinetics and does not have its normal activity. Seques- Chain Reactions (1934), the English trans- tering agents are often chelating agents. lation of which was published in 1935. He shared the 1956 Nobel Prize for chemistry sesqui- Prefix indicating a 2/3 ratio in a with Sir Cyril HINSHELWOOD. compound. A sesquioxide, for example, would have the formula M2O3. A semiconductor A crystalline material sesquicarbonate is a mixed carbonate and that conducts only under certain condi- hydrogencarbonate of the type tions. Its conductivity, unlike that of met- Na2CO3.NaHCO3.2H2O, which contains als, increases with temperature because the 2 CO3 units and 3 sodium ions. highest occupied and lowest unoccupied energy levels are very close. A semiconduc- shell A group of electrons that share the

199 sheradizing same principal quantum number n. Early sievert Symbol: Sv The SI derived unit of work on x-ray emission studies used the absorbed dose equivalent in living tissue, terms K, L, M, and these are still some- equal to that produced by ionizing radia- times used for the first three shells: n = 1, tion which, when multiplied by certain di- K-shell; n = 2, L-shell; n = 3, M-shell. See mensionless factors, gives a value of one also atom. joule per kilogram (1 J kg–1). The dimensionless factors are used to modify the ab- sheradizing A technique used to obtain sorbed dose to take account of the fact that corrosion-resistant articles of iron or steel different types of radiation cause different by heating with zinc dust in a sealed rotat- biological effects. The sievert is named for ing drum for several hours at about 375°C. the Swedish radiologist and physicist Rolf At this temperature the two metals com- Sievert (1896–1966). bine, forming internal layers of zinc–iron and zinc–steel alloys and an external layer sigma bond See orbital. of pure zinc. Sheradizing is principally used for small parts, such as springs, washers, sigma orbital See orbital. nuts, and bolts. It is named for its inventor, English metallurgist Sherard Cowper- silane Any of a small number of hy- Coles (1866–1936). drides of silicon: SiH4, Si2H6, Si3H8, etc. Silanes can be made by the action of acids short period See period. on magnesium silicide (Mg2Si). They are unstable compounds and ignite sponta- SI See SI units. neously in air. Only the first few members of the series are known and there is not the side reaction A chemical reaction that range of ‘hydrosilicon’ compounds to com- takes place to a limited extent at the same pare with the hydrocarbons. time as a main reaction. Thus the main product of a reaction may contain small silica See silicon(IV) oxide. amounts of other compounds. silica gel A gel made by coagulating Sidgwick, Nevil Vincent (1873–1952) sodium silicate sol. The gel is dried by heat- British chemist. The first notable contribu- ing and used as a catalyst support and as a tion by Sidgwick to chemistry was his book drying agent. The silica gel used in desicca- The Organic Chemistry of Nitrogen tors and in packaging to remove moisture (1910). He subsequently became interested is often colored with a cobalt salt to indi- in the electronic structure of atoms, includ- cate whether it is still active (blue = dry; ing work on coordination compounds and pink = moist). the hydrogen bond. He summarized his research in an influential book The Elec- silicates A large number of compounds tronic Theory of Valency (1927). He spent containing metal ions and complex sili- the rest of his career trying to understand con–oxygen combinations. The negative 4– compounds in terms of valence theory. ions in silicates are of the form SiO4 , 6– This culminated in his monumental two- Si2O7 , etc., and many are polymeric, con- volume book The Chemical Elements and taining SiO4 units linked in long chains, their Compounds (1950). sheets, or three-dimensional arrays. Alumi- nosilicates and borosilicates are similar siemens Symbol: S The SI derived unit materials containing aluminum or boron of electrical conductance, equal to a con- atoms in the structure. Silicates occur nat- ductance of one ampere per volt.1S = 1A urally in many rocks and minerals. v–1. In earlier metric systems the equivalent of the siemens was known as the mho. It is silicic acid A colloidal or jellylike hy- named for German electrical engineer and drated form of silicon(IV) oxide (SiO2), inventor Ernst von Siemens (1816–92). made by adding an acid (such as hy-

200 O

Si = O O O

Si O4– as in Be SiO (phenacite) 4 2 4

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

12– Si O 6– as in BaTiSi O (bentonite) Si O as in Be Al Si O (beryl) 3 9 3 9 6 18 3 2 6 18

single chain : pyroxenes

double chain : amphiboles sheet : micas

Silicate: examples of silicate structures

201 silicide drochloric acid) to a soluble SILICATE. Being a metalloid, silicon is somewhat amphoteric and consequently silica is weakly silicide A compound of silicon with a acidic, dissolving in fused alkalis to form more electropositive element. the appropriate silicate. Symbol: Si; m.p. 1410°C; b.p. 2355°C; silicon A hard brittle gray semiconduct- r.d. 2.329 (20°C); p.n. 14; most common ing metalloid; the second element of group isotope 28Si; r.a.m. 28.0855 14 (formerly IVA) of the periodic table. It has the electronic configuration of neon silicon bronze See bronze. with four additional outer electrons; i.e., [Ne]3s23p2. silicon carbide (carborundum; SiC) A Silicon accounts for 27.7% of the mass black very hard crystalline solid made by of the Earth’s crust and occurs in a wide heating silicon(IV) oxide with carbon in an variety of silicates with metals, and in electric furnace. Sand and coke may be em- clays, micas, and sand, which is largely sil- ployed for lower grades. It is used as an icon(IV) oxide (SiO2). The element is ob- abrasive and as a refractory material. tained on a small scale by the reduction of SiO2 by carbon or calcium carbide. It is silicon(IV) chloride (silicon tetrachlo- employed in certain alloys, but its major ride; SiCl4) A colorless fuming liquid use by far is in semiconducting electronic which rapidly hydrolyzes in moist air or components and chips. For semiconductor water. It is used to produce pure silica for applications very pure silicon is produced making special kinds of glass. by direct reaction of silicon with an HCl/Cl2 mixture to give silicon tetrachlo- silicon dioxide See silicon(IV) oxide. ride (SiCl4), which can then be purified by fractional distillation. This compound is silicones Polymeric synthetic silicon then decomposed on a hot wire in an at- compounds containing chains of alternat- mosphere of hydrogen. For ultrapure sam- ing silicon and oxygen atoms, with organic ples zone refining is used. Silicon has two groups bound to the silicon atoms. Sili- allotropes: the familiar gray one has a dia- cones are used as lubricants and water re- mond lattice; the other is an amorphous pellants and in waxes and varnishes. brownish powder. Silicone rubbers are superior to natural Silicon does not react with hydrogen rubbers in their resistance to both high and except under extreme conditions in the low temperatures and to chemicals. See presence of chlorine, in which case com- also siloxanes. pounds such as trichlorosilane (HSiCl3) are obtained. The hydride SiH4 itself may be silicon(IV) oxide (silicon dioxide; silica; prepared in the laboratory by hydrolysis of SiO2) A hard crystalline compound oc- magnesium silicide, in which case a num- curring naturally chiefly in three crystalline ber of other hydrides up to Si6H14 are also forms: quartz (hexagonal), tridymite obtained, or more conveniently by reduc- (rhombic), and crystobalite (tetragonal or tion of silicon tetrachloride with lithium cubic). Sand is mostly fine particles of tetrahydridoaluminate: quartz. Fused silica is a glassy substance → SiCl4 + LiAlH4 SiH4 + LiCl + AlCl3 used in laboratory apparatus. Silicon(IV) The silanes are mildly explosive. oxide is used in the manufacture of glass, in Silicon combines with oxygen when refractory materials, and in metallurgical heated in air to form silicon(IV) oxide furnaces. (SiO2). The mineral silicates constitute a vast collection of materials with a wide silicon tetrachloride See silicon(IV) range of structures including chains, rings, chloride. lattices, twisted chains, sheets, and vary- ingly cross-linked structures all based on siloxanes Compounds containing Si–O–Si 2– different assemblies of [SiO4] tetrahedra. groups with organic groups bound to the 202 sintering silicon atoms. The silicones are polymers ble iodide solution to a solution of silver(I) of siloxanes. nitrate. Silver(I) iodide occurs in nature as iodargyrite in the form of hexagonal crys- silver A soft ductile malleable brilliant tals. It is virtually insoluble in ammonia so- white precious transition metal, the second lution, and is used in the photographic element of group 11 (formerly IB) of the industry. Silver(I) iodide will absorb am- periodic table. Silver occurs native and in monia gas. It is trimorphic, contracting such minerals as argentine (silver sulfide, when heated and expanding when cooled. Ag2S) and chlorargyrite (AgCl). It is also found in gold, copper, and lead ores, from silver(I) nitrate (AgNO3) A white crys- which it is extracted as a by-product during talline poisonous solid prepared by dis- refining. Silver is not as reactive as copper, solving silver in dilute nitric acid and but is more reactive than gold. It darkens in crystallizing the solution. Silver(I) nitrate is air due to the formation of silver sulfide. It dimorphous, crystallizing in the ortho- is used in jewelry, dental amalgams, elec- rhombic and hexagonal systems. It under- trical conductors, and batteries. Until re- goes thermal decomposition to yield silver, cently it was also used in coinage alloys, dinitrogen oxide, and oxygen. Probably tableware, and as a reflective surface for the most important silver salt, it is used in mirrors. Silver compounds are extensively the medical industry as an antiseptic and used in photography. for the treatment of warts, and in the pho- Symbol: Ag; m.p. 961.93°C; b.p. tographic industry to make light-sensitive 2212°C; r.d. 10.5 (20°C); p.n. 47; most emulsions. In the laboratory, it is used both common isotope 107Ag; r.a.m. 107.8682. as an analytical and volumetric reagent. silver(I) bromide (AgBr) A pale yellow silver(I) oxide (argentous oxide; Ag2O) precipitate obtained by adding a soluble A brown amorphous solid formed by the bromide solution to a solution of silver(I) addition of sodium or potassium hydrox- nitrate. The halide dissolves in concen- ide solution to a solution of silver(I) ni- trated ammonia solution but not in dilute trate. Silver(I) oxide turns moist red litmus ammonia solution. It is used extensively in blue and decomposes (at 160°C) to give sil- photography for making light-sensitive ver and oxygen. Silver(I) oxide dissolves in emulsions. Silver(I) bromide crystallizes in concentrated ammonia solution, forming + a similar manner to sodium(I) chloride. the complex ion [Ag(NH3)2] . It is used in Unlike silver(I) chloride and silver(I) io- organic chemistry. dide, silver(I) bromide does not absorb ammonia gas. silver(II) oxide (argentic oxide; AgO) A black diamagnetic substance prepared by silver(I) chloride (AgCl) A compound the oxidation of either silver or silver(I) prepared as a curdy white precipitate by oxide using ozone. Alternatively it can be the addition of a soluble chloride solution prepared as a precipitate by the addition of to a solution of silver(I) nitrate. It occurs in a solution of potassium persulfate nature as the mineral chlorargyrite. Sil- (K2S2O8) to one of silver(I) nitrate. ver(I) chloride is insoluble in water but dissolves in concentrated hydrochloric acid simple cubic crystal See cubic crystal. and in concentrated ammonia solution. It is rather unreactive but is affected by single bond A covalent bond between sunlight, and is thus used extensively in two elements that involves one pair of elec- photography to make light-sensitive emul- trons only. It is represented by a single line, sions. Silver(I) chloride crystallizes in the for example H–Br, and is usually a sigma sodium chloride lattice pattern. bond, although it can be a pi bond. Com- pare multiple bond. See also orbital. silver(I) iodide (AgI) A pale yellow precipitate prepared by the addition of a solu- sintering A process in which certain

203 SI units powdered substances (e.g. metals, ceram- Smalley to direct his laser beam at a ics) coagulate into a single mass when graphite target, leading to the discovery of heated to a temperature below the sub- a new alloprope of carbon, now known as stance’s melting point. Sintered glass is a BUCKMINSTERFULLERENE. porous material used for laboratory filtration. smelting An industrial process for extracting metals from their ores at high tem- SI units (SI; Système International peratures. Generally the ore is reduced d’Unités) The modern coherent rational- with carbon (for zinc and tin) or carbon ized internationally adopted metric system monoxide (for iron). Copper and lead are of units. It has seven BASE UNITS and two di- obtained by reduction of the oxide with the mensionless units, formerly called supple- sulfide; for example: → mentary units. DERIVED UNITS are formed 2Cu2O + Cu2S 6Cu + SO2 by multiplication and/or division of base A flux is also used to combine with impu- units. Standard prefixes are used for deci- rities and form a SLAG on top of the molten mal multiples and submultiples of SI units, metal. along with standard symbols for both units and prefixes. smithsonite See calamine. slag (basic slag) Glasslike compounds of soap See detergents. comparatively low melting point formed during the extraction of metals when the soapstone A soft type of TALC which has impurities in an ore react with a flux. The a greasy feel and which is easy to carve to fact that the slag is a liquid of compara- make ornaments. It was formerly known tively low density means that it can be sep- as steatite. arated from the liquid metal on which it floats. In a blast furnace, the flux used is soda See sodium carbonate; sodium hy- limestone (CaCO3) and the main impurity droxide. is silica (SiO2). The slag formed is mainly calcium silicate: soda ash See sodium carbonate. → CaCO3 + SiO2 CaSiO3 It is used as railroad ballast and in fertiliz- soda glass See glass. ers and concrete. soda lime A gray solid produced by slaked lime See calcium hydroxide. adding sodium hydroxide solution to calcium oxide, to give a mixture of Ca(OH)2 slaking See calcium hydroxide. and NaOH on evaporation. It is used in the laboratory as a drying agent and as an ab- slip planes The planes of weakness in a sorbent for carbon dioxide. crystal, e.g. the boundaries between the octahedral layers in graphite. sodamide (NaNH2) A white ionic solid formed by passing dry ammonia over slurry A thin paste of suspended solid sodium at 300–400°C. The compound re- particles in a liquid. acts with water to give sodium hydroxide and ammonia. It reacts with red-hot car- Smalley, Richard Errett (1943– ) bon to form sodium cyanide and with ni- American chemist. In 1981 Smalley de- trogen(I) oxide to form sodium azide. vised a procedure to produce microclusters Sodamide is used in the manufacture of of a hundred or so atoms, by vaporizing a sodium hydroxide and in the explosives in- metal using a laser. The released atoms are dustry. cooled by a jet of helium and condense into variously sized clusters. In 1985 a visiting soda water A solution of carbon diox- British chemist, Harold KROTO, persuaded ide dissolved in water under pressure, used

204 BASE AND DIMENSIONLESS SI UNITS

Physical quantity Name of SI unit Symbol for SI unit length meter m mass kilogram(me) kg time second s electric current ampere A thermodynamic temperature kelvin K luminous intensity candela cd amount of substance mole mol *plane angle radian rad *solid angle steradian sr *supplementary units

DERIVED SI UNITS WITH SPECIAL NAMES

Physical quantity Name of SI unit Symbol for SI unit frequency hertz Hz energy joule J force newton N power watt W pressure pascal Pa electric charge coulomb C electric potential difference volt V electric resistance ohm Ω electric conductance siemens S electric capacitance farad F magnetic flux weber Wb inductance henry H magnetic flux density tesla T luminous flux lumen lm illuminance (illumination) lux lx absorbed dose gray Gy activity becquerel Bq dose equivalent sievert Sv

DECIMAL MULTIPLES AND SUBMULTIPLES USED WITH SI UNITS

Submultiple Prefix Symbol Multiple Prefix Symbol 10–1 deci- d 101 deca- da 10–2 centi- c 102 hecto- h 10–3 milli- m 103 kilo- k 10–6 micro- µ 106 mega- M 10–9 nano- n 109 giga- G 10–12 pico- p 1012 tera- T 10–15 femto- f 1015 peta- P 10–18 atto- a 1018 exa- E 10–21 zepto- z 1021 zetta- Z 10–24 yocto- y 1024 yotta- Y

205 sodium in fizzy drinks. When the pressure is re- sodium acetate See sodium ethanoate. leased, the liquid effervesces with bubbles of carbon dioxide gas. See also carbonic sodium aluminate (NaAlO2) A white acid. solid produced in the laboratory by adding excess aluminum to a hot concentrated so- sodium A soft silver-white reactive al- lution of sodium hydroxide. Once the reac- kali metal; the second element of group 1 tion has been initiated, sufficient heat is (formerly IA) of the periodic table. It has liberated to keep the reaction going; hy- the electronic configuration of neon plus drogen is also produced. In solution the – one additional outer 3s electron. The ele- aluminate ions have the structure Al(OH)4 ment undergoes electronic excitation in and NaAl(OH)4 is known as sodium(I) flames and in sodium-vapor lamps to give tetrahydroxoaluminate(III). Addition of a distinctive yellow color arising from in- sodium hydroxide to an aluminum salt so- tense emission at the so called ‘sodium-D’ lution produces a white gelatinous precipi- line pair. The element’s ionization poten- tate of aluminum hydroxide, which tial is rather low and the sodium atom thus dissolves in excess alkali to give a solution readily loses its electron (i.e. the metal is of sodium(I) tetrahydroxoaluminate(III). strongly reducing). The chemistry of Sodium aluminate is used in cleaning prod- sodium is largely the chemistry of the ucts, in the treatment of waste effluent, and monovalent Na+ ion. in zeolite production. It is also used as a Sodium occurs widely as NaCl in sea- mordant. water and as deposits of halite in dried-up lakes etc. (2.6% of the Earth’s crust). The sodium azide (NaN3) A white solid prepared by passing nitrogen(I) oxide over element is obtained commercially via the heated sodamide. On heating, sodium Downs process by electrolysis of NaCl azide decomposes to give sodium and ni- melts in which the melting point is reduced trogen. It is used as a reagent in organic by the addition of calcium chloride; chemistry and in the manufacture of explo- sodium is produced at the steel cathode. sives (detonators). The metal is extremely reactive, vigorously so with the halogens and also with water, sodium bicarbonate See sodium hy- in the latter case to give hydrogen and drogencarbonate. sodium hydroxide. It is used as a coolant in fast-breeder nuclear reactors. The chem- sodium bisulfate See sodium hydrogen- istry of sodium is very similar to that of the sulfate. other members of group 1. Nearly all sodium compounds are solu- sodium bisulfite See sodium hydrogen- ble in water. Sodium hydroxide is pro- sulfite. duced commercially by the electrolysis of brine using diaphragm cells or mercury- sodium borate See disodium tetrabo- cathode cells; chlorine is a coproduct. rate decahydrate. Solutions of sodium metal in liquid ammonia are blue and have high electrical sodium bromide (NaBr) A white solid conductivities; the main current carrier of formed by the action of bromine on hot such solutions is the solvated electron. sodium hydroxide solution. Alternatively, Such solutions are used in both organic and it can be made by the action of dilute hy- inorganic chemistry as efficient reducing drobromic acid on sodium carbonate or agents. hydroxide. Sodium bromide is soluble in The metal itself has a body-centered crystal water and forms crystals similar in shape to structure. sodium chloride. It is used in medicine, an- Symbol: Na; m.p. 97.81°C; b.p. 883°C; alytical chemistry, and photography. r.d. 0.971 (20°C); p.n. 11; most common Sodium bromide forms a hydrate, 23 isotope Na; r.a.m. 22.989768. NaBr.2H2O. 206 sodium dichromate(VI) sodium carbonate (soda ash; Na2CO3) A white amorphous powder, which aggre- gates on exposure to air owing to the formation of hydrates. Industrially it is prepared by the SOLVAY PROCESS. On crystallizing from aqueous solution large translucent crystals of the decahydrate (Na2CO3.10H2O) (washing soda) are formed. These effloresce quickly in air, forming the monohydrate Na2CO3.H2O. Large quantities of sodium carbonate are chloride ion used in the manufacture of sodium hy- sodium ion droxide. Sodium carbonate is also used in photography, as a food additive, and in the Sodium-chloride structure textile trade. Washing soda is used as a domestic cleanser. The salt produces an alka- each edge and in the center of the cube. line solution in water by hydrolysis: Electrostatic attraction holds the oppo- → Na2CO3 + 2H2O 2NaOH + H2CO3 sitely charged ions together. It is used in volumetric analysis to stan- The lattice can also be thought of as dardize strong acids. two interpenetrating face-centered cubes, one composed of sodium ions and the sodium chlorate(V) (NaClO3) A white other of chloride ions. Other compounds solid formed by the action of chlorine on (e.g. sodium bromide and potassium chlor- hot concentrated sodium hydroxide solu- ide) having their ions arranged in the same tion, or by the electrolysis of a concen- positions are also described as having the trated sodium chloride solution. It is sodium-chloride structure. soluble in water. Sodium chlorate is a powerful oxidizing agent. If heated it yields sodium cyanide (NaCN) An extremely oxygen and sodium chloride. The chlorate poisonous white solid formed either by the is used in explosives, in matches, as a weed action of carbon on sodamide at high tem- killer, and in the textile industry. perature or by passing ammonia over sodium at 300–400°C to form sodamide sodium chloride (common salt; salt; and then reacting it with carbon. The in- NaCl) A white solid prepared by the neu- dustrial salt is prepared by absorbing hy- tralization of hydrochloric acid with aque- drogen cyanide into a sodium hydroxide ous sodium hydroxide. It occurs in sea solution and purifying the solid by recrys- water and natural brines. Natural solid de- tallization from liquid ammonia. It is used posits of rock salt (halite) are also found in as a source of cyanide and hydrocyanic certain places. It is an essential dietary min- acid. In the cyanide process it is used in the eral. The compound dissolves in water extraction of silver and gold. Its aqueous with the absorption of heat, its solubility solutions are alkaline due to salt hydroly- changing very little with temperature. It is sis. used to season and preserve food. Industri- ally, it is used as a raw material in the man- sodium dichromate(VI) (Na2Cr2O7) ufacture of sodium carbonate via the An orange deliquescent solid that is very SOLVAY PROCESS, sodium hydroxide via soluble in water. It is made from finely electrolysis, and soap. Sodium chloride is powdered chromite, which is heated with almost insoluble in alcohol. calcium oxide and sodium carbonate in a furnace. If an alkali is added to a solution sodium-chloride structure A form of of the dichromate, the chromate is pro- crystal structure that consists of a face-cen- duced. At high temperatures, sodium tered cubic arrangement of sodium ions dichromate decomposes to give the chro- with chloride ions situated at the middle of mate, chromic oxide, and oxygen. It is used

207 sodium dihydrogen orthophosphate as an oxidizing agent, particularly in or- tem but in forms that closely resemble ganic chemistry, and in volumetric analysis cubes and isometric octahedrals. Sodium to estimate iron(II) ions and iodide ions. It hexafluoroaluminate is also used as a flux is also used as a mordant. in the manufacture of aluminum. sodium dihydrogen orthophosphate sodium hydride (NaH) A white crys- See sodium dihydrogen phosphate(V). talline solid prepared by passing a stream of pure dry hydrogen over sodium at sodium dihydrogen phosphate(V) 350°C; the sodium is usually suspended in (sodium dihydrogen orthophosphate; an inert medium. Electrolysis of fused NaH2PO4) A white solid prepared by sodium hydride yields hydrogen at the titrating phosphoric acid with sodium hy- anode, suggesting the presence of the H– droxide solution using methyl orange as ion. Sodium hydride reacts vigorously with the indicator. On evaporation white crys- water to form sodium hydroxide solution tals of the monohydrate are formed. It is and hydrogen. It is used as a powerful re- used in some baking powders. The mono- ducing agent to convert water to hydrogen, hydrate crystallizes in the orthorhombic concentrated sulfuric acid to hydrogen sul- system. fide, and iron(III) oxide to iron. Sodium hydride bursts into flames spontaneously sodium dioxide See sodium superox- on contact with the halogens at room tem- ide. perature. It dissolves in liquid ammonia to give sodamide, NaNH2. sodium ethanoate (sodium acetate; CH3COONa) A white solid prepared by sodium hydrogencarbonate (sodium bi- the neutralization of ethanoic acid with carbonate; baking soda; NaHCO3)A sodium carbonate or sodium hydroxide. white solid formed either by passing an ex- Sodium ethanoate reacts with sulfuric acid cess of carbon dioxide through sodium car- to form sodium hydrogensulfate and bonate or hydroxide solution, or by ethanoic acid; with sodium hydroxide it precipitation when cold concentrated solu- gives rise to sodium carbonate and tions of sodium chloride and ammonium methane. Sodium ethanoate is used in the hydrogencarbonate are mixed. Sodium hy- dyeing industry. drogencarbonate decomposes on heating to give sodium carbonate, carbon dioxide, sodium fluoride (NaF) A toxic white and water. With dilute acids, it yields car- solid formed by the action of hydrofluoric bon dioxide. It is used as an antacid and as acid on sodium carbonate or sodium hy- a constituent of baking powder, and in ef- droxide (the reaction between metallic fervescent beverages and fire extinguishers. sodium and fluorine is too violent). It is also used in the textile, ceramics, and Sodium fluoride is soluble in water and re- paper-making industries. Its aqueous solu- acts with concentrated sulfuric acid to tions are alkaline as a result of salt hydrol- yield hydrogen fluoride. It is used as a con- ysis. Sodium hydrogencarbonate forms stituent of ceramic enamels, as an antisep- monoclinic crystals. tic, as an agent to prevent fermentation, and in minute amounts to fluoridate water sodium hydrogensulfate (sodium bisul- supplies. fate; NaHSO4) A white solid formed either by the partial neutralization of sulfuric sodium hexafluoroaluminate (Na3- acid with sodium hydroxide solution, by AlF6) A compound found in nature as the action of concentrated sulfuric acid on the mineral cryolite. It is usually white or sodium nitrate, or by heating equimolar colorless, but may be reddish or brown be- quantities of sodium chloride and concen- cause of impurities. It is used to make trated sulfuric acid. In aqueous solution enamels, opaque glass, and glazes for ce- sodium hydrogensulfate is strongly acidic. ramics. It crystallizes in the monoclinic sys- It crystallizes as the monohydrate

208 sodium peroxide

(NaHSO4.H2O), which dehydrates on sodium metasilicate See sodium sili- warming. If heated strongly the pyrosulfate cate. (Na2S2O7) is formed, which decomposes to give the sulfate and sulfur(VI) oxide. sodium monoxide (Na2O) A whitish Sodium hydrogensulfate is used as a cheap deliquescent solid formed by burning source of sulfuric acid and in the dyeing in- sodium in a deficiency of oxygen or alter- dustry. natively by reducing sodium peroxide or sodium hydroxide with the requisite sodium hydrogensulfite (sodium bisul- amount of sodium. It reacts violently with water to form sodium hydroxide and with fite; NaHSO3) A white powder prepared by saturating a solution of sodium carbon- acids to form solutions of their salts. ate with sulfur(IV) oxide. It is isolated from Sodium monoxide forms cubic crystals. It dissolves in liquid ammonia to give a mix- the aqueous solution by precipitation with ture of sodamide and sodium hydroxide. alcohol. If heated it undergoes thermal decomposition to give sodium sulfate, sul- sodium nitrate (Chile saltpeter; NaNO3) fur(VI) oxide, and sulfur. Sodium A white solid formed by the neutralization hydrogensulfite is used to sterilize equip- of nitric acid with either sodium carbonate ment used in the brewing and winemaking or sodium hydroxide. It occurs naturally in industries, and also in medicine as an anti- large quantities in South America in im- septic. pure mineral deposits called caliche. Sodium nitrate is very soluble in water and sodium hydroxide (caustic soda; NaOH) on crystallization forms colorless deliques- A white deliquescent slightly translucent cent crystals. On heating it decomposes to solid. In solution it is a strong alkali and give sodium nitrite and oxygen. When electrolyte. In the laboratory it can be pre- heated with concentrated sulfuric acid, ni- pared by reacting sodium, sodium monox- tric acid is produced. Sodium nitrate is ide, or sodium peroxide with water. used as a fertilizer and as a source of ni- Industrially it can be prepared by the elec- trates and nitric acid. The salt crystallizes trolysis of sodium chloride using a cell hav- in the rhombohedral system and is isomor- ing a mercury cathode or a cell in which the phous with the iodate. anode and cathode are separated by a diaphragm. Sodium hydroxide dissolves sodium nitrite (NaNO2) A yellowish- readily in water with the evolution of heat. white solid formed by the thermal decom- Its solutions have a soapy feel and are very position of sodium nitrate. It is readily corrosive. It is used to absorb acidic gases, soluble in water. The anhydrous salt forms such as carbon dioxide and sulfur(IV) orthorhombic crystals. When treated with cold dilute hydrochloric acid, sodium ni- oxide, and to remove heavy metals from trite forms nitrous acid. It is used in or- sewage and industrial effluents. Industri- ganic chemistry and industrially as a ally it is also used in soap and paper man- corrosion inhibitor. ufacture and in the purification of bauxite. sodium orthophosphate See trisodium sodium iodide (NaI) A white solid phosphate(V). formed by reacting sodium carbonate or sodium hydroxide with hydroiodic acid; sodium peroxide (Na2O2) A yellowish- the solution is then evaporated. Sodium io- white ionic solid formed by the direct com- dide forms colorless crystals having a cubic bination of burning sodium with excess shape. It is used as a source of iodine and oxygen. It reacts with water to form in medicine and photography. Acidified so- sodium hydroxide and hydrogen peroxide; lutions of sodium iodide exhibit reducing the latter decomposes rapidly in the alka- properties owing to the formation of hy- line solution to give oxygen. Sodium per- droiodic acid. oxide is used as a bleaching agent and an

209 sodium silicate oxidizing agent for such materials as wool less crystals of the heptahydrate (Na2- and wood pulp. Sodium peroxide can con- SO3.7H2O). When treated with dilute min- vert nitrogen(II) oxide to sodium nitrate eral acids, sulfur(IV) oxide is evolved. At and iodine to sodium iodate. high temperatures, sodium sulfite undergoes thermal decomposition to give sodium silicate (sodium metasilicate; sodium sulfate and sodium sulfide. It is Na2SiO3.5H2O) A colorless crystalline used as a bleaching agent for textiles and solid used in detergents and other cleaning paper, and as an antioxidant in some compounds, in refractories, and in cements canned goods. for ceramics. A concentrated aqueous solution is known as water glass and is used as sodium superoxide (sodium dioxide; a sizing agent and for making precipated NaO2) A pale yellow solid obtained by silica and silica gel. heating sodium peroxide in oxygen at 490°C. It reacts with water to give hydro- sodium sulfate (Na2SO4) A white solu- gen peroxide, sodium hydroxide solution, ble solid often formed in the laboratory by and oxygen. Commercial sodium peroxide heating a mixture of sodium chloride and contains 10% sodium superoxide. concentrated sulfuric acid. Industrially it is more frequently prepared by reacting mag- sodium thiosulfate(IV) (hypo; Na2S2O3) nesium sulfate with sodium chloride. It A white solid prepared either by boiling forms two hydrates, the decahydrate sodium sulfite with flowers of sulfur or by (Na2SO4.10H2O) and the heptahydrate passing sulfur(IV) oxide into a suspension (Na2SO4.7H2O). Sodium decahydrate, of sulfur in boiling sodium hydroxide. found in nature as the mineral mirabilite, Sodium thiosulfate is readily soluble in also known as Glauber’s salt, is used in the water and crystallizes as large colorless manufacture of glass and as a purgative in crystals of the pentahydrate medicine. It effloresces on exposure to air (Na2S2O3.5H2O). It reacts with dilute to form the anhydrous salt, which is used acids to give sulfur and sulfur(IV) oxide. It as a drying agent in organic chemistry. At is used in photography as the fixative room temperature sodium sulfate crystal- ‘hypo’ (it was formerly known as sodium lizes in the orthorhombic system; around hyposulfate) and industrially as an an- 250°C there is a transition to the hexago- tichlor. In volumetric analysis, solutions of nal system. The decahydrate crystallizes in sodium thiosulfate are usually prepared the monoclinic form. from the pentahydrate. On heating it disproportionates to give sodium sulfate and sodium sulfide (Na2S) A yellow-red sodium sulfide. solid formed by the reduction of sodium sulfate using carbon (or carbon monoxide soft iron Iron that has a low carbon or hydrogen) at a high temperature. It is a content and is unable to form permanent corrosive material and deliquesces to re- magnets. lease hydrogen sulfide. Sodium sulfide forms hydrates containing 4½, 5, and 9 soft solder See solder. H2O. In aqueous solution it is readily hydrolyzed, the solution being strongly alka- soft water See hardness. line. Its reducing properties make it useful in metallurgy and in the production of sol A COLLOID consisting of solid parti- dyestuffs and wood pulp. cles distributed in a liquid medium. A wide variety of sols are known; the colors often sodium sulfite (Na2SO3) A white solid depend markedly on the particle size. The formed by reacting the exact amount of term aerosol is used for solid or liquid sulfur(IV) oxide with either sodium car- phases dispersed in a gaseous medium. bonate or sodium hydroxide. It is readily soluble in water and crystallizes as color- solder An alloy used in joining metals.

210 solvation

The molten solder wets the surfaces to be solubility product Symbol: Ks If an joined, without melting them, and solidi- ionic solid is in contact with its saturated fies on cooling to form a hard joint. The solution, there is a dynamic equilibrium surfaces must be clean and free of oxide, a between solid and solution: result usually achieved with the aid of a AB(s) ˆ A+(aq) + B–(aq) flux. Soft solders consist of lead with up to The equilibrium constant for this is given 60% tin and melt in the range 183–250°C. by Soft-soldering is used, for example, in [A+][B–]/[AB] plumbing and making electrical joints. The concentration of undissolved solid Brazing solders are copper–zinc alloys that [AB] is also constant, so + – have higher melting points and produce Ks = [A ][B ] stronger joints than soft solders; silver can Ks is the solubility product of the salt (at a be added to produce silver solders. given temperature). For a salt A2B3, for instance: + 2 – 3 solid The state of matter in which the Ks = [A ] [B ] , etc. particles occupy fixed positions, giving the Solubility products are meaningful only for substance a definite shape. The particles sparingly soluble salts. If the product of are held in these positions by bonds. Three ions exceeds the solubility product, precip- kinds of attraction fix the positions of the itation occurs. particles: electrovalent, covalent, and intermolecular. Since these bonds act over short solute A material that is dissolved in a distances the particles in solids are packed solvent to form a solution. closely together. The strengths of these solution A liquid system of two or more three types of bonds are different and so, species that are intimately dispersed within therefore, are the mechanical properties of each other at a molecular level. The system different solids. is therefore totally homogeneous. The major component is called the solvent solid solution A solid composed of two (generally liquid in the pure state) and the or more substances mixed together at the minor component is called the solute (gas, molecular level. Atoms, ions, or molecules liquid, or solid). of one component in the crystal are at lat- The process occurs because of a direct tice positions normally occupied by the intermolecular interaction of the solvent other component. Certain alloys are solid with the ions or molecules of the solute. SOLUTIONS of one metal in another. Iso- This interaction is called solvation. Part of morphic salts can also sometimes form the energy of this interaction appears as a solid solutions, as in the case of crystalline change in temperature on dissolution. See alums. also solid solution; solubility. solubility Symbol: S The amount of one solvation The attraction of a solute substance that can dissolve in another to species (e.g. an ion) for molecules of sol- form a saturated solution under specified vent. In water, for example, a positive ion conditions of temperature and pressure. will be surrounded by water molecules, Solubilities are stated as moles of solute per which tend to associate around the ion be- kilogram of solvent (molality), or as kilo- cause of attraction between the positive grams of solute per cubic meter of solvent charge of the ion and the negative part of (density). The solubility of a solid in a liq- the polar water molecule. The energy of uid generally increases with temperature, this solvation (hydration in the case of whereas that of a gas in a liquid generally water) is the ‘force’ needed to overcome the decreases. An increase in pressure on a gas attraction between positive and negative above a liquid leads to a proportional in- ions when an ionic solid dissolves. The at- crease in the solubility of the gas. See also traction of the dissolved ion for solvent concentration. molecules may extend for several layers. In

211 Solvay process the case of transition metal elements, ions Aprotic. A solvent that can neither accept may also form complexes by coordination nor yield protons. An aprotic solvent is to the nearest layer of molecules. therefore the opposite to an amphiprotic solvent. Solvay process (ammonia–soda process) An industrial process for mak- solvent extraction (liquid–liquid extrac- ing sodium carbonate. The raw materials tion) A method of removing a substance are calcium carbonate and sodium chloride from solution by shaking it with and dis- (with ammonia). The calcium carbonate is solving it in a better solvent that is imisci- heated: ble with the original solvent. → CaCO3 CaO + CO2 Carbon dioxide is bubbled into a solution solvolysis A reaction between a com- of sodium chloride saturated with ammo- pound and the solvent in which it is dis- nia, precipitating sodium hydrogencarbon- solved. See also hydrolysis. ate and leaving ammonium chloride in solution. The sodium hydrogencarbonate sorption See absorption. is then heated: → 2NaHCO3 Na2CO3 + H2O + CO2 specific Denoting a physical quantity The ammonia is regenerated by heating the per unit mass. For example, volume (V) per ammonium chloride with the calcium unit mass (m) is called specific volume (v): oxide: V/m = v → 2NH4Cl + CaO CaCl2 + 2NH3 + In certain physical quantities the term H2O does not have this meaning: for example, specific gravity is more properly called rel- solvent A liquid capable of dissolving ative density. other materials (solids, liquids, or gases) to form a solution. The solvent is generally specific gravity See relative density. the major component of the solution. Sol- vents can be divided into classes, the most specific heat capacity Symbol: c The important being the following: amount of heat needed to raise a unit mass Polar. A solvent in which the molecules of a substance by one degree. It is measured possess a moderate to high dipole moment in joules per kilogram per Kelvin and in which polar and ionic compounds (J kg–1K–1) in SI units. are easily soluble. Polar solvents are usu- α ally poor solvents for nonpolar com- specific rotatory power Symbol: m pounds. For example, water is a good The rotation of plane-polarized light in de- solvent for many ionic species, such as grees produced by a 10-centimeter length sodium chloride or potassium nitrate, and of solution containing one gram of a given polar molecules, such as the sugars, but substance per milliliter of stated solvent. does not dissolve paraffin wax. The specific rotatory power is a measure of Nonpolar. A solvent in which the mol- the optical activity of substances in solu- ecules do not possess a permanent dipole tion. It is measured at 20°C using the D- moment and consequently will solvate line of sodium. nonpolar species in preference to polar species. For example, some organic com- specific volume See specific. pounds are good solvents for iodine and paraffin wax, but do not dissolve sodium spectra See spectrum. chloride. Amphiprotic. A solvent that undergoes spectral line A particular wavelength of self-ionization and can act both as a proton light emitted or absorbed by an atom, ion, donor and as an acceptor. Water is a good or molecule. See line spectrum. example and ionizes according to: + – 2H2O = H3O + OH spectral series A group of related lines 212 spin in the absorption or emission spectrum of troscopy, in various forms, is used for a substance. The lines in a spectral series analysis of mixtures, for identifying and arise when the transitions all occur be- determining the structures of chemical tween one particular energy level and a set compounds, and for investigating energy of different levels. See also Bohr theory. levels in atoms, ions, and molecules. In the visible and longer wavelength ultraviolet, spectrograph An instrument for pro- transitions correspond to electronic energy ducing a photographic record of a spec- levels in atoms and molecules. The shorter trum. wavelength ultraviolet corresponds to transitions in ions. In the x-ray region, spectrographic analysis A method of transitions in the inner shells of atoms or analysis in which the sample is excited elec- ions are involved. The infrared region cor- trically (by an arc or spark) and emits radi- responds to vibrational changes in mol- ation characteristic of its component ecules, with rotational changes at longer atoms. This radiation is passed through a wavelengths. slit, dispersed by a prism or a grating, and 2. Any of various techniques for analyzing recorded as a spectrum, either photograph- the energy spectra of beams of particles or ically or photoelectrically. The photo- for determining mass spectra. graphic method was widely used for qualitative and semiquantitative work but spectrum (plural spectra) 1. A range of photoelectric detection also allows wide electromagnetic radiation emitted or ab- quantitative application. sorbed by a substance under particular circumstances. In an emission spectrum, light spectrometer 1. An instrument for ex- or other radiation emitted by the object is amining the different wavelengths present analyzed to determine the particular wave- in electromagnetic radiation. Typically, lengths produced. The emission of radia- spectrometers have a source of radiation, tion may be induced by a variety of which is collimated by a system of lenses methods; for example, by high tempera- and/or slits. The radiation is dispersed by a ture, bombardment by electrons, absorp- prism or grating, and recorded photo- tion of higher-frequency radiation, etc. In graphically or by a photocell. There are an absorption spectrum a continuous flow many types for producing and investigat- of radiation is passed through the sample. ing spectra over the whole range of the The radiation is then analyzed to deter- electromagnetic spectrum. Often spec- mine which wavelengths are absorbed. See trometers are called spectroscopes. See also also band spectrum; continuous spectrum; spectrophotometer. line spectrum. 2. Any of various other instruments for an- 2. In general, any distribution of a prop- alyzing the energies, masses, etc., of parti- erty. For instance, a beam of particles may cles. See mass spectrometer. have a spectrum of energies. A beam of ions may have a mass spectrum (the distri- spectrophotometer A form of spec- bution of masses of ions). See mass spectrometer able to measure the intensity of trometer. radiation at different wavelengths in a spectrum, usually in the visible, infrared, spelter Commercial zinc containing or ultraviolet regions. about 3% impurities, mainly lead. spectroscope See spectrometer. spin A property of certain elementary particles whereby the particle acts as if it spectroscopy 1. The production and were spinning on an axis; i.e. it has an an- analysis of spectra. There are many spec- gular momentum. Such particles also have troscopic techniques designed for investi- a magnetic moment. In a magnetic field the gating the electromagnetic radiation spins line up at an angle to the field direc- emitted or absorbed by substances. Spec- tion and precess around this direction. Cer-

213 spin–orbit coupling tain definite orientations to the field direc- mites and stalactites can eventually meet to π tion occur such that msh/2 is the compo- form a single rock pillar. nent of angular momentum along this direction. Here ms is the spin quantum standard cell A voltaic cell whose e.m.f. number, which for an electron has values is used as a standard. See Clark cell; We- +1/2 and –1/2, and h is the Planck con- ston cadmium cell. stant. standard electrode A half cell used for spin–orbit coupling The coupling be- measuring electrode potentials. The hydro- tween the spin angular momentum and the gen electrode (standard hydrogen half cell) orbital angular momentum of an electron. is the basic standard but, in practice, This is a FINE STRUCTURE feature of atomic calomel electrodes are usually used. spectra. standard hydrogen half cell See hy- spin paired See electron pair. drogen electrode. spin quantum number See atom; spin. standard pressure An internationally agreed value of 101 325 Pa (approximately square-planar See complex. 100 kPa), equal in non-SI units to a baro- metric height of 760 millimeters of mer- ° stabilization energy The difference in cury at 0 C or one ATMOSPHERE. energy between the delocalized structure and the conventional structure for a com- standard solution A solution that con- pound. The stabilization energy can be de- tains a known mass of reagent in a definite volume of solution. A standard flask or termined by comparing the experimental volumetric flask is used for this purpose. value for the heat of formation with that The solutions may be prepared by direct calculated for a conventional structure. determination of mass for primary standards. If the reagent is not available in a stabilizer A substance added to prevent pure form or is deliquescent the solution chemical change (i.e. a negative catalyst). must be standardized by titration against another known standard solution. See pri- See steel. stainless steel mary standard. stalactites and stalagmites Pillars of standard state The standard conditions calcium carbonate found hanging from the used as a reference system in thermody- ceiling (stalactites) and standing on the namics: pressure is 101 325 Pa; tempera- floor (stalagmites) in limestone caverns. ture is 25°C (298.15 K); concentration is They form when water containing dis- one mol. The substance under investiga- solved carbon dioxide forms weak car- tion must also be pure and in its usual bonic acid, which is able to react with state, given the above conditions. limestone rock (calcium carbonate) to give a solution of calcium hydrogencarbonate. standard temperature An internation- This solution accumulates in drops on the ally agreed value for which many measure- roof of caves. As it evaporates, the reaction ments are quoted. It is the melting is reversed, causing calcium carbonate to temperature of water, 0°C (273.15 K). See be deposited. Over a very long period, also STP. these deposits build up to create a stalactite hanging from the cavern roof. Water that stannane (tin(IV) hydride; SnH4) A col- drips onto the floor from the tip of this sta- orless poisonous volatile reactive gas pre- lactite also evaporates to leave a deposit of pared by the action of lithium calcium carbonate, and this deposit gradu- tetrahydroaluminate(III) on tin(II) chlor- ally grows to form a stalagmite. Stalag- ide. It is unstable, decomposing immedi-

214 step ately at 150°C. Stannane is used as a re- Gases have no fixed shape or volume. ducing agent. They expand spontaneously to fill the container and are easily compressed. The mol- stannate See tin. ecules have almost free random motion. A PLASMA is sometimes considered to be stannic chloride See tin(IV) chloride. a fourth state of matter. stannic compounds Compounds of stationary phase See chromatography. tin(IV). statistical mechanics The subject that stannite See tin. links the microscopic and macroscopic as- pects of a system by using statistical meth- stannous chloride See tin(II) chloride. ods to relate the properties of the very large number of microscopic particles, such as stannous compounds Compounds of atoms and molecules, to the thermody- tin(II). namics of the system. The derivation of the MAXWELL–BOLTZMANN DISTRIBUTION in the Stark effect The splitting of atomic second half of the 19th century can be con- spectral lines due to the presence of an ex- sidered to be the start of statistical me- ternal electric field. This phenomenon was chanics. discovered by the German physicist Jo- hannes Stark (1874–1957) in 1913 and can steam distillation A method of isolat- be explained using classical electron theory ing or purifying substances by exploiting and the BOHR THEORY of the atom. Dalton’s law of partial pressures to lower the boiling point of the mixture. When two states of matter The three physical con- immiscible liquids are distilled, the boiling ditions or phases in which substances point will be lower than that of the more occur: solid, liquid, and gas. The addition volatile component and consequently will or removal of energy (usually in the form be below 100°C if one component is water. of heat) enables one state to be converted The method is particularly useful for re- into another. The major distinctions between the covering materials from tarry mixtures. states of matter depend on the kinetic energies of their particles and the distances be- steatite See soapstone. tween them. In solids, the particles have low kinetic energy and are closely packed; steel An alloy of iron with small in gases they have high kinetic energy and amounts of carbon and, often, other met- are very loosely packed; kinetic energy and als. Carbon steels contain 0.05–1.5% car- separation of particles in liquids are inter- bon – the more carbon, the harder the steel. mediate. Alloy steels also contain small amounts of Solids, for instance, have fixed shapes other elements, such as chromium, man- and volumes, i.e. they do not flow like liq- ganese, and vanadium. Their properties de- uids and gases, and they are difficult to pend on the composition. Stainless steels, compress. In solids the atoms or molecules for instance, are resistant to corrosion. The occupy fixed positions in space. In most main nonferrous constituent is chromium cases there is a regular pattern of atoms, (10–25%) with up to 0.7% carbon. meaning that the solid is crystalline. Liquids have fixed volumes (i.e. low step An elementary stage in a chemical compressibility) but flow to take up the reaction, in which energy may be trans- shape of their container. Their atoms or ferred from one molecule to another, molecules move about at random, but they bonds may be broken or formed, or elec- are quite close to one another and their trons may be transferred. For example, in motion is hindered. the reaction between hypochlorite ions and

215 steradian iodide ions in aqueous solution there are lent bond which occurs between carbon three steps: and hydrogen. Its actual form was not Step 1 solved until the work of William LIPSCOMB – → OCl (aq) + H2O(l) HOCl(aq) + in the 1950s. OH–(aq) Step 2 stoichiometric coefficient See chemi- I–(aq) + HOCl(aq) → HOI(aq) + Cl–(aq) cal equation. Step 3 – → – OH (aq) + HOI(aq) H2O(l) + OI (aq) stoichiometric compound See stoichiometry. steradian Symbol: sr The SI dimensionless unit of solid angle, equal to that sub- stoichiometry The proportions in tended at the center of a sphere by the area which elements form compounds. A stoi- of a square on the sphere’s surface whose chiometric compound is one in which the sides are equal in length to the radius of the atoms have combined in small whole num- sphere. bers. stereochemistry The branch of chem- storage battery See accumulator. istry concerned with the shapes of molecules and the way these affect the STP (NTP) Standard temperature and chemical properties. pressure. Conditions used internationally when measuring quantities that vary with stereoisomerism See isomerism. both pressure and temperature (such as the density of a gas). The values are 101 325 stereospecific Describing a chemical re- pascals (Pa) (approximately 100 kPa) and action involving an asymmetric atom (usu- 0°C (273.15 kelvin). See also standard ally carbon) that results in only one pressure; standard temperature. geometric isomer. See isomerism; optical activity. straight chain See chain. steric effect An effect in which the strong acid An acid that is almost com- shape of a molecule influences its reac- pletely dissociated into its component ions tions. A particular example occurs in mol- in solution. Compare weak acid. ecules containing large groups, which hinder the approach of a reactant (steric strong base A base that is completely or hindrance). almost completely dissociated into its component ions in solution. steric hindrance See steric effect. strontia See strontium oxide. still An apparatus for distillation. strontium A soft reactive alkaline earth Stock, Alfred (1876–1946) German metal; the fourth member of group 2 (for- chemist. Stock began studying boron hy- merly IIA) of the periodic table. The elec- drides in 1909. Boron had previously been tronic configuration is that of krypton with little studied and was thought to react only two additional outer 5s electrons. Stron- with strongly electronegative elements, tium is of low abundance in the Earth’s such as oxygen. However Stock found that crust, strontium occurring in the minerals magnesium boride and acid produced strontianite (SrCO3) and celestite (SrSO4). B4H10, the first of the several boron hy- The element is produced industrially by drides that he discovered. It was clear from roasting the carbonate to give the oxide this and the other hydrides, B2H6 and and then reducing the oxide with alu- B10H14, that the bond between boron and minum: → hydrogen could not be the familiar cova- 3SrO + 2Al Al2O3 + 2Sr 216 structural formula

Strontium has a low ionization poten- color. The hexahydrate (SrCl2.6H2O) is tial, has large atoms, and is therefore very prepared by the neutralization of hy- electropositive. Its chemistry is therefore drochloric acid by strontium hydroxide or characterized by high reactivity. The prop- carbonate. erties of strontium fall into sequence with other alkaline earths. Thus it reacts directly strontium hydrogencarbonate (stron- with oxygen (finely divided strontium will tium bicarbonate; Sr(HCO3)2) A com- ignite in air), nitrogen, sulfur, the halogens, pound present in solutions, formed by the and hydrogen to form respectively the action of carbon dioxide on a suspension in oxide SrO, nitride Sr3N2, sulfide SrS, cold water of strontium carbonate, to halides SrX2, and hydride SrH2, all of which it reverts on heating: which are largely ionic in character. The SrCO3 + CO2 + H2O = Sr(HCO3)2 oxide SrO and the metal react readily with water to form the hydroxide Sr(OH)2, strontium hydroxide (Sr(OH)2)A which is basic and between Ca(OH)2 and solid that normally occurs as the octahy- Ba(OH)2 in terms of solubility. The car- drate (Sr(OH)2.8H2O), which is prepared bonate and sulfate are both insoluble. by crystallizing an aqueous solution of As the metal is very electropositive the strontium oxide. Strontium hydroxide is salts are never much hydrolyzed in solution readily soluble in water and its aqueous so- and the ions are largely solvated as lutions are strongly basic. It is used in the 2+ [Sr(H2O)6] . purification of sugar. Strontium is used in special alloys and to help create high vacuums. Its com- strontium monoxide See strontium pounds flame bright red and thus find use oxide. in fireworks and flares. The fission of uranium in atomic bombs creates the long- strontium nitrate (Sr(NO3)2) A color- lived radioactive isotopes 89Sr and 90Sr, less crystalline compound, often used in which can accumulate in bone (in prefer- pyrotechnics to produce a bright red flame. ence to calcium) and cause cancer. Symbol: Sr; m.p. 769°C; b.p. 1384°C; strontium oxide (strontia; strontium r.d. 2.54 (20°C); p.n. 38; most common monoxide SrO) A grayish-white powder isotope 88Sr; r.a.m. 87.62. prepared by the thermal decomposition of strontium carbonate, hydroxide, or nitrate. strontium bicarbonate See strontium Strontium oxide is soluble in water, form- hydrogencarbonate. ing an alkaline solution because of the attraction between the oxide ions and strontium carbonate (SrCO3) A white protons from the water: 2– → – insoluble solid that occurs naturally as the O + H2O 2OH mineral strontianite. It can be prepared by It is used as a drying agent and to make passing carbon dioxide over strontium various strontium salts, as well as in pig- oxide or hydroxide or by passing the gas ments, soaps, and lubricants. through a solution of a strontium salt. Strontium carbonate is then formed as a strontium sulfate (SrSO4) A white precipitate. It is used as a slagging agent in sparingly soluble salt that occurs naturally certain metal furnaces and to produce a red as the mineral celestite. It can be prepared color in fireworks, and also as a phosphor by dissolving strontium oxide, hydroxide, for cathode-ray screens. or carbonate in sulfuric acid. It is used as a substitute for barium sulfate in paints. It is strontium chloride (SrCl2) A white del- also used in fireworks to impart a red color iquescent solid obtained directly from and in paints and glazes. strontium and chlorine or by passing chlorine over heated strontium oxide. It is used structural formula The formula of a in fireworks and flares to produce a red compound showing the numbers and types

217 structural isomerism of atoms present, together with the way in sulfite A salt or ester of sulfurous acid. which these are arranged in the molecule. Often this arrangement can be shown by sulfonamide A type of organic com- grouping the atoms, as in the structural pound with the general formula formula for hydrogen peroxide (HOOH), R.SO2.NH2. Sulfonamides, which are or by illustrating their orientation toward amides of sulfonic acids, are active against each other in space. Compare empirical bacteria, and some are used in pharmaceu- formula; molecular formula. ticals (‘sulfa drugs’). structural isomerism See isomerism. sulfonate A salt or ester of sulfonic acid. See sulfonic acid. sublimate A solid formed by sublimation. sulfonation A reaction introducing the –SO2OH (sulfonic acid) group into an or- sublimation The conversion of a solid ganic compound. Sulfonation of aromatic into a vapor without the solid first melting. compounds is usually accomplished by re- For instance, at standard pressure iodine, fluxing with concentrated sulfuric acid for solid carbon dioxide, and ammonium several hours. The attacking species is SO3 chloride sublime. At certain conditions of (sulfur(VI) oxide) and the reaction is an ex- external pressure and temperature an equi- ample of electrophilic substitution. librium can be established between the solid phase and vapor phase. sulfonic acid A type of organic compound containing the –SO2.OH group. subshell A subdivision of an electron The simplest example is benzenesulfonic shell. It is a division of the orbitals that acid C6H5SO2OH). Sulfonic acids are make up a shell into sets of orbitals that are strong acids. Electrophilic substitution can degenerate (i.e. have the same energy) in introduce other groups onto the benzene the free atom. For example, in the third or ring; the –SO2.OH group directs sub- M-shell there are the 3s, 3p, and 3d sub- stituents into the 3-position. shells. The 3p subshell has three p orbitals and the 3d sub shell has five d orbitals, in- sulfonium compound An organic dicating that they are 3- and 5-degenerate, compound of general formula R3SX, respectively. where R is an organic radical and X is an electronegative radical or element; it con- + substitution reaction A reaction in tains the ion R3S . An example is diethyl- which an atom or group of atoms in an or- methylsulfonium chloride, (C2H5)2.CH3.- ganic molecule is replaced by another atom S+Cl–, made by reacting diethyl sulfide with or group. chloromethane. substrate A material that is acted on by sulfoxide An organic compound of gen- a catalyst. eral formula RSOR′, where R and R′ are organic radicals. An example is dimethyl sulfate A salt or ester of sulfuric acid. sulfoxide, (CH3)2SO, commonly used as a solvent. sulfide A compound of sulfur with a more electropositive element. Simple sul- sulfur A nonmetallic solid element, yel- fides contain the ion S2–. Polysulfides can low in its common forms; the second mem- also be formed containing ions with chains ber of group 16 (formerly VIA) of the 2– of sulfur atoms (Sx ). periodic table. It has the electronic configuration [Ne]3s23p4. sulfinate See dithionite. Sulfur occurs in elemental form in Sicily and some southern states of the USA, and sulfinic acid See dithionous acid. in large quantities in combined forms such

218 sulfur dioxide as sulfide ores (e.g. iron pyrite, FeS2) and nating agents and chlorinating agents in sulfate minerals (e.g. anhydrite, (CaSO4)). organic chemistry. The chlorides are also It forms about 0.5% of the Earth’s crust. industrially important for hardening rub- Elemental sulfur is extracted commercially bers. Sulfur hexafluoride may be prepared by the Frasch process, in which super- by direct reaction of the elements but SF4 is heated water is forced down the outer of prepared by fluorinating SCl2 with NaF. In three concentric tubes leading into the de- a limited supply of chlorine, sulfur com- posit. This causes the sulfur to melt. Air is bines to give sulfur monochloride, S2Cl2; in then blown down the central tube, causing excess chlorine at higher temperatures the the molten sulfur to be forced out of the dichloride is formed, well, where it is collected and cooled at the → 2S + Cl2 S2Cl2 well head. In contrast, the large amount of → S2Cl2 + Cl2 2SCl2 sulfur obtained from the smelting of sulfide In addition to these binary halides, sul- ores or roasting of sulfates is not obtained fur forms two groups of oxyhalides; the as elemental sulfur but is rather used disulfur dihalide oxides (thionyl halides), rectly as SO2/SO3 for conversion to sulfuric SOX2 (F, Cl, and Br, but not I), and the sul- acid via the contact process, which has fur dihalide dioxides (sulfuryl halides), many uses. SO2X2 (F and Cl, but not Br and I). Sulfur exhibits allotropy and its struc- Most metals react with sulfur to form ture in all its phases is quite complex. The sulfides, of which there are a large number common crystalline modification, rhombic of structural types. With electropositive el- sulfur, is in equilibrium with a triclinic ements of groups 1 and 2 the sulfides are modification above 96°C. Both have struc- largely ionic (S2–) in the solid phase. Hy- tures based on S rings, but the crystals are 8 drolysis occurs in solution thus: quite different. If molten sulfur is poured S2– + H O → SH– + OH– into water a dark red ‘plastic’ variety is ob- 2 Most transition metals form sulfides, tained in a semielastic form. Its structure which although formally treated as FeS, appears to be a helical chain of S atoms. CoS, NiS, etc., are largely covalent and fre- Unlike selenium and tellurium, sulfur does quently nonstoichiometric. not appear to have a gray ‘metal-like’ allotrope. In addition to the complexities of the Sulfur reacts directly with hydrogen but binary compounds with metals, sulfur the position of the equilibrium at normal forms a range of binary compounds with temperatures precludes the use of this reac- elements such as Si, As, and P, which may tion as a preparative method for hydrogen be polymeric and generally of rather com- sulfide. The element burns readily in oxy- plex structure, e.g. (SiS4)n, (SbS3)n, N4S4, gen with a characteristic blue flame to form As4S4, P4S3. Sulfur is used in fungicides and in the sulfur(IV) oxide (SO2) and traces of sul- vulcanizing process for rubber. It also fur(VI) oxide (SO3). Sulfur(IV) oxide is used as a food preservative, and to sterilize forms a wide range of organic sulfur com- fruit juices and barrels used to age wines pounds, most of which have the typically and spirits. Sulfur also forms a large num- revolting smell of H2S. ber of oxyacid species, some containing Symbol: S; m.p. 112.8°C; b.p. 444.6°C; peroxide groups and some with two or r.d. 2.07; p.n. 16; most common isotope more S atoms. It forms four distinct fluo- 32S; r.a.m. 32.066. rides, S2F2, SF4, SF6, S2F10; three chlorides, S2Cl2, SCl2, SCl4; a bromide S2Br2, but no sulfur dichloride dioxide (sulfuryl chlor- iodide. Apart from SF6, which is surpris- ide; SO2Cl2) A colorless fuming liquid ingly stable and inert, the halides are gen- formed by the reaction of chlorine with sul- erally susceptible to hydrolysis, commonly fur(IV) oxide in sunlight. It is used as a to give SO2, sometimes H2S, and the hy- chlorinating agent. drogen halide. The halides SF4, S2Cl2, and SCl2 are frequently used as selective fluori- sulfur dioxide See sulfur(IV) oxide. 219 sulfuretted hydrogen sulfuretted hydrogen See hydrogen passing sulfur(IV) oxide and oxygen over sulfide. hot vanadium(V) oxide, which acts as a catalyst, and then cooling the product in sulfuric(IV) acid See sulfurous acid. ice. Sulfur(VI) oxide reacts vigorously with water to form sulfuric acid. See also con- sulfuric acid (oil of vitriol; H2SO4)A tact process. colorless oily liquid manufactured by the CONTACT PROCESS. The concentrated acid is sulfur trioxide See sulfur(VI) oxide. diluted by adding it slowly to water, with careful stirring. Concentrated sulfuric acid sulfuryl Describing a compound con- acts as an oxidizing agent, giving sulfur(IV) taining the group =SO2. oxide as the main product, and also as a dehydrating agent. The diluted acid acts as sulfuryl chloride See sulfur dichloride a strong dibasic acid, neutralizing bases dioxide. and reacting with active metals and carbonates to form sulfates. superconductivity The property of Sulfuric acid is used in the laboratory to zero electrical resistance exhibited by cer- dry gases (except ammonia) and to prepare tain metals and metallic compounds at low nitric acid. In industry, it is used to manu- temperatures. Metals superconduct at tem- facture fertilizers (e.g. ammonium sulfate), peratures close to absolute zero but a num- explosives, chemicals, rayon, pigments, ber of ceramic metal oxides are now and detergents. It is also used to clean met- known that superconduct at higher (94K als, and as the electrolyte in vehicle batter- or better) temperatures. Synthetic organic ies. superconductors have also been produced. sulfur monochloride See disulfur supercooling The cooling of a liquid at dichloride. a given pressure to a temperature below its melting temperature at that pressure with- sulfurous acid (sulfuric(IV) acid; trioxo- out solidifying it. The liquid particles lose sulfuric(IV) acid; H SO ) A weak acid 2 3 energy but do not spontaneously fall into found only in solution, made by passing sulfur(IV) oxide into water. The solution is the regular geometrical pattern of the solid. unstable and smells of sulfur(IV) oxide. It A supercooled liquid is in a metastable is used as a reducing agent, converting state and will usually solidify if a small iron(III) ions to iron(II) ions, chlorine to crystal of the solid is introduced to act as a chloride ions, and orange dichromate(VI) ‘seed’ for the formation of crystals. As soon ions to green chromium(III) ions. as this happens, the temperature returns to the melting temperature until the substance sulfur(IV) oxide (sulfur dioxide; has completely solidified. SO2) A colorless choking gas prepared by burning sulfur or by heating metal sul- superfluidity A property of liquid he- fides in air, or by treating a sulfite with an lium at very low temperatures (2.186 K) acid. It is a powerful reducing agent, used when it makes a transition to a superfluid as a bleach. It is also used for sterilizing state in which it has a high thermal con- and as a food preservative. It dissolves in ductivity and flows without friction. See water to form sulfurous acid (sulfuric(IV) also helium. acid) and combines with oxygen, in the presence of a catalyst, to form sulfur(VI) superheating The raising of a liquid’s oxide. This latter reaction is important in temperature above its boiling temperature, the manufacture of sulfuric(VI) acid. accomplished by increasing the pressure. sulfur(VI) oxide (sulfur trioxide; SO3) A supernatant Denoting a clear liquid fuming volatile white solid prepared by that lies above a sediment or a precipitate.

220 Système International d’Unités superoxide An inorganic compound suspension A system in which small – containing the O2 ion. particles of a solid or liquid are dispersed in a liquid or gas. superphosphate A mixture – mainly calcium hydrogen phosphate and calcium sylvite See potassium chloride. sulfate – used as a fertilizer. It is made from calcium phosphate and either sulfuric acid symbol, chemical See chemical symbol. or phosphoric(V) acid. symmetry The property of an object supersaturated solution See saturated that certain operations, called symmetry solution. operations, change the object to a state in which it is indistinguishable from its origi- supersaturated vapor See saturated nal state. Examples of symmetry opera- vapor. tions are rotation about an axis, reflection supramolecular chemistry A branch through a plane, and inversion through a of chemistry concerned with the synthesis point. A symmetry element is a geometrical and study of large structures consisting of feature of the object with respect to which molecules assembled together in a definite the symmetry operation is carried out. For pattern. In a supramolecule the molecular example, an axis of rotation, a plane of re- units (which are sometimes known as syn- flection, and a point through which inver- thons) are joined by intermolecular bonds sion can be performed are symmetry – i.e. by hydrogen bonds or by ionic attrac- elements. A symmetry operation has to be tions. A particular interest in supramolecu- associated with a symmetry element and lar research is the idea of ‘self-assembly’ – vice versa. See also molecular symmetry. i.e. that the molecules form well-defined structures spontaneously as a result of their syn-anti isomerism See isomerism. geometry and chemical properties. In this way, supramolecular chemistry is ‘chem- synthesis The preparation of chemical istry beyond molecules’. compounds from simpler compounds. One type of supramolecule is a helicate, which has a double helix made of two synthesis gas A mixture of carbon chains held by copper ions along its axis. monoxide and hydrogen produced by The structure is analogous to the double steam reforming of natural gas. helix of DNA. See also host–guest chem- → istry. CH4 + H2O CO + 3H2 Synthesis gas is a useful starting ma- supramolecule See supramolecular chem- terial for the manufacture of a number of istry. organic compounds. surfactant A substance that lowers sur- synthon See supramolecular chemistry. face tension and has properties of wetting, foaming, detergency, dispersion, and emul- Système International d’Unités See SI sification, e.g. a soap or other detergent. units.

221 T

talc (French chalk) An extremely soft hibit electron transfer between ions. Taube greenish or white mineral, a hydrous sili- showed experimentally that ligand bridges cate of magnesium (Mg3Si4O10(OH)2). Pu- form between interacting complexes, thus rified it is sold as talcum powder. It is also allowing electrons to be transferred. For used as a lubricant, an electrical insulator, his work in this field Taube was awarded a filler (in paint, rubber, and paper) and an the 1983 Nobel Prize for chemistry. ingredient of some ceramics. tautomerism Isomerism in which each tantalum A hard ductile rare silvery isomer can convert into the other, so that transition metal, the third member of the two isomers are in equilibrium. The group 5 (formerly VB) of the periodic isomers are called tautomers. Tautomerism table. It occurs chiefly in the minerals often results from the migration of a hy- colombite and tantalite, in association with drogen atom. niobium. It is strong, highly resistant to corrosion, and easily worked. Tantalum is technetium A silver-gray radioactive used in turbine blades and cutting tools, transition metal, the second element of and in surgical and dental work. It is also group 7 (formerly VIIB) of the periodic used in electronic components and in fila- table. It does not occur naturally on Earth ments for light bulbs subject to shock and although it has been detected in the spectra vibration. of stars. It is produced artificially by bom- Symbol: Ta; m.p. 2996°C; b.p. 5425 ± barding molybdenum with deuterium nu- 100°C; r.d. 16.654 (20°C); p.n. 73; most clei or neutrons, and also during the fission common isotope 181Ta; r.a.m. 180.9479. of uranium. It is used in medicine as a radioactive label. tartar emetic (potassium antimonyl tar- Symbol: Tc; m.p. 2172°C; b.p. 4877°C; ¼ trate; KSbO(C4H4O6). H2O) A poiso- r.d. 11.5 (est.); p.n. 43; r.a.m. 98.9063 nous substance used as an emetic in (99Tc); most stable isotope 98Tc (half-life medicine and as a mordant in dyeing and 4.2 × 106 years). as an insecticide. telluride A compound of tellurium and Taube, Henry (1915– ) American in- another element. See tellurium. organic chemist. Taube has developed a number of experimental techniques for tellurium A brittle silvery semiconduct- studying the kinetics and mechanism of in- ing metalloid element; the fourth member organic reactions, in particular electron- of group 16 (formerly VIA) of the periodic transfer reactions. Transition metals such table. It is found chiefly in combination as iron, copper, cobalt, and molybdenum with metals in minerals known as tel- form coordination compounds of a type lurides. Commercially it is recovered pri- first described by Alfred WERNER. In a typ- marily during the refining of copper, lead, ical coordination compound a metal ion is and gold. Tellurium is used mainly as an attached to a number of ligands, such as additive to improve the qualities of stain- water or ammonia. It was thought that the less steel and various metals. It is also used ligands would keep the ions apart and in- in semiconductors, and as a catalyst in the

222 thallium petroleum industry. Tellurium compounds compounds are also used in lasers and in are poisonous. color television sets. Symbol: Te; m.p. 449.5°C; b.p. Symbol: Tb; m.p. 1356°C; b.p. 3123°C; 989.8°C; r.d. 6.24 (20°C); p.n. 52; most r.d. 8.229 (20°C); p.n. 65; only natural iso- common isotope 128Te; r.a.m. 127.6. tope 159Tb; r.a.m. 158.92534. temperature scale A practical scale ternary compound A chemical com- against which temperature can be mea- pound formed from three elements; e.g. sured. A temperature scale is determined Na SO or LiAlH . by one or more fixed points, which are re- 2 4 4 producible and are assigned an agreed tem- tervalent (trivalent) Having a valence of perature. On the Celsius scale, for three. example, the fixed points are the temperature of pure melting ice (the ice tempera- tesla Symbol: T The SI derived unit of ture) and the temperature of pure boiling magnetic flux density, equal to a flux den- water (the steam temperature). The difference between these fixed points is subdi- sity of one weber of magnetic flux per –2 vided into temperature interval units. The square meter. 1 T = 1 Wb m . It is named International Practical Temperature Scale for the Croatian-born electrical engineer has 11 fixed points that cover the range and inventor Nicola Tesla (1857–1943). 13.81 kelvin (K) to 1337.58 K. tetracarbonyl nickel(0) See nickel car- temporary hardness A type of water bonyl. hardness caused by the presence of dissolved calcium, iron, and magnesium hy- tetraethyl lead See lead tetraethyl. drogencarbonates. This form of hardness can be removed by boiling the water. Tem- tetragonal crystal See crystal system. porary hardness arises because rainwater combines with carbon dioxide from the at- tetrahedral compound A compound mosphere to form weak carbonic acid. such as methane in which an atom has four This acid reacts with carbonate rocks, such bonds directed toward the corners of a reg- as limestone, to produce calcium hydro- ular tetrahedron. The angles between the gencarbonate, which then goes into solu- bonds in such a compound are about 109°. tion. When this solution is heated, the complete reaction sequence is reversed so tetrahydrate A crystalline hydrated that calcium carbonate is precipitated. If compound containing four molecules of this compound is not removed, it will ac- water of crystallization per molecule of cumulate as scale in boilers and hot-water compound. See also complex. pipes and reduce their efficiency. Compare permanent hardness. See also water soften- tetravalent (quadrivalent) Having a va- ing. lence of four. tera- Symbol: T A prefix used with SI thallium A soft malleable bluish-white units denoting 1012. For example, 1 ter- awatt (TW) = 1012 watts (W). metallic element belonging to group 13 (formerly IIIA) of the periodic table. It is terbium A soft ductile malleable silver- found in lead, zinc, and cadmium ores, and gray rare element of the lanthanoid series in pyrites (e.g. FeS2). Thallium and its com- of metals. It occurs in association with pounds are highly toxic and were used pre- other lanthanoids in minerals such as viously as a rodent and insect poison. gadolinite, monazite, and xenotime. It is Various compounds are now used in pho- also found in apatite. One of its few uses is tocells, infrared detectors, and low-melting as a dopant in solid-state devices. Terbium glasses.

223 thermal dissociation

Symbol: Tl; m.p. 303.5°C; b.p. 1457°C; The third law of thermodynamics states r.d. 11.85 (20°C); p.n. 81; most common that the entropy of a substance tends to isotope 205Tl; r.a.m. 204.3833. zero as its thermodynamic temperature ap- proaches zero. thermal dissociation The decomposi- Often a zeroth law of thermodynamics tion of a chemical compound into compo- is given: that if two bodies are each in ther- nent atoms or molecules by the action of mal equilibrium with a third body, then heat. Often it is temporary and reversible. they are in thermal equilibrium with each other. This is considered to be more funda- thermite (thermit) A mixture of alu- mental than the other laws because they as- minum powder and (usually) iron(III) sume it. See also Carnot cycle; entropy. oxide. When ignited the oxide is reduced; the reaction is strongly exothermic and thermodynamic temperature Symbol: molten iron is formed: T A temperature that measures changes → 2Al + Fe2O3 Al2O3 + 2Fe in the entropy of a closed system with re- Thermite is used in incendiary bombs spect to heat energy. The thermodynamic and for welding steel. See also Gold- temperature interval unit is the kelvin. See schmidt process. also absolute temperature. thermochemistry The branch of chem- thin-layer chromatography A tech- istry concerned with heats of reaction, sol- nique widely used for the analysis of mix- vation, etc. tures. Thin-layer chromatography employs a solid stationary phase, such as alumina or thermodynamic engine See heat en- silica gel, spread evenly as a thin layer on a gine. glass plate. A base line is carefully scratched near the bottom of the plate thermodynamics The study of heat and using a needle, and a small sample of the other forms of energy and the various re- mixture is spotted onto the base line using lated changes in physical quantities such as a capillary tube. The plate is then stood up- temperature, pressure, density, etc. right in solvent, which rises up to the base The first law of thermodynamics states line and beyond by capillary action. The that the total energy in a closed system is components of the spot of the sample will conserved (constant). In all processes en- dissolve in the solvent and tend to be car- ergy is simply converted from one form to ried up the plate. However, some of the another, or transferred from one system to components will cling more readily to the another. solid phase than others and will not move A mathematical statement of the first up the plate so rapidly. In this way, differ- law is: ent fractions of the mixture eventually be- ∆Q = ∆U + ∆W come separated. When the solvent has Here, ∆Q is the heat transferred to the almost reached the top, the plate is re- system, ∆U the change in internal energy moved and quickly dried. The plate is de- (resulting in a rise or fall of temperature), veloped to locate the positions of colorless and ∆W is the external work done by the fractions by spraying with a suitable chem- system. ical or by exposure to ultraviolet radiation. The second law of thermodynamics can The components are identified by compar- be stated in a number of ways, all of which ing the distance they have traveled up the are equivalent. One is that heat cannot pass plate with standard solutions that have from a cooler to a hotter body without been run simultaneously, or by computing some other process occurring. Another is an RF value. the statement that heat cannot be totally converted into mechanical work, i.e. a heat thionyl Describing a compound con- engine cannot be 100% efficient. taining the group =SO.

224 tin thiosulfate A salt containing the ion ilizations when the ores used were rela- 2– S2O3 . See sodium thiosulfate(IV). tively rich. Currently worked ores are as low as 1–2% tin and considerable concen- thixotropy The change of viscosity with tration must be carried out before roasting. movement. Fluids that undergo a decrease The metal itself is obtained by reduction in viscosity with increasing velocity are using carbon: → said to be thixotropic. Nondrip paint is an SnO2 + C Sn + CO2 example of a thixotropic fluid. Tin is an expensive metal and several processes are therefore used to recover it thoria See thorium dioxide. from scrap tin-plate. These may involve dry chlorination to give the volatile SnCl4, thorium A soft ductile gray toxic ra- or electrolytic methods using an alkaline dioactive element of the actinoid series of electrolyte: – → 2– – metals. It has several long-lived radioiso- Sn + 4OH Sn(OH)4 + 2e (anode) 2– → 2+ – topes and is found in a variety of minerals, Sn(OH)4 Sn + 4OH (cathode) including monazite and pitchblende. Tho- Sn2+ + 2e → Sn (cathode) rium is used in magnesium alloys, incan- Tin does not react directly with hydro- descent gas mantles, refractory materials, gen but an unstable hydride, SnH4, can be and nuclear fuel elements. prepared by reduction of SnCl4. The low Symbol: Th; m.p. 1750°C; b.p. 4790°C stability is due to the rather poor overlap of (approx.); r.d. 11.72 (20°C); p.n. 90; the diffuse orbitals of the tin atom with the r.a.m. 232.0381; most stable isotope 232Th small H-orbitals. Tin forms both tin(II) (half-life 1.41 × 1010 years). oxide and tin(IV) oxide. Both are amphoteric, dissolving in acids to give tin(II) and thorium dioxide (thoria; ThO2)A tin(IV) salts, and in bases to form stannites white insoluble compound, used as a re- and stannates, – → 4– fractory, in incandescent gas mantles, and SnO + 4OH [SnO3] + 2H2O as a replacement for silica in some types of stannite (relatively unstable) – → 4– optical glass. SnO2 + 4OH [SnO4] + 2H2O stannate thulium A very rare soft malleable duc- The halides, SnX2, may be prepared by tile silvery-gray element of the lanthanoid dissolving tin metal in the hydrogen halide series of metals. It occurs in association or by the action of heat on SnO plus the hy- with other lanthanoids in minerals such as drogen halide. Tin(IV) halides may be pre- gadolinite and xenotime. It is used in arc pared by direct reaction of halogen with lighting. Thulium-170, an artificially cre- the metal. Although tin(II) halides are ion- ated radioactive isotope, is used as a power ized in solution their melting points are all source in portable x-ray machines. low, suggesting considerable covalence in Symbol: Tm; m.p. 1545°C; b.p. all but the fluoride. The tin(IV) halides are 1947°C; r.d. 9.321 (20°C); p.n. 69; only volatile and essentially covalent with slight natural isotope 169Tm; r.a.m. 168.93421. polarization of the bonds. Tin(II) compounds are readily oxidized to tin(IV) com- tin A white lustrous malleable metal; the pounds and are therefore good reducing fourth member of group 14 (formerly IVA) agents for general laboratory use. of the periodic table. Tin is the first dis- Tin has three crystalline modifications tinctly metallic element of the group even or allotropes, α-tin or ‘gray tin’ (diamond though it retains some amphoteric proper- structure), stable below 13°C; β-tin or ties. Its electronic structure has outer s2p2 ‘white tin’, stable between 13°C and electrons ([Kr]4d105s25p2). The element is 160°C; and γ-tin, stable only above 160°C. of low abundance in the Earth’s crust The latter two are metallic, with close- (0.004%) but is widely distributed, largely packed structures. Tin also has several iso- as the mineral cassiterite (SnO2). The metal topes. It is used in a large number of alloys has been known since early Bronze Age civ- including Babbit metal, bell metal, Britan-

225 tincal nia metal, bronze, gun metal, tin plate, and tin(IV) sulfide (SnS2) A yellowish solid pewter as well as several special solders. that can be precipitated by reacting hydro- Symbol: Sn; m.p. 232°C; b.p. 2270°C; gen sulfide with a solution of a soluble r.d. 7.31 (20°C); p.n. 50; most common tin(IV) salt. A crystalline form sometimes isotope 118Sn; r.a.m. 118.710. known as mosaic gold is obtained by heating a mixture of tin filings, sulfur, and am- tincal A naturally occurring form of monium chloride. Tin(IV) sulfide is used as borax. See disodium tetraborate decahy- a pigment. drate. titania See titanium(IV) oxide. tin(II) chloride (stannous chloride; See titanium(IV) chlor- SnCl2) A transparent solid made by dis- titanic chloride solving tin in hydrochloric acid. Tin(II) ide. chloride is a reducing agent, it combines with ammonia, and forms hydrates with titanic oxide See titanium(IV) oxide. water. It is used as a mordant. titanium A silvery lustrous light strong transition metal, the first element of group tin(IV) chloride (stannic chloride; SnCl4) A colorless fuming liquid. Tin(IV) chloride 4 (formerly subgroup IVB) of the periodic is soluble in organic solvents but is hy- table. It occurs in various ores as tita- drolyzed by water. It dissolves sulfur, nium(IV) oxide and also in combination phosphorus, bromine, and iodine, and it with iron and oxygen. The most commercially important minerals are rutile (TiO ) dissolves in concentrated hydrochloric acid 2 2– and ilmenite (FeTi)3). The mineral titanite to give the anion SnCl6 . ((CaTiO)SlO4) is often used for gemstones. Titanium is extracted by conversion of ti- tin(IV) hydride See stannane. tanium(IV) oxide to the chloride, which is reduced to the metal by heating with tin(II) oxide (stannous oxide; SnO) A sodium. Titanium is chiefly reactive only at dark green or black solid. It can also be ob- high temperatures. Its strength, low den- tained in an unstable red form, which turns sity, and resistance to corrosion make it es- black on exposure to air. Tin(II) oxide can pecially useful for alloys used in the be made by precipitating the hydrated aviation and aerospace industries. Tita- oxide from a solution containing tin(II) nium pins, plates, and joints are also used ions and dehydrating the product at in dentistry and medicine to replace dam- ° 100 C. It is used in the manufacture of cos- aged teeth or bones. It forms compounds metics and ceramics. with oxidation states +4, +3, and +2, the +4 state being the most stable. Titanium tin(IV) oxide (stannic oxide; tin dioxide; compounds are used in pigments and cos- SnO2) A colorless crystalline solid, which metics. is usually discolored owing to the presence Symbol: Ti; m.p. 1660°C; b.p. 3287°C; of impurities. It occurs in nature as the r.d. 4.54 (20°C); p.n. 22; most common mineral cassiterite. It exists as hexagonal isotope 48Ti; r.a.m. 47.867. or rhombic crystals or in an amorphous form. Tin(IV) oxide is insoluble in water. It titanium(IV) chloride (titanic chloride; is used for polishing glass and metal and in titanium tetrachloride; TiCl4) A volatile cosmetics. colorless liquid formed by heating titanium(IV) oxide with carbon in a stream of tin(II) sulfide (SnS) A gray solid that dry chlorine at 700°C. The product is puri- can be prepared from tin and sulfur. Above fied by fractional distillation. Titanium(IV) 265°C tin(II) sulfide slowly turns into chloride fumes in moist air forming the tin(IV) sulfide and tin: oxychlorides of titanium. It undergoes hy- → 2SnS(s) SnS2(s) + Sn(s) drolysis in water but this can be prevented 226 transition state by the presence of excess hydrochloric investigate chemical reactions or physical acid. Crystallization of such a solution processes (e.g. diffusion). See isotopes. gives crystals of the di- and pentahydrates. Titanium(IV) chloride is used in the prepa- transactinide elements Those elements ration of pure titanium and as an interme- following 103Lw (i.e. following the actinide diate in the production of titanium series). To date, element 104 (ruther- compounds from minerals. fordium) through to element 112 and element 114 have been synthesized; so far, titanium dioxide See titanium(IV) oxide. elements 111, 112, and 114 are unnamed. All the transactinides are unstable and titanium(IV) oxide (titania; titanic oxide; have very short half-lives. There is specula- titanium dioxide; TiO2) A white ionic tion that islands of stable nuclei with very solid that occurs naturally in three crys- high mass numbers exist, because theoreti- talline forms: rutile (tetragonal), brookite cal calculations predict unusual stability (orthorhombic), and anatase (tetragonal). for elements 114 and 118. It reacts slowly with acids and is attacked by the halogens at high temperatures. On trans-isomer See isomerism. heating it decomposes to give titanium(III) oxide and oxygen. Titanium(IV) oxide is transition elements (group 3–12 ele- amphoteric: with hot concentrated sulfuric ments) A class of elements occurring in acid, it forms titanyl sulfate, TiOSO4; the periodic table in three series: from scan- when fused with alkalis, it forms titanates. dium to zinc; from yttrium to cadmium; It is used as a white pigment. and from lanthanum to mercury. The transition elements are all metals. They owe titanium tetrachloride See titanium(IV) their properties to the presence of d elec- chloride. trons in the atoms. In the first transition series, calcium has the configuration titrant See titration. 3s23p64s2. The next element scandium has 3s23p63d14s2, and the series is formed by titration A procedure in volumetric filling the d levels up to zinc at 3d104s2. The analysis in which a solution of known con- other two transition series occur by filling centration (called the titrant) is added to a of the 4d and 5d levels. Often the elements solution of unknown concentration from a scandium, yttrium, and lanthanum are burette until the equivalence point or end considered with the lanthanoids rather point of the titration is reached. See volu- than the transition elements, and zinc, cad- metric analysis. mium, and mercury are also treated as a separate group. Transition elements have tonne Symbol: t An m.k.s. unit of mass certain characteristic properties resulting equal to 103 kilograms in SI units (i.e. one from the existence of d levels: megagram). 1. They have variable valences – i.e. they can form compounds with the metal in topaz A hydrous aluminosilicate min- different oxidation states (Fe2+ and Fe3+, eral containing some fluoride ions, used for etc.). making refractories, glasses, and glazes. A 2. They form a vast number of inorganic clear pale yellow or brown form is used as complexes, in which coordinate bonds a gemstone. are formed to the metal atom or ion. 3. Compounds of transition elements are torr A unit of pressure equal to one often colored. mmHg. In SI units it is equal to 133.322 See also d-block elements; lanthanoids; pascals. It was named for Italian physicist metals; zinc group. Evangelista Torricelli (1609–47). transition state (activated complex) Sym- tracer An isotope of an element used to bol: ‡ A short-lived high-energy mol-

227 transition state theory ecule, radical, or ion formed during a reac- triammonium phosphate See ammo- tion between molecules possessing the nec- nium phosphate. essary activation energy. The transition state decomposes at a definite rate to yield triatomic Denoting a molecule, radical, either the reactants again or the final prod- or ion consisting of three atoms. For exam- ucts. The transition state can be considered ple, O3 and H2O are triatomic molecules. to be at the top of the potential energy profile. tribasic acid An acid with three replace- For the reaction, able hydrogen atoms (such as phos- ‡ → X + YZ = X…Y…Z XY + Z phoric(V) acid, H3PO4). See acid. the sequence of events is as follows. X ap- proaches YZ and when it is close enough triclinic crystal See crystal system. the electrons are rearranged producing a weakening of the bond between Y and Z. tridymite See silicon(IV) oxide. A partial bond is now formed between X and Y producing the transition state. De- trigonal bipyramid See complex. pending on the experimental conditions, the transition state either then breaks trigonal crystal See crystal system. down to form the products or reverts back to the reactants. trihydrate A crystalline hydrated compound that contains three molecules of transition state theory An alternative water of crystallization per molecule of name for ACTIVATED COMPLEX THEORY. compound. transition temperature A temperature triiron tetroxide (ferrosoferric oxide; at which some definite physical change oc- magnetic iron oxide; Fe3O4) A black curs in a substance. Examples of such tran- solid prepared by passing either steam or sitions are change of state, change of carbon dioxide over red-hot iron. It may crystal structure, and change of magnetic also be prepared by passing steam over behavior. heated iron(II) sulfide. Triiron tetroxide occurs in nature as the mineral magnetite. transmutation A change of one element It is insoluble in water but will dissolve in into another by radioactive decay or by acids to give a mixture of iron(II) and bombardment of the nuclei with particles. iron(III) salts in the ratio 1:2. Generally it is chemically unreactive; it is, however, a transport number Symbol: t In an elec- fairly good conductor of electricity. See trolyte, the transport number of an ion is also iron(II) oxide; iron(III) oxide. the fraction of the total charge carried by that type of ion in conduction. trimer A molecule (or compound) formed by addition of three identical mol- transuranic elements Those elements ecules. of the ACTINOID series that have higher atomic numbers than uranium, i.e. neptu- trimethylaluminum (aluminum tri- nium through lawrencium. The methyl; (CH3)3Al) A colorless liquid pro- transuranic elements are formed by adding duced by the sodium reduction of dimethyl neutrons to the lower actinoids by high-en- aluminum chloride. It ignites sponta- ergy bombardment. They generally have neously on contact with air and reacts vio- such short half-lives that macroscopic iso- lently with water, acids, halogens, lation is impossible. Neptunium, pluto- alcohols, and amines. Aluminum alkyls are nium, americium, and possibly curium are used in the Ziegler process for the manu- also formed in trace quantities by natural facture of high-density polyethene. neutron bombardment from spontaneous fission of 235U. trimolecular Describing a reaction or

228 turquoise step that involves three molecules interact- tritium Symbol: T, 3H A radioactive iso- ing simultaneously with the formation of a tope of hydrogen of mass number 3. The product. For example, the final step in re- nucleus contains 1 proton and 2 neutrons. action between hydrogen peroxide and Tritium decays with emission of low-en- acidified potassium iodide is trimolecular: ergy beta radiation to give 3He. The half- + – → HOI + H + I I2 + H2O life is 12.3 years. It is useful as a tracer in It is uncommon for reactions to take studies of chemical reactions. Compounds place involving trimolecular steps. The ox- in which 3H atoms replace the usual 1H idation of nitrogen(II) oxide to atoms are said to be tritiated. A positive tri- nitrogen(IV) oxide, + → tium ion, T , is a triton. 2NO + O2 2NO2 is often classified as a trimolecular reaction triton See tritium. but many chemists believe it to involve two bimolecular reactions. trivalent (tervalent) Having a valence of three. trioxoborix(III) acid See boric acid. trioxosulfuric(IV) acid See sulfurous tungsten A hard gray malleable transi- acid. tion metal, the third element of group 6 (formerly VIB) of the periodic table. It oc- trioxygen See ozone. curs naturally in the minerals wolframite ((Fe,Mn)WO4) and scheelite (CaWO4). It triple bond A covalent bond formed be- was formerly called wolfram. It is used as tween two atoms in which three pairs of the filaments in electric lamps and in vari- electrons contribute to the bond. One pair ous steel alloys. Tungsten has the highest forms a sigma bond (equivalent to a single melting point of all the metals. Tungsten bond) and two pairs give rise to two pi salts are used in paints and in tanning. bonds. It is conventionally represented as Symbol: W; m.p. 3410 ± 20°C; b.p. three lines, thus H–C≡C–H. See multiple 5650°C; r.d. 19.3 (20°C); p.n. 74; most bond. common isotope 184W; r.a.m. 183.84. triple point The only point at which the tungsten carbide Either of two carbides gas, solid, and liquid phases of a substance (W2C and WC) produced by heating pow- can coexist in equilibrium. The triple point dered tungsten with carbon. The carbides of water (273.16 K at 101 325 Pa) is used are extremely hard and are used in industry to define the kelvin. to make cutting tools or as an abrasive. WC has a very high melting point trisodium phosphate(V) (sodium or- (2770°C) and will conduct electricity. W C thophosphate; Na PO ) A white solid 2 3 4 also has a very high melting point prepared by adding sodium hydroxide to ° disodium hydrogenphosphate. On evapo- (2780 C) but is a less efficient conductor of ration white hexagonal crystals of the do- electricity. It is very resistant to chemical attack and behaves in a manner very simi- decahydrate (Na3PO4.12H2O) may be obtained. These crystals do not effloresce lar to that of tungsten. It is strongly at- or deliquesce. They dissolve readily in tacked by chlorine to give tungsten water to produce alkaline solutions owing hexachloride, WCl6. to salt hydrolysis. Trisodium phosphate is used as a water softener. turquoise A naturally occurring hydrated copper aluminum phosphate, used tritiated See tritium. as a green-blue gemstone.

229 U

ultracentrifuge A high-speed centrifuge unit A reference value of a quantity, used for separating out very small parti- used to express other values of the same cles. Because the sedimentation rate de- quantity. See also SI units. pends on the particle size, the ultracentrifuge can be used to measure the unit cell The smallest group of atoms, molecular mass of colloidal particles and ions, or molecules that, when repeated at large molecules (e.g. proteins). regular intervals in three dimensions, will produce the lattice of a crystal system. ultrahigh vacuum See vacuum. unit processes (chemical conversions) ultraviolet (UV) A form of electromag- The recognized steps used in chemical netic radiation, shorter in wavelength than processes, e.g. alkylation, distillation, hy- visible light. Ultraviolet wavelengths range drogenation, pyrolysis, nitration, etc. between about 1 nm and 400 nm. Ordi- nary glasses are not transparent to these univalent (monovalent) Having a va- waves; quartz is a much more effective ma- lence of one. terial for making lenses and prisms for use with ultraviolet. Like light, ultraviolet radi- universal gas constant See gas con- ation is produced by electronic transitions stant. between the outer energy levels of atoms. However, having a higher frequency, ultra- universal indicator (multiple-range indi- violet photons carry more energy than cator) A mixture of indicator dyestuffs those of light and can induce photolysis of that shows a gradual change in color over compounds. See also electromagnetic radi- a wide pH range. A typical formulation ation. contains methyl orange, methyl red, bro- mothymol blue, and phenolphthalein and unimolecular Describing a reaction (or changes through a red, orange, yellow, step) in which only one molecule is in- green, blue, and violet sequence between volved. For example, radioactive decay is a pH 3 and pH 10. Several commercial unimolecular reaction: preparations are available as both solu- Ra → Rn + α tions and test papers. Only one atom is involved in each disintegration. unsaturated solution See saturated so- In a unimolecular chemical reaction, lution. the molecule acquires the necessary energy to become activated and then decomposes. unsaturated vapor See saturated vapor. The majority of reactions involve only uni- or BIMOLECULAR steps. The following reac- unun- A prefix used in the names of new tions are all unimolecular: chemical elements. Synthetic elements with → N2O4 2NO2 high proton numbers were given tempo- → PCl5 PCl3 + Cl2 rary names based on the proton number, → CH3CH2Cl C2H4 + HCl pending official agreement on the actual 230 uranyl

° Element Number Symbol Symbol: U; m.p. 1132.5 C; b.p. ° ° nil 0 n 3745 C; r.d. 18.95 (20 C); p.n. 92; most 238 un 1 u stable isotope U ; r.a.m. 238.0289. bi 2 b tri 3 t uranium hexafluoride (UF6) A crys- quad 4 q talline volatile compound, used in separat- pent 5 p ing uranium isotopes by differences in the hex 6 h rates of gas diffusion. sept 7 s oct 8 0 uranium–lead dating A technique for enn 9 e dating certain rocks depending on the decay of the radioisotope 238U to 206Pb (half-life 4.5 × 109 years) or of 235U to 207Pb (half-life 7.1 × 108 years). The decay name to be used. These names were formed of 238U releases alpha particles and an esti- from the word elements in the table above. mate of the age of the rock can be made by The suffix -ium was also used (because the measuring the amount of helium present. elements were metals). So element 104, for Another method is to measure the ratio of example, had the temporary systematic radioactive lead present to the amount of name unnilquadium (un + nil + quad + nonradioactive lead. See also radioactive ium). The names for elements using this system are shown in the table below. dating. uraninite See pitchblende. uranium(IV) oxide (uranium dioxide; urania; UO2) An extremely poisonous uranium A toxic radioactive silvery ele- radioactive black crystalline solid. It occurs ment of the actinoid series of metals. Its in pitchblende (uraninite) and is used in ce- three naturally occurring radioisotopes, ramics, photographic chemicals, pigments, 238U (99.283% in abundance), 235U and as a nuclear fuel. (0.711%), and 234U (0.005%), are found in numerous minerals including the ura- uranium(VI) oxide (uranium trioxide; nium oxides pitchblende, uraninite, and UO3) An extremely poisonous radioac- carnotite. The readily fissionable 235U is a tive orange solid, used as a pigment in ce- major nuclear fuel and nuclear explosive, ramics and in uranium refining. while 238U is a source of fissionable 239Pu. 2+ Spent uranium fuel rods have been used to uranyl The radical UO2 , as in uranyl make armor-piercing shells. nitrate UO2(NO3)2.

P.N. Temporary name Symbol Name Symbol 101 unnilunium Unu mendelevium Md 102 unnilbiium Unb nobelium No 103 unniltriium Unt lawrencium Lr 104 unnilquadium Unq rutherfordium Rf 105 unnilpentium Unq dubnium Db 106 unnilhexium Unh seaborgium Sg 107 unnilseptium Uns bohrium Bh 108 unniloctium Uno hassium Hs 109 unnilennium Une meitnerium Mt 110 ununnilium Uun darmstadtium Ds 111 unununium Uuu unnamed 112 ununbiium Uut unnamed

231 V

vacancy See defect. uct is 6 and for the three oxygen atoms (valence 2) the product is also 6. vacuum A space containing gas below The valence of an element is generally atmospheric pressure. A perfect vacuum equal to either the number of valence elec- contains no matter at all, but for practical trons or eight minus the number of valence purposes soft (low) vacuum is usually de- electrons. Transition metal ions display fined as down to about 10–2 pascal, and variable valence. hard (high) vacuum as below this. Ultra- high vacuum is lower than 10–7 pascal. valence band The highest energy level in a semiconductor or nonconductor that vacuum distillation The distillation of can be occupied by electrons. See energy liquids under a reduced pressure, so that level; semiconductor. the boiling point is lowered. Vacuum distillation is a common laboratory technique valence-bond theory A technique for for purifying or separating compounds calculating the electronic structure of mol- that would decompose at their ‘normal’ ecules. In valence-bond theory the starting boiling point. point is to consider the atoms in a molecule and the ways in which valence electrons in valence (valency) The combining power the atoms can pair up in chemical bonds. of an element or radical, equal to the num- Each way of pairing up the electrons is called a valence bond structure. A WAVE- ber of hydrogen atoms that will combine FUNCTION is written for each valence bond with or displace one atom of the element. structure. The total wavefunction Ψ for the For simple covalent molecules the valence molecule is a linear combination of the is obtained directly, for example C in CH 4 wavefunctions for the valence-bond struc- is tetravalent; N in NH is trivalent. For 3 tures. The wavefunction incorporates RES- ions the valence is regarded as equivalent ONANCE between the various valence bond to the magnitude of the charge; for exam- 2+ 2– structures. ple Ca is divalent, CO3 is a divalent radical. The rare gases are zero-valent be- valence electron An outer electron in cause they do not form compounds under an atom that can participate in forming normal conditions. Because the valence for chemical bonds. many elements is constant, the valence of some elements can be deduced without ref- valency See valence. erence to compounds formed with hydrogen. Thus, because the valence of chlorine vanadium A strong silvery toxic transi- in HCl is 1, the valence of aluminum in tion metal, the first element of group 5 AlCl3 is 3; because oxygen is divalent (formerly VB) of the periodic table. It oc- (H2O), silicon in SiO2 is tetravalent. The curs in complex ores, especially those of product of the valence and the number of iron, lead, and uranium, in small quanti- atoms of each element in a compound must ties. It is used in alloy steels to give them be equal. For example, in Al2O3 for the added strength and heat resistance. Vana- two aluminum atoms (valence 3) the prod- dium forms compounds with oxidation

232 vapor density states +5, +4, +3, and +2. It forms colored those of chemical bonds and typically have ions. energies of less than one joule per mole. Symbol: V; m.p. 1890°C; b.p. 3380°C; The van der Waals force, named for r.d. 6.1 (20°C); p.n. 23; most common iso- Dutch physicist Johannes van der Waals tope 51V; r.a.m. 50.94. (1837–1923), arises because of three effects: permanent dipole–dipole interactions vanadium(V) oxide (vanadium pentox- found for any polar molecule; dipole–in- ide; V2O5) An oxide of vanadium exten- duced dipole interactions, where one di- sively used as a catalyst in oxidation pole causes a slight charge separation in processes, as in the contact process. bonds that have a high polarizability; and dispersion forces, which result from tem- van der Waals, Johannes Diderik porary polarity arising from an asymmetri- (1837–1923) Dutch physicist. In his doc- cal distribution of electrons around the toral thesis of 1873 entitled On the Conti- nucleus. Even atoms of the rare gases ex- nuity of the Liquid and Gaseous States van hibit dispersion forces. der Waals studied the kinetic theory of gases and liquids. This work provided the van’t Hoff factor Symbol: i The ratio foundation for most of his subsequent of the number of particles present in a so- work. He derived an equation of state for lution to the number of undissociated mol- gases which is more realistic than the ideal ecules added. It is used in studies of gas laws discovered by Robert BOYLE and colligative properties, which depend on the others because it takes into account both number of entities present. For example, if the non-zero sizes of the molecules and the n moles of a compound are dissolved and existence of attractive intermolecular dissociation into ions occurs, then the forces. The equation of state he found is number of particles present will be in. Os- called the van der Waals equation and the motic pressure (π), for instance, will be intermolecular forces are called van der given by the equation Waals forces. This work resulted in van der πV = inRT Waals winning the 1910 Nobel Prize for It is named for the Dutch chemist Jacobus physics. van’t Hoff (1852–1911). van der Waals equation An equation van’t Hoff isochore The equation: ∆ 2 of state for real gases. For n moles of gas d(logeK)/dT = H/RT the equation is showing how the equilibrium constant, K, (p + n2a/V2)(V – nb) = nRT of a reaction varies with thermodynamic where p is the pressure, V the volume, and temperature, T. ∆H is the enthalpy of reac- T the thermodynamic temperature. a and b tion and R is the gas constant. are constants for a given substance and R is the gas constant. The equation gives a bet- vapor A gas formed by the vaporization ter description of the behavior of real gases of a solid or liquid. Some particles near the than the perfect gas equation (pV = nRT). surface of a liquid acquire sufficient energy The equation contains two corrections: in collisions with other particles to escape b is a correction for the non-negligible size from the liquid and enter the vapor; some of the molecules; a/V2 corrects for the fact particles in the vapor lose energy in colli- that there are attractive forces between the sions and re-enter the liquid. At a given molecules, thus slightly reducing the pres- temperature an equilibrium is established, sure from that of an ideal gas. See also gas which determines the vapor pressure of the laws; kinetic theory. liquid at that temperature. van der Waals force An intermolecular vapor density The ratio of the mass of a force of attraction arising from weak elec- certain volume of a vapor to the mass of an trostatic interactions between molecules. equal volume of hydrogen (measured at the Such forces are considerably weaker than same temperature and pressure). Determi-

233 vaporization nation of vapor densities is one method of count of molecular vibrations requires that finding the relative molecular mass of a anharmonicity is taken into account. A di- compound (equal to twice the vapor den- atomic molecule can have a vibrational- sity). Victor Meyer’s method, Dumas’ rotational spectrum only if it has a perma- method, or Hofmann’s method can be nent dipole moment. A polyatomic mol- used. ecule can have a vibrational-rotational spectrum only if the normal modes of vi- vaporization The process by which a bration cause the molecule to have an os- liquid or solid is converted into a gas or cillating dipole moment. The analysis of vapor by heat. Unlike boiling, which oc- vibrational–rotational spectra is greatly fa- curs at a fixed temperature, vaporization cilitated by the use of group theory. This can occur at any temperature. Its rate in- type of spectroscopy provides information creases as the temperature rises. about interatomic distances and the force constants of chemical bonds. vapor pressure The pressure exerted by a vapor. The saturated vapor pressure is vicinal positions Positions in a mol- the pressure of a vapor in equilibrium with ecule at adjacent atoms. For example, in its liquid or solid. It depends on the nature 1,2-dichloroethane the chlorine atoms are of the liquid or solid and the temperature. in vicinal positions, and this compound can thus be named vic-dichloroethane. verdigris Any of various greenish basic salts of copper. True verdigris is basic cop- viscosity Symbol: η The resistance to per acetate, a green or blue solid of variable flow of a fluid. composition used as a paint pigment. The term is often applied to green patinas vitreous Resembling glass, or having the formed on metallic copper. Copper cook- structure of a glass. ing vessels can form a coating of basic copper carbonate, CuCO3.Cu(OH)2. The volatile Easily converted into a vapor. verdigris formed on copper roofs and domes is usually basic copper sulfate, volt Symbol: V The SI derived unit of CuSO4.3Cu(OH)2.H2O, while basic cop- electrical potential, potential difference, per chloride, CuCl2.Cu(OH)2, may form in and e.m.f., defined as the potential differ- regions near the sea. ence between two points in a circuit between which a constant current of one vermilion See mercury(II) sulfide. ampere flows when the power dissipated is one watt. 1 V = 1 W A–1. It is named for the vesicant A substance that causes blister- Italian physicist Alessandro Volta (1745– ing of the skin. Mustard gas is an example. 1827). vibrational spectroscopy The spectro- voltaic cell See cell. scopic investigation of the vibrational energy levels of molecules. In the infrared voltameter See coulombmeter. region of the electromagnetic spectrum vibrational transitions are accompanied by volume strength See hydrogen perox- rotational transitions. Infrared spectra of ide. molecules are series of bands, with each band being associated with a vibrational volumetric analysis One of the classi- transition and every line in that band being cal wet methods of quantitative analysis. It associated with a rotational transition that involves measuring the volume of a solu- accompanies the vibrational transition. tion of accurately known concentration Some features of vibrational spectra can that is required to react with a solution of be analyzed by regarding the vibrations as the substance being determined. The solu- simple harmonic motion but a realistic action of known concentration (the standard

234 VSEPR solution) is added in small portions from a pulsion) A method of predicting the shape burette. The process is called a titration of molecules. In this theory one atom is and the observed point of complete reac- taken to be the central atom and pairs of tion is called the end point. End points are valence electrons are drawn round the cen- observed with the aid of indicators or by tral atom. The shape of the molecule is de- instrumental methods, such as via conduc- termined by minimizing the Coulomb tion or light absorption. Volumetric analy- repulsion between pairs and the repulsion sis can also be applied to gases. The gas is between pairs due to the Pauli exclusion typically held over mercury in a graduated principle. Thus, for three pairs of electrons tube, and volume changes are measured on an equilateral triangle is favored and for reaction or after absorption of components four pairs of electrons a tetrahedron is fa- of a mixture. vored. The VSEPR theory has had considerable success in predicting molecular VSEPR (valence-shell electron-pair re- shapes. See lone pair.

235 W

washing soda See sodium carbonate. tion of water molecules into some ionic crystals as water of crystallization. water (H2O) A colorless liquid that Water is decomposed by reactive metals freezes at 0°C and, at atmospheric pres- (e.g. sodium) when cold and by less active sure, boils at 100°C. In the gaseous state metals (e.g. iron) when steam is passed water consists of single H2O molecules. over the hot metal. It is also decomposed Due to the presence of two lone pairs the by electrolysis. atoms do not lie in a straight line, the angle between the central oxygen atom and the water gas A mixture of carbon monox- two hydrogen atoms being 105°; the dis- ide and hydrogen produced when steam is tance between each hydrogen atom and the passed over red-hot coke or made to com- oxygen atom is 0.099 nm. When ice forms, bine with hydrocarbons, e.g. → hydrogen bonds some 0.177 nm long de- C(s) + H2O(g) CO(g) + H2(g) → velop between the hydrogen atom and oxy- CH4(g) + H2O(g) CO(g) + 3H2(g) gen atoms in adjacent molecules, giving ice The production of water gas using its tetrahedral crystalline structure with a methane is an important step in the prepa- density of 916.8 kg m–3 at STP. Different ration of hydrogen for ammonia synthesis. ice structures develop under higher pres- Compare producer gas. sures. When ice melts to form liquid water, the tetrahedral structure breaks down, but water glass See sodium silicate. some hydrogen bonds continue to exist; liquid water consists of groups of associ- water of crystallization Water present ated water molecules, (H2O)n, mixed with in definite proportions in crystalline com- some monomers and some dimers. This pounds. Compounds containing water of mixture of molecular species has a higher crystallization are called HYDRATES. Exam- density than the open-structured crystals. ples are copper(II) sulfate pentahydrate The maximum density of water, 999.97 kg (CuSO4.5H2O) and sodium carbonate dec- –3 ° m , occurs at 3.98 C. This accounts for ahydrate (Na2CO3.10H2O). The water can the ability of ice to float on water and for be removed by heating. the fact that water pipes burst as ice ex- When hydrated crystals are heated the pands on freezing. water molecules may be lost in stages. For Although water is predominantly a co- example, copper(II) sulfate pentahydrate valent compound, a very small amount of changes to the monohydrate (CuSO4.H2O) ˆ + ° ionic dissociation occurs (H2O H + at 100 C, and to the anhydrous salt – ° OH ). In every liter of water at STP there is (CuSO4) at 250 C. The water molecules in approximately 10–7 mole of each ionic crystalline hydrates may be held by hydro- species. It is for this reason that, on the pH gen bonds (as in CuSO4.H2O) or, alterna- scale, a neutral solution has a value of 7. tively, may be coordinated to the metal ion As a polar liquid, water is the most as a complex aquo ion. powerful solvent known. This is partly a result of its high dielectric constant and water softening The removal from partly its ability to hydrate ions. This latter water of dissolved calcium, magnesium, property also accounts for the incorpora- and iron compounds, thus reducing the

236 white arsenic

HARDNESS of the water. The compounds are weak acid An acid that is not fully dis- potentially damaging because they can ac- sociated in solution. cumulate in pipes and boilers. They also react with, and therefore waste, soap. weak base A base that is only partly or Temporary hardness can be removed by incompletely dissociated into its compo- boiling the water. Permanent hardness can nent ions in solution. be removed in a number of ways: by distillation; by the addition of sodium carbon- weber Symbol: Wb The SI unit of mag- ate (which causes dissolved calcium, for netic flux, equal to the magnetic flux that, example, to precipitate out as calcium car- linking a circuit of one turn, produces an bonate); and by the use of ion-exchange e.m.f. of one volt when reduced to zero at products such as Permutit (utilizes zeolites) uniform rate in one second. 1 Wb = 1 V s. and Calgon (utilizes polyphosphates). It is named for German physicist Wilhelm Weber (1804–91). watt Symbol: W The SI unit of power, defined as a power of one joule per second. Werner, Alfred (1866–1919) French- 1 W = 1 J s–1. born Swiss chemist. Werner’s main contribution to chemistry consists of his wavefunction Symbol ψ. A function elucidation of structure and valence in in- that specifies the state of a system in the organic molecules. In a series of papers Werner put forward a theory of what he WAVE MECHANICS formulation of quantum called ‘coordination compounds’. He dis- mechanics. Thus, a wavefunction appears tinguished between the primary and sec- in the SCHRÖDINGER EQUATION, with each ondary valence of a metal, with the wavefunction which is a solution to the secondary valence being associated with Schrödinger equation for a given quantum how many ligands can surround a metal mechanical system being an EIGENFUNC- atom. Werner put forward his ideas in TION of the equation. The physical inter- ψ New Ideas on Inorganic Chemistry (1911). pretation of , known as the Born He won the 1913 Nobel Prize for chem- interpretation since it was put forward by istry for his work on coordination com- Max Born in 1926, is that the square of the pounds. wavefunction is a measure of the probability of finding a particle in a small volume Weston cadmium cell (cadmium element at a given point. cell) A standard cell that produces a constant e.m.f. of 1.0186 volts at 20°C. It con- waveguide See microwaves. sists of an H-shaped glass vessel containing a negative cadmium-mercury amalgam λ wavelength Symbol: The distance be- electrode in one leg and a positive mercury tween the ends of one complete cycle of a electrode in the other. The electrolyte, a wave. Wavelength is related to the speed saturated cadmium sulfate solution, fills (c) and frequency (v) thus: the horizontal bar of the vessel to connect c = vλ the two electrodes. The e.m.f. of the cell varies very little with temperature, being wave mechanics See quantum theory. given by the equation E = 1.0186 – 0.000 037 (T – 293), where T is the thermody- wave number Symbol: σ The reciprocal namic temperature. of the wavelength of a wave. It is the number of wave cycles in unit distance, and is wet cell A type of cell, such as a car bat- often used in spectroscopy. The unit is the tery, in which the electrolyte is a liquid so- meter–1 (m–1). The circular wave number lution. (symbol: k) is given by: k = 2πσ white arsenic See arsenic(III) oxide.

237 white cast iron white cast iron See cast iron. low-pressure processes for making aldehy- des from ethene and propene. white gold See palladium. witherite See barium carbonate. white lead See lead(II) carbonate hydroxide. wolfram A former name for tungsten. white phosphorus See phosphorus. Wood’s metal A fusible alloy containing 50% bismuth, 25% lead, and 12.5% white spirit A liquid hydrocarbon re- tin and cadmium. Its low melting point sembling kerosene obtained from petro- (70°C) leads to its use in fire-protection de- leum, used as a solvent and in the vices. manufacture of paints and varnishes. Woodward–Hoffmann rules A set of Wilkinson, Sir Geoffrey (1921–96) rules concerning the course of chemical re- British inorganic chemist. Wilkinson is actions proposed by the American chemists noted for his studies of inorganic com- Robert Burns Woodward and Roald Hoff- mann in the late 1960s. These rules can be plexes. He shared the Nobel Prize for stated in terms of molecular orbital theory chemistry in 1973 with Ernst FISCHER for (particularly FRONTIER ORBITAL THEORY). work on sandwich compounds. A theme of This gives rise to the Woodward– Wilkinson’s work in the 1960s was the Hoffmann rules being referred to as the study and use of complexes containing a conservation of orbital symmetry. The metal–hydrogen bond. Thus complexes of rules were initially developed for organic rhodium with triphenyl phosphine reactions but can also be applied to inor- ((C6H5)3P) can react with molecular hy- ganic reactions. drogen. The compound RhCl(P(C6H5)3), known as Wilkinson’s catalyst, was the work function See photoelectric effect. first such complex to be used as a homogeneous catalyst for adding hydrogen to the wrought iron A low-carbon steel ob- double bonds of alkenes (hydrogenation). tained by refining the iron produced in a This type of compound can also be used as blast furnace. Wrought iron is processed a catalyst for the reaction of hydrogen and by heating and hammering to reduce the carbon monoxide with alkenes (hydro- slag content and to ensure its even distrib- formylation). It is the basis of industrial ution.

238 XYZ

xenon A colorless odorless monatomic called bremsstrahlung (German for brak- gas, the fifth member of the rare-gases; i.e. ing radiation). group 18 (formerly VIIIA or 0) of the periodic table. It occurs in trace amounts in air x-ray crystallography The study of the from which it is recovered by fractional internal structure of crystals using the tech- distillation. Xenon is used in electron nique of x-ray diffraction. tubes, strobe lighting, arc lamps, and lasers. Xenon compounds with fluorine x-ray diffraction A technique used to and oxygen are known. determine crystal structure by directing x- Symbol: Xe; m.p. –111.9°C; b.p. rays at the crystals and examining the dif- –107.1°C; mass density 5.8971 (0°C) fraction patterns produced. At certain kg m–3; p.n. 54; most common isotope angles of incidence a series of spots are pro- 132Xe; r.a.m. 131.29. duced on a photographic plate; these spots are caused by interaction between the x- x-radiation An energetic form of elec- rays and the planes of the atoms, ions, or tromagnetic radiation. The wavelength molecules in the crystal lattice. The posi- range is 10–11 m to 10–8 meter. X-rays are tions of the spots are consistent with the normally produced when high-energy elec- Bragg equation nλ = 2dsinθ. trons are absorbed in matter. The radiation can pass through matter to some extent, x-rays See x-radiation. hence its use in medicine and industry for investigating internal structures. It can be yellow phosphorus See phosphorus. detected with photographic emulsions and devices like the Geiger-Müller tube. yocto- Symbol: y A prefix used with SI X-ray photons result from electronic units, denoting 10–24. For example, 1 yoc- transitions between the inner energy levels tometer (ym) = 10–24 meter (m). of atoms. When high-energy electrons are absorbed by matter, an x-ray line spectrum yotta- Symbol: Y A prefix used with SI results. The structure depends on the sub- units, denoting 1024. For example, 1 yot- stance and is thus used in x-ray spec- tameter (Ym) = 1024 meter (m). troscopy. The line spectrum is always formed in conjunction with a continuous ytterbium A soft malleable ductile sil- background spectrum. The minimum (cut- very element belonging to the lanthanoid λ off) wavelength 0 corresponds to the max- series of metals. It occurs in association imum x-ray energy, Wmax. This equals the with other lanthanoids in minerals such as maximum energy of electrons in the beam gadolinite, monazite, and xenotime. Ytter- producing the x-rays. Wavelengths in the bium has been used to improve the me- λ continuous spectrum above 0 are caused chanical properties of steel. It is also used when electrons decelerate and lose energy in ceramics. rapidly, such as when they collide with a Symbol: Yb; m.p. 824°C; b.p. 1193°C; nuclear target or pass through an electric r.d. 6.965 (20°C); p.n. 70; most common field. X-radiation emitted in this process is isotope 174Yb; r.a.m. 173.04.

239 yttrium yttrium A silvery metallic element, the dinate to a pi-electron system rather than second element of group 3 (formerly IIIB) to individual atoms. of the periodic table. It is found in almost every lanthanoid mineral, particularly zeolite A member of a group of hydrated monazite. Yttrium is used in various alloys, aluminosilicate minerals, which occur in in yttrium–aluminum garnets used in the nature and are also manufactured for their electronics industry and as gemstones, as a ion-exchange and selective-absorption catalyst, and in superconductors. A mix- properties. They are used for water soften- ture of yttrium and europium oxides is ing and for sugar refining. The zeolites widely used as the red phosphor on televi- have an open crystal structure and can be sion screens. used as molecular sieves. Symbol: Y; m.p. 1522°C; b.p. 3338°C; See also ion exchange; molecular sieve. r.d. 4.469 (20°C); p.n. 39; r.a.m. 88.90585. zepto- Symbol: z A prefix used with SI units, denoting 10–21. For example, 1 zep- Zeeman effect The splitting of atomic tometer (zm) = 10–21 meter (m). spectral lines by a magnetic field. This effect was found by the Dutch physicist zero order Describing a chemical reac- Pieter Zeeman (1865–1943) in 1896. Some tion in which the rate of reaction is inde- of the patterns of line splitting that be ex- pendent of the concentration of a reactant; plained both by classical electron theory i.e. and the BOHR THEORY of electrons in rate = k[X]0 atoms. The Zeeman splitting that can be The concentration of the reactant re- explained in these ways is known as the mains constant for a period of time al- normal Zeeman effect. There exist more though other reactants are being complicated Zeeman splitting patterns that consumed. The hydrolysis of 2-bromo-2- cannot be explained either by classical elec- methylpropane using aqueous alkali has a tron theory or the Bohr theory. This more rate expression, complicated type of Zeeman effect is rate = k[2-bromo-2-methylpropane] known as the anomalous Zeeman effect. It i.e. the reaction is zero order with re- was subsequently realized that the anomalous Zeeman effect occurs because of elec- spect to the concentration of the alkali. The tron spin and that the normal Zeeman rate constant for a zero reaction has the –3 –1 effect occurs only for transitions between units mol dm s . singlet states. At very high magnetic fields a splitting zero point energy The energy pos- pattern known as the Paschen–Back effect sessed by the atoms and molecules of a sub- occurs. In this pattern, named for the Ger- stance at absolute zero (0 K). man spectroscopists Friedrich Paschen and Ernst Back in 1912, the basic pattern re- zetta- Symbol: Z A prefix used with SI 21 turns to that of the normal Zeeman effect, units, denoting 10 . For example, 1 21 but with each line split up into a set of zettameter (Zm) = 10 meter (m). closely spaced lines. This occurs because the total orbital and spin angular momen- Zewail, Ahmed H. (1946– ) Egyptian- tum vectors of the atom, denoted L and S born American chemist. Zewail has respectively, precess independently about pioneered the technique called femto- the direction of the magnetic field. chemistry for studying chemical reactions. Zewail and his colleagues have studied the Zeise’s salt A complex of platinum and detailed dynamics of many chemical reac- ethane (ethylene) first synthesized by W. C. tions using this technique. This has justi- Zeise in 1827. It has the formula fied the picture of chemical reactions given PtCl3(CH2CH2) and was the first complex by Svante ARRHENIUS, Henry EYRING, and in which the metal ion was known to coor- others as well as giving many surprising

240 zinc group discoveries. Zewail won the 1999 Nobel solves easily in water. It is used in dentistry, Prize for chemistry. as a flux, and as a dehydrating agent in organic reactions. zinc A bluish-white hard brittle reactive transition metal, the first element of group zinc group A group of metallic elements 12 (formerly IIB) of the periodic table. It in the periodic table, consisting of zinc occurs naturally chiefly as the sulfide (zinc (Zn), cadmium (Cd), and mercury (Hg). blende) and carbonate (smithsonite). It is The elements all occur at the ends of the extracted by roasting the ore in air and three transition series and all have outer then reducing the oxide to the metal using d10s2 configurations; Zn [Ar]3d104s2, Cd carbon. Zinc is used to galvanize iron, in [Kr]4d105s2, Hg[Xn]5d106s2. The elements alloys (e.g. brass), and in dry batteries. It all use only outer s-electrons in reaction reacts with acids and alkalis but only cor- and in combination with other elements rodes on the surface in air. Zinc ions are unlike the coinage metals, which immedi- identified in solution by forming white pre- ately precede them. Thus all form diposi- cipitates with sodium hydroxide or ammo- tive ions, and oxidation states higher than nia solution, both precipitates being +2 are not known. In addition mercury soluble in excess reagent. Zinc compounds forms the mercurous ion, sometimes writ- are used in paints, cosmetics, and medi- ten as Hg(I), but actually the ion +Hg-Hg+. cines. See also zinc group. Although the elements fall naturally at the Symbol: Zn; m.p. 419.58°C; b.p. end of the transition series their properties 907°C; r.d. 7.133 (20°C); p.n. 30; most are more like those of main-group elements common isotope 64 Zn; r.a.m. 65.39. than of transition metals: 1. The melting points of the transition zincate A salt containing the ion elements are generally high whereas those 2– ° ZnO2 . of the zinc group are low (Zn 419.5 C, Cd 329.9°C, Hg –38.87°C). zinc-blende (sphalerite) structure A 2. The zinc-group ions are diamagnetic form of crystal structure that consists of a and colorless whereas most transition ele- zinc atom surrounded by four sulfur atoms ments have colored ions and many para- arranged tetrahedrally; each sulfur atom is magnetic species. similarly surrounded by four atoms of zinc. 3. The zinc group does not display the Zinc sulfide crystallizes in the cubic sys- variable valence associated with transition tem. Covalent bonds of equal strength and metal ions. However the group does have length result in the formation of a giant the transition-like property of forming molecular structure. If the zinc and sulfur many complexes or coordination com- 2+ atoms are replaced by carbon atoms the di- pounds, such as [Zn(NH3)4] and 2– amond structure is produced. Wurtzite [Hg(CN)4] . (another form of zinc sulfide) is similar but Within the group, mercury is anom- 2+ belongs to the hexagonal crystal system. alous because of the Hg2 ion mentioned previously and its general resistance to re- zinc carbonate See calamine. action. Zinc and cadmium are electropositive and will react with dilute acids to zinc chloride (ZnCl2) A white crys- release hydrogen whereas mercury will talline solid prepared by the action of dry not. When zinc and cadmium are heated in hydrogen chloride or chlorine on heated air they burn to give the oxides, MO, zinc. The anhydrous product undergoes which are stable to further strong heating. sublimation. The hydrated salt may be pre- In contrast mercury does not react readily pared by the addition of excess zinc to di- with oxygen (slow reaction at the boiling lute hydrochloric acid and crystallization point), and mercuric oxide, HgO, decom- of the solution. Hydrates with 4, 3, 5/2, poses to the metal and oxygen on further 3/2, and 1 molecule of water exist. Zinc heating. All members of the group form dichloride is extremely deliquescent and dis- alkyls and diaryls, R2M. 241 zincite zincite (spartalite) A red-orange mineral as a white amorphous precipitate by bub- form of zinc oxide, ZnO, often containing bling hydrogen sulfide through an alkaline also some manganese. It is an important solution of a zinc salt or by the addition of ore of zinc. See zinc oxide. ammonium sulfide to a soluble zinc salt solution. Zinc sulfide occurs naturally as the zinc oxide (ZnO) A compound pre- mineral zinc blende. It dissolves in dilute pared by the thermal decomposition of acids to yield hydrogen sulfide. Impure zinc nitrate or carbonate; it is a white pow- zinc sulfide is phosphorescent. It is used as der when cold, yellow when hot. Zinc a pigment and in the coatings on lumines- oxide is an amphoteric oxide, almost insol- cent screens. uble in water, but dissolves readily in both acids and alkalis. If mixed with powdered zircon See zirconium. carbon and heated to red heat, it is reduced to the metal. Zinc oxide is used in the paint zirconium A hard lustrous silvery tran- and ceramic industries. Medically it is used sition element the second element of group in zinc ointment. 4 (formerly IVB) of the periodic table. It occurs chiefly in the mineral zircon zinc sulfate (ZnSO4) A white crystalline (ZrSiO4) some deposits of which are of solid prepared by the action of dilute sulfu- gemstone quality. It is extracted by chlori- ric acid on either zinc oxide or zinc car- nation, followed by reduction with mag- bonate. On crystallization the hydrated nesium. It is used in some strong alloy salt is formed. The heptahydrate steels and as a protective coating. It is very ° (ZnSO4.7H2O) is formed below 30 C, the corrosion-resistant. ° ° ° hexahydrate (ZnSO4.6H2O) above 30 C, Symbol: Zr; m.p. 1850 C; b.p. 4380 C; ° and the monohydrate (ZnSO4.H2O) at r.d. 6.506 (20 C); p.n. 40; most common 100°C; at 450°C the salt is anhydrous. isotope 90Zr; r.a.m. 91.224. Zinc sulfate is extremely soluble in water. It is used in the textile industry. zwitterion (ampholyte ion) An ion that has both a positive and negative charge on zinc sulfide (ZnS) A compound that can the same species. Zwitterions occur when a be prepared by direct combination of zinc molecule contains both a basic group and and sulfur. Alternatively it can be prepared an acidic group.

242 APPENDIXES Appendix I 18 Kr Ar Xe He Ne Rn 2 36 18 86 10 54 18 I 0 (or VIIIB) VIIIA (or 0) F Cl At Br 17 35 17 85 9 53 symbol Lr Lu 17 71 103 VIIB VIIA S O Se 16 Po Te Uuh 34 116 16 84 8 52 Yb No 16 VIB VIA 70 102 P N Bi Sb 15 As 33 115 15 83 7 51 Tm Md 15 VB VA 69 101 C Si Sn 14 Pb Ge Uuq 32 114 14 82 6 50 Er Fm 14 IVB IVA 68 100 B In Al Tl 13 Ga 31 113 13 81 5 49 Es Ho 13 IIIB IIIA 67 99 12 Cd Zn Hg Uub 30 112 80 48 Cf Dy 12 IIB IIB 66 98 11 Ag Cu Au Uuu 29 111 79 47 Bk Tb IB IB 11 65 97 Pt 10 Ni Pd Ds 28 110 78 46 Gd Cm 10 64 96 9 Ir Rh Co Mt 27 109 77 45 Eu 9 Am 63 95 8 Fe Hs Os Ru 26 108 76 44 VIII (or VIIIB) VIII (or VIIIA) - giving group, atomic number, and chemical Pu 8 Sm 62 94 7 Tc Re Bh Mn 25 107 75 43 7 Np Pm 61 93 VIIB VIIA 6 W Sg Cr Mo 24 106 74 42 U 6 Nd VIB VIA 60 92 5 V Ta Db Nb 23 105 73 41 groups. Older group designations are shown below. Pr Pa 5 VB VA 59 91 4 Ti Rf Zr Hf 22 104 72 40 The above is the modern recommended form of table using 18 Ce Th 4 IVB IVA 58 90 - Lu 3 Y Sc 21 39 Ac - Lr 57-71 La 89-103 La Ac 3 IIIB IIIA 57 89 2 Sr Be Ba Ca Ra Mg 20 88 12 56 4 38 2 IIA IIA 1 K H Li Fr Cs Rb Na Periodic Table of the Elements Actinides 1 19 87 11 55 3 37 Lanthanides

1 IA IA 6 7 3 6 2 5 1 4 Period 7 European convention Modern form N. American convention

244 Appendix II

The Chemical Elements (* indicates the nucleon number of the most stable isotope)

Element Symbol p.n. r.a.m Element Symbol p.n. r.a.m actinium Ac 89 227* europium Eu 63 151.965 aluminum Al 13 26.982 fermium Fm 100 257* americium Am 95 243* fluorine F 9 18.9984 antimony Sb 51 112.76 francium Fr 87 223* argon Ar 18 39.948 gadolinium Gd 64 157.25 arsenic As 33 74.92 gallium Ga 31 69.723 astatine At 85 210 germanium Ge 32 72.61 barium Ba 56 137.327 gold Au 79 196.967 berkelium Bk 97 247* hafnium Hf 72 178.49 beryllium Be 4 9.012 hassium Hs 108 265* bismuth Bi 83 208.98 helium He 2 4.0026 bohrium Bh 107 262* holmium Ho 67 164.93 boron B 5 10.811 hydrogen H 1 1.008 bromine Br 35 79.904 indium In 49 114.82 cadmium Cd 48 112.411 iodine I 53 126.904 calcium Ca 20 40.078 iridium Ir 77 192.217 californium Cf 98 251* iron Fe 26 55.845 carbon C 6 12.011 krypton Kr 36 83.80 cerium Ce 58 140.115 lanthanum La 57 138.91 cesium Cs 55 132.905 lawrencium Lr 103 262* chlorine Cl 17 35.453 lead Pb 82 207.19 chromium Cr 24 51.996 lithium Li 3 6.941 cobalt Co 27 58.933 lutetium Lu 71 174.967 copper Cu 29 63.546 magnesium Mg 12 24.305 curium Cm 96 247* manganese Mn 25 54.938 darmstadtium Ds 110 269* meitnerium Mt 109 266* dubnium Db 105 262* mendelevium Md 101 258* dysprosium Dy 66 162.50 mercury Hg 80 200.59 einsteinium Es 99 252* molybdenum Mo 42 95.94 erbium Er 68 167.26 neodymium Nd 60 144.24

245 Appendix II The Chemical Elements Element Symbol p.n. r.a.m Element Symbol p.n. r.a.m neon Ne 10 20.179 scandium Sc 21 44.956 neptunium Np 93 237.048 seaborgium Sg 106 263* nickel Ni 28 58.69 selenium Se 34 78.96 niobium Nb 41 92.91 silicon Si 14 28.086 nitrogen N 7 14.0067 silver Ag 47 107.868 nobelium No 102 259* sodium Na 11 22.9898 osmium Os 76 190.23 strontium Sr 38 87.62 oxygen O 8 15.9994 sulfur S 16 32.066 palladium Pd 46 106.42 tantalum Ta 73 180.948 phosphorus P 15 30.9738 technetium Tc 43 99* platinum Pt 78 195.08 tellurium Te 52 127.60 plutonium Pu 94 244* terbium Tb 65 158.925 polonium Po 84 209* thallium Tl 81 204.38 potassium K 19 39.098 thorium Th 90 232.038 praseodymium Pr 59 140.91 thulium Tm 69 168.934 promethium Pm 61 145* tin Sn 50 118.71 protactinium Pa 91 231.036 titanium Ti 22 47.867 radium Ra 88 226.025 tungsten W 74 183.84 radon Rn 86 222* uranium U 92 238.03 rhenium Re 75 186.21 vanadium V 23 50.94 rhodium Rh 45 102.91 xenon Xe 54 131.29 rubidium Rb 37 85.47 ytterbium Yb 70 173.04 ruthenium Ru 44 101.07 yttrium Y 39 88.906 rutherfordium Rf 104 261* zinc Zn 30 65.39 samarium Sm 62 150.36 zirconium Zr 40 91.22

246 Appendix III

The Greek Alphabet

A α alpha N ν nu B β beta Ξξxi Γγgamma O ο omikron ∆δdelta Ππ pi E ε epsilon P ρ rho Z ζ zeta Σσsigma H η eta T τ tau Θθ theta Υυupsilon I ι iota Φφ phi K κ kappa X χ chi Λλlambda Ψψpsi M µ mu Ωωomega

Appendix IV

Fundamental Constants speed of light c 2.997 924 58 × 108 m s–1 µ π × –7 permeability of free space o 4 10 = 1.256 637 0614 × 10–6 H m–1 ε µ –1 –2 × –12 –1 permittivity of free space 0= 0 c 8.854 187 817 10 F m charge of electron or proton e ±1.602 177 33 × 10–19 C × –31 rest mass of electron me 9.109 39 10 kg × –27 rest mass of proton mp 1.672 62 10 kg × –27 rest mass of neutron mn 1.674 92 10 kg electron charge-to-mass ratio e/m 1.758 820 × 1011 C kg–1 × –15 electron radius re 2.817 939 10 m Planck constant h 6.626 075 × 10–34 J s Boltzmann constant k 1.380 658 × 10–23 J K–1 Faraday constant F 9.648 531 × 104 C mol–1

247 Appendix V

Webpages

Chemical society webpages include:

American Chemical Society www.chemistry.org

Royal Society of Chemistry www.rsc.org

The International Union of Pure and www.iupac.org Applied Chemistry

Information on nomenclature is available at:

Queen Mary College, London www.chem.qmul.ac.uk/iupac

Advanced Chemistry Development, Inc www.acdlabs.com/iupac/nomenclature

The definitive site for the chemical elements is:

WebElements Periodic Table www.webelements.com

Bibliography

There are a number of comprehensive texts covering inorganic chemistry. These include:

Cotton, F. A.; Murillo, C.; Wilkinson, G.; Bochmann, M. & Grimes, R. Advanced Inor- ganic Chemistry. 6th ed. New York: Wiley, 1999

Greenwood, N. N. & Earnshaw, A. Chemistry of the Elements. Oxford, U.K.: Butter- worth-Heinemann, 1997

Shriver, D. F. & Atkins, P. W. Inorganic Chemistry. 3rd ed. Oxford, U.K.: Oxford Uni- versity Press, 1999

Additional useful sources are:

Emsley, J. Nature's Building Blocks – An A-Z Guide to the Elements. Oxford, U.K.: Oxford University Press, 2001

King, R.B. (ed.) Encyclopedia of Inorganic Chemistry. New York: Wiley, 1994

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