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Home , Charles Janet

The Past and Future of the

This stalwart symbol of the field of chemistry always faces scrutiny and debate

Eric R. Scerri

t graces the walls of lecture halls making up one period. The lengths elements, Döbereiner found that the Iand laboratories of all types, from of periods vary: The first has two ele- weight of the middle element—such as universities to industry. It is one of ments, the next two eight each, then 18 in the triad formed by , the most powerful icons of science. It and 32, respectively, for the next pairs selenium and —had an atom- captures the essence of chemistry in of periods. Vertical columns make up ic weight that was the approximate one elegant pattern. The periodic table groups, of which there are 18, based average of the weights of the other two provides a concise way of understand- on similar chemical properties, related elements. Sulfur’s atomic weight, in ing how all known chemical elements to the number of electrons in the outer Döbereiner’s time, was 32.239, where- react with one another and enter into shell of the atoms, also called the va- as tellurium’s was 129.243, the average chemical bonding, and it helps to ex- lence shell. For instance, group 17, the of which is 80.741, or close to the then- plain the properties of each element halogens, all lack one electron to fill measured value for selenium, 79.264. that make it react in such a fashion. their valence shells, all tend to acquire The importance of this discovery lay But the periodic system is so funda- electrons during reactions, and all form in the marrying of qualitative chemical mental, pervasive and familiar in the acids with . properties, such as degree of reactivity, study of chemistry that it is often taken with numerical data on the elements. for granted. A century after the death The Classics It suggested that there might be some of the leading discoverer of the period- There have been many changes to the underlying numerical order that could ic system, the Russian chemist Dimitri table since Mendeleev’s first, which serve to relate the elements to one an- Mendeleev, it seems time to revisit the showed eight groups, 12 rows and 66 other in a systematic way. origins and modern status of this now- elements, was published in 1869. But Döbereiner also discovered other standard chemical classification. There neither did Mendeleev’s table spring triads, such as , and were a number of historic precursors from a vacuum. Historians of chem- , and , and po- to Mendeleev’s periodic system. But istry have long recognized two ideas tassium. Other chemists discovered there are also current ongoing debates that contributed to the evolution of the yet more triads and began to make ta- regarding the best way to display the periodic system: the notion of triads bles that also attempted to relate triads periodic system, and whether there is of elements and Prout’s hypothesis, among one another. But some of these really a “best way” of doing so. whereby the atomic weights of the el- contributions degenerated into mere The periodic system of elements ements are integral multiples of the numerology, especially when they ne- gets its moniker because it graphs how atomic weight of hydrogen, the light- glected chemical relations between the certain properties of chemicals repeat est of all the elements. elements. For example, in his 1857 ar- after regular intervals. In the modern In 1817 the German chemist Johann ticle, German chemist Ernst Lenssen table of 117 elements, each is placed Döbereiner noticed that several groups suggested the existence of a triad con- across rows in order of increasing of three elements formed triads with sisting of , and , atomic number—the number of pro- two interesting features. Not only was even though there was no conceivable tons in the nucleus of one atom of each the middle element of a triad of inter- chemical connection between these el- element. There are seven rows, each mediate chemical reactivity, but it also ements. Nevertheless, the lure of the had an intermediate atomic weight. search for triads encouraged chemists Differing from atomic number, a val- to determine atomic weights more ac- Eric R. Scerri is a lecturer in chemistry and the ue that had not yet been ascertained, curately, a pursuit that served chemis- history and philosophy of science at University of atomic weight had been measured try in many other ways. California, Los Angeles. He is the founder and edi- since the start of the 1800s. The idea A little earlier, in 1815, the London tor-in-chief of the journal Foundations of Chem- was to determine the weight of each based physician, William Prout, pro- istry and the author of The Periodic Table: Its indivisible unit of matter relative to posed another general principle. In a Story and Its Significance (Oxford University Press, 2007). He received his Ph.D. in the history hydrogen, whose weight was taken few papers, which he published anony- and philosophy of science from King’s College Lon- as 1.00. Because formulas for many mously, Prout wrote that the fact that don. Address: Department of Chemistry & Bio- compounds were unknown, atomic the atomic weights of many elements chemistry, UCLA, Los Angeles, CA 90095-1569. weights remained imprecisely mea- seemed to be integral multiples of the Internet: [email protected] sured for some time. But in triads of weight of hydrogen suggested that all

52 American Scientist, Volume 96 © Theodore Gray 2007 www.periodictable.com Figure 1. With its compact form displaying a wealth of information about the elements, the periodic table has long been a standard-issue reference in the field of chemistry. Theodore Gray created this museum display to improve on the appeal of the table by adding samples of each element behind its entry. (This close-up shows, at left, the triad sulfur, selenium and tellurium, studies of which were a precursor to the periodic system.) The story of the periodic table is one of gradual improvements, from early measurements of atomic weight to current-day proposals for new layouts of the elements that look nothing like a two-dimensional chart. the elements were composite multiples curate atomic-weight data began to to positive charge, or the number of of hydrogen. He also went on to claim accumulate. The notion of triads was protons in the nucleus of any atom. that this would imply the essential unity found to be too approximate and even On re-examining the notions of triads of all matter. But some elements such as then only applied to carefully selected and Prout’s hypothesis in the light of , which then had a value of 12.6 groups of three elements. Meanwhile, atomic number, one finds a remarkable relative to hydrogen, seemed to point Prout’s hypothesis showed too many sense in which both notions have made against Prout’s hypothesis. Prout’s sup- nonintegral exceptions. In the lan- what another famous philosopher of porters regarded such facts as anoma- guage of philosopher of science Karl science, Imre Lakatos, has termed a lies that would eventually disappear Popper, both ideas had been refuted theoretical comeback. In terms of with the more accurate determination by the second half of the 19th century. atomic number, the elements have ex- of atomic weights. At the start of the 20th century, it act multiples of the number of protons As in the case of triads, attempts to was found that atomic number, rather in the hydrogen atom—as hydrogen confirm or refute Prout’s hypothesis than atomic weight, serves as the more has only one proton, everything is a contributed to renewed efforts on the correct criterion for ordering the ele- multiple of it. And perhaps in a deeper part of chemists to measure atomic ments in a linear sequence. Research- sense, modern astrophysics has shown weights. However, although these ers such as the British physicist Henry that almost all of the elements are liter- ideas were fruitful in some ways, they Moseley found that they could use x- ally formed from hydrogen and he- were also found wanting as more ac- ray diffraction to relate atomic number lium atoms, which combined together www.americanscientist.org 2008 January–February 53 during the Big Bang at the not be further simplified. start of the universe, as well This is the notion of ele- as in the interiors of stars ments as first emphasized and supernovae. by Antoine Lavoisier in the Moreover, if we consider 1700s, when he called them atomic numbers instead of ”simple substances.” atomic weights for the tri- But there is a second no- ads discovered in the 19th tion, which Mendeleev century, it turns out that sometimes called ”real ele- the atomic number of the ments,” in order to indicate middle element is exactly their more fundamental the average of the other two status. In this sense, the el- elements. Indeed, about half ements represent abstract of all the possible triads in substances that lack what the modern periodic table we normally regard as are exact in this sense. How- properties and that repre- ever, many other potential sent the form that elements triads are not even approxi- take when they occur in mately correct in that the compounds. For example, atomic number of the mid- sodium and as dle element is nowhere near simple substances—a grey the average of the other two metal and a greenish gas re- elements. spectively—are not literally The reason for this be- present in the compound havior is that the periodic sodium chloride (table table shows a repetition salt). Mendeleev would in the length of all periods have said that sodium and (with the exception of the chlorine are present in the first very short period which compound as the abstract consists of just the elements or “real elements.” hydrogen and ). The Let me emphasize that second period consists of Corbis these abstract elements are eight elements (lithium to Figure 2. Dimitri Ivanovich Mendeleev was born in 1834 in Tobolsk, still real, and indeed should ) followed by another Siberia, the youngest of 14 children. He studied in St. Petersburg, Russia, be regarded as somehow period of eight elements (so- where he became a professor of chemistry at the university in 1863. He being more fundamental dium to ), followed by published his initial periodic table in 1869. Although his table was not than the elements as simple two periods of 18 elements, the first, his version is the one that had the biggest impact on the scientific substances that can actu- presumably followed by two community. He also championed the system, defending its validity and ally be isolated. Mendeleev periods of 32 elements and devoting time to its elaboration. Mendeleev died just over 100 years ago, gave just one attribute to in 1907. A statue of him with his table stands in St. Petersburg. so on. As a result of these the abstract element, name- repetitions, atomic number ly atomic weight. It is the triads are exact in half of all possible deleev—the undisputed champion of atomic weight of sodium, for example, cases. Take the element chlorine as an the periodic table—was a critic of the that preserves its identity when so- example. In order to encounter another use of triads and especially of Prout’s dium enters into chemical combina- element with similar chemical proper- hypothesis concerning the existence of tion. Just as Mendeleev implied that ties we need to advance 18 places to primary matter. Mendeleev was firmly the abstract version of the concept of get to the element . To reach convinced of the individuality and dis- element was more real, so he empha- yet another element sharing these same tinct existence of the elements. He is sized that his periodic classification chemical properties it is necessary to ad- rightly famous for having left gaps in was primarily concerned with the ab- vance a further 18 places to the element his periodic tables for elements that stract elements. . Bromine lies exactly between had not been isolated and for success- As some authors have argued, this chlorine and iodine in terms of atomic fully predicting many of their proper- more philosophical view of the ele- number, precisely because the length of ties, especially in the case of , ments may have been the crucial sense the two periods between these elements and . in which Mendeleev went further than is exactly the same—18 elements. But There are aspects of Mendeleev’s his competitors, who restricted their in other cases of potential triads, the system that are not very well known attention to the elements as simple second and third elements are not in but that nevertheless were quite fun- substances. It also seems to provide periods of the same length, so the triads damental to his approach. Mendeleev a means of understanding how Men- don’t work. repeatedly emphasized that there is a deleev was able to challenge the val- dual sense of the concept of element. ues of the atomic weights of many ele- Bridging the Gaps In the first case, elements are the final ments and the manner in which several Despite this modern reprieve and stage of chemical analysis, or some- elements had been accommodated into explanation, in the mid-1800s, Men- thing that can be isolated and that can- the periodic system. This was achieved

54 American Scientist, Volume 96 FMFNFOU BUPNJD DIFNJDBMSFBDUJWJUZ XFJHIU UPXBSEXBUFS

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Figure 3. The elements lithium, sodium and form a triad of the kind studied by chemist Johann Döbereiner in 1817. Elements formed a triad when the average of the then-known atomic weights of the first and third ones closely approximated that of the center member. The reactivity of the middle element was also known to be intermediate to that of the first and last. In the modern periodic table, this particular triad is part of the group of alkali metals. Actual samples of these elements (right) are stored in oil because they are highly reactive with water and air. by ignoring, to some extent, the more two chemists has successfully accom- is in perfect agreement with the lengths obvious, more superficial properties of modated the then-known elements into of periods in the chemist’s periodic ta- the elements as simple substances. their respective periodic systems rather ble. The simple quantum mechanical Mendeleev is often given most than for any foretelling. theory does not, however, account for credit for his fame as the “father of the Theoretical physics has provided a the repetition of all period lengths ex- periodic table” because he predicted partial explanation for the form and cept for the first one. Indeed, this prob- elements that were undiscovered at existence of Mendeleev’s table and its lem has continued to elude theoretical the time. But just how impressive were modern descendents. From the view- those predictions? As far as the ele- point of physics, the electrons orbiting ments gallium, germanium and scan- the nucleus of an atom are responsible FMFNFOU BUPNJDXFJHIU dium are concerned, they were quite for its chemical properties. 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As one historian of physicist Erwin Schrödinger’s famous BMVNJOJVN "M  chemistry has wondered, how is it that equation for the electron can be charac- TJMJDPO 4J  we are prepared to forgive Mendeleev terized by a set of quantum numbers. QIPTQIPSVT 1  so many failures? When this set is supplemented with an TVMGVS 4  In addition, it is by no means clear additional for spin, DIMPSJOF $M  that successful predictions were in fact it is possible to predict that subsequent QPUBTTJVN ,  so decisive in the acceptance of the pe- main shells of the atom can contain a riodic table by the scientific community maximum of 2, 8, 18 or 32 electrons. This DBMDJVN $B  in Mendeleev’s era. For example, the UJUBOJVN 5J  Davy medal, which predates the Nobel Figure 4. One early attempt at a classification DISPNJVN $S  Prize as the highest accolade in chemis- system for the elements related their atomic NBOHBOFTF .O  weights to that of hydrogen, whose weight try, was jointly awarded to Mendeleev JSPO 'F  and Julius Lothar Meyer, his leading was taken to be 1.0. Around 1815 London competitor, who did not make any pre- physician William Prout hypothesized that, DPCBMU $P  because the atomic weights of many elements dictions. Indeed, there is not even a OJDLFM /J  seemed to be integral multiples of that of hy- DPQQFS $V  mention of Mendeleev’s predictions in drogen, perhaps all elements were, in fact, the published speech that accompa- composite multiples of hydrogen. The atomic [JOD ;O  nied the joint award of the Davy prize. weights shown in this table were typical val- BSTFOJD "T  It therefore seems that this prize was ues that were available in Prout’s time, but TUSPOUJVN 4S  awarded for the manner in which the they are not accurate by modern standards. www.americanscientist.org 2008 January–February 55 physicists until quite recently. Appro- that the periodic table continues to be periodic table itself. Some even ques- priately enough, it was a Russian physi- an area of active research by physicists tion whether a two-dimensional table cist, the late Valentin Ostrovsky, who as well as chemists even though it has is the best way to arrange the elements. recently published a theory to explain existed for nearly 140 years. Chemists frequently express the view this feature, although it is not yet gener- that there is no one best representation ally accepted. Although the theory is Fertile Ground and that the question of representation too mathematically complicated to ex- Chemists, physicists and philosophers is a matter of convenience and conven- plain here, Ostrovsky’s work and some of science continue to debate the relative tion. More recently this view has been other competing accounts demonstrate virtues of different forms to display the questioned by philosophers of science,

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Figure 5. The standard modern periodic table, called the medium-long form (top), shows the elements in rows in order of increasing atomic number, or the number of protons in the nucleus of an atom of each element. Each row makes up a period, the lengths of which vary. Each column represents a group in which 4DFSSJQFSJPEJDUBCMF  the elements have similar chemical properties, related to the number of electrons in the outer, or valence,  ) )F -J #F # $ / 0 shell of their atoms. The lanthanides and actinides, in the separated bottom two rows, are pulled out after the  ' /F /B .H "M 4J 1 4 elements barium (Ba) and (Ra), for the sake of compactness. Highlighted on this table (pink) are tri- $M "S , $B 4D 5J 7 $S .O 'F $P /J $V ;O (B (F "T 4F ads, originally related by their atomic weights, but that work in terms of atomic numbers (beige). An alternate #S ,S 3C 4S : ;S /C .P 5D 3V 3I 1E "H $E *O 4O 4C 5F table is the left-step form (middle), which puts helium in the alkaline earth group as these all have two elec- * 9F $T #B -V )G 5B 8 3F 0T *S 1U "V )H 5M 1C #J 1P trons in their valence shells. It also more naturally "U 3O 'S 3B -S 3G %C 4H #I )T .U %T 3H 6VC 6VU 6VR 6VI 6VI follows the order of the filling of electron shells. The author has proposed another form (bottom) that puts 6VT 6VP hydrogen in the halogens group and places this group at the leftmost edge of the table. This form dispenses with the anomalous-seeming two-element period in -B $F 1S /E 1N 4N &V (E 5C %Z )P &S 5N :C the medium-long table and is based on maximizing "D 5I 1B 6 /Q 1V "N $N #L $G &T 'N .E /P atomic-number triads, such as the new one formed by hydrogen (H), fluorine (F) and chlorine (Cl) (pink).

56 American Scientist, Volume 96 some of whom believe that there may should one even seek to create any be one best way to arrange the elements new perfect triad? This feature is rath- in groups of columns. They argue that er important because it is based solely disputes concerning the placement of on atomic number, the only criterion certain troublesome elements, such as of the elements regarded as basic sub- hydrogen and helium, in the periodic stances rather than simple substances. system have one correct solution, even As mentioned earlier, Mendeleev went if this is not yet apparent to current-day to great lengths to emphasize that the science. periodic system was primarily a clas- Consequently, they maintain that sification of the elements as basic sub- some displays of the periodic system stances (“real elements”). may, in truth, be superior to others. This more-philosophical view of Whereas the conventionally displayed the elements has come to the rescue table, called the medium-long form, of chemistry as its own field, rather has many virtues, it places helium than simply a part of physics, on among the noble-gas elements. Some more than one occasion. It suggests have argued that in spite of appear- that chemistry possesses an essential ances, helium should in fact be placed philosophical foundation even though at the head of group 2, the alkaline it is popularly presumed to reduce to earth group, which includes , quantum physics and thus to be de- and calcium. Helium has void of a philosophical character. In two outer-shell electrons as do the ele- the early years of the 20th century, ments in the alkaline earth group. when isotopes of many elements were Figure 6. Canadian chemist Fernando Dufour In addition, the filling of electron discovered, it suddenly seemed as if has taken the periodic system from two to three shells follows a particular ordering, the number of “elements,” in the sense dimensions, dispensing with the idea of a table which is more naturally displayed of simplest substances, that can be iso- altogether. His system, produced in 1990 and with this grouping, called the left-step lated had multiplied. Some chemists called the ElemenTree, emphasizes chemical periodic table. This form of the peri- believed that this proliferation would similarities that span different groups on the odic system was first proposed by the signal the demise of the periodic table, standard table. (Photo courtesy of the author.) Frenchman Charles Janet in the 1920s which would give way to a table of the and has recently been revived by U.S. isotopes. “ElemenTree,” they also serve to em- chemical educator Gary Katz, among However, some chemists such as phasize chemical similarities that are not others. Further support for this rep- Austrian Friedrich Paneth reconcep- embodied in the conventional two-di- resentation also lies in the fact that it tualized the notion of elements in such mensional table. renders the periodic system more or- a way as to avoid the abandonment For example, the elements in group derly than the conventional layout. In of the chemist’s periodic table. Paneth 13 of the conventional-format table, the left-step table there are two very appealed to Mendeleev’s distinction such as boron, aluminum and gal- short periods of two elements, instead between “real elements” and elements lium, all display a combining power, of one, with the result that all period as simple substances. By concentrating or valence, of three. However, there lengths, without fail, are repeated. on the “real elements” as Mendeleev are a number of other elements that In a recently-accepted paper, I have had done, but now characterizing also show this property, such as the proposed another periodic table in them by their atomic numbers, the elements in group 3 of the conventional which hydrogen is placed at the head chemist could ignore the fact that the table, including scandium, and of the halogen group. Moreover, this “elements” occur as many hundreds . In Dufour’s system all these table has been rearranged so that the of isotopes. The isotopes could be re- elements fall onto the same two-dimen- group that is now headed by hydrogen garded as mere simple substances. sional plane which may be pictured as appears at the left-hand edge of the ta- Moreover, isotopes of the same ele- a slice through the three-dimensional ble. The main outcome of this arrange- ment, with a few exceptions such as classification system. ment is to introduce greater regularity those of hydrogen, tend to show iden- Another design that Philip Stewart into the display of the periodic system, tical chemical properties, thus justify- of the University of Oxford has revived which may reflect the regularity of ing this approach. and argued for is the spiral-form peri- the periodic law more faithfully. This Perhaps the most radical development odic system, and it has received a good modified periodic table displays two to take place in contemporary research deal of recent attention. As Stewart periods of eight elements at the start of on the periodic table has been a willing- contends, the conventional table fails the periodic system and dispenses al- ness to challenge tradition by question- to emphasize the continuity in the se- together with the anomalous-seeming ing whether the periodic system should quence of the elements. Spiral systems very short period of two elements. be displayed in a two-dimensional form stress continuity rather than implying The main motivation for this layout and whether it should even be displayed breaks between the noble gases at the is that it to the formation of a as a table. At least three distinct three- right-hand edge and the alkali metals new perfect triad involving hydrogen. dimensional periodic systems have been at the left edge. In addition, the perfect triad involv- developed and successfully marketed as ing helium is retained, unlike in the educational tools. In some cases, such as Hindsight left-step table, where it is lost. But why Canadian chemist Fernando Dufour’s Could it be that our reliance on the www.americanscientist.org 2008 January–February 57 CHEMICAL GALAXY II A NEW VISION OF THE PERIODIC SYSTEM OF THE ELEMENTS

SYMBOL NAME No. R.A.M. Ac 89 (227) Ag 47 107.87 Al Aluminum 13 26.982 ? Am 95 (243) 118 : see Sb Ar Argon 18 39.948 As 33 74.922 At 85 (210) Au 79 196.97 ? B Boron 5 10.811 Ba Barium 56 137.33 117 Be Beryllium 4 9.0122 Bh 107 (262) Bi 83 208.98 Rn Bk 97 (247) 86 Br Bromine 35 79.904 C 6 12.011 At Ca Calcium 20 40.078 85 Cd 48 112.41 Fr Ce 58 140.12 87 Cf 98 (251) Cl Chlorine 17 35.453 Cm 96 (247) Xe Co 27 58.933 54 I Cr 24 51.996 Ra 53 Cs Cesium 55 132.91 88 Cu 29 63.546 Db 105 (262) Cs Ds 110 (271) Ac 55 Dy 66 162.50 89 Er 68 167.26 Th Kr Br Es 99 (254) 90 Eu 63 151.97 36 35 Pa F Fluorine 9 18.998 91 Ba Fe 26 55.845 56 Fm 100 (257) U Rb Fr 87 (223) 92 La 37 Ga Gallium 31 69.723 57 Cl Gd 64 157.25 Np Ce Ar ? Ge Germanium 32 72.640 18 17 Gold: see Au 93 58 116 H Hydrogen 1 1.0079 Pr Sr He Helium 2 4.0026 Pu 59 K Hf 72 178.49 94 38 Hg 80 200.59 Nd 19 Ho 67 164.93 60 F Po Hs 108 (265) Am Ne 84 I Iodine 53 126.90 95 Pm 10 9 In 49 114.82 61 Ir 77 192.22 Ca Na Iron: see Fe Cm 20 11 Te K Potassium 19 39.098 Sm 52 Kr 36 83.800 96 62 La 57 138.91 He : see Pb Mg Li 2 Se Li Lithium 3 6.9410 Eu 34 Lr 103 (260) Bk 63 12 3 Lu Lutetium 71 174.97 97 S Md 101 (258) Be Mercury: see Hg Gd 4 O 16 Mg Magnesium 12 24.305 64 n Mn 25 54.938 0 8 Mo 42 95.940 Cf Mt 109 (266) N Nitrogen 7 14.007 98 Tb Na Sodium 11 22.990 65 Nb 41 92.906 Nd 60 144.24 H N ? Ne Neon 10 20.180 1 7 115 Ni 28 58.693 Es Dy P No 102 (259) 99 66 15 As Bi Np 93 237.05 Sb 83 O 8 15.999 33 51 Os 76 190.23 C P 15 30.974 6 Pa 91 231.04 Ho Pb Lead 82 207.20 67 Pd 46 106.42 B Pm 61 (145) Fm 5 Si Po 84 (209) 100 14 Potassium: see K Pr 59 140.91 Er Pt 78 195.08 Pu 94 (244) 68 Ge Ra Radium 88 (226) Al 32 Rb 37 85.468 13 Re 75 186.21 Sn Rf 104 (261) Rg 111 (272) Md Sc 50 Rh 45 102.91 Tm 21 101 Pb Rn 86 (222) 69 Ga Ru 44 101.07 31 82 Uuq S Sulfur 16 32.065 114 Sb Antimony 51 121.76 Sc Scandium 21 44.996 Zn Se Selenium 34 78.960 Ti 30 Sg 106 (266) Y 22 Cu In Si Silicon 14 28.086 Yb 39 29 49 Silver: see Ag 70 Ni Sm 62 150.36 No V Co 28 Sn 50 118.71 102 23 Cr Fe 27 Sodium: see Na Mn 26 Cd Sr Strontium 38 87.620 24 25 48 Tl Ta 73 180.95 Ag 81 Tb 65 158.93 Lu Zr Tc 43 (99) 40 47 Te Tellurium 52 127.60 71 ? Th 90 232.04 Pd Hg 113 Ti 22 47.867 46 80 Tin: see Sn Rh Tl 81 204.38 Nb 45 Tm 69 168.93 41 Ru Au : see W Lr Mo Tc 44 79 U 92 238.03 103 42 43 Uub Uub Ununbium 112 (227) 112 Uuq Ununquadium 114 (289) Hf Pt V 23 50.942 78 W Tungsten 74 183.84 72 Xe Xenon 54 131.29 Ir Rg Y Yttrium 39 88.906 77 Yb 70 173.04 111 Zn 30 65.390 Os Zr 40 91.224 Ta 76 73 Re Ds W 75 110 74 Mt Rf 109 104 Hs Copyright © 2007 P J Stewart 108 Bh www.chemicalgalaxy.co.uk Db 107 Artwork by www.borndigital.co.uk 105 Sg 106 Published by Jensan Scientifics, LLC www.sciencemall-usa.com Printed in the U.S.A.

ELECTRONIC CONFIGURATION IN GROUND STATE: EXCEPTIONS TO THE REGULAR BUILD UP OF SUBSHELLS From Sc to Zn: nth element has n 3d electrons, but Cr=[Ar].3d5.4s1; Cu=[Ar].3d10.4s1 From Y to Cd: nth element has n 4d electrons, but Nb=[Kr].4d4.5s1; Mo=[Kr].4d5.5s1; Tc=[Kr].4d6.5s1; Ru=[Kr].4d7.5s1; Rh=[Kr].4d8.5s1; Pd=[Kr].4d10.5s0; Ag=[Kr].4d10.5s1 From Lu to Hg: nth element has n 5d electrons, but Pt=[Xe].4f14.5d9.6s1; Au=[Xe].4f14.5d10.6s1 From La to Yb: nth element has n 4f electrons and no 5d, but La=[Xe].4f0.5d1.6s2; Ce=[Xe].4f1.5d1.6s2; Gd=[Xe].4f7.5d1.6s2 From Ac to No: nth element has n 5f electrons and no 6d, but: Ac=[Rn].5f0.6d1.7s2; Th=[Rn].5f0.6d2.7s2; Pa=[Rn].5f2.6d1.7s2; U=[Rn].5f3.6d1.7s2; Np=[Rn].5f4.6d1.7s2; Cm=[Rn].5f7.6d1.7s2m

Figure 7. Philip Stewart of Oxford University has championed a spiral format for the periodic system. Such continuous forms get rid of the implied breaks between periods in the conventional table. Spiral forms have been considered for more than 100 years, but this poster shows Stewart’s adaptation, which he calls a “chemical galaxy,” as the increasing length of the periods can be accommodated by a format similar to the radiating arms of a spiral galaxy. (Image courtesy of Philip Stewart and Carl Wenczek of Born Digital Ltd.) two-dimensional forms of the periodic comes rather obvious, as was noted by riod table for the 21st century. The Chemical table are due to the predominance, the English chemist William Crookes Educator 6:324–332. until recently, of the two-dimensional more than 100 years ago. Leach, M. R. Periodic Table Formulations. In The Chemogenesis Web Book. http://meta- textbook page surface and the two- The periodic table began with the synthesis.com/webbook/35_pt/pt.html dimensional nature of the walls of lec- recognition of triads of elements and Ostrovsky, V. 2001. What and how physics ture theaters? After all, a three-dimen- arose at the time of Prout’s hypothesis contributes to understanding the periodic sional system is not so easily displayed of the unity of all matter. From these law. Foundations of Chemistry 3:145–182. in a book or indeed on the wall of a numerical and philosophical origins it Scerri, E. R. 2007. The Periodic Table: Its Story lecture hall. But could it also be that has evolved into an enormously prac- and Its Significance. New York: Oxford Uni- with the rise of new technologies in the tical tool used not just by chemists, but versity Press. 21st century, Mendeleev’s famous icon by all scientists and engineers. But its Scerri, E. R. 2005. Some aspects of the meta- might be transformed into something philosophical aspects have not been physics of chemistry and the nature of the that even he might not recognize if he completely eclipsed, and, as I argue elements. Hyle 11:127–145. http://www. were still here to see it? here, they continue to underwrite the hyle.org/journal/issues/11-2/scerri.htm In fact, as far as spiral forms are con- periodic system and sometimes sur- Stewart, P. 2004. The spiral periodic system. cerned, Mendeleev did consider such face to assist in the solution of practi- Education in Chemistry 41:165. arrangements but did not devise a suc- cal issues concerning its identity and van Spronsen, J. W. 1969. The Periodic System cessful version. As Stewart has written, graphical representation. of Chemical Elements: A History of the First if Mendeleev had paid more attention to spiral forms, he might have added References For relevant Web links, consult this the prediction of the whole family of Gray, T. Periodic table arrangement promoted ­issue of American Scientist Online: noble gases to his other famous predic- by Eric Scerri. http://www.periodictable. tions of isolated elements. If one uses a com/index.scerri.html http://www.americanscientist.org/ IssueTOC/issue/1041 spiral display of the elements, the pos- Katz, G. 2001. The periodic table: An eight pe- sible existence of the noble gases be-

58 American Scientist, Volume 96 Hundred Years. New York: Elsevier.

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