C. R. Bury: His Contributions to Physical Chemistry Mansel Davies The Edward Davles Chemical Laboratories, University College of Wales, Aberystwyth, U.K.
OF CHEMICAL EI)UCATION offered a notaThe JOURNAL ble contribution to the his~orvof . vhvsical chemistrv in the papers centered on the work on G. N. Lewis (&nuaryMarch, 1984). The present article relates closelvon the Lewis-Langmuir atomic structure studies, to the ~ e w i s - ~ a l v i n work on color and molecular structure (both detailed in those papers), and also to the work of McBain, the pioneer colloid scientist of Stanford University. I must disclose an interest. C. R. Bury was my teacher in physical chemistry a t the University College of Wales, Aberystwyth, and, for two years, 1933-35, he was my research supervisor. Bury, although an Oxford graduate and a distinctlv" Enelish character.. nresented ohvsical chemistw with . a strong emphasis on American work. Apart from the contributions of Lewis. Lanamuir. and McBain. we were introduced to thermodyna&s via "Lewis and Randall," and we learned of (then) current studies by T. W. Richards, Grinnell Jones, J. H. Hildebrand, C. P. Smyth, etc. And two of Bury's major publications appeared in the Journal of the American Chemical Society.
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The Langrnulr Model and Its Background Firstly, one must recall that the Lewis-Langmuir presentation of valencv and chemical bondine was entirelv in terms of electron pairs in the octet model. The preoccupation with the number 8 had a lone histom. - . eoine back to Newlands' "law of octaves" and to ibegg's papers of 1904. J. J. Thomson was perhaps the first t o indicate (1904), from considerations of electrical stability, that the electrons would have a structured pattern within the atom. Thomson used "corpuscles" for what we now call electrons: making this substitution, he wrote (I),
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...whatever the number ofelrctrons thepositionsofequilibrium
would heafrymmetrical distribution over rhesurfaceotasphrre. Such a dirtrihution would inderd crchnically be one of rquilibrium, but a mathematical calculation shows that unless the number of electrons is quite small, say seven or eight at most, this arrangement is unstable and so can never persist. When the number of electrons is greater than this limiting number, the electrons break un into two erouns. " . One eroun ,. . eantainine the smaller number of rlrctronli is on the surface of a small hod" concentric with the rphrrr: the rrmainder arc on the surface of a larger concentric hdy. When the number ofelectruns is still further increased.. . the electrons now divided themselves into three groups. . . Here we see a physical justification for an ''octet7' and, equally important, the suggestion of successive groups of electrons within the atomic structure. Langmuir (2)was the first to offer adetailed picture of the electron arrangements in atoms based on their chemical behavior. In an important account of this work Sharrock (3) makes two relevant obsenrations. He points out the close similarity between the character, quality, and lively research discussions in the G. E. Schenectady Research Laboratory and those in G. N. Lewis's Physical Chemistry Department a t Berkeley. Specifically he shows, from a studs of laboratory notebooks,~that~ a n g m u i ru,ho, , for a number nf years. had already a keen interest in atomic electron structures, suecrested in 1917 to his cnlleaeue Saul Dushman that he (Dushman) should not write 2,9; K: 2, 8, 9; but rather
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Charles R. Bury (1890-1968). Photographtaken ca. 1928.
Na: 2,8,1; K: 2, 8, 8,l. This appears in Dushman's notes of November 9,1916. Subsequently Dushman published (4) in 1917 a 25-page account, "The Structure of the Atom." In that, inter alia, he advanced representations of the different valencies of tungsten as the result of "tautomeric" electron structures which he gave: WOa: 6-valent: 2,8,10,24,30 (or: 24 + 6) WCIS: 5-valent: 2,8,10,25,29 (24 + 5 ) WC14: 4-valent: 2,8,10,26,28 (24 + 4) WCI2: 2-valent: 2,8, lo, 28,26 (24 + 2) This instance suffices to show that Dushman was one of the first to present "transition element" behavior in terms of the rearrangement of inner electron groups. In Langmuir's model the electrons are given equal volume elements (or "cells") in a succession of shells; the number of the cells in successive shells is fixed as 2:8:18:32. In his fourth postulate, Langmuir states that each cell can contain two electrons, except the innermost, which can contain only one, and that (note carefully) there can be no electrons in the outer shell until all the inner shells contain their maximum numbers of electrons. Bury's Contrlbutlon 1\11chemisrs will recognize the accumulated defects in this postulate. It was Hury in 1Y21 who correcred this model and gave the pattern which has heen universally accepted ever since (5).His paper is entitled (perhaps with a view to proVolume 63 Number 9
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moting its publication) "Langmuir's Theory of the Arrangement of Electrons in Atoms and Molecules." Bury writes, Thus, successive layers can contain 2, 8, 18 and 32 electrons. Groups of 8 and 18 electrons in a layer are stable, even when that layer can contain a larger number of electrons. The maximum number of electrons in the outer layer of an atom is 8: more than8 electrons can exist inashell only when there isan accumulation of electrons in an outer layer. During the change of an inner layer from astable group of 8 to one of 18, or from 18 to 32, there occurs a transition series of elements which can have more than one structure. The brevity, clarity, and correctness of this statement is remarkable. In the space of seven pages, Bury then proceeds to give a summary of the valence properties of all the elements UD to uranium, accountine for the numbers of the transition elements i n t h e first anb second long periods (up to xenon). and in the "rare earths." which he correctlv starts with ceri"m (at. no. 58, which can he 2, 8, 18, 19, 8;3) and finishes with lutetium (at. no. 71: 2, 8. 18. 32. 8. 3). This is quite different from Langmnir, who writes 2-16 for argon, in place of 2.8.8. Bum's deductions have formed the basis of all iater discussions of the chemistry of the elements in the Periodic Table. Two further quotations and comments. "Between lutetium and tantalum an element of atomic number 72 is to he ex~ected.This would have the structure (2. . . 8.. 18.. 32.. 8.. 4). and would resemble zirconium." Nothing could he clearer; element 72 would not he a rare earth but would be related to and probably associated with zirconium. Two years later such an element was discovered by Coster and Hevesy and was named hafnium. Again, from Bury: Only 5 elements of this (last)period are known, and the ehemiof onlv two of them. thorium and uranium. are cal .orooerties . known well. In this period aseeond 18-32 transition series may be expected. . . . Possibly an element, not yet discovered, of atomic number 94, two more than that of uranium, is the first of a series of 7 transition elements that would he metals, something like the ruthenium group but more electropositive These 1921 predictions are probably the first reasoned anticipations of the chemistry of trans-uranic elements. However, the anticipations were not supported and uranium is now represented as the fourth actinide, having the preferred structure (2,8,18,32,21,9,2) rather than Bury's (2,8,18,32, 18,8, 6). Bohr's Contrlbutlon I have asked several senior chemists. "To whom do we owe the accepted interpretations of the ~ e r i o d i Tahle c in terms of electron structure?" T h e almost invariable answer has been, "Ah, well, it is based on Bohr's extended electronic model." Scarcely ever is Bury's name mentioned. An explanation is needed. On March 24, 1921, Nature published (6) a long letter from Bohr.' The new proposals begin to be outlined on the second page: "Now on the correspondence principle. . . ."Later, the total supporting weight for the deduction, and its conclusions, is given: this (correspondence) principle offers a simple argument . . . (which) suggests that after the first two electrons are bound in one-quantum orbits, the next eight electrons will be bound in twoquanta orbits, the next eighteen in three-quanta orbits, and the neat thirty-two in four-quanta orbits. -
It is a characteristically verbose effort of some 3700 words; the
first six sentences are of 89. 59. 59, 54. 94, and 101 words. The late C. W. Davies. in 1921 a junior colleague of Bury's and close to him, wrote that he had heard this stated at thl time. (Letter from CWD. Oct. 3, 1974. to Sharrock and the present author). 742
Journal of Chemical Education
It is important to note that the only electronic structures given by Bohr are those for the inert gases, up to and including radon: 2, 8, 18, 32, 18, 8. T h e other most relevant statements are: The assumption. . .appears.. .to offer.. .an immediate interpretation of the appearance of such families of elements within the periodic tahle, where the chemical properties of successive elements differ only very slightly from each other. . . . Thus in the family of the rare earths we may be assumed to he witnessing the successive formation of an inner group of thirty-two electrons.. . where formerly the corresponding group possessed only eighteen electrons. In a similar way we may suppose the appearance of the iron, palladium, and platinum families to be witnessing stages of the formation of groups of eighteen electrons. These extracts include all the mention Bohr makes of specific chemical elements and their electron structures. Bury's paper was received in Philadelphia on April 28, 1921. It must have left Aberystwyth, a t latest, about April 18, some three weeks after Bohr's letter appeared. Bury acknowledges seeing Bohr's letter. There is reason to believe that the Bury paper was first written well before March 242 but sent for publication soon thereafter. Bury's contrihution was later mentioned by Bohr (7), Lewis (81, and others. A 1931 volume of Glasstone's (9) . . has a Daee . ,. headinr.~ . 'The . Ruhr-Uury 'I'heorv." Subsequently. Bury's major rule in the inrerpreratio~~ of thc Peridir'l'able has been forrotten. 'l'w ohe el prizewinners, intimately concerned with t i i s aspect of chemistry, were unaware of Bury's contrihution, and a host of senior physical chemists have never heard his name. (Incidentally, it is pronounced "Berry". As he characteristically told one chairman who was introducing him as "Mr. Bewry," "Pardon me, but you bury your dead.") Bury and Colloldal Micelles In the early 1920's McBain studied soap solutions and interpreted certain abrupt changes in their physical properties with concentration as due to the appearance of multimolecular aggregates that might also contain ions (10). Bury clarified this situation (11) . . bv. em~hasizinethat several of McBain's results were complicatedbhy hydrolysis effects hut that the ahruutness of the changes arose from the laree value of n (e.g., n >'20) in the idealize2 aggregation, nA This provided the simple basis for the critical concentration for micelle formation. As a model instance he illustrated the formation of micelles in aqueous solutions of hutyric acid; later studies (12) suggested that here n was of order 80 f 10.
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Bury on Color and Constltutlon Manv collea~ues.hut Melvin Calvin in articular (13). have indicatedthat G. N. Lewis had a continuing concern with the origin of color in relation to atomic and molecular properties. The Lewis-Calvin work in this context is of major significance and Calvin gives an intimate picture of the initiation of their work in 1937. "We began with t h a e ' l e t ' s write a paper on the color of organic substances,' he said. In order to do that, we had to review the subject of color. . . ." The whole of this paragraph (ref 13, p 16) is both lively and relevant. But what is missine. I venture to suerest. is anv mention of the fact that BU& had already, ii"l93'5, lished in the Journal of the American Chemical Societv (14) ;I lhen gtnerally arcrprahle rorrelation of color and elecrroniccmstitut:on fm nlmmt all thr ian~iliesufuraanird~estuffs with the exception of the indigo group. It so happens that I daily saw Bury prepare and eventually write this three-page paper, which is a model of concision and clarity. His thesis is that the intense light absorption is due to the presence of a t least one electron delocalized hetween two, often distant, molecular sites. DBbner's violet (the chloride of p,p-diaminotriphenylcarbinol) may serve as an example:
HzN+'bhH2
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The thesis is suppoted by reference to the critical role played by suhstituents in many different structures: closely equivalent "resonating" electronic structures must he present. In addition to the triphenylmethane dyes, Bury instances the diphenylmethane dyes, indamines, indophenols, auramines, acridine, pyronine, azine, oxyazine and thiazine dyes, azodyes, anthraquinones, flavones, indanthrenes, the alizarins, etc. Not only did the later work of Lewis and Calvin conform to Bury's anticipations, hut several other substantial studies explored and extended this thesis. Mention may he made of Schwarzenhach's sequence of papers in Heluetica Chemica Acta, the work of Hamer and Mills in Journal of the Chemical Society (London), and (with particular pleasure) that of L. G. S. Brooker and colleagues at theEastman KodakRochester Laboratories. Visiting there in the early '60's and listening to Brooker's exposition of his well-developed studies on dye sensitization, I mentioned that I knew Bury well. The result was instantaneous; Brooker opened the door from the
office into his laboratory and called, "Hey, there's a visitor here who knows Bury; come to hear about him." Well, I have now given the essentials of Bury's contrihutions to physical chemistry, and by these he would wish to he remembered. Not of all Nohel-acclaimed chemists can it be said that everv colleee student of chemistrv is taueht the " content of their work. Any student given an account of the chemical elements of the Periodic Table in relation to their electronic structures is being offered what Bury first descrihed-whether they or their teachers are aware of i t or not. He died, in his 79th year, in 1968." Literature Cited 111 Thornson, J. .I. "Eleetrieify and Matter"; The Sillimsn Lectures, Yale. May 1903: Cmstnhle: Wertminster. 1904. (21 Langrnuir. I. J . A m m Chem Soc. 1919,41,868. (91 Shsrrock. P. M.Sc (History of Science) thoris. University af Montreal, November 1976. (41 Durhrnan,S. C
