August 4, 1923] 



NA TURE 



J 79 



The Electron in Relation to Chemistry. 



•"PHE Faraday Society's conference on "The 

 ^ Electronic Theory of Valency," held at Cam- 

 bridge on July 13 and 14, may be regarded as marking 

 a new stage In the welding together of physics and 

 chemistry, which has been so notable a feature of 

 the recent history of these two sciences. The con- 

 ference was attended by about 120 delegates from 

 different universities, about half of whom were drawn 

 from outside Cambridge. Some forty visitors were 

 entertained in Trinity Hall, through the kindness 

 of the master and fellows, to whom a deep debt of 

 gratitude is due for contributing in this way to the 

 pleasant social features of the conference. The 

 foreign guests included Prof. G. N. Lewis, Prof. 

 W. A. Noyes, Prof. Lyman, and Prof. Victor Henri 

 of Zurich ; the physicists included Sir J. J. Thomson, 

 Sir Ernest Rutherford, Sir WiUiam Bragg, Prof. 

 Barton, Prof. W. L. Bragg, Prof. Porter, Prof. 

 Rankine, Dr. F. W. Aston, and Mr. R. H. Fowler ; 

 the chemists, in addition to Sir Robert Robertson, 

 the president of the Faraday Society, included Sir 

 William Pope, Prof. Heilbron, Prof. Lapworth, Prof. 

 Lowry, Prof. Robinson, Prof. Smithells, Prof. Thorpe, 

 Mr. C. R. Bury, Dr. Flurscheim, Dr. W. E. Garner, 

 Dr. Henstock, Dr. Kenner, Mr. W. H. Mills, Mr. 

 E. K. Rideal, and Dr. N. V. Sidgwick. 



The conference was held in the new Department 

 of Physical Chemistry, which is housed very ap- 

 propriately in a block of buildings lying between 

 the Chemical Laboratory in Downing Street and the 

 Cavendish Laboratory in Free School Lane. These 

 buildings, which were formerly in the occupation of 

 the Department of Engineering, now provide ideal 

 quarters for work in physical chemistry. They 

 have been completely refitted and are admirably 

 suited to their new use ; they are also so com- 

 modious that there is a reserve of floor-space which 

 has been loaned to workers from other departments, 

 pending the time when it may be required for further 

 extensions of physico-chemical work. Tea was served 

 before the conference opened, in the large laboratory 

 of the Hopkinson wing, which was erected in 1900 

 as a memorial to the late Prof. Bertram Hopkinson 

 and his son, while the discussions were held in the 

 lecture theatre adjoining. 



The Friday afternoon session, dealing mainly with 

 the application of the electronic theory to the problems 

 of inorganic chemistry, was presided over by Sir 

 J. J. Thomson, who in his opening address referred 

 to the fact that, while the force which retains the 

 electrons in an atom is proportional to the positive 

 charge on the nucleus, the disruptive force which 

 tends to make them scatter is proportional to the 

 third power of their number, so that a limit is set 

 to the number of electrons which can be crowded 

 into one atom. The law of force is such that when 

 the number of electrons is small, the octet is a 

 particularly stable grouping ; but with a different 

 law of force, a sextet might be more stable than an 

 octet. The problem of molecular structure can be 

 attacked most readily by the study of cases of 

 substitution ; thus the electric moment introduced 

 on replacing hydrogen by chlorine can be calculated, 

 and measurements of the specific inductive capacity 

 of molecules of different types have shown that this 

 moment is constant in magnitude. 



Prof. G. N. Lewis, in presenting his paper on 

 " Valence and the Electrons," directed attention to 

 the reconciliation which has recently taken place 

 between the views of physicists and chemists in 

 reference to the structure of the atom. Since 



NO. 2805, VOL. 112] 



physicists have now adopted a model in three 

 dimensions, it is possible to regard the orbit of the 

 electron as having a fixed orientation, although the 

 electron itself is in rapid motion. The chemist's 

 theory of static electrons has, therefore, been merged 

 quite naturally into a scheme of static orbits. Prof. 

 Lewis directed attention to the fact that, in Bohr's 

 atomic structures, each of the rare gases from neon 

 _ to niton, and all the stable elementary ions, possess 

 eight electrons in the outer shell, thus affording full 

 justification for what came to be known as " the 

 octet theory." The fundamental phenomenon of 

 chemistry is, however, the formation of pairs of 

 electrons ; and of some hundred thousand known 

 substances only about half-a-dozen contain uneven 

 numbers of electrons. This pairing may perhaps be 

 due to magnetic forces, since unpaired electrons 

 always give rise to a magnetic moment. When four 

 of these electron-pairs are grouped at the corners of 

 a regular tetrahedron the still more stable configura- 

 tion of the octet is obtained. 



Prof. Lewis attaches great importance to the view 

 that the sharing of a pair of electrons constitutes a 

 chemical bond between two atoms. When this bond 

 is broken, the electron-pair usually remains attached 

 to one atom, which acquires a negative charge, while 

 the associated atom (which loses its share of the 

 electron-pair) acquires a positive charge on disruption. 

 This contrast is described in Langmuir's nomenclature 

 as the conversion of a " co valence " into an " electro- 

 valence " ; and most English readers have accepted 

 this nomenclature as an essential feature of the 

 " Lewis-Langmuir hypothesis." Prof. Lewis, how- 

 ever, regards the ionised bond as being no longer a 

 bond at all, and even objects to the use of the term 

 " valence " to express the electrical state of the 

 atom, although for nearly seventy years bismuth and 

 aluminium have been described, like phosphorus and 

 nitrogen, as tervalent elements. 



The two following papers, by Mr. R. H. Fowler on 

 " Bohr's Atom in Reference to the Problem of 

 Covalency " and by Dr. N. V. Sidgwick on " The 

 Nature of the Non-polar Link," were of interest as 

 exhibiting two parallel lines of thought in the applica- 

 tion of Bohr's theory of the structure of atoms to 

 the unsolved problem of the electronic structure of 

 molecules. The close agreement between the con- 

 clusions reached on this subject at Oxford and at 

 Cambridge is noteworthy. As might perhaps have 

 been anticipated, the Cambridge physicist was much 

 more apologetic than the Oxford chemist, since he 

 evidently realised more fully the risks that must 

 be taken when forsaking the mathematical concepts, 

 verified by observations of spectra, on which the 

 structure of the atom is based, for purely qualitative 

 conceptions of molecular structure, which are at 

 present beyond the range of mathematical analyses 

 and of experimental verification. The chemist, on 

 the other hand, boldly translating the shared electrons 

 of Lewis into shared orbits (compare Dr. N. P. 

 Campbell's letter in Nature of April 28, p. 569), was 

 ready at once to gather a harvest of new conceptions 

 from this speculative extension of Bohr's theory. A 

 study of the printed papers shows, however, no 

 important discrepancies between the views of the 

 two authors as to the results of extending the theory 

 of orbits from atomic to molecular structure. 



In the discussion following upon the reading of 

 these papers, Sir J. J. Thomson pointed out that two 

 electrons are not necessary to make a bond, since 

 H2+ is one of the most persistent aggregates met 



