54 



NATURE 



[September 8, 192 1 



depends on the number of electrons in the outer 

 layer of its atom will also recur periodically, which 

 is precisely that remarkable property of the 

 elements which is expressed by the periodic law of 

 Mendeleeff, or the law of octaves of Newlands. 



The valency of the elements, like their period- 

 icity, is a consequence of the principle that equi- 

 librium becomes unstable when there are more 

 than eight electrons in the outer layer of the atom. 

 For on this view the chemical combination between 

 two atoms, A and B, consists in the electrons of A 

 getting linked up with those of B. Consider an 

 atom like that of neon, which has already eight 

 electrons in its outer layer ; it cannot find room 

 for any more, so that no atoms can be linked to 

 it, and thus it cannot form any compounds. Now 

 take an atom of fluorine, which has seven electrons 

 in its outer layer; it can find room for one, but 

 only one, electron, so that it can unite with one, 

 but not with more than one, atom of an element 

 like hydrogen, which has one electron in the outer 

 layer. Fluorine, accordingly, is monovalent. The 

 oxygen atom has six electrons ; it has, therefore, 

 room for two more, and so can link up with two 

 atoms of hydrogen : hence oxygen is divalent. 

 Similarly nitrogen, which has five electrons and 

 three vacant places, will be trivalent, and so on. 

 On this view an element should have two valen- 

 cies, the sum of the twp being equal to eight. 

 Thus, to take oxygen as an example, it has only 

 two vacant places, and so can find room only for 

 the electrons of two atoms ; it has, however, six 

 electrons available for filling up the vacant places 

 in other atoms, and as there is only one vacancy 

 to be filled in a fluorine atom the electrons in an 

 oxygen atom could fill up the vacancies in six 

 fluorine atoms, and thereby attach these atoms to 

 it. A fluoride of oxygen of this composition re- 

 mains to be discovered, but Its analogue, SFg, 

 first made known by Moissan, is a compound of 

 this type. The existence of two valencies for an 

 element is in accordance with views put forward 

 some time ago by Abegg and Bodlander. Prof. 

 Lewis and Dr. Irving Langmuir have developed, 

 with great ingenuity and success, the consequences 

 which follow from the hypothesis that an octet of 

 electrons surrounds the atoms in chemical com- 

 pounds. 



The term " atomic weight " has thus acquired 

 for the chemist an altogether new and much wider 

 significance. It has long been recognised that it 

 has a far deeper import than as a constant useful 

 in chemical arithmetic. For the ordinary pur- 

 poses of quantitative analysis, of technology, and 

 of trade, these constants may be said to be now 

 known with sufficient accuracy ; but, in view of 

 their bearing on the great problem of the essential 

 nature of matter and on the " superlatively grand 

 question. What is the inner mechanism of the 

 atom? " they become of supreme importance. 

 Their determination and study must now be ap- 

 proached from entirely new points of view and by 

 the conjoint action of chemists and physicists. 

 The existence of isotopes has enormously widened 



NO. 2706, VOL. 108] 



the horizon. At first sight it would appear that 

 we should require to know as many atomic 

 weights as there are isotopes, and the chemist may 

 well be appalled at such a prospect. All sorts of 

 difliculties start up to aftright him, such as the 

 present impossibility of isolatmg isotopes in a state 

 of individuality, their possible instability, and the 

 inabihty of his quantitative methods to establish 

 accurately the relatively small differences to be 

 anticipated. All this would seem to make for com- 

 plexity. On the other hand, it may eventually 

 tend towards simplification. If, with the aid of 

 the physicist, we can unravel the nature and con- 

 figuration of the atom of any particular element, 

 and determine the number and relative arrangement 

 of the constituent protons and electrons, it may 

 be possible to arrive at the atomic weight by 

 simple calculation, on the assumption that the 

 integer rule is mathematically valid. This, how- 

 ever, is almost certainly not the case, owing to the 

 influence of "packing," The little differences, in 

 fact, may make all the difference. The case is 

 analogous to that of the so-called gaseous laws in 

 which the departures from their mathematical ex- 

 pression have been the means of elucidating the 

 physical constitution of the gases and of throwing 

 light upon such variations in their behaviour as 

 have been observed to occur. There would appear, 

 therefore, ample scope for the chemist in deter- 

 mining with the highest attainable accuracy the 

 departures from the whole-number rule, since it 

 is evident that much depends upon their exact 

 extent. 



These considerations have already engaged the 

 attention of chemists. For some years past a 

 small International Committee, originally ap- 

 pointed in 1903, has made and published an annual 

 report in which they have noted such determina- 

 tions of atomic weight as have been made during 

 the year preceding each report, and they have 

 from time to time made suggestions for the 

 amendment of the tables of atomic weights, pub- 

 lished in text-books and chemical journals, and in • 

 use in chemical laboratories. In view of recent 

 developments, the time has now arrived when the 

 work of this International Committee must be re- 

 organised and its aims and functions extended. 

 The mode in which this should be done has beeiT 

 discussed at the meeting in Brussels, in June last, 

 of the International Union of Chemistry Pure and: 

 Applied, and has resulted in strengthening the con- 

 stitution of the Committee and in a wide extension 

 of its scoge. 



The crisis through which we have recently 

 passed has had a profound effect upon the world. 

 The spectacle of the most cultured and most highly 

 developed peoples on this earth, armed with every 

 offensive appliance which science and the inventive 

 skill and ingenuity of men could suggest, in the 

 throes of a death struggle must have made the 

 angels weep. That dreadful harvest of death is 

 past, but the aftermath remains. Some of it is 

 evil, and the evil will persist for, it may be, genera- 



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