September 23, 1921] 



SCIENCE 



263 



outer layer. Fluorine, accordingly, is mono- 

 valent. 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 valencies, 

 the sum of the two being equal to eight. 

 Thus, to take oxygen as an example, it has 

 only two vacant places, and so can only find 

 room 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 com- 

 pound 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. Professor Lewis and 

 Mr. 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 compounds. 



The term " atomic weight " has thus ac- 

 quired for the chemist an altogether new and 

 much wider significance. It has long been 

 recognized that it has a far deeper import 

 than as a constant useful in chemical arithme- 

 tic. For the ordinary purposes of quantitative 

 analysis, of technology, and of trade, these 

 constants may be said to be now known with 

 sufiicient accuracy. But in view of their bear- 

 ing 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 im- 

 portance. Their determination and study 

 must now be approached from entirely new 

 standpoints and by the conjoint action of 

 chemists and physicists. The existence of 

 isotopes has enormously widened 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 be 

 appalled at such a prospect. All sorts of 

 difficulties start up to affright him, such as 

 the present impossibility of isolating isotopes 

 in a state of individuality, their possible 

 instability, and the inability of his quantita- 

 tive methods to establish accurately the rel- 

 atively small differences to be anticipated. 

 All this would seem to make for complexity. 

 On the other hand, it may eventually tend 

 towards simplification. If, with the aid of 

 the physicist we can unravel the nature and 

 configuration of the atom of any particular 

 element, 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 mathe- 

 matically valid. This, however, is almost 

 certainly not the ease, 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 mathe- 

 matical expression have been the means of 

 elucidating the physical constitution of the 

 gases and of throwing light upon such varia- 

 tions in their behavior as have been observed 

 to occur. There would appear, therefore, 

 ample scope for the chemist in determining 

 with the highest attainable accuracy the de- 

 partures 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, origin- 

 ally appointed in 1903, has made and pub- 

 lished an annual report in which they have 

 noted such determinations 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, published 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 



