Constitution of Atoms. 283 



The inference from radioactivity work, that most of our 

 elements are not "pure " but consist of mixtures of isotopes, 

 and that the A value of any pure element, or single isotope, 

 is appreciably integral (0 = 16 being taken as standard) has 

 been splendidly confirmed by Aston's recent work. Of the 

 eighteen elements already examined by his mass-spectrum 

 method, only hydrogen gives an atomic weight (1*008), 

 which is not integral to within one in a thousand. The 

 cause of this exception requires further investigation, but in 

 the meantime it may be set aside as related in some way (as 

 suggested by Aston) to the fact that hydrogen is unique in 

 containing no electrons in its nucleus. 



It is unique also in other ways, and especially in the value 

 of the ratio A/N, which in PI is 1 and in no other atom is 

 less than 2 (nor more than 2' 6). 



If we write A — 2N — n, we find from Aston's work that 

 ?i = in pure elements, He, C, N, 0, S, and also in the lower 

 isotopes of B, Ne, Si, and Ar. There can be no doubt that 

 this also holds for the lower isotopes of Li, Mg, Ca, and, 

 perhaps, some of the other light atoms. It follows, therefore, 

 that the group (Ji 2 e) may be regarded as a secondary unit of 

 positive charge, with mass 2, and that (b 2 e)s expresses the 

 composition of the nucleus of any of those atoms. 



In the higher isotopes of B, Ne, Si and Ar, in the pure 

 elements F and P, and in both the isotopes of 01, n has small 

 values, ranging from 1 to 4 ; in As it is 9, in the two Br 

 isotopes it is 9 and 11, and in the six isotopes of Kr it ranges 

 between 6 and 14. The values are higher still in Xe (20 to 

 27) ; and in Hg it apparently ranges between 37 and 44. 

 The numerous isotopes of elements with N values from 81 

 to 92, contained in the three radioactive series, have all n 

 values from 42 to 54. There is thus a general tendency for 

 n to increase with N, modified by the fact that it may 

 vary considerably among isotopes with the same N. 



If the b 2 e group be still taken as the unit of positive 

 charge, there must be added to it in most cases n electrically 

 neutral couplets (be), each having unit mass. We thus can 

 express the nuclei of all atoms from He to U by the 

 general formula (b 2 e)-$(be)n, n having any integral value from 

 to 54 ; and even the unique case of H is included if n be 

 given the special value of — 1. 



If we distinguish the nucleus from the shell electrons by 

 enclosing the former within square brackets, \_(b s e)^(be)n]eTs 

 becomes the perfectly general formula for any electrically 

 neutral atom, while positively or negatively charged ions 



