ARTICLES 



MASS-SPECTRA AND THE ATOMIC 

 WEIGHTS OF THE ELEMENTS 



By F. W. Aston, M.A., D.Sc. A.I.C., Trinity College, Cambridge. 



Some ten years ago two widely different sets of experiments 

 were in progress the results of which were to shake, and in the 

 end destroy, one of the best established articles of scientific 

 faith : Dalton's postulate that the atoms of an element were 

 equal to each other in weight. One of these was the work of 

 Sir Ernest Rutherford and his colleagues on radio-active disinte- 

 gration, the other was Sir J. J. Thomson's analysis of Positive 



Rays. 



The results of the first of these researches led inevitably 

 to the conclusion that it was possible to obtain quite a number 

 of elements whose chemical properties were identical with 

 those of lead, but whose atomic weights not only differed from 

 each other in a measurable degree, but also from the accepted 

 atomic weight of lead. Other branches of radio-active trans- 

 formation proved the same remarkable result in the case of 

 thorium. The theory that elements of different atomic weight 

 could still be chemically identical and occupy the same posi- 

 tion in the periodic table was largely developed by Prof. Soddy, 

 who, in view of the latter property, called them " isotopes." 

 As far as lead is concerned, the existence of its isotopes has 

 recently been triumphantly vindicated by the production, in 

 quantities ample for chemical determination, of varieties 

 chemically indistinguishable but yielding atomic weights differ- 

 ing by quite unmistakable amounts. As the present article 

 is concerned with the constitution of the lighter and non- 

 radio-active elements, it will be sufficient to state that the 

 possibility of the existence of isotopes among the group of 

 elements studied in radio-active change is now no longer in 

 question. 



The first suggestion of the existence of isotopes among the 

 lighter elements was afforded by the anomalous behaviour of 

 the monatomic gas neon when subjected to positive-ray analysis 

 by Sir J. J. Thomson. The method of analysis then used has 



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