PROTIUM— DEUTERIUM— TRITIUM 

 THE HYDROGEN TRIO ' 



By Hugh S. Taylor 

 David B. Jones professor of chemistry, Princeton University 



[With 3 plates] 



Three months before the outbreak of war in 1914 an international 

 scientific race had just been concluded. Soddy of Aberdeen had 

 found that radio-lead from thorium sources had an atomic weight of 

 about 208. Richards and Lembert in Harvard and Honigschmidt 

 in Vienna had shown independently that rjidio-lead from uranium 

 sources had an atomic weight of about 206. Ordinary lead was 

 known to be about 207. Soddy's concept that substances could exist 

 with identical, or practically identical, chemical and spectroscopic 

 properties but different atomic weights was established. Soddy 

 suggested a name for such substances, isotopes, because, though dif- 

 ferent in mass, they occupied the same place in the chemist's periodic 

 table of the elements. We know now that isotopes of the same ele- 

 ment have the same net positive charge on the nucleus and the same 

 system of external electrons. It is the net nuclear charge, not the 

 mass of the nucleus, which determines the position in the periodic 

 table. 



Aston, who after the war returned to the Cavendish Laboratory 

 in Cambridge, England, developed a mass spectrograph to determine 

 masses of individual charged particles, and in November of the year 

 1919 supplied definite proof that the rare gas, neon, existed in at 

 least two isotopic forms of masses 20 and 22. He thus extended the 

 concept of isotopes to elements which were not radioactive in their 

 origins. There followed a decade of activity in which, with the 

 mass spectrograph progressively refined, an increasingly large num- 

 ber of elements were shown to be isotopically complex. There are, 

 for example, 11 isotopes of tin. Some elements persistently proved 

 to be simple. Carbon, oxygen, and hydrogen were among those so 

 regarded at the end of the 10-year period. 



1 Reprinted by permission from The Scientific Monttily, vol. 39, pp. 361-372, October 

 1934. 



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