THE SMALLEST PARTICLES OF MATTER 17 



The question of isotopes is no longer academic. In order to 

 make the first of the "atomic bombs" tested at Alamogordo, New 

 Mexico, and later used at Hiroshima, the fissionable uranium 

 isotope 235 had to be separated from the non-fissionable isotope 

 238 by an ingenious application of the process of differential dif- 

 fusion of uranium hexafluoride. This work was based on the 

 pioneer diffusion experiments of Thomas Graham, in the course 

 of which he coined the word "colloid" and established much of 

 the nomenclature of colloid chemistry. The bomb dropped on 

 Nagasaki was of the new fissionable material plutonium, synthe- 

 sized (indirectly) by bombarding the isotope U 238 with neutrons. 



Another outstanding case of the importance of isotopes is found 

 in our lightest element, hydrogen. The discovery by Professor 

 Harold C. Urey (Nobel prize, 1934) that common water contains a 

 "heavy" fraction, was quickly followed by the isolation of deu- 

 terium, whose atomic weight is double that of hydrogen. Natural 

 hydrogen contains 0.02 per cent of deuterium, whose nuclei con- 

 tain one proton and one neutron, and have a net nuclear charge 

 of one. Triple hydrogen, called tritium, with a radioactive half- 

 life of about 30 years, has been artificially produced; it may be 

 readily stored by combining it with lithium as lithium tritide. 



Moseley and X-ray Spectra 



Henry G. Moseley was killed at Gallipoli in 1915 at the age of 

 28 as a result of the unwise drafting of highly trained scientists; 

 his death was both a calamity and a disgrace. He had already 

 brilliantly begun investigating the relation between nuclear charge 

 and atomic number — the property which fixes the position of an 

 element in the Periodic Table. Moseley used the crystal spec- 

 trometer developed by Sir William H. Bragg and his son, Sir 

 Lawrence Bragg, who jointly received the Nobel prize in 1915. A 

 direct correlation was discovered between the wave lengths of 

 x-rays emitted where different metals were used as targets in the 

 x-ray tubes, and the atomic number of the metal. The x-ray 

 spectra varied uniformly and regularly, all the lines being similar; 

 but they shifted in frequency in passing from one element to the 

 next, because they all depend upon the number and arrangement 

 of the planetary electrons circulating about the nucleus, whose net 

 positive charge Moseley supposed to be identical with the atomic 

 number. This was later proved by Sir James Chadwick (Nobel 

 prize, 1935). 



