178 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1924 



early observed that some of the radioactive elements which showed 

 distinct radioactive properties were chemically so alike that it was 

 impossible to efl'ect their separation when mixed together. Similar 

 elements of this kind were called " isotopes " by Soddy, since they 

 appeared to occupy the same place in the periodic table. For 

 example, a number of radioactive elements in the uranium and 

 thorium series have been found to have physical and chemical prop- 

 erties identical with those of ordinary lead, but yet to have atomic 

 weights differing from ordinary lead, and also distinctive radio- 

 active properties. The nuclear theory of the atom offers at once a 

 simple interpretation of the relation between isotopic elements. 

 Since the chemical properties of an element are controlled by its 

 nuclear charge and little influenced by its mass, isotopes must cor- 

 respond to atoms with the same nuclear charge but of different 

 nuclear mass. Such a view also offers a simple explanation why 

 the radioactive isotopes show different radioactive properties, for it 

 is to be anticipated that the stability of a nucleus will be much 

 influenced by its mass and arrangement. 



Our knowledge of isotopes has been widely extended in the last 

 few years by Aston, who has devised an accurate direct method for 

 showing the presence of isotopes in the ordinary elements. He has 

 found that some of the elements are " pure " — i. e., consist of atoms 

 of identical mass — while others contain a mixture of two or more 

 isotopes. In the case of the isotopic elements, the atomic mass, as 

 ordinarily measured by the chemist, is a mean value depending on 

 the atomic masses of the individual isotopes and their relative abun- 

 dance. These investigations have not only shown clearly that the 

 number of distinct species of atoms is much greater than was sup- 

 posed but have brought out a relation between the elements of great 

 interest and importance. The atomic masses of the isotopes of most 

 of the elements examined have been found, to an accuracy of about 

 one in a thousand, to be whole numbers in terms of oxygen, 16. This 

 indicates that the nuclei are ultimately built up of protons of mass 

 very nearly one and of electrons. It is natural to suppose that this 

 building unit is the h5'drogen nucleus, but that its average mass in 

 the complex nucleus is somewhat less than its mass in the free state 

 owing to the close packing of the charged units in the nuclear struc- 

 ture. We have already seen that the helium nucleus of mass 4 is 

 probably a secondary unit of great importance in the building up of 

 many atoms, and it may be that other simple combinations of pro- 

 tons and electrons of mass 2 and 3 occur in the nucleus, but these 

 have not been observed in the free state. 



While the masses of the majority of the isotopes are nearly whole 

 numbers, certain cases have been observed by Aston where this rule 



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