PHYSICS 



charges but different atomic weights, should be 

 chemically inseparable. As a matter of fact that 

 is found to be the case. Such pairs of elements, 

 which have the same chemical properties, but 

 whose atoms have different masses, are known as 

 isotopes. The recent experiments of Aston have 

 shown that many of the common elements, such 

 as chlorine, consist in reality of a mixture of 

 isotopes. When the different isotopes are sorted 

 out it appears that the various atomic species have 

 atomic masses which are whole numbers in terms 

 of oxygen = 16, except hydrogen, whose atomic 

 mass in terms of this unit is 1008. Hydrogen 

 has been shown not to be a mixture of isotopes, 

 so that this deviation from unity is not due to the 

 admixture of a relatively small proportion of an 

 isotope of mass 2 or 3. 



We have seen that the heavy radioactive 

 atoms contain helium and electrons as part of 

 the structure of their nuclei. W 7 hat of the light 

 atoms ? This problem has been successfully 

 attacked by Rutherford by the very direct 

 method of bombarding the nuclei of the light 

 atoms by a particles and examining the pro- 

 perties of the fragments which are ejected as a 

 result of what is practically a head-on collision. 

 These fragments in their general properties re- 

 semble the a particles which cause their ejection. 

 The two can, in fact, only be distinguished when 



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