PROBLEMS OF MODERN SCIENCE 



hydrogen. Thus the successive atomic numbers 

 of the light elements are H = i, He = 2, Li = 3, 

 Be = 4, B = 5, C = 6, N = 7, O = 8, and so on. 

 The nuclear charge of boron, for example, is $e 

 where e is the magnitude of the charge on the 

 electron. The boron atom is made up of this 

 nucleus associated with the 5 electrons which are 

 required to make the structure neutral. The 

 configuration of these electrons will be determined 

 almost entirely by the charge on the nucleus ; so 

 that we see that it is the nuclear charge which will 

 determine the properties of the atoms, the mass 

 of the nucleus being a relatively secondary matter. 

 The structure of the nuclei of the various atoms 

 furnishes a very fascinating problem. The first 

 evidence in this direction was obtained from the 

 phenomena of radioactivity. You will recall that 

 radioactive substances emit two kinds of electrically 

 charged rays, the a and (3 rays respectively. Of 

 these the a rays have been proved to be identical 

 with the nucleus of the helium atom ; so that it 

 is clear that the nuclei of the heavy radioactive 

 atoms contain a number of helium nuclei among 

 their constituents. The (3 rays are rapidly mov- 

 ing negative electrons, and there is convincing 

 evidence that these also come out of the nuclei of 

 the radioactive atoms. As a result of the expul- 

 sion of these rays the radioactive elements are 



spontaneously transforming themselves into other 

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