ARTICLES 



51 



negative charges are allocated in concentric shells of equal 

 thickness around the nucleus, each shell being divided into a 

 number of equal cells proportional to the area of the shell. The 

 first shell has two cells. The second shell, having four times 

 the surface, contains eight cells, the third eighteen, and the 

 fourth thirty- two cells. Moreover, each cell, except the two 

 innermost, may contain either one or two electrons, which are 

 usually arranged radially. But before a given cell can contain 

 more than one electron, all the cells in the inner shells must be 

 completely filled. Consequently the electrons in each shell, 

 except the first, are arranged in two layers which may be 

 numbered I, lla, lib, Ilia, lllb, IVa and IV^. 



The following table shows the arrangement of the electrons 

 in the atoms of each element : 



