RADIUM, ATOMIC ENERGY, AND ATOMIC STRUCTURE 553 



and it is found that, for all of the next 8 elements from lithium to 

 neon, the tendency is either to lose electrons until none are left, 

 or to gain electrons until a stable ring of 8 is formed. The ar- 

 rangement of the electrons in this second shell has been pictured 

 by Lewis as shown in Fig. 123. The electrons are assumed to 

 occupy the corners of an imaginary cube, in the center of which is 

 the atomic nucleus. 



Li 



Be 



B 



N 



Ne 



FIG. 123 



The valence of any one of these elements is therefore repre- 

 sented either by the number of electrons that can be lost (positive 

 valence) or by the number required to form a stable ring of 8 

 (negative valence). The next eight elements (sodium to argon) 

 exhibit the same behavior. We have not the space here to dis- 

 cuss in detail the electron arrangement in these and later elements. 

 It will suffice to mention that not only are all of the relationships 

 suggested by the Periodic System (Chapter XXIII) confirmed, but 

 many of the points of difficulty in Mendelejeff's tabulation are 

 satisfactorily explained. 



Co-Valence. When combination takes place between two 

 atoms (e.g., Li and F) by loss and gain of electrons, we are left 



