THE NATURE OF MATTER 



169 



noted in the groups of elements under the periodic law that the 

 elements in the first group, after the inert substances of group o, 

 like neon, argon, have a valence of one, those of the second two, 

 the third three, and these are all positive. Substances in the 

 fourth group may behave either as positives or negatives, and 

 their valence is four. The fifth, sixth, and seventh groups have 

 decreasing negative valences, three, two, one respectively, or 

 they may rarely behave as positives, with valence of five, six, 



,o 



o 



o 



o 



o 

 o 



o 

 o 



c 



o 



o 



o 



6 



c 



o 



o' 



o 



o 



o 



FIG. 67. (a) Diagram of the sodium atom, with a group of protons at the 

 center, two electrons indicated by dotted lines in the first sphere, eight in the next 

 lying at the corners of a cube (suggested by lines) in the second sphere, and one 

 electron of the next sphere. (6) Diagram of the fluorine atom. 



seven respectively. Now this is all easily explicable on the 

 basis of the structure of their atoms. Thus sodium has eleven 

 protons and electrons arranged as shown in Figure 67^, while 

 fluorine has nine arranged as in Figure 676. Sodium has only 

 one lonesome electron in its outer sphere. It needs seven more 

 to fill up this sphere to satisfaction. Fluorine has its outer 

 sphere full except for one electron. Now if fluorine takes this 

 lonely electron in the outer sphere of sodium into its outer 

 sphere to make up the eight, then this electron will jointly be a 

 member of the sodium atom and of the fluorine atom. The two 

 atoms are tied together and are united to form sodium fluoride. 



