30 CONTEMPORARY SCIENCE 



if we consider that the atomic number of silicon is 14 

 while that of sulfur is 16. Thus if we should replace 

 the nucleus of the sulfur atom in a molecule of sulfur 

 fluoride by the nucleus of a silicon atom, without dis- 

 turbing any of the surrounding electrons, we would have 

 removed two positive charges and would obtain a nega- 

 tive ion with two negative charges of the formula SiF 6 . 



In the presence of potassium ions we would then have 

 the familiar salt potassium fluosilicate. The theory is 

 thus capable of explaining typical complex salts. In fact, 

 it is applicable to the whole field of inorganic compounds 

 covered by the work of Werner, and helps to simplify 

 the theory of such compounds. There is no time, how- 

 ever, to go into this subject. 



The simple theory of atomic structure which we have 

 discussed thus far explains perfectly what has usually 

 been called "the maximum positive and negative valance." 

 The maximum positive valence represents the number of 

 electrons which the atom possesses in excess of the num- 

 ber needed to form one of the particularly stable configu- 

 rations of electrons. On the other hand, the maximum 

 negative valence is the number of electrons which the 

 atom must take up in order to reach one of these stable 

 configurations. 



For example, magnesium has a positive valence of two, 

 since its atomic number is 12 while that of neon is 10. 

 Sulfur has a positive valence of 6 since it has 6 electrons 

 more than neon ; but it has a negative valence of two be- 

 cause it must take up two more electrons before it can 

 assume a form like that of the argon atom. 



It is clear, however, that this theory of valence is not 

 yet complete. 1 It is not applicable to those cases where 

 we have usually taken valences of 4 for sulfur, or 3 and 



1 The theories of Kossel, Lacomble, Teudt, etc., which have 

 recently been proposed in Germany, have not advanced beyond 

 this point and are therefore very unsatisfactory as a general 

 theory of valence. 



