26 FALK— DEVELOPMENT OF THE . [April 24, 



In the same way, the chemical affinity between two combining 

 atoms would correspond to the intensity factor in chemical energy. 

 This chemical affinity can be measured quantitatively only by the 

 change in free energy of the reaction in question. Stability rela- 

 tionships, or chemical affinity discussions, do not enter directly into 

 valence questions, although the existence of substances is controlled 

 entirely by these. The separation of these two problems, valence 

 and chemical affinity, makes it clear that while a great number of 

 substances may be predicted from a consideration of valence struc- 

 tures alone, questions of chemical affinity, or relative stability, limits 

 the number of these substances which are actually known or may 

 be prepared. 



As a result of the comparative study of large numbers of com- 

 pounds, it has become possible to say which would probably exist 

 under ordinary conditions and whether some would react more 

 rapidly than others. These qualitative factors do not give any in- 

 formation concerning the real quantitative measures of relative 

 stability. 



Valence is therefore essentially a classifying principle. While it 

 is based directly upon and derived from the atomic theory, it may 

 also be used without considering atoms. The conception of atoms 

 and molecules is based upon the experimental laws of definite and 

 multiple proportions, and if, instead of atoms and molecules, com- 

 bining and formula weights are used, the same relations will be 

 found, although perhaps not pictured as readily. 



The most important feature of the present development is that 

 in speaking of the valence of an element, it is not sufficient to give 

 a number. It is just as important to state whether this number is 

 positive or negative, as the valence or the number of unit atoms or 

 groups held in combination involves also the question whether these 

 are electropositive or electronegative. This is brought out clearly 

 in the Periodic System of INIendeleeff, especially for Groups 4 to 7, 

 where the types of combination with hydrogen and with oxygen 

 represent the maximum negative and positive valences of the ele- 

 ments of these groups. 



Since the valence of an atom may be positive or negative depend- 



