PRINCIPLES OF STRUCTURE 



TABLE III 



DISTANCES AND MAIN VALENCY FORCES BETWEEN THE ATOMS IN 



ORGANIC COMPOUNDS 



31 



valency of an atom species corresponds to the co-ordination number 

 with regard to another atom species (as, e.g., in CH4), the mutual 

 saturation of the valencies excludes the possibility of unlimited lattices 

 such as those shown in Figs. 25 and 26 (p. 27). Although such 

 molecules no longer possess free valencies, they can still be arranged 

 in a crystal lattice (see Mark and Schossberger, 1937). The binding 

 forces, however, are now of a different nature; in contrast to the pri- 

 mary valencies they are called secondary valencies. They are explained 

 in theoretical physics by means of dipole moments, in much the same 

 way as the orientation and attraction of water molecules by an ion (see 

 Fig. 19, p. 19). In practice these forces between the molecules cause 

 the cohesion. The secondary valence forces are, therefore, identical with 

 the Van der Waals cohesive forces. In molecule lattices they are of 

 the same nature as in liquids and they can therefore be derived from 

 the heat of sublimation or vaporization of the compound. It then be- 

 comes apparent that each atom or radical occurring in the structural 

 formulae of organic chemistrv contributes a certain amount to the 

 cohesion. At a first approximation the cohesion of a molecule species 

 is composed additively of these partial contributions, and can be 

 calculated by adding up the various increments, in exactly the same 

 way as the molecular volume (according to Kopp's rule), the molecular 



