IXTERMOLECULAR FORCES AXD INTERACTIOX ENERGY 



241 



common ions the cavity radius is about 30-35% greater than the ionic ra- 

 dius. The accepted values of ionic radii and van der Waals' radii, determined 

 by diffraction studies, include this thermal increase in interaction distance; 

 thus the true radii are somewhat smaller. However, the cavity size in the 

 solid state may not be the same as in the liquid state or in solution. 



Fig. 6-10. Relationship between the thermal cavity size 

 and the ionic size in water. 



Orientation Factors in Repulsion Forces 



The orientation of groups or molecules relative to one another is an im- 

 portant factor in determining the magnitude of the repulsion energy, 

 just as it is a factor to be considered in evaluating the attraction energy. 

 For example, we have seen that two dipoles will attract one another with 

 forces dependent on their relative orientations; likewise the repulsion wiU 

 vary with the orientation. Hence the equilibrium energy q)^ and distance dg 

 will depend on orientation. The importance of this factor is indicated in 

 the results of Mason and Hirschfelder (1957) for the interaction of hydrogen 

 molecules (Table 6-14), where it is seen that the repulsion energy is more 

 sensitive than the attraction (dispersion) energy to orientation. Although 



