262 PHENOMENA, ATOMS, AND MOLECULES 



the nuclei. The electric field around such ions is in general very much 

 greater than around polar and non-polar molecules, and falls off still more 

 slowly as the distance increases. The force between two ions in free space 

 varies inversely as the square of the distance between the ions. But in 

 presence of other molecules, and particularly in presence of other ions, the 

 deformation of the non-polar molecules, the orientation of the polar mole- 

 cules, and the concentration differences among the ions all tend to decrease 

 the forces between ions. The increased concentration of positive ions 

 around negative ions and vice versa causes the force between two given 

 ions to decrease with increasing distance more rapidly than according to 

 the inverse square law. Debye and Hiickel ® have recently been able to 

 develop a mathematical theory of these effects which has cleared up at 

 one stroke most of the perplexing problems involved in the theory of 

 electrolytic dissociation. 



Attractive Forces. 



From this analysis we see that there are three fundamental factors that 

 cause attractions between molecules : 



a. Deformation 



b. Orientation 



c. Segregation 



Debye has shown that two uncharged molecules, each consisting of a 

 rigid structure of positive and negative particles, would on the average 

 exert no force on one another if all orientations of the molecules are equally 

 probable. But, since all substances have dielectric constants greater than 

 unity, the particles must become deformed when they get into each other's 

 fields. This deformation is always such as to cause an attraction between the 

 molecules. 



In general, there will also be a tendency for molecules to be oriented 

 when they approach one another sufficiently closely. This orientation 

 always increases the attractive force. 



Debye has also pointed out that if, around a given ion in an electrolyte, 

 there were no concentration of ions of opposite sign, there would be no 

 resulting attraction between an ion and its neighbors. But since there is in 

 fact around any ion an increased concentration of ions of opposite sign, 

 there is a resulting attractive force which opposes the osmotic pressure of 

 the ions, due to their thermal agitation. It is this effect which we have 

 denoted by the term Segregation. It may occur not only with ions, but also 

 with dipole molecules such as those of water. By the segregation of dipole 

 molecules and their orientation about a given molecule or ion electric forces 

 may be transmitted through distances larger than would otherwise be 

 possible. 



