Chapter III 



— 21 



Solutions 



In addition to the forces described above, dipoles and higher poles ex- 

 perience a torque making certain orientations more stabile than others. For 

 this reason the electric axis in the field of an ion tends to orient toward the 

 ion. In the field of a second dipole the axes tend to become parallel. These 

 forces are disturbed by thermal agitation so that complete orientation is pre- 

 vented under most conditions but the tendency tov^ard alignment is re- 

 flected in an attractive force termed the orientation efifect. 



When two molecules approach each other until their electronic charges 

 encounter the fields of their nuclei there are strong attractive forces be- 

 tween them. As they come even closer together their electronic clouds no 

 longer screen their nuclear charges ; the nuclei repel each other by the 

 electrostatic Coulomb forces. The strength of primary chemical bonds 

 cannot be accounted for by the attractive forces mentioned above at the 

 equilibrium point but the polarization effect does explain some types of 

 residual force. 



Even spherically symmetrical molecules are polarized in an external 

 field and the forces between the inducing particle and the induced dipole are 

 shown in numbers 4 and 5 of Table 3. Spherically symmetrical molecules 

 with a zero average field may show temporary asymmetry and hence give 

 rise to a fluctuating field. These transient fields induce transient electric 

 moments producing the dispersion forces listed in number 6 of Table 3. 



Table 4. — Contributions of the three constituents of the van der Waals forces to inter- 

 molecular forces of the liquid state {selected data front Hober (1945) and 



London (1937) : — 



Table 4 presents values for the contributions of the orientation, induc- 

 tion, and dispersion effects to the van der Waals forces between molecules 

 for six well known compounds. Common values for the van der Waals 

 constants a and b are also included. 



Although the details of atomic interaction cannot be treated here it 

 seems well established that under certain conditions, as atoms or molecules 

 come together there is a tendency toward an equilibrium distribution of 

 outer shell electrons in the internuclear region that constitutes the "electron 

 sharing" postulated by electronic theories of valence. These forces are 

 responsible for all attractions between non-ionic particles exemplified by 

 numbers 3, 5, and 6 of Table 3. 



A detailed analysis of the general theory of molecular forces is given 

 by London (1937). The above treatment follows closely that of Bateman 

 in Hober (1945). 



The development of the modern views of intermolecular forces has 

 come through many stages. Early chemists speaking in terms of chemical 



