UNION OF ANTIBODY WITH ANTIGEN 



119 



+ -+ -+ - + 



Schemes of dipole association 



First step 



Second step 



Third step 



Attraction of o dipole by an ion 

 Fig. 9-1. Schemes showing dipole-dipole association and attraction of a 

 dipole by an ion. 



may attract other dipoles as a result of coulomb forces, or attract 

 positive or negative ions (Fig. 9-1). 



Van der Waals forces constitute the most general intermolecular 

 attraction and may operate between any two molecules. They depend 

 not upon permanent but upon instantaneous dipole moments. A mole- 

 cule which has no permanent dipole moment, for example methane 

 (CH4), may have at a certain instant an instantaneous dipole mo- 

 ment when the center of charge of the rapidly moving negative elec- 

 trons surrounding the carbon nucleus lies to one side of the center 

 of charge of the positive nucleus. This instantaneous dipole moment 

 produces an instantaneous electric field which may influence another 

 molecule in the immediate neighborhood. As a result the electrons 

 of the second molecule move relative to their nucleus in such a way 

 as to produce a force of attraction for the first molecule. 



Van der Waals forces decrease very rapidly with distance, being 

 inversely proportional to the seventh power of the distance, and are 



