32 PHYSICAL FORCES AND CHEMICAL BONDS 





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Fig. 13. A particle is represented with its charges distributed so that the 

 negative charges tend to be concentrated near one end and the positive charges 

 near the other. The particle is thus equivalent electrically to the dipole indi- 

 cated on the right. 



together than with single bonds. Indeed, the measured separation 

 (1.39 A) between the carbon atoms in benzene is actually intermediate 

 between the 1.54 A for a single bond and 1.34 A for a double bond be- 

 tween carbon atoms. 



The next kind of bond to be discussed is that called variously the 

 molecular energy or the van der Waals energy. This is a bond between 

 electrically neutral atoms or molecules, or groups of atoms or molecules. 

 Electric neutrality does not necessarily imply zero electric charge ; it may 

 also arise from the equality of plus and minus charges. If the particle 

 we are now discussing happens to have some of the negative charges 

 separated from an equal amount of its positive charges, it is still neutral 

 as a whole, but forms what is called a dipole, as in Fig. 13. Here the 

 separation of the charges is indicated schematically by the circled plus 

 and minus signs. Such an entity, composed of plus and minus charges, is 

 called an electric dipole, by analogy with the similar situation with 

 magnetic poles. Now, dipoles can interact electrically, too, although the 

 attraction of one charge is offset to quite an extent by the repulsion 

 exerted by the other charge of the dipole. Thus dipole forces are usually 

 quite weak. We shall not enter into a discussion of these dipole forces 

 other than to say that there are several varieties of them, all contributing 

 to the van der Waals energy, and all decreasing much more rapidly with 

 particle separation than by the regular interaction described at the be- 

 ginning of this chapter. Because these dipoles are involved in the disper- 

 sion of light, the energy involved is also frequently called dispersion 

 energy. 



The final bond to be discussed is the hydrogen bond. Consider, for the 

 moment, the OH - ion. We recall that oxygen picks up two electrons to 

 reach the nearest stable atomic configuration, and that hydrogen nor- 



