CHEMICAL BONDS 31 



makes a difference in quantum physics. The ultimate test rests on the 

 experimental value of the binding energy, and the result is unequivocally 

 in favor of quantum physics. Exchange energy does, indeed, exist. 



Fig. 11. A schematic representation 

 of the benzene molecule: C 6 H 6 . 



For biology, the exchange frequently appears in another form which 

 is called by another name: resonance. To illustrate this, consider the 

 benzene molecule shown in Fig. 11. Each carbon atom has four valence 

 electrons and therefore makes four covalent bonds. Each carbon atom is 

 bonded to one hydrogen atom. Thus, of the 24 bonding electrons initially 

 present, 18 are left for the intercarbon bonds, making three bonds per 

 atom. Figure 12 shows that there are two different ways of achieving 

 this bonding, both using the idea of forming double bonds between half 

 the pairs of atoms. As before, quantum physics tells us to count both 

 possibilities in computing the binding energy, and experiment confirms 

 this prediction. In this situation, there is a model for the exchange which 

 results in the name of "resonance" energy; the molecule is said to 

 resonate between the two forms. One can think that the electrons form- 

 ing the double bond spend half the time on one side of each carbon atom, 

 and half the time on the other side. By racing back and forth between 

 these two configurations, the electrons tend to pull the carbon atoms closer 



H H 



Fig. 12. Two equivalent representations of the benzene molecule, with all 

 the bonds indicated. There are three bonds remaining per carbon atom after 

 the bond to hydrogen has been established. The three double bonds can be 

 placed in the two ways shown, and chemists speak of the structures as resonat- 

 ing from one form to the other. 



