We can use the concept of resonance to explain these characteristics of the benzene
molecule. There are two Kekulé structures with exactly the same energy: they
differ only in the positions of the double bonds. As a result of resonance between
these two equivalent structures (14), the electrons shared in the C£C double bonds
are delocalized over the whole molecule, thereby giving each bond a length intermediate
between that of a single and that of a double bond. Resonance makes all six
CßC bonds identical; this equivalence is implied by representing the double bonds
in the resonance hybrid with a circle (15). It can be seen from (16) why there can be
only one 1,2-dichlorobenzene. Finally, as we have remarked, an important consequence
of resonance is that it stabilizes a molecule by lowering its total energy. This
stabilization makes benzene less reactive than expected for a molecule with three
carbon–carbon double bonds.