<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> Resonance has a deeper significance in the mathematical formalism of valence bond theory (VB). Quantum mechanics requires that the wavefunction of a molecule obeys its observed symmetry . If a single contributing structure does not achieve this, resonance is invoked . </P> <P> For example, in benzene, valence bond theory begins with the two Kekulé structures and constructs the actual wave function of the molecule as a linear superposition of the wave functions representing the two structures . As both Kekulé structures have equal energy, they are equal contributors to the overall structure--the superposition is an equally weighted average, or a 1: 1 linear combination of the two--but this need not be the case . The symmetric combination gives the ground state while the antisymmetric combination gives the first excited state as shown . </P>

The number of canonical structures shown by the compound