Page "Molecular orbital theory" Paragraph 7

An example is the MO description of benzene,, which is composed of a hexagonal ring of six carbon atoms.

In this molecule, 24 of the 30 total valence bonding electrons are located in 12 σ ( sigma ) bonding orbitals, which are located mostly between pairs of atoms ( C-C or C-H ), similarly to the electrons in the valence bond description.

However, in benzene the remaining six bonding electrons are located in three π ( pi ) molecular bonding orbitals that are delocalized around the ring.

Two of these electrons are in an MO that has equal contributions from all six atoms.

The other four electrons are in orbitals with vertical nodes at right angles to each other.

As in the VB theory, all of these six delocalized π electrons reside in a larger space that exists above and below the ring plane.

All carbon-carbon bonds in benzene are chemically equivalent.

In MO theory this is a direct consequence of the fact that the three molecular π orbitals combine and evenly spread the extra six electrons over six carbon atoms.