Notice that the layout of the energy levels in benzene is a regular hexagon with its apex point ing downwards. It turns out that the energy level diagram for the molecular orbitals resulting from the combination of any regular cyclic arrangement of p orbitals can be deduced from the appropriately sided polygon with an apex pointing downwards. The horizontal diameter (the red line) represents the energy of a carbon p orbital and any energy levels on this line repre sent non-bonding molecular orbitals. All molecular orbitals with energies below this line are bonding; all those above are antibonding.

It’s worth noting a few points about these energy level diagrams: n = 8

• The method predicts the energy levels for the molecular orbitals of planar, cyclic arrangements of identical atoms (usually all C) only.

• There is always one single molecular orbital lower in energy than all the others. This is because there is always one molecular orbital where all the p orbitals combine in phase.

• If there is an even number of atoms, there is also a single molecular orbital highest in energy; otherwise, there will be a pair of degenerate molecular orbitals highest in energy.

• All the molecular orbitals come in degenerate pairs except the one lowest in energy and (for even-numbered systems) the one highest in energy.