Molecular orbital theory is the key to understanding the Huckel's rule that an aromatic ring should have 4n + 2 π electrons. If you examine the molecular orbital diagram, you will see one, single lowest energy molecular orbital corresponding to π system hybridization, with successive energy levels corresponding to pairs of orbitals, if the total numbers of electron pairs is odd (number of electrons = 4n + 2), all bonding electrons will be paired; if not, then single electrons will go into the top level with spins parallel, which is not as stable.

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