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When two AOs combine linearly (overlap) to form MOs (Figs ), the bonding MO formed by positive LCAO has lower energy than each of the starting AOs, while the antibonding MO formed by negative LCAO possesses higher energy than the AOs [4, 5]. As a result, the electrons from the starting AOs flow into the lower‐energy‐level bonding MO upon the formation of the molecule and the antibonding MO with a higher energy level remains empty. The overall energy decreases. The diagrams showing correlations of AOs and the resulting MOs and their relative energy levels are called molecular orbital diagrams. Figs show the MO diagrams for the formation of H₂ from hydrogen atoms, the formation of F₂ from fluorine atoms, and the formation of a π‐bond from p orbitals. All the MO diagrams clearly indicate the energy gains (driving forces) for the formation of a molecule (or a bond) from individual atoms. Figure 1.11 shows MO diagrams for the formation of a conjugate π‐bond from p orbitals. The MO diagrams also indicate that any MOs whose energies are lower than those of the starting AOs are bonding orbitals, responsible for the formation of the molecule. If an MO has the same energy as that of a starting AO, it is a nonbonding orbital which does not make any contribution to the formation of the molecule. Any MOs whose energies are higher than those of the starting AOs are antibonding orbitals, responsible for the dissociation of the molecule if they are populated with electrons [5].

FIGURE 1.11 Formation of conjugate π bonds from p orbitals in (a) the allyl radical (CH₂=CHCH₂˙) and (b) the 1,3‐butadiene (CH₂=CHCH=CH₂) molecule.

FIGURE 1.12 Resonance stabilization of benzene.