Molecular Orbitals from p Atomic Orbitals

To determine the molecular orbitals of many other molecules, we need to examine how p orbitals combine to give molecular orbitals. The p orbitals can overlap in two ways: head-to-head or sideways. Head-to-head overlap of p atomic orbitals results in a bonding and antibonding molecular orbital, where the electron density is centred along the internuclear axis, making them σ orbitals (Figure1.1 “Head-to-head overlap of p orbitals”).

Figure1.1. Head-to-head overlap of p orbitals.

Sideways overlap of the remaining four p atomic orbitals can occur along the two other axes, generating four π molecular orbitals having electron density on opposite sides of the internuclear axis (Figure1.2 “Sideways overlap of p orbitals”).

Figure1.2. Sideways overlap of p orbitals.

The head-to-head overlap giving σ molecular orbitals results in greater overlap, making its bonding molecular orbital the most stable and lowest energy, while the σ* antibonding is least stable and has the highest energy (Figure1.3 “Molecular orbital energy diagram for homonuclear diatomic molecules made from atoms of atomic number 8-10″). Sideways overlap gives four π molecular orbitals, two lower-energy degenerate-bonding molecular orbitals, and two higher-energy antibonding orbitals.

Figure1.3. Molecular orbital energy diagram for homonuclear diatomic molecules made from atoms of atomic number 8-10.

The energy diagram we have just generated fits experimentally with O₂, F₂, and Ne₂, but does not fit for B₂, C₂, and N₂. In the latter, homonuclear diatomic molecules (B₂, C₂, and N₂), interactions take place between the 2s and 2p atomic orbitals that are strong enough to swap the ordering of the σ_2p and π_2p molecular orbitals (Figure 1.4).

Figure 1.4. Molecular orbital energy diagram for homonuclear diatomic molecules made from atoms of atomic number 5-7.