MO diagrams of interacting orbitals

The following MO diagram shows how the orbitals of O₂ are formed from those of the oxygen atoms.

The p₁ and the p₂^* levels are the bonding and antibonding levels, respectively, formed from the 2p orbitals that are perpendicular to the molecular axis. The two 2p orbitals lying along the molecular axis form s₅ and s₆^*. Strictly speaking these 2p orbitals also contribute to s₃ and s₄^*, and the 2s orbitals contribute to s₅ and s₆^*. These contributions are relatively small, however, and are therefore not shown.

Such MO diagrams as the one above can be drawn for many other molecules as well. They can be obtained from quantum chemical calculations, or from applying the rules of qualitative MO theory. Their main purpose is to show why a molecule is formed by showing which electrons get a lower energy when the molecule is formed. For O₂ we see that the electrons in s₅ have a lower energy than two electrons in 2p orbitals of oxygen atoms. The two electrons in p₂^* have a higher energy, but this is more than compensated by four electrons in p₁.

The occupation of the orbitals tells us the bond order of a bond, and what type of bond it is. For O₂ we have a bond order of two (i.e., a double bond), and there is a s and a p bond. Each pair of electrons in a bonding orbital increases the bond order by one. Each pair of electrons in an antibonding orbital decreases the bond order by one. The bonding s₃ an the antibonding s₄^* lead to no net bonding, but, because s₆^* is vacant, s₅ is a s bond. There are two more electrons in the p bonding then in the antibonding orbitals, so we have one p bond as well.

MO diagrams

• Basics of MO diagrams.
• MO diagrams of interacting orbitals (this page).
• MO diagrams and bond angles.