MO Theory and Covalent Bonding

Molecular orbital theory describes the distribution of electrons throughout a molecule rather than localizing them to specific bonds between atoms.

Constructive interference between in-phase atomic orbitals corresponds to greater electron density between the positively charged nuclei, making the molecule more stable. This bonding molecular orbital is lower in energy than either of the original atomic orbitals.

Destructive interference between out-of-phase atomic orbitals corresponds to lower electron density in a nodal plane between the nuclei, making the molecule less stable. This antibonding molecular orbital is higher in energy than the atomic orbitals and is marked with a star or asterisk.

Molecular orbitals are classified by the way the atomic orbitals overlap. Head-on combination of atomic orbitals along the internuclear axis, such as the overlap between two s orbitals or two end-to-end p orbitals, results in sigma molecular orbitals. The sigma orbital electron density is centered around the internuclear axis.

Sideways overlap, such as the side-on overlap of two p orbitals, results in pi molecular orbitals. Here, the electron density is concentrated on opposite sides of the internuclear axis.

The orientation of the three different p orbitals means that typically, one pair overlaps end-to-end and the other two pairs overlap sideways. The pi bonding orbitals are typically equal in energy, or degenerate, as are the pi antibonding orbitals.

Molecular orbital theory predicts the stability of covalent bonds from the bond order of the molecule, which is the number of electrons in bonding orbitals minus the number of electrons in antibonding orbitals divided by two.

A bond order of greater than zero indicates that one or more covalent bonds can exist, whereas a bond order of zero means that bonds should not exist.

Molecular orbital theory is also useful for polyatomic molecules like benzene. The Lewis model of benzene cannot accurately represent its delocalized electrons, whereas molecular orbital theory assigns those electrons to three pi bonding molecular orbitals covering the entire carbon ring.