Theories of Covalent Bonding

Diatomic Molecules

Molecular bond theory can explain the properties of homonuclear diatomic molecules such as the number of unpaired electrons and paramagnetic properties originating from the unpaired electrons.

A homonuclear diatomic molecule is a molecule consisting of two atoms of the same element, bound together by one or more covalent bonds. Oxygen and hydrogen are examples of elements that form homonuclear diatomic molecules. Molecular orbital theory is frequently used to explain the properties of homonuclear diatomic molecules. Fractional bond orders can account for the stability of the H2+ and H2 ions, which cannot be explained by Lewis structures or valence bond theory. Molecular orbital theory has this key advantage over Lewis structures and valence bond theory.

Oxygen (O2) molecules have eight molecular orbitals: σ2s\sigma_{2s} and σ2s{\sigma_{2s}}^*, σ2p\sigma_{2p} and σ2p{\sigma_{2p}}^*, and two each of π2p\pi_{2p} and π2p{\pi_{2p}}^*. Each oxygen atom, however, has six valence electrons, so an O2 molecule has 12 valence electrons. The electrons fill the orbitals in order of increasing energy, so there are two electrons in σ2s\sigma_{2s}, two in σ2s{\sigma_{2s}}^*, two in σ2p\sigma_{2p}, and two in each of the π2p\pi_{2p} orbitals. Once these five orbitals are filled with five pairs of electrons (10 total), there are still two more electrons to account for. Electrons enter orbitals of the same energy level singly before pairing up, so the last two electrons are split into the two π2p{\pi_{2p}}^* orbitals. This means that according to molecular orbital theory, an O2 molecule has two unpaired electrons. These unpaired electrons explain why O2 is paramagnetic. Unpaired electrons induce a small magnetic field.
The molecular orbital diagram reveals the electron configuration for a molecule of diatomic oxygen, O2. Because of the relative energies of the molecular orbitals, the 16 electrons in O2 are all paired except for the two in the π2p{\pi_{2p}}^* antibonding orbitals. These unpaired electrons give rise to O2's experimentally observed paramagnetism.