This SparkNote presents two approaches to describing orbitals in molecules: the Valence Bond (VB) model and the Molecular Orbital (MO) model. The VB model, which is an extension of Lewis structures, assumes that covalent bonds are the overlap of individual atomic orbitals. An initial difficulty with this approach is that the geometries of atomic orbitals are inconsistent with actual molecular geometries. In order to resolve this issue, we introduce hybrid orbitals, which are formed by melding atomic orbitals. We demonstrate how the VB model easily accounts for double and triple bonds, which result from the sideways overlap of unhybridized p-orbitals.
While the MO model is more complicated, it is superior to the VB model in its ability to give a qualitative assessment of orbital energies and electronic delocalization. The MO model does away with the notion that electrons are confined to their original atomic orbitals. Instead, this theory holds that electrons reside in orbitals that "belong" to the entire molecule. Atomic orbitals are therefore replaced by bonding and antibonding molecular orbitals. To a large extent, the energies of these orbitals determine the stability of the bond. This stability in turn depends on the relative size of the constituent atoms, their relative electronegativities, and the degree of physical overlap of the orbitals.
Finally, we illustrate how the simplicity of the VB model and the generality of the MO model can be used in conjunction to describe complex molecular systems like benzene in a consistent way. In such a scheme, sigma bonds are considered to be localized while the delocalized pi system is given a separate MO treatment.