Summary: Orbitals
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.
