Molecular orbital theory rests on the notion that atomic orbitals are combined to form molecular orbitals. Because electron density from each atom is spread out over the span of the entire molecule, the electrons are lowered in energy. This accounts for the stabilization that occurs during bonding. The amount of the stabilization depends on the amount of overlap between atomic orbitals and the difference in energy between them. Atomic orbitals that overlap effectively produce stable molecular orbitals. One condition for overlap is that the overlapping atomic orbitals must be of similar energies.
The bonding in homonuclear and heteronuclear diatomic molecules will be discussed to show how molecular orbital theory works. Due to the complexities of describing the molecular orbitals in polynuclear molecules, we will introduce the notion of bonding through hybridized atomic orbitals to account for the bonding in such systems.
Molecular orbital theory can give us information about both ionic and covalent molecules and naturally predicts which molecules will be ionic and which will be covalent. It is a powerful and complex tool available to chemists for predicting the properties of molecules. In this SparkNote, only a brief introduction to molecular orbital theory will be given. The interested reader should consult the further reading section accompanying this document to gain access to more detailed treatments.