Qualitative MO theory

Accurate molecular orbitals are often a linear combination of many atomic orbitals, and their determination generally has to be done with a computer. A lot of insight can be obtained, however, by looking at the case in which just two orbitals combine to form an orbital. This is a situation that can be treated analytically. It is also a situation that at least qualitatively describes many interactions in chemistry. For example, the formation of the bonding orbital of H₂ is mainly due to the interaction of the 1s orbitals of the hydrogen atoms. An accurate description of the bonding orbital may involve many other atomic orbitals, but, unless one's interested in numerical values, the 1s orbitals suffice. Similarly why helium does not form molecules can be explained by looking at the interaction of only two 1s orbitals of helium. In fact, most interaction between two atoms, molecules, or molecular fragments can be described by the interaction of one orbital on each atom, molecule, or molecular fragment.

• Derivation of the secular equation.
• Identical interacting orbitals.
• Different interacting orbitals.
• The three rules of qualitative MO theory.
• Two applications of the rules of qualitative MO theory.