So far we have used two models for chemical bonding and molecular geometry: Lewis' electron pairs to explain how atoms are held together, and valence- shell electron-pair repulsion theory (VSEPR theory) to account for the geometry of bonding. These are simple ideas, but we have pushed them about as far as they will go. The cracks and seams in the bonding theory have begun to show through-the need for delocalized electrons is one example-and rather than trying to patch over the cracks, it is wiser to develop a better theory of bonding that will include the old ideas and permit us to go beyond them. Now that we have the atomic orbitals of Chapter 8, we can use these to develop a theory of molecular orbitals that will do a much better job of accounting for the structures and properties of molecules.
The central idea in molecular orbital theory (or MO theory) is that of combining atomic orbitals (or AO's) from all the atoms in a molecule into the same number of molecular orbitals. The process is illustrated on page 2 for methane.