Molecular orbital theory

Molecular Orbital (MO) theory is introduced as a further theoretical advance beyond valence bond theory. VB theory gives a satisfactory electronic description of a large number of molecules, but — as O2's paramagnetism (section 5.4.7) makes clear — it gives an INCORRECT description in some cases. Rather than building a molecule's bonding atom-by-atom from individual overlapping orbitals, MO theory instead treats the WHOLE molecule as the fundamental unit under consideration: a molecular orbital plays exactly the same conceptual role for a molecule that an atomic orbital plays for a single atom. Just as an atomic orbital is derived from the square of an atom's wave function (giving the probability of finding an electron at a point around that atom), a molecular orbital is derived from the square of a MOLECULE's wave function, giving the probability of finding an electron at a point somewhere within the whole molecule. Like atomic orbitals, molecular orbitals have definite, distinct e …