Key ideas of MO theory

Seven key ideas summarise molecular orbital theory as a whole. First, molecular orbitals (MOs) are to a MOLECULE what atomic orbitals (AOs) are to a single ATOM — each is a region of space, derived from a wave function, describing the probability of finding an electron there. Second, MOs are formed by combining the AOs of different atoms, and the total NUMBER of MOs produced always exactly equals the number of AOs that combined to form them (so, for instance, combining two 1s AOs always yields exactly two MOs — one bonding, one antibonding — never more or fewer). Third, only AOs of comparable energy AND matching symmetry (section 5.5.2's three conditions) can combine into MOs at all. Fourth, an MO that ends up LOWER in energy than the starting AOs is classed as a bonding MO, while an MO that ends up HIGHER in energy than the starting AOs is classed as an antibonding MO. Fifth, electrons fill the available MOs starting from the LOWEST-energy MO first, working upward — exactly the same Aufbau-style logic used for filling atomic orbitals. Sixth, each individual MO — again exactly like an atomic orbital — can hold at most TWO electrons, and those two electrons must have OPPOSITE (paired) spins. Seventh, and finally, …