Formation of molecular orbitals

Molecular orbitals are built from atomic orbitals through the Linear Combination of Atomic Orbitals (LCAO) method. Physically, this combination is understood as the INTERFERENCE of the electron waves belonging to the two combining atoms — and, exactly like any other kind of wave, these electron waves can interfere either CONSTRUCTIVELY (reinforcing one another) or DESTRUCTIVELY (partially cancelling one another). Correspondingly, there are exactly two ways two atomic orbitals can combine: by ADDING their wave functions together, or by SUBTRACTING one from the other. Addition of the two atomic orbitals' wave functions produces a molecular orbital that is LOWER in energy than either starting atomic orbital — this is called a Bonding Molecular Orbital (BMO). Subtraction of the two atomic orbitals' wave functions instead produces a molecular orbital that is HIGHER in energy than either starting atomic orbital — this is called an Antibonding Molecular Orbital (AMO). Physically, a bonding MO concentrates a comparatively LARGE electron density directly BETWEEN the two bonded nuclei — which is exactly the extra electron density that glues the two positively-charged nuclei together — so placing an electron into a bonding MO …