Hybridisation

Simple orbital-overlap arguments (as in section 10.8.2) work well for small, symmetric diatomic molecules such as hydrogen and fluorine, but they fail for polyatomic molecules such as methane, ammonia, or beryllium chloride. Take methane: it has been shown experimentally that CH₄ has a perfectly tetrahedral structure, with all FOUR C-H bonds completely equivalent (same length, same strength, same 109°28' angles to each other). Yet carbon's UN-hybridised valence orbitals -- 2s² 2pₓ¹ 2p_y¹ (with 2p_z empty in the ground state) -- have DIFFERENT energies and shapes from one another, and there are only two unpaired p electrons available for bonding, not four. Simple overlap of these unequal, mismatched atomic orbitals with hydrogen's 1s orbitals cannot explain methane's four IDENTICAL bonds. …