These four tetrahedral hybrid atomic orbitals are less stable than the s and three p orbitals from which they came, because a small amount of energy is required to bring the s-orbital energy up to the energy of p before they can be hybridisied. This energy is regained several times over whn bonds are formed between these hybrid orbitals and orbitals from other atoms, so the mixing is possible. The four tetrahedral orbitals are called sp hybrid atomic orbitals.
Bonding in methane is illustrated at the right. Each of the four sp hybrid orbitals can combine with a hydrogen 1s atomic orbital to form a localised bonding and antibonding pair of MO's. The antibonding MO's are of no importance for methane because thwey are never occupied. But when one of the bonding orbitals is filled with a pair of electrons, a bond is formed between C and H. The sp hybridisation leads to the observed molecular geometry with bond angles of 109.5. Formation of four such bonds uses all of the 1s orbitals of the four hydrogen atoms, and the s, p, p and p orbitals from carbon. Filling the bonding orbitals requires all four H electrons and all four second-shell electrons from C. The 1s orbital of carbon and its electron pair are not involved in the bonding process.