Eight electrons are required to fill an s and three p orbitals, and when the total number of electrons used for the bonds and lone pairs is eight, a stable molecular structure results. This is called the octet rule and is useful when qualitatively discussing the molecular structures of main group compounds. Of course, this rule is not applied to a hydrogen molecule, H2, but is applicable to covalent molecules, such as simple two-atomic molecules O2 or CO and even to complicated organic compounds. For the