1. There are two major theories for describing chemical bonding between atoms: valence bond theory and molecular orbital theory. 2. Valence bond theory describes bonding as the overlap of atomic orbitals to form shared pairs of electrons between atoms. For H2, the 1s orbitals of the two hydrogen atoms overlap to form a sigma bond with shared electron density between the nuclei. 3. Molecular orbital theory uses atomic orbitals to construct molecular orbitals delocalized over the whole molecule. For H2, combining the 1s atomic orbitals yields a lower energy bonding molecular orbital and a higher energy antibonding molecular orbital, with two electrons filling the bonding orbital to form the stable H2 molecule.