In our studies of atoms and molecules we have seen that molecular orbitals are formed by the interaction of atomic orbitals. In molecular orbitals of H2 for example, the binding orbital is separated from the antibonding orbital by several electron-Volts ( 1eV = 23.06 kcal / mol ). When three orbitals from three atoms ( as in the case of H3 ) combine to form MOs, we get three orbitals ( , and ), a bonding, a nonbonding and an antibonding orbital. When N atomic orbitals from N atoms interact, they give N molecular orbitals. When N becomes very large, as in the case of a solid, there will be a large number of orbitals in a given range of energy, say between and . This collection of orbitals or energy levels is referred to as an (energy) band. The study of band structure forms a central theme in the study of electronic structure of solids.