Introduction to Band Structure

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.