The term band structure identifies the size of various bands, the space between adjacent bands and the occupation of the levels in the bands by electrons. In the solids established by systems such as uncommon gas atoms, all of the bands (corresponding to full shells in atoms) are totally filled up with electrons and the unfilled bands (corresponding unoccupied orbitals in the atoms) are fully empty. There is no discernible movement of these electrons when the system is subjected to an external electric field and the substance behaves as an insulator. The same is true for diamond (Fig 1) where the filled bonding sp3 band is widely separated from the unfilled antibonding sp3 by a considerable gap and the substance is a well known insulator.