The Group 4A elements carbon (an insulator, in the diamond form), silicon, and germanium (semiconductors) have similar structures. Each atom is surrounded by four other atoms at the corners of a tetrahedron (Figure 12.16). Typically, we would represent the bond- ing in these elements using a localized bond model. However, the band model for bonding can also be used with advantage to explain the conductivity of these elements. In the band model, the valence orbitals (the ns and np orbitals) of each atom are combined to form molecular orbitals that are delocalized over the solid. However, unlike metals where there is a continuous band of molecular orbitals, in semiconductors there are two distinct bands, a lower-energy valence band and a higher-energy conduction band, separated by a band gap [Figure 12.14, right, and Figure 12.17]. In the Group 4A elements, the orbitals of the valence band are completely filled and the conduction band is empty. The band gap is an energy barrier to the promotion of electrons from the valence band to the higher-energy conduction band.