Electrons, crystalline solid materials energy band model

The valence bond theory is useful in explaining the structure and the geometry of molecules, but it is not suitable to explain the properties of semiconductor materials. The energy-band model for electrons can be applied to all crystalline solids and allows identifying a conductor, an insulator or a semiconductor material. Indeed, the properties of a solid are determined by the difference of energy between the different bands and the distribution of the electrons contained within. 

---