Distinguish between a metal and an insulator on the basis of energy band diagrams ?
Metals: (i) For metals, the valence band is completely filled and the conduction band can have two possibilities—either it is partially filled with an extremely small energy gap between the valence and conduction bands or it is empty, with the two bands overlapping each other as shown below:
(ii) On applying an small even electric field, metals can conduct electricity.
Insulators: (i) For insulators, the energy gap between the conduction and valence bands is very large. Also, the conduction band is practically empty, as shown below:
(ii) When an electric field is applied across such a solid, the electrons find it difficult to acquire such a large amount of energy to reach the conduction band. Thus, the conduction band continues to be empty. That is why no current flows through insulators.