Which band is characterized as being full in insulators?

In insulators, the valence band is characterized as being full. The valence band consists of the highest energy levels that are fully occupied by electrons in a material. This band contains all the electrons that are involved in bonding and conduction through the solid when it is at absolute zero temperature.

In insulators, there is a significant energy gap between the valence band and the conduction band, meaning that the conduction band is empty at room temperature. Since the valence band is full, electrons do not have available states to move into and cannot easily gain energy to jump into the conduction band. This inability to promote electrons to the conduction band is what results in the material having very low electrical conductivity.

Understanding this is crucial because in semiconductors or conductors, the valence band may not be completely full or there may be a much smaller energy gap, allowing for easier conduction of electricity. In contrast, the full valence band in insulators signifies the lack of free charge carriers, underpinning their insulating properties.