Answer
The structure of the energy bands determines whether a material is a conductor or an insulator.
Work Step by Step
This is explained in Figure 42.19. In an insulating material, the energy gap is large, and the conduction band is mostly empty. There are no easily accessible energy states for electrons in the filled valance band to go when a voltage is applied, so the material is insulating.
In contrast, in a conductor, the conduction band is already partially filled, and the energy gap between the valence band and conduction band is small.
