According to the band theory, why is a material such as diamond an insulator?

Question

Quizplus · Accepted Answer

In diamond, the valence band is fully occupied with electrons tightly bound to the carbon atoms in covalent bonds. The energy gap between the valence band and the conduction band is very large, which means that very few electrons can acquire enough energy to overcome this gap and move into the conduction band where they would be free to conduct electricity. Therefore, diamond is an insulator.

According to the Band Theory, Why Is a Material Such