Final answer:
In an insulator, the energy gap between the valence band and conduction band is larger than in a semiconductor. Insulators have a band gap greater than approximately 1 eV, making them poor conductors of electricity.
Step-by-step explanation:
In an insulator, the forbidden energy gap between the valence band and conduction band is larger than in a semiconductor. Insulators have a band gap greater than approximately 1 eV, which means a substantial amount of energy is required to move electrons from the filled valence band to the empty conduction band. As a result, insulators are poor conductors of electricity.
For example, the energy gap of diamond, which is an insulator, is several electron-volts.
This large band gap in insulators makes it difficult for electrons to move between bands, and they remain bound to their atoms, resulting in the inability to conduct electricity effectively.