What is the energy gap between valence band and conduction band in crystal of insulators

a. Both the bands are overlapped with each other

b. Very small

c. Infinite

d. Very large

Ans. d

Explanation :  When insulators (non metal atoms) interact to form a solid their atomic orbitals mix to form two bunches of orbitals separated by a large band gap. Electrons cannot therefore be promoted to an empty level, where they could move freely.