In an insulator, the forbidden energy gap between the valence band and the conduction band is of the order of
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 5 eV
Solids are classed as metals, semiconductors and insulators by the energy gap \( E_g \) between the valence band and the conduction band. In an insulator, this gap is large, \( E_g\gt3\,\mathrm{eV} \). So thermal energy cannot lift electrons from the valence band into the empty conduction band, and no conduction is possible. A gap of about 5 eV is of this order. For example, diamond, an insulator, has a gap of 5.4 eV.
NCERT (2026-27):
Classification of Metals, Conductors and Semiconductors – “… Case II: In this case, … a large band gap Eg exists (Eg > 3 eV). There are no electrons in the conduction band, and therefore no electrical conduction is possible. … This is the case of insulators.”