C, Si and Ge have the same lattice structure. Why is C an insulator w…

The correct answer is Option (3) → The band gap of C is as high as 5.4 eV.

The three solids share one lattice structure. So the difference in their resistivity comes from the energy gap between their valence and conduction bands.

Option (1) C has the tendency of catenation. — Incorrect. Catenation is the chemical tendency of carbon atoms to link into chains. It does not decide whether electrons can reach the conduction band.

Option (2) C is electrically neutral. — Incorrect. Si and Ge crystals are electrically neutral too, so neutrality cannot make C behave differently from them.

Option (3) The band gap of C is as high as 5.4 eV. — Correct. The gap of C (diamond) is 5.4 eV, above the 3 eV limit for insulators. So thermal energy cannot lift electrons into its conduction band. The gaps of Si (1.1 eV) and Ge (0.7 eV) lie in the semiconductor range of 0.2 eV to 3 eV.

NCERT (2026-27):
Extrinsic Semiconductor – “… The difference in the resistivity of C, Si and Ge depends upon the energy gap between their conduction and valence bands. For C (diamond), Si and Ge, the energy gaps are 5.4 eV, 1.1 eV and 0.7 eV, respectively.”

Summary – “11. For insulators E_g > 3 eV, for semiconductors E_g is 0.2 eV to 3 eV, while for metals E_g ≈ 0.”