C, Si and Ge have the same lattice structure but C is an insulator while Si and Ge are intrinsic semiconductors because

(A) ionisation energy will be least for C, followed by Si and highest for Ge
(B) ionisation energy will be least for Ge, followed by Si and highest for C
(C) number of free electrons for conduction in Ge and Si are negligibly small but significant for C
(D) number of free electrons for conduction in Ge and Si are significant but negligibly small for C

Choose the correct answer from the options given below:

(B) and (D) only

The correct answer is Option (2) → (B) and (D) only

Carbon (diamond), Silicon, and Germanium all have the same crystal lattice structure (diamond cubic), but their electrical properties differ due to the size of their band gaps and ionization energies.

(B) Ionisation energy is highest for C, intermediate for Si, and lowest for Ge. A higher ionisation energy means electrons are more tightly bound and less available for conduction. This explains why C is an insulator while Si and Ge are semiconductors.

(D) The number of free electrons in Ge and Si at room temperature is significant enough for conduction because of their smaller band gaps, while in C, very few electrons can move to the conduction band due to its large band gap. This makes C an insulator.