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:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (B) and (D) only
(A) Ionisation energy will be least for C, followed by Si and highest for Ge. — Incorrect. The order is reversed. The outer electrons of the smaller carbon atom are held most tightly.
(B) Ionisation energy will be least for Ge, followed by Si and highest for C. — Correct. The outer electrons are bound less tightly as the atom becomes larger (C, then Si, then Ge).
(C) Number of free electrons for conduction in Ge and Si are negligibly small but significant for C. — Incorrect. It is the other way round.
(D) Number of free electrons for conduction in Ge and Si are significant but negligibly small for C. — Correct. Because less energy is needed to free an electron in Ge and Si, they have a significant number of free electrons, whereas carbon (diamond) has a negligible number and is an insulator.
NCERT (2026-27):
Intrinsic Semiconductor – “… Example 14.1 C, Si and Ge have same lattice structure. Why is C insulator while Si and Ge intrinsic semiconductors?
Solution The 4 bonding electrons of C, Si or Ge lie, respectively, in the second, third and fourth orbit. Hence, energy required to take out an electron from these atoms (i.e., ionisation energy Eg) will be least for Ge, followed by Si and highest for C. Hence, number of free electrons for conduction in Ge and Si are significant but negligibly small for C.”