Which of the following is correct for a hexagonal crystal system?

The correct answer is Option (3) → a = b ≠ c, α = β = 90°, γ = 120°

In the hexagonal system two edges of the unit cell are equal and the third is different (a = b ≠ c); the two angles involving the c axis are right angles (α = β = 90°) and the angle between the equal edges is γ = 120°. Graphite, ZnO and CdS are examples.

Why the other options are incorrect

• Option (1) — Incorrect. a = b ≠ c with all angles 90° is the tetragonal system.

• Option (2) — Incorrect. a ≠ b ≠ c with all angles 90° is the orthorhombic system.

• Option (4) — Incorrect. a ≠ b ≠ c with α = γ = 90°, β ≠ 90° is the monoclinic system.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Primitive and Centred Unit Cells – …