(i) (a) Hexagonal unit cell : a = b ≠ c
α = β = 90°, γ = 120°
Examples are graphite, ZnO.
(b) Monoclinic unit cell : a ≠ b ≠ c
α = γ = 90°, β ≠ 90°
Examples are Na2SO4·10H2O, monoclinic S.
(ii) (a) Face-centred unit cell : Lattice points are at the corners and centre of each face.
No. of atoms per unit cell =8×

1

8

+6×

1

2

=4
(b) End-centred unit cell : Lattice points are at the corners and at the centres of two end faces.
No. of atoms per unit cell = 8×

1

8

+2×

1

2

=2