(i) (a) Hexagonal unit cell : a = b ≠ c α = β = 90°, γ = 120° Examples are graphite, ZnO. (b) Monoclinic unit cell : a ≠ b ≠ cα = γ = 90°, β ≠ 90°Examples are $\mathrm{Na_2SO_4\cdot 10H_2O}$, 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 $=8\times\frac{1}{8}+6\times\frac{1}{2}=4$(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 = $8\times\frac{1}{8}+2\times\frac{1}{2}=2$