GIVEN :-

Edge of unit cell, a = 4.3 A⁰ = 4.3 x 10^-10 m
Density of sodium, Density = 963 kg / m³
Atomic weight, M = 23 kg / k. mol
Avogadro's number, N = 6.02 x 10²⁶ atoms / k. mol.

Density = nM / N (a)³
So, n = [Density N (a³)] / M
i.e. n = [(963 kg / m³) (6.02 x 10²⁶ atoms / k.mol) (4.3 x 10^-10m)] / (23 kg / k.mol)
i.e. n = 46.09 / 23 atoms = 2 atoms

As, number of atoms per unit cell in BCC is 2, sodium crystallizes in a BCC cell.