In NaCl crystal, Cl- ions are in FCC arrangement. Calculate the number of Cl- ions in its unit cell.

In NaCl crystal lattice, Chloride ions have fcc arrangement while Sodium ions are present at all octahedral voids. FCC arrangement means Chloride ions are present at the corners and at the centre of each face. There are 8 corners and 6 faces in crystal lattice. An atom present at the corner is shared by 8 unit cells. So, the contribution of each atom present at the corner is 1/8. An atom present at the face is shared by 2 unit cells. So, the contribution of each atom on face is 1/2.

$\text{Number of chloride ions in a unit cell} =(8\times \frac{1}{8})+(6\times \frac{1}{2})$
= 1 + 3
= 4