The unit cell of sodium chloride is

Explanation

Sodium chloride crystallises in a face centred cubic arrangement in which each sodium ion is surrounded octahedrally by six chloride ions and each chloride by six sodium ions, giving a coordination number of six for both. Caesium chloride, whose larger cation allows eight neighbours, adopts a body centred arrangement instead. The unit cell is simply the smallest repeating block from which the whole lattice can be built.