The Sodium chloride structure
NaCl is an ionic crystal and in this crystal the sodium and chlorine ions are located side-by side in the FCC lattice. Like diamond it is also an FC lattice with a two atom basis. It can also be thought of as having two FCC sub-lattices . Cl ion occupies the FCC lattice and the second atom of the basis is the Na ion which is at a distance (a/2, 0, 0) from the Cl ion. Every ion in the NaCl crystal has six nearest neighbour ions at a distance of a/2. The ionic radius of sodium is about 0.98A° wheras that of chlorine ion is 1.81 A°.There are 4 molecules of NaCl in every unit cell.