The coordination number of a constituent particle (atom, ion or molecule) in a crystal is the number of constituent particles which are the immediate neighbours of that particle in the crystal. In ionic crystals, coordination number of an ion is the number of oppositely charged ions surrounding that particular ion. In fcc arrangement of $\mathrm{NaCl}$ crystal, $\mathrm{Cl}^{-}$ ions occupy the corners and face centres of the cube while $\mathrm{Na}^{+}$ions are present at the edges centres and body centres and both have coordination number of 6 .