In crystal structure of sodium chloride, the arrangement of ${\mathrm{Cl}}^{-}$ ions is

The $NaCl$ unit cell is made up of ${\mathrm{Na}}^{+}$ions and ${\mathrm{Cl}}^{-}$ions. The number of ${\mathrm{Na}}^{+}$ and ${\mathrm{Cl}}^{-}$ ions in the unit cell of $NaCl$ is calculated using four different types of site-unique centre location, face site, edge site, and corner site. A corner site would be shared by seven more unit cells when calculating the number of ions. As a result, one corner would be one-eighth of an ion. It is shared by additional unit cells for a face site and by three additional unit cells for an edge site. A FCC unit cell with four cations and four anions is $NaCl$. This can be demonstrated by counting the ions and multiplying the results according to their positions.

${\mathrm{Na}}^{+}:1_{\text{center }}+{12}_{\text{edge }}\times \frac{1}{4}=4$

${\mathrm{Cl}}^{-}:4_{\text{face }}\times \frac{1}{2}+8_{\text{corner }}\times \frac{1}{8}=4$