In rock-salt structure (edge length= a), the distance between centers of cation and anion is

In rock-salt structure, anion-cation occupy the edge of cube. Hence

$r_{\mathrm{a}}+2r_{\mathrm{c}}+r_{\mathrm{a}}=a$ or $r_{\mathrm{c}}+r_{\mathrm{a}}=a/2$.

White crystals of rock salt (NaCl), an ionic compound, are found in nature. It is obtained either from the evaporation of saltwater or the mineral form halite. Repeating the face-centered cubic unit cell creates the structure of NaCl.