$Na _{2} O$ has antifluorite structure. $O ^{2-}$ ions constitute a ccp
lattice and $Na ^{+}$ ions occupy the tetrahedral voids, each $O ^{2-}$ ion is in contact with $8 Na ^{+}$ ions and each $Na ^{+}$ ion is in contact with $4 O ^{2-}$ ions. Thus $C$. $N$. of sodium in $Na _{2} O =4$.