Question

An ionic compound $A{B}_2$ possesses $Ca{F}_2$ type crystal structure. Write the coordination number of $A^{2+}$ and $B^{-}$ ions in crystals $A{B}_2$.

Answer 1

Calcium fluoride has cubic structure, where each $C{a}^2+$ ion (cation) is surrounded by 8 $F^{-}$ ion and each $F^{-}$ ion (anion) is surrounded by 4 $C{a}^2+$ ions. So the coordination number of $C{a}^2+$ is 8 and coordination number of $F^{-}$ is 4.

Therefore, in case of $A{B}_2$ (which have similar crystal structure with $Ca{F}_2$), coordination number of $A^2+$ is 8 and coordination number is $B^{-}$ is 4.

Rock salt has a cubic crystal structure. Here $N{a}^{=}$ and $C{l}^{-}$ have FCC structure . Both cation and anion are surrounded by 6 oppositely charged ions. So the coordination number of each $N{a}^{+}$ (cation) and $C{l}^{-}$ (anions) is 6.