Question

What is the coordination of cation and anion in flourite structure $Ca{F}_2$?

• A. $1:1$
• B. $8:4$
• C. $4:8$
• D. $6:6$

Answer 1

The correct option is B $8:4$

In fluorite structure, the $C{a}^{2+}$ forms the fcc lattice.

Thus, $C{a}^{2+}$ occupies the corners and face centres of the cube.
The number of $C{a}^{2+}$ ions in each unit cell is
$\frac{1}{8}\times 8+\frac{6}{2}=4$

$F^{-}$ ion occupies all the tetrahedral voids of the fcc. There are 8 tetrahedral voids in fcc.
Thus, the number of $F^{-}$ ions in each unit cell is 8.

Here, each $F^{-}$ ion in tetrahedral void is surrounded 4 $C{a}^{2+}$ ions and each $C{a}^{2+}$ ion is surrunded by 8 $F^{-}$ ion.

Thus, the coordination number of $F^{-}$ is 4 and coordination number of $C{a}^{2+}$ is 8
$C.N.ofCa{F}_2=8:4$