Question

The coordination number of cation and anion in fluorite $\left(Ca{F}_2\right)$ and $CsCl$ are, respectively :

• A. $8:4$ and $6:3$
• B. $6:3$ and $4:4$
• C. $8:4$ and $8:8$
• D. $4:2$ and $2:4$

Answer 1

The correct option is C $8:4$ and $8:8$

Fluorite, $Ca{F}_2$ is a cubic crystal (face-centered cubic), with coordination number of cation as 8 and that of anion as 4 (typical of a fluorite structure), hence, the ratio 8:4. Other examples of this type are: $Sr{F}_2,Cd{F}_2,Th{O}_2$ etc.

$CsCl$ has a cubic structure where Cs cation sits on cube void and Chlorine at the simple cubic lattice, with coordination number of 8 for cation and anion each. Hence, cation-anion coordination number ratio is 8:8. Other examples of this type are metallic crystals $Fe,V,Cr,TlCl$ etc.