Question

The $C{a}^{2+}$ and $F^{-}$ ions are located in $Ca{F}_2$ crystal, respectively at face centred cubic lattice points and in

• A. tetrahedral voids
• B. half of tetrahedral points
• C. octahedral points
• D. half of octahedral voids

Answer 1

Answer: (A)

In $Ca{F}_2$ radius ratio $=0.73$
It suggested its crystal structure to be body centered cubic. In this each $C{a}^{2+}$ ion is surrounded by eight $F^{-}$ions, i.e., co-ordination number is eight and for fluorine it is $4$ .
In fluorite the $C{a}^{2+}$ ions are too small to touch each other, hence the structure is not strictly close packed structure relative positions of $C{a}^{2+}$ ions are like ions in a cubic close packed structure in which all the tetrahedral holes are occupied by the $F^{-}$ ions.