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  4. The Ca2+ and F- ions are located in CaF2 crystal, respectively at face centred cubic lattice points and in

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Q. The $Ca^{2+}$ and $F^-$ ions are located in $CaF_2$ crystal, respectively at face centred cubic lattice points and in

AIIMSAIIMS 2015The Solid State

Solution:

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