Correct Answer - 2
In `CaF_(2)` (fluorite) structure,the `Ca^(2+)` ions are located at the face centred cubic lattice points and ,therfore ,have cubic closet packed arrangement ,The fluroide ions `(F^(-)`ions) occupy all the eight tetrahedral voids.Thus there are four `Ca^(2+)` ions and eight `F^(-)` ions in the unit cell of calcium fluoride.
since `F^(-)` ion is present in the tetrahedral void ,it is surrounded by four `Ca^(2+)` ions.if we imagine that the unit cell is divided into eight subcubes then each `Ca^(2+)` ion sitting at the face centre is shared by eight minicubes ,Since the centre of each subcube is captured by `F^(-)` ions we can say that each `Ca^(2+)` ions is surrounded by `8F^(-)` ions.
