Question

A unit cell of calcium fluoride has four calcium ions. The number of fluoride ions in the unit cell is:

• A. $2$
• B. $4$
• C. $6$
• D. $8$

Answer 1

The correct option is D $8$

Formula for calcium fluoride is $Ca{F}_2$

Number of Calcium ions $=4$

So, to maintain electrical neutrality, we have

$4\times (+2)+\left(x\right)\times (-1)=0$

where $(+2)$ is charge on calcium on and $(-1)$ is charge on fluoride ion

$\Rightarrow 8-x=0\Rightarrow x=8$

So, number of $F^{-}$ ions in unit cell $=8$