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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3ss182

Which statements about $CaF X_{2}$ and $Na X_{2} O$ are correct?

AIn $CaF X_{2}$ the $Ca$ - $F$ distance equals $\frac{3}{4} a$ .
BIn $CaF X_{2}$ , $Ca X^{2 +}$ ions form the fcc lattice and $F X^{-}$ ions fill all tetrahedral voids.
CThe coordination number of $O X^{2 -}$ in $Na X_{2} O$ is 4.
DA unit cell of $Na X_{2} O$ contains 8 $Na X^{+}$ and 4 $O X^{2 -}$ ions.

Answer: A, B, D

CaF X_{2}

has fcc

Ca X^{2 +}

(4 per cell) and 8

F X^{-}

in the 8 tetrahedral voids (true).

Na X_{2} O

is antifluorite with 8

Na X^{+}

and 4

O X^{2 -}

per cell (true). Each

O X^{2 -}

has 8

Na X^{+}

neighbours, not 4 (false). A tetrahedral-void ion is

\frac{3}{4} a

from the lattice ion (true).

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