Question

In $Ca{F}_2$ type (fluorite structure), $C{a}^{2+}$ ions form__(A)___ structure and $F$ ions are present in all __(B)___ voids. The coordination number of $C{a}^{2+}$ is ___(C)___ and $F^{-}$ is___(D)____.
(A), (B), (C) and (D) respectively are:

• A. (A) - $ccp$; (B) - octahedral; (C) - $8$; (D) - $4$
• B. (A) - $bcc$; (B) - tetrahedral; (C) - $4$; (D) - $8$
• C. (A) - $ccp$; (B) - tetrahedral; (C) - $8$; (D) - $4$
• D. (A) - $ccp$; (B) - octahedral; (C) - $4$; (D) - $8$

Answer 1

Answer: (C)

(A) - $ccp$; (B) - tetrahedral; (C) - $8$; (D) - $4$

In ${CaF}_2$, the calcium ions form the $ccp$ (A) arrangement, i.e., these ions occupy all the corner positions and the centre of each face of the cube.

As there are two tetrahedral sites available for every calcium ion, the fluoride ions occupy all the tetrahedral sites (B). The stoichiometry of the compound is 1:2.

Each fluoride ion is surrounded by four (D) calcium ions whereas each calcium ions is surrounded by eight (C) fluoride ions which are disposed towards the corners of a cube. The coordination of the compound is 8:4.

Hence, the correct answer is option $C$.