Question

In the calcium fluoride structure the coordination number of the cation and anions are respectively

• A. 6, 6
• B. 8, 4
• C. 4, 4
• D. 4, 8

Answer 1

Answer: (B)

In $Ca{F}_2,$ the $C{a}^{2+}$ ions are arranged in ccp arrangement, i.e., $C{a}^{2+}$ ions are present at all the comers and at the centre of each face of the cube. The fluoride ions occupy all the tetrahedral sites. In the $Ca{F}_2$ structure, each $F^{-}$ ion is surrounded by four $C{a}^{2+}$ ions while each $C{a}^{2+}$ ion is surrounded by eight $F^{-}$ ions. Hence, the coordination numbers of $C{a}^{2+}$ and $F^{-}$ ions are 8 : 4.