Question

In calcium fluoride, having the fluorite structure, the coordination numbers for calcium ion $(C{a}^{2+})$ and fluoride ion $(F^{-})$ are

• A. 4 and 2
• B. 6 and 6
• C. 8 and 4
• D. 4 and 8

Answer 1

Answer: (C)

Calcium fluoride crystallizes in a Face-Centered Cubic unit cell (FCC) having an edge length of $5.463$ Angstroms. The cell is displayed here with calcium cations (in blue) defining FCC lattice sites, and fluoride anions (in green) occupying all tetrahedral sites. Fluoride anions have 4 neighbors of opposite charge arranged at vertices of an tetrahedron. Calcium cations have EIGHT neighbors of opposite charge arrange at corners that outlinea smaller cube. So the Ca: $F$ coordination ratio is $8:4$ or $2:1$. In $4Ca{F}_2$ the the co ordination number of $C{a}^{+2}$ is 8