Question

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

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

Answer 1

Answer: (A)

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 outline a 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