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  4. In calcium fluoride, having the fluorite structure, the coordination numbers for calcium ion (Ca2+) and fluoride ion (F-) are

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

NEETNEET 2016The Solid State

Solution:

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 $4 CaF _{2}$ the the co ordination number of $Ca ^{+2}$ is 8