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In calcium fluoride, having the fluorite structure, the coordination numbers of \(\mathrm{Ca^{2+}}\) ion and \(\mathrm{F^-}\) ion are

Question and options

In calcium fluoride, having the fluorite structure, the coordination numbers of Ca2+\mathrm{Ca^{2+}} ion and F−\mathrm{F^-} ion are
  1. 4 and 2
  2. 6 and 6
  3. 8 and 4
  4. 4 and 8

Answer and explanation

Correct answer: C - 8 and 4
In fluorite CaF2\mathrm{CaF_2}, Ca2+\mathrm{Ca^{2+}} forms FCC with F−\mathrm{F^-} in all tetrahedral holes: each Ca2+\mathrm{Ca^{2+}} has 8 F−\mathrm{F^-} neighbours and each F−\mathrm{F^-} has 4 Ca2+\mathrm{Ca^{2+}}, so 8 and 4. Coordination 6 and 6 is the rock-salt (NaCl) arrangement, not fluorite.

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