Question

Specify the coordination number of Calcium and Fluoride ions in the compound Calcium fluoride $Ca{F_2}$.

Answer 1

Coordination number of compounds defined as the total number of spheres or particles closest to the other particles in the crystal lattice.

Complete answer:
Calcium fluoride is made up of cation of calcium ion $C{a^{ + 2}}$ and two anions of fluorine ion ${F^ - }$. They collectively form a compound which is represented as $A{B_2}$ .
These types of structures are also known as Fluoride structure.
In Calcium fluoride compound, Calcium ions $C{a^{ + 2}}$ has a tendency to present in cubic close packing $\left( {ccp} \right)$. In this arrangement, Calcium ions $C{a^{ + 2}}$ are surrounded by eight atoms of fluorine ion ${F^ - }$.
While packing into the $\left( {ccp} \right)$ arrangement one ion of calcium is present on each face of the cube to complete the crystal lattice.
In the $\left( {ccp} \right)$ arrangement, one ion of calcium $C{a^{ + 2}}$ is surrounded by eight Fluorine ion ${F^ - }$ which are present at the tetrahedral corner.
Fluorine ion ${F^ - }$ in the Calcium fluoride compound arranged in the tetrahedral arrangement and occupies all the apices of a tetrahedron. Therefore, each atom of ${F^ - }$ is surrounded by four $C{a^{ + 2}}$ ions.
From the above discussion we see that one ion of $C{a^{ + 2}}$ is surrounded by eight ${F^ - }$ ions. Hence coordination number of calcium in calcium fluoride compound is $8$.
Similarly, one ion of ${F^ - }$ is surrounded by four $C{a^{ + 2}}$ ions. Hence coordination number of fluorine in calcium fluoride compound is $4$.

Note:
Sodium oxide is arranged in the same constituent $A{B_2}$ but it is arranged in the opposite manner of calcium fluoride therefore it is known as anti fluoride structure.
The compound packed as $\left( {ccp} \right)$ arrangement are alphabetically represented as $ABCABCA....$