Question

# Coordination number of $C{s^ + }$ and $C{l^ - }$ in $CsCl$ crystal are: A. $8,8$B. $4,4$ C. $6,6$D. $8,4$

Before calculating the coordination number of $CsCl$ lattice first we need to know the type of unit cell it has and the cesium ions in $CsCl$ forms the simple cubic arrangement and the chloride ions occupy the interstitial sites or is present at the center of the lattice, It can be vice-versa as well that the cesium ions occupy the center of the lattice and the chloride ions show the simple cubic arrangement.
Hence, the center ion is the $C{s^ + }$ ion which is surrounded or directly touching the eight $C{l^ - }$ ions present at the corners of the cube. Similarly, each $C{l^ - }$ ion present at the corners is also surrounded by or directly touching to the eight $C{s^ + }$ ions present at the center of the surrounding unit cells. Thus, the coordination numbers in this type of crystal are both $8$.  This can be easily explained with the help of a diagram as follows:
Therefore, the correct answer is $8,8$ (Option A).
Note:A student may confuse about that why $CsCl$ is not a body-centered lattice as one of the ions sits at the body center and the other at the corner well this is because for a lattice to be body-centered it requires the same ion to occupy the corners and the center of the lattice.