Question

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

Answer 1

The structure of CsCl is of primitive cubic lattice. Coordination number is the number of ions that surrounds an ion of the opposite charge. In CsCl the $C{{s}^{+}}$ ion is the centre ion which forms the simple cubic arrangement and it is surrounded by 8 $C{{l}^{-}}$ ions which occupies the interstitial sites.

Complete step by step solution:
From your chemistry lessons you have learned about the coordination number and the arrangement of molecules in the crystal lattice. Coordination number is defined as the number of ions which immediately surrounds the ion of an opposite charge within the crystal lattice. In a crystal if we talk about the coordination then it refers to the total number of atoms, molecules or ions but in the atom the term ligancy is used to refer to the coordination number of an atom. There are different ways to calculate the ligancy of a molecule as compared to the coordination number of the atoms in a crystal lattice. The central atom which is bonded to the ions, molecules or atoms is called ligands. In this question CsCl forms a simple cubic crystal of an ionic compound in which the composition of the cation $(C{{s}^{+}})$ and the anion $(C{{l}^{-}})$ are of similar size which is in a ratio of 1:1. In this crystal the cesium $(C{{s}^{+}})$ ion is a central ion and forms a simple cubic arrangement and the chloride ions $(C{{l}^{-}})$ occupies the interstitial sites in such a way that each of the chloride ion is surrounded by 8 cesium ion and each of the cesium ions are surrounded by 8 chloride ions. Therefore the coordination number of the $C{{s}^{+}}$ and $C{{l}^{-}}$ is 8 and 8.

Thus the correct option will be (A).

Note: A lattice is defined as the ordered array of the points which describes the arrangement of a particle which forms the crystal. And the unit cell of the crystal is called the lattice point. The structure of CsCl is not a BCC structure because the cesium ion and the chloride ions are not identical to each other. Radii of $C{{s}^{+}}$ is 174pm and $C{{l}^{-}}$ is 181pm.