Question

What is the coordination number of $C{{l}^{-}}$ in $CsCl$ structure?

Answer 1

The bonds are not clear in the solid-state structures for crystals. In that case, the value of the coordination number of the central atom equals the total number of neighboring atoms to a specific atom in a crystal depends on the location of the atom in the crystal.

Complete step by step solution:
The coordination number is the number of ions that immediately surrounded an ion of the opposite charge within a crystal lattice. In a crystal it refers to the total number of atoms, ions, or molecules to the atom, the term which refers to the coordination number of an atom is Ligancy.
The central atom which is bonded to the atoms, ions, or molecules is called ligands. The ligancy of molecules is calculated differently when compared to the coordination number of a central atom in a crystal.
When a simple cubic crystal forms an ionic compound by a composition of cations and anions of similar size in a 1:1 ratio. For example, Cesium chloride, CsCl with $C{{s}^{+}}\And C{{l}^{-}}$ having radii of 174pm, and 181pm respectively.
Cesium ions form a simple cubic arrangement and chloride ions occupy the interstitial sites such that each chloride ion surrounded by 8 cesium ions and each Cesium ion is surrounded by 8 chloride ions. So the coordination ratio of both ions is 8:8.

The coordination number of $C{{l}^{-}}$ in $CsCl$ structure is 8.

Note: One $C{{s}^{+}}\And C{{l}^{-}}$ion is present in the unit cell of a simple cubic with cesium ion in the center or cesium forming a unit cell, or cesium ions overlapping unit cells formed by chloride ions of simple cubic unit cells. CsCl is not a BCC structure cesium ion and chloride ions are not identical to each other.