The diagram at the right represents a portion of the CsCl lattice which can be described as a simple cubic array of chloride ions (green) with cesium ions (yellow) in all cubic sites.The coordination number of an ion in an lattice of an ionic solid equals the number of ions of opposite charge that it touches.
The coordination number of Cs+ (green) is 8.
The coordination number of Cl– is 8.
The coordination number of Cl– is 8.
In general for an ionic solid having equal numbers of anions and cations, the coordination numbers of the anion and the cation are the same.
The formula for an ionic solid represents the ratio of the number of cations to the number of anions in the overall lattice and thus in the unit cell.
For CsCl there is one Cs+ (yellow) in each unit cell at its center, andthere is also one Cl– (eight Cl– at the corners each contribute 1/8)