The coordination number of both ions in a cesium chloride crystal is 8.

Here's why:

* Structure: Cesium chloride (CsCl) crystallizes in a simple cubic structure. This means that each Cs+ ion is surrounded by 8 Cl- ions, and each Cl- ion is surrounded by 8 Cs+ ions.

* Coordination Number: The coordination number refers to the number of nearest neighbors an ion has in a crystal lattice.

Therefore, both Cs+ and Cl- ions in CsCl have a coordination number of 8.