Here's why:

* Cesium (Cs) is an alkali metal in Group 1 of the periodic table. It has a +1 charge when it forms an ion.

* Bromine (Br) is a halogen in Group 17 of the periodic table. It has a -1 charge when it forms an ion.

To form a neutral compound, one cesium ion (Cs+) combines with one bromide ion (Br-), resulting in the formula CsBr.