Here's why:

* Cesium (Cs) is an alkali metal in group 1, meaning it forms a +1 cation (Cs⁺).

* Sulfur (S) is a nonmetal in group 16, meaning it forms a -2 anion (S²⁻).

* To balance the charges, you need two cesium ions (2 x +1 = +2) to combine with one sulfide ion (-2).