Here's why:

* Cesium (Cs) is in Group 1 of the periodic table, meaning it has a +1 charge as an ion (Cs⁺).

* Iodine (I) is in Group 17 of the periodic table, meaning it has a -1 charge as an ion (I⁻).

To form a neutral compound, one cesium ion (Cs⁺) combines with one iodide ion (I⁻), resulting in the formula CsI.