Here's why:
* Cesium (Cs) is in group 1 of the periodic table, making it an alkali metal with a +1 charge.
* Iodine (I) is in group 17 of the periodic table, making it a halogen with a -1 charge.
To form a neutral compound, one cesium atom (Cs+) combines with one iodine atom (I-) resulting in CsI.
