Here's why:

* Aluminum (Al) is a metal in group 3 of the periodic table, and it typically forms a +3 charge in ionic compounds.

* Iodine (I) is a nonmetal in group 17 of the periodic table, and it typically forms a -1 charge in ionic compounds.

To balance the charges, you need three iodide ions (3 x -1 = -3) for every one aluminum ion (+3), resulting in the formula AlI₃.