* Aluminum (Al) is a metal in group 3 of the periodic table. It forms a +3 ion (Al³⁺).

* Chlorine (Cl) is a non-metal in group 17 of the periodic table. It forms a -1 ion (Cl⁻).

To balance the charges, you need three chlorine ions for every one aluminum ion. Therefore, the correct formula for the compound formed by aluminum and chlorine is AlCl₃, which is called aluminum chloride.