Here's why:

* Aluminum (Al) is in group 3 of the periodic table, meaning it typically forms a +3 ion (Al³⁺).

* Chlorine (Cl) is in group 17, meaning it typically forms a -1 ion (Cl⁻).

* To balance the charges in AlCl₃, you need three chloride ions (3 x -1 = -3) to balance the +3 charge of the aluminum ion.