Here's why:

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

* Chlorine (Cl) is in group 17 of the periodic table, meaning it has a -1 charge when it forms an ion (Cl⁻).

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