Here's why:

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

* Chlorine (Cl) is a nonmetal in group 17, meaning it has a -1 charge when it forms an ion.

* To form a neutral compound, we need three chloride ions (3 x -1 = -3) to balance the +3 charge of the aluminum ion.

Therefore, the formula for aluminum chloride is AlCl₃.