Here's why:

* Aluminum is a metal in group 13 of the periodic table. This means it has 3 valence electrons (electrons in its outermost shell).

* Chlorine is a nonmetal in group 17, with 7 valence electrons. It needs one more electron to complete its octet.

* To form a stable compound, aluminum will lose its 3 valence electrons, becoming a +3 ion (Al³⁺). Each chlorine atom will gain one electron, becoming a -1 ion (Cl⁻).

* To balance the charges, we need two chlorine ions for every aluminum ion, resulting in the formula AlCl₃.

Therefore, aluminum can form both AlCl₃ and AlCl₂. While AlCl₂ is less common and less stable than AlCl₃, it can exist in certain conditions.