Here's why:

* Aluminum (Al) is a metal and has a +3 charge when it forms ionic bonds.

* Chlorine (Cl) is a nonmetal and has a -1 charge when it forms ionic bonds.

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

Therefore, the correct formula for the binary compound is AlCl₃.