The ratio of Aluminum atoms to Chlorine atoms in a molecule of Aluminum Chloride (AlCl3) is 1:3.

Here's why:

* Aluminum (Al) has a +3 charge.

* Chlorine (Cl) has a -1 charge.

To balance the charges, you need one Aluminum atom (with +3 charge) and three Chlorine atoms (each with -1 charge). This gives a total charge of +3 - 3 = 0, resulting in a neutral compound.