Ok, so it's to do with the number of electrons in the atom's outer shell.
Al is in
Group 3, so you know it must have
3 outer electrons. Therefore, the stable ion of Al would be
Al3+ (because it wants a
full outer shell so it 'loses' 3 electrons, giving it a charge of +3).
Cl is in
Group 7, so it has 7 electrons in its outer shell. Now, this means one chlorine atom is very reactive as it wants to
gain one more electron to get a full outer shell. When it gains another electron (e-), the charge of that ion now becomes more negative (because electrons are negatively charged). Seeing as it's only gained one electron, the charge decrease by one, therefore giving it a charge of -1.
Now that you know an aluminium ion has a charge of +3 and a chloride ion has a charge of -1, any molecule that it makes must have a charge of 0.
To make 0, you need to cancel any charges of the ions making up that molecule: Al3+ and Cl-. +3-3 = 0
So three -1 charges are needed to cancel out the +3 charge on the aluminium ion --> three Cl- ions needed --> AlCl3
Really hoped this helped