How can I write the formula for aluminum chloride?

Aluminum Chloride has a formula of ##AlCl_3##.

Aluminum typically is a cation and has an charge of ##Al^(+3)##. Chlorine typically is an anion and has a charge ##Cl^(-1)##.

In order to bond ionically the charges must be equal and opposite. It will take three -1 chloride ions to balance one +3 aluminum ion forming an aluminum chloride molecule of ##AlCl_3##.

I hope this was helpful. SMARTERTEACHER