Here's how we arrive at this:
* Aluminum (Al) is a metal in group 3A, meaning it forms a +3 cation (Al³⁺).
* Bromine (Br) is a nonmetal in group 7A, meaning it forms a -1 anion (Br⁻).
To form a neutral compound, we need three bromide ions (3 x -1 = -3) to balance the +3 charge of the aluminum ion.
