The formula of the compound formed between aluminum ions (Al³⁺) and phosphate ions (PO₄³⁻) is AlPO₄. Here's how to determine that:

* Aluminum ions (Al³⁺) have a +3 charge.

* Phosphate ions (PO₄³⁻) have a -3 charge.

To form a neutral compound, the charges must balance. Since the charges are equal and opposite, one aluminum ion and one phosphate ion are needed. This results in the formula AlPO₄.