The formula for the compound formed between aluminum ions (Al³⁺) and phosphate ions (PO₄³⁻) is AlPO₄. Here's how to figure it out:

* Aluminum ions (Al³⁺): Aluminum has a +3 charge.

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

To form a neutral compound, the charges must balance. Since the charges are equal and opposite, one aluminum ion combines with one phosphate ion. Therefore, the formula is AlPO₄.