Here's how to determine the formula of the ionic compound formed between aluminum (Al) and phosphorus (P):

1. Determine the Charges:

* Aluminum (Al) is in group 3A, so it typically forms a +3 ion (Al³⁺).

* Phosphorus (P) is in group 5A, so it typically forms a -3 ion (P³⁻).

2. Balance the Charges:

* Since the charges are already equal and opposite (+3 and -3), the formula is simply AlP.

Therefore, the formula of the ionic compound formed between aluminum and phosphorus is AlP.