Final answer:
The Phosphate ion (PO₄³⁻) has a -3 charge due to the valence of phosphorus and the number of oxygen atoms bonded to it.
Step-by-step explanation:
The ion that has a -3 charge is the Phosphate ion (PO₄³⁻). The phosphate ion consists of one phosphorus atom bonded to four oxygen atoms.
Phosphorus has a valence of +5, and there are four oxygen atoms with a valence of -2 each. Using the formula for calculating the charge on oxyanions, which is valence electrons on the central atom minus twice the number of oxygen atoms, we get a charge of -3 for the phosphate ion.