PO4³⁻(phosphate anion) is a polyatomic anion which first of all means that he has more then one atom in his structure. ³⁻means that he is negatively charged, so that's why he is an anion. ³⁻ also means that he has 3 extra electrons, 3 more electrons than there are protons in the nuclei of a phosphorus atom plus oxygen atoms.
