Final answer:
In the phosphate ion (PO4^3-), phosphorus has a formal charge of 0 and an oxidation number of +5.
Step-by-step explanation:
In the Lewis structure of the phosphate ion (PO43-), phosphorus is surrounded by four oxygen atoms. Phosphorus has a formal charge of 0, because it follows the octet rule by sharing its valence electrons with the oxygen atoms. Its oxidation number in this compound is +5. This is due to oxygen being more electronegative, and it effectively takes the electrons from the bonds; since there are four O atoms, each with an oxidation number of -2, this results in a total of -8 from oxygen, which, when combined with the +3 charge of the ion, yields an oxidation state of +5 for phosphorus in phosphate.