Final answer:
The oxidation number of phosphorus in the PO₃-⁴ ion is +2.
Step-by-step explanation:
The oxidation number of phosphorus in the PO₃-⁴ ion can be determined using the known oxidation numbers of the other elements in the compound. Oxygen typically has an oxidation number of -2. Since there are three oxygen atoms in PO₃-⁴, the total oxidation number due to oxygen is -6. The overall charge of the ion is -4. To find the oxidation number of phosphorus, you can set up an equation.
The algebraic sum of the oxidation numbers of all the atoms in the ion must be equal to the overall charge. Let x be the oxidation number of phosphorus. The equation would be: x + (-6) = -4. By solving this equation, we find that the oxidation number of phosphorus in the ion PO₃-⁴ is +2.
Therefore, the correct answer is c) +2.