The oxidation number of an atom in a molecule or an ion is the charge that atom would have if all of its bonds were ionic. Determining oxidation numbers requires systematic consideration of the charges on all atoms and their bonds. Here's how to identify the oxidation number of phosphorus (P) in phosphate (PO4^3-) step-by-step:

1. Determine the total charge of the phosphate ion:

The phosphate ion has a charge of -3, indicated by the superscript "3-".

2. Assign the oxidation number of oxygen:

Oxygen typically has an oxidation number of -2 in most compounds, including phosphate. Since there are four oxygen atoms in PO4^3-, the total contribution of oxygen atoms is 4 x (-2) = -8.

3. Calculate the oxidation number of phosphorus:

Let the oxidation number of phosphorus be "x". To balance the overall charge of the phosphate ion, the oxidation number of phosphorus must compensate for the charge of oxygen atoms and the net charge of the ion.

Therefore, x + (-8) = -3

Solving for "x", we get x = +5

Therefore, the oxidation number of phosphorus in phosphate (PO4^3-) is +5.