The oxidation number of **Phosphorus (P) is the unknown here**. Let us consider it as **x**.

The Oxidation Number of Hydrogen (H) is + 1

The Oxidation Number of Oxygen (O) is -2

The H_{3}PO_{2} is a neutral molecule, so the overall charge is zero (0).

Using the values in the formula,

H_{3}PO_{2} = 3(1) + x + 2(-2) = 0

H_{3}PO_{2 }= 3 + x -4 = 0

x -1 = 0

x = 1

The Oxidation Number of P in H_{3}PO_{2} is **+ 1**.

To confirm, substitute the value obtained in the formula, the answer should be zero as the charge on the molecule is zero.

H_{3}PO_{2} = 3(1) + 1 + 2(-2) = 4-4 = 0