- Sodium (Na): +1

- Hydrogen (H): +1

- Phosphorus (P): +5

- Oxygen (O): -2

To calculate the oxidation number of the disodium phosphate molecule as a whole, we consider the charges of its constituent atoms and balance them out:

2 (+1) + 1 (+1) + 1 (+5) + 4 (-2) = 0

Therefore, the oxidation number of disodium phosphate is 0.