Determine the oxidation number of:?

Question

P in PH3

P in PO43-

P in HP2O73-

Answer 1

P = +3  (P is less electronegative than H, so it gets the positive oxidation number and H is -1)

P = +5  (4 oxygens = -8, so P = +5 to make the overall charge -3)

P = +7  (7 oxygens = -14, H = +1, so 2P = +14 to make the overall charge -3)

Answer 2

P is -III in PH3 (just as N is -III in ammonia) (N.B. Phosphine is obtained by reducing phosphorus, not oxidising it.)

P is +V in (PO4)3-

P is +V in (HP2O7)3-