1. Understand Oxidation Numbers
* Oxidation numbers represent the hypothetical charge an atom would have if all its bonds were 100% ionic.
* They are used to track electron transfer in chemical reactions.
2. Know the Rules
* Oxygen (O): Usually has an oxidation number of -2, except in peroxides.
* Hydrogen (H): Usually has an oxidation number of +1, except in metal hydrides where it's -1.
* The sum of oxidation numbers in a polyatomic ion equals the charge of the ion.
3. Apply the Rules
* Oxygen: There are four oxygen atoms, each with an oxidation number of -2, giving a total of -8.
* Hydrogen: There's one hydrogen atom with an oxidation number of +1.
* Overall Charge: The hydrogen phosphate ion has a charge of -2.
4. Solve for Phosphorus
Let 'x' be the oxidation number of phosphorus:
x + (+1) + (-8) = -2
x - 7 = -2
x = +5
Therefore, the oxidation number of phosphorus in the hydrogen phosphate ion (HPO₄²⁻) is +5.
