Oxidation numbers
An oxidation number is the charge an atom would have if all its bonds were ionic. It is a bookkeeping tool for following electrons through a reaction.
Rules, applied in order:
- An atom in a pure element is 0: Na, O2, Cl2.
- A single-atom ion equals its charge: Na+ is +1.
- Fluorine is always -1 in compounds.
- Hydrogen is +1 when bonded to nonmetals.
- Oxygen is usually -2.
- The numbers add up to zero in a neutral compound, or to the charge of a polyatomic ion.
The order matters. An earlier rule wins when two conflict. In hydrogen peroxide, H2O2, hydrogen is +1 by rule 4, so each oxygen must be -1 for the total to reach zero.
Worked example: sulfur in H2SO4
Hydrogen is +1 and there are two, giving +2. Oxygen is -2 and there are four, giving -8. The total must be zero.
(+2) + S + (-8) = 0
S = +6
Worked example: manganese in MnO4−
Four oxygens give -8. The total must equal the ion’s charge, -1.
Mn + (-8) = -1
Mn = +7
Common mistake
Forgetting to multiply by the subscript. In H2SO4 the oxygens contribute 4 × (-2) = -8 in total, not -2.