How is the oxidation number of a monoatomic ion related to its charge?
How? Well, the oxidation number of a mono-atomic ion is EQUAL to its charge.
When oxygen undergoes reduction, we would write:
The oxygen atom has undergone reduction by accepting 2 electrons to give the oxide anion. From where do the electrons come? Well typically a metal:
To write the overall redox equation, we sum the oxidation and reduction equations, weighting one of the half equations so that electrons are removed:
Elements thus have a formal oxidation state of