Oxidation Number
The hypothetical charge an atom would carry if all its bonds were treated as fully ionic.
Formula / equation
Σ oxidation numbers = overall charge
Unit
Dimensionless
In depth
Assigned by a set of priority rules — fluorine always −1, oxygen usually −2, hydrogen usually +1 — oxidation numbers make electron transfer visible even in covalent species. A rise indicates oxidation and a fall reduction, which is the basis of one method of balancing redox equations.
Examples in the real world
Manganese is +7 in MnO₄⁻, +4 in MnO₂ and +2 in Mn²⁺; carbon ranges from −4 in CH₄ to +4 in CO₂.