AP® Chemistry · Unit 4 · Topic 4.9
Oxidation Numbers
An oxidation number is the charge an atom would have if every bond were fully ionic, assigned by a set of rules so you can track electron movement even in covalent molecules.
Atoms in covalent molecules don't actually carry whole charges, but pretending they do gives you a bookkeeping system for electrons. Oxidation numbers are assigned using a priority order: free elements are 0, monatomic ions equal their charge, fluorine is always -1, group 1 and 2 metals are +1 and +2, hydrogen is usually +1 (except -1 in metal hydrides like NaH), and oxygen is usually -2. Whatever is left over goes to the remaining atom, found by forcing the sum of all oxidation numbers to equal the overall charge of the molecule or ion — zero for a neutral compound, the ion's charge otherwise.
This matters because real reactions often involve electrons moving from one atom to another without those atoms ever becoming free ions. Oxidation numbers let you spot that movement: if an atom's number goes up across a reaction, it lost electron 'ownership' and was oxidized; if it goes down, it was reduced. That's exactly the tool you need once you get to balancing redox equations and identifying oxidizing and reducing agents, which sit right after this topic. Without oxidation numbers, a reaction like methane burning to CO2 looks like simple rearrangement; with them, you can see carbon being stripped of electron density step by step.
The rules are a hierarchy, not a set of absolutes, because real electronegativity differences sometimes override the defaults. Oxygen's -2 assignment assumes it's the more electronegative atom pulling electrons away, which fails in peroxides (equal oxygen-oxygen bonds split the usual assignment) and fails completely against fluorine, the one element more electronegative than oxygen itself.
Key terms
- Oxidation number
- The hypothetical charge an atom would have if all its bonds were fully ionic.
- Oxidation
- A process in which an atom's oxidation number increases, meaning it loses electron ownership.
- Reduction
- A process in which an atom's oxidation number decreases, meaning it gains electron ownership.
- Peroxide
- A compound containing an oxygen-oxygen single bond, such as H2O2, where oxygen is assigned -1.
- Polyatomic ion
- A charged group of covalently bonded atoms that acts as a single unit, like MnO4-.
Watch it explained
- How it worksHow do the assignment rules let you track electrons through a messy reaction?
- Worked exampleFind the oxidation number of every atom in K2Cr2O7 and in MnO4 minus.
- Common mistakeWhy isn't the oxidation number of oxygen always minus 2?
- Going furtherWhat happens to the oxidation numbers of carbon along the series methane, methanol, and CO2?
Check yourself
End of topic.