AP Chemistry Oxidation Numbers
The priority-ordered rules, worked examples, and how they identify redox reactions.
Redox reactions are identified by changes in oxidation number, but figuring out an atom's oxidation number in the first place follows a specific set of rules applied in priority order. Here's the rule set, worked through real compounds and polyatomic ions, including the two exceptions that trip up almost everyone.
The Rules, in Priority Order
Apply these from top to bottom, an earlier rule always overrides a later one when they'd otherwise conflict:
- A free (uncombined) element is 0. The oxidation number of Fe in solid iron, or of each O in O₂ gas, is 0.
- A monatomic ion's oxidation number equals its charge. Na⁺ is +1, Cl⁻ is −1.
- Fluorine is always −1 in every compound, no exceptions.
- Oxygen is usually −2, except in peroxides (like H₂O₂), where each oxygen is −1 instead, since the two oxygens are bonded to each other rather than each pulling a full −2 from a separate atom.
- Hydrogen is usually +1, except in metal hydrides (like NaH), where it's −1, since a metal is less electronegative than hydrogen and loses electron-holding priority to it.
- Group 1 metals are always +1 and Group 2 metals are always +2 in compounds.
- The oxidation numbers in a neutral compound sum to 0; in a polyatomic ion, they sum to the ion's overall charge. This is the rule that lets you solve for an unknown.
Worked Example: Finding an Unknown in a Neutral Molecule
Nitric acid, HNO₃. Hydrogen is +1 (rule 5), oxygen is −2 each (rule 4), three oxygens total −6. The molecule is neutral, so everything must sum to 0: 1 + x + (−6) = 0, solving for nitrogen's oxidation number, x = +5.
Worked Examples: Finding an Unknown in a Polyatomic Ion
Permanganate, MnO₄⁻. Four oxygens at −2 each = −8. The ion's overall charge is −1, so: x + (−8) = −1, giving manganese an oxidation number of +7.
Dichromate, Cr₂O₇²⁻. Seven oxygens at −2 each = −14. The ion's overall charge is −2, so: 2x + (−14) = −2, giving 2x = 12, so each chromium has an oxidation number of +6. (Both of these ions also appear in the polyatomic ions reference, if the names and formulas themselves need a refresher.)
Connecting to Redox Reactions
Once you can assign oxidation numbers, identifying a redox reaction is direct: a species is oxidized if its oxidation number increases between reactants and products, and reduced if it decreases. If every atom's oxidation number stays the same on both sides, nothing was actually oxidized or reduced, no matter how the equation looks. This is the exact mechanism behind the Unit 4 rule that redox reactions are identified by oxidation number changes rather than by inspecting which species "gave" or "took" electrons directly.
Common Mistakes
- Treating oxygen as always −2. The peroxide exception (−1) is easy to forget, and it changes the entire calculation for every other atom in the compound.
- Forgetting the metal hydride exception for hydrogen. Hydrogen is +1 almost everywhere, but flips to −1 when bonded to a less electronegative metal.
- Getting the sum-to-zero (or sum-to-charge) sign wrong. Double-check whether the species is neutral (sum to 0) or an ion (sum to its actual charge, including the sign).
- Confusing oxidation number with real ionic charge for covalent compounds. Oxidation number is a bookkeeping tool, not a claim about literal charge, except in true ionic compounds.
Related Resources
- Unit 4 Review: Chemical Reactions
- AP Chemistry Polyatomic Ions
- Unit 9 Review: Thermodynamics and Electrochemistry
- AP Chemistry Naming Compounds
- AP Chemistry Periodic Table
- AP Chemistry Study Guide
Frequently Asked Questions
What is an oxidation number?
An oxidation number (also called oxidation state) is a bookkeeping value assigned to an atom that tracks how electrons are distributed, as if every bond were fully ionic. It's not always a real charge, but it lets you track electron transfer in a reaction even when the bonding is covalent.
Why is fluorine always -1 but oxygen only usually -2?
Fluorine is the most electronegative element, so it never gives up electron-holding priority to anything, making its oxidation number always -1 in every compound. Oxygen is highly electronegative but not the most electronegative, so in the rare case it bonds to something that pulls even harder on electrons (like in a peroxide, where two oxygens bond to each other), its oxidation number changes to -1 instead of the usual -2.
How do you find oxidation numbers in a polyatomic ion?
Use the same rules as a neutral compound, except the oxidation numbers have to add up to the ion's overall charge instead of zero. For dichromate (Cr₂O₇²⁻), the two chromium atoms and seven oxygens (each -2) must sum to -2 overall, which is what lets you solve for chromium's unknown oxidation number.
Is oxidation number the same as ionic charge?
Only for true ionic compounds made of monatomic ions, like NaCl, where sodium really is +1 and chlorine really is -1. For covalent compounds and polyatomic ions, oxidation number is an assigned bookkeeping value based on electronegativity, not a literal charge, even though it's calculated and used the same way.
How do oxidation numbers identify a redox reaction?
A species is oxidized if its oxidation number increases (loses electron-holding priority) and reduced if its oxidation number decreases (gains it). If no atom's oxidation number changes between reactants and products, the reaction isn't a redox reaction at all, regardless of how complicated it looks.
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