Elemental State: The oxidation number of any atom in its pure elemental form (e.g., , , ) is always zero. This is because there is no net transfer of electrons between identical atoms.
Monatomic Ions: For simple ions consisting of a single atom, the oxidation number is equal to the charge of the ion (e.g., is , is ).
Oxygen and Hydrogen: Oxygen is typically assigned in compounds (except in peroxides where it is ). Hydrogen is usually when bonded to nonmetals and when bonded to metals (hydrides).
Summation Rule: In a neutral molecule, the sum of all oxidation numbers must be zero. In a polyatomic ion, the sum must equal the overall charge of the ion.
Step 1: Split the Reaction: Divide the overall unbalanced equation into two separate half-reactions: one for oxidation and one for reduction. This allows for independent balancing of mass and charge.
Step 2: Balance Atoms: First, balance all atoms except Oxygen and Hydrogen. Then, balance Oxygen by adding molecules and balance Hydrogen by adding ions (in acidic solution).
Step 3: Balance Charge: Add electrons () to the side with the higher total positive charge so that the net charge on both sides of the half-reaction is equal.
Step 4: Combine and Simplify: Multiply the half-reactions by integers so that the number of electrons lost equals the number of electrons gained. Add the reactions together and cancel out species appearing on both sides.
| Feature | Oxidation | Reduction |
|---|---|---|
| Electron Change | Loss of electrons | Gain of electrons |
| Oxidation Number | Increases | Decreases |
| Role in Reaction | Reducing Agent | Oxidizing Agent |
| Half-reaction side | Electrons on Product side | Electrons on Reactant side |
Verify Charge Balance: The most common mistake in redox exams is balancing the atoms but failing to balance the net charge. Always perform a final check to ensure the total charge on the reactant side equals the total charge on the product side.
Identify the Change First: Before attempting to balance, calculate the oxidation numbers for every element. This immediately identifies which species are undergoing redox and prevents wasting time on spectator ions.
Check for Disproportionation: Be alert for reactions where the same element is both oxidized and reduced. In these cases, the element will appear in two different product species with different oxidation states.
Sanity Check: If you find yourself adding a massive number of electrons (e.g., 50), re-evaluate your oxidation number calculations; most standard redox problems involve transfers of 1 to 10 electrons.
