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Oxidation number practice
Oxidation numbers are bookkeeping values that help track electron transfer. Their sum equals the overall charge of the species, whether the species is neutral or an ion.
By ExamCat · Original study lesson · Published September 24, 2026
Understand the method
Start with reliable rules: a free element has oxidation number zero, and a monatomic ion uses its charge. Oxygen is usually −2, with exceptions such as peroxides. Hydrogen is usually +1 with nonmetals, but −1 in metal hydrides.
Worked example, step by step
Find sulfur’s oxidation number in sulfate, SO₄²⁻.
- Assign the usual −2 to each oxygen in this ion.
- Four oxygens contribute −8.
- Let sulfur be x and set x − 8 = −2, using the ion’s charge.
- Solve x = +6. This formal number is not a claim that a free S⁶⁺ ion is present in the solution.
A mistake to watch for
Setting the sum to zero for every formula fails for charged species. Always write the overall charge before solving. When applying rules, check whether the compound belongs to an exception.
Try it yourself: three questions
Write an answer before opening the explanation. For calculations, keep your units and intermediate steps; for concepts, state the reason for your choice.
1. Find nitrogen’s oxidation number in NO₃⁻.
Show answer and explanation
x + 3(−2) = −1, so x = +5.
2. What is oxygen’s oxidation number in O₂?
Show answer and explanation
Zero, because this is oxygen in elemental form rather than oxygen combined in a compound.
3. Iron changes from Fe²⁺ to Fe³⁺. Is it oxidized or reduced?
Show answer and explanation
Oxidized: its oxidation number increases by one, corresponding to loss of one electron. Reduction would lower the oxidation number.
Build your next practice session
Mark the first step where your reasoning differed from the explanation. Turn that step into a question, answer it without looking, then change one condition in the example and explain what would change. A correct guess is a reason to revisit the method.
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Further reading
OpenStax Chemistry 2e provides textbook background. The worked scenario and practice questions on this page were written independently for ExamCat.
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