Oxidation Number Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Oxidation Number.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

A number assigned to each atom in a compound using a set of rules, representing the hypothetical charge that atom would carry if all bonds.

An imaginary 'electron bookkeeping' system. If an atom 'owns' more electrons than usual, its oxidation number is negative; fewer means positive.

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How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Oxidation numbers help track electron transfer in redox reactions. An increase means oxidation; a decrease means reduction.

Common stuck point: Oxidation number is not the same as ionic charge — it's an assigned value that follows rules, even in covalent compounds.

Sense of Study hint: When assigning oxidation numbers, apply the rules in order of priority. First, free elements have oxidation number 0. Then, monatomic ions have oxidation number equal to their charge. Next, oxygen is usually -2 (except in peroxides: -1), and hydrogen is usually +1 (except in metal hydrides: -1). Finally, the sum of all oxidation numbers in a neutral compound must equal 0, and in a polyatomic ion must equal the ion's charge.

Worked Examples

Example 1

easy
Assign oxidation numbers to each element in \text{H}_2\text{SO}_4.

Solution

  1. 1
    Rule: H is +1 (in compounds with nonmetals). O is -2 (in most compounds).
  2. 2
    Let S = x. Sum of oxidation numbers = 0 (neutral compound).
  3. 3
    2(+1) + x + 4(-2) = 0 \Rightarrow 2 + x - 8 = 0 \Rightarrow x = +6.

Answer

\text{H: } +1,\; \text{S: } +6,\; \text{O: } -2
Oxidation numbers are assigned using a set of rules. They help us track electron transfer in redox reactions and predict the products of chemical reactions.

Example 2

medium
Assign oxidation numbers to each atom in \text{KMnO}_4 and determine the oxidation number of manganese.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy
What is the oxidation number of nitrogen in \text{NO}_3^-?

Example 2

hard
In the reaction \text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2, determine the oxidation number of iron and carbon on each side. Which element is oxidized and which is reduced?
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Background Knowledge

These ideas may be useful before you work through the harder examples.

oxidationreductionredox