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 were treated as fully ionic (all shared electrons assigned to the more electronegative atom).

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

Read the full concept explanation →

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 Number starts by comparing the reactant-product pattern, charges, states, and conserved atoms.

Common stuck point: Students often know a formula related to oxidation number but skip the recognition step: Does the balanced equation match a recognizable pattern of reactants, products, ions, oxygen, or electron transfer? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Does the balanced equation match a recognizable pattern of reactants, products, ions, oxygen, or electron transfer?

Worked Examples

Example 1

easy
Assign oxidation numbers to each element in H2SO4\text{H}_2\text{SO}_4.

Answer

H: +1,  S: +6,  O: −2\text{H: } +1,\; \text{S: } +6,\; \text{O: } -2

First step

1
Rule: H is +1+1 (in compounds with nonmetals). O is −2-2 (in most compounds).

Full solution

  1. 2
    Let S = xx. Sum of oxidation numbers = 0 (neutral compound).
  2. 3
    2(+1)+x+4(−2)=0⇒2+x−8=0⇒x=+62(+1) + x + 4(-2) = 0 \Rightarrow 2 + x - 8 = 0 \Rightarrow x = +6.
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 KMnO4\text{KMnO}_4 and determine the oxidation number of manganese.

Example 3

medium
Find the oxidation number of Cl in ClO3−\text{ClO}_3^-.

Example 4

medium
Find the oxidation number of S in SO32−\text{SO}_3^{2-}.

Example 5

hard
In Cr2O72−→2Cr3+\text{Cr}_2\text{O}_7^{2-} \rightarrow 2\text{Cr}^{3+}, how many electrons does each Cr atom gain, and what does this tell you about the reaction?

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 NO3−\text{NO}_3^-?

Example 2

hard
In the reaction Fe2O3+3CO→2Fe+3CO2\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?

Example 3

easy
What is the oxidation number of an element in its pure form, e.g. O2O_2?

Example 4

easy
Oxidation number of oxygen in H2OH_2O?

Example 5

easy
Oxidation number of H in HClHCl?

Example 6

easy
Oxidation number of NaNa in NaCl?

Example 7

easy
What must oxidation numbers sum to in a neutral compound?

Example 8

easy
Oxidation number of a monatomic ion equals what?

Example 9

easy
Oxidation number of Cl in NaCl?

Example 10

easy
Oxidation number of oxygen in peroxide H2O2H_2O_2?

Example 11

medium
Find the oxidation number of S in SO2SO_2.

Example 12

medium
Find the oxidation number of S in H2SO4H_2SO_4.

Example 13

medium
Find the oxidation number of N in NO3−NO_3^-.

Example 14

medium
Find the oxidation number of Cr in Cr2O72−Cr_2O_7^{2-}.

Example 15

medium
Find the oxidation number of Mn in MnO4−MnO_4^-.

Example 16

medium
Is oxidation number the same as ionic charge in CH4CH_4?

Example 17

medium
Find the oxidation number of C in CO32−CO_3^{2-}.

Example 18

medium
Find the oxidation number of Fe in Fe2O3Fe_2O_3.

Example 19

medium
Find the oxidation number of P in PO43−PO_4^{3-}.

Example 20

challenge
In 2H2+O2→2H2O2H_2 + O_2 \rightarrow 2H_2O, identify which element is oxidized.

Example 21

challenge
In Zn+Cu2+→Zn2++CuZn + Cu^{2+} \rightarrow Zn^{2+} + Cu, which species is reduced?

Example 22

challenge
Find the oxidation number of S in the thiosulfate ion S2O32−S_2O_3^{2-} (average).

Example 23

easy
What is the oxidation number of nitrogen in N2\text{N}_2?

Example 24

easy
Find the oxidation number of hydrogen in NaH\text{NaH}.

Example 25

easy
Find the oxidation number of Ca in CaO\text{CaO}.

Example 26

easy
Find the oxidation number of Al in Al2O3\text{Al}_2\text{O}_3.

Example 27

easy
What is the oxidation number of O in OF2\text{OF}_2?

Example 28

medium
Find the oxidation number of N in NH3\text{NH}_3.

Example 29

medium
Find the oxidation number of P in H3PO4\text{H}_3\text{PO}_4.

Example 30

medium
Find the oxidation number of N in N2O\text{N}_2\text{O}.

Example 31

medium
Find the oxidation number of Cl in HClO4\text{HClO}_4.

Example 32

medium
Find the oxidation number of C in CO\text{CO}.

Example 33

medium
Find the oxidation number of V in VO2+\text{VO}_2^+.

Example 34

medium
Find the oxidation number of C in CH3OH\text{CH}_3\text{OH} (methanol).

Example 35

medium
Find the oxidation number of S in Na2S2O3\text{Na}_2\text{S}_2\text{O}_3 (average per S).

Example 36

medium
Find the oxidation number of I in IO3−\text{IO}_3^-.

Example 37

medium
Identify the oxidized element in 4Fe+3O2→2Fe2O34\text{Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3.

Example 38

hard
Find the oxidation number of Mn in MnO2\text{MnO}_2.

Example 39

hard
In H2O2→H2O+12O2\text{H}_2\text{O}_2 \rightarrow \text{H}_2\text{O} + \tfrac{1}{2}\text{O}_2, what is unusual about the role of oxygen?

Example 40

hard
In MnO4−+5Fe2++8H+→Mn2++5Fe3++4H2O\text{MnO}_4^- + 5\text{Fe}^{2+} + 8\text{H}^+ \rightarrow \text{Mn}^{2+} + 5\text{Fe}^{3+} + 4\text{H}_2\text{O}, identify the species oxidized and reduced.

Example 41

hard
Find the oxidation number of S in S4O62−\text{S}_4\text{O}_6^{2-} (tetrathionate, average).

Example 42

challenge
In 3Cl2+6OH−→5Cl−+ClO3−+3H2O3\text{Cl}_2 + 6\text{OH}^- \rightarrow 5\text{Cl}^- + \text{ClO}_3^- + 3\text{H}_2\text{O}, classify the reaction with respect to chlorine.
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Background Knowledge

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

oxidationreductionredox