Oxidation states and identifying redox
Key idea: Oxidation states sum to the formula's total charge. An increase shows oxidation and a decrease shows reduction; a visible reaction without these changes is not redox.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Compare one atom’s oxidation state with the whole formula’s sum.
- Check oxygen’s state in an element, a compound and an ion.
- Add oxygen’s contribution and match the ion charge.
- Find both elements’ states before and after, then name each process.
- Correct the sum and find manganese’s oxidation state.
- Use the ion’s total to find nitrogen’s oxidation state.
- Check whether the visible reaction changes oxidation states.
- Find silver’s states on both sides, then classify the reaction.
- Explain why precipitation occurs without oxidation or reduction.
- Find both metal states and identify the oxidising agent.
- Show the sums and explain oxidation, reduction and the oxidising agent.
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ContinueSome redox reactions show no oxygen transfer and no ions. Oxidation states let you recognise redox in any equation. An oxidation state is a number assigned to an atom by a set of rules: it helps you track changes in electron ownership even when the equation shows no ions and no oxygen moving.
Oxidation states
Apply these in order:
- An element on its own, such as Zn, O₂ or Cl₂, has oxidation state 0.
- A simple ion has an oxidation state equal to its charge: Na⁺ is +1, Cl⁻ is −1, Fe³⁺ is +3.
- In compounds, oxygen is usually −2 and hydrogen is usually +1.
- In their compounds, Group 1 metals have oxidation state +1 and Group 2 metals +2. Chlorine in a metal chloride has oxidation state −1. For example, sodium is +1 and chlorine is −1 in NaCl.
- The oxidation states in a formula add up to its overall charge: 0 for a neutral compound, and the ion’s charge for a polyatomic ion.
An oxidation state belongs to one atom of the named element. A subscript tells you how many atoms contribute to the sum; it does not multiply the oxidation state of each atom. In a covalent compound, an oxidation state is an assigned number, not an actual ion charge.
Write every oxidation state with its sign, such as +6, even when it is positive. Then:
- an increase in oxidation state is oxidation;
- a decrease in oxidation state is reduction;
- a reaction is redox only if at least one element’s oxidation state changes.
Check your understanding 1 Read the formula before choosing a rule
Give the oxidation state of each oxygen atom in O₂ and in CO₂, then give the total oxidation-state sum for SO₄²⁻. Enter signed integers where the value is non-zero.
Show answer
Oxygen is 0 in O₂ and −2 per atom in CO₂. The sulfate ion’s total is −2. Choose the element rule first, then use the formula’s overall charge for any sum.
Worked example 1
Find an unknown oxidation state
Problem
Show solution step by step
Fix the known states
Method
Give each oxygen −2.Reason
Oxygen is −2 in sulfate, and sulfur is the only unknown.Working
Answer
Four oxygens contribute 4 × (-2) = -8.Set the correct total
Method
Make the sum equal the ion’s charge.Reason
Sulfate is an ion with charge 2−, not a neutral compound.Working
Answer
x + (-8) = -2Solve with the sign
Working
Answer
x = +6. Sulfur is +6 in sulfate.
Quick check
Guided practice 2
Is this reaction redox?
Problem
Track each element
Hints
Hint 1
elements
Before the reaction both iron and chlorine are elements.Hint 2
compound
In FeCl₃ the three chlorines are −1 each, and the total must be 0.Show solution step by step
Before
Reason
Both reactants are elements.Working
Answer
Fe: 0; Cl₂: 0.After
Method
Use chlorine to find iron.Reason
Chlorine is −1 as chloride, and a neutral compound sums to 0.Working
Answer
x + 3(-1) = 0, so iron is +3.Compare
Working
Answer
Iron: 0 → +3, oxidised. Chlorine: 0 → −1, reduced. The states change, so the reaction is redox.
Quick check
Spot the mistake 3
Summing to zero for an ion
Learner working
Find the first wrong line
Show solution step by step
Find the first error
Method
Check the total the learner used.Reason
MnO₄⁻ is an ion with charge 1−, so its oxidation states must add to −1, not 0.Working
Answer
The oxygen contribution, 4(-2) = -8, was correct; the total was not.Correct it
Working
Answer
x - 8 = -1, so manganese is +7, matching the “(VII)” in the name.
Common mistake
Check your understanding 2 Use the corrected rule on another ion
Find the oxidation state of nitrogen in NO₃⁻. Enter the total for the ion, the combined contribution from oxygen and the nitrogen state as signed integers.
Show the steps Optional working
Show answer
The total is −1; oxygen contributes 3(-2) = -6. Therefore x - 6 = -1 and nitrogen is +5. Check: + 5 - 6 = -1.
Reactions that are not redox
A reaction is redox only if an oxidation state changes. Precipitation, neutralisation and many other reactions that look dramatic involve no change at all.
Check your understanding 3 A visible change and an electron change
For AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq), each nitrate ion has charge 1− and chlorine in a metal chloride is −1. Check silver’s state before and after, then decide whether this reaction is redox. Sodium remains +1, chlorine −1 and nitrate is unchanged.
Show answer
Silver is +1 before and after. None of the elements changes oxidation state, so this reaction is not redox.
Check your understanding 4 A reaction that is not redox
Silver nitrate solution is added to sodium chloride solution: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq). A precipitate forms, so a learner says “something must have been oxidised”. Assign oxidation states to silver, sodium and chlorine before and after, and decide whether the learner is right.
Show answer
Silver is +1 in both AgNO₃ and AgCl; sodium is +1 in both NaCl and NaNO₃; chlorine is −1 in both NaCl and AgCl. Nitrate ions also stay unchanged (nitrogen +5 and oxygen −2). No oxidation state changes, so this precipitation reaction is not redox. Compare silver, sodium and chlorine on both sides, and check that nitrate stays unchanged. A visible change alone does not show redox; an oxidation-state change does.
Check your understanding 5 Track metals inside nitrate compounds
For Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s), each nitrate ion has charge 1−. Find each metal’s oxidation state before and after, then identify the oxidising agent.
Show answer
Copper: 0 → +2, oxidised. Silver: +1 → 0, reduced. Silver ions accept electrons and are the oxidising agent. Construct the compound sums and full explanation in the next activity.
Check your understanding 6 Copper and silver nitrate
Copper reacts with silver nitrate solution: Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s). Each nitrate ion has charge 1−. Find the oxidation states of copper and silver before and after, showing the compound sums. Use the changes to identify oxidation, reduction and the oxidising agent.
Show answer
Copper: 0 in Cu. In Cu(NO₃)₂ each nitrate ion has charge 1−, so x + 2(-1) = 0 and copper is +2. Silver: in AgNO₃, x + (-1) = 0, so silver is +1; it is 0 in Ag. Copper rises from 0 to +2, so copper is oxidised. Silver falls from +1 to 0, so silver is reduced. The oxidising agent is the silver ions in silver nitrate. Compare both compound sums, the direction of each state change and the agent named for accepting electrons. The coefficient 2 counts two silver atoms; each changes from +1 to 0, not from +2 to 0.
Common mistakes
- Leaving off the sign, as in 6 instead of +6.
- Making the oxidation states in an ion add up to zero instead of to its charge.
- Giving the oxidation state of the wrong element, such as oxygen instead of sulfur.
- Calling a reaction redox because it shows a visible change, such as a precipitate.
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Syllabus and review details
- SEC G3 Combined Science Chemistry component 2027 · 2027
Chemistry content structure and subject content, PDF pages 26–36
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