Variable oxidation numbers: iron(II) and iron(III)
Use Fe²⁺ and Fe³⁺ to interpret names, formulas and electron changes without changing the element.
Extended only (Supplement) · Chemistry 0620, 2026–2028.
Before you begin
- Calculate oxidation numbers from formula charge accounting.
- Identify electron loss and gain in redox half-equations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What oxidation number belongs to a monatomic Fe³⁺ ion?
What you will learn
- Describe variable oxidation numbers of transition-element ions using iron(II) and iron(III).
- Connect ionic charges with compound formulas and one-electron conversion.
One element can form more than one ionic charge
Transition elements can form ions with variable oxidation numbers. The required iron examples are iron(II), Fe²⁺, and iron(III), Fe³⁺. Their +2 and +3 values refer to iron in the stated ions and compounds, not two or three iron atoms. Both remain iron because each has 26 protons; their electron counts differ.
For neutral chloride formulas, Fe²⁺ requires two Cl⁻ to form FeCl₂, while Fe³⁺ requires three Cl⁻ to form FeCl₃. With oxide ions O²⁻, FeO and Fe₂O₃ give iron oxidation numbers +2 and +3 respectively. Use the atom-weighted total, rather than assuming every iron compound must have the same charge because it contains the same element.
Connect the change to redox
Fe²⁺→Fe³⁺+e⁻ describes iron(II) losing one electron and being oxidised. Fe³⁺+e⁻→Fe²⁺ is the reverse reduction. The change from +2 to +3 is one electron, not three additional electrons lost from Fe²⁺. A neutral iron atom would instead need to lose three electrons in total to form Fe³⁺.
The Roman numeral helps distinguish compounds and reactions when a transition element has more than one oxidation number. It does not uniquely establish a solution colour or identify every possible compound from a metal name alone. Use the stated species and equation to identify agent roles, rather than assigning the metal one permanent redox role in all contexts.
Pause and explain
Which pair correctly illustrates variable iron oxidation numbers?
Worked example
Compare FeCl₂ and FeCl₃ and write the change from Fe²⁺ to Fe³⁺.
Show the worked solution
- Each chloride is −1, so FeCl₂ has iron +2 and FeCl₃ iron +3.
- Fe²⁺ loses one electron to give Fe³⁺.
- Fe²⁺→Fe³⁺+e⁻ is oxidation; iron's 26-proton identity remains unchanged.
Answer Iron(II) in FeCl₂; iron(III) in FeCl₃; Fe²⁺→Fe³⁺+e⁻.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Find iron's oxidation numbers in FeO and Fe₂O₃ using O=−2.
Give me a hint
Set each neutral formula's total to zero.
Compare my reasoning
- FeO needs x−2=0, giving iron +2.
- Fe₂O₃ needs 2x+3(−2)=0.
- Thus x=+3 in Fe₂O₃, demonstrating different iron oxidation numbers.
FeO: +2 iron. Fe₂O₃: +3 iron.
Look for these in your work
- The oxygen multiplier is included.
- Both neutral formulas sum to zero.
Write the reduction half-equation Fe³⁺ to Fe²⁺ and check total charge.
Give me a hint
An electron must be gained for the charge to decrease.
Compare my reasoning
- Write Fe³⁺+e⁻→Fe²⁺.
- Left charge is +3−1=+2, matching the right.
- One iron atom is present on each side and the electron gain is reduction.
Fe³⁺+e⁻→Fe²⁺; charge +2 on both sides.
Look for these in your work
- Atom and charge accounting agree.
- The electron gain is labelled reduction.
Given iron atomic number 26, compare electron counts in neutral Fe, Fe²⁺ and Fe³⁺ and assess a claim that Fe²⁺→Fe³⁺ removes one proton.
Give me a hint
Positive ions have fewer electrons, not different proton identities.
Compare my reasoning
- Neutral Fe has 26 electrons; Fe²⁺ has 24 and Fe³⁺ 23.
- The change from Fe²⁺ to Fe³⁺ loses one electron.
- All three have 26 protons; removing a proton would change element identity rather than this ionic redox state.
26, 24 and 23 electrons respectively; one electron is lost, with 26 protons unchanged.
Look for these in your work
- Each count follows the written charge.
- Ionic change is separated from changing the nucleus.
Common mistakes
- Reading II or III as an iron atom count.
- Assigning +2 to every iron compound.
- Removing a proton when Fe²⁺ changes to Fe³⁺.
- Calling +2 to +3 a loss of three further electrons.
What can you explain now?
How many electrons are lost when Fe²⁺ becomes Fe³⁺?
Compare with the explanation
One electron.
The charge increases by one; this is distinct from forming Fe³⁺ directly from neutral iron.
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Make it stick
Interpret II and III in two iron compounds, balance their charge sums and write both directions of the Fe²⁺/Fe³⁺ half-equation pair.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.