Use Roman numerals for oxidation numbers
Interpret Roman numerals in compound names as the oxidation-number magnitude of the named element.
Section 6 · Electrochemistry
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Check atom balance, total charge and oxidation-state changes separately. Written practice is self-checked
Before you begin
- Recognise common ions and write formulas for simple compounds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does the Roman numeral in iron(III) chloride represent?
What you will learn
- Use Roman numerals to indicate an element's oxidation number
- Distinguish an oxidation-state label from a subscript or ionic charge
- Connect compound names with oxidation-number calculations
The Roman numeral distinguishes oxidation states
In names such as iron(II) chloride and iron(III) chloride, the Roman numeral indicates the oxidation number of iron: +2 or +3. It is written in parentheses after the element name.
The numeral is not a subscript and does not state how many atoms of the element appear in the formula. The formula subscripts and overall charge still need to balance independently.
Use the formula to verify the named oxidation number
For FeCl₂, each Cl is −1, so neutral charge requires Fe to be +2: iron(II) chloride. For FeCl₃, iron is +3: iron(III) chloride.
The Roman numeral reports the magnitude of the oxidation number; use the compound's charge balance to determine its sign and check the name against the formula.
Pause and explain
What is the oxidation number of iron in FeCl₂?
Worked example
Name Fe₂O₃ using a Roman numeral for iron.
Show the worked solution
- Each O is −2, so three oxygen atoms contribute −6.
- Two iron atoms must total +6; each Fe is +3.
- Write the oxidation number as Roman numeral III.
Answer Iron(III) oxide.
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Test this topic →From guided practice to a new situation
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.
Write the name of CuCl₂ using a Roman numeral for copper.
Give me a hint
Each chloride is −1 and the compound is neutral.
Compare my reasoning
- Two Cl atoms contribute −2.
- Copper must be +2.
- Use Roman numeral II after copper.
Copper(II) chloride.
Look for these in your work
- Oxidation number calculated
- Roman numeral selected
- Name format correct
Determine the Roman numeral in manganese(VII) oxide and verify the formula Mn₂O₇.
Give me a hint
Use seven O atoms at −2 and a neutral total.
Compare my reasoning
- O contributes −14.
- Two Mn atoms together contribute +14, so each is +7.
- Write VII after manganese.
Mn is +7, so the name uses manganese(VII) oxide.
Look for these in your work
- O contribution calculated
- Per-atom oxidation number found
- Roman numeral written
A student says the III in iron(III) chloride means the formula must contain three iron atoms. Correct the statement.
Give me a hint
Compare the numeral's naming role with the formula subscripts.
Compare my reasoning
- The Roman numeral identifies iron's oxidation number.
- In iron(III), the oxidation number is +3.
- Subscripts, not Roman numerals in the name, state atom counts.
Iron(III) means iron has oxidation number +3; it does not mean three iron atoms. Formula subscripts give atom counts.
Look for these in your work
- Oxidation number role stated
- Value interpreted
- Subscript distinction explained
Common mistakes
- Using the Roman numeral as a formula subscript
- Writing iron(II) for Fe₂O₃
- Ignoring the total charge when assigning a name
- Forgetting the parentheses around the Roman numeral
What can you explain now?
What does the Roman numeral in iron(III) chloride represent?
Compare with the explanation
The oxidation number of iron is +3
The formula and charge balance give iron's +3 oxidation number.
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Make it stick
What does a Roman numeral report, and how does charge balance verify it?
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