Roman Numerals in Chemical Names
15 questions · Naming, Translation & Formula Writing · Cambridge IGCSE Chemistry 0620
Section A
Translation (5 Qs)
Section B
Naming (5 Qs)
Section C
Formulas (5 Qs)
🔑 The Crossover Method (Formula Writing)
Step 1: Write ions with charges
Fe³⁺ Cl⁻
Step 2: Swap numbers as subscripts
Fe₁Cl₃
Step 3: Drop the 1 and simplify
FeCl₃ ✅
Decode what the Roman numeral tells you about the compound.
What does the Roman numeral (II) in Copper(II) sulfate represent?
💡 Hint: Think: what property of the copper ion does the Roman numeral describe?
What does the Roman numeral (III) in Iron(III) chloride tell you?
💡 Hint: Which ion has the Roman numeral? What is its charge?
Why do we need Roman numerals when naming transition metal compounds but NOT when naming sodium chloride (NaCl)?
💡 Hint: Think about how many charges sodium can form vs. how many iron can form.
A student writes "lead oxide" without a Roman numeral. Explain why this name is incomplete.
💡 Hint: Lead forms more than one type of ion. Which ones?
The (IV) in Manganese(IV) oxide tells you the oxidation state of manganese is +4. Using this, calculate the overall charge of the oxide ion present.
💡 Hint: The compound is neutral. If Mn = +4, what must the oxide ions total to?
Determine the oxidation state from the formula and write the full IUPAC name.
Give the full chemical name for FeCl₃.
💡 Hint: Chloride is Cl⁻ (charge = −1). There are 3 chloride ions. What must the iron charge be?
Give the full chemical name for PbO₂.
💡 Hint: Oxide is O²⁻ (charge = −2). Two oxide ions are present. What is the charge on lead?
Give the full chemical name for CuO.
💡 Hint: One oxide ion O²⁻. One copper ion. What must the copper charge be?
Give the full chemical name for Fe₂O₃.
💡 Hint: Three O²⁻ ions and two Fe ions. Total negative charge = −6. Split across 2 Fe atoms.
Give the full chemical name for SnCl₄.
💡 Hint: Four Cl⁻ ions. Total anionic charge = −4. What is the charge on tin (Sn)?
Use the Roman numeral and the anion charge to construct the correct chemical formula.
Write the formula for Manganese(IV) oxide.
💡 Hint: Mn is +4. Oxide ion is O²⁻. Use the crossover method: swap the numbers as subscripts.
Write the formula for Copper(I) oxide.
💡 Hint: Cu is +1. Oxide ion is O²⁻. Apply the crossover method.
Write the formula for Iron(III) sulfate.
💡 Hint: Fe³⁺ and sulfate SO₄²⁻. Crossover the charges as subscripts, then simplify.
Write the formula for Lead(II) nitrate.
💡 Hint: Pb²⁺ and nitrate NO₃⁻. What subscripts do you need?
Write the formula for Chromium(III) chloride.
💡 Hint: Cr³⁺ and Cl⁻. Apply the crossover method and simplify if needed.
⚡ Cambridge Exam Strategy
- → Always check the number of anions to calculate the cation's charge.
- → Use the crossover method and simplify all formulas to their lowest ratio.
- → Polyatomic ions (SO₄²⁻, NO₃⁻) need brackets when subscript > 1.
- → Transition metals need Roman numerals. Group 1 & 2 metals do NOT.
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