Introduction
So far you've named simple binary ionic compounds like NaCl or MgO, where the metal always forms the same charge. But many metals — especially the transition metals and a few others like lead, tin, and copper — can form more than one stable cation. When this happens, we need a way to show which charge is present in the formula. This is where Roman numerals come into the naming system.
Why do we need Roman numerals?
Consider iron. It can form Fe²⁺ or Fe³⁺ ions, each combining differently with chloride:
- Fe²⁺ + 2Cl⁻ → FeCl₂
- Fe³⁺ + 3Cl⁻ → FeCl₃
If we just called both compounds "iron chloride," there would be no way to tell them apart from the name alone. IUPAC nomenclature exists precisely to avoid this kind of ambiguity (Talbot, p. 172), so we add the charge of the cation in Roman numerals, in brackets, straight after the metal's name:
- FeCl₂ = iron(II) chloride
- FeCl₃ = iron(III) chloride
The rule
Binary ionic compounds are named cation first, then anion, with the anion ending in "-ide" (Talbot, p. 126). When the cation is a metal capable of more than one charge, insert the Roman numeral for that charge in brackets immediately after the metal name — with no space before the bracket.
To find the Roman numeral, work backwards from the formula using the anion's known charge:
- Identify the anion and its charge (e.g., Cl⁻ = 1−, O²⁻ = 2−).
- Use the formula's ratio to calculate the total negative charge.
- Divide by the number of cations to get the cation's charge.
- Convert that charge into a Roman numeral.
Worked Examples
Example 1: CoCl₂ Cobalt is a transition metal, so it can have more than one oxidation state. Chlorine is always 1−. There are two chlorides, so the total negative charge is 2−. This must be balanced by one Co²⁺ ion. → Name: cobalt(II) chloride (Talbot, p. 130)
Example 2: PbO₂ Oxygen is 2−, and there are two oxygens, giving 4− total. One lead ion must balance this, so lead is 4+. → Name: lead(IV) oxide
Example 3: Cu₂O Oxygen is 2−, only one oxygen present. Two copper ions share this charge equally, so each copper is 1+. → Name: copper(I) oxide
Example 4: BaCl₂ (contrast case) Barium is in Group 2, so it always forms Ba²⁺ — there's no ambiguity. No Roman numeral is needed. → Name: simply barium chloride (Talbot, p. 130)
A quick shortcut
Metals in Groups 1 and 2, along with aluminium and a few others, have fixed charges, so they never need a Roman numeral. Transition metals (and metals like Pb, Sn, Cu) usually do need one, since their charge can vary between compounds.
Key Takeaways
- Some metals form more than one stable cation charge — mainly transition metals, plus Pb, Sn, and Cu.
- Roman numerals in brackets after the metal name show the cation's charge, e.g., iron(III) oxide.
- Work out the charge by balancing it against the known charge of the anion.
- Group 1 and Group 2 metals have fixed charges, so they never need Roman numerals.
- This system exists so every compound name is unambiguous — a core goal of IUPAC nomenclature (Talbot, p. 172).