Naming Compounds with Transition Metals (Roman Numerals)

Iron can form two different chlorides: one is FeCl₂, a pale green solid, and the other is FeCl₃, a dark brown-black solid. Calling both “iron chloride” would be ambiguous. Chemists solve this with a Roman numeral in brackets that shows the charge on the metal ion: iron(II) chloride and iron(III) chloride. This guide explains when you need Roman numerals, how to work out the right number from a formula, and how to go the other way.

Why some metals need Roman numerals

Main-group metals usually form only one ion:

• Group 1: always +1 (Na⁺, K⁺).
• Group 2: always +2 (Mg²⁺, Ca²⁺).
• Aluminium: always +3 (Al³⁺).

So sodium chloride, magnesium oxide and aluminium sulfate need no Roman numerals — there’s only one possibility.

Many transition metals (and some post-transition metals such as tin and lead) can form more than one ion, because they can lose different numbers of electrons from their 4s and 3d (or equivalent) orbitals (see transition metals and transition metal electron configurations).

Metal	Common ions
Iron	Fe²⁺, Fe³⁺
Copper	Cu⁺, Cu²⁺
Chromium	Cr²⁺, Cr³⁺ (and +6 in chromates)
Manganese	Mn²⁺ (and higher states in MnO₂, MnO₄⁻)
Cobalt	Co²⁺, Co³⁺
Tin	Sn²⁺, Sn⁴⁺
Lead	Pb²⁺, Pb⁴⁺
Mercury	Hg₂²⁺ (mercury(I)), Hg²⁺

For these, the name must show which ion is present. This is the Stock system, named after the German chemist Alfred Stock.

The rule

Write the metal’s name, then its charge (oxidation state) as a Roman numeral in brackets, with no space, then the anion name:

iron(III) oxide, copper(II) sulfate, lead(II) nitrate

Step-by-step: name from formula

1. Identify the anion and its charge. Learn the common ones (see common ions and their charges).
2. Calculate the total negative charge: anion charge × number of anions.
3. The metal’s total positive charge must balance it. Divide by the number of metal atoms to find the charge on each.
4. Write the name with the charge as a Roman numeral.

Worked example 1: FeCl₃

• Anion: chloride, Cl⁻ (−1).
• Total negative: 3 × (−1) = −3.
• One Fe must be +3.
• Name: iron(III) chloride.

Worked example 2: Cu₂O

• Anion: oxide, O²⁻ (−2).
• Total negative: −2.
• Two Cu share +2, so each is +1.
• Name: copper(I) oxide (the red solid formed in the Benedict’s test — see food tests).

Worked example 3: Fe₂O₃

• Anion: oxide, O²⁻.
• Total negative: 3 × (−2) = −6.
• Two Fe share +6, so each is +3.
• Name: iron(III) oxide (the main component of rust).

Worked example 4: CuSO₄

• Anion: sulfate, SO₄²⁻ (−2).
• One Cu must be +2.
• Name: copper(II) sulfate.

Worked example 5: PbO₂

• Anion: oxide × 2 = −4.
• One Pb must be +4.
• Name: lead(IV) oxide.

Worked example 6: Mn(NO₃)₂

• Anion: nitrate, NO₃⁻ (−1). Total: 2 × (−1) = −2.
• One Mn must be +2.
• Name: manganese(II) nitrate.

Worked example 7: Cr₂(SO₄)₃

• Anion: sulfate (−2). Total: 3 × (−2) = −6.
• Two Cr share +6 → each +3.
• Name: chromium(III) sulfate.

Step-by-step: formula from name

1. Read the metal’s charge from the Roman numeral.
2. Find the anion’s charge.
3. Balance the charges using the lowest whole numbers (see the criss-cross method).
4. Use brackets around polyatomic ions if there’s more than one.

Examples

Name	Ions	Formula
iron(II) sulfate	Fe²⁺, SO₄²⁻	FeSO₄
iron(III) sulfate	Fe³⁺, SO₄²⁻	Fe₂(SO₄)₃
copper(I) chloride	Cu⁺, Cl⁻	CuCl
copper(II) hydroxide	Cu²⁺, OH⁻	Cu(OH)₂
tin(IV) chloride	Sn⁴⁺, Cl⁻	SnCl₄
cobalt(II) nitrate	Co²⁺, NO₃⁻	Co(NO₃)₂
vanadium(V) oxide	V⁵⁺, O²⁻	V₂O₅

Vanadium(V) oxide is the catalyst in the Contact process for making sulfuric acid. The “V⁵⁺” is really a bookkeeping charge — the bonding in V₂O₅ has a lot of covalent character — but the Roman numeral still gives the oxidation state correctly (see how to find oxidation numbers and the oxidation number calculator).

Metals that don’t need Roman numerals

Some transition metals have only one common ion, and by convention their names usually omit the numeral:

• Zinc: always Zn²⁺ → zinc chloride, ZnCl₂.
• Silver: almost always Ag⁺ → silver nitrate, AgNO₃.
• Scandium: Sc³⁺.

Including the numeral isn’t wrong — “silver(I) nitrate” is correct — but it’s unusual.

The old system: -ous and -ic

Before the Stock system, chemists used endings on the metal’s name (often its Latin name):

• -ous for the lower charge.
• -ic for the higher charge.

Stock name	Old name	Formula
iron(II) chloride	ferrous chloride	FeCl₂
iron(III) chloride	ferric chloride	FeCl₃
copper(I) oxide	cuprous oxide	Cu₂O
copper(II) oxide	cupric oxide	CuO
tin(II) fluoride	stannous fluoride	SnF₂
lead(II) acetate	plumbous acetate	Pb(CH₃COO)₂

You’ll still see these names on older labels, in toothpaste ingredients (stannous fluoride) and in iron supplements (ferrous sulfate, ferrous fumarate — see iron in the body). The weakness of the old system is that it doesn’t tell you the actual charge — you have to remember that ferrous means +2 but cuprous means +1.

Roman numerals and colour

The charge on a transition metal ion often changes its colour, which is a practical reason to name it precisely:

• Fe²⁺ compounds are often pale green; Fe³⁺ compounds are yellow–brown.
• Cu⁺ compounds are often white or red; Cu²⁺ compounds are typically blue or green.
• Cr³⁺ compounds are green; chromate(VI) is yellow and dichromate(VI) orange.

How to decide whether a numeral is needed

A quick decision process helps in exams:

1. Is the metal in group 1 or group 2, or is it aluminium? If so, no numeral is needed — each has only one common charge.
2. Is it zinc, silver or cadmium? By convention, no numeral is usually used, although including it isn’t wrong.
3. Is it any other transition metal, or tin, lead or mercury? Then include the numeral.

When you’re given a formula, always calculate the charge from the anions rather than guessing. The most common error is reading the subscript on the metal as its charge. In Fe₂O₃, the 2 tells you how many iron atoms there are; the three oxide ions carry −6 in total, so each iron must be +3. In Cu₂O, the single oxide carries −2, shared between two copper atoms, so each is +1. A few seconds of arithmetic avoids the mistake every time.

Common mistakes

• Using the number of atoms instead of the charge. Fe₂O₃ is iron(III) oxide, not iron(II) oxide — the “2” is the number of iron atoms.
• Adding Roman numerals to group 1 and 2 metals. “Sodium(I) chloride” is unnecessary.
• Forgetting the anion’s charge. For Fe(OH)₃, hydroxide is −1, so iron is +3.
• Leaving a space: write iron(III), not iron (III).
• Forgetting brackets around polyatomic ions in formulas: Fe₂(SO₄)₃, not Fe₂SO₄₃.

Practice

Name: (a) CuCl₂ (b) FeO (c) SnO₂ (d) Co₂O₃ (e) Hg₂Cl₂ Write formulas: (f) iron(III) nitrate (g) lead(II) oxide (h) copper(I) sulfide

Answers: (a) copper(II) chloride (b) iron(II) oxide (c) tin(IV) oxide (d) cobalt(III) oxide (e) mercury(I) chloride — mercury(I) exists as the Hg₂²⁺ ion (f) Fe(NO₃)₃ (g) PbO (h) Cu₂S

Key takeaways

• Metals with more than one possible charge need a Roman numeral showing the charge: iron(II), iron(III).
• Find the charge by balancing it against the total anion charge, then dividing by the number of metal atoms.
• Group 1, group 2, aluminium, zinc and silver usually don’t need numerals.
• Old names use -ous (lower) and -ic (higher): ferrous = iron(II), ferric = iron(III).