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General Chemistry I · Atoms, Molecules & Ions

Naming Ionic Compounds

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

Ionic compounds are named by naming the cation first and the anion second, but the cation half splits into two cases. Fixed-charge metals — the alkali metals (Group 1), alkaline earth metals (Group 2), plus aluminum (Al³⁺), zinc (Zn²⁺), and silver (Ag⁺) — always form one ion, so their name is simply the metal's name (sodium, calcium, aluminum). Variable-charge metals (most transition metals, plus a few post-transition metals like lead and tin) can form more than one ion, so the charge must be stated with a Roman numeral in parentheses: iron(III) means Fe³⁺, iron(II) means Fe²⁺. The anion is either a simple monatomic ion ending in -ide (Cl⁻ chloride) or a polyatomic ion whose name is memorized (SO₄²⁻ sulfate).

Why this matters

Ionic compounds are everywhere — salts, minerals, and many materials — and their names are unambiguous codes for their composition. The Roman numeral is not decorative: iron(II) chloride (FeCl₂) and iron(III) chloride (FeCl₃) are different substances with different properties, and mislabeling them can matter in medicine (iron supplements) and industry. Naming ionic compounds is also the foundation for understanding acids, bases, and precipitation reactions later in the course.

The college version

Key Ideas

  • Order: cation first, anion second.
  • Monatomic cations: metal name unchanged (Na⁺ sodium, Mg²⁺ magnesium).
  • Monatomic anions: element stem + "-ide" (Cl⁻ chloride, O²⁻ oxide, S²⁻ sulfide, N³⁻ nitride).
  • Fixed-charge metals (no Roman numeral): Group 1 (+1), Group 2 (+2), Al³⁺, Zn²⁺, Ag⁺.
  • Variable-charge metals (need Roman numeral): Fe²⁺/Fe³⁺, Cu⁺/Cu²⁺, Pb²⁺/Pb⁴⁺, Sn²⁺/Sn⁴⁺, Co²⁺/Co³⁺, etc.
  • Roman numeral = the charge (oxidation state) of the metal, never a subscript.
  • Polyatomic ions keep their name (NH₄⁺ ammonium, OH⁻ hydroxide, NO₃⁻ nitrate, SO₄²⁻ sulfate, CO₃²⁻ carbonate, PO₄³⁻ phosphate); use parentheses if more than one is needed.
  • The compound is electrically neutral overall.

Equations and Variables

No equations, but the guiding rule is charge balance:

total positive charge = total negative charge

For a metal M with charge +m and a nonmetal/polyatomic X with charge −n, the neutral formula is MₙXₘ (criss-cross the charges as subscripts, then reduce to the lowest ratio).

How It Works or Problem-Solving Method

Naming from a formula:

  1. Identify the cation and anion.
  2. If the metal has a variable charge, work backward from the total anion charge to find the cation charge, then state it as a Roman numeral.
  3. Write: metal name ( + Roman numeral if variable) + anion name.

Writing a formula from a name:

  1. Translate each part to a symbol and charge (the Roman numeral is the metal's charge).
  2. Criss-cross the charge magnitudes into subscripts.
  3. Reduce to the lowest whole-number ratio; add parentheses around a polyatomic ion if its subscript is 2 or more.

Worked Example

Problem 1 (fixed charge): Name K₂O and CaCl₂.

K₂O: K⁺ (potassium, Group 1, fixed +1) + O²⁻ (oxide) → potassium oxide. CaCl₂: Ca²⁺ (calcium, Group 2, fixed +2) + Cl⁻ (chloride) → calcium chloride.

Problem 2 (variable charge): Name FeCl₃ and FeCl₂.

FeCl₃: three Cl⁻ give a total of 3−, so Fe must be 3+ → iron(III) chloride. FeCl₂: two Cl⁻ give 2−, so Fe is 2+ → iron(II) chloride.

Problem 3 (polyatomic): Name Na₂SO₄ and write the formula for ammonium phosphate.

Na₂SO₄: Na⁺ (sodium) + SO₄²⁻ (sulfate) → sodium sulfate. Ammonium phosphate: NH₄⁺ and PO₄³⁻. Criss-cross → (NH₄)₃PO₄ (three ammonium ions balance one phosphate).

Problem 4 (writing from a Roman-numeral name): Write the formula for copper(II) oxide.

Copper(II) = Cu²⁺; oxide = O²⁻. Criss-cross → Cu₂O₂, reduced to CuO.

Common Confusions

  • "The Roman numeral is the number of atoms." Wrong — it is the charge (oxidation state) of the metal. In Fe₂O₃, iron is still iron(III) even though there are two Fe atoms.
  • "All metals need a Roman numeral." Wrong — Group 1, Group 2, Al, Zn, and Ag have fixed charges and are named without one (sodium chloride, not "sodium(I) chloride").
  • "NaCl should be written Na₂Cl₂ for balance." Wrong — the formula is reduced to the lowest whole-number ratio (NaCl), which is already neutral.
  • "Polyatomic ions can have their subscripts changed inside." Wrong — a polyatomic ion is a fixed unit (SO₄²⁻ stays intact); use parentheses and an outside subscript — (NH₄)₂SO₄, never NH₈SO₄.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Naming an ionic compound is like introducing two people at a party: "first name, then surname" — the metal goes first, the nonmetal second. Some metals are predictable and always bring the same gift (Group 1 and Group 2 metals always give away 1 or 2 electrons), so you don't need to label them. But transition metals are like people with several different outfits — iron can come as Fe²⁺ or Fe³⁺ — so you must put a tag on them (iron(II) vs. iron(III)) so everyone knows exactly which "outfit" showed up. Limit of the analogy: the Roman numeral isn't a choice the atom makes — it's a fixed charge determined by how many electrons were transferred, and it's deduced from the formula, not guessed.

Key takeaways

  • Cation first, anion second.
  • Monatomic anion = stem + "-ide" (Cl⁻ chloride, O²⁻ oxide).
  • Fixed charges: Group 1 = +1, Group 2 = +2, Al³⁺, Zn²⁺, Ag⁺ (no Roman numeral).
  • Transition metals usually need a Roman numeral = their charge (Fe²⁺ = iron(II)).
  • Roman numeral is the charge, not a subscript count.
  • Polyatomic ions keep their name; wrap in parentheses if subscript > 1.
  • Formula must be electrically neutral; reduce subscripts to the lowest ratio.
  • Metal cation first, nonmetal/polyatomic anion second.
  • Anion ending: -ide (chloride, oxide, sulfide).
  • Fixed charges (no numeral): +1 Gp 1, +2 Gp 2, Al³⁺, Zn²⁺, Ag⁺.
  • Variable charges: use Roman numeral = metal's charge.
  • FeCl₃ = iron(III) chloride; FeCl₂ = iron(II) chloride.
  • Polyatomic ions keep their names; use parentheses when more than one.
  • Neutral overall; lowest whole-number ratio.

Keep learning

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Practice General Chemistry I

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Related

You’ll learn to

  • Name and write formulas for ionic compounds containing fixed-charge metals.
  • Use Roman numerals to name compounds of variable-charge (transition) metals.
  • Incorporate polyatomic ions into ionic names and formulas correctly.
  • Explain why ionic formulas are always the lowest whole-number ratio that is electrically neutral.

Sources & references

  1. OpenStax, *Chemistry 2e*, Ch. 2.7, "Chemical Nomenclature."
  2. OpenStax, *Chemistry 2e*, Ch. 2.6, "Molecular and Ionic Compounds."

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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