AP Chemistry Naming Compounds
Ionic and molecular nomenclature rules, worked through every common case.
Compound naming splits into two genuinely different systems depending on what's bonded: ionic compounds (metal + nonmetal, or a polyatomic ion) follow one set of rules, molecular compounds (nonmetal + nonmetal) follow a completely different one. Mixing the two up is the single most common naming mistake. Here's each system, worked through real examples.
Naming Binary Ionic Compounds
Cation name, then anion name with an -ide ending. No prefixes, ever, ionic formulas are already fixed by charge balance, so the name doesn't need to specify how many atoms there are.
Worked examples: NaCl → sodium chloride. MgO → magnesium oxide. Al₂O₃ → aluminum oxide (still no prefixes, even though there are two aluminums and three oxygens, the 2:3 ratio is implied entirely by charge balance).
Naming Ionic Compounds with Polyatomic Ions
Same pattern, cation first, then the anion, except the anion is a polyatomic ion used exactly as it's named, never modified.
Worked examples: Na₂SO₄ → sodium sulfate. NH₄Cl → ammonium chloride (here the polyatomic ion is the cation instead, but the rule is identical either way).
Naming Ionic Compounds with Variable-Charge Metals
Most transition metals (and a handful of others, like tin and lead) can form more than one possible charge, so their name needs a Roman numeral showing which charge is actually present in that specific compound. Find it by working backward from the anion's charge, exactly the same reasoning used to assign an oxidation number.
Worked example: FeCl₃. Chlorine is always −1, and there are three of them, for a total of −3. To balance that, iron must be +3, so the name is iron(III) chloride.
Worked example: Cu₂O vs. CuO. Oxygen is −2 in both. In Cu₂O, two coppers must together balance −2, so each copper is +1: copper(I) oxide. In CuO, one copper alone must balance −2, so copper is +2: copper(II) oxide, a different compound entirely, even though both contain only copper and oxygen.
Naming Molecular (Covalent) Compounds
Two nonmetals can often combine in more than one ratio, so molecular compound names use Greek numerical prefixes on both elements to specify the exact atom count: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), hepta- (7), octa- (8). The first element only gets a prefix if there's more than one of it; the second element always gets a prefix, including mono- when there's exactly one.
Worked examples: CO → carbon monoxide (mono- on the second element only). CO₂ → carbon dioxide. N₂O₄ → dinitrogen tetroxide (tetra- drops its final "a" before the vowel in "oxide," the same elision that makes CO "monoxide" instead of "monooxide").
Common Mistakes
- Using Greek prefixes on an ionic compound. Al₂O₃ is aluminum oxide, never "dialuminum trioxide," prefixes belong to molecular compounds only.
- Forgetting the Roman numeral on a variable-charge metal. "Iron chloride" is ambiguous between FeCl₂ and FeCl₃, the Roman numeral isn't optional once the metal has more than one possible charge.
- Modifying a polyatomic ion's name to match the compound. Sulfate stays "sulfate" whether it's sodium sulfate, ammonium sulfate, or aluminum sulfate, the ion's name never changes.
- Adding mono- to the first element of a molecular compound. It's "carbon monoxide," never "monocarbon monoxide."
Related Resources
- AP Chemistry Polyatomic Ions
- AP Chemistry Oxidation Numbers
- Unit 1 Review: Atomic Structure and Properties
- Unit 2 Review: Compound Structure and Properties
- How to Write Net Ionic Equations
- AP Chemistry Empirical & Molecular Formula
- AP Chemistry Study Guide
Frequently Asked Questions
What's the difference between naming ionic and molecular compounds?
Ionic compounds are named cation-first, anion-second, using the anion's -ide (or polyatomic ion) name, with no prefixes for how many atoms are present. Molecular compounds between two nonmetals use Greek numerical prefixes (mono-, di-, tri-...) on both elements to show the exact atom count, since the same two nonmetals can combine in more than one ratio.
How do you know when a metal needs a Roman numeral in its name?
When the metal can form more than one possible charge, most transition metals and a few others like tin and lead. You determine the Roman numeral by working backward from the compound's formula: the anion's total negative charge tells you what the cation's charge must be to balance it.
Is "mono-" ever used for the first element in a molecular compound name?
No. Mono- is only used for the second element, and only to distinguish it from other possible ratios, like carbon monoxide (CO) versus carbon dioxide (CO₂). The first element never gets a mono- prefix even when there's only one of it.
Why does N₂O₄ become "dinitrogen tetroxide" and not "dinitrogen tetraoxide"?
When a prefix ending in a vowel (tetra-, penta-) comes before a word starting with a vowel (oxide), the prefix drops its final vowel for smoother pronunciation. Tetra- + oxide becomes tetroxide; mono- + oxide becomes monoxide, the same rule at work in carbon monoxide.
How do you name a compound with a polyatomic ion?
The same way as any other ionic compound, cation name followed by the polyatomic ion's own name, unchanged. Na₂SO₄ is sodium sulfate; you don't rename or modify sulfate just because it's part of a larger formula.
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