Chemical Nomenclature

Core Concepts

Binary ionic compounds

Name the cation first, then the anion with its ending changed to -ide: NaCl is sodium chloride, MgO is magnesium oxide, CaF₂ is calcium fluoride, K₂S is potassium sulfide. Cations keep their element name; simple anions take the element's stem plus -ide (chlorine → chloride, oxygen → oxide, nitrogen → nitride).

Metals with variable charge: the Roman numeral (Stock) system

Many transition metals form more than one cation: iron gives Fe²⁺ and Fe³⁺, copper gives Cu⁺ and Cu²⁺, tin gives Sn²⁺ and Sn⁴⁺. The charge is written as a Roman numeral in parentheses after the metal name: FeCl₂ is iron(II) chloride and FeCl₃ is iron(III) chloride; Cu₂O is copper(I) oxide and CuO is copper(II) oxide. To find the charge, work backward from the anion: in Fe₂O₃, three O²⁻ ions total −6, so the two iron ions must be +3 each — iron(III) oxide. A few metals always have one charge and take no numeral: Zn²⁺, Ag⁺, Cd²⁺, Al³⁺ (and the group 1 and 2 metals). An older system uses -ous (lower charge) and -ic (higher charge): ferrous = Fe²⁺, ferric = Fe³⁺; cuprous = Cu⁺, cupric = Cu²⁺.

Compounds with polyatomic ions

Polyatomic ions keep their own names. Na₂SO₄ is sodium sulfate, NH₄NO₃ is ammonium nitrate, Ca(OH)₂ is calcium hydroxide, KCN is potassium cyanide, NaHCO₃ is sodium hydrogen carbonate (also called sodium bicarbonate). The -ate/-ite endings signal oxygen content: nitrate (NO₃⁻) has one more oxygen than nitrite (NO₂⁻); sulfate (SO₄²⁻) has one more than sulfite (SO₃²⁻). Prefixes extend the pattern: per- means one more oxygen than -ate (perchlorate, ClO₄⁻), and hypo- means one fewer than -ite (hypochlorite, ClO⁻).

Molecular (binary covalent) compounds

When two nonmetals combine, use Greek prefixes to state how many atoms of each element are present: mono- 1, di- 2, tri- 3, tetra- 4, penta- 5, hexa- 6, hepta- 7, octa- 8, nona- 9, deca- 10. The second element ends in -ide, and mono- is usually omitted on the first element: CO₂ carbon dioxide, CO carbon monoxide, N₂O₅ dinitrogen pentoxide, P₂O₅ diphosphorus pentoxide, SF₆ sulfur hexafluoride, N₂O dinitrogen monoxide. A handful of common compounds keep traditional names: H₂O water, NH₃ ammonia, CH₄ methane.

Acids

Binary acids (H + one nonmetal) use hydro- + element stem + -ic acid: HCl hydrochloric acid, HBr hydrobromic acid, H₂S hydrosulfuric acid. Oxyacids (H + a polyatomic ion) follow the anion's ending: -ate anions give -ic acids (NO₃⁻ nitrate → HNO₃ nitric acid; SO₄²⁻ sulfate → H₂SO₄ sulfuric acid; CO₃²⁻ carbonate → H₂CO₃ carbonic acid), and -ite anions give -ous acids (NO₂⁻ nitrite → HNO₂ nitrous acid; SO₃²⁻ sulfite → H₂SO₃ sulfurous acid). Memorize the special names: H₃PO₄ phosphoric acid, HC₂H₃O₂ acetic acid, HClO₄ perchloric acid, HClO hypochlorous acid.

Hydrates

A hydrate is an ionic compound that includes fixed water molecules in its crystal. Name the salt, then add a Greek prefix and the word hydrate: CuSO₄·5H₂O copper(II) sulfate pentahydrate; BaCl₂·2H₂O barium chloride dihydrate.

How It Works / Step-by-Step Process

1. Classify the compound: ionic (metal/polyatomic + anion), binary molecular (two nonmetals), acid (starts with H), or hydrate (contains ·nH₂O).
2. Ionic: name the cation (add a Roman numeral if the metal has variable charge), then the anion (-ide or polyatomic name).
3. Molecular: name both elements with Greek prefixes; the second element ends in -ide.
4. Acid: binary → hydro-…-ic; oxyacid → match the anion ending (-ate → -ic; -ite → -ous).
5. Hydrate: salt name + Greek prefix + "hydrate."

Common Confusions