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Inorganic Nomenclature

Chemical names are a code: they tell you exactly what is in a compound and in what ratio. Learn the patterns once and you can name — or write the formula for — almost any inorganic compound.

Ionic Compounds

Name the cation first, then the anion. A monatomic cation keeps the element name (sodium, calcium); a monatomic anion takes the -ide ending (chloride, oxide, sulfide). NaCl is sodium chloride; MgO is magnesium oxide.

Metals that form more than one charge — most transition metals — need a Roman numeral for the charge: Fe²⁺ is iron(II), Fe³⁺ is iron(III). Find the charge from the anion: in FeCl₃, three Cl⁻ give −3, so iron is +3: iron(III) chloride. (The older -ous/-ic names, like ferrous and ferric, are no longer used.)

Polyatomic Ions

Memorize these — they appear everywhere:

NH₄⁺ ammonium, OH⁻ hydroxide, CN⁻ cyanide
NO₃⁻ nitrate, NO₂⁻ nitrite, CO₃²⁻ carbonate, HCO₃⁻ hydrogen carbonate (bicarbonate)
SO₄²⁻ sulfate, SO₃²⁻ sulfite, PO₄³⁻ phosphate, ClO₃⁻ chlorate, ClO₄⁻ perchlorate, ClO⁻ hypochlorite
CrO₄²⁻ chromate, Cr₂O₇²⁻ dichromate, MnO₄⁻ permanganate, C₂H₃O₂⁻ acetate

The -ate / -ite pattern: -ate is the more common, more oxygenated ion; -ite has one fewer oxygen. Per- -ate adds one more oxygen; hypo- -ite removes one. When a polyatomic ion needs a subscript, put it in parentheses: Ca(OH)₂, Al₂(SO₄)₃.

Example: Name Fe₂O₃. Three O²⁻ give −6, so the two irons must total +6: each iron is +3. The name is iron(III) oxide.

Example: Write the formula for calcium phosphate. Ca²⁺ and PO₄³⁻ must balance to zero: the lowest common multiple of 2 and 3 is 6, so three Ca²⁺ (+6) and two PO₄³⁻ (−6): $$\text{Ca}_3(\text{PO}_4)_2$$

Molecular (Covalent) Compounds

Two nonmetals share electrons, and the name uses Greek prefixes to give the numbers: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), hepta- (7), octa- (8), nona- (9), deca- (10). The first element usually drops mono-; the second element always takes -ide. CO is carbon monoxide, CO₂ is carbon dioxide, PCl₅ is phosphorus pentachloride.

Example: N₂O₅ — two nitrogens, five oxygens: dinitrogen pentoxide.

Acids

Binary acids (hydrogen + one other element) take hydro- -ic acid: HCl is hydrochloric acid, HBr is hydrobromic acid, H₂S is hydrosulfuric acid.

Oxoacids (hydrogen + a polyatomic ion with oxygen) follow the ion: an -ate ion gives an -ic acid, an -ite ion gives an -ous acid. H₂SO₄ (sulfate) is sulfuric acid, H₂SO₃ (sulfite) is sulfurous acid, HNO₃ (nitrate) is nitric acid, HNO₂ (nitrite) is nitrous acid, HClO₄ (perchlorate) is perchloric acid.

Example: H₃PO₄ contains the phosphate ion, PO₄³⁻ — an -ate ion, so the acid is phosphoric acid.

Hydrates

A hydrate is an ionic compound with water molecules locked into its crystal. Name the compound, then add a prefix + "hydrate" for the waters, joined with a dot in the formula: CuSO₄·5H₂O is copper(II) sulfate pentahydrate. Others: Na₂CO₃·10H₂O is sodium carbonate decahydrate (washing soda); MgSO₄·7H₂O is magnesium sulfate heptahydrate (Epsom salts).

Practice Problems

Name CuCl₂.
Name SO₃.
Write the formula for aluminum sulfate.
Name HClO₄.

Answers: 1. Copper(II) chloride 2. Sulfur trioxide 3. Al₂(SO₄)₃ 4. Perchloric acid