Chemical Nomenclature

Core Concepts

Naming binary ionic compounds

For a compound of one cation and one anion, the name is simply the cation name followed by the anion name with an -ide ending:

• NaCl → sodium chloride (Na⁺ + Cl⁻)
• CaO → calcium oxide (Ca²⁺ + O²⁻)
• AlN → aluminum nitride (Al³⁺ + N³⁻)

Monatomic anion names: F⁻ fluoride, Cl⁻ chloride, Br⁻ bromide, I⁻ iodide, O²⁻ oxide, S²⁻ sulfide, N³⁻ nitride, P³⁻ phosphide.

Transition metals and Roman numerals

Metals with more than one common charge use a Roman numeral in parentheses: iron(II) = Fe²⁺, iron(III) = Fe³⁺; copper(I) = Cu⁺, copper(II) = Cu²⁺; lead(II) = Pb²⁺, lead(IV) = Pb⁴⁺. Group 1 and 2 metals, aluminum, zinc, and silver have only one common charge and take no numeral (Na⁺, Ca²⁺, Al³⁺, Zn²⁺, Ag⁺). To find the charge from a formula, use charge balance (worked example below). Older names (ferrous = iron(II), ferric = iron(III)) still appear on labels; the systematic names are preferred.

Compounds with polyatomic ions

Polyatomic ions keep their names inside ionic compounds: NO₃⁻ nitrate, NO₂⁻ nitrite, SO₄²⁻ sulfate, SO₃²⁻ sulfite, PO₄³⁻ phosphate, CO₃²⁻ carbonate, HCO₃⁻ hydrogen carbonate (bicarbonate), OH⁻ hydroxide, NH₄⁺ ammonium, CN⁻ cyanide, CH₃CO₂⁻ acetate. Note the oxygen pattern: -ate has one more oxygen than -ite (nitrate vs nitrite). Examples: NaNO₃ sodium nitrate, Ca(OH)₂ calcium hydroxide, (NH₄)₂SO₄ ammonium sulfate.

Molecular compounds: Greek prefixes

Nonmetal–nonmetal compounds use prefixes to state atom counts: mono- 1, di- 2, tri- 3, tetra- 4, penta- 5, hexa- 6, hepta- 7, octa- 8, nona- 9, deca- 10. The second element keeps the -ide ending:

• CO₂ → carbon dioxide
• N₂O → dinitrogen monoxide
• N₂O₄ → dinitrogen tetroxide
• SF₆ → sulfur hexafluoride

The prefix mono- is usually omitted on the first element (CO is carbon monoxide, not monocarbon monoxide), and the first element's name is not changed to -ide.

Acids

Binary acids (H + one nonmetal) use the pattern hydro- + stem + -ic acid: HCl hydrochloric acid, H₂S hydrosulfuric acid. Oxoacids are named from their oxoanions: anions ending in -ate become -ic acids (NO₃⁻ nitrate → HNO₃ nitric acid; SO₄²⁻ sulfate → H₂SO₄ sulfuric acid); anions ending in -ite become -ous acids (NO₂⁻ nitrite → HNO₂ nitrous acid; SO₃²⁻ sulfite → H₂SO₃ sulfurous acid). Watch the exceptions: phosphorus keeps its full stem (H₃PO₄ phosphoric acid), and chlorine forms four acids (perchloric, chloric, chlorous, hypochlorous).

Hydrates

Ionic compounds that trap water in their crystals are named with a Greek prefix + hydrate: CuSO₄·5H₂O is copper(II) sulfate pentahydrate, and Na₂CO₃·10H₂O is sodium carbonate decahydrate. The dot separates the salt from its water of crystallization; the water is not bonded covalently to the ions.

Worked Example: Formula → Name with Charge Balance

Problem. Name Fe₂O₃ and Cu(NO₃)₂.

Solution. For Fe₂O₃, the three oxide ions contribute 3(-2) = -6. The two iron ions must total +6, so each iron has charge x:

2x + 3(-2) = 0 ⇒ 2x = 6 ⇒ x = +3

So Fe₂O₃ is iron(III) oxide. For Cu(NO₃)₂, each nitrate is −1, two give −2, so copper is +2: copper(II) nitrate. Note the parentheses: Cu(NO₃)₂ shows two complete nitrate groups; writing CuNO₃₂ would be wrong.

Worked Example: Name → Formula

Problem. Write formulas for sodium sulfate, magnesium hydroxide, and dinitrogen monoxide.

Solution. Sodium sulfate: Na⁺ + SO₄²⁻ → charge balance needs two sodiums:

2(+1) + (-2) = 0 ⇒ Na₂SO₄

Magnesium hydroxide: Mg²⁺ + OH⁻ → Mg(OH)₂ (parentheses show two hydroxide groups). Dinitrogen monoxide: di- = 2 nitrogen, mono- = 1 oxygen → N₂O. The name-to-formula direction requires knowing the ion charges and the prefix meanings — the same rules, applied backward.

Worked Example: Naming Acids

Problem. Name HNO₃, HNO₂, and HCl.

Solution. HNO₃ comes from the nitrate ion (NO₃⁻, -ate) → nitric acid. HNO₂ comes from nitrite (NO₂⁻, -ite) → nitrous acid. HCl is a binary acid of hydrogen and chloride → hydrochloric acid. The pattern lets you name an acid from its anion without memorizing each acid separately.

Common Confusions