Naming Molecular Compounds and Acids

Core Idea

Binary molecular compounds, made of two nonmetals, are named with Greek prefixes that state how many atoms of each element are present, as in dinitrogen tetroxide for N₂O₄. Ionic compounds do not use these prefixes because their charges fix the ratio. Acids follow their own rules: binary acids in water take the form hydro-…-ic acid, and oxoacids take -ic or -ous endings that track the -ate or -ite name of their anion.

How It's Best Learned

First decide whether a compound is ionic or molecular, since that choice selects the naming system. Drill the ten prefixes with real molecules such as CO, CO₂, SF₆ and P₄O₁₀. For acids, start from the anion name you already know and convert it: sulfate to sulfuric acid, sulfite to sulfurous acid, chloride to hydrochloric acid.

Common Misconceptions

• Every compound name uses prefixes, so CaCl₂ is calcium dichloride. (Atom-count prefixes are used for molecular compounds; CaCl₂ is ionic, its charges fix the ratio, and it is named calcium chloride.)
• The first element always gets a prefix, including mono-. (Mono- is dropped for the first element: CO is carbon monoxide, not monocarbon monoxide.)
• HCl is always called hydrochloric acid. (Pure HCl gas is hydrogen chloride; the acid name applies to its solution in water.)

Explainer

You already know how to name ionic compounds. You name the cation, then the anion, and the charges decide the formula. Molecular compounds need a different system. Two nonmetals can combine in several ratios, so the name must say how many atoms of each are present. Nitrogen and oxygen alone form NO, NO₂, N₂O, N₂O₄ and N₂O₅.

For a binary molecular compound, write the element names in the order they appear in the formula. The element farther left or lower in the periodic table usually comes first. Give the second element an -ide ending, as in oxide, chloride or fluoride. Then add Greek prefixes for the number of atoms: mono (1), di (2), tri (3), tetra (4), penta (5), hexa (6), hepta (7), octa (8), nona (9) and deca (10).

Two small rules keep names tidy. First, leave off mono- for the first element. CO is carbon monoxide, and CO₂ is carbon dioxide. Second, in common usage the final a or o of a prefix is often dropped before a vowel, so you write monoxide, pentoxide and tetroxide. Strict IUPAC usage drops only the o of mono-. Some examples: SF₆ is sulfur hexafluoride, P₄O₁₀ is tetraphosphorus decaoxide and CCl₄ is carbon tetrachloride.

Acids have their own naming pattern because their names describe what happens in water. A binary acid is hydrogen combined with one nonmetal. Dissolved in water, it is named with hydro-, the stem of the nonmetal, and -ic acid. HCl(aq) is hydrochloric acid, and HBr(aq) is hydrobromic acid. As a pure gas, though, HCl is simply hydrogen chloride.

Oxoacids contain hydrogen, oxygen and another element. Name them from their anions. If the anion ends in -ate, the acid ends in -ic. If the anion ends in -ite, the acid ends in -ous. Sulfate (SO₄²⁻) gives sulfuric acid, H₂SO₄, and sulfite (SO₃²⁻) gives sulfurous acid, H₂SO₃. Nitrate gives nitric acid, and nitrite gives nitrous acid. The chlorine family adds the prefixes per- and hypo-: perchlorate gives perchloric acid (HClO₄), and hypochlorite gives hypochlorous acid (HClO).

The decision tree is short. Is it an acid in water? Use the acid rules. Is it ionic? Name the ions. Is it two nonmetals? Use prefixes.