2.7 Chemical Nomenclature

Chemical nomenclature is the system chemists use to name compounds. Once you know the rules, you can look at a name like "iron(II) chloride" and immediately know what elements are in it and how they're arranged. You can also work backward: see a formula and produce the correct name.

The rules depend on what type of compound you're dealing with. Ionic compounds (metal + nonmetal) follow one set of rules, molecular compounds (nonmetal + nonmetal) follow another, and acids have their own naming pattern. This guide walks through each one.

Chemical Nomenclature

Systematic Nomenclature for Inorganic Compounds

There are three main categories of compounds you need to name, and each has its own logic.

Ionic Compounds are made of a cation (positive ion) and an anion (negative ion). The naming pattern is straightforward: say the cation name first, then the anion name.

• The cation keeps its element name. Sodium stays "sodium," calcium stays "calcium."
• If the cation is a transition metal that can form more than one charge, you indicate the charge with Roman numerals in parentheses. For example, iron can be $\text{Fe}^{2+}$ or $\text{Fe}^{3+}$, so you write iron(II) chloride or iron(III) chloride to distinguish them.
• The anion's name depends on whether it's a single atom or a group of atoms:
  • Monatomic anions take the root of the element name plus "-ide." Chlorine becomes chloride ($\text{Cl}^-$), oxygen becomes oxide ($\text{O}^{2-}$), sulfur becomes sulfide ($\text{S}^{2-}$).
  • Polyatomic anions have their own names you need to memorize: nitrate ($\text{NO}_3^-$), sulfate ($\text{SO}_4^{2-}$), carbonate ($\text{CO}_3^{2-}$), and others.

Molecular Compounds are made of two or more nonmetals bonded together. Instead of charges, you use prefixes to show how many atoms of each element are present.

• The prefixes: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), hepta- (7), octa- (8), nona- (9), deca- (10)
• Drop "mono-" from the first element. Carbon monoxide is correct, not "monocarbon monoxide."
• The second element always ends in "-ide."
• Examples: carbon dioxide ($\text{CO}_2$), dinitrogen pentoxide ($\text{N}_2\text{O}_5$), sulfur trioxide ($\text{SO}_3$)

Acids are named based on the anion they produce when dissolved in water. There are three patterns to learn:

Ionic vs. Molecular Compound Formulas

Recognizing whether a compound is ionic or molecular is the first step in naming it correctly.

• Ionic compounds contain a metal and a nonmetal. Their formulas represent the simplest whole-number ratio of ions, not individual molecules. $\text{NaCl}$ means a 1:1 ratio of sodium ions to chloride ions arranged in a repeating crystal lattice.
• Molecular compounds contain only nonmetals. Their formulas show the actual number of atoms in one molecule. $\text{CO}_2$ means each molecule literally has one carbon atom and two oxygen atoms, held together by covalent bonds (shared electrons).
• Binary compounds are compounds made of exactly two elements. They can be either ionic ($\text{NaCl}$) or molecular ($\text{CO}_2$).

Quick check: look at the elements involved. Metal + nonmetal = ionic. Nonmetal + nonmetal = molecular.

Writing Chemical Formulas from Compound Names

For ionic compounds, follow these steps:

1. Write the symbol and charge of the cation, then the symbol and charge of the anion.
2. Balance the charges so the compound is electrically neutral. The total positive charge must equal the total negative charge.
3. Use subscripts to show how many of each ion you need. If the subscripts can be reduced, reduce them.

Worked examples:

• Sodium chloride: $\text{Na}^+ \text{Cl}^-$ → charges already balance → $\text{NaCl}$
• Calcium chloride: $\text{Ca}^{2+} \text{Cl}^-$ → need two $\text{Cl}^-$ to balance one $\text{Ca}^{2+}$ → $\text{CaCl}_2$
• Aluminum oxide: $\text{Al}^{3+} \text{O}^{2-}$ → need 2 Al and 3 O to balance (total charge: +6 and -6) → $\text{Al}_2\text{O}_3$

For molecular compounds, the prefixes in the name tell you exactly what to write:

1. Identify each element and its prefix.
2. Convert the prefix to a number and use it as the subscript.

• Dinitrogen pentoxide: di = 2 nitrogen, penta = 5 oxygen → $\text{N}_2\text{O}_5$
• Carbon dioxide: no prefix on carbon = 1, di = 2 oxygen → $\text{CO}_2$

Additional Nomenclature Concepts

• Chemical formula: A shorthand way to represent a compound's composition using element symbols and numerical subscripts. $\text{H}_2\text{O}$ tells you there are 2 hydrogen atoms and 1 oxygen atom.
• IUPAC (International Union of Pure and Applied Chemistry): The organization that sets the official rules for naming chemical compounds. The naming conventions in this guide follow IUPAC standards.
• Polyatomic ions: Charged groups of two or more atoms bonded together that act as a single unit. You'll need to memorize the common ones (nitrate, sulfate, carbonate, phosphate, hydroxide, etc.).
• Oxyanions: A specific type of polyatomic ion that contains oxygen. They're named with "-ate" or "-ite" suffixes, where "-ate" has more oxygen atoms and "-ite" has fewer. For example, sulfate ($\text{SO}_4^{2-}$) has more oxygen than sulfite ($\text{SO}_3^{2-}$).