4.3 Chemical Nomenclature | Chemistry: Atoms First 2e

Learning Objectives

Derive systematic names and formulas for simple ionic and molecular inorganic compounds
Name ionic compounds containing variable-charge metal ions and ionic hydrates
Distinguish and correctly name binary acids and oxyacids

Core Concepts & Principles

Chemical nomenclature provides a standardized, systematic set of rules governed by IUPAC to name chemical compounds uniquely. To name any inorganic compound, first classify it: is it ionic, molecular, or an acid?

1. Simple Ionic Compounds (Fixed Charge)

Ionic compounds consist of a metal cation and a nonmetal anion.

Rule: Name the metal cation first, followed by the nonmetal anion with its ending changed to the suffix -ide (e.g., $\text{NaCl}$ is sodium chloride, $\text{MgO}$ is magnesium oxide).

2. Ionic Compounds with Variable-Charge Metals

Most transition metals and some main group metals can form cations with multiple possible charges.

Rule: Specify the exact charge of the metal using a Roman numeral in parentheses immediately following the metal name (e.g., $\text{FeCl}_2$ is iron(II) chloride; $\text{FeCl}_3$ is iron(III) chloride).

3. Ionic Hydrates

Hydrates are ionic compounds that incorporate trapped water molecules within their crystal lattices.

Rule: Name the anhydrous ionic compound normally, then append a Greek prefix indicating the number of water molecules followed by the word "hydrate" (e.g., $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$ is copper(II) sulfate pentahydrate).

4. Molecular (Covalent) Compounds

Molecular compounds form between two nonmetals. Because nonmetals can combine in multiple variable ratios, atom counts must be explicitly stated.

Rule: Name the more metallic element (further left/bottom on the periodic table) first. Name the second element with an -ide ending. Use Greek prefixes (mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-) to designate atom counts. Omit mono- for the first element (e.g., $\text{CO}_2$ is carbon dioxide; $\text{N}_2\text{O}_4$ is dinitrogen tetroxide).

5. Acids

Acids release hydrogen ions ( $\text{H}^+$ ) when dissolved in water:

Binary Acids ( $\text{H}$ + single nonmetal): Use the prefix hydro-, change the nonmetal root ending to -ic, and add the word acid (e.g., $\text{HCl}(aq)$ is hydrochloric acid).
Oxyacids ( $\text{H}$ + polyatomic oxygen-containing ion): Omit "hydrogen", take the anion root name, replace -ate with -ic or -ite with -ous, and add acid (e.g., $\text{HNO}_3$ is nitric acid from nitrate; $\text{HNO}_2$ is nitrous acid from nitrite).

The Golden Rule of Nomenclature

Always identify the bonding type first. Never use Greek prefixes (di-, tri-) for ionic compounds, and never use Roman numerals for molecular covalent compounds!

Problem-Solving Routines & Methods

How to Systematically Name Any Inorganic Compound

1
Step 1: Identify compound class. Does it start with a metal (ionic), two nonmetals (molecular), or hydrogen (acid)?
2
Step 2: If ionic, determine if the metal has a fixed or variable charge. Calculate the cation charge using the anion if variable, and insert Roman numerals.
3
Step 3: If molecular, write element names directly and apply Greek prefixes for atom counts, ending the second element in -ide.
4
Step 4: If an acid, check if it is aqueous binary (hydro-...-ic acid) or an oxyacid (-ic/-ous acid derived from the oxoanion).

Pro-Tip: Always verify overall electrical neutrality for ionic formulas before finalizing names.

Fixed-Charge Ionic Compound Formula Naming

\text{Name} = \text{Cation Name} + \text{Anion Name (-ide)}

Standard naming structure for binary ionic compounds.

Variables & Constants

\text{Cation Name}

=exact name of metal;

\text{Anion Name}

=nonmetal root + -ide suffix

Practice & Concept Checks

Concept Check

Why are Greek prefixes used when naming molecular compounds, but Roman numerals are used for ionic compounds with variable-charge metals?

Concept Check

What is the difference in naming and formulation between hydrogen chloride gas,

\text{HCl}(g)

, and hydrochloric acid,

\text{HCl}(aq)

Key Terms & Vocabulary

NomenclatureGeneral Principles

A systematic collection of rules and conventions used for naming chemical compounds.

Example: IUPAC guidelines for inorganic chemistry.

binary compoundsCompound Classes

Chemical compounds composed of exactly two different elements.

Example:

\text{NaCl}

\text{CO}_2

, and

\text{H}_2\text{O}

acidsAcids & Bases

Substances that release hydrogen ions (

\text{H}^+

) when dissolved in aqueous solutions.

Example:

\text{HCl}

and

\text{H}_2\text{SO}_4

hydratesIonic Compounds

Ionic compounds that contain water molecules bound as an integral part of their crystal lattices in specific ratios.

Example:

\text{CuSO}_4 \cdot 5\text{H}_2\text{O}

(copper(II) sulfate pentahydrate).

binary acidAcids & Bases

An acid composed of hydrogen combined with a single nonmetallic element, dissolved in water.

Example:

\text{HCl}(aq)

(hydrochloric acid).

oxyacidsAcids & Bases

Acids containing hydrogen, oxygen, and at least one other element, typically derived from an oxoanion.

Example:

\text{HNO}_3

(nitric acid) and

\text{H}_3\text{PO}_4

(phosphoric acid).