How to Name Ionic Compounds

Ionic compounds consist of cations (positive ions) and anions (negative ions). Ionic compound nomenclature or naming is based on the names of the component ions. In all cases, ionic compound naming gives the positively charged cation first, followed by the negatively charged anion. Here are the principal naming conventions for ionic compounds, along with examples to show how they are used:

Roman Numerals in Ionic Compound Names

A Roman numeral in parentheses, followed by the name of the element, is used for elements that can form more than one positive ion. There is no space between the element name and the parenthesis. This notation is usually seen with metals since they commonly display more than one oxidation state or valence. You can use a chart to see the possible valences for the elements.

• Fe²⁺ Iron(II)
• Fe³⁺ Iron(III)
• Cu⁺ Copper(I)
• Cu²⁺ Copper(II)

Example: Fe₂O₃ is iron(III) oxide.

Naming Ionic Compounds Using -ous and -ic

Although Roman numerals are used to denote the ionic charge of cations, it is still common to see and use the endings -ous or -ic. These endings are added to the Latin name of the element (e.g., stannous/stannic for tin) to represent the ions with lesser or greater charge, respectively. The Roman numeral naming convention has wider appeal because many ions have more than two valences.

• Fe²⁺ Ferrous
• Fe³⁺ Ferric
• Cu⁺ Cuprous
• Cu²⁺ Cupric

Example: FeCl₃ is ferric chloride or iron(III) chloride.

Naming Ionic Compounds Using -ide

The -ide ending is added to the name of a monoatomic ion of an element.

• H^- Hydride
• F^- Fluoride
• O^2- Oxide
• S^2- Sulfide
• N^3- Nitride
• P^3- Phosphide

Example: Cu₃P is copper phosphide or copper(I) phosphide.

Naming Ionic Compounds Using -ite and -ate

Some polyatomic anions contain oxygen. These anions are called oxyanions. When an element forms two oxyanions, the one with less oxygen is given a name ending in -ite and the one with more oxygen are given a name that ends in -ate.

• NO₂^- Nitrite
• NO₃^- Nitrate
• SO₃^2- Sulfite
• SO₄^2- Sulfate

Example: KNO₂ is potassium nitrite, while KNO₃ is potassium nitrate.

Naming Ionic Compounds Using hypo- and per-

In the case where there is a series of four oxyanions, the hypo- and per- prefixes are used in conjunction with the -ite and -ate suffixes. The hypo- and per- prefixes indicate less oxygen and more oxygen, respectively.

• ClO^- Hypochlorite
• ClO₂^- Chlorite
• ClO₃^- Chlorate
• ClO₄^- Perchlorate

Example: The bleaching agent sodium hypochlorite is NaClO. It is also sometimes called the sodium salt of hypochlorous acid.

Ionic Compounds Containing bi- and di- Hydrogen

Polyatomic anions sometimes gain one or more H⁺ ions to form anions of a lower charge. These ions are named by adding the word hydrogen or dihydrogen in front of the name of the anion. It is still common to see and use the older naming convention in which the prefix bi- is used to indicate the addition of a single hydrogen ion.

• HCO₃^- Hydrogen carbonate or bicarbonate
• HSO₄^- Hydrogen sulfate or bisulfate
• H₂PO₄^- Dihydrogen phosphate

Example: The classic example is the chemical name for water, H2O, which is dihydrogen monoxide or dihydrogen oxide. Dihydrogen dioxide, H₂O₂, is more commonly called hydrogen dioxide or hydrogen peroxide.