The rules for naming ionic compounds are a simple set of instructions that tell you how to convert a chemical formula into a written compound name. Here is the list of rules along with examples of name of binary and polyatomic compounds. The list includes some of the uncommon rules and exceptions you need to know.
Rules for Naming Ionic Compounds
- First, balance the chemical formula for mass and charge. An ionic compound is electrically neutral. In other words, the positive electrical charges of the cations exactly balance the negative electrical charges of the anion.
- Write the element name of the cation first. This is almost always a metal. One significant exception is the ammonium cation (NH4+), which is a polyatomic cation consisting of nonmetals. It forms ionic compounds, such as ammonium chloride (NH4Cl).
- If the cation element has more than one possible oxidation state (charge), follow the element name by parentheses containing the appropriate Roman numeral. There is no space between the element name and the parentheses. The elements this applies to are some transition metals, including the lanthanides and actinides. For example, the iron cation is either iron(II) or iron(III). Metals in group 1 (alkali metals) and group 2 (alkaline earth metals) always go by their element name, with no Roman numerals.
- Unlike covalent compound names, there is no prefix indicating the number of atoms in the cation. So, Hg2Cl2 is mercury(II) chloride and not dimercury(II) dichloride. The exception is when you indicate relative composition. So, K2MgCl4 is magnesium dipotassium tetrachloride, distinguishing it from KMgCl3, which is magnesium potassium trichloride.
- If there are multiple metal atoms in the formula, the formula and the name list the elements in alphabetical order. Sometimes this order is different for the formula and the name. So, KMgCl3 is magnesium potassium trichloride and not potassium magnesium trichloride.
- The anion name follows the cation. A space separates the two parts of the ionic compound name. If the compound is a binary compound or the anion is a single type of atom, the anion name is a combination of the element name with the -ide ending. For example, AgCl has the name silver chloride.
- If the anion consists of multiple types of atoms, use its polyatomic ion name.
Examples of Ionic Compound Names and Formulas
Here are examples of binary chemical formulas and names:
Classic Ionic Compound Naming System
When reading older texts and articles, you’ll see the classic names of ionic compounds. The rules for naming anions are the same. But, istead of using Roman numerals, the cation names reflect the metal oxidation state using the endings -ous (lesser charge) or -ic (greater charge).
- Fe2+ – ferrous
- Fe3+ – ferric
- Cu+ – cuprous
- Cu2+ – cupric
- Sn2+ – stannic
- Sn4+ – stannous
- Pb2+ – plumbous
- Pb4+ – plumbic
- Cr2+ – chromous
- Cr3+ – chromic
- Au+ – aurous
- Au3+ – auric
Replacing this system with the present one avoids unnecessary confusion and allows for more than just two oxidation states. For example, the old system did not cover naming the Cr6+ (hexavalent chromium) oxidation state.
Ionic Compound Naming Worksheet
Practice naming ionic compounds. This worksheet tests your ability to give names and formulas. The worksheet and answer key are in PDF format for easy downloading and printing.
- Fernelius, W. Conard (November 1982). “Numbers in chemical names”. Journal of Chemical Education. 59 (11): 964. doi:10.1021/ed059p964
- International Union of Pure and Applied Chemistry, Division of Chemical Nomenclature (2005). Neil G. Connelly (ed.). Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005 (New ed.). Cambridge: RSC Publ. ISBN 978-0-85404-438-2.
- Kotz, John C.; Treichel, Paul M; Weaver, Gabriela C. (2006). Chemistry and Chemical Reactivity (6th ed.). Belmont, CA: Thomson Brooks/Cole. ISBN 978-0-534-99766-3.
- Zumdahl, Steven S. (1989). Chemistry (2nd ed.). Lexington, Mass.: D.C. Heath. ISBN 978-0-669-16708-5.