How to Use Roman Numerals in Chemistry Nomenclature

How to Use Roman Numerals in Chemistry Nomenclature
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Compounds composed of ions are generally easy to name if the metal ions are alkali metals or alkaline earth metals. This is because they have only one ion form. However, its a different case when the compound is a transition metal compound. Any transition metal compound is composed of a positive transition metal ion and a negative anion. A transition metal can have several ion forms, such as iron, which can ionize to form either Fe2+ or Fe3+. We can specify which form of the ion is present in the ionic compound using Roman numerals to indicate their positive charge.

Naming a Transition Metal Ionic Compound Using Roman Numerals

    Determine the symbol for the transition metal in the chemical formula. This is usually the first symbol written in the formula, while the symbol for the anion is written second. For example, if we have the compound FeCl2, the symbol Fe stands for the transition metal and the symbol Cl stands for the anion.

    Determine what transition metal is represented by the symbol in the formula using the periodic table. In our example, Fe is the transition metal, and using the periodic table, we can determine its name as iron.

    Determine the charge of the transition metal ion. In order to do this, use the subscript of the anion as the positive charge of the transition metal ion and the subscript of the metal ion as the negative charge of the anion. For our example of FeCl2, the charge on the metal is Fe2+ since the subscript on the anion is 2, while the anion is Cl-, since the subscript on the metal ion is 1.

    In naming the transition metal ion, add a Roman numeral in parenthesis after the name of the transition metal ion. The Roman numeral must have the same value as the charge of the ion. In our example, the transition metal ion Fe2+ would have the name iron(II).

    Add the name of the anion to the transition metal ion. In our example, FeCl2 would have the name iron(II) chloride since the anion is Cl-, which has the name chloride.

    Things You'll Need

    • Periodic table
    • List of transition metal ion names


    • Find a good list of transition metal ions. This can greatly help in the naming process. These lists are usually available in any standard chemistry textbook, but some periodic tables also list the possible ion forms of transition metals.


    • Transition metal compounds must be treated as toxic whenever handling them in the laboratory or otherwise. Always use safety equipment and follow safety guidelines when handling these.

About the Author

Joshua Suico is a university teacher specializing in chemistry and the life sciences. He holds a Master of Science degree in chemistry. During his college days, he once intentionally dropped sodium pellets into a sink for fun and for science.