Rules for Naming Binary Ionic Compounds Containing a Metal Ion With a Variable Charge

A binary ionic compound is composed of ions of two different elements - one of which is a metal, and the other a nonmetal. For example, iron(III) iodide, FeI3, is composed of iron ions, Fe3+ (elemental iron is a metal), and iodide ions, I- (elemental iodine is a nonmetal). Rule 1. The cation is written first in the name; the anion is written second in the name. Rule 2. The name of the cation is the same as the (neutral) element from which it is derived. The charge on the cation is indicated using a Roman numeral, in parentheses, immediately following the name of the cation (e.g., Fe3+ = "iron(III)"). Rule 3. The anion is named by adding the suffix -ide to the root of the element name (e.g., I- = "iodide"). Note: Greek prefixes are not used to indicate the number of atoms of each element in the formula unit for the compound (e.g., FeI3 is named "iron(III) iodide" not "iron(III) triiodide").