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, sodium iodide, NaI, is composed of sodium ions, Na⁺ (elemental sodium is a metal), and iodide ions, I^- (elemental iodine is a nonmetal).

Rule 1. The positive ion (cation) is written first in the name; the negative ion (anion) is written second in the name.

Rule 2. The name of the cation is the same as the name of the (neutral metal) element from which it is derived (e.g., Na⁺ = "sodium", Ca²⁺ = "calcium", Al³⁺ = "aluminum").

Rule 3. The anion is named by adding the suffix -ide to the root of the (nonmetal) element name (e.g., iodine = I, "iodide" = I^-; sulfur = S, "sulfide" = S^2-).

Note: Greek prefixes are not used to indicate the number of atoms of each element in the formula unit for the compound (e.g., Na₂O is named "sodium oxide" not "disodium oxide", or "disodium monoxide").