Polyatomic ions are ions which consist of more than one atom. For example, nitrate ion, NO ₃ ^-, contains one nitrogen atom and three oxygen atoms. The atoms in a polyatomic ion are usually covalently bonded to one another, and therefore stay together as a single, charged unit.

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

Rule 2. When the formula unit contains two or more of the same polyatomic ion, that ion is written within parentheses and a subscript is written outside the parentheses to indicate the number of polyatomic ions. Exception: parentheses and a subscript are not used unless more than one of a polyatomic ion is present in the formula unit (e.g., calcium sulfate = "CaSO ₄" not "Ca(SO ₄)"; ammonium carbonate = "(NH ₄) ₂CO ₃" not "(NH ₄) ₂(CO ₃)").

Rule 3. If the cation is a metal ion with a fixed charge, 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", Ca ²⁺ = "calcium", Al ³⁺ = "aluminum"). If the cation is a metal ion with a variable charge, the charge on the cation is indicated by using a Roman numeral, within parentheses, immediately following the name of the cation (e.g., Fe ³⁺ = "iron(III)", Fe ²⁺ = "iron(II)").

Rule 4. If the anion is a monatomic ion, the anion is named by adding the suffix -ide to the root of the 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, or polyatomic ions, in the formula unit for the compound (e.g., Ca(NO ₃) ₂ is named "calcium nitrate" not "calciuim dinitrate").