When polyatomic ions are included, the number of ionic compounds increases significantly. Indeed, most ionic compounds contain polyatomic ions. Well-known examples are sodium hydroxide (NaOH), calcium carbonate (CaCO₃), and ammonium nitrate (NH₄NO₃). A list of the more important polyatomic ions is given in the following table. A great many of them are oxyanions.

The properties of compounds containing polyatomic ions are very similar to those of binary ionic compounds. The ions are arranged in a regular lattice and held together by coulombic forces of attraction. The resulting crystalline solids usually have high melting points and all conduct electricity when molten. Most are soluble in water and form conducting solutions in which the ions can move around as independent entities. In general polyatomic ions are colorless, unless, like CrO₄^2– or MnO₄^–, they contain a transition- metal atom. The more negatively charged polyatomic ions, like their monatomic counterparts, show a distinct tendency to react with water, producing hydroxide ions; for example,

PO₄^3– + H₂O → HPO₄^2– + OH^–

It is important to realize that compounds containing polyatomic ions must be electrically neutral. In a crystal of calcium sulfate, for instance, there must be equal numbers of Ca²⁺ and SO₄^2– ions in order for the charges to balance. The formula is thus CaSO₄. In the case of sodium sulfate, by contrast, the Na⁺ ion has only a single charge. In this case we need two Na⁺ ions for each SO₄^2–ion in order to achieve electroneutrality. The formula is thus Na₂SO₄.

EXAMPLE What is the formula of the ionic compound calcium phosphate?

Solution It is necessary to have the correct ratio of calcium ions, Ca²⁺, and phosphate ions, PO₄^3–, in order to achieve electroneutrality. The required ratio is the inverse of the ratio of the charges on ions. Since the charges are in the ratio of 2:3, the numbers must be in the ratio of 3:2. In other words the solid salt must contain three calcium ions for every two phosphate ions:

The formula for calcium phosphate is thus Ca₃(PO₄)₂.