Ionic Compounds Containing Polyatomic Ions

When polyatomicContaining two or more atoms. 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 latticeThe points in space that define the ordered, repeating arrangement of atoms, ions, or molecules in a crystal. 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 solubleAble to dissolve in a solvent to a significant extent. 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-metalAn element characterized by a glossy surface, high thermal and electrical conductivity, malleability, and ductility. 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 saltAn ionic compound that can be formed by replacing the hydrogen ion of an acid with a different cation. must contain three calcium ions for every two phosphate ions:

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