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.
|Hydrogen carbonate (bicarbonate)||HCO3-|
|Acetate||C2H3O2- or CH3COO-|
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 CrO42– 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,
PO43– + H2O → HPO42– + 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 SO42– 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 SO42–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, PO43–, 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.