The coordination number is the number of atoms, molecules or ions directly bonded to a central atom in a molecular species.
1. Examples: in methane CH4 the coordination number of carbon est 4; that of cobalt is 6 in the complex cation hexaamminecobalt Co(NH3)63+.
2. The expression is used in a different sense in the geometrical description of ionic crystals; here it is the number of nearest neighbours to the ion in question. Example: in sodium chloride the ions form a regular lattice in which each ion has a coordination number of 6.