Defined by Alfred Werner in 1893, a coordination number is a term used in chemistry to denote the number of bonds a central metal ion has in a coordination compound. Coordination numbers range from two to 16, with four and six being the most common. Though they are normally used for transition metal atoms in a compound, they can also refer to nonmetallic substances as well.
Neutral atoms — which are atoms that have no charge — or cations — atoms with a positive charge — can both be the central atom in a coordination compound. These atoms are usually transition metals, which are those elements that react with halogens. Iron, copper, gold, and chromium are all examples of transition metals.
Transition metals are listed in groups three through 12 on the Periodic Table of Elements. Their reaction with other substances is usually obvious because solutions and compounds undergo a color change. For example, Ferric Chloride (FeCl) is usually green-black, but when dissolved in a solution it turns yellow. Conversely, halogens include fluorine, chlorine, and iodine. They always have seven valence electrons, or electrons capable of sharing in bonds with other atoms.
Attached to the metal ion is a ligand. Either an atom or a molecule, a ligand can be neutral or negatively charged, an anion. Ligands are usually halogens.
It is the attachments of ligands to the main metal ion that results in the coordination number. For example, Ag[NH3]2+, or diamminesilver ion, is a simple coordination compound with a coordination number of two. Ag, or silver, is the central metal ion. Attached to the silver atom are two ammonia molecules, NH3, one on either side of the atom. The plus sign in the chemical name shows that the compound is positively charged.
Coordination compounds are often used as catalysts, or substances that start or change the rate of a chemical reaction. The actual compound can be positively charged, negatively charged, or neutral. Compounds with coordination numbers may be referred to by the actual number or by a geometric name which reflects both the coordination number and the basic shape of the compound.
For example, a compound with a coordination number of two will be linear, so these compounds are called collinear. Collinear compounds are uncommon in metals, but relatively common for non-metals. Coordination numbers of three are called trigonal planar. These are rare, but are found when ligands are particularly large. Tetrahedral compounds have coordination numbers of four, while fives are either trigonal-bipyramidal or square-pyramidal.