Coordination number

As a coordination number (CN ) is defined as the number of nearest neighbors of a structural unit ( atom, ion, molecule) in a crystal (eg ion crystal, metal grid) or the number of directly bonded to a central atom atoms in a complex. A crystal structure is described by specifying the position of their building blocks, that is, by their coordination in the unit cell.

The most common coordination numbers 4 and 6 come on, where 2, 3, 8, and 12 occur in complexes sometimes other values ​​. The coordination number is, among others, by the ratio of the radii of the ions ( the ionic crystal ) or the binding properties depend ( in complexes ). The coordination number in crystals increases with the pressure to most, such as the transition from graphite (KZ: 3) Diamond (KZ: 4).

Specifying the coordination number in the crystal lattice occurs in brackets. Example sodium chloride may be mentioned Na Cl, which corresponds to a rectangular grid structure. Each sodium ion has 6 neighboring chloride ions and each chloride as 6 neighbor sodium ions as coordination polyhedron is obtained in both cases, an octahedron.

In certain crystal structures can be one and the same structural unit are also found on items with different coordination numbers.

Examples

In the following table, the coordination numbers of some types of gratings are illustrated.