Coordination number

In chemistry, crystallography, and materials science the coordination number of a central atom in a molecule or crystal is the number of its near neighbors. This number is determined somewhat differently for molecules than for crystals.

For molecules and polyatomic ions the coordination number of an atom is determined by simply counting the other atoms to which it is bonded (by either single or multiple bonds). For example, [Cr(NH₃)₂Cl₂Br₂]⁻ has Cr³⁺ as its central cation, and has a coordination number of 6.

However the solid-state structures of crystals often have less clearly defined bonds, and in these cases a count of neighboring atoms is employed. The simplest method is one used in materials science. The usual value of the coordination number for a given structure refers to an atom in the interior of a crystal lattice with neighbors in all directions. In contexts where crystal surfaces are important, such as materials science and heterogeneous catalysis, the value for an interior atom is the bulk coordination number, while the value for an atom at a surface of the crystal is the surface coordination number.

In chemistry, coordination number (C.N.), defined originally in 1893 by Alfred Werner, is the total number of neighbors of a central atom in a molecule or ion. Although a carbon atom has four chemical bonds in most stable molecules, the coordination number of each carbon is four in methane (CH₄), three in ethylene (H₂C=CH₂, each C is bonded to 2H + 1C = 3 atoms), and two in acetylene (HC≡CH). In effect we count the first bond (or sigma bond) to each neighboring atom, but not the other bonds (pi bonds).

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