Pauling's rules

The Pauling linking rules are five rules that were published in 1929 by Linus Pauling. They are used to determine the crystal structure of ionic crystals built up and thus represent one of the foundations of the crystal chemistry dar.

1 A coordination polyhedron of anions is formed about each cation. The cation - anion distance is determined by the sum of the ionic radii, coordination number (abbreviated KZ) by the radius ratio ( rK / rA).

Examples of radii ratios and related coordination polyhedra ( the densest sphere packings have a coordination number of 12 ) are:

2 An ionic structure is stable if the sum of the thicknesses of the electrostatic bonds of each anion to all next cations in magnitude equal to the charge of this anion. This means that a stable ionic structure receives local electrical neutrality. Expressed mathematically, the following applies:

Cations with a O2 - anion, the bond strengths are for example:

Common edges 3 and in particular surfaces of two polyhedra reduce the stability of the structure. This effect is greater the greater is the charge of the cation and the smaller is the coordination number. This effect stems from the fact that the cations closer get in face-sharing coordination polyhedra than your edges or tip link. The repulsive interaction is stronger. It is especially pronounced when the radius ratio is near the lower limit for the Polyederstabilität.

4 In a crystal with different cations those with high valence ( high charge ) and low coordination number strive to not share polyhedral elements. Therefore, in the aluminosilicates Si4 - and Al3 tetrahedra mostly on corners, rarely interconnected by edges.

5 The number of different constituents ( or components ) of a crystal tends to be small. This means, for example, that for chemically similar atoms / ions like environments are formed.