Pauling Rule

"Ions are essentially Charged, Incompressible, Non-Polarizable Spheres"

More sophisticated models assume ions are composed of two parts:

* a central hard, unperturbable core, where most electron density is concentrated
* a soft, polarizable outer sphere, which contains very little electron density


Pauling Rule 1: Coordination Polyhedra


"A coordination polyhedron of anions is formed around every cation (and vice-versa) - it will only be stable if the cation is in contact with each of its neighbours.

* Ionic crystals may thus be considered as sets of linked polyhedra.
* The cation-anion distance is regarded as the sum of the ionic radii."

Common coordination polyhedra are:-






Molecular Materials

* Absolute coordination numbers are controlled by valency (VSEPR)

Non-Molecular Materials

* Valency has only an indirect bearing on coordination number

e.g. NaICl, MgIIO, ScIIIN, TiIVC all have the Rock Salt (6:6) Structure despite change in valency and from predominantly ionic to covalent bonding
* Ionic Size does influence coordination number

The Coordination Number of the Cation will be Maximized subject to the criterion of Maintaining Cation-Anion Contact

Determined by comparison of the ratio of the ionic radii, r+/r- with values derived from the geometric contact criterion

The Radius Ratio Rules






http://www.chem.ox.ac.uk/icl/heyes/structure_of_solids/Lecture3/Lec3.html#anchor1