Structures of Simple Ionic Compounds

Metals have structures which may be discussed in terms of the close packing of spheres, and as a result have the high coordination numbers of the close packed systems. Ionic solids, however, have lower coordination numbers, and the discussion of the structure simply in terms of close packed species has to be adapted.

The idea of an ionic solid, though, depends on being able to define an ion. The Ionic Model treats a solid as being made up of oppositely charged spheres that interact by the coulombic forces between them, and the short range repulsive forces which occur between closed shell species at small separations.

The use of the idea of close packed spheres, and the holes within these structures is very useful in the discussion of the structures of ionic solids.

The structures of many ionic solids of the formula AB and AB2 may be visualized in terms of the close packed arrangement of the negatively charged anions, with the positively charged cations occupying the holes within the structure.

Characteristic structures of ionic solids

The simple structures come from the arrangement of the anions (though sometimes the cations) in the positions of the spheres in the fcc or hcp lattices, and the cations go into some or all of the octahedral and tetrahedral holes within the lattices.


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The fluoride anions (green) occupy all of the tetrahedral holes in the fcc lattice of calcium cations (red).

There is now an fcc array of calcium cations, and the fluoride anions occupy all of the tetrahedral holes.