Ionic Solids

The structural units of an ionic crystal are cations and anions, bound together by strong electrostatic (Coulombic) attractive forces. To maximise this attraction, each cation surrounds itself with as many anions as geometrically possible, and vice versa -- so ionic crystals possess a definite, well-defined crystal structure; many are cubic close packed. The arrangement of Na+ and Cl- ions in the NaCl crystal is the textbook example.

Four characteristics follow directly from this ionic bonding:

1. High melting points -- breaking apart a 3D network of strong electrostatic attractions takes a great deal of thermal energy.
2. No electrical conductivity in the solid state -- the ions are locked into fixed lattice positions and cannot move to carry charge.
3. Good electrical conductivity when molten or dissolved -- once the rigid lattice breaks down (on melting) or the ions are freed by a solvent (on dissolving), they become mobile and can carry current. …