Why Are Most Ionic Substances Brittle?

Ionic compounds usually shatter under pressure because when stress is applied to the ionic crystal, the like charges are forced closer to each other. Ionic compounds form crystal lattices instead of amorphous solids.

Squeezing the crystal forces the like-charged ions in the lattice to slide into alignment; this electrostatic repulsion is enough to split the ionic crystal. The strong attraction between the anions and cations in an ionic bond also causes it to have high melting and boiling points, since high temperatures are need to overcome the attraction. Ionic compounds cannot conduct electricity in a solid state. They have to be dissolved in an aqueous solution so that ions can dissociate and carry the charge through the solution.