Some Properties of Ionic Solids

As noted above, ionic solids are generally hard and brittle. Both of these properties reflect the strength of the coulombic force. Hardness measures resistance to deformation. Because the ions are tightly bound to their oppositely-charged neighbors and, a mechanical force exerted on one part of the solid is resisted by the electrostatic forces operating over an extended volume of the crystal.

But by applying sufficient force, one layer of ions can be made to slip over another; this is the origin of brittleness. This slippage quickly propagates along a plane of the crystal (more readily in some directions than in others), weakening their attraction and leading to physical cleavage. Because the "ions" in ionic solids lack mobility, the solids themselves are electrical insulators.