Ionic compounds are hard crystalline solids because of the strong electrostatic forces between their constituent ions. These forces result from the attraction between positive and negative charges. The greater the charge on the ions, the stronger the electrostatic forces will be. The lattice energy, which is the energy required to separate all of the ions in a crystal lattice, is a measure of the strength of these electrostatic forces.

Ionic compounds typically have high lattice energies because they contain ions with high charges, such as sodium and chloride ions. The strong electrostatic forces between these ions hold the crystal lattice together tightly, making it difficult to break apart. This is why ionic compounds are hard solids.

In addition to being hard, ionic compounds are also brittle. This means that they tend to break apart when subjected to stress. This is because the electrostatic forces between the ions are strong, but they are not directional. This means that they do not provide any support against shear forces. When an ionic compound is subjected to stress, the ions can move past each other, causing the crystal to break.

The hardness and brittleness of ionic compounds make them useful for a variety of applications, such as in ceramics and glass. Ceramics are made by heating an ionic compound until it melts and then allowing it to cool slowly. This allows the ions to rearrange themselves into a regular crystal structure. The resulting material is hard and brittle, but it is also resistant to heat and wear. Glass is made by heating an ionic compound until it melts and then rapidly cooling it. This prevents the ions from rearranging themselves into a regular crystal structure. The resulting material is hard and brittle, but it is also transparent.