Ionic compounds are generally hard and brittle due to their crystal lattice structure and the nature of ionic bonds. The ions in an ionic compound arrange themselves in a repeating three-dimensional pattern, forming a rigid crystal lattice. The electrostatic forces of attraction between oppositely charged ions are strong, providing the compound with hardness. However, when subjected to stress, the layers of ions can shift, causing like-charged ions to come in contact, leading to repulsion and breakage along specific planes. This results in brittleness, as the crystal structure tends to fracture rather than deform under external forces.