The reason why ionic compounds are hard and brittle is because the ions in the
compound are already held rigidly in place in a lattice due to the
strong electrostatic forces of attraction in between the cations and the
anions as well as the repulsion in the ions with like charges .
However, if a large enough force is applied along a certain plane, this
causes the ions to shift along that layer, displacing that layer with
respect to the next, and there will hence be a repulsion between like
charges within the ionic compound. This repulsion will shatter the
crystal lattice of the ionic compound..