Ionic compounds exhibit brittleness due to their unique structural characteristics. When an external force is applied, the ions within the compound can shift, leading to a situation where similar charges come into proximity. This results in repulsion, ultimately causing the structure to break.

Ionic compounds, such as table salt ($\text{NaCl}$), consist of positively and negatively charged ions arranged in a regular, repeating pattern known as a crystal lattice. This lattice structure is exceptionally strong owing to the electrostatic forces of attraction between the oppositely charged ions, which tightly bind them together. However, this very structural integrity also contributes to the brittleness of ionic compounds.

When force is exerted on an ionic compound, it can displace the ions in the lattice. If the displacement is significant, ions of the same charge may end up adjacent to one another. Since like charges repel each other, this proximity generates a repulsive force that can fracture the crystal lattice. Consequently, the ionic compound may shatter or crack under stress. This phenomenon explains why ionic compounds are categorized as brittle; they are unable to endure substantial stress or strain without breaking.

It is essential to recognize that although ionic compounds are brittle, they are also generally very hard. This hardness stems from the strong electrostatic attractions between the ions in the lattice. As a result, while ionic compounds are prone to shattering or cracking under pressure, they exhibit resistance to scratching or denting. This combination of hardness and brittleness is a defining characteristic of many ionic compounds.