Here's why:

* Ionic Bonding: KCl is formed by the electrostatic attraction between positively charged potassium ions (K+) and negatively charged chloride ions (Cl-). This strong attraction between oppositely charged ions is the defining characteristic of ionic bonding.

* Crystal Lattice: The ions arrange themselves in a highly ordered, repeating pattern called a crystal lattice. This structure contributes to the solid's rigid and brittle nature.

* High Melting and Boiling Points: Due to the strong electrostatic forces holding the ions together, ionic solids have high melting and boiling points.

Let me know if you'd like more details about the properties of ionic solids!