Explain why most metals are malleable and ductile but ionic crystals are not

Metals are malleable due to the layers of atoms which can move over each other. Ionic crystals are made of rigid lattice structures

Explanation:

The molecular structure of metals consists of metallic ions in a sea of de-localized electrons. The ions are closely packed in a regular arrangement. The layers of ions are held together due to the electrostatic forces between the ions and electrons. The layers of ions are not bonded to each other directly, which allows them to move when force is applied. This is why metals are malleable

Ionic crystals are strongly bonded lattice structures with oppositely charged ions strongly attracted to each other. As the ions are bonded directly to each other, the application of a force has the potential to break existing bonds, making the structure brittle.