Malleability and ductility are properties of metals that allow them to be deformed under stress without breaking. These properties are due to the way that metal atoms are arranged in a metallic lattice. In a metallic lattice, the atoms are arranged in a regular, repeating pattern and are held together by metallic bonds. Metallic bonds are relatively weak, which allows the atoms to move past each other when the metal is deformed.

Covalent bonds, on the other hand, are much stronger than metallic bonds. This is because covalent bonds involve the sharing of electrons between atoms, which creates a very strong bond. The strength of covalent bonds prevents the atoms from moving past each other when the material is deformed, which makes covalent materials hard and brittle.