Question

Which would you infer to be harder - ionic or covalent compounds?

Answer 1

You can’t give a single answer without specifying the kind of covalent solid. Hardness depends on the bonding pattern (network vs discrete molecules) and structure, not just whether the bonding is ionic or covalent.

- Ionic crystals (e.g. NaCl, MgO) are typically hard and have high melting points; they are brittle because shear brings like-charged ions into contact and they repel.
- Covalent network solids (e.g. diamond, SiC, quartz) are extremely hard — often harder than typical ionic solids — because each atom is locked into a rigid, directional network of strong covalent bonds.
- Covalent molecular solids (e.g. iodine, many organic crystals) are usually soft because molecules are held together by weak van der Waals forces.

Short summary: ionic solids are generally hard and brittle, but the hardest materials are covalent network solids.