Question

Covalent bonding occurs in both molecular and covalent-network solids. Why do these two kinds of solids differ so greatly in their hardness and melting points?

Short answer

The difference in hardness and melting points between molecular and covalent-network solids arises due to the difference in the forces holding them together. Molecular solids have weaker intermolecular forces, resulting in lower hardness and melting points. In contrast, covalent-network solids have stronger covalent bonds, leading to much higher hardness and melting points.

Step-by-step solution

Molecular and Covalent-Network Solids

Molecular solids are made up of discrete molecules held together by relatively weak intermolecular forces like dipole-dipole interactions, london dispersion forces and hydrogen bonding. On the other hand, covalent-network solids consist of atoms bonded together by a network of strong covalent bonds throughout the entire structure.

Hardness and Melting Point Differences

Hardness and melting point properties depend on the strength of the forces holding the solid together. In molecular solids, the relatively weak intermolecular forces can be more easily broken, which results in lower hardness and lower melting points. Covalent solids, however, have strong covalent bonds that require a lot of energy to break. This results in much higher hardness and melting points in covalent-network solids compared to molecular solids.

Examples

For instance, sugar (sucrose) is a molecular solid with a melting point of \(186^\circ C\), while diamond, a covalent-network solid, has a melting point of approximately \(3820^\circ C\) and is one of the hardest known materials. In summary, the difference in hardness and melting points between molecular and covalent-network solids comes from the difference in the forces holding them together. Molecular solids have weaker intermolecular forces, leading to lower hardness and melting points, while covalent-network solids have stronger covalent bonds, resulting in much higher hardness and melting points.