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?

Molecular solids are formed when molecules are held together by weak intermolecular forces, such as van der Waals forces, dipole-dipole interactions, or hydrogen bonding. These forces are weaker than covalent bonds between atoms within the molecules. Examples of molecular solids include water ice, carbon dioxide, and iodine.

Covalent network solids are composed of atoms that are bonded together by strong covalent bonds in a continuous, extended network. Examples of covalent network solids are diamond, which is made of carbon atoms, and silicon dioxide (SiO2), also known as quartz.

The main difference between molecular and covalent network solids is the type of bonding that holds them together. In molecular solids, the bonding between the molecules is much weaker compared to the bonding within the molecules. In covalent network solids, all the atoms are held together by strong covalent bonds, forming a continuous, extended network.

The hardness of a solid is related to the strength of the bonds holding it together. Since molecular solids are held together by weaker forces, they tend to be softer compared to covalent network solids, which are held together by strong covalent bonds. For example, water ice and carbon dioxide are both relatively soft and can easily be broken or reshaped, whereas diamond is one of the hardest materials known to man due to its rigid covalent network structure.

The melting point of a solid depends on the energy required to break the bonds holding it together. Molecular solids have weaker intermolecular forces holding their molecules together, so they require less energy to break these forces and change from solid to liquid state. Consequently, these solids have lower melting points compared to covalent network solids. Covalent network solids, on the other hand, have high melting points because a significant amount of energy is required to break the strong covalent bonds in their extended networks. In conclusion, molecular and covalent network solids differ greatly in their hardness and melting points due to the differences in their atomic or molecular structures and the types of forces holding them together. Molecular solids are held together by weaker intermolecular forces, making them softer and giving them lower melting points, while covalent network solids have strong covalent bonds resulting in higher hardness and melting points.