91Ó°ÊÓ

Molecular solids are formed by molecules held together by weak intermolecular forces like van der Waals forces or hydrogen bonding. Covalent-network solids, on the other hand, are formed by atoms connected through strong covalent bonds throughout the whole network.

(a) Covalent bonds are stronger than intermolecular forces; therefore, this statement is incorrect as it suggests that molecular solids have stronger bonding than covalent-network solids. (b) This statement suggests that the molecules in molecular solids are held together by weak intermolecular forces. This statement is correct, as molecular solids are held together by weak forces like van der Waals forces or hydrogen bonds. (c) Polarizability refers to the ease with which an atom's electron cloud can be distorted. This statement is not relevant to the difference in hardness and melting points between molecular and covalent-network solids. Additionally, polarizability is not an overarching factor which distinguishes these two types of solids. (d) Density is a measure of mass per unit volume. This statement is also not relevant to the difference in hardness and melting points between molecular and covalent-network solids. Density may differ between individual substances, but there is no general trend that supports this statement.

Based on our analysis, the best explanation for the difference in hardness and melting points between molecular and covalent-network solids is statement (b): The molecules in molecular solids are held together by weak intermolecular interactions.