Problem 9 - Chemistry: The Central Science

Question

Covalent bonding occurs in both molecular and covalentnetwork solids. Which of the following statements best explains why these two kinds of solids differ so greatly in their hardness and melting points? (a) The molecules in molecular solids have stronger covalent bonding than covalent-network solids do. (b) The molecules in molecular solids are held together by weak intermolecular interactions. (c) The atoms in covalent-network solids are more polarizable than those in molecular solids. (d) Molecular solids are denser than covalent-network solids.

Step-by-Step Solution

Answer

The best explanation for the difference in hardness and melting points between molecular and covalent-network solids is that the molecules in molecular solids are held together by weak intermolecular interactions (Statement b).

Step 1: Understanding Molecular and Covalent-Network Solids

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.

Step 2: Analyzing the Statements

(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.

Step 3: Choosing the Best Explanation

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.