Question

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

Short answer

The best statement that explains why molecular and covalent network solids differ so greatly in their hardness and melting points is (b) "The molecules in molecular solids are held together by weak intermolecular interactions.".

Step-by-step solution

Statement (a)

"The molecules in molecular solids have stronger covalent bonding than covalent-network solids do." Although molecular solids do have covalent bonding, covalent network solids have much stronger covalent bonds since they form an extended network throughout the entire solid, leading to higher hardness and melting points. Therefore, this statement is incorrect.

Statement (b)

"The molecules in molecular solids are held together by weak intermolecular interactions." This statement is true. Molecular solids are held together by weak intermolecular forces like van der Waals interactions and hydrogen bonds. These weak forces are easier to break, leading to lower melting points and hardness compared to covalent network solids. This statement seems to correctly explain the main difference between the two.

Statement (c)

"The atoms in covalent-network solids are more polarizable than those in molecular solids." Polarizability is not the main factor that contributes to the hardness and melting points of molecular and covalent network solids. Although it could affect the strengths of the interactions between the atoms, it doesn't address the key difference between the types of bonding present in each type of solid. So, this statement is incorrect.

Statement (d)

"Molecular solids are denser than covalent-network solids." The density of a solid is not the main reason for the differences in the hardness and melting points of molecular and covalent network solids. It is not necessarily true that molecular solids are denser than covalent network solids, so this statement is incorrect.

Conclusion

Based on our analysis, the best statement that explains why molecular and covalent network solids differ so greatly in their hardness and melting points is (b) "The molecules in molecular solids are held together by weak intermolecular interactions.".

Key concepts

Molecular Solids

Molecular solids are formed from individual molecules held together by intermolecular forces rather than covalent bonds extending throughout the solid. The key players in maintaining the structure of molecular solids are weak forces like van der Waals forces and hydrogen bonds. These forces are relatively easy to overcome, resulting in lower melting points compared to covalent network solids.

• Intermolecular forces determine the physical properties such as softness and low melting points.
• Examples include ice (solid form of water), dry ice (solid CO₂), and sulfur.

These types of solids are often softer and less dense. Their ability to melt at lower temperatures makes them suitable for applications requiring gentle phase transitions, like sublimation and deposition.

Covalent Network Solids

Covalent network solids are a fascinating category where atoms are bonded together in a continuous network, forming one giant molecule. Unlike molecular solids, each unit in these solids is interconnected through strong covalent bonds, significantly enhancing their structural integrity and resilience.

• Diamond and quartz are quintessential examples of covalent network solids.
• The extensive bonding results in extremely high melting points and exceptional hardness.

The strength of covalent bonds here makes these solids very durable and capable of withstanding a lot of stress without breaking.

Intermolecular Interactions

Intermolecular interactions are the forces that hold molecules together in a solid state, outside of covalent bonds. While influencing the solidity, they are typically much weaker than the covalent bonds you would find in a network solid. Some key types of intermolecular interactions include:

• Van der Waals Forces: Weak attractions between dipoles or induced dipoles in molecules.
• Hydrogen Bonds: Stronger than van der Waals but still weaker than covalent bonds, these occur in molecules where hydrogen is covalently bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine.

These interactions play a significant role in dictating the melting points and hardness of molecular solids but offer less contribution to covalent network solids.

Melting Points

The melting point of a solid is a reflective measure of its internal bonds' strength. Covalent network solids showcase impressively high melting points due to the persistent and strong covalent bonds extending across the solid. In contrast, molecular solids have much lower melting points.

• Molecular solids melt at lower temperatures due to weaker intermolecular forces.
• Network solids require more energy (heat) to break their extended covalent bonds.

Understanding why different solids melt at different temperatures helps in applications where temperature sensitivity is crucial, such as manufacturing processes.

Hardness of Solids

The hardness of a solid is directly linked to the strength and arrangement of its chemical bonds. Network solids, thanks to their robust covalent bonding, exhibit tremendous hardness, whereas molecular solids often feel much softer.

• The hardness of diamonds is due to the dense network of carbon-carbon covalent bonds.
• Molecular solids like paraffin wax are comparably soft due to weaker intermolecular forces.

Evaluating the hardness of materials is essential in sectors like construction and machining, where material strength is paramount.