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Ch.12 - Solids and Modern Materials
All textbooksBrown 15th EditionCh.12 - Solids and Modern MaterialsProblem 67
Chapter 12, Problem 67

Classify each of the following statements as true or false: (a) Although both molecular solids and covalent-network solids have covalent bonds, the melting points of molecular solids are much lower because their covalent bonds are much weaker. (b) Other factors being equal, highly symmetric molecules tend to form solids with higher melting points than asymmetrically shaped molecules.

Verified step by step guidance
1
Step 1: Understand the nature of molecular solids and covalent-network solids. Molecular solids are composed of molecules held together by intermolecular forces, such as London dispersion forces, dipole-dipole interactions, or hydrogen bonds. Covalent-network solids consist of atoms connected by a continuous network of covalent bonds.
Step 2: Analyze statement (a). Molecular solids have lower melting points compared to covalent-network solids because the forces holding the molecules together (intermolecular forces) are generally weaker than the covalent bonds in covalent-network solids. Therefore, the statement is true, but not because the covalent bonds in molecular solids are weaker, but because the intermolecular forces are weaker.
Step 3: Consider the role of symmetry in molecular structure for statement (b). Symmetric molecules can pack more efficiently in a solid state, leading to stronger intermolecular interactions and higher melting points. Asymmetric molecules may not pack as efficiently, resulting in weaker interactions and lower melting points.
Step 4: Evaluate statement (b) based on the understanding of molecular packing. Given that symmetric molecules tend to pack more efficiently, the statement that they form solids with higher melting points than asymmetrically shaped molecules is generally true.
Step 5: Summarize the analysis. Statement (a) is true with the clarification that the lower melting points of molecular solids are due to weaker intermolecular forces, not weaker covalent bonds. Statement (b) is true as symmetric molecules tend to have higher melting points due to efficient packing.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Molecular Solids vs. Covalent-Network Solids

Molecular solids are composed of molecules held together by intermolecular forces, such as van der Waals forces, which are generally weaker than the covalent bonds found in covalent-network solids. This difference in bonding strength results in molecular solids typically having lower melting points compared to covalent-network solids, where atoms are bonded in a continuous network, leading to much higher melting points.
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Melting Point and Molecular Symmetry

The melting point of a solid is influenced by the strength and nature of the interactions between its constituent particles. Highly symmetric molecules can pack more efficiently in a solid state, leading to stronger intermolecular interactions and, consequently, higher melting points. In contrast, asymmetrical molecules may not pack as closely, resulting in weaker interactions and lower melting points.
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Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between molecules. These forces include hydrogen bonds, dipole-dipole interactions, and London dispersion forces. The strength of these forces significantly affects the physical properties of substances, including their melting and boiling points, with stronger intermolecular forces leading to higher melting points in solids.
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Related Practice
Textbook Question

The coordination number for the Al3+ ion is typically between four and six. Use the anion coordination number to determine the Al3 + coordination number in the following compounds: (a) AlF3 where the fluoride ions are two coordinate.

Textbook Question

The coordination number for the Al3+ ion is typically between four and six. Use the anion coordination number to determine the Al3 + coordination number in the following compounds: (b) Al2O3 where the oxygen ions are six coordinate.

Textbook Question

The coordination number for the Al3+ ion is typically between four and six. Use the anion coordination number to determine the Al3 + coordination number in the following compounds: (c) AlN where the nitride ions are four coordinate.

Textbook Question

Both covalent-network solids and ionic solids can have melting points well in excess of room temperature, and both can be poor conductors of electricity in their pure form. However, in other ways their properties are quite different. (a) Which type of solid is more likely to dissolve in water?

Textbook Question

For each of the following pairs of semiconductors, which one will have the larger band gap: (a) CdS or CdTe? (b) GaN or InP? (c) GaAs or InAs?