Problem 11 - Chemistry The Central Science In Si Units

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 molecules in molecular solids are held together by weak intermolecular interactions (option b).

Step 1: Understand Covalent Bonding

Covalent bonding involves the sharing of electron pairs between atoms. This type of bonding can occur in both molecular and covalent-network solids, but the way atoms are bonded and interact differs significantly.

Step 2: Analyze Molecular Solids

In molecular solids, the entities are distinct molecules. Within these molecules, atoms are held together by strong covalent bonds. However, the intermolecular forces (such as Van der Waals forces or hydrogen bonds) holding these molecules together in the solid state are relatively weak.

Step 3: Examine Covalent-Network Solids

Covalent-network solids are constructed from atoms interconnected by a continuous network of strong covalent bonds, without distinct molecules. This extensive bonding network results in materials that often have high hardness and high melting points.

Step 4: Compare Key Factors

When comparing the two types of solids, the key difference lies in the strength and nature of the attractions between entities: intermolecular forces for molecular solids versus extensive covalent bonds for covalent-network solids.

Step 5: Analyze the Answer Choices

(a) Incorrect - Molecular solids do not necessarily have stronger covalent bonds than covalent-network solids. (b) Correct - Molecular solids are characterized by weaker intermolecular forces. (c) Incorrect - Polarizability is not the primary factor for the differences in properties. (d) Incorrect - Density is not directly related to hardness and melting point.

Key Concepts

Molecular Solids

In the fascinating world of chemistry, molecular solids are structures where molecules are the individual components that make up the solid. These molecules are held together within the solid by intermolecular forces, which can include van der Waals forces, dipole-dipole interactions, and hydrogen bonds.

While the covalent bonds within the molecules themselves are strong, what sets molecular solids apart is the relative weakness of the forces between these molecules. As a result, molecular solids tend to exhibit certain characteristics:

• Low melting and boiling points: Because the intermolecular forces are relatively weak, it doesn't take much energy to break them apart, leading to lower melting and boiling points.
• Soft and brittle nature: The lack of strong forces between the molecules makes these solids less durable, making them softer and more brittle.
• Poor electrical conductivity: Molecular solids generally do not conduct electricity, as there are no free charges to carry a current.

Molecular solids often include substances like ice, sugar, and dry ice (solid carbon dioxide), where the focus is on the distinctive molecular structure and weak bonding forces between them.

Covalent-Network Solids

Covalent-network solids showcase a fascinating structure where atoms are bonded in a continuous, extensive network. Unlike molecular solids, these do not comprise discrete molecules. Instead, the entire solid is essentially one large molecule.

The covalent bonds in these solids are very strong and are responsible for several key properties:

• High melting and boiling points: The strength and complexity of the network of covalent bonds require a significant amount of energy to break, leading to high melting and boiling points.
• Hardness: Due to the extensive bonding, covalent-network solids are typically much harder compared to molecular solids.
• Variable electrical conductivity: While many covalent-network solids do not conduct electricity, forms of carbon like graphite do, thanks to the presence of delocalized electrons in specific types of covalent bonds.

Covalent-network solids include examples like diamond, quartz, and silicon carbide, where the continuous network of bonds imparts incredible strength and unique electrical properties.

Intermolecular Forces

Intermolecular forces play a crucial role in defining the physical properties of molecular solids. These are forces of attraction that occur between molecules, distinguishing from the covalent bonds present within each molecule.

These forces can be broken down into several types:

• Van der Waals forces: Also known as London dispersion forces, these are the weakest types of intermolecular forces, arising from temporary dipoles induced in atoms or molecules.
• Dipole-Dipole interactions: These occur between molecules with permanent dipoles, where the positive end of one molecule is attracted to the negative end of another.
• Hydrogen bonds: A type of strong dipole-dipole interaction specifically involving hydrogen bonded to atoms like nitrogen, oxygen, or fluorine, which increases the strength of the force.

The various strengths of these forces greatly influence properties such as melting and boiling points in molecular solids. Understanding these forces sheds light on why molecular solids often have lower melting points and less hardness compared to covalent-network solids.