Problem 27 - Chemistry and Chemical Reactivity

Question

Which of the following allotropes of carbon is not a network solid? (a) graphite (c) buckyballs \(\left(\mathrm{C}_{60}\right)\) (b) diamond (d) graphene

Step-by-Step Solution

Answer

Buckyballs (C60) are not a network solid.

Step 1: Understanding the Question

The question asks us to identify which of the given allotropes of carbon is not a network solid. Network solids are materials where atoms are bonded covalently in a continuous network extending throughout the material. Thus, we need to know the structure of the given allotropes.

Step 2: Analyze Each Allotrope

Let's consider each option: - **Graphite**: Consists of layers where carbon atoms are bonded in a hexagonal lattice through covalent bonds, typical of a network solid. - **Diamond**: Each carbon atom forms four covalent bonds with other carbon atoms, creating a very strong three-dimensional network. - **Buckyballs (C60)**: These molecules have a spherical structure made of carbon atoms bonded in a closed network, but these bonds do not extend in a continuous network through a material; instead, they form discrete molecular units. - **Graphene**: A single layer of carbon atoms arranged in a two-dimensional hexagonal lattice, similar to a single layer of graphite.

Step 3: Identify the Non-Network Solid

Based on the structural analysis: - Graphite, diamond, and graphene are network solids due to their continuous covalent bonding structure. - Buckyballs (C60), however, form discrete molecular structures and do not extend as a network solid.

Key Concepts

Network Solids

Network solids are fascinating materials where atoms are interconnected through an endless web of covalent bonds. These extend throughout the bulk of the material, forming a sturdy and unified structure. Such solids exhibit remarkable characteristics:

• High melting points: Network solids require significant energy to break their extensive covalent bonds, resulting in exceptionally high melting temperatures.
• Hardness: The strong covalent interactions make many network solids, like diamond, incredibly hard.
• Insolubility: They are generally insoluble in water or organic solvents due to the strong, extensive bonding.

Examples of network solids include diamond and graphite. These materials are vital in many applications due to their unique properties depending on the atomic arrangement.

Graphite Structure

Graphite features an intriguing structure. It consists of layers of carbon atoms arranged in a hexagonal pattern. Each carbon atom forms three covalent bonds with neighboring atoms in the same plane:

• The fourth electron of each carbon is delocalized, enabling the electrical conductivity of graphite.
• The layers in graphite are held together by weak van der Waals forces, allowing them to slide over each other with ease.

This sliding ability makes graphite an effective lubricant and an excellent material for pencil leads. Its layered structure makes it a network solid, though with unique electronic properties due to the free-moving electrons.

Diamond Structure

Diamonds are renowned for their remarkable structure as a network solid. Each carbon atom forms four covalent bonds with other carbon atoms, resulting in a three-dimensional matrix. This structure imparts several distinct characteristics:

• Extreme hardness: The rigid tetrahedral bonding makes diamond the hardest known natural material.
• Brilliance: The tight bonding and high refractive index give diamonds their famous sparkling quality.
• Insulating properties: Unlike graphite, diamond lacks free electrons, rendering it an electrical insulator.

Such attributes make diamond not just a precious gemstone but an essential material in industrial cutting and grinding tools.

Fullerenes

Fullerenes, such as buckyballs ( C_{60}), present a unique form of carbon allotrope. Unlike network solids, fullerenes comprise discrete molecules arranged in a spherical or cylindrical shape. Key properties of fullerenes include:

• Discrete molecular form: Each fullerene molecule forms a closed cage-like structure.
• Unique electrical properties: They can exhibit semiconductor behavior under certain conditions.
• Potential applications: Fullerenes show promise in fields such as nanotechnology and materials science.

While they are made of carbon atoms like other allotropes, the distinct molecular structure of fullerenes means they don't form continuous three-dimensional networks, setting them apart from other carbon network solids.