Allotropes of Carbon

This lesson covers the different structural forms (allotropes) of carbon — diamond, graphite, graphene, fullerenes and carbon nanotubes. Understanding their structures, properties and uses is required by the Edexcel GCSE Chemistry (1CH0) specification.

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What Are Allotropes?

Allotropes are different structural forms of the same element in the same physical state. The atoms are bonded together in different arrangements, giving each allotrope different physical properties.

Carbon has several allotropes, including diamond, graphite, graphene, fullerenes (e.g. buckminsterfullerene, C₆₀) and carbon nanotubes.

Exam Tip: Don't confuse allotropes with isotopes. Allotropes = same element, different structures. Isotopes = same element, different numbers of neutrons.

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Diamond

(Covered in detail in the previous lesson — brief recap here.)

• Each carbon bonded to 4 others in a tetrahedral arrangement.
• Giant covalent structure — rigid 3D lattice.
• Very hard, very high melting point.
• Does not conduct electricity (no delocalised electrons).
• Used in cutting tools, drill bits, jewellery.

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Graphite

(Covered in detail in the previous lesson — brief recap here.)

• Each carbon bonded to 3 others in flat hexagonal layers.
• Weak intermolecular forces between layers — layers slide.
• 1 delocalised electron per carbon — conducts electricity along the layers.
• Soft, slippery, very high melting point.
• Used as a lubricant, in pencils, and as electrodes.

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Graphene

Graphene is a single layer of graphite — a one-atom-thick sheet of carbon atoms arranged in hexagons.

Structure

• Each carbon is bonded to 3 others in a flat hexagonal arrangement (like one layer of graphite).
• There is 1 delocalised electron per carbon atom.
• The sheet is only one atom thick — making it a two-dimensional material.

Properties

Property	Description	Explanation
Strength	Extremely strong (stronger than steel for its weight)	Strong covalent bonds throughout the 2D lattice
Electrical conductivity	Excellent conductor	Delocalised electrons free to move across the sheet
Thermal conductivity	Excellent conductor of heat	Delocalised electrons transfer energy efficiently
Transparency	Transparent (almost)	Only one atom thick — absorbs very little light
Flexibility	Flexible	Thin single layer can bend without breaking
Weight	Extremely light	Only one atom thick

Uses and Potential Applications

• Electronics and computing (transistors, flexible screens).
• Composite materials (to make strong, lightweight structures).
• Membranes for filtration and desalination.
• Conducting coatings.

Exam Tip: Graphene is often described as a "wonder material." The key properties to remember are: extremely strong, conducts electricity (delocalised electrons), one atom thick, transparent, flexible.

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Fullerenes

Fullerenes are molecules of carbon atoms arranged in hollow shapes — spheres (balls) or tubes. The carbon atoms are arranged in hexagons and pentagons.

Buckminsterfullerene (C₆₀)

The most famous fullerene is buckminsterfullerene (C₆₀), also called a "buckyball."

• Contains exactly 60 carbon atoms.
• Arranged in a sphere made of 20 hexagons and 12 pentagons (like a football).
• Each carbon is bonded to 3 others.
• It is a molecule (not a giant structure) — it has a definite formula.
• Relatively low melting point compared to diamond/graphite (it is a simple molecular substance in terms of intermolecular forces between C₆₀ molecules).

Properties of Fullerenes

• Hollow structure — can trap atoms or molecules inside ("cage" molecules).
• Can be used for drug delivery — drug molecules can be placed inside the hollow and delivered to specific parts of the body.
• Can act as catalysts or carriers for catalysts.
• Can act as lubricants (the spherical shape allows them to roll).

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Carbon Nanotubes

Carbon nanotubes are cylindrical fullerenes — imagine rolling a sheet of graphene into a tube.

Structure

• Carbon atoms arranged in hexagons, forming a hollow cylinder.
• Each carbon bonded to 3 others.
• Can be single-walled or multi-walled (tubes within tubes).
• Delocalised electrons (like graphite and graphene).

Properties

Property	Description	Explanation
Strength	Extremely strong (high tensile strength)	Strong covalent bonds throughout the tube wall
Electrical conductivity	Excellent conductor (along the tube)	Delocalised electrons free to move
Thermal conductivity	Excellent conductor of heat	Efficient energy transfer via electrons
Weight	Very light	Hollow structure, very small diameter
Diameter	Very small (1-100 nm typically)	Nanoscale structures

Uses

• Reinforcing materials (e.g. in composites for tennis rackets, bicycle frames, body armour).
• Electronics (nanoscale wires and components).
• Drug delivery systems.
• Catalysts (high surface area).

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Comparison of Carbon Allotropes

graph TD
    C["Carbon Allotropes"] --> D["Diamond"]
    C --> Gr["Graphite"]
    C --> Ge["Graphene"]
    C --> F["Fullerenes (C₆₀)"]
    C --> NT["Carbon Nanotubes"]
    D -->|"4 bonds per C<br/>3D lattice<br/>Hard, insulator"| D2[" "]
    Gr -->|"3 bonds per C<br/>Layers<br/>Soft, conductor"| Gr2[" "]
    Ge -->|"3 bonds per C<br/>Single layer<br/>Strong, conductor"| Ge2[" "]
    F -->|"3 bonds per C<br/>Hollow sphere<br/>Drug delivery"| F2[" "]
    NT -->|"3 bonds per C<br/>Hollow tube<br/>Strong, conductor"| NT2[" "]

Allotrope	Bonds per C	Structure	Conduct?	Key Use
Diamond	4	3D giant covalent	No	Cutting tools
Graphite	3	Layered giant covalent	Yes	Lubricant, electrodes
Graphene	3	Single flat layer	Yes	Electronics, composites
C₆₀ (buckyball)	3	Hollow sphere (molecule)	Limited	Drug delivery
Carbon nanotube	3	Hollow cylinder	Yes	Reinforcing materials

Exam Tip: Notice that diamond is the only allotrope where each carbon makes 4 bonds (no delocalised electrons). All other allotropes have 3 bonds per carbon with 1 delocalised electron, which is why they can conduct electricity.

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Key Points

• Allotropes are different structural forms of the same element.
• Diamond: 4 bonds per C, giant 3D lattice, very hard, does not conduct.
• Graphite: 3 bonds per C, layers with weak forces between them, soft/slippery, conducts electricity.
• Graphene: single layer of graphite, extremely strong, excellent conductor, transparent.
• Buckminsterfullerene (C₆₀): hollow sphere of 60 carbon atoms, can trap molecules inside, drug delivery.
• Carbon nanotubes: cylindrical fullerenes, extremely strong, excellent conductors, many uses.
• All allotropes except diamond can conduct electricity due to delocalised electrons.

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Practice Questions

1. What is meant by the term "allotrope"?
2. Explain why graphene can conduct electricity.
3. Describe the structure of buckminsterfullerene (C₆₀).
4. Give two uses of carbon nanotubes and explain how their properties make them suitable.
5. Explain why diamond does not conduct electricity but graphite does.

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Worked Example: Comparing All Five Allotropes

Question: Summarise the structure and bonding of diamond, graphite, graphene, buckminsterfullerene and carbon nanotubes, and explain why four of the five conduct electricity.

Answer:

Allotrope	Bonds per C	Structure	Free electrons per C	Conducts?
Diamond	4	3D tetrahedral giant covalent	0	No
Graphite	3	Hexagonal layers (giant covalent)	1 (delocalised)	Yes (along layers)
Graphene	3	Single hexagonal layer	1 (delocalised)	Yes
Buckminsterfullerene (C₆₀)	3	Hollow sphere (60 C atoms)	1 per C (delocalised across cage)	Limited
Carbon nanotubes	3	Hollow cylinder (rolled graphene)	1 (delocalised)	Yes