Properties of Different Structures: Diamond Graphite and Fullerenes
Giant Covalent Structures
Some substances have atoms bonded together by covalent bonds in a huge network extending in all directions. These are called giant covalent structures (or macromolecules). They have very high melting points because many strong covalent bonds must be broken.
The most important examples at GCSE are diamond, graphite and silicon dioxide.
Diamond
Diamond is a form of carbon in which each carbon atom is bonded to four other carbon atoms by strong covalent bonds, forming a rigid tetrahedral arrangement.
Properties of Diamond
- Very high melting point (~3550°C) — many strong covalent bonds to break
- Very hard — the rigid 3D network of bonds makes it one of the hardest known substances
- Does not conduct electricity — all four outer electrons on each carbon are used in bonding, so there are no delocalised electrons to carry charge
- Does not dissolve in water or other common solvents
Uses
Diamond is used in cutting tools (drill bits, glass cutters) and jewellery because of its hardness and brilliance.
Graphite
Graphite is another form of carbon. Each carbon atom bonds to three other carbon atoms, forming flat hexagonal layers.
Structure
- Each carbon uses 3 of its 4 outer electrons for covalent bonding
- The fourth electron on each carbon is delocalised — free to move along the layers
- The layers are held together by weak intermolecular forces (van der Waals)
Properties of Graphite
- Very high melting point — strong covalent bonds within layers must be broken
- Soft and slippery — the weak forces between layers allow them to slide over each other easily
- Good conductor of electricity — the delocalised electrons can move along the layers, carrying charge
- Good conductor of heat — delocalised electrons transfer energy
Uses
Graphite is used in pencil leads (layers slide off onto paper), lubricants (slippery layers), and electrodes (conducts electricity and has a high melting point).
Diamond vs Graphite: Same Element, Different Properties
Diamond and graphite are both made entirely of carbon atoms. They are different forms of the same element — called allotropes. Their different properties arise because of their different structures and bonding:
| Property | Diamond | Graphite |
|---|---|---|
| Bonds per carbon | 4 covalent | 3 covalent + 1 delocalised |
| Structure | 3D tetrahedral network | Flat hexagonal layers |
| Hardness | Very hard | Soft/slippery |
| Electrical conductivity | No | Yes (along layers) |
| Melting point | Very high | Very high |
Fullerenes
Fullerenes are molecules of carbon atoms arranged in hollow shapes — spheres or tubes. Unlike diamond and graphite, fullerenes form distinct molecules.
Buckminsterfullerene (C₆₀)
The most famous fullerene is buckminsterfullerene (C₆₀), also known as a buckyball. It is a hollow sphere made of 60 carbon atoms arranged in 20 hexagons and 12 pentagons (like a football).
Properties:
- Each carbon is bonded to 3 others
- It is a simple molecular substance
- Relatively low melting point compared to diamond/graphite
- Soluble in some solvents
Carbon Nanotubes
Carbon nanotubes are cylindrical tubes of carbon atoms, essentially rolled-up sheets of graphene. They have remarkable properties:
- Very strong for their weight — high tensile strength
- Excellent electrical conductors — delocalised electrons along the tube
- Good thermal conductors
Uses of carbon nanotubes:
- Reinforcing composite materials (e.g. tennis rackets, aircraft)
- Electronics and semiconductors
- Drug delivery systems in medicine
- Catalysts (molecules can be trapped inside)
Graphene
Graphene is a single layer of graphite — a sheet of carbon atoms one atom thick, arranged in hexagons. It is the thinnest material known and has extraordinary properties:
- Very strong despite being only one atom thick
- Excellent electrical conductor — delocalised electrons move freely across the sheet
- Transparent and flexible
- Used in electronics, composite materials and research
Exam Tips
- Be able to explain properties in terms of structure and bonding, not just list them
- Diamond does not conduct electricity because there are no delocalised electrons; graphite does because it has delocalised electrons
- Fullerenes are molecules with specific formulae (e.g. C₆₀), unlike diamond and graphite which are giant structures
- Know at least two uses for each allotrope of carbon