Allotropes of carbon: diamond, graphite and fullerenes
Allotropes are different forms of the same element. In diamond each carbon bonds to four others, making it the hardest; in graphite each bonds to three, in slippery, conducting layers.
Diamond and graphite compared
| Feature | Diamond | Graphite |
|---|---|---|
| Bonding | Each C is bonded to 4 others in a three-dimensional tetrahedral network | Each C is bonded to 3 others in flat layers (bond angle 120°); the layers are held weakly |
| Appearance | Colourless and transparent (impurities can colour it); sparkles when cut | Black, with a metallic lustre |
| Hardness | The hardest natural substance | Soft and slippery: the layers slide over each other |
| Electricity and heat | Does not conduct electricity, as all four electrons are in bonds; conducts heat about five times better than copper | Conducts electricity: the fourth electron of each carbon is free to move |
| Density | 3.51 g cm⁻³ | 2.2 g cm⁻³ |
| Melting point (in a vacuum) | About 3500 °C | About 3700 °C |
| Uses | Cutting and grinding hard materials, glass cutters, rock drills, jewellery | Dry lubricant for hot machine parts, electrodes in dry cells and arcs, pencil leads, containers for melting metals |
How diamonds form
Natural diamonds form deep inside the earth at about 1500 °C and about 70,000 atmospheres of pressure. In India they are found at Panna in Madhya Pradesh and Wajrakarur in Andhra Pradesh. Since the 1950s, synthetic diamonds have been made from graphite. It is heated to about 1500 °C under 50,000 to 65,000 atmospheres, with a metal such as nickel or iron. Most are used as abrasives (grinding materials) and in drilling tools. Graphite can be turned into diamond only under such very high pressure and temperature.
Fullerenes
Fullerenes were discovered in 1985 by Robert Curl, Harold Kroto and Richard Smalley, who won the Nobel Prize in Chemistry in 1996. They are closed cages of carbon atoms, shaped like a football. The best known, buckminsterfullerene, has 60 carbon atoms: C₆₀. Fullerenes form when carbon vapour condenses in an inert gas. Scientists hope to use them in superconductors, catalysts and new materials.
Non-crystalline (amorphous) forms of carbon
| Form | How it is made | Use |
|---|---|---|
| Charcoal | Heating wood strongly without air | Activated charcoal, cleaned with steam, removes colours and bad smells from water |
| Coke | Heating coal strongly without air | A reducing agent in metallurgy (getting metals from ores) |
| Carbon black | Burning hydrocarbons in limited air: CH₄ + O₂ → C + 2H₂O | Black pigment in inks; rubber tyres |
Activity: compare graphite, coal and charcoal
- 1Take a pencil lead (graphite), a small lump of coal and a piece of charcoal.
- 2Look at each and rub it on paper.Graphite is shiny grey-black and leaves a smooth mark. Coal and charcoal are dull black and leave rough, sooty marks
- 3Feel each between your fingers.Graphite feels slippery. Charcoal is light and breaks easily
- 4★Put each in turn in a circuit with a cell and a bulb.Graphite lights the bulb; coal and charcoal conduct very poorly or not at all
- 5Record your results in a table.
★ key step
How to draw the structures
- Diamond: draw one carbon atom in the centre joined to four others at the corners of a tetrahedron (a pyramid with a triangular base).
- Graphite: draw two or three flat sheets of joined hexagons, one above another, with dotted lines between the sheets for the weak forces.
- Fullerene C₆₀: draw a ball made of joined hexagons and pentagons, like a football.
- Label: carbon atom, covalent bond, weak forces between layers.
The glass cutter at a hardware shop has a tiny diamond tip, and pencil leads are made of graphite. Many home water filters use activated charcoal to remove smells and colours.
2 marks: “Graphite is used as a lubricant in machines that run very hot. Which property of graphite makes this possible, and why are oils not used there?” Graphite's layers are held by weak forces, so they slide over each other and make it slippery. It stays solid at high temperatures, where ordinary oils would burn or break down.
3 marks: “Compare diamond and graphite in structure, hardness and electrical conductivity. Give one use of each that depends on these properties.” Diamond: each C bonded to 4 others in a 3D network; hardest; no conduction; used for cutting glass. Graphite: each C bonded to 3 others in layers; soft and slippery; conducts; used for electrodes (or as a lubricant).
1 marks: “Name the allotrope of carbon that is shaped like a football.” Fullerene (buckminsterfullerene, C₆₀).
Diamond: 4 bonds, 3D, hard, no current. Graphite: 3 bonds, layers, slippery, conducts.