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A diamond ring and a pencil are made of exactly the same element. So is the soot on a chimney, the “buckyballs” discovered in the 1980s, and graphene, one of the strongest materials ever measured. Different forms of the same element are called allotropes, and carbon has more remarkable ones than any other element.
The difference between them isn’t the atoms — it’s how the atoms are connected.
Diamond
In diamond, every carbon atom forms four single covalent bonds to four neighbours, arranged at the corners of a tetrahedron. This repeats in all three directions, making one giant, rigid network.
- Hardest natural material — 10 on the Mohs scale. See the hardest element.
- Electrical insulator — every outer electron is locked in a bond; none are free to move.
- Excellent conductor of heat — better than copper, because the stiff lattice passes vibrations efficiently. That’s why jewellers can test diamonds with a heat probe.
- Transparent and highly refractive, which gives cut diamonds their sparkle.
- Very high melting point (it converts to graphite first at normal pressure).
Natural diamonds form deep in the Earth’s mantle under high pressure and temperature. Synthetic diamonds, made since the 1950s, now dominate industrial uses like cutting, drilling and polishing, and are widely sold as gemstones.
Graphite
In graphite, each carbon atom bonds to only three others, forming flat sheets of hexagons like chicken wire. The fourth outer electron from each atom is delocalized — free to move along the sheet.
- Soft and slippery — the sheets are held together only by weak forces, so they slide over each other. That’s why graphite marks paper (the “lead” in pencils is graphite mixed with clay) and works as a dry lubricant.
- Conducts electricity along the sheets, thanks to the delocalized electrons. It’s used for electrodes, brushes in electric motors, and as the anode in lithium-ion batteries.
- Grey, opaque and shiny.
Graphite is actually the more stable form at everyday conditions. Diamond is thermodynamically unstable at the surface, but converting it to graphite is so slow that it never noticeably happens. “Diamonds are forever” is chemically true only in practice.
Graphene
Peel a single layer off graphite and you have graphene: one atom thick, a flat sheet of carbon hexagons. Andre Geim and Konstantin Novoselov first isolated it in 2004 — famously using sticky tape to peel layers off graphite — and won the 2010 Nobel Prize in Physics.
- Among the strongest materials ever tested in tension, for its thickness.
- Exceptional conductor of electricity and heat.
- Nearly transparent and highly flexible.
Researchers are developing graphene for electronics, sensors, composites, coatings and batteries.
Fullerenes
In 1985 Harold Kroto, Robert Curl and Richard Smalley discovered C₆₀, a hollow ball of 60 carbon atoms arranged in 12 pentagons and 20 hexagons — exactly the pattern of a classic football. They named it buckminsterfullerene (“buckyballs”), after the architect Buckminster Fuller, whose geodesic domes it resembles. They received the 1996 Nobel Prize in Chemistry. Other fullerenes, like C₇₀, followed.
Carbon nanotubes
Roll a graphene sheet into a seamless cylinder and you get a carbon nanotube, a few nanometres wide but potentially very long. Depending on how the sheet is rolled, a nanotube can conduct electricity like a metal or act as a semiconductor. Nanotubes are used to strengthen composites, and studied for electronics and energy storage.
Amorphous carbon
Soot, charcoal, carbon black and coke have no long-range order — tiny fragments of graphite-like layers jumbled together. Activated charcoal has an enormous internal surface area (a teaspoon can have the surface area of a football pitch), which makes it an excellent filter for water purification, gas masks and poisoning treatment. Carbon black is what makes car tyres black and helps them resist wear.
Side by side
| Allotrope | Bonds per atom | Structure | Conducts electricity? | Hardness |
|---|---|---|---|---|
| Diamond | 4 | 3D network | No | Hardest |
| Graphite | 3 | Stacked sheets | Yes (along sheets) | Very soft |
| Graphene | 3 | Single sheet | Yes, extremely well | Very strong in tension |
| C₆₀ fullerene | 3 | Hollow sphere | Poorly (as a solid) | Soft |
| Nanotube | 3 | Rolled sheet | Metal or semiconductor | Very strong |
Other elements with allotropes
Carbon isn’t alone. Oxygen has O₂ and ozone (O₃); phosphorus has white, red and black forms; sulfur has dozens of allotropes; tin has metallic white tin and brittle grey tin. See isotopes vs. ions vs. isomers vs. allotropes.
Quick answers
What is an allotrope? One of several different structural forms of the same element in the same physical state.
Why does graphite conduct but diamond doesn’t? Graphite has one delocalized electron per carbon atom; in diamond every outer electron is in a fixed bond.
Can graphite be turned into diamond? Yes, with very high pressure and temperature — that’s how most synthetic industrial diamonds are made.
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