Science · Chemistry

Allotropes of carbon

Chemistry — materials

Carbon's allotropes are the textbook demonstration that structure, not composition, decides a material's properties: the hardest natural substance and one of the softest are the same element. Diamond bonds every atom tetrahedrally to four neighbours; graphite stacks flat hexagonal sheets held together so weakly that they slide. Three more forms were added in living memory — buckminsterfullerene, the sixty-atom football found by Kroto, Curl and Smalley in 1985; carbon nanotubes; and graphene, a single sheet one atom thick, peeled off graphite with sticky tape in Manchester in 2004. Each of the last two discoveries brought a Nobel Prize.

How to spot them

▸The hardest natural substance and a soft lubricant, made of the same element
▸One of them was named after an architect's geodesic domes
▸One was first isolated with ordinary adhesive tape, and won a physics Nobel six years later
▸Tetrahedral sp3 bonding in one form, flat hexagonal sheets in another
▸Nanotubes, lonsdaleite, amorphous soot and a one-atom-thick sheet all belong here

Key facts

The hardest natural substance and one of the softest solids are the same element

Diamond and graphite — tetrahedral sp3 bonding against weakly stacked sp2 sheets

Same element, yet one form conducts electricity and another is an insulator (1985)

Buckminsterfullerene, C60, found by Harold Kroto, Robert Curl and Richard Smalley

Diamond, graphite, graphene, nanotubes and buckyballs are all the same element arranged differently (2004)

Graphene, a single sheet one atom thick, peeled from graphite by Geim and Novoselov

A hexagonal form named after the crystallographer Kathleen Lonsdale

Lonsdaleite, found in the Canyon Diablo meteorite in 1967

Nobel Prizes in both Chemistry (1996) and Physics (2010) went to new forms of this one element (1996)

Twelve pentagons and twenty hexagons, exactly like a stitched football

Andre Geim and Konstantin Novoselov shared the 2010 Nobel Prize in Physics for a single-atom sheet (2010)

Geim is the only person to hold both a Nobel and an Ig Nobel, the latter for levitating a frog

Rolled-up sheets a nanometre across, with extraordinary tensile strength

Carbon nanotubes, brought to wide attention by Sumio Iijima in 1991

Key ideas

Carbon's four valence electrons can hybridise as sp3 (four tetrahedral single bonds, as in diamond) or sp2 (three in-plane bonds plus a delocalised electron, as in graphite and graphene), and that single choice accounts for the difference between the hardest known solid and a lubricant that conducts electricity
Graphite is the thermodynamically stable form at ordinary conditions; diamond is metastable, but the conversion is so slow as to be unobservable

Quick-fire facts

•The main allotropes are diamond, graphite, graphene, fullerenes, carbon nanotubes, amorphous carbon and lonsdaleite
•Diamond is 10 on the Mohs scale, the hardest natural material, and the best-known thermal conductor of any bulk solid
•Graphite conducts electricity along its sheets, is used as a lubricant and as the 'lead' of pencils, and moderates neutrons in some nuclear reactors
•Buckminsterfullerene, C60, was found in 1985 at Rice University and named after Buckminster Fuller; Kroto, Curl and Smalley took the 1996 Nobel Prize in Chemistry
•A C60 cage has 12 pentagonal and 20 hexagonal faces — the pattern of a traditional football, and of Fuller's geodesic domes
•Graphene was isolated in 2004 by Andre Geim and Konstantin Novoselov at Manchester by repeatedly cleaving graphite with adhesive tape; they shared the 2010 Nobel Prize in Physics
•Geim had earlier won an Ig Nobel Prize for magnetically levitating a live frog, making him the only holder of both awards
•Lonsdaleite, hexagonal diamond, is named after Kathleen Lonsdale and turns up at meteorite impact sites
•Carbon nanotubes were described in detail by Sumio Iijima in 1991 and have among the highest tensile strengths of any known material
•Pencil graphite was long called plumbago, and the Borrowdale deposit in Cumberland was the only large source of solid graphite ever found

Drill it — 10 cards

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Sources

H. W. Kroto, J. R. Heath, S. C. O'Brien, R. F. Curl and R. E. Smalley — C60: Buckminsterfullerene Nature 318, 162–163 (1985) — the discovery paper
The Nobel Foundation — The Nobel Prize in Physics 2010 Geim and Novoselov, for groundbreaking experiments on graphene
Los Alamos National Laboratory — Carbon — Periodic Table of Elements