Carbon

:Alternative meaning: Carbon (API)

Allotropes

The allotropes of carbon are the different molecular configurations (allotropes) that pure carbon can take.

Related Topics:
Allotropes - Carbon

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The three relatively well-known allotropes of carbon are amorphous carbon, graphite, and diamond. Several exotic allotropes have also been synthesized or discovered, including fullerenes, carbon nanotubes, lonsdaleite and aggregated diamond nanorods.

Related Topics:
Amorphous carbon - Graphite - Diamond - Fullerene - Carbon nanotube - Lonsdaleite - Aggregated diamond nanorods

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In its amorphous form, carbon is essentially graphite but not held in a crystalline macrostructure. It is, rather, present as a powder which is the main constituent of substances such as charcoal, lamp black (soot) and activated carbon.

Related Topics:
Graphite - Charcoal - Lamp black - Soot - Activated carbon

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At normal pressures carbon takes the form of graphite, in which each atom is bonded to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons. The two known forms of graphite, alpha (hexagonal) and beta (rhombohedral), both have identical physical properties, except for their crystal structure. Graphites that naturally occur have been found to contain up to 30% of the beta form, when synthetically-produced graphite only contains the alpha form. The alpha form can be converted to the beta form through mechanical treatment and the beta form reverts back to the alpha form when it is heated above 1000 °C.

Related Topics:
Graphite - Hexagon - Aromatic hydrocarbon - Rhombohedral - C

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Because of the delocalization of the pi-cloud, graphite conducts electricity. The material is soft and the sheets, frequently separated by other atoms, are held together only by van der Waals forces, so easily slip past one another.

Related Topics:
Pi-cloud - Electricity - Van der Waals force

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At very high pressures carbon forms an allotrope called diamond, in which each atom is bonded to four others. Diamond has the same cubic structure as silicon and germanium and, thanks to the strength of the carbon-carbon bonds, is together with the isoelectronic boron nitride (BN) the hardest substance in terms of resistance to scratching. The transition to graphite at room temperature is so slow as to be unnoticeable. Under some conditions, carbon crystallizes as Lonsdaleite, a form similar to diamond but hexagonal.

Related Topics:
Diamond - Silicon - Germanium - Bond - Isoelectronic - Boron nitride - Graphite - Lonsdaleite

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Fullerenes have a graphite-like structure, but instead of purely hexagonal packing, also contain pentagons (or possibly heptagons) of carbon atoms, which bend the sheet into spheres, ellipses or cylinders. The properties of fullerenes (also called "buckyballs" and "buckytubes") have not yet been fully analyzed. All the names of fullerenes are after Buckminster Fuller, developer of the geodesic dome, which mimics the structure of "buckyballs".

Related Topics:
Buckyball - Buckytube - Buckminster Fuller - Geodesic - Dome

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A nanofoam allotrope has been discovered which is ferromagnetic.

Related Topics:
Nanofoam - Ferromagnetic

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Carbon allotropes include:

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• amorphous carbon
• carbon nanofoam (discovered in 1997)
• carbon nanotube
• diamond
• fullerene
• graphite
• lonsdaleite
• ceraphite
• aggregated diamond nanorods (synthesised in 2005)

The system of carbon allotropes spans a range of extremes.

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Between diamond and graphite:

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• Diamond is the hardest mineral known to man (although aggregated diamond nanorods are now believed to be even harder), but graphite is one of the softest.
• Diamond is the ultimate abrasive, but graphite is a very good lubricant.
• Diamond is an excellent electrical insulator, but graphite is a conductor of electricity.
• Diamond is usually transparent, but graphite is opaque.
• Diamond crystallizes in the cubic system but graphite crystallizes in the hexagonal system.

Between amorphous carbon and nanotubes:

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• Amorphous carbon is among the easiest materials to synthesize, but carbon nanotubes are extremely expensive to make.
• Amorphous carbon is completely isotropic, but carbon nanotubes are among the most anisotropic materials ever produced.