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Components of Cells
The Macromolecules
Forms of Carbon

Carbon

Pure, elemental carbon is found widely dispersed in nature, and can show 3-5 allotropic forms, depending on how you count them.

Graphite

is one of the softest substances known, and is used in a variety of ways. It is the "lead" in a pencil and a lubricant in locks or where metal rubs against metal.

It exists in forms called alpha or hexagonal and beta or rhombohedral, each of which are almost exactly the same except for their crystalline structures.

In each case, pure carbon atoms are covalently bonded to other pure carbon atoms to form sheets, or layers, of infinitely linked carbon atoms. These sheets of atoms stack one above another at a distance apart of 3.37 x 10-8 centimeters. This is far enough that there is little bonding between the individual layers. Consequently the sheets of atoms slip past one another with great ease, hence the lubricating properties of graphite.

The hexagonal alpha form of graphite can be converted to the beta form by mechanical treatment. The rhombohedral beta form can be converted to the alpha form by heating it above 1000oC.

In naturally occurring graphites there is usually a mixture of these two forms, some containing as much as 30% of the rhombohedral (beta) form. It is also possible to make graphite synthetically, and when this is done, it is mainly in the alpha form.

"White" and Amorphous Carbon

These are two, vaguely characterized forms of possibly pure carbon.

So called, "white" carbon was discovered in 1969 when graphite was heated under very low pressures and high temperatures (above 2500 degrees Kelvin). Tiny crystals of small, transparent material were seen clinging to the edges of the regular graphite. While similar to graphite found in a meteor crater in Germany, "white" carbon was birefringent. Very little is known about this allotropic form.

Amorphous carbon is a form that is produced when coal, coal gas or wood is burnt at high temperature without much oxygen present. Under these conditions combustion is not complete and a solid, sooty crystalline form of carbon is created that probably consists of microscopic crystals of other graphite varieties.

Diamond

In 1955 graphite was converted into diamond. In the presence of a catalyst such as iron, chromium, manganese or cobalt, graphite was heated and subjected to very high pressures. These extreme conditions caused the carbon atoms in the graphite to rearrange themselves and take on a new conformation.

Instead of forming flat layers of carbon sheets (that are not interconnected), carbon atoms in diamond are arranged into an infinite three-dimensional array in which every carbon atom is linked to four other carbon atoms. Each of these covalent bonds is at an equal angle to each of the other covalent bonds, thus forming a four sided shape that resembles a pyramid.

In this way, every carbon atom in a diamond is strongly and covalently linked to four more carbon atoms at a distance of about 1.54 x 10-8 centimeters, to form a very strong lattice-work that can only be destroyed or disrupted by very, very powerful forces.

Diamonds are, therefore, very hard, chemically inactive (relatively), good insulators and have a melting point so high that it can only be deduced, not reached!

Fullerenes

Sometimes called buckyballs or buckytubes , these molecules are a new form of carbon that has only recently been created.

Fullarenes resemble graphite in that they are composed entirely of carbon atoms that are joined together in sheets of hexagonal rings (see above). But the difference is (and it is a big difference), some of the carbon rings have five atoms, or more, in them (forming pentagons and heptagons). This forces the sheet of linked rings into the shape of a hollow sphere or tube.

These molecules were named after the architect Richard Buckminster Fuller , who used a similar principle, but on a much larger scale, to create geodesic domes.

A popular fullerene has 60 carbon atoms in it, and takes the form of a "truncated icosahedron", a shape much more commonly recognized as a football!


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© 2004, Professor John Blamire