Carbon is found in three allotropes graphite, diamonds, amorphous carbon, nanofoam, and fullerenes. All five allotropes occur, because in each of the forms the structures, in which the carbon atoms are arranged, are different.
Graphite is a soft, dark gray or black, lustrous material with either a rhombohedral or hexagonal crystalline structure. The carbon atoms that make up graphite are held together by covalent bonding, meaning that the atoms share electrons. Graphite exists in two forms, alpha and beta. Alpha and beta graphite have very similar physical properties, the only difference is in their crystal structures. The beta form of graphite is found naturally, where synthetic graphite contain only the alpha form.
|
Melting Point |
3652-3697 °C |
|
Boiling Point |
4200 °C |
|
Density |
1.9-2.3 g/cm3 |
The second allotrope of carbon is the diamond. The diamond is the hardest natural substance known. It has been found that a diamond is so hard that it can be cut only by another diamond, earning a ten on the Mohs Hardness Scale. Diamonds get their hardness from their very strong crystal lattice, made up of hexagons arranged in repeating three-dimensional pattern.
|
Melting Point |
3550 °C |
|
Boiling Point |
4827°C |
|
Density |
3.5-3.53 g/cm3 |
Fullerenes are stable molecules constructed of twelve pentagonal faces and any number greater than one of hexagonal faces. The pentagonal faces of the fullerenes, is what bends the structure into a spheres or cylinders. C60 is the most common fullerene, also known as the buckminsterfullerene. It takes on a spherical shape, resembling a soccer ball, and is made up of sixty carbon atoms.
|
Melting Point |
800 °C (Sublimes) |
|
Boiling Point |
N/A |
|
Density |
1.69 g/cm3 |
The amorphous allotrope of carbon does not have any crystalline structure, having no continuous pattern of carbon atoms. The amorphous carbon allotrope is formed when a material that contains carbon goes through a combustion reaction in which there is not enough oxygen for a complete reaction. Some forms of amorphous carbon include charcoal, coal, and coke.
Nanofoam is the most recently discovered allotrope of carbon. It is constructed of groups of carbon atoms, which come together forming a loose three-dimensional web. Each of these groups are formed up of 4000 carbon atoms linked in a crystal lattice similar to that of the graphite allotrope. Unlike fullerenes these are made up of heptagons, which give it a negative curvature, making it opposite, in shape, to the fullerenes. The most unusual physical property of the nanofoam allotrope is it becomes magnetic at -138°C.