The primary objective of this article is the study of the element carbon itself and its polymorphs, i.e., graphite, diamond, fullerenes, and other less common forms. These allotropes have the same building block, the carbon atom, but their physical form, i.e., the way the building blocks are put together, is different. In other words, they have distinct molecular or crystalline forms.
The capability of an element to combine its atoms to form such allotropes is not unique to carbon. Other elements in the fourth column of the periodic table, silicon, germanium, and tin, also have that characteristic. However carbon is unique in the number and the variety of its allotropes.
The properties of the various carbon allotropes can vary widely. For instance, diamond is by far the hardest-known material, while graphite can be one of the softest. Diamond is transparent to the visible spectrum, while graphite is opaque; diamond is an electrical insulator while graphite is a conductor, and the fullerenes are different from either one. Yet these materials are made of the same carbon atoms; the density is the result of different arrangements of their atomic structure.
Just as carbon unites easily with itself to form polymorphs, it can also combine with hydrogen and other elements to give rise to an extraordinary number of compounds and isomers. The compounds of carbon and hydrogen and their derivatives form the extremely large and complex branch of chemistry known as organic chemistry. More than harf-a-million organic compounds are identified and new ones are continuously discovered. In fact, far more carbon compounds exist than the compounds of all other elements put together.
While organic chemistry is not a subject of this article, it can’t be overlooked since organic compounds play a major part in the processing of carbon polymorphs. Some examples of organic precursors are shown in table 2.1 bellow:
Table 2.1 Organic precursors of carbon products
| Precursors | Products |
| Methane | Pyrolytic graphite |
| Hydrocarbons
Fluorocarbons Acetone, etc |
Diamond-like carbon
Polycrystalline diamond |
| Rayon
Polyacrylonitrile |
Carbon fibers |
| Phenolics
Furfuryl alcohol |
Carbon-carbon
Vitreous carbon |
| Petroleum fractions
Coal tar pitch |
Molded graphites
Carbon fibers |
| Plants | Coal |
In order to understand the formation of the allotropes of carbon from these precursors and the reasons for their behavior and properties, it is essential to have a clear picture of the atomic configuration of the carbon atom and the various ways in which it bonds to other carbon atoms. These are reviewed in this chapter.