Carbon is the 6th element in the periodic table with the atomic number 6 and is represented by the symbol ‘C’.  Carbon is the most abundant material on earth and the 4th most common element in the universe. All living organisms are made up of allotropes of carbon. By mass, the second most abundant element in the human body is carbon. Fossil fuels, polymers, and most medicines contain carbon compounds.

Overview of Carbon Allotropes

An element found in two or more forms is known as allotropy, and these forms are known as allotropes. Even though their physical properties are different, they have similar chemical properties.

Carbon’s valency and its ability to catenate contribute to having multiple numbers of allotropic forms.

The allotropes of carbon can be categorised into two:

• Crystalline carbon allotropes:

1. 1. Diamond
   2. Graphite
   3. Fullerenes
   4. Graphene
   5. Lonsdaleite
   6. Carbophene
   7. Diamane

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• Amorphous carbon allotropes:

1. Coal
2. Coke
3. Carbon Nanotube
4. Carbon Nanoform
5. Carbon Nanobud
6. Glassy Carbon

Let us discuss some of them one by one.

Buckminsterfullerenes

• The buckminsterfullerenes were first discovered in 1985 by scientists and researchers from the University of Sussex and Rice University.

• Fullerenes are positively curved molecules, having C2n atoms where n≥30. Fullerenes can have varying sizes made of carbon only, such as hollow spheres, tubes, or ellipsoids.

• The behaviour and structure of fullerenes change with temperature and pressure.
• Fullerenes are soluble in an organic solvent.
• Fullerene exhibits ferromagnetism.
• Fullerenes are used to make conductors, lubricants, cosmetic products, etc.
• Fullerenes can absorb gases.

Graphene

• Graphene is a single layer of the structure of graphite.
• Graphene has excellent thermal and electrical properties.
• Graphene can be produced by epitaxy on a conducting substrate. It can also be achieved by repeated peeling from the surface of graphite. 
• Graphene can replace silicon. Graphene can be used in high-performance and advanced electronic devices. 

Lonsdaleite

• Lonsdaleite is also called a hexagonal diamond.
• Lonsdaleite is formed from graphite in meteorites that fall on the earth. 
• The graphite present in meteorites converted into lonsdaleite due to the tremendous heat and pressure of the impact of meteorites. At the same time, its hexagonal crystal lattice is maintained. 
• Lonsdaleite can be made in the laboratory, and it is done by compressing graphite in a static press. 

Carbophene

• Carbophene is a covalent organic framework of two-dimension. 
• 4-6 carbophene can be obtained from 1-3-5 trihydroxy benzene. 
• Carbophen contains 4-C and 6-C rings in the ratio of 1:1. 
• The angles between the three respective σ-bonds are 90°, 120° and 150°.

 Diamane

• Diamane is the two-dimensional form of a diamond. 
• Diamane is formed at high temperatures and pressures. Without pressure, the material goes back to graphene. 
• The second method to make diamane is to add hydrogen atoms to the material. But hydrogen bonds are small and weak. So, we use fluorine to strengthen the bonds, known as f- diamane.

Coal

• Coal and soot are called amorphous carbon.

• Amorphous carbon does not have a crystalline structure. 

• Amorphous carbon has short-range order. 

• Amorphous carbon contains microscopic crystals similar to graphite or diamond.

• Coals are a product of pyrolysis. Pyrolysis is decomposing a substance over a while by the action of heat.

Carbon Nanotubes

• Carbon nanotubes can also be called buckytubes.

• Carbon nanotubes are cylindrical molecules, useful in several applications, such as nano-electronics, materials applications, optics, etc.

• Carbon nanotubes have unique electrical properties, good conductors of heat and extraordinary strength.

• Nanotube has a diameter of a few nanometers but is several centimetres in length. 

• There are two kinds of nanotubes, the first is single-walled nanotubes, and the second is multi-walled nanotubes.

Carbon Nanobuds

• In Carbon nanobuds, fullerene-like “buds” are attached with the covalent bond to the outer walls of the carbon nanotubes. 
• Carbon nanobuds have various properties of carbon nanotubes and fullerenes.
• Carbon nanotubes are excellent field emitters.

Carbon Nanoform

• Andrei V. Rode discovered carbon nanofoam in 1997.
• Carbon nanofoam contains a low-density cluster of carbon atoms, hanging together in a loose 3-dimensional web.
• Every cluster can be approximately 6 nanometers wide. A collection contains about 4000 atoms connected in graphite-like sheets. It has a negative curvature due to heptagons in between regular hexagonal patterns. 
• Carbon nanofoam clusters are the opposite of buckminsterfullerenes. In buckminsterfullerenes, there is a positive curvature because of the insertion of pentagons.
• Carbon nanofoam has a large-scale structure and is a poor conductor of electricity.

Glassy Carbon

• Glassy carbon is also known as vitreous carbon.
• Glassy carbon is non-graphitising. 
• Glassy carbon is used as a suitable electrode material, as a component of prosthetic devices and for high-temperature crucibles.
• Bernard Redfern manufactured glassy carbon in the 1950s.
• The manufacturing of glassy carbon requires the treatment of the organic precursors at about 3000 °C temperature. 
• Glassy carbon is impermeable to gases.
• Glassy carbon is chemically inert.
• Glassy carbon showed that the oxidation rates are lower than any other carbon allotropes for some. 
• Glassy carbon is resistant to acids. Ordinary graphite is reduced to a powder, whereas the glassy carbon remains unaffected if treated by concentrated nitric acid and sulfuric acid.

Conclusion

For the past few centuries, carbon has been used as a source of energy. Even in this era of technology, carbon is significant in every field of science and technology. Without the element, life may not have thrived on this planet. So, it’s imperative for us, human beings, to know about its various forms.