Carbon occurs in free state (not combined with any other element) in three allotropic forms. Allotropes are different forms of the same element in the same physical state. Earlier only two allotropic forms - graphite and diamond were known. Another allotropic form, fullerene, has been discovered few years back.
Diamonds are formed inside the earth under the conditions of high temperature (about 1500°C) and high pressure (about 70,000 atmospheres). In a diamond crystal, each carbon atom is linked to four other carbon atoms by covalent bonds in a tetrahedral fashion. This results in a three dimensional arrangement.
The three-dimensional network of covalently bonded carbon atoms provides a rigid structure to diamonds. This rigidity makes diamond a very hard substance. It is the hardest natural substance known. The only other substance harder than diamond is silicon carbide which is also known as carborandum but diamond is a natural substance whereas carborandum is a synthetic.
Diamonds are basically colourless. However, some impurities impart colour to them.
The density of diamond is high. It has a value of 3.51 g cm–3. The melting point of diamond (in vacuum) is also very high (3500°C) because a large amount of heat energy is required to break the three-dimensional network of covalent bonds.
Since all the four electrons are covalently bonded and there are no free electrons in diamond, hence it does not conduct electricity. But diamond is a good conductor of heat. Its thermal conductivity is five times that of copper. Thus, it can easily dissipate the heat energy released by friction when it is used as an abrasive.
Because of its properties, diamond has the following uses:
In contrast to diamond, graphite is soft, black and slippery solid. It has a metallic luster. It is also a good conductor of electricity and heat.
Both graphite and diamond contain only carbon atoms, then why do they exhibit such different properties? The answer is in the structure of graphite. In contrasts to diamond, which has a three-dimensional tetrahedral arrangement of carbon atoms, graphite contains layers of carbon atoms. In each layer, a particular carbon atom is linked to three other carbon atoms in a trigonal planar arrangement with a bond angle of 120°.
Thus, three electrons of carbon are covalently bonded to the other three carbon atoms. The fourth electron, which does not participate in bonding, is free. These electrons of various carbon atoms are free to move along between the layers and hence are able to conduct electricity.
The bonding between these layers of carbon atoms is weak. Hence, these layers can slide one over the other. This property makes graphite a good solid lubricant.
The density of graphite is less than that of diamond. It has a value of 2.2g cm-3. The melting point of graphite (in vacuum) is about 3700°C. Graphite can be converted to diamond by applying very higher atmospheric pressure and temperature.
Because of its properties, graphite has the following uses:
Fullerenes were discovered in 1985 by Robert F. Curl, Harold W. Kroto and Richard E. Smalley. They were awarded the Nobel Prize in Chemistry in 1996 for this discovery. Fullerenes have closed structure like a football. A typical fullerene, named as buckminsterfullerene has 60 carbon atoms.
Fullerenes are formed when vaporized carbon condenses in an atmosphere of an inert gas. The discovery of fullerenes has opened up a new field in Chemistry. Fullerenes of various other sizes are being synthesized and their properties and uses are being studied. New materials, which contain metals enclosed in the fullerenes are being synthesized. It is hoped that these materials would find uses as superconducting materials, new catalysts, polymers.
In addition to the three allotropic forms, carbon also exists in three microcrystalline or amorphous forms of graphite. They are charcoal, coke and carbon black.
Charcoal is formed when wood is heated strongly in the absence of air. It has a large surface area. Activated charcoal is a pulverized form whose surface has been made free from any adsorbed materials by heating with steam. It is widely used for adsorbing coloured impurities and bad odours from water and other substances.
Coke is an impure form of carbon. It is formed when coal is strongly heated in the absence of air. It is used as a reducing agent in metallurgy.