Allotropes of Carbon

Allotropes of Carbon

The substances, which have same chemical properties, but different physical properties, are called allotropes and this property is called allotropy.
Allotropies of Carbon — Diamond, graphite, charcoal

Diamond

Diamonds are formed inside the Earth under the conditions of high temperature (about 1,500°C) and high pressure (about 70,000 atmospheres). The three dimensional network of covalently bonded carbon atoms provides a rigid structure to diamonds. This rigidity makes diamond a very hard substance.

The density of diamond is high. It has a value of 3.51 g cnf3. The melting point of diamond (in vacuum) is also very high, i.e., 3,500°C, because a large amount of heat energy is required to break the three-dimensional network of covalent bonds.

Diamond has the following uses :
» It is used in cutting and grinding of other hard materials.
» It is also employed in instruments used for cutting of glass and drilling of rocks.
» It is used in jewellery. Beautiful ornaments are made with diamonds. The high refractive index of diamond (2.5) makes it very brilliant when it is properly cut and polished.

Graphite

In contrast to diamond, graphite is soft, black, and slippery solid. It has a metallic lustre. It is also a good conductor of electricity and heat.

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.

Graphite can be converted to diamond by applying very higher atmospheric pressure and temperature.

Graphite has the following uses :
» It is used as a dry lubricant for moving machine parts which operate at a high temperature and where other ordinary oil lubricants cannot be used.
» It is used for making electrodes in dry cells and in electric arcs.
» It is used for making pencil leads. Because of its soft nature and layered structure, it leaves black marks on paper. Hence, it is used for writing as leads in pencils.
» It is used for making containers which are used for melting metals.

Fullerenes

Fullerenes are formed when vaporized carbon condenses in an atmosphere of an inert gas.

These materials would find uses as superconducting materials, new catalysts, polymers, etc.

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.