Allotropes of carbon and its structure, properties and uses
Some of the allotropes of carbon are given below
- Graphite [ Graphene]
- Amorphous carbon
- Buckminsterfullerene [ Carbon nanotube, Carbon nanobuds]
- Glassy carbon
- Carbon nanofoam
- Lonsdaleite (hexagonal diamond)
- Linear acetylene carbon (LAC)
- Hyperenes – have penta coordinate
It is a precious gem and hardest substance but chemically it is a crystalline (cubical) allotrope of carbon. Naturally it occurs in cemberlite stone. There are various types of diamond which are found all over the world by the name of Culinan (3032 carat), hopp (445 carat), kohinoor (186 carat), pit (136.2 carat) etc. Artificial diamond was firstly prepared by Moisson in 1893.
Pure diamond is transparent, colorless and chemically inactive. There are close links of C-atoms in it and consequently a great hardness and involvent of its valence electrons in bond formation occurs that’s why it is a bad conductor of heat and electricity. Diamond is a deadly poisonous substance, transparent to X-rays and hence a pure diamond is distinguished from an artificial diamond which is not transparent to X-rays. Due to the presence of various impurities the colours of diamonds are different, some diamonds are black which is called Bort. Due to an optical phenomenon of total internal reflection an extremely brighten characteristic in the diamond appears. Usually it is chemically inactive but when it is heated with potassium dichromate (K2Cr2O7) and conc. H2SO4 at 200°C, it evolves CO2. It burns at 700-900°C in air and CO2 is evolved, also small ash of silica and Fe2O3 are formed.
Each carbon atom in a diamond is covalently bonded to four other carbons in a tetrahedron. These tetrahedra together form a 3-dimentional network of six-membered carbon rings (similar to cyclohexane) in the chair conformation. There is zero-angle strain
The stable network of covalent bonds and hexagonal rings is the reason for the great strength of carbon.
- Diamond is the hardest known natural mineral. It holds polish and lustier extremely well. No known naturally occurring substances can cut (or even scratch ) a diamond (10 on mohs scale)
- Diamond is a non-conductor of heat and electricity.
- However, the thermodynamic stability of diamond is less than that of graphite.
Uses of diamond :
- Used as a gem in jewelry
- Embedded in cutting tools and polish papers.
- Used in containment for high pressure experiment (diamond avail)
- Possible use in microchips
The colourless diamonds are used in making gems and costly ornaments, while coloured diamond like black diamonds (Bort) are used in making rock-drilling machines, glass-cutting devices, gem-cutters etc.
Graphite is another allotropes of carbon which is a soft crystalline substance with a greasy feel and metallic luster. The structure of it appears like a hexagonal lattice layer in Which electrons (charge carriers) are found to move in entire crystal lattice, that’s why graphite is a good conductor of heat and electricity. Since on marking on the paper it appears black so it is also called Black Lead. Graphite is chemically inactive and it doesn’t react with strong acids, alkalies, chlorine, dil. HNO3, dil. H2SO4 etc. But when it is heated in air it burns and CO2 gas is formed. A mixture of potassium dichromate (K2Cr2O7) and cone. H2SO4 oxidizes it to CO2.
Each carbon atom uses only three of its valance electron to bond covalently with three other carbon atoms in a planner fashion. Each carbon atom contributes one electron to a delocalized system of electron that is also part of the chemical bonding
The delocalized electrons are free to move throughout the plane. For this reason, graphite conducts electricity along the planes of carbon atoms, but it does not conduct electricity in a direction at right angles to the plane.
- It is a good conductor of electricity.
- It is thermodynamically more stable than diamond.
- Graphite is soft (2 on mohs scale)
- Graphite lubricity is due to adsorbed air and water between the layers.
- Its specific gravity is 2.3 which make it lighter than diamond.
- Chromic acid oxidizes graphite to CO2. Burns without producing smokes.
- In electrodes.
- As a dry lubricant
- In lead pencils
- As pyro lytic carbon (graphite layers bound by larger number of crystal defects) in blood-contacting implants such a prosthetic heart valves
- In pure glassy (isotopic) synthetic forms pyrolytic graphite and carbon fibre graphite are highly heat resistance (up to 3000°C). Used as reentry shields for missile nose cones.
- Under high temperature and pressure (2000°C and 5 G Pa) it is transformed to diamond.
- A single layer of graphite is called graphene
- It is produce by-
i) Epitaxy (layering on surface) on an insulating or conducting substrate.
ii) Exfoliation – repeated peeling
Learn by video:
It is carbon that does not have any crystalline structure. There is no long range pattern of atomic position. coal and soot or carbon black are informally called amorphous carbon.
♦Fullerenes, Buckminsterfullerene or Buckyballs:
A fullerene is a allotropes of carbon which have the form of a hollow sphere, tube, ellipsoid and many other shapes. The spherical fullerene are also called as Buckminsterfullerene or buckyballs, it is a type of fullerene with the formula C60. Buckminsterfullerene has a cage like fused ring structure, resemble the balls used in association football. It is made of twelve pentagons and twenty hexagons with a carbon atom at each vertex of each polygon and a bond along each polygon edge.
Are cylindrical fullerenes, It have unusual properties, which are valuable for electronics, nanotechnology, optics and other fields of materials science and technology.
♦Glassy carbon or vitreous carbon:
A non graphitizing carbon
It has long density cluster-assembly of carbon atoms strong together in a loose three dimensional web.
Each cluster is about 6nm wide and consists of~4000 carbon atoms linked in graphite like sheets
These are given negative curvature by the inclusion of heptagons amongst pentagons and hexagons. Fullerene has positive curvature as it contains only pentagons and hexagons. It is lighter than an aerogel.
♦Lonsdaleite (hexagonal diamond):
It is the hexagonal allotropes of carbon. It s the carbon allotrope diamond. It is believed to form from graphite present in meteorites upon their impact to earth
♦Linear acetylene carbon (LAC):
A one dimensional carbon polymer with the structure
These are berocarbon compound compounds with planner penta coordinate carbon units.