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Group 14: Carbon-Allotropes

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Group 14: Carbon-Allotropes - Lesson Summary

Allotropes are different forms of an element which have different physical properties but identical chemical properties. Different allotropes of the same element have different arrangements of the atoms.

Carbon exists in both crystalline as well as the amorphous forms.

There are three important crystalline allotropic forms of carbon - diamond, graphite and fullerenes.

Diamond, the first allotrope of carbon, is the hardest substance on earth.

In diamond, all carbon atoms exhibit sp³ hybridization. Each carbon is bonded to four other carbon atoms situated tetrahedrally around it. Since there are no free electrons in a diamond crystal, it is a bad conductor of electricity.

Graphite has a layered structure. These layers are held together by the weak van der Waals forces of attraction. Each layer of graphite is made from planar, hexagonal rings of carbon atoms, as shown in the diagram.

Each carbon in graphite is sp² hybridized. Each atom forms three sigma bonds with its three neighboring carbon atoms.

Fullerenes can be prepared by heating graphite in an electric arc in the presence of inert gases.

Fullerenes are cage-like molecules C60 or buckminsterfullerene looks like a soccer ball. It is sometimes referred to as "buck ball".

Buckminsterfullerene contains 60 carbon atoms arranged as a hollow sphere. This ball shaped molecule has 60 vertices, each occupied by a carbon. All the carbons in the Buckminster fullerene molecule are sp² hybridized.

Each carbon atom forms three sigma bonds with its three neighboring atoms. The remaining electron in each unhybridized p- orbital of carbon is delocalized in molecular orbitals, giving the aromatic character to the molecule.

Graphite is the thermodynamically most stable allotrope of carbon. Both diamond and fullerenes have higher heats of formation, reflecting their lower stability. The heat of formation of diamond is 1.90kJ per mole, and the heat of formation for fullerene is 30.1kJ per mole.

The amorphous forms of carbon include carbon black, coke and charcoal. They can all be considered as impure forms of graphite or fullerenes.

Carbon black is formed by burning hydrocarbons in a limited supply of air, or by incomplete combustion. It is an almost pure form of amorphous carbon containing 98%-99% carbon.

Coke is formed when wood is heated at high temperatures in the absence of air, called the destructive distillation of wood.

To form charcoal, coal is heated at high temperatures in the absence of air.

Carbon has a wide variety of uses in its elemental forms.

Diamond is widely valued in jewellary is used as an abrasive in sharpening tools, die-making and in the manufacture of tungsten filaments for light bulbs.

Graphite is used as a dry lubricant in machines running at high temperature where oil cannot be used, fishing rods, aircraft and canoes.

Being good a conductor, graphite is used to construct electrodes for use in batteries and in industrial electrolysis. Activated charcoal is very porous and can adsorb many times of its own volume of gases. It is used for adsorption of poisonous gases, removal of organic contaminants from water and for odour control in air conditioners.

Carbon black is used as a black pigment in black ink and as filler in automobile tires. Coke is used as a fuel and as a reducing agent in metallurgy.

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