Allotropes of Carbon

Physical properties of diamond

• It is very hard, because of all the strong covalent bonds holding the structure together.
• It is also brittle (i.e. strong but breaks easily), because all of the bonds are directional and stress will tend to break the structure.
• It is a good insulator, as all of the valence (outer shell) electrons are used in bonding, i.e. none of the electrons are free to move
• It is insoluble in water, due to the truly weak Van der Waal's forces of attractions between the carbon atoms and the water molecules, whereas the carbon atoms are bonded very tightly to one another.
• Very high melting point, because of all the strong covalent bonds holding the structure together - it requires a lot of energy to break these bonds.

                        

Fig 1: Structure of Diamond

Graphite

Again the carbon atoms are bonded together to make a giant structure but in this case all of the carbons are bonded to only three neighbour and are sp2 hybridised. As the sp2 hybridisation results in planar structures, there are giant 2 dimensional layers of carbon atoms and each layer is only weakly linked to the next layer by Van der Waal's forces.

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Physical properties of graphite:

• It is soft and slippery, because of the weak intermolecular forces of attraction, which allow the layers of molecules to slide easily over one another. And so this is why graphite powder is used as a lubricant.

• It is also brittle, as the intra-molecular forces (between atoms within the molecules) are stronger than the intermolecular forces (between molecular layers), making it strong and, at the same time, easily breakable.

• It is a good conductor of electricity, because only three of the four valence (outer shell) electrons are used in bonding. This means that all carbon atoms within graphite will each have one free electron. All of these delocalized electrons together make graphite a good electrical conductor.

• It is insoluble in water, as there are only very weak Van der Waal's attractions between the carbon atoms and the water molecules, whereas the carbon atoms are bonded very tightly to one another.

• Like diamond, graphite also has a very high melting point, due to the giant molecular covalent structure - it requires a fairly high amount of energy to break these bonds.

                             

  Fig 3: Structure of graphite        

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Fullerenes

These are small molecules of carbon in which the giant structure is closed over into spheres of atoms (bucky balls) or tubes (sometimes called nano-tubes). The smallest fullerene has 60 carbon atoms arranged in pentagons and hexagons like a football. This is called Buckminsterfullerene.

The name 'Buckminster fullerene' comes from the inventor of the geodesic dome (Richard Buckminster Fuller) which has a similar structure to a fullerene. Fullerenes were first isolated from the soot of chimneys and extracted from solvents as red crystals.

Fullerenes are insoluble in water but soluble in methyl benzene. They are bad conductors as the individual molecules are only held to each other by weak van der Waal's forces.

Physical properties of fullerenes:

• They are soft and slippery, as there are few covalent bonds holding the molecules together, but only weak Vander Waals forces between molecules.
• They are also brittle, due to the weak Van der Waal’s forces of attraction between the molecular layers.
• They are good electrical insulators, because there is no movement of electrons available from one molecule to the next. The exception could be the formation of nano-tubes that are capable of conducting electricity along their length. These are the subject of some experiments in micro electronics
• They are insoluble in water, as there are only very weak Van der Waal's attractions between the carbon atoms and the water molecules whereas the carbon atoms are bonded very tightly to one another in the molecules.
• And they have low melting points, typical of covalent crystals where only Van der Waal's interactions have to be broken for melting.

Fig 5: Structure of Buckminsterfullerene (C60)

Bibliography:

• IB Chemistry. (2007, April 5). IB Chemistry higher level notes. Retrieved January 20, 2008, from http://ibchem.com/IB/ibnotes/full/bon_htm/14.4.htm

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