14.4 - Allotropes of carbon
14.4.1 Describe and explain the structures and properties of diamond, graphite and fullerene. Students should recognize the type of hybridization present in each allotrope and the delocalization of electrons in graphite and C60 fullerene.
Allotropy
Allotropes are different physical forms of the same element. All elements are made up uniquely of their own atoms and therefore any physical differences must be a consequence of how the atoms are joined together - their arrangement within the bulk structure.
Many elements exhibit allotropy as there are often varous ways in which the atoms can be linked together into molecules and also different ways in which the molecules can be arranged to make larger structures.
In the case of carbon, the atoms form either giant macromolecular structures (diamond and graphite) in which all of the atoms in the bulk structure are joined together by covalent bonds making giant molecules, or smaller molecules (buckminster fullerene) in which there are only discrete molecules made up of 60 carbons in a structure resembling a football (hence the nickname 'bucky balls')
Diamond
Each carbon in a diamond crystal is bonded to four other carbon atoms making a giant macromolecular array (lattice). As each carbon has four single bonds it is sp3 hybridised and has tetrahedral bond angles of 109º 28'
Properties of diamond
- hardest substance known to man
- brittle (not malleable)
- insulator (non-conductor)
- insoluble in water
- very high melting point
Physical properties of diamond explained by considering the structure and bonding
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Property
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Explanation
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Diamond structure
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Hard
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Many strong covalent bonds holding the structure together |
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Brittle
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All of the bonds are directional and stress will tend to break the structure (In a malleable substance, such as for example a metal, the bonding is non-directional and can still act if the particles are displaced with respect to one another). | |
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Insulator
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All of the valence (outer shell) electrons are used in bonding. The bonds are sigma and the electrons are located between the two carbon nuclei being bonded together. None of the electrons are free to move | |
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Insoluble
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There are only very weak Van der Waal's attractions between the carbon atoms and the water molecules whereas the carbon atoms are bodned very tightly to one another. | |
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Very high melting point
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Many strong covalent bonds holding the structure together - it requires massive amounts of energy to pull it apart |
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.
Physical properties of graphite explained by considering the structure and bonding
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Property
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Explanation
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Graphite structure
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Soft and slippery
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Many strong covalent bonds holding the structure together but only in 2 dimensions. The layers are free to slide easily over one another. Graphite powder is used as a lubricant. |
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Brittle
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All of the bonds are directional within a layer and stress across a layer will tend to break them. Graphite rods used for electrolysis easily break when dropped. | |
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Electrical conductor
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Only three of the valence (outer shell) electrons are used in sigma bonding. The other electron is in a 'p' orbital which can overlap laterally with neighbouring 'p' orbitals making giant molecular pi orbitals that extend over the whole of each layer. Electrons are free to move within these delocalised pi orbitals. | |
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Insoluble in water.
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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. | |
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V. high melting point
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Many strong covalent bonds holding the layers together - it requires massive amounts of energy to pull it apart |
Fullerenes
These are small molecules of carbon in which the giant structure is closed over into spheres of atoms (bucky balls) or tubes (sometimes caled 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 geodhesic dome (Richard Buckminster Fuller) which has a similar structure to a fullerene. Fullerenes were first isolated from the soot of chimineys and extracted from solvents as red crystals.
The bonding has delocalised pi molecular orbitals extending throughout the structure and the carbon atoms are a mixture of sp2 and sp3 hybridised systems.
Fullerenes are insoluble in water but soluble in methyl benzene. They are non- conductors as the individual molecules are only held to each other by weak van der Waal's forces.
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Buckminster fullerene
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Structure
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As the molecule is totally symmetrical with all bond lengths and angles being equal, it is likely/inevitable that the hybridisation of the carbon atoms is somewhere between that of sp2 and sp3. Another example of a theory (hybridisation in this case) having to be modified to accomodate the observed experimental data. |
Physical properties of fullerenes explained by considering the structure and bonding
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Property
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Explanation
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Fullerene structure
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Soft and slippery
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Few covalent bonds holding the molecules together but only weak Vander Waals forces between molecules. |
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Brittle
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Soft weak crystals typical of covalent substances | |
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Electrical insulator
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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 | |
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Insoluble in water.
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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. | |
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Low m.p. solids
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Typical of covalent crystals where only Van der Waal's interactions have to be broken for melting. |
Useful links
Structures of carbon allotropes
