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Background:
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 various 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). Each carbon atom has tetrahedral bond angles of 109º 28'.
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
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