Carbon can exist in various physical forms in the nature with widely varying physical properties, but identical chemical properties. Such characteristic is called allotropy and the different forms are called allotropic forms or allotropes. The difference among allotropes lies in the manner in which the carbon atoms are bonded to one another. Allotropes are exclusively formed by carbon atoms only i.e. it is an elemental form, not a hydrocarbon form at all. The three solid allotropes of carbon are –
(C) Fullerene (recently discovered)
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Diamond is the hardest natural substance known. It has extraordinary brilliance and is transparent in nature.
Structure – Diamond is a huge molecule of carbon atoms only. Each carbon atom in diamond is bonded with four other carbon atoms by covalent bond. The four surrounding carbon atoms are at the four vertices (four corners) of a regular tetrahedron, which are further surrounded by four other carbon atoms. Such tetrahedral network of carbon atoms gives a very rigid three dimensional structure of diamond. Due to such closely packed network structure, diamond has a high density and is very hard in nature.
1. Due to its three dimensional rigid network structure –
(a) it is the hardest natural substance known.
(b) it has a high density.
(c) it has very high melting point (4000°C). Thus, a large amount of energy is needed to break the network structure.
2. It is bad conductor of electricity as no free electrons are present.
- As diamond is very hard, it is used for cutting glass and grinding other hard materials.
- Due to its extraordinary brilliance and transparency, it is used for making jewellery.
- It is used for making ‘dies’ for drawing thin wires from metals.
- It is used as a tool for removing cataract from eyes by eye surgeons.
Diamond can be made artificially when pure carbon is treated at very high pressure and temperature. Synthetic diamond can be distinguished from natural diamond by its smaller size only.
It is grayish black opaque substance.
Structure – It consists of two dimensional hexagonal layers or sheets of carbon atoms instead of three dimensional structure of diamond. Each carbon atom in the graphite crystal is linked with three close neighbour carbon atoms by covalent bonds and form a flat sheet of two dimensional hexagonal rings. The two adjacent layers or sheets of graphite are quite far apart so that no covalent bond can exist between two layers. The various layers are held together by weak vanderwaal forces. Such weak forces make graphite smooth and slippery as one layer can easily slide over the other. Three valence electrons are used for bond formation and fourth valence electron of each carbon atom is free to move. Due to the ‘presence of free electrons’, graphite crystals are good conductor of electricity. The density of graphite is much lower than diamond. This is because of presence of wide spacing (340pm) between the layers and less closely packed structure than diamond.
1. Due to wide spacing and weak vanderwaal forces –
(a) it has low density and lighter than diamond.
(b) it is soft and slippery in nature.
2. It is good conductor of heat and electricity as it has free electrons to move.
- Due to its soft, slippery nature and black colour, it is used for making the cores of pencils (pencil lead) and black paints.
- It is used for making electrodes for dry cells and electric accessories as it is good conductor of electricity.
- Powdered graphite is used as a lubricant for the fast moving parts of machinery even at very temperature due to its soft and non-volatile nature.
Fullerenes are another newly discovered class of carbon allotropes. The first one to be identified was C-60 Buckminsterfullerene which has carbon atoms arranged in the shape of a football. It is a special molecule in which 60 carbon atoms are arranged in interlocking hexagonal (20) and pentagonal (12) rings of carbon atoms in which each corner of every panel represents a carbon atom. Since the shape of C-60 Buckminsterfullerene was looked like a geodesic dome
designed by the U.S. architect Buckminster Fuller, the molecule was named Fullerene.
Fullerene differs from other two allotropes as –
(a) graphite and diamond are gaint molecules having an unending network of carbon atoms, whereas, fullerene is a small molecule.
For e.g., C-60 Buckminsterfullerene molecule is made up of only 60 carbon atoms.
(b) Diamond is a very hard substance whereas graphite is soft in nature. On the contrary, fullerene is neither very hard nor soft. This allotrope of carbon is still under investigation. So, other properties are to be discovered. The chemical properties of all three allotropes are similar. They all form carbon dioxide when burn in oxygen which turns lime water milky.