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The term allotropy comes from the Ancient Greek word where ‘allos’ means ‘other’ and ‘tropos’ represents ‘manner or form’. It refers to the property of chemical elements to exist in variable to different forms in a similar physical state. Allotropes refer to different structural changes in an element i.e. the atoms of an element are bonded together in a variable manner. This can be seen in allotropes of carbon such as diamond (here the carbon atoms are bonded to form a crystal lattice or a tetrahedra) graphite (in this the carbon atoms forms a sheet of hexagonal lattice) and fullerene (in which the carbon atoms form a spherical or a tubular structure). Only elements are considered under allotropy and not compounds. Polymorphism considers compounds and it is only restricted to solid materials like crystals. Within the similar physical state allotropy refers to variable forms of an element. In a few elements allotropes possess different crystalline structures and different molecular formulas, as well as a difference in physical states is also observed in two different allotropes of oxygen like dioxygen O2 and Ozone O3 both of them can exist in solid, liquid and gas. Elements showing allotropes include tin, carbon, phosphorus, sulphur and oxygen In 1841 a Swedish chemist by the name Jons Jakob Berzilius discovered the concept of allotropy.
Properties of Allotropes
Allotropes exhibit different physical and chemical properties.
The changes in allotropic forms is the result of the same force which influences other structures like- light, pressure and temperature.
The stability of different allotropes depends upon some specific conditions.
Allotropes of the same elements possess different properties like allotropes of carbon, i.e. diamond and graphite possess variable appearances, hardness values, reactivity, melting and boiling points.
Allotropes of Carbon
Carbon is one of the elements in the periodic table which shows allotropy. The allotropes of carbon is categorised into two distinct forms:
Amorphous Carbon Allotropes
Crystalline Carbon Allotropes
Graphite
In 1789 Abraham Gottlob Wener discovered the most common allotrope of carbon and named it as graphite. It is the most stable and purest allotrope of carbon. It comprises a flat two dimensional layer of carbon atoms arranged hexagonally. It is characterised by soft, black and slippery soil.
In every layer a single carbon atom is bonded to three other carbon atoms with the help of C-C covalent bonds, and every carbon is sp2 hybridised. The remaining fourth bond occurs as a π-bond. As these π-electrons are delocalised they become mobile and help in conducting electricity. There are generally two forms graphite α and ß. The α form possesses layers that are arranged in a series of ABAB with respect to the third layer placed above the first layers. Whereas in ß form it is arranged in ABCABC format.
Properties of Graphite
Due to a perfect stacking of layers one above the other this allotrope of carbon can act as a lubricant.
Graphite also possesses a metallic lustre which helps in conducting electricity, also graphite is a good conductor of heat and electricity.
At high temperatures where oil cannot be used there graphite is used as a dry lubricant.
It can be used in making crucibles which possess the property of inertness to dilute acids and alkalis.
Graphite is thermodynamically more stable than diamond.
It possesses a honeycomb-like layered structure, where each layer is made of planar hexagonal rings of carbon atoms. These layers are held together via Van der Waals forces of attraction.
In graphite 3 carbon forms sigma bonds and 1 carbon forms pi-bond.
Diamond
It is the purest form of carbon. In diamond the carbon atoms are arranged in the lattice in the firm of face centred cubic crystal structure. Diamond possesses many unique physical quantities which arise due to stronger covalent bonding between the atoms. Each carton atom is bonded covalently and tetrahedrally to four other carbon atoms. This tetrahedron together makes up a 3d network of six-membered carbon rings in a chair conformation and thereby providing a zero bond angle strain. This 3d covalently bonded network has been proven to be the main reason for strong strength of diamond.
Properties of Diamond
It is very hard and strong as a substance.
It possesses high melting points
They possess a high relative density, and are transparent to X-rays.
Diamonds have a high value of refractive index.
It is a bad conductor of electricity but a good conductor of heat.
They are mostly insoluble in all solvents.
Amorphous carbon
It refers to carbon which does not possess a crystalline structure. The ratio of sp2 to sp3 hybridised bonds present in a material depicts the properties of amorphous carbon. The materials rich in sp3 hybridised bonds are known as tetrahedral amorphous carbon or diamond like carbons.
Fullerene
In 1985 Professor Harry Kroto from University of Sussex discovered C60 and named it as buckminsterfullerene. Until this discovery there were only two allotropes of carbon: diamond and graphite. It is a spherical structure made of 60 carbon atoms where 20 carbon atoms are arranged as hexagon and 12 of them as pentagon, the carbon atoms are sp2 hybridised. Its shape resembles football that’s why it is sometimes also referred to as a ‘buckyball’. They possess low intermolecular forces and low melting points. It comprises a sea of electrons which helps them conduct electricity. They have the ability to dissolve in an organic solvent.
Conclusion
This system of carbon allotropes possess a wide range of extremities. As in diamond the bond that holds all the carbon atoms together is very strong and makes diamond the hardest of all, whereas these same bonds are weaker in graphite. In diamond the bonds are inflexible and form a 3d network whereas in graphite the bond forms hexagonal sheets, these sheets can slide over each other and make graphite soft in nature. We hope this article has provided a clear concept on allotropes, their properties along with different allotropes of carbon.
