Allotropes

Allotropy is the property by the virtue of which an element exists in more than one form and each form has different physical properties, but identical chemical properties. These different forms are called allotropes. Carbon, phosphorus and sulphur exhibit the property of allotropy.

CRYSTALLINE FORMS OF CARBON:

Crystalline forms are those forms in which the carbon atoms are arranged in a definite order due to which they assume definite shape or form crystals.

(a) Diamond:

It is the purest form of carbon. In India, diamonds are found in Panna mines in Madhya Pradesh and Golcondamines in Karnataka, also at Wajrakrur (A.P.). Diamonds are found in South Africa, Brazil, Congo, Angola, Tanzania and the U.S.A. Some famous diamonds are the Kohinoor, the Hope, the Pitt, and Nizam etc. The weight of diamond is expressed in Carats. 1 Carat = 200 mg

Note: The coloured diamonds are called gems.

(i) Structure of Diamond: 

Diamond crystals found in nature are generally octahedral (eight faced). In the structure of diamond, each carbon is linked to four other carbon atoms forming a regular and tetrahedral arrangement and this network of carbon atoms extends in three dimensions and is very rigid. This strong bonding is the cause of its hardness and its high density. This regular, symmetrical arrangement makes the structure very difficult to break. To separate one carbon from the structure, we have to break four strong covalent bonds.

(ii) Properties of Diamond:

• Purity: Diamond is the purest form of carbon.
• Hardness and density: It is the hardest known substance with high density and high melting point.
• This is due to the strong and short distance C – C covalent bonds binding the carbon atoms in its structure.
• Conductivity: Diamond is a bad conductor of heat and electricity.
• Solubility: Diamond is insoluble in all the known solvents.
• Transparency and Brilliance: Because of high refractive index (2.5), diamond can reflect and refract light.
• It is therefore, a colourless transparent and lustrous brilliant substance.

Interesting Notes:

• Black diamonds are also called as carbonado.
• Among the diamonds black diamonds are the most hardest.

Uses of Diamond: 

• They are used in jewellery because of their ability to reflect and refract light.
• Black Diamonds are used in cutting glass and drilling rocks.
• Diamond has an extraordinary sensitivity to heat rays and due to this reason, it is used for making high precision thermometers.
• Diamond has the ability to cut out harmful radiations and due to this reason it is used for making protective windows for space probes.
• Diamond dies are used for drawing thin wires. Very thin tungsten wires of diameter less than one-sixth of the diameter of human hair have been drawn using diamond dies.
• Surgeons use diamond knives for performing delicate operations.

GRAPHITE:

Graphite is an allotrope of carbon, which is black or bluish grey with a metallic lustre and a greasy feel. It occurs in igneous and metamorphic rocks, such as marble.NCERT solutions for class 8 Science prepared by Physics Wallah will help you to solve your NCERT text book exercise. 

(i) Structure of Graphite:

Each carbon is bonded to only three neighboring carbon atoms in the same plane forming layers of hexagonal networks separated by comparatively larger distance. The different layers are held together by weak forces, called Vanderwaals forces. The layers can therefore, easily slide over one another. This makes graphite lubricating, soft and greasy to touch.

Within each layer of graphite, every carbon atom is joined to three others by strong covalent bonds. This forms a pattern of interlocking hexagonal rings. The carbon atoms are difficult to separate from one another. So graphite also has a high melting point.

However, the bonds between the layers are weak. The layers are able to slide easily over one another, rather like pack of cards. This makes graphite soft and slippery. When we write with a pencil, layers of graphite flake off and stick to the paper.

Properties of Graphite:

• Dark grey crystalline solid with metallic lusture (shining nature).
•  Its density is 2.3 g cm3
• Due to weak vanderwal’s force between the hexagonal layers, The layers are slippery to touch hence used as a lubricant.
•  It is good conductor of electricity due to the presence of free electrons.
•  Transmission of light – It is opaque to X-rays.
•  Insoluble in solvents normally.
•  The melting point of graphite is above 35000 C

Uses of graphite:

• It is used for making pencil lead, printer’s ink, black paint etc.
• It is used as dry lubricant for heavy machinery.
• It is used in making crucibles for melting substances.
• It is used as an electrode in batteries and electric furnaces.
• It is used in nuclear reactors as moderator to regulate nuclear reactions.
• It is also used in making artificial diamonds.

Difference between diamond and graphite 

FULLERENES:

Fullerene is naturally occurring allotrope of carbon in which 60 carbon atoms are linked to form a stable structure. Previously, only two forms of carbon (diamond and graphite) were known. The third allotrope of carbon, called fullerene, was discovered in 1985 by Robert Curl, Herald Kroto and Richard Smalley. It contains 60 or more number of carbon atoms.

Uses of Fullerenes:

It is hoped fullerenes or their compounds may find uses as-

• Superconductors
• Semiconductors 
• Lubricants
• Catalysts 
• Highly tensile fibres for construction industry. 
• nhibiting agents in the activity of the AIDS virus.