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Allotropes of Carbon

Allotropy is the property of some chemical elements to exist in two or more different forms having the same chemical properties but different physical properties. The different forms of an element are called allotropes.
Carbon exists in two different physical forms, namely crystalline and amorphous (see Figure 7.1.)

Crystalline Forms of Carbon

Diamond Diamond is a crystalline form of carbon wherein each carbon atom is bonded to four other carbon atoms in a tetrahedral structure by a strong covalent bond. This makes diamond the hardest substance.
Uses of diamond
  1. It is used as a gemstone in jewellery.
  2. It is used for cutting, drilling and polishing hard materials such as glass.
  3. It is used as an abrasive.
  • Diamond is sold in carat (a unit of weight equal to th of a gram).
  • Diamond has the highest thermal conductivity
Graphite In graphite, the carbon atoms are arranged in layers in a regular hexagonal form. The successive layers are held together by weak van der Waal’s forces.
The carbon present in the earth’s crust crystallises at a very high pressure to form a diamond. Graphite occurs in nature as an opaque, black solid called Plumbago. It is believed that at volcanic temperature, wood present in the layers of earth is converted into graphite.


Uses of graphite
  1. It is used as a lubricant.
  2. It is used for making furnace electrodes.
  3. It is used as a moderator in atomic reactors.
  4. It is used in the manufacture of paints and pencils.
Fullerene The structure of fullerene, a recently discovered allotropic form of carbon, differs from the structure of diamond and graphite. It has a discrete, hollow molecular structure. Different forms of fullerene are, namely, C-60, C-70, C-76, C-78 and C-84. However, C-60 is the most common and abundantly available form. The structure of fullerene resembles that of a soccer ball, also known as buckminster fullerene. C-60 consists of a fused system of five- and six-membered rings. It has 32 faces, of which 12 are pentagons and 20 are hexagons.
Unlike diamond and graphite, fullerenes are soluble in organic solvents.
Uses of fullerene
  1. It is used as a super-conductor.
  2. It is used in cancer therapy.

Amorphous Forms of Carbon

Wood charcoal: Charcoal is obtained by burning logs of wood in a limited supply of air. The charcoal which has more pores for the adsorption of gases is called the activated charcoal. Since, both these combustion reactions are exothermic reactions, charcoal is used in iron boxes and in ovens.
Animal charcoal (also known as activated charcoal): When crushed bones are heated in the absence of air, animal charcoal is formed. This is a mixture of carbon and calcium phosphate. It has the property of absorbing colouring matter and is used to decolourise crude sugar.
Uses of charcoal
  1. It is used as a fuel.
  2. It is used as a reducing agent in the extraction of iron from its ore.
  3. It is used in the manufacture of gun powder.
  4. It is used to decolourise and to remove bad odour from crude sugar.
  5. It is used in the purification of water.
  6. It is used in the manufacture of calcium carbide, silicon carbide (carborundum) and water gas.
  7. It is used in gas masks.
Lampblack or Soot: It is a soft, black powder. It is a non-conductor of heat and electricity.
Uses of lampblack or soot
  1. It is used in the preparation of Indian inks and printing inks.
  2. It is used in the vulcanisation of rubbers.
  3. It is used in the preparation of shoe polishes.
  4. It is used in the manufacture of black paints.
  5. It is used in the manufacture of carbon papers and typewriter ribbons.

Coal: It is one of the impure forms of carbon. Coal is called fossil fuel because it was formed from the remains of vegetation that grew 400 million years ago. It is often referred to as ‘buried sunshine’, because the plants which form coal capture energy from the sun through photosynthesis.
Coal is formed when organic material is buried under sediment and exposed to high temperatures and high pressure for millions of years. This is shown in Figure.
Different types of coal are as follows:
  1. Anthracite
  2. Bituminous
  3. Lignite
  4. Peat
Description: 33391.png

Calorific value is defined as the quantity of heat liberated by the complete combustion of a unit weight or a unit volume of fuel.


1 cal = 4.18 J


1 kcal = 4186.8 J


1 BTU (British thermal unit) = 1055.05 J



Uses of coal
  1. Coal is used as a fuel.
  2. Bituminous coal is used to make coal tars.
  3. Anthracite coal is used in steam engines and in industries.
  4. Coal is used in the preparation of coke.
  5. Coal is used in the manufacture of synthetic petrol.
  6. Coal is used in the preparation of coal gas, water gas and producer gas that are used as gaseous fuels.
  7. Coal is used as a reducing agent in the extraction of iron from its ore.
Products of coal: When coal is subjected to destructive distillation, it yields the following products:
  1. Coke: When bituminous coal is heated to around 1573 K in the absence of air, coke is obtained.
  2. Water gas (CO + H2): When super-heated steam is passed over a red hot coke, a gaseous fuel, i.e. water gas, is obtained.
  3. Producer gas (N2 + CO in the ratio 2:1): It is prepared by passing a controlled amount of air through red hot coke. Producer gas is also a gaseous fuel.
  4. Calcium carbide (CaC2): When calcium oxide (CaO) and coke are heated, calcium carbide is obtained.
    Description: 31590.png
    Calcium carbide is a crystalline solid. It reacts with water to form acetylene.
    CaC2 + 2H2O Description: 39363.jpg Ca(OH)2 + C2H2­
    Acetylene is used in oxyacetylene flame for welding purposes. Acetylene is also used in the manufacture of acetic acid and plastics such as polyvinylchloride (PVC).
  5. Coal gas: It is a gaseous fuel obtained by the dry distillation of coal. Coal gas is a mixture of hydrogen (5–55%), methane (25–30%), carbon monoxide (8–10%) and ethylene (3–5%).
    Uses of coal gas
    1. It is used as a domestic fuel for cooking purposes.
    2. It is used as an illuminant in lighting homes at night.
    3. It is used as a fuel in industries.
    4. It is used in metallurgical processes for creating an inert atmosphere.
  6. Coal tar: It is a black viscous liquid. It is composed of a complex mixture of a number of carbon compounds. A few years ago, coal tar was the chief source of carbon compounds used in dyes, explosives, paints, synthetic fibres, drugs and pesticides (currently, many such compounds are synthesised from petrochemicals, which are obtained from petroleum).
  7. Silicon carbide (SiC): It is a binary compound of silicon and carbon. It is also called ‘carborundum’. Silicon Carbide is produced when silicon dioxide and coke are heated in an electric furnace. Description: 27695.png Silicon carbide is a very hard substance. It does not dissolve in water. It has a high melting point.
    Uses of silicon carbide
    1. It is used as an abrasive in the manufacture of sand papers.
    2. It is used as a grinding stone to cut and sharpen tools.

Destructive distillation refers to the decomposition of a solid by heating it in a closed container and collecting the volatile constituents given off. Destructive distillation is a process by which organic substances such as wood, coal etc. are decomposed by heat in the absence of air and distilled to produce useful products such as coke, charcoal, oils and gases. The other elements which exhibit allotropy are as follows:

Element Allotropes
Carbon • Fullerenes (buckyballs)
• Carbon nanotubes (cylindrical nanostructure)
Phosphorus • White phosphorus (crystalline solid P4)
• Red phosphorus (polymeric solid)
• Scarlet phosphorus
• Violet phosphorus
• Black phosphorus
Oxygen • Dioxygen (O2, colourless)
• Ozone (O3, blue)
• Tetraoxygen (O4, metastable)
• Octaoxygen (O8, red)
Sulphur • Plastic sulphur (polymeric solid)
• Rhombic sulphur (large crystals composed of S8 molecules)
• Monoclinic sulphur (fine needle-like crystals)
Selenium • Red selenium (cyclo-Se8)
• Grey selenium (polymeric Se)
• Black selenium
Boron • Amorphous boron (brown powder)
• Crystalline boron (blackish and hard)
Germanium • α-germanium
• β-germanium (at high pressures)
Arsenic • Yellow arsenic (molecular non-metallic As4)
• Grey arsenic (polymeric As, metalloid)
• Black arsenic (metalloid)
Antimony • Blue-white antimony (metalloid)—the stable form
• Yellow antimony (non-metallic)
• Black antimony (non-metallic)
Silicon • Amorphous silicon (brown powder)
• Crystalline silicon (greyish colour with metallic luster)

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