Allotropes of Carbon: Structures, Properties & Examples

Allotropes of Carbon are referred to elements that occur in two or more forms, however, in the same state of matter. Carbon and Sulphur are the two common chemical elements that have allotropes. Carbon is capable of forming many allotropes cause of its valency. A well-known crystalline form of carbon is diamond, followed by graphite and fullerenes. A large number of allotropes have been discovered and researched including ball shapes and sheets. Larger scale structures of carbon include nanotubes, nanobuds and nanoribbon.

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Allotropes of Carbon: Details

Carbon is a non-metal with the symbol \({\rm{C}}\) and atomic number \(6\). In the modern periodic table, Carbon is placed in the second period and \(14\) group. The name Carbon is derived from the Latin word ‘carbo’, which means ‘coal’. Carbon allotropy example – Diamond, graphite, coal, coke and fullerene are made of Carbon elements only and are referred to as allotropes of Carbon.

Allotropes Definition

The phenomenon of the existence of an element in two or more forms has different physical properties, but identical chemical properties are called allotropy, and the different forms are called Allotropes. A few allotropy examples are given in this article.

What is the Crystalline Form of Carbon?

Carbon exhibits the phenomenon of allotropy and exists in the following two types of allotropic forms:

I. Crystalline form of Carbon: Four Crystalline Allotropes of Carbon having well-defined crystal structures are: 

1. Diamond
2. Graphite
3. Fullerenes
4. Carbon Nanotubes

II. Amorphous allotropic forms of Carbon: Some important amorphous allotropic forms of Carbon are:

1. Coal
2. Coke
3. Wood charcoal
4. Animal charcoal
5. Sugar charcoal
6. Lampblack
7. Gas Carbon

What Makes Diamond Precious?

Diamond: Structure

Diamond possesses a three-dimensional network of carbon atoms that join together through strong covalent bonds. Each Carbon atom is in the state of \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybridisation and linked tetrahedrally to four neighbouring Carbon atoms.

This network extends in three dimensions. All Carbon-Carbon \(\left( {{\rm{C – C}}} \right)\) bonds are equal and equal to \(154\,{\text{pm}}\), and each \({\rm{C – C – C}}\) bond angle is \({\rm{10}}{{\rm{9}}^{\rm{^\circ }}}{\rm{2}}{{\rm{8}}^{\rm{‘}}}.\)

Diamond: Properties

1. It is the purest and the densest variety of Carbon. Its density is \(3.51\,{\text{g}}\,{\text{c}}{{\text{m}}^{ – 3}}\)
2. It is the hardest natural known substance and possesses a very high melting point \(\left({3843\,{\text{K}}} \right)\) It is insoluble in all solvents.
3. It is transparent and processes a high refractive index.
4. It is a bad conductor of electricity. This is because all the valence electrons of each Carbon are involved in the Carbon-Carbon Sigma \(\left( \sigma  \right)\) bonds, and no unpaired electrons are left in the crystal.
5. Chemically, diamond is resistant to almost all acids, alkalis, and salts. However, it is acted upon by fused Sodium Carbonate. When heated with a mixture of potassium dichromate and sulphuric acid to \(475\,{\text{K}}\), it slowly gets oxidized to Carbon dioxide.
6. The value of a diamond depends upon its size and colour. Bluish white diamonds are more precious than those having a low colour. Black diamonds are the cheapest and are not used in jewellery.

Diamond: Uses

1. Diamonds are used to cut instruments like a glass cutter, saw for cutting marbles, and rock drilling equipment.

2. Diamonds are used for making jewellery due to their extraordinary brilliance.

3. Eye surgeons use Sharp-edged diamonds as a tool to remove a cataract from the eyes with great precision.

4. Diamond dies are used for drawing very thin wires of metals like tungsten.

Graphite: A Non-metallic Conductor

In graphite, each Carbon atom is in a state of \({\text{s}}{{\text{p}}^{\text{2}}}\) hybridization and is covalently attached to three neighbouring Carbon atoms lying in the same plane. Thus, planar hexagonal rings are formed. The Carbon-Carbon bond length in this ring is \(142\,{\text{pm}}\).

The hexagonal rings constitute layers. The layers are held together by weak van der Waal’s force and separated by \(142\,{\text{pm}}\). These layers can slide over one another, hence graphite is soft and possesses lubricating properties.

Graphite: Properties

1. It is a dark grey substance with a metallic lustre.
2. It is very soft and greasy to touch.
3. Since only three electrons of each Carbon are used in making hexagonal rings in graphite, the fourth valence electron of each Carbon is to move. This makes graphite a good conductor of heat and electricity.
4. It does not attack by dilute acids, alkalis, and chlorine. A mixture of potassium dichromate and sulphuric acid oxidizes it slowly to Carbon dioxide.

Graphite: Uses

1. It is used in making electrodes and Carbon arcs.

2. It is used as a lubricant for machines working at high temperatures.

3. It is used in the manufacture of lead pencils. The powdered graphite is mixed with clay and pressed into sticks. These sticks are used to make pencils.

4. It is used as a moderator in atomic reactors.

5. It is used as a reducing agent in steel manufacturing.

6. It is used in high-strength composite materials.

7. It is used in the manufacture of Crucible, which can withstand high temperatures.

Fullerenes: A Synthetic Allotrope of Carbon

Fullerenes are the only pure form of Carbon because they do not have dazzling edges or surface bonds that attract other atoms, as in the case of graphite or diamond.

Fullerene is collectively discovered in \(1985\) by R.E. Smalley and R.F. Curl of Rice University, Houston, Texas(U.S.A) and H.W. Kroto of the University of Sussex, Brighton (U.K). For this discovery, they shared the Nobel prize in \(1996\).

Fullerene is a large spherical molecule of composition \({{\rm{C}}_{{\rm{2n}}}}\) where \({\rm{n}} \ge {\rm{30}}.\) Fullerene is practically produced by heating graphite in an electric arc in an inert gas such as helium or argon when a sooty material is formed by condensation of \({{\rm{C}}_{{\rm{n}}}}\) small molecules.

The sooty material so formed mainly consists of \({{\rm{C}}_{{\rm{60}}}}\) with a smaller quantity of \({{\rm{C}}_{{\rm{70}}}}\) and traces of other fullerenes. The \({{\rm{C}}_{{\rm{60}}}}\) and \({{\rm{C}}_{{\rm{70}}}}\) Fullerenes can be readily separated from the fullerenes soot by extraction with benzene or toluene followed by chromatography over alumina.

Structure of \({{\rm{C}}_{{\rm{60}}}}\) Fullerenes

The \({{\rm{C}}_{{\rm{60}}}}\) fullerenes also named Buckminsterfullerene or simply fullerene, in honour of American architect, Robert Buckminster Fuller. Fullerene is a saucer-ball-shaped molecule that has \({\rm{60}}\) vertices with a Carbon atom at each vertex. It contains \({\rm{20}}\) six-membered rings and \({\rm{12}}\) five-membered rings.

Six-member rings are fused both to other six-membered rings and five-membered rings, but the five-membered rings are connected only to six-membered rings. It contains both single and double bonds with Carbon-Carbon distances of \(142\,{\text{pm}}\) and \(138.3\,{\text{pm}}\), respectively.

Buckminsterfullerene Uses

Buckminsterfullerene is used as lubricants, in drug delivery systems, and as catalysts. Fullerene is also used as a conductor. Some types of fullerene are also used as an absorbent for gases. It is used in making cosmetic products.

Carbon Nanotubes

A Carbon nanotube consists of a two-dimensional array of hexagonal rings of Carbon, just as in a layer of graphite. The layer is then rolled into a cylindrical and capped at each end with half of a \({{\rm{C}}_{{\rm{60}}}}\) fullerenes.

Carbon nanotubes are very tough, about \(100\) times as strong as steel. These are electrically conducted only along the length of the tube.

Which Allotrope of Carbon is formed from Carbonisation of Vegetable Matter?

Coal occurs in huge deposits in almost all parts of the world and is believed to be formed by the slow Carbonisation of the vegetable matter buried in the layers of the earth long ago. It is an impure form of Carbon and is available in several varieties. These are peat (\(60%\) Carbon), lignite (\(70%\) Carbon), bituminous coal (\(78%\) Carbon), semibituminous coal (\(83%\) Carbon) and anthracite (\(90%\) Carbon).

It is extensively used as a fuel in boilers, engines, and furnaces. It is also used in the manufacture of fuel gases and synthetic petrol.

Which Form of Carbon is Used as a Reducing Agent?

Coke is obtained by destructive distillation of coal and contains about \(80\) to \(95%\) Carbon. It is a greyish black hard solid. It is mainly used as a fuel and also as a reducing agent in metallurgical operations.

Which Form of Carbon is Used for Teeth Whitening?

Wood Charcoal

It is obtained by burning wood in a limited supply of air. It is a black, soft, and porous solid. It absorbs colouring matter and odoriferous gases. It is used as a fuel, a deodorant, and decolorizing matter. It is also used in making gunpowder.

Animal Charcoal (Bone Black)

It is obtained by the destructive distillation of animal bones and contains about \(10%\) Carbon. It is a better absorbent than wood charcoal and is mainly used for decolorizing organic substances and crude sugar solutions. Sugar charcoal

Sugar Charcoal

It is the purest form of charcoal and is obtained by heating cane sugar in the absence of air.

Lampblack (Soot)

It is a fine black powder and is obtained by burning Carbon-rich substances such as tar, turpentine oil, kerosene oil, petroleum, etc., an insufficient supply of air. It is a velvety black powder containing about \(89.99%\) Carbon. It is used in making printing ink, black paints, boot polishes, and as a filler in the rubber industry.

Gas Carbon

Carbon scraped from the retorts’ walls for the destructive distillation of coal is called gas Carbon. It is a good conductor of electricity and is used for making electrodes.

Summary

Life would not have existed on the earth without the element Carbon. From living to non-living things, Carbon is present in one or other forms. In this article, you have gained knowledge about the different crystalline allotrope of Carbon like diamond, graphite, fullerene, etc. and details of their structure, properties, and uses.

FAQs on Allotropes of Carbon

Here are a few frequently asked questions related to Allotropes of Carbon.

Q.1: What are Allotropes?
Ans: The phenomenon of the existence of an element in two or more forms has different physical properties, but identical chemical properties are called allotropy, and the different forms are called Allotropes.

Q.2: Is coke an Allotrope of Carbon?
Ans: Yes, coke is an amorphous allotropic form of Carbon. Coke is obtained by destructive distillation of coal and contains about 80 to 95 Carbon.

Q.3: What is the hardest allotrope of Carbon?
Ans: Diamond is the hardest Allotrope of Carbon.

Q.4: What is the purest form of Carbon?
Ans: Fullerene is the purest form of Carbon because it does not have dazzling edges or surface bonds that attract other atoms as in the case of graphite or diamond.

Q.5: How is fullerene obtained?
Ans: Fullerene is obtained by heating graphite in an electric arc in an inert gas such as helium or argon when a sooty material is formed by condensation of C_n small molecules. The C₆₀ and C₇₀ fullerenes present in sooty material formed are edges separated from the fullerenes soot by extraction with benzene or toluene followed by chromatography over alumina.