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The World of Carbon

Allotropes of Carbon

At the beginning of the 19th century, compounds obtained from plants and animals were regarded as organic compounds.

Organic compounds that contain both hydrogen and carbon are called hydrocarbons. These are parent fundamental compounds.

Classification of hydrocarbons

Hydrocarbons are classified on the basis of their structure and bonding.

The given figure shows the classification of hydrocarbons.

Open chain hydrocarbons - An open chain is an arrangement of atoms that does not form a ring, i.e. it has two open ends. It may consist of straight or branched chain compounds.

Open chain hydrocarbons are again classified into two sub-groups.

(a) Saturated open chain hydrocarbons - The saturated (contains only single bonds) open chain hydrocarbons form a homologous series called the paraffin series or the alkane series.

Their formula can be summarised as CnH2n+ 2, where n is the number of carbon atoms in the molecule.

Examples- Methane (CH4), ethane (C2H6),propane (C3H8), Butane (C4H10)

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Alkanes are also known as 'paraffins'. The word ‘paraffin’, which is derived from a Latin word meaning "little activity", means that such compounds are very unreactive.

(b) Unsaturated open chain hydrocarbons - Unsaturated open chain hydrocarbons include the alkene or olefin series, the diene series and the alkyne series.

1. Alkene: The alkene series consists of chain hydrocarbons that contain a double bond between two carbon atoms. The general formula for the series is CnH2n, where n is the number of carbon atoms.

Examples: Ethene (CH2=CH2), Propene (CH3-CH=CH2), Butene (CH3-CH2-CH=CH2)

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Alkenes are also called olefins because they form oily liquids on reaction with chlorine gas.

2. Alkyne: Members of the alkyne series contain a triple bond between two carbon atoms in a molecule. Alkynes have the general formula CnH2n −2.

Examples: Ethyne (HC≡ CH), Propyne (CH3-C≡CH), 2-Butyne (CH3-C≡C-CH3)

Closed Chain Hydrocarbons - The hydrocarbons which contain a closed chain of carbon atoms are called cyclic hydrocarbons. These compounds contain carbon atoms joined in the form of a ring. They are further sub-divided into two sub-groups.

(a) Alicyclic Hydrocarbons - These cyclic compounds contain single bonds. These are closed chain or cyclic compounds but they behave more like aliphatic or open chain compounds in spite of their rings.Upon cyclisation, an alkane hydrocarbon chain would result in the formation of alicyclic compounds.


(b) Aromatic Compounds - These organic compounds contain at least one Benzene ring. Aromatic compounds have alternate single and double bonds. The substitution of one or all hydrogen atoms in a Benzene ring can form aromatic compounds.


Do You Know?

Aromatic compounds get their name from the fact that many of these compounds have a sweet scent.

Do you know that the number of carbon compounds outnumber the compounds of all the other elements taken together? The number of carbon compounds is so vast that there is one branch of chemistry dedicated only to the study of carbon and its compounds. This branch is known as organic chemistry. 

Do you know why carbon is a part of so many compounds? Which properties of carbon help it to be a part of so many compounds?

Here, we will try to find the answers to these questions.

The most important property of carbon that allows it to be a part of so many compounds is catenation. Catenation is the ability of an element to combine with itself through covalent bonds.

Carbon has a unique ability to combine with other atoms of carbon to form long chains and rings. Carbon forms very strong bonds with itself. Hence, the chains and rings formed by carbon through catenation are very stable. Thus, these chains and rings are not only long, but also very stable. This is because carbon can form strong single, double, and triple bonds with other atoms of carbon. This can give rise to branches, chains, and rings as shown in the figure below.


Formation of long chained, branched, and cyclic structures of carbon.

Now, carbon has a valency of four. This means that it can bond with four other carbon atoms or four other monovalent atoms, which gives rise to compounds having specific properties. These properties depend on carbon as well as on elements other than carbon.

Do You Know:

Apart from carbon, a very few elements display the property of catenation so extensively. Silicon can join with other atoms of silicon, but the maximum length of the chains that can be obtained in the case of silicon is limited to seven or eight atoms. In addition, the compounds formed by silicon by catenation are not very stable.

Carbon is the versatile element present in food, clothes, medicines, papers, etc. In addition, all living structures are carbon based. The earth’s crust has only 0.02% carbon in the form of minerals and the atmosphere has 0.03% carbon dioxide. In spite of the small amount of carbon available in nature, the importance of carbon is immense.

Position of carbon in the periodic table

Carbon has an atomic number 6 and an atomic mass as 12 u. It contains 2 and 4 electrons in the K and L shell respectively. It belongs to the group IVA of the periodic table and has a valency of 4.

Occurrence: Carbon occurs in free as well as in combined state.

In free state, carbon occurs as diamond, graphite, and coal. Diamond and graphite are pure forms of carbon while coal is an impure form of carbon in which the percentage of carbon varies from 24% to 90%.

In combined state, carbon occurs in

  • bio-molecules such as cellulose, carbohydrates, fats, proteins, etc.

  • minerals in the form of carbonates

  • calcium carbonate (CaCO3) in lime, marble, and chalk

  • magnesium carbonate (MgCO3) in magnesite

  • calcium and magnesium carbonate (MgCO3.CaCO3) in dolomite

  • calamine (ZnCO3) as zinc carbonate


  • It is present in large amount in petrol, kerosene oil, diesel oil, grease, and wax.

  • Natural gas, marsh gas, petroleum gas, and coal gas contain carbon in the form of its compounds.

Allotropy: The phenomenon due to which an element exhibits different physical forms is called allotropy.

Reasons for allotropy are different arrangement of atoms in each allotrope, different methods by which each allotrope is prepared, and different amount of energy associated in the formation of each allotrope.

Let us study about the different allotropes of carbon.

Carbon is considered to have two main kinds of allotropes i.e., crystalline and amorphous.

Following is a classification of the crystalline and amorphous forms of carbon:

Well known allotropes of carbon are:

  1. Diamond
  2. Graphite
  3. Buckminsterfullerene

Among these, diamond and graphite have been known for a very long time and are quite common. Buckminsterfullerene, on the other hand, was discovered just recently. Thus, not much is known about it.

Crystalline forms of carbon


Crystal structure: In diamond, each atom of carbon of the given crystal unit is surrounded by four other carbon atoms, which are joined by covalent bonds, such that they form a regular tetrahedron. The diamond crystal is a compact structure in which atoms of a single unit lie in different planes. Thus, the atoms cannot slip because of their different positions in different planes and hence, diamond is the hardest naturally occurring substance.

Physical properties: The physical properties of diamond are:

  • It is the hardest naturally occurring substance.
  • Pure diamond is colourless, transparent, but brittle solid.
  • It is chemically inert under ordinary conditions as there are no free electrons available.
  • It is a good conductor of heat but is does not conduct electricity.
  • Its refractive index is 2.42.
  • Its specific gravity is 3.52.
  • It is the densest form of carbon, its density being 3.5.
  • It is transparent to X-rays, ultraviolet rays, and visible light rays.

Chemical properties: The chemical properties of diamond are:

  • Action of air or oxygen: When diamond i…

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