The compounds obtained from 'Carbon' are widely used as clothes, medicines, books, food, fertilizer, fuel etc. all living structures are carbon based. 
    The amount of carbon present in the earth's crust and in the atmosphere is quite merge. The earths crust has only 0.02% carbon in the form of
    mineral (like carbonates, hydrogen-carbonates, coal and petroleum) and the atmosphere has 0.03% of carbon dioxide. In spite of this small
    amount of carbon available in nature, the importance of carbon seems to be immense.
    Carbon forms a large number of compounds with hydrogen which are known as hydrocarbons. In addition to hydrogen,
    carbon compound may also contain some other element such as oxygen, halogen, nitrogen, phosphorus, sulphur etc. 
    The number of compounds of carbon is more than three million which is much larger than the compounds formed by all other element put together.


    Carbon forms covalent bonds in its compounds with other atoms. In each compound the valency of carbon is four. That is, carbon has tetravalent character.
    But what is covalent bond and what is meaning of tetravalent ?


    A chemical bond formed between two atoms of the same element or two atoms of different elements by sharing of electron is called a covalent bond. 
    Necessary conditions of the formation of covalent bond :
*  The combining atoms should have nonmetallic character.
*  The combining atoms should contain 4 to 7 electrons in their respective valence shell.
*  In hydrogen there is only 1 valence electron, but it also forms covalent bond.
*  The combining atoms need 1, 2, 3 or 4 electrons to complete their octet (hydrogen completes its duplet) 
*  The combining atoms should contribute equal number of electrons to form pair of electrons to be shared.
*  After sharing the pair of electrons each combining atoms should attain stable electronic configuration like its nearest noble gas.


    On the basis of the number of electrons shared by two combining atoms, the covalent bond are of three types.
*   Single Covalent Bond : A single covalent bond is formed by the sharing of one pair of electrons between the two atoms.
    It is represented by one short line (–––) between the two atoms.
    Example : H–H, Cl –Cl, H–Cl, CH3–CH3.

*   Double Covalent Bond : A double covalent bond is formed by the sharing of two pairs of electron between the two combining atoms.
    It is represented by putting (=) two short lines between the two bonded atoms.
    Examples : O = O (O2), CO2 (O = C = O), H2C = CH2      

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