o Covalent Bond: When a bond is formed by sharing of electrons by the two atoms the bond formed between them is called covalent bond. Each pair of shared electrons is indicated by a line -. Once the covalent bond has been formed, the two bonding electrons are attracted by the two nuclei instead of one, so the bonded state becomes more stable than the non-bonded state.
For eg. The formation of Chlorine molecule (Cl2)
    17Cl      +     17Cl    →   Cl2
     2,8,7               2,8,7
Both the chlorine atoms have 7 electrons in their outermost shell. So here the exchange of electrons is not possible. For the formation of Cl2 molecule, each Cl atom shares one pair of electron. The bond thus formed is called a single covalent bond.

           The formation of O2 molecule
                 8O       +    8O      →     O2
                2,6            2,6
     To attain octet, each oxygen atom shares 2 pairs of electrons. Thus, a double covalent bond is formed.

    

The formation of N2 molecule
7N       +    7N      →     N2
2,5             2,5
To attain the octet configuration, the nitrogen atoms share 3 pairs of electrons. Thus a triple bond is formed.



    The number of covalent bonds formed by an atom is termed as its covalency. In the formation of Cl2 molecule, the covalency of each Cl atoms is one, in O2 molecule, the covalency of O atom is 2 and in N2, it is 3. The double and triple bonds are together called multiple bonds.

Ø  Properties of Covalent Compounds: The covalent compounds are found to possess the following general properties, which are opposite to those of ionic compounds:
(a) These substances are made up of individual covalent molecules with weak inter-molecular forces.
(b) Because of the weak inter-molecular forces, the substances are gases, liquids or soft solids at room temperature.
(c) These substances when put into water do not undergo ionisation. Hence these substances in the fused state or in solution do not conduct electricity. However, in presence of water, some polar covalent compounds can ionise and conduct electricity.
(d) These substances have low melting and boiling points because less energy is needed to break down the weak inter-molecular forces.
(e) These substances are commonly soluble in benzene and other organic solvents but insoluble in water.