When two atoms of different elements are linked by a covalent bond, the shared electrons are not attracted equally by the two nuclei of bonding atoms. Due to unequal distribution of electron cloud, one end of the molecule acquire partial positive charge and the other end acquires equal partial negative charge.
The polarity of bond gives partial ionic character. The percentage of partial ionic character depends upon the difference of E.N. of two atoms join with each other through a covalent bond.
1. HCl molecule:
E.N. of H = 2.1
E.N of Cl = 3.0
Due to difference of E.N., the ability Cl-atom to attract shared pair of electron is greater than that of
H-atom. Therefore, covalent bond in HCl has ionic character.
In HCl, covalent bond is 17% to 18% ionic.

2. HF molecule:
E.N. of H = 2.1
E.N. of F = 4.0
In this example difference of E.N. is greater as compare to HCl. Therefore, ionic character of HF is 64%.
Ionic character of covalent compound affects properties of these compounds. For example viscosity, M.P., B.P. of compounds having ionic character are higher than that of non-polar compounds.
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