POLAR COVALENT BONDS
Covalent bonds between different atoms may acquire ionic characters which can be explained on the basis of concept of electronegativity. Electronegativity of an element defined as the power of its atoms to attract the bonding or d pair of electrons towards itself
POLAR CHARACTER OF COVALENT BOND
When covalent bond is formed between two similar atoms, the shared pair of electrons lies midway between the nuclei of the two atoms, because both the atoms have same attraction for the bonding electrons. The electron cloud constituting the covalent bond is symmetrically distributed around the atoms. Such a covalent bond is called a non-polar covalent bond.
For example, molecules like H2, Cl2, O2 and N2 contain non-polar bonds.
In case, the covalent bond is formed between two dissimilar atoms, one of which has a larger value of electronegativity, the bonding pair of electrons is displaced towards the more electronegative atom. In other words, electron cloud containing the bonding electrons gets distorted and the charge density concentrates around the more electronegative atom.
Due to the unequal distribution of electron charge density, the more electronegative atom acquires a partial negative charge (indicated as whereas the less electronegative atom acquires a partial positive charge indicated as 8+. Thus, a covalent bond develops a partial ionic character as a result of the difference of electronegativities of the atoms comprising the bond. Such a bond is called polar covalent bond (as shown in Fig. 7.10).
For example, the bond between hydrogen and chlorine atoms in HCl molecule is polar because the shared electron pair is displaced towards chlorine atom which is more electronegative.
The extent of ionic character in a covalent bond depends upon the difference of electronegativities of the two atoms forming a bond. Greater the difference of electronegativities,greater is the percentage of ionic character in a bond. For example, hydrogen fluoride is more polar than hydrogen chloride because the difference of between H and F is more than that between H and Cl
It has been observed that the bond has 50% ionic character and 50% covalent character if the difference of electronegativities of the participating atoms is 1. 7. On the other hand, the covalent character dominates if the difference of electronegativities is less than 1.7 while ionic character dominates if the difference of electronegativities is greater than 1.7.