Polar and Non-polar Molecules

Polar Molecules

When compounds are formed from elements whose atoms have different electronegativities, the electrons forming the bonds will have a tendency to be attracted closer to the more electronegative element. In such instances covalent molecules develop some ionic character and are called polar molecules. As a result of polarity, a small negative charge (d^-) appears on the more electronegative atom and a small electropositive charge (d⁺) appears on the less electronegative atom.

Thus polar molecules are formed when the combining atoms have different electronegativites, due to which the shared pair of electrons is displaced towards the more electronegative atom resulting in separation of charges. The degree of polarity depends on the difference in the electronegativites of the constituting elements.

To illustrate polarity in a molecule the example of hydrogen chloride is studied. Here a pair of electrons is shared between the hydrogen and the chlorine atoms (Fig. 3.14).

The chlorine atom is much more electronegative than hydrogen atom. Hence, the shared pair electron from the hydrogen and chlorine atom has the tendency to be closer to the chlorine atom. This makes the chlorine atom slightly more negative (d^-), while the hydrogen atom has a slightly more positive charge (d⁺).

In the case of ammonia, the electrons of hydrogen are slightly closer to nitrogen than to the hydrogen. Hence, nitrogen gets a partial negative charge (d^-) charge and hydrogen gets partial positive charge (d⁺) charge (Fig. 3.15).

Similarly in the case of water, the electrons of hydrogen are more close to the oxygen than to the hydrogen. Hence, oxygen gets (d-) charge and hydrogen gets a (d+) charge.

Non-Polar Molecules

When diatomic molecules are formed from identical atoms, the electrons forming the bonds are shared equally between the atoms e.g.,H₂, N₂, O₂, Cl₂ etc. Such molecules are termed as non-polar as there is no separation of charges.

When compounds are formed between two different types of atoms having more or less identical electronegativities, then such molecules are also termed as non-polar molecules.

Example: Methane

Differences Between Polar and Non-polar Covalent Compounds

Non-Polar covalent compound	Polar covalent compound
Shared pair of electrons is equally distributed between the combining atoms	Shared pair of electrons is displaced towards the more electronegative atom and not equally distributed between the combining atoms
There is no separation of charges	Separation of charges takes place
The molecules are symmetrical	The molecules are not symmetrical