A polar molecule is an asymmetric molecule with a permanent electric dipole moment that arises from the finite separation of the centres of the net positive charge and the net negative charge in the molecule, even in the absence of an external electric field. Dipole moments of polar molecules are of the order of 10-30 C∙m.
(a) polar molecules (b) nonpolar molecules
Examples : Gaseous hydrogen halides (HF, HCl, etc.); NH3, NO2 , N2O, water molecules; all hetero- nuclear diatomic molecules (with any covalent bond between two different atoms).
A nonpolar molecule is one which does not have a permanent electric dipole moment because in the absence of an external electric field, the centres of the net positive charge and the net negative charge in the molecule coincide. Thus, it is a symmetric molecule.
Examples : H2 , CO2 , N2 , O2 , methane, polyethylene, polystyrene.
[Note : The asymmetric charge distribution in a polar molecule arises from differences in electronegativity and other features of bonding. A heteronuclear polyatomic molecule may be nonpolar if the vector sum of the bond dipole moments is zero, as in CO2 , CH4 , CCl4 , etc. The absence or presence of a dipole moment in a polyatomic molecule can be a revealing clue to the structure of the molecule. For example, BF3 with a planar trigonal sym-metric structure is nonpolar, while PF3 with a trigonal pyramidal structure is polar. Of the two isomers of dichloroethylene, cis is polar while trans is nonpolar.]
