A polar covalent bond with positive and negative charge centres at its ends is called a dipole. The polarity of a dipole is measured by its dipole moment. Mathematically it is expressed as dipole moment, `mu=q xx d` where q and d are the net charge and the distance between the two charges respectively. Dipole moment is a vector quantity. The net dipole moment of a polyatomic molecule is the resultant of the various bond moments present in the molecule. The values of dipole moment are expressed in Debye (D) or in SI units in terms of coulomb- metre (Cm). One of the most important applications of dipole moment is in the determination of geometry and shape of molecules besides prediction of a number of properties of the molecules.
The molecules `BF_3` and `NH_3` both are covalent compounds but `BF_3` is non-polar while `NF_3` is polar . The reason is that