Dipole Moment

    It has been pointed out earlier that a covalent bond between two atoms acquires a partial polar character if the values of electro negativity of the two bonded atoms differ. The two charged ends of the bond behave as electrical dipole and the degree of polarity is measured in terms of dipole moment. Dipole moment is defined as the product of the magnitude of charge on anyone of the atoms and the distance between them.

Dipole moment is represented by a Greek letter 'µ'. It can be expressed mathematically, as:

µ = e × d

        Where, e = charge on anyone of the atoms

         d = distance between the atoms.

    Since the charge 'e'
is of the order of 10^–10 e. s. u. & 'd'
is of the order of 10^–8 cm, then 'µ' which is the product of 'd'
and 'e'
is of the order of 10^–18, e. s. u.-cm.
This unit is called Debye
and is represented by D. Thus,

                1D = 1 ´ 10^–18 e. s. u.-cm.

        For example, dipole moment of HCl is 1.03 ´ 10^–18 e. s. u.-cm and is expressed as 1.03 D.

    Dipole moment can be determined experimentally and its value can give an idea of the polar character of a molecule. It is a vector quantity as it has a direction as well as magnitude. The direction of dipole moment is usually represented by an arrow → pointing from positive end towards the negative end.