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Dipole Moment

It has been pointed out earlier that a covalent bond between two atoms acquires a partial polar character if the values of electronegativity of the two bonded atoms differ. The two charged ends of the bond behave as electrical dipole just like ends of the magnet bar has two opposite poles. The degree of polarity is measured in terms of dipole moment. Dipole moment is defined as the product of the magnitude of charge on any one 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 x d

where, e = charge on any one of the atoms

d = distance between the atoms.

Since the charge ‘e’ is of the order of 10-10 e.s. u. and ‘d’ is of the order of 10-8 cm, then J.!, 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.

In SI units, dipole moment may be expressed in terms of Coulomb-meter abbreviated as C m.

1 D = 1 x 10-18 e.s.u.-cm. = 3.335 x 10-30 C m

For example, dipole moment of HCl is 1.03 x10-18 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 à pointing from positive end towards the negative end.

DIPOLE MOMENT AND MOLECULAR STRUCTURE

(a) Diatomic molecules. A diatomic molecule has two atoms bonded to each other by covalent bond. In such a molecule the dipole moment of the bond also gives the dipole moment of the molecule. For example, dipole moment of HCl molecule is the same as that of H-Cl bond (µ. = 1.03 D).

Greater the electronegativity difference between the atoms, more will be the dipole moment of such molecules. For example, the dipole moment of HF is 1.91 D while that of HCl is 1.03 D (F is more electronegative than Cl).

(b) Polyatomic molecules. A polyatomic molecule has more than two atoms bonded by covalent bonds. In such cases the idea of a dipole can be applied to individual bonds within

the molecule. The dipole moment of individual bond in a polyatomic moletule is referred to as bond dipole. The dipole moment of the molecule depends upon the orientations of various bond dipoles.

For example, carbon dioxide (C02) and water (HzO) are both triatomic molecules but dipole moment of carbon dioxide is zero whereas that of water is 1.83 D. This can be explained

on the basis of their structures.

 

Carbon dioxide is a linear molecule in which the two C = O bonds are oriented in the opposite directions at an angle of 180°. The bond dipole of C = O bond is 2.3 D but due to linear structure of CO2, the bond dipoles of two C = O bonds cancel each other. Therefore, the resultant dipole moment of the molecule is zero. Hence, CO2 is a non-polar molecule. On the other hand, water molecule has a bent structure in which two O-H bonds are oriented at an angle 104.5°.

Therefore, the bond dipoles of two O-H bonds do not cancel each other. Hence, the resulting dipole moment of H2O molecule is not zero but has a value equal to 1 .83 D.

In the similar way, the resulting dipole moment of BF3 molecule is zero. The bond dipoles of three B-F bonds give a net sum of zero because the resultant of any two is equal and opposite to the third (parallelogram law of forces) as shown in Fig. 7.13. Thus, BF3 is a non-polar molecule.

Fig. 7.13. Dipole moment of BF3 molecule.

On the other hand, the bond dipoles of three N-H bonds in HN3 molecule, do notcancel each other. Hence, the molecule has a net dipole moment

The dipole moments of some selected molecules are given below in tabular form

It may be noted that the molecules having zero dipole moment (µ= 0) are called Non-Polar Molecules. On the other hand the molecules having dipole moment greater than zero (µ> 0) are Polar Molecules.