What is meant by dipole moment?

Dipole Moment (µ) definition:

            “Product of charge +ve or –ve and distance between the centres of charges is called dipole moment”

Dipole moment is the measure of separation of charges and also the measure of the polarity of bond and polarity of the molecule.

Dipole moment Formula:

                                    µ = q x r           

 where

           µ = dipole moment

            q = charge +ve or –ve

            r = distance between the centre of charges i.e bond length.    

Dipole moment is a vector quantity having magnitude “q x r” and direction from +ve pole to –ve pole. e.g.

Units of dipole moment:

            The S.I unit for dipole moment is Coulomb-metre (C-m) beside it other units are electrostatic unit-centimetre (esu-cm) and debye (D). Their relation ship is:

                                    1 D = 3.336 x 10-30 C.m

                                    1 D = 1 x 10-18 esu-cm                                   

                                    1 esu-cm = 3.336 x 10-12 C.m

How do you find the dipole moment?

Measurement of dipole moment:

            The dipole moment is measured by an electric condenser (Dipole meter). Parallel plates of the condenser are charged by the battery.

Polar molecules are dipoles. When they are placed between plates with an electric field off they are randomly oriented but when an electric field is on then the polar molecules are oriented such that their +ve poles towards –ve plate and their –ve poles towards +ve plates. The molecules hung between the two plates. The electric potential drops. Drop in potential is proportional to the polarity of the molecule. So dipole moment is measured in terms of the drop in potential.

Applications of Dipole Moment:

  • Prediction of Polar nature:

It is used to predict whether a molecule is polar or non-polar. e.g. dipole moment of CH4, CO2, and Cl2  is Zero so they are non-polar. on the other hand dipole moments of CHCl3, H2O, and HCl are not Zero so they are polar molecules.

  • Measure of Polarity:

It is the measure of the polarity of the bond as well as that of the molecule. More is the dipole moment more is the polarity of molecule e.g.  Consider halogen acids their order of polarity is  

HF       >          HCl     >          HBr     >          HI   

Dipole moment     1.91D              1.03D              0.79D        0.38D

  • Calculation of percent ionic Character:

Percent ionic character of a compound can be calculated as follows

µ obs = It is the observed dipole moment which is measured by the dipole meter

µ cal = It is the dipole moment which is calculated by the formula µ = q x r

          considering molecule to be 100% ionic.

e.g.            For H – F    

   µ obs = 1.91 D  or         1.91 x 3.336 x 10-30 C.m

   r = Bond length of H – F = 9.17 x 10-11 m

q = 1.6 x 10-19 C consider molecule to be 100% ionic, then complete transfer of electron take place and charge will be equal to the charge of electron.

                              µ cal = q x r = 1.6 x 10-19     x   9.17 x 10-11 = 1.467 x 10-29 C.m

  • Molecular Shapes and Bond Angles:

Dipole moment helps in deciding the molecular shape as well as bond angles.

The dipole moment of a molecule is obtained by vector addition of its bond moments.  Molecular geometry is set such that the calculated dipole moment of the molecule should match the observed dipole moment of the molecule.

  • Shape CO2 molecule:
  • µCO2 = 0, this is only possible when CO2 has linear geometry. So two C=O bond moments on vector addition cancel each other and the resultant vector is zero.
  • Shape CS2 molecule:

µCS2 = 0, this is only possible when CS2 has linear geometry. So two C=S bond moments on vector addition cancel each other and the resultant vector is zero.

  • Shape of H2O molecule:

µH2O = 1.84D, this is only possible when H2O has an angular or bent structure with bond angle of 104.5 o. So two O─H bond moments, on vector addition give the resultant vector as 1.84D.

  • Shape of SO2 molecule:

µSO2 = 1.62D, this is only possible when SO2 has an angular or bent structure with

bond angle 118 o. So  S─O and S=O bond moments, on vector addition give

the resultant vector as 1.62D.

Shape of BF3 molecule:

µBF3 = 0, this is only possible when BF3 has a trigonal planner structure with a bond angle 120 o. So three B─F bond moments, on vector addition give the resultant vector zero.     

  • Shape of NH3 molecule:

µNH3 = 1.47D, this is only possible when NH3 has a pyramidal structure with a bond angle 107.5o . So three N─H bond moments, on vector addition give the resultant vector 1.47D.


Recommended video:

Dipole moment

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