What is dipole moment and how to apply it

Dipole moment is the product of magnitude of charge and the distance between them. Here, we will study about the applications of dipole moment.

What is dipole moment

  1. D.M. is the product of magnitude of charge on each end & the distance between the centres of the nuclei of bonded atoms.
  2. It’s unit is Debye. 1 D = 10^-18 esu cm.
  3. It is a vector quantity pointing from + to –
  4. In case of diatomic molecules, greater is the distance between the bonded atoms, greater is the D.M.
  5. H-F > H-Cl > H-Br > H-I ( Dipole moment )
  6. For polyatomic molecules, D.M. is the resultant of the different bonds. For example, CCl4 has zero D.M. due to symmetrical molecule
  1. H2O is a bent molecule with a resultant D.M. of 1.84 D
  1. BeF2 and CS2 is a linear molecule having 0 resultant dipole moment.
  2. BF3, CCl4 are symmetrical, Therefore, having 0 D.M.
  3. On the contrary, NF3 < NH3 (D.M.). Let’s discuss why ?

Comparison of dipole moments of NH3 and NF3

  1. Both NH3 and NF3 have pyramidal shapes with 1 pair of lone pair on N atom.
  2. F is more electronegative. Therefore, N -F bond is more polar. Therefore, we expect the net D.M. of NF3 is greater than NH3.
  3. But, actually D.M. of NF3 is less than NH3
  4. Dipole points from N to lone pair.
  5. F is more E.N. than N. Therefore, dipole points from N to F.
  6. Resultant of N-H bonds adds to the bond moment of lone pair.
  7. But the net dipole moment of 3N-F bonds partly cancels the resultant moment of lone pair.
  8. Hence, the net dipole moment of NF3 is less than that of NH3

Applications of D.M.

  1. D.M. helps in determining polarity of bonds of diatomic molecules. Higher the Electronegativity difference, more is the polarity of the covalent bond. H-F > H-Cl > H-Br > H-I ( Polarity of bonds )
  2. It helps to calculate the percentage ionic character. % ionic character = Observed D.M. / Ionic D.M. *100
  3. D.M. helps in calculating the symmetry or shape of the molecule. Symmetrical molecules have zero dipole moment.
  4. It helps to distinguish between cis and trans forms. Bond moments cancels each other which results in zero D.M.
  5. D.M. finds application in distinguishing ortho, meta and para isomers. D.M. of para isomer is zero. But the D.M. of ortho isomer is greater than meta isomer.

Check out the post on covalent bonding here

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