Electronic PolarizationPublished on 24/2/2012 and last updated on 31/7/2018
As due to influence of external electric field the center of nucleus and center of electrons cloud are separated, there will be an attraction force developed between them according to Coulomb’s law. Say, at the distance of separation of center of nucleus and electron cloud, x, the equilibrium is established. That means at distance of separation x, the forces acting on the nucleus or electron cloud due to external electric field and due to Coulomb law become same and opposite. It is obvious that the radius of nucleus is much much more than that of electron cloud. So in respect of electron cloud the nucleus can be considered as point charge. Hence, the electrostatic force acting on the nucleus would be +E.Z.e. Now the nucleus has been shifted from the center of electrons cloud by a distance x.
According to Gauss’s theorem, the force due to negative electron cloud acting upon the positive nucleus would only be due to the portion of the cloud enclosed by the sphere of radius x. The portion of the cloud outside the sphere of radius x does not apply any force on the nucleus. Now, the volume of the sphere of radius x is (4/3)πx3 and the volume of the sphere of radius R is (4/3)πR3. Now total negative charge of the electron cloud is -Ze and we have already considered that it is uniformly distributed throughout the volume of the cloud.
Hence, the quantity of negative charge enclosed by the sphere of radius x is, Only this much charge will apply coulombic force on the nucleus. So, according to Coulomb’s law, the force would be
At equilibrium condition, Now the dipole moment of the nucleus is Zex as dipole moment is the product of charge of the nucleus and the distance of displacement. Now, putting the expression of x in the expression of dipole moment, we get, The polarization is defined as the number of dipole moments per unit volume of the material. If N is the number of dipole moments per unit volume, the polarization would be, From the above expression it is found that the electronic polarization or atomic polarization is dependent upon the radius (or volume) of the atom and the number of atoms presents in unit volume of the material.