- Ionic Polarization Definition: Ionic polarization is the shift of negative ions toward the positive side and positive ions toward the negative side in a molecule when an external electric field is applied.
- Formation of Sodium Chloride: Sodium chloride (NaCl) forms through an ionic bond between sodium and chlorine, resulting in positive and negative ions that create a dipole moment
- Permanent Dipole Moments: Some molecules have a permanent dipole moment due to their asymmetrical structure, present even without an external electric field.
- External Electric Field Effect: Applying an external electric field causes ions in a molecule to shift, leading to ionic polarization.
- Types of Polarization: In ionic compounds, both ionic and electronic polarization occur when an electric field is applied, with the total polarization being the sum of both.
Ionic Polarization
To understand ionic polarization, let’s first see how a sodium chloride (NaCl) molecule forms. NaCl is created by an ionic bond between sodium and chlorine atoms. The sodium atom loses one electron, becoming a positive ion, while the chlorine atom gains an electron, becoming a negative ion. The electrostatic force between these ions holds them together, forming a NaCl molecule. Each NaCl molecule has a positive end (sodium) and a negative end (chlorine) due to the charges of the ions.
The sodium chloride molecule has a dipole moment because of the distance between the sodium and chlorine ions. In NaCl, there is a single dipole moment pointing from the negative ion to the positive ion. In ionic compounds with more than two atoms, multiple dipole moments exist, each pointing from negative to positive ions. The overall dipole moment of such a molecule is the vector sum of its individual dipole moments.
If a molecule has a center of symmetry, its dipole moments cancel out, resulting in no net dipole moment. Only asymmetrical molecules have a net dipole moment, called a permanent dipole moment, which exists even without an external electric field. For example, a molecule with two atoms has a single dipole moment from the negative to the positive ion, while a symmetrical molecule has no net dipole moment because its dipole moments cancel each other.
In symmetrical molecules, dipole moments cancel each other, resulting in no net dipole moment. In asymmetrical molecules, a net dipole moment exists due to the uneven distribution of charges. When an external electric field is applied, negative ions in the molecules shift toward the positive side of the field, and positive ions shift toward the negative side. This shift is called ionic polarization.
This is called ionic polarization. If there are N number of polarized molecules present in unit volume of the material. The ionic polarization of the material is given by
Where, µionic is the average induced dipole moment of the molecule due to externally applied electric field. This is obviously proportional to the strength of applied electric field. So, 
Again, when external field is applied there will be slight shifting of positive nucleus and negative electrons of each atom of the molecules. Because of that there will be an electronic dipole moment in each atom of the molecules. This electronic dipole moment is also proportional to the number of molecules per unit volume and strength of applied electric field. The proportionality constant or polarizability for that say, α electronic.
It is needless to say that whenever an electric field is applied in a dielectric of ionic compound, there would be two types of polarization occur in it. These are ionic polarization and electronic polarization. The total polarization is the sum of these two polarizations.
