Fajan gives the following four rules for the Polarisation:
Polarization will be increased by:
1. High charge and small size of the cation.
2. High charge and large size of the anion.
3. The polarizability of an anion is related to the deformability of its electron cloud (i.e. its “softness”)
4. An incomplete valence shell electron configuration. Noble gas configuration of the cation produces better shielding and less polarizing power.
What are the effects of Polarization?
1. Due to higher lattice energies of Bromides and iodides they have higher stability than expected from theoretical calculations. This results in formation of a polar covalent Bond. This covalent bond is more stable than ionic bond.
2. With increase in degree of polarization, the solubility of ionic compounds in polar solvents decreases.
3. With increase in degree of polarization, the hardness of ionic compounds decreases.