Question

Explain Fajan's rule.

Answer 1

Hint:With the help of Fajan’s rule, you can decide, the most ionic compound among alkali metal chlorides or any other series of compounds. Thus, Fajan’s rule comes as a handy tool.

Complete answer:
Fajan's rule generalizes the ability of the ionic compounds to show covalent character due to polarization.
The formation of an ion is easy if
1. The ion formed has stable electronic structure
2. The ion formed has small charge
3. Anion is formed from a small atom whereas the cation is formed from the large atom.
Compounds will have some covalent character if the above criteria are not satisfied.

The conditions that favor an increase in the covalent character are as follows:
1. The ions should have high charge
2. The cation should have small size and the anion should have large size.
3. Cations should have 18-electron structures; examples include non-inert gas type cuprous cations.
High charge and small size of cation results in high charge density and high polarizing power. This increases the covalent character.
Larger the size of the anion, less effective is the nuclear charge and loosely bound is the valence shell electron. This increases the polarizability of the anion. Greater is the polarizability of the anion, greater is the covalent character.

Note:
 Polarizing power is the extent to which a cation can polarize an anion. The polarizing power increases with increase in the charge density, the ratio of the charge to volume. Polarisability gives the extent, or the ease, to which you can polarize an ion.