Fajans rules are a set of guidelines proposed by Kazimierz Fajans in 1923 to predict the polarization of covalent compounds. These rules are based on the concept of the relative sizes of cations and anions, and the degree of covalent character in a compound. The rules are as follows:

1. Size of cation: A smaller cation is more polarizing than a larger cation.

2. Charge of cation: A cation with a higher charge is more polarizing than a cation with a lower charge.

3. Size of anion: A larger anion is more easily polarized than a smaller anion.

4. Charge of anion: An anion with a higher charge is more easily polarized than an anion with a lower charge.

5. Covalent character: Compounds with more covalent character are more easily polarized than compounds with less covalent character.

Explanation:

1. Size of cation: The smaller the cation, the greater the polarization of the anion. This is because a smaller cation has a greater charge density, which means that it can attract the electrons of the anion more strongly.

2. Charge of cation: A cation with a higher charge has a greater polarization effect on the anion. This is because a cation with a higher charge has a greater positive charge density, which means that it can attract the electrons of the anion more strongly.

3. Size of anion: A larger anion is more easily polarized than a smaller anion. This is because a larger anion has a greater electron cloud, which means that it is more easily distorted by the electric field of the cation.

4. Charge of anion: An anion with a higher charge is more easily polarized than an anion with a lower charge. This is because an anion with a higher charge has a greater negative charge density, which means that it can be attracted more strongly by the positive charge of the cation.

5. Covalent character: Compounds with more covalent character have greater electron density in the bond, which means that the electrons are more easily distorted by the electric field of the cation. Therefore, compounds with more covalent character are more easily polarized than compounds with less covalent character.

In summary, Fajans rules are a set of guidelines that help predict the degree of polarization in covalent compounds based on the relative sizes and charges of the cation and anion, as well as the degree of covalent character in the compound.