According to Fajan, covalent character of a compound is more when there are

small cations and large anion

The correct answer is option 1. small cations and large anion.

Fajan's rules describe the factors that influence the degree of covalent character in ionic compounds. The key factors are:

Size of the Cation:

A smaller cation has a higher charge density (charge per unit volume). Higher charge density allows the cation to exert a stronger attractive force on the electrons of the anion. This stronger attraction results in greater polarization of the electron cloud of the anion towards the cation.

Size of the Anion:

A larger anion has a more diffuse electron cloud. A diffuse electron cloud is more easily polarizable because the electrons are farther from the nucleus and less tightly held. When a cation with high charge density approaches a large anion, it can distort the electron cloud of the anion more effectively.

Charge on the Ions:

Higher charges on ions increase their polarizing power. For example, a 2+ or 3+ cation will have a stronger polarizing effect than a 1+ cation of the same size.

Based on these principles, Fajan's rules predict that covalent character increases under conditions where:

Cations are small: Small cations have higher charge densities, enhancing their ability to polarize nearby anions.

Anions are large: Large anions have more easily polarizable electron clouds, which can be more significantly distorted by the nearby cation.

Let us apply this to the options given:

1. Small cations and large anions

This option correctly identifies the conditions under which covalent character is maximized according to Fajan's rules. Small cations (high charge density) and large anions (easily polarizable) lead to strong polarization and thus increased covalent character in ionic compounds.

2. Small cations and small anions

Small anions are less easily polarizable, so this combination does not maximize covalent character.

3. Large cations and large anions

Large cations have lower charge density and weaker polarizing power, so covalent character is reduced compared to small cations.

4. Large cations and small anions

Small anions are less easily polarizable, so this combination does not maximize covalent character.

Therefore, based on Fajan's rules, the correct answer is 1. small cations and large anions

This combination maximizes the polarizing power of the cation and the polarizability of the anion, leading to greater covalent character in the ionic compound.