Question

According to Fajan, covalent character of a compound is more when there are (1) small cations and large anion (2) small cations and small anion (3) large cations and large anion (4) large cations and small anion

Short answer

Option (1) small cations and large anions.

Step-by-step solution

Explanation of Fajan's Rule

Fajan's Rule states that the covalent character of a compound is higher when the positive ion (cation) has a small size and high charge, and the negative ion (anion) has a large size and high charge.

Analyze the Options

Compare the given options to Fajan's Rule: (1) small cations and large anions, (2) small cations and small anions, (3) large cations and large anions, (4) large cations and small anions.

Apply Fajan's Rule

Covalent character is strongest with small cations and large anions because small cations can polarize large anions more effectively, leading to a higher covalent character.

Conclusion

Therefore, the correct option is (1) small cations and large anions.

Key concepts

Covalent Character

Covalent character refers to the extent to which electrons are shared between atoms in a bond. When components of a molecule share electrons more equally, the molecule has pronounced covalent character. Fajan's Rule helps in predicting whether a bond will be more ionic or covalent.
Factors that impact covalent character include:

• **Charge on the ions**: Higher charges result in stronger attractions, enhancing covalent character.
• **Size of the ions**: Generally, smaller cations and larger anions exhibit higher covalent character.

The presence of covalent character in a compound affects its physical properties like solubility, melting and boiling points.

Cation-Anion Size

The size of both the cation and anion plays a critical role in determining the nature of the bond.
**Cations**: Smaller cations have high charge density, meaning the charge is concentrated over a smaller area. This high charge density polarizes the anion more effectively.
**Anions**: Larger anions are more easily distorted due to the loose hold their nuclei have on the outer electrons. This distortion leads to higher covalent character.
According to Fajan's Rule:

• **Small cations and large anions** create a scenario where the cation's strong electric field distorts the electron cloud of the large anion, enhancing the covalent character.
• **Large cations and small anions** have less covalent character because the large cation’s field is weaker and less effective in polarizing the anion.

Polarization Effect

Polarization refers to the distortion of the electron cloud of an anion by a cation. A high degree of polarization increases the covalent character of the bond.
Factors affecting polarization include:

• **High charge of cation**: Ions with higher charge have a stronger polarizing power.
• **Small size of cation**: Smaller cations can come closer to the anion, exerting a stronger force.
• **Large size of anion**: Larger anions are more easily distorted due to their loosely held outer electrons.

For example, in the case of small cations like Al³⁺ and large anions like I^-, the polarization effect is strong, leading to significant covalent character in the compound.