Fajan’s Rule - Practice Questions & MCQ

The covalent character in ionic bond is determined by Fajan’s rule. It simply says that no ionic bond is completely ionic, there is always some covalent character in ionic bond. When a cation approaches an anion, then the electron cloud of the anion is distorted and shifted towards the cation, this distortion is known as the polarisation of the anion. 

The ability of the cation to distort the anion is known as polarising power and the ability of the anion to get distorted is known as polarisability.

The covalent character in ionic bond depends on the following factors:

• Size of the cation: Smaller the size of the cation, larger will be its polarisability.

• Size of the anion: Larger the size of the anion, larger will be its polarisability.

• Charge on cation and anion: More the charge on cation more will be its polarising power. Further, more the charge on anion, more will be its polarisability. 

Thus covalent character for chlorides follows this order:

NaCl < MgCl₂ < AlCl₃

In this case, the charge on the cation increases, thus its polarising power also increases.

Further, for cation size, the covalent character follows the below order:

LiCl > NaCl > KCl > CsCl 

In this case, as the size of the cation increases, its polarising power decreases.