Fajans Rule is a concept in inorganic chemistry that predicts whether a chemical bond will be covalent or ionic based on the charge and size of the ions involved. It was first discussed by Kazimierz Fajans in 1923. The rule is based on the concept of polarization, which states that the covalent nature of an anion increases with an increase in its polarization. The rule is based on three main factors or postulates: the size of the ion, electronic configuration, and the charge of the cation. The smaller the size of the cation and the larger the size of the anion, the greater the covalent character of the ionic bond. The covalent character of an ionic bond also increases as the charge of the cation increases. The polarizing power of the cation and the polarizability of the anion are two factors that affect the extent of polarization of an anion.

Fajans Rule can be summarized as follows:

• Ionic Characteristics: Large Cation, Large Anion, Small Charge
• Covalent Characteristics: Small Cation, Small Anion, Large Charge

Fajans Rule is important in predicting the nature of chemical bonds and is commonly used in inorganic chemistry.