Fajan’s Rule

Fajans' Rule is one of the most important concepts, simply because it tells a lot about the nature of the chemical bond, more precisely about the transition from ionic to covalent bonds. Presented by Polish chemist Kazimierz Fajans in 1923, this rule has been of much help in presenting a systematic approach toward the factors that impact the characteristics of bonds formed between atoms. The essential postulate of Fajans' Rule is that the degree of covalency in what is normally considered to be an ionic bond can vary enormously depending on the size and charge of the ions involved.

JEE Main 2025: Chemistry Formula | Study Materials | High Scoring Topics | Preparation Guide

JEE Main 2025: Syllabus | Sample Papers | Mock Tests | PYQs | Study Plan 100 Days

NEET 2025: Syllabus | High Scoring Topics |  PYQs

This Story also Contains

1. Fajans' Rule
2. Some Solved Examples
3. Summary

Fajans' Rule

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

• Size of the cation: The smaller the size of the cation, the larger will be its polarisability.
• Size of the anion: The larger the size of the anion, the larger will be its polarisability.
• Charge on cation and anion: The more the charge on a cation more polarising power. Further, the more the charge on an anion, the more will be its polarisability.

Thus covalent character for chlorides follows this order:{NaCl}<{MgCl}₂<{AlCl}₃

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.

Recommended topic video on (Fajan's rule)

Some Solved Examples

Example 1
Question: Which one is the least ionic in the following compounds?
1) AgCl
2) KCl
3) NaCl
4) CsCl

Solution: The covalent character in ionic bonds is greater when the size of the cation is smaller and the charge on the cation is greater. Since Ag+ has a pseudo-noble gas configuration, AgCl will have the greatest covalent character. Hence, the answer is option (1) AgCl.

Example 2
Question: Arrange the following in decreasing order of covalent character: LiCl, KCl, NaCl, RbCl.
1) KCl > LiCl > NaCl > RbCl
2) NaCl > KCl > LiCl > RbCl
3) LiCl > NaCl > KCl > RbCl
4) RbCl > NaCl > KCl > LiCl

Solution: The more the polarisation, the more the covalent character. As the size of the cation increases, the ability of the cation to distort the electron cloud of the anion decreases, resulting in a decrease in polarisation and covalent character. Therefore, the order is LiCl > NaCl > KCl > RbCl. Hence, the answer is option (3).

Example 3
Question: Polarisability of halide ions increases in the order:
1) F^-, I^-, Br^-, Cl^-
2) Cl^-, Br^-, I^-, F^-
3) I^-, Br^-, Cl^-, F^-
4) F^-, Cl^-, Br^-, I^-

Solution: The polarisability of any anion is dependent on its size and charge. The greater the size, the greater the polarisability. Therefore, the correct order is F^- Cl^-, Br^-, I^-. Hence, the answer is option (4).

Example 4
Question: Arrange the following in the decreasing order of their covalent character: (A) LiCl, (B) NaCl, (C) KCl, (D) CsCl.
1) (A) > (C) > (B) > (D)
2) (B) > (A) > (C) > (D)
3) (A) > (B) > (C) > (D)
4) (A) > (B) > (D) > (C)

Solution: Covalent character increases with the increase in charge density of the cation. Therefore, the order of covalent characters is LiCl > NaCl > KCl > RbCl. Hence, the answer is option (3).

Summary

In other words, Fajans' Rule is one of the important theories explaining the nature of chemical bonds, mainly with respect to the transition of ionic to covalent character. It was based on factors affecting polarizing power and polarizability dependent upon the size and charge of cations and anions. Coupled with this rule, there are three main postulates that provide a guideline for assessing the covalency character of ionic bonds.