Fajan’s Rule

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.

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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.