Fajan’s rule-definition-postulates and examples

Fajan’s rule definition in chemistry 

It is a common experience that many times an ionic bond has some covalent character and a covalent bond has some ionic character.

Fajan’s rule is related to the covalent character in ionic bond or ionic compounds.

It has been experimentally observed that a polar covalent bond is more stable than a pure covalent or a pure ionic bond .

The higher the degree of ionic polarization , the greater is the stability of the polar covalent bond .

The extent of polarization depends , evidently, on the polarizing power of the cation and the polarizability of the anion.

The rules regarding to the polarization, is known as Fajan’s rule .

Fajan’s rule postulates with example in chemistry   

Fajan gave some postulates or condition to explain how does covalent character gradually increase in ionic bond or ionic compounds . The main postulates of Fajan’s rule are discussed below.

Postulate-I : Size of cation .

The cation with smaller size have higher polarizing power, that is , they cause polarization of electron charge cloud of an anion to a greater extent.

Hence, with decreasing cationic size, the covalent character of the concerned compounds for a particular anion increases.

For example, the cationic size [ M ²⁺ ]of group-II alkaline metal elements gradually increases from top to bottom along the group . Hence covalent character decreases from top to bottom.

Hence, covalent character of group-II metal halides decreases from top to bottom along the group.

Postulate-II: Size of anion .

The anions with large size have high polarizability . That is, their electron charge cloud can be deformed by cation easily.

Hence, with increasing anionic size , the covalent character of the related compound increases.

For example, the size of halide ions gradually increase from top to bottom and hence the covalent character of halogen compounds for a particular cation are increased  from top to bottom .

For example, covalent character of different calcium halides increases from F ^– to  I ^–.

Postulate-III: Charge of cation or anion or both.

The degree of polarization of anion by cation enhance with enhancing the extent of charge on cation or anion or both.

Because, the electrostatic forces which cause the polarization, would increase with increase in the charge on the ions .

Therefore, with increase in charge on ions, the covalent character of the producing compounds increases. 

For example, AlCl₃ is more covalent than MgCl₂ and  MgCl₂  is more covalent than NaCl .

Postulate-IV : The number of electrons in the outer most inert shell of cation.

It has been found that, cations with pseudo inert gas configuration, such as, ns² p⁶ d ¹⁰, or with inert pair configuration, such as, (n-1)d ¹⁰ ns ², have high polarizing power ,while cations with noble gas configuration, such as, ns² np⁶, have low polarizing power.

That is due to greater effective nuclear charge [ low shielding effect of f-orbital ] in the former cases and smaller effective nuclear charge in the later cases. 

For the above reason, chloride compounds of Cu, Ag and Au [with electronic  configuration, such as, ns² p⁶ d ¹⁰] are more covalent than chloride compounds of Na , K and Rb [ with electronic configuration ,ns² np⁶ ] respectively.

Postulate-V : Dielectric constant of medium.

The value of dielectric constant of polar medium is high . In polar medium, due to high dielectric constant, polarization does not occur . 

But in non-polar medium, due to lower dielectric constant, polarization occurs easily.

That is, in non-polar medium, the covalent character of a compound increases.

Examples of Fajan’s rule

Why does MgCl₂ is more covalent than NaCl?

The degree of polarization of anion by cation enhance with enhancing the extent of charge on cation or anion or both.

Because, the electrostatic forces which cause the polarization, would increase with increase in the charge on the ions .

Therefore, with increase in charge on ions, the covalent character of the producing compounds increases.

Since, the extent of charge on Mg 2+ cation is higher than Na + cation , so MgCl₂ is more covalent than NaCl.

Why does AgI is more covalent than AgCl ?

The anions with large size have high polarizability. That is, their electron charge cloud can be deformed by cation easily.

Hence, with increasing anionic size, the covalent character of the related compound increases.

For example, the size of halide ions gradually increase from top to bottom and hence the covalent character of halogen compounds for a particular cation are increased  from top to bottom.

Consequently, AgI is more covalent than AgCl .

Summary 

Fajan’s rule definition in chemistry .

Fajan’s rule postulates with example in chemistry . 

Examples of Fajan’s rule

Why does AgI is more covalent than AgCl ?

Why  does MgCl₂ is more covalent than NaCl ?

Actinide contraction definition and consequences .