Polarizability-Polarizing power-application of Fajan’s rule

What is polarizability of anions in chemistry ?

Polarizability is defined as the property of an anion to become polarized through the deformation of electron charge cloud by cation.

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

Polarizability of  an anion increase with increase in anionic size or anionic charge.

 For example, the polarizability of I^– ion is higher than F ^– ion, due to large size of iodide ion.

If the polarizability of an anion increase, then covalent character of the resulting  compound increases. Such as, AgI is more covalent than AgF .

What is polarizing power of cations in chemistry?

Polarizing power of cation is defined as the capability of cation to distort the electron charge cloud of an anion.

Now, it has been experimentally found that, cation with smaller size or with high positive charge have its high polarizing power.

For example, Al³⁺ ion have more polarizing power than Na⁺ ion, due to small size and high charge of Al³⁺ ion with respect to Na⁺ ion.

What is ionic potential in polarization of cation?

The polarizing power of cation is expressed by using a symbol, phi(ɸ) which is known as ionic potential. The value of phi (ɸ) is equal the ratio of cationic charge and cationic radius.

                                               ɸ = cationic charge / cationic radius.

The polarizing power of cation increases with increase in the value of ɸ and hence the covalent character of ionic compound increases.

If we move from left to right along a period, the extent of cationic charge increases but ionic radius decreases and hence the value of ɸ increases.

Again, if we move from top to bottom along a group, the extent of cationic charge does not change but ionic radius increases and hence the value of ɸ decreases.

The ɸ value increases with increase in oxidation number for same metal under different oxidation states.

Application of Fajan’s rule with examples

Fajan’s rule, explained satisfactorily that how does covalent character increase in ionic compound due to polarization of anion by cation.

Now, with increase in covalent character in ionic compounds, the chemical as well as physical properties of [ such as, solubility, color, thermal stability existence of compounds etc ] compounds are changed.

The changed of different chemical as well as physical properties of compounds due to polarization or enhancing of covalent character are discussed below.

( I ) Effect on solubility.

Ionic compounds are experimentally more soluble than covalent compounds in polar solvent.

Again, covalent compounds are more soluble than ionic compounds in non-polar solvent.

Now, according to Fajan’s rule, the order of polarization of halide ions are , I ,> Br , > Cl  , > F [ polarization of anion increase with increase in anionic size ] .

So, the order of silver halide compounds  with increasing their covalent character are , AgI > AgBr > AgCl > AgF.

Therefore, the descending order of the above compounds, according to their ionic character are , AgF > AgCl > ABr > AgI.

Consequently, in polar solvent [ water ] , the solubility order of these silver halide 
compounds are,  AgF > AgCl > ABr > AgI . 

But in non-polar solvent [ alcohol ] , the solubility order is just opposite to the previous order, AgF < AgCl < ABr < AgI.

( II )  Thermal stability of metal carbonates.

If we move from top to bottom along a specific group [Gr-II ] in periodic table, the cationic size increases. Hence, the value of ф decreases. 

So, the order of cationic size of group-II metal ions are, Be²⁺ < Mg²⁺ < Ca²⁺ .

Now, with decrease in ф  value the ionic character increases. Hence, the order of ionic character of group-II metal carbonates are, BeCO₃ < MgCO₃  < CaCO₃ .

Again, with increase in ionic character of compound , their thermal stability increases.

Hence, the order of thermal stability of group-II  metal carbonates are,

BeCO₃ < MgCO₃  < CaCO₃ .

Therefore, if we move from top to bottom along a group, the stability of metal carbonates gradually increases.

(III) Melting point of compounds.

According to Fajan’s rule, cations with smaller size or with high positive charge have high polarizing power.

That is, the polarization of anions by cation increases with decrease in cationic size. Now, with increase in polarization of anions by cation, the covalent character of ionic compounds increases.

Since the covalent character of ionic compounds increases, hence the melting point of compound decreases.

For example, the melting point of alkali metal chlorides gradually increases from top to bottom along the group.

Because, the cationic charge from Li+ to Fr+ remain same but the cationic size gradually increases from Li+ to Fr+. Due to small size, lithium ion has high polarizing power.

Since the polarizing power of alkali metals decrease from Fr+ to Li+, hence covalent character of alkali metal chloride increases from Fr+ to Li+.

That is, LiCl is more covalent and hence less ionic. Consequently, the melting points of alkali metal chlorides are increases from LiCl to FrCl.  The increasing order of alkali metal chlorides are, LiCl < NaCl < KCl < RbCl < CsCl < FrCl.

( IV ) Color of different salts of same metal.

 If the size of an anion increase, then the tendency of an anion to become polarized ,is increased. That is, polarization of anionic electron charge cloud by cation is increased.

As a result, temporary displacement of electrons occur from complete anionic orbital to incomplete orbital of cation.

For this process, the amount of energy required depends on the extent of anionic polarization.

Generally, anion with high polarization, required less energy and anion with less polarization need high energy.

Now, the necessary energy for this process, is collected from the visible range of light.

Therefore, the color of the relating compound depends on the energy absorbed from 

which part of the visible range of light.

From the above discussion, it is clear that, with increase in covalent character,the color of the compound becomes more intense or deep.

For example, HgCl₂ is white but HgI₂ is red. Again, AgCl is white while AgI is yellow.

( V ) Non-existence of compounds.

It has been found that, because of the high polarization of anion , the resulting compound does not exist .

For example, PbCl₄ does exist but  PbI₄ have no existence. Because of the high polarization of I^– ion two I^– ion among the four are united to form I₂ molecule.

Consequently, Pb⁴⁺ cation becomes reduced to form Pb²⁺ ion .

That is, PbI₄ has no existence. It dissociate to PbI₂ and I₂ molecule.

On the other hand, the anionic size of Cl^– ion is less than I^– ion . Hence, the extent of polarization of Cl^– ion is also less than that of I^– ion.  

So,Pb⁴⁺ ion is attached with four Cl^– ion to form PbCl₄ . That is, PbCl₄ can exist but PbI₄ have no existence.




Summary:

What is polarizability of anions in chemistry ?

What is polarizing power of cations in chemistry ?

Application of Fajan’s rule with examples.

Fajan's rule postulates and examples