Fajans' Rules of Polarization and Polarizing Power 

Fajans Rules:

Ionic compounds have a degree of covalent bonding, there are rules indicate the extent to which an ionic bond has covalent character caused by polarization of the ions, these rules called Fajans' rules:

The covalent character increase in case of:

1.     Increase of charge of cation

2.     Increase of charge of anion

3.     Small size of cation

4.     Large size of anion

5.     Non-inert gas configuration of cation

Note:

Noble gas configuration of the cation produces better shielding and less polarizing power

The Polarizing power of cation:

Polarizing power of a cation can be estimated from the ratio of its charge to ionic radius and depends on

a)     High charge of cation

b)     Small size of the cation

c)     Non-inert gas configuration of cation

Solved Question 1: Explain why NaCl is polar ionic compound (the bond in NaCl is ionic) while AgCl  is polar covalent bond (the type of bond in AgCl is covalent)??

Answer: The chloride is the same for the two compounds but Ag⁺ has non-inert gas configuration while Na⁺ has inert gas configuration

           Ag⁺ 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶  4d¹⁰

           Na⁺ 1s² 2s² 2p⁶

So, Ag⁺ has more polarizing power than Na⁺, AgCl is not dissolved in water due to covalent character and high polarization energy

The inert gas configuration is:

Atomic no.	Element	No. of electrons/shell
2	Helium(He)	2
10	Neon(Ne)	2, 8
18	Argon(Ar)	2, 8, 8
36	Krypton(Kr)	2, 8, 18, 8
54	Xenon(Xe)	2, 8, 18, 18, 8
86	Radon(Rn)	2, 8, 18, 32, 18, 8

Solved Question 2: Explain why NaCl is mainly ionic while NaI has covalent character??

Answer: Because the size of I^- is greater than the size of Cl^- so it can be polarized more than Cl^-   (I^- is more polarizable than Cl^-)       

Solved Question 3: compare between Li⁺ ,Be⁺² ,and B⁺³ in the type of bond??

(Which of the following cations will form compounds with covalent character Li⁺, Be⁺² or B⁺³??)

(or which of the following compounds has more polarizing power??)

Answer: Li⁺ has small size so and small charge so it has low polarizing power

Be⁺² form compounds with covalent character because it is larger in charge

B⁺³ form ionic bonds with greater covalent character and has high polarizing power because it is larger in charge.

Solved Question 4: Explain why the bond in AlCl₃ is covalent while in_, AlF₃ is ionic ?

Answer: AlCl₃ the bond is covalent due to the large size of Cl^- so it is polarizable and the large charge of the Al⁺³

AlF₃ the bond is ionic because F^- is small and so less polarizable

AgI the bond is mainly covalent because Ag⁺ has high polarizing power due to large size and non-inert gas configuration and also I^- is more polarizable due to large size.