Fajan's Rule for determining polarisibility of molecules

1. Presented by
Dr. Neelam
Department of chemistry

2. Fajan’s Rule
POLARISATION : (Fajan’s Rule) (Covalent nature in ionic bond)
Polarisation Power :
The ability of cation to polarise a nearby anion is called Polarisation power of
cation.
Polarisation ∝ Covalent character

3. Fajan’s rule : (Factors Affecting Polarisation)
(a) Size of cation : - Polarisation of the anion increases as the size of cation
decreases.
 Greatest polarising power of Be2+, shows its maximum covalent
character
In a period.

5. (b) Size of anion : - If the size of the anion increases for a given cation, the
covalent character increases/Nature Polarisation ∝ size of anion.
(c) Charge on cation and anion : -
 Polarisation ∝ charge on cation anion
(I) Charge on cation ∝ Polarisation (covalent character)
- Charge on cation increases
- Covalent character increases
-Ionic character decreases (M.P. decreases)
-Charge on anion ∝ polarisation ∝ covalent nature ∝

6. (d) Electronic configuration of cation : -
Polarisation capacity of cation having pseudo inert gas configuration is high. If
the size of cations is same than that of cation having inert gas configuration.
Cu+ and Na+ both the cation (Pseudo & inert) have same charge and size but
polarising power of Cu + is more than Na + because-
So CuCl has more covalent character than NaCl.

7. Covalent character ∝
Boiling point
1
Covalent character ∝
1
Covalent character ∝
Solubility
1
Covalent character ∝
Ionic character
1
Covalent character ∝ ∝ Size of anion ∝ charge of cation
Size of cation
1
Covalent character ∝ Size of anion ∝ polarising power
Melting point

8. Covalent character ∝ Size of anion = F- < O2- < N3-
Ionic character ∝ Size of cation = Be2+ < Mg2+ < Ca2+ < Sr2+ < Ba2+
Size of cation = Al3+ < Na+ < Sn4+ < Pb2+
Size of cation = C4+ < B3+ < Be2+ < Li+

9. 9