Fajan's Rule

Fajan's Rule

Kazimierz Fajans in 1923, gave some important points to predict whether a chemical bond is expected to be predominantly ionic or covalent.They are given below-
1. Size of Cations
2. Size of Anions
3. Charge on ions (Cations and Anions)
4. 18 electron configuration

1. Size of Cations
Smaller the size of cation, greater the covalent character.
Example: LiCl, NaCl, KCl, RbCl, CsCl
LiCl is most covalent among the given IA chlorides as the size of Li⁺ is smaller than that of other IA cations.
The order of covalent character is-
LiCl > NaCl > KCl > RbCl > CsCl

2. Size of Anions
Larger the size of anions, greater the covalent character.
Example: LiF, LiCl, LiBr, LiI
LiI is most covalent among the given IA halides as the sixe of iodide ion is larges.
So, the order of covalent character is-
LiI > LiBr > LiCl > LiF

3. Charge on ions (Cations and Anions)
Higher the charge on ions, greater the covalent character.
Example: NaCl, MgCl₂, AlCl₃
AlCl₃ is most covalent among the given molecules as the chage on Al is +3 highest (charge on Na is +1 and on Mg is +2).
So, the order of covalent character is-
AlCl₃ > MgCl₂ > NaCl

4. 18 electron configuration
Molecules which follows 18 electron configuration is more covalent in nature than those molecules which follows 8 electron configuration.
Example: NaCl and CuCl CuCl is more covalent as it follows 18 electron configuration while NaCl is more ionic because it follows 8 electron configuration.

According to Fajan's rules, the covalent nature of ionic compounds is favoured by

a. Large cation and small anion
b. Large cation and large anion
c. Small cation and large anion
d. Small cation and small anion

Maximum covalent character is associated with the compound

a. NaI
b. MgI₂
c. AlCl₃
d. AlI₃

Amongst LiCl, RbCl, BeCl₂ and MgCl₂ the compounds with the greatest and the least ionic character, respectively, are

a. LiCl and RbCl
b. RbCl and BeCl₂
c. RbCl and MgCl₂
d. MgCl₂ and BeCl₂

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