Fajan Rule

Fajan Rule

source: chem.uwi.mona.edu.jm

Fajan Rule :

Covalent character developed in an ionic compound may be determined by fajan’s rule. According to this rule covalent character is more , when :

a) Ions have high charge i.e cation has greater positive charge and anion has greater negative charge.

b) Cation has small ionic radius.

c) Anion has large ionic radius.

d) Compounds of cation  having s²,p⁶,d¹⁰ configuration of outer most shell are largely covalent as compared to cation having the same size and charge but outer electronic configuration of s²p⁶ type.

Explanation:

In ionic compound , cation attracts the electrons of anion towards itself. The ability of cation to attract the electrons of anion is called polarizing power. The electrons of anion get attracted towards the cation . This  is called  polarizability  of the anion. Due to this, anion is distorted. It is called polarization. Due to polarization sharing of electrons takes place and ionic compound has some covalent character.

“Greater is the polarization, greater is the covalent character.”

Question 1:Write decreasing order of  Covalent character in LiCl, NaCl, KCl.

Solution :  LiCl > NaCl > KCl

Reason : According to Fajan  Rule ,“Smaller is the cation, more is the covalent character.”

Question 2: Write decreasing order of  Covalent character in NaCl , MgCl₂ , AlCl₃.

Solution :               AlCl₃ > MgCl₂> NaCl

Reason : According to Fajan  Rule , ” cation has more positive charge is more covalent.”

Q 3: Write increasing order of melting point of  AlCl₃ , MgCl₂ , NaCl

Solution :

AlCl₃ > MgCl₂ > NaCl

180⁰ > 710⁰ > 800⁰

Reason : Bigger is the anion, greater is the covalent character. Melting point of covalent compound is less than electrovalent compound.

Q 4: Write decreasing order of  Covalent character in AlBr₃, AlCl₃, AlF₃ .

Solution : AlBr₃ > AlCl₃ > AlF₃

Reason : Bigger is the anion, greater is the covalent character.