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Fajan's rule - Chemical bonding

Fajan's rule - Chemical bonding


This is the most important theory in chemical bonding and many quastions are solved by understand this theory.

According to Lewis theory of bonding there are two types of bond.

1. Covalent bond :
                                In which sharing of electrons take place and covalent compounds having low boiling point and low melting point.

2. Ionic bond : 
                          In which electron transfer takes place and ionic compounds having high boiling and melting point due to strong electrostatic force between +ve and -ve ions.

Let's understand importance of Fajan's rule by taking following example....

NaBr and AlBr3 both are ionic compounds so it should have high melting and boiling point. In case of NaBr M.P. is 747 ℃ but AlBr3 having very low melting point i.e. 97.5 ℃ even it is a ionic in nature.

How this happen is understand by Fajan's rule.


We know that the +ve ion is small while -ve ion is large in size. So effective nuclear charge is greater in +ve ion and it is attract the electron cloud towards it. Hence +ve charge decrease and also -ve charge decrease and covalent character enter the ionic compound.
     But actually all the compounds are not purely ionic and not purely covalent. 

There are two terms related with this theory :

1) Polarising power : 
                                     The ability of a cation to distort an anion is called its polarising power. and it is defined for positive ion.
    
2) Polarizability :          
                               The tendency of anion to become polarized by the cation is called its polarizability and it is defined for anion. If polarizability is greater then the compound show more covalent character. And therefore its melting and boiling point decreases.

➝ Polarising power is express in form of Ø which is known as polarising power.
                           
               Ø = charge on cation / radius of cation

If charge is more on +ve ion so it is attract the -ve ion greatly and ionic potential increase so the polarising power increases and covalent character increases

If radius of cation deceased then charge density increase and it is distort anion more and so covalent character increases.


Ex. Which is more covalent in nature ?     OR
       Which having more melting point ?  
       BaCl2 , BeCl2

Ans.  -ve ion Cl- is same 
         Ba+2 and Be+2 belong to same group (2)
         Here size of Ba+2 is greater than Be+2 so polarising power decrease in BaCl2 and covalent character decreases and ionic character more than BeCl2 so it has more melting point than BeCl2.

Ex. CaF2 and CaI2
       Here negative ions are different and from same group (17).
       Size of I- is more than F- so tendency of distort is more in I- than F- and covalent character increase in CaI2 and ionic character is more in CaF2.

I hope it is helpful for you. If any quastion please comment below.

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