POLARIZATION OF IONS

POLARIZATION OF IONS:



1.1. INTRODUCTION:

      •  If two oppositely charged ions are brought tighter, the nature of the bond between them depends upon  the effect of one ion on the other.  
      •  If the degree of polarization is quite small, an ionic bond is formed, while if the degree of polarization is larger, a covalent bond result.  
      •  The ability of a cation to distort an anion is known as its polarization power and the tendency of the anion to become polarized by the cation to become polarized by the cation is known as its polarizability 


The two identical atom have equal tendency to attract the shared pair of electron and hence the shared pair of electron lies exactly in the middle of the nucleic of two atoms. 

    However, in the case of covalent bond formation between atoms having different electronegativity, the atom with higher electronegativity will have grater tendency to attract the shared pair of electron more toward itself then the other atom


  •      In ionic compound, there is an electrostatic attractive force between the cation and anion.
  •      The positively charges cation attract their valence electrons of anion, while repelling the nucleus.

  • This causes distortion in the electron cloud of the anion and its electron density drifts toward the cation ,which result in some sharing of the valence electrons between these ions. Thus a partial covalent character is developed between them.

    This phenomenon is called as polarization



1.2. FACTOR AFFECTING THE POLARIZATION:  


A. Charge on the ion:  

      • Charge on the cation increase, it’s tendency to polarize the anion increase and covalent nature increase in ionic compound. 
      • With the increasing charges of anion its ability to get polarized by the cation, also increase. 

(Example):  

          Polarization of  Charge on cation and anion  


     NaCl                MgCl2                   AlCl3                        SiCl4              Na +                     Mg ++                     Al +++                       Si ++++     

------------increasing        ----------------------------------------------------------------------->


NOTES:

      • Charge of cation increase.  
      •  Covalent character increase. 
      • Ionic character decrease          
      •   Melting point decrese.


B) Size of the cation: 

      

     Smaller the size of cation more will be polarizing power and higher in its covalent nature.   

 

         

→ In a group,    

BeCl2                                          

MgCl2  

                              CaCl2  

                              SrCl2  

                              BaCl2   

  •  Size of cation increases 
  •  Covalent character decrease           
  •  Ionic Character increase





Greater polarizing power of Be2+, shows its maximum covalent character.  

→ In Period: 

       Na+                 Mg++                      Al+++                       Si++++   

                               ------------------------------------------------------------------------>

      •      Size of cation decrease  
      •   Covalent Character increase


C) Size of the anion:

                                          Larger the size of the anion, polarized by the cation and covalent character increase.


EXAMPLE:

        •  In case of Halides of calcium, the covalent character increase from F- anion to I- anion.  

→ In Period:  

    CaF2  

    CaCl2  

    CaBr2  

     CaL2 

  •           Size of anion increase.
  •          Covalent character increase.
  •            Ionic character decrease


→ In Group

AlF3               AlC3                  AlBr3                 AlI3

                                   ----------------------------------------------------------------------------------------------->

Covalent character increase as the size of the halide ion increase


D). Electronic configuration of cation:

      •   Polarization capacity of cation having period inert gas configuration is high.
      •  If the size of cation is same than that of cation having inert gas configuration

Example:

  •       CuCl   (M.P.  4420C)    →       Cu+          (Covalent)- 2,8,18                         
  •        NaCl   (M.P 8000 C)    →         Na+                       ( Ionic )- 2,8 

Cu+ and Na+ both the cation (Pseudo and inert ) having same charge and size but polarizing power of Cu+ is more than Na .So CuCl has more covalent character than NaCl.



 





























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