FANJAN’S RULE

FANJAN’S RULE  :


Fanjan’s Rule:

  • Introduction
  •     Factor favoring polarization
  •   Application of polarization. 


INTRODUCTION:

      Ionic and covalent bonds are the two extreme type of bonding and almost always the bonds formed intermediated type. This is explained in the term of polarizing power of the cation and polarizability of the anion and the shape of the ions.

      • Fanjan’s rule in inorganic, formulated by Kazimier Fajan in 1993.
      • Fanjan’s rule can be used to predict whether a chemical bond is expected to be predominately ionic or covalent and depend on the relative charges and sizes of the cation and anion.

IONIC  

COVALENT 

Low positive charge 

High positive charge 

Large cation 

Small cation 

Small anion 

Large anion 


The scientist, Fanjan introduced mainly four important rules, which summarize the factor favoring polarization and hence covalency.

 A). A small positive ion favors covalency:

      •  Small positive ions is concentrated over a small area.
      •  The ions highly polarizing and very distorting the negative ion.
      • High charge density of the small cation distorts the electron cloud of the anion, such that extent that the electron clouds of the anion and cation overlap, making the bond resemble a covalent bond 

 Example:

LiCl  and KCl

   B) .A large negative ion favor Covalency:

       Large ions are high polarizable. which easily distorted by the positive ion, because of the outermost electrons shields from the charge on the nucleus by filled shell of electrons.

                Example: 

                                 LiI  <  LiBr <  LiF

 

C) .Large charge on either ion, on both ions, favor covalency: 

  • High charge increases the amount of polarization.

                 Example:

Na+Cl-    >    Mg2+Cl2  >   Al3+Cl3  

D). Polarization and hence covalency, is favored if the positive ion does not have a noble gas configuration:

      • A noble gas configuration is the most effective at shielding the nuclear charge, so ions without the noble gas configuration will have high charge at their surfaces, and thus be highly polarizing



APPLICATION OF FANJAN’S RULE:  

      • The compound with higher covalent character will have a lower melting and boiling point.

    Example:     

  Melting point:  BeCl2 = 4500C  and   CaCl2 = 7820C 

      • Greater the covalent character, lower is its solubility in polar solvent.

Example:

 Ag  >    AgCl   >   AgBr  >    Ag

      •     Greater the covalent character, thermal stability decrease.

Example:

      BeCo3  <    MgCO3    <    CaCO3     <  BaCO3









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