More stable states of Cu is +2 while that of Au is +3 -explain  why?

Both copper and gold are the members of same family of group IB in periodic table. Their outer electronic configuration are similar which given below,

So, it is expected that they will show the same stable oxidation state of +1. But in case of Cu the stable oxidation state is +2  when that of ‘Au’ is +3 . 

This anomaly can be explained only with the help of crystal field theory (  C.F.T ) .

 According to C.F.T theory metal ion d-orbitals in valence shell loss their degeneracy in presence of ligands  during  complexation and they split into different sets depending on the nature of arrangements of the ligands, that is, geometry of the complex  ( octahedral, tetrahedral  etc).

Due to such splitting of the d-orbitals,  complexation of transitional metal ion containing  one to nine d-electron gives extra stability to the system , which is known as crystal field stabilization energy ( C.F.S.E ). 
In  +1 state both Cu and Au are d¹⁰ system  and according to C.F.T there is no extra stability due to complexation. Thus +1 state can not be a very stable one. 

But in +2 or +3 states they become transitional element. In case of copper the energies of 3d and 4s sub shells are very close. 

Thus it is very easy  for Cu ⁺¹ to become Cu ⁺² by losing one electron  because the energy obtained by C.F.S.E  exceeds second ionization  potential and again for 3d series the C.F.S.E for a d ⁸ system of  square planar geometry is not so large which can permit removal of the third electron . 

For  5d series the C.F.S.E  for  a   d ⁸ system   ( Au ⁺³ ) in square planar complexes is very much large, that can permit the removal of third electron from Au ⁺²  ion. 

Thus for Au metal atom , +2 oxidation state is unstable and +3  oxidation state is very stable.

                                              Au ⁺²    ⇌  Au ⁺¹    +   Au  ⁺³

                                  Small  CFSE        no CFSE      Large CFSE   

Practice problems: 

What is crystal field theory ?

What is crystal field stabilization energy ?

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