1. Hard acids are characterized by:

 

    # Low electronegativity

    # Small size

    # High charge

 

eg. s,f-block, left side of d-block in high oxidation state. High charge results in small size.

Na⁺, Mg²⁺, Fe³⁺, Al³⁺

 

 

2. Hard bases are characterized by:

 

     # High electronegativity

     # Small size

 

eg. The combination of high electronegativity and small size results in non-polarizable electron cloud around the donor atom. Only oxygen and fluorine are in specified range. So hard bases only include those containing F and O. 

O^2-, F^-, H₂O, CO₃^2-, PO₄^3-

 

 

3. Soft acids are characterized by:

 

      # Intermediate to high electronegativity

      # Large size

      # Low charge

 

eg.Large size species generally have large number of electrons, some of which are far from nucleus. This leads to polarization or distortion.

Cu⁺, Hg²⁺, Au⁺, Ag⁺, Pb²⁺

 

4. Soft bases are characterized by:

 

      # Intermediate to high electronegativity

      # Large size

 

eg. S^2-, I^-, Br^-, PEt₃