Soft and hard acids and base (SHAB) concept

 SOFT AND HARD ACID-BASE (SHAB) CONCEPT

This concept was given by R.G. Pearson in1963. He introduced the idea of soft and hard acids and bases. He divided the acids and bases according to the concept of polarizability.

1. Hard acid:

Hard acid is one in which the acceptor atom

• is small size
• has a high positive charge
• no valence electron that can easily be excited
• low polarizability
• is lighter
• less number of valence electron
  
• has a high oxidation state 

For example:   Li⁺¹, Ca⁺², Al⁺³, H⁺ , Be⁺², Mg⁺²

 2. Soft acid:

Soft acid is one in which the acceptor atom

• is of large size
• has a greater number of valence electron
• is heavier
• has high polarizability
• ha low oxidation state
• has zero or low positive charge
• has several valence electrons that can be easily excited.
  

For example heavy transition metals  Ag⁺¹, Hg⁺², Cu⁺², Au⁺

3. Hard base:

A hard base is one in which donor atom

• has low polarizability
• has high electronegativity
• is not easily oxidized
• is associated with empty orbitals of high energy

 For example: F^-1, Cl^-1, OH^-1

4. Soft base:

The soft base is one in which the donor atom

• has high polarizability
• has low electronegativity
• can be easily oxidized
• is associated with the energy of low orbitals

For example P, S^-2, CN^-1, I^-1