What is a hard and soft acid?

Answer 1

The term hard and soft acid is a qualitative one which refers to the polarizability of the acid (and typically refers only to Lewis acids). A soft acid is one that is highly polarizable, whereas a hard acid is one that is not polarizable.

For more information, and a list of many examples of hard and soft acids, see the Web Links to the left.

Answer 2

Soft acids:

The acceptor atoms are larger, have lower positive charge, have unshared pair of electrons in their valence shell. Also they have higher polarisability and lower electronegativity.

Soft bases:

The donor atoms are smaller, have higher positive charge, donot have unshared pair of electrons in their valence shell. Also they have higher polarisability and lower electronegativity. They are easily oxidised and the valence electrons are loosely held.

Hard acids:

The acceptor atoms are smaller in size, have higher positive charge, no unshared pair of electrons in their valence shell. Also they have lower polarisability and higher electronegativity.

Hard bases:The donor atoms have higher electronegativity, lower polarisability, difficult to oxidise. The valence electrons are held tightly