Hard and soft acids and bases

Hard and soft acids and bases

Today I want to talk about the theory of hard and soft acids and bases, because for a long time I myself kept mixing up which reactions proceed under which conditions. I think it won’t hurt to share a clear explanation here — it will save you the same confusion I had

Did you know that the ease of an acid–base reaction depends not only on the strength of the acid and base, but also on another property called the hardness (or softness) of the acid or base? Let’s take a closer look at the characteristic features of hard and soft acids and bases

Soft bases are donor atoms that have low electronegativity and high polarizability. They are easily oxidized; their valence electrons are held weakly

Hard bases are donor atoms that have high electronegativity and low polarizability. They are difficult to oxidize; their valence electrons are held tightly

Soft acids are acceptor atoms that have a large atomic radius and carry a low positive charge. Their valence shells contain unshared electron pairs (p or d). They possess high polarizability and low electronegativity

Hard acids are acceptor atoms that have a small radius and carry a high positive charge. Their valence shells do not contain unshared electron pairs. They have low polarizability and high electronegativity