Question

What are the limitations of Lewis theory?

Answer 1

1. According to Lewis's theory, an acid is which accepts electrons, and a base is one which donates a pair of electrons.
2. Ammonia (${\mathrm{NH}}_3$) is considered a base as it can donate electrons.
3. Boron trifluoride (${\mathrm{BF}}_3$) is a Lewis acid as it can accept a pair of electrons.

The limitations of Lewis's theory are shown below.

• The Lewis concept may or may not include the general idea of all coordination reactions and compounds.
• It could not explain the behavior of protonic acids like Hydrochloric acid and Sulphuric acid which do not form coordination compounds with bases. Hence, were not regarded as acids.
• Lewis could not explain the relative strengths of acids and bases.
• This concept was not applied to acid-base reactions. It could not explain the energy release during covalent bond formation.
• Many Lewis acids do not exhibit catalytic activity.
• It violates the octet rule for the molecules with less than eight electrons ie ${\mathrm{BF}}_3$ and molecules with more than eight electrons ie. ${\mathrm{PCl}}_5$ on the central atom.
• Examples of Lewis acids and bases are given below.
  $$\begin{gathered} {\mathrm{BF}}_3+{\mathrm{NH}}_3\to {\mathrm{BF}}_3:{\mathrm{NH}}_3 \\ \mathrm{acid}\mathrm{base}\mathrm{adduct} \end{gathered}$$