(i)    All the captions are Lewis acids because they have a tendency to accept electron pairs. Larger captions of alkali and alkaline earth metals behave as relatively weaker Lewis acids due to their lesser tendency to accept electrons. Smaller captions like H⁺, Li^+, Be⁺⁺, etc. are likely to act as stronger Lewis acids due their tendency to accept electrons form stronger bonds. Also higher charged captions have a higher tendency to accept electrons relative to the lower changed action of a given metal. For example, Fe⁺⁺⁺ behaves as a stronger Lewis acid as compared to Fe++. Similarly, Sn⁴⁺ is a stronger Lewis acid than Sn²⁺.

(ii)    Electron deficient compounds or compounds in which the central atom can expand its valence shell can behave as Lewis acids.

For example, BF₃, AICI₃, GaBr₃,. GeCI₂ are electron deficient compounds and have tendency to accept electron pairs to complete their octet .
                                              BF₃ + F ^- →   [BF4]^-
Compounds like SiF₄, S_nCI₄, SbCI₃, SbCI₅, TeBr₄, SO₃, etc., represent Lewis acids where central atom can expand its valence shell.
                                                SnCI4 + 2CI^-→ [SnCI6]^-2
                                                SO3 + R2O →  [ (R2O) (SO3)]
                                                SiF4 + 2F- →  [SiF6]2-
Lewis Bases. These can be classified into the following categories.
(i)    All the anions can act as Lewis bases because they have a tendency to donate electron paired. For example, oxides (O^2-), hydroxides (OH^-), halides (x^- ), amides (NH₂^-), etc., act as Lewis bases. The greater the charge density on the anion, the stronger the base.

(ii)    Substances having unshared pairs of electrons act as Lewis bases. For example, water H₂O`:) ammonia (:NH₃), ethers (R₂O:) , Ketones (R₂C = O:), etc , act as electron donors.

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