Lewis goes further
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Lewis proposed an even simpler theory:
The base donates a pair of electrons to the acid. The acid accepts a pair of electrons.
While Brønsted Lowry theory is suitable for the majority of school studies, it is worth noting that the Lewis theory of acids and bases extends beyond Brønsted Lowry to include an even wider set of substances. Rather than the reaction of an acid and a base involving a proton transfer (Brønsted Lowry) the Lewis definition is simply the sharing of a electron pair.
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Can you think of an example of chemistry that is not explained by Brønsted Lowry theory that could be explained by Lewis theory?
The acidity in water of non‐metal oxides such as SO2, SO3, CO2, NO, NO2 can be explained by Lewis theory.
Some transition metal ions make water more acidic, eg iron(III) as explained by the inductive effects of the central metal in the complex giving the equilibrium:
Fe(H2O)63+(aq) + H2O(l) Fe(H2O)5(OH)2+(aq) + H3O+ (aq)
This provides further evidence for the limitations of Brønsted Lowry theory.
In the example of carbon dioxide being bubbled through water to give carbonic acid, can you identify the Lewis acid and the Lewis base?
CO2(g) + H2O(l) H2CO3(aq) Water is the Lewis base sharing its electrons with CO2 which is the Lewis acid.
In the example of boron trifluoride, BF3 (which cannot be in aqueous solution) reacting with ammonia, NH3 what is the Lewis acid and what is the Lewis base?
BF3 + NH3 F3B—NH3 where BF3 acts as the Lewis acid accepting the electron pair donated by ammonia, the Lewis base, forming a dative (coordinate) bond.
This is more advanced theory, mostly applicable in advanced studies of chemistry, likely beyond the experience of school studies.