A Brønsted-Lowry acid is a species that donates protons, while a Brønsted-Lowry base is a
species that accepts protons.Brønsted-Lowry acids and bases always occur in pairs, called conjugate acid-base pairs. The two
members of a conjugate pair are related by the transfer of a proton. For example, H 3 O+ is the
conjugate acid of H 2 O, and NO 2 − is the conjugate base of HNO 2 :
Conversely, one can also say that H 2 O is the conjugate base of H 3 O+, and that HNO 2 is the conjugate
acid of NO 2 −.
LEWIS DEFINITION
At approximately the same time as Brønsted and Lowry, Gilbert Lewis also proposed definitions for
acids and bases. Lewis defined an acid as an electron-pair acceptor, and a base as an electron-pair
donor. Lewis’s are the most inclusive definitions. Just as every Arrhenius acid is a Brønsted-Lowry
acid, every Brønsted-Lowry acid is also a Lewis acid (and likewise for bases). However, the Lewis
definition encompasses some species not included within the Brønsted-Lowry definition. For
example, BCl 3 and AlCl 3 can each accept an electron pair and are therefore Lewis acids, despite their
inability to donate protons. We shall, however, focus our attention on Brønsted-Lowry acids and
bases.
BASIC CONCEPT
A Lewis acid is an electron-pair acceptor, and a Lewis base is an electron-pair donor.