A hydrogen of sulfuric acid (H2SO4) is the acid, and the nitrogen of
ammonia (NH3) is the base. They react to form the hydrogen sulfate
anion (HSO4c- ) and the ammonium ion (NH4⊕). The ammonium ion is
the conjugate acid of ammonia. The bisulfate ion is the conjugate base
of the sulfuric acid
Like Brønsted and Lowry, G. N. Lewis defined acids and bases
in a broader scheme than Arrhenius did. Lewis noted that there are a
number of reactions that look like acid-base reactions but do not
involve the transfer of a proton. Instead, they involve the interaction
of a pair of nonbonding electrons. From that observation, he defined
an acid as a molecule that forms a covalent bond by accepting a pair of
electrons and a base as a molecule that forms a covalent bond by
donating a pair of electrons. Below is a simplified statement of the
Lewis definition of acids and bases.
A Lewis acid is an electron-pair acceptor.
A Lewis base is an electron-pair donor.
Reconciling the Acid-Base Theories
To prevent confusion over the terms acceptor and donor, stop and look at the
three definitions of acids and bases. Keep in mind that although all three
definitions consider the same concept, they do so from different viewpoints.
Arrhenius and Brønsted-Lowry look at acids and bases from the viewpoint ofproton transfers. Lewis looks at them from the viewpoint of electron pairs. The
two viewpoints mesh when you remember that a proton is a positive hydrogen
ion that has no electron, and is thus capable of accepting a pair of electrons.