How do Lewis acids and bases differ from Bronsted/Lowry acids and bases?

•A Brønsted-Lowry acid is any substance (molecule or ion) that can donate a hydrogen ion (#H^(+)#).

•A Brønsted-Lowry base is any species that can accept a hydrogen ion (#H^(+)#).

#color(blue) ("Take the reaction below for example:")#

#HNO_2+ H_2O rightleftharpoons H_3O^(+) + NO_2^(-)#

Nitrous acid (#HNO_2#) is the Brønsted-Lowry acid because it donates a hydrogen ion to water. Water is the Brønsted-Lowry base because it accepts the hydrogen ion.

#color(red)("On the other hand")#, Lewis acids and bases are defined in terms of either being an electron-pair donor or an electron-pair acceptor.

• A Lewis acid is defined as an electron-pair acceptor. This means that acids can accept a lone pair of electrons from a Lewis base because the acid has vacant valence orbitals.

• A Lewis acid must have a vacant valence orbital and it can be a cation, such as #Al^(3+)#, or a neutral molecule, such as #CO_2#.

• A Lewis base is an electron-pair donor. This means that a Lewis base has the ability to donate two of its electrons to a Lewis acid.

Here's a general depiction of a Lewis acid-base reaction:

In the diagram above, #A# is a Lewis acid because it is accepting a pair of electrons from #B# as denoted by the curved arrow.

#B# is a Lewis base it is donating its electrons to #A#.