Page "Acid" Paragraph 18

In the first reaction a fluoride ion, F < sup >−</ sup >, gives up an electron pair to boron trifluoride to form the product tetrafluoroborate.

Fluoride " loses " a pair of valence electrons because the electrons shared in the B — F bond are located in the region of space between the two atomic nuclei and are therefore more distant from the fluoride nucleus than they are in the lone fluoride ion.

BF < sub > 3 </ sub > is a Lewis acid because it accepts the electron pair from fluoride.

This reaction cannot be described in terms of Brønsted theory because there is no proton transfer.

The second reaction can be described using either theory.

A proton is transferred from an unspecified Brønsted acid to ammonia, a Brønsted base ; alternatively, ammonia acts as a Lewis base and transfers a lone pair of electrons to form a bond with a hydrogen ion.

The species that gains the electron pair is the Lewis acid ; for example, the oxygen atom in H < sub > 3 </ sub > O < sup >+</ sup > gains a pair of electrons when one of the H — O bonds is broken and the electrons shared in the bond become localized on oxygen.

Depending on the context, a Lewis acid may also be described as an oxidizer or an electrophile.