Editing Acid (section)

===Lewis acids===
{{main|Lewis acids and bases}}
A third, only marginally related concept was proposed in 1923 by [[Gilbert N. Lewis]], which includes reactions with acid–base characteristics that do not involve a proton transfer. A '''Lewis acid''' is a species that accepts a pair of electrons from another species; in other words, it is an electron pair acceptor.<ref name="Ebbing" /> Brønsted acid–base reactions are proton transfer reactions while Lewis acid–base reactions are electron pair transfers. Many Lewis acids are not Brønsted–Lowry acids. Contrast how the following reactions are described in terms of acid–base chemistry:
:[[File:LewisAcid.png|374px]]
In the first reaction a [[fluoride|fluoride ion]], F<sup>−</sup>, gives up an [[lone pair|electron pair]] to [[boron trifluoride]] to form the product [[tetrafluoroborate]]. Fluoride "loses" a pair of [[valence electron]]s because the electrons shared in the B—F bond are located in the region of space between the two atomic [[atomic nucleus|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 [[Oxidizing agent|oxidizer]] or an [[electrophile]]. Organic Brønsted acids, such as acetic, citric, or oxalic acid, are not Lewis acids.<ref name="Oxtoby8th" /> They dissociate in water to produce a Lewis acid, H<sup>+</sup>, but at the same time, they also yield an equal amount of a Lewis base (acetate, citrate, or oxalate, respectively, for the acids mentioned). This article deals mostly with Brønsted acids rather than Lewis acids.