In 1923, the Brønsted–Lowry definition of acids and bases was proposed by Johannes Brønsted and Thomas Lowry. According to this theory, a Brønsted acid is defined as a species that donates a proton in a chemical reaction and gets converted to its conjugate base. A Brønsted base is defined as a species that accepts a proton in a chemical reaction and gets converted into its conjugate acid. These transfers of protons are caused by the displacement of electrons in these reactions, which is represented by curved arrows.

Consider the following reaction between acetic acid and ammonia as shown in figure 1.

Figure 1. Reaction between acetic acid and ammonia

In this reaction, the transfer of two-electron pairs is shown by curved arrows, symbolizing the proton transfer from the acetic acid to ammonia in this acid—base reaction. The two curved arrows in this reaction depict the two electron-pair shifts. One electron pair is transferred from ammonia to the acetic acid, and another is transferred from the O—H bond of the acetic acid to the oxygen. This, in turn, results in the transfer of a proton from an acetic acid molecule to an ammonia molecule. Thus, the acetate ion and ammonium ion are formed as the conjugate base and conjugate acid in this reaction.