Final answer:
A Brønsted-Lowry base is defined as a species that accepts a proton from another molecule.
Step-by-step explanation:
A Brønsted-Lowry base is defined as a species that can accept a proton (H+) from another molecule. This definition was proposed by chemists Johannes Brønsted and Thomas Lowry in 1923, who expanded upon the earlier Arrhenius definition to encompass a broader range of acid-base reactions beyond those occurring in aqueous solutions alone. According to the Brønsted-Lowry theory, an acid-base reaction is characterized as the transfer of a proton from an acid to a base.
To better understand, let's consider the example of ammonia (NH3) reacting in water, an example of a Brønsted-Lowry base in action. When ammonia is in solution with water, NH3 accepts a hydrogen ion from H2O, resulting in the formation of the ammonium ion (NH4+) and a hydroxide ion (OH−). This demonstrates that ammonia acts as a base because it accepts a proton from the water molecule, which is acting as an acid.