acceptor

Meaning: An acceptor takes something from a donor. General chemistry uses the word in three ways. 1. Proton acceptor: a Brønsted-Lowry base, such as NH₃ becoming NH₄⁺. 2. Electron-pair acceptor: a Lewis acid, such as BF₃, H⁺ or a metal ion like Fe³⁺. It has an empty orbital and forms a coordinate covalent bond with the donor (the Lewis base or ligand). 3. Electron acceptor: the oxidizing agent. It gains electrons, so it is reduced and its oxidation number falls. In biochemistry, O₂ is the final electron acceptor of the electron transport chain, and NAD⁺ is an electron acceptor in catabolism.

On the MCAT: A passage gives a reaction or a complex-ion formation and asks which species is the acceptor. For acid-base, look for the species gaining H⁺ or accepting a lone pair. For redox, find the species whose oxidation number decreases. In a galvanic cell, the species that accepts electrons is reduced at the cathode.

Common confusion: Students call the electron acceptor "the oxidized species." It is the opposite: the acceptor is reduced, and that is what makes it the oxidizing agent. Also, "acid" does not always mean proton donor. A Lewis acid is an electron-pair acceptor and may carry no H⁺ at all.