What term is used to describe a molecule that can act as both an acid and a base?

The term used to describe a molecule that can act as both an acid and a base is amphoteric. Amphoteric substances have the capacity to either donate a proton (H⁺), functioning as an acid, or accept a proton, thereby acting as a base. This dual functionality is characteristic of certain molecules and ions, such as water (H₂O) or amino acids, where the presence of both acidic and basic groups allows them to respond to different environments by either donating or accepting protons depending on the conditions.

In contrast, the other terms listed do not pertain to this dual capability. Electrophilic refers to species that are electron-deficient and can accept electron pairs but does not imply any acid-base behavior. Diatomic specifically refers to molecules made up of two atoms, such as O₂ or N₂, without any indication of their acid-base characteristics. Monovalent describes an atom or ion that has a valence of one, which relates to connectivity rather than acid-base behavior. Thus, amphoteric is the only term among those provided that accurately describes a molecule capable of acting as both an acid and a base.