Which one of the following compounds is NOT considered an acid ?

In chemical theory, acids and bases are defined by their ability to donate or accept protons (Brønsted-Lowry) or electron pairs (Lewis). Boron trifluoride (BF3) and Aluminium chloride (AlCl3) are classic Lewis acids because they are electron-deficient molecules that can accept an electron pair to complete their octet. Phenol (C6H5OH) is a Brønsted-Lowry acid because it can donate a proton (H+) to a base. In contrast, Ammonia (NH3) is characterized as a base. Under the Brønsted-Lowry definition, it acts as a proton acceptor. Under the Lewis definition, it is a Lewis base because the nitrogen atom possesses a lone pair of electrons that it can donate to form a covalent bond with an electron-deficient species. Therefore, NH3 is the only compound among the options that is not considered an acid.