Lewis Acid-Base Theory

Ammonia is a base in the Brønsted-Lowry sense because it acts as a proton acceptor. For example, it accepts a proton from a water molecule to form the ammonium ion and hydroxide ion. In the Lewis Acid-Base theory, the emphasis is on the electron pair. Ammonia is a base because it acts as an electron-pair donor toward the proton. The proton is an acid because it acts as an electron-pair acceptor. Lewis theory is more general than Brønsted-Lowry theory, because it applies to systems that do not involve protons. For example, ammonia acts as an electron-pair donor toward boron tri-fluoride, a molecule that has a vacant 2p orbital. Ammonia also acts as a Lewis base toward metal ions, as in this example of a cobalt 3 hexamine complex. The ammonia electron pairs are donated into vacant metal orbitals. Carbon dioxide acts as a Lewis acid toward water. One of the unshared electron pairs on oxygen is donated to the carbon atom as one of the CO double-bond electron pairs is moved onto the oxygen of carbon dioxide. A proton from the water molecule moves to the oxygen of the carbon dioxide molecule, completing the formation of carbonic acid.