Previously we said that an acid produces H+ when dissolved in water, and a base produces OH– when dissolved in water. Then the acid and base (meaning H+ and OH–) can react (without redox) to make water. This is a pretty good definition, but it is kind of narrow.
Lewis explained that in many reactions that form new bonds, both electrons in the new bond come from one atom (or one reactant) only, instead of one electron coming from each. He called all these reactions acid-base reactions. The picture shows water forming from the elements, in a redox process, and water forming from hydrogen ion and hydroxide ion, in an acid base process. It also shows how the tetrafluoroborate ion, BF4– can form from boron trifluoride and fluoride ion.
In general, if we can draw a good Lewis structure by making two molecules or ions share an electron pair, it's likely that the reaction can happen. For instance, BH3 can react with NH3, because N has an extra lone pair, and B only has 6 electrons and 3 connected atoms. So a Lewis acid is something than can fit two more electrons from a different molecule. It can share another molecule's lone pair. A Lewis base is any molecule or ion with a lone pair to share. It's easy to see what can be a Lewis base just by drawing a Lewis structure. Lewis acids are usually cations, like H+ or Al3+. Boron is a famous Lewis base because it often makes electron-deficient compounds, like BH3, in which it only has six electrons. Try drawing Lewis structures for an acid-forming anhydride combination reaction. Is the anhydride a Lewis acid or base?
