In the periodic table, all elements in group 13 have three valence electrons. Consequently, they form trivalent compounds in which the central atom has a sextet of electrons and a vacant orbital.
These electron-deficient compounds achieve their octets by receiving electrons from other species in a chemical reaction.
For example, aluminum chloride reacts with ammonia by accepting the lone pair from the nitrogen atom.
This makes aluminum chloride — the electron-pair acceptor — a Lewis acid, and ammonia — the electron-pair donor — a Lewis base.
As the two oppositely charged species attract each other, the electron pair is transferred from the base to the acid. This completes the octet of the Lewis acid, resulting in a Lewis acid—base adduct.
Now, consider another Lewis acid—base reaction between the electron-deficient boron trifluoride and ammonia.
The electrostatic potential map of boron trifluoride indicates a significant positive charge centered on the boron atom.
In the case of ammonia, the nitrogen atom carries a negative charge on account of its nonbonding electron pair.
The opposite charges cause boron trifluoride and ammonia to react. During the reaction, the lone pair of ammonia fills the valence shell of boron to give it an octet of electrons.
In the adduct, boron carries a formal negative charge and nitrogen carries a formal positive charge — thus conserving the net charge of the compound.
Now, examine the reaction between a protic acid, like hydrochloric acid, and a base — ammonia. By the Brønsted–Lowry definition, HCl is an acid owing to its ability to donate a proton.
Although HCl is not an electron-deficient compound, in its reaction with ammonia, the hydrogen atom of HCl loses its shared electrons to chlorine while simultaneously accepting a pair of electrons from ammonia. HCl is, therefore, also a Lewis acid.
