Lewis Acids and Bases:

Lewis acids

 Lewis acids are ions/electron deficient molecules along with an unfilled valence shell. They are categorized as acids since they can accept a lone pair of electrons from other molecule to fill their valence shell. Lewis acids involve all the Brønsted-Lowry acids we described earlier, as well as ions (Example H⁺, Mg₂⁺), and neutral species like BF₃ and AlCl₃.

Both Al and B are in the periodic table's Group 3A and have 3 valence electrons in their outer shell. The meaning of this is that these elements can create three bonds. Though, there is still room for a fourth bond. For instance in BF₃, boron is surrounded via six electrons (three bonds consisting of two electrons each). Though, the valence shell of boron can accommodate eight electrons and thus a fourth bond is possible if the fourth group can give both electrons for the new bond. Because both boron and aluminum are in the periodic table's Group 3A, they are electropositive and will react with electron-rich molecules in order to acquire this fourth bond. Several transition metal compounds can as well act as Lewis acids (example TiCl₄ and SnCl₄).