A Lewis base is a chemical species that can donate a pair of electrons to form a covalent bond with another species. In simpler terms, a Lewis base is a substance that tends to give away electron pairs. This electron donation forms the basis of a new chemical bond.
A Lewis acid, on the other hand, is a chemical species that can accept a pair of electrons to form a covalent bond. In essence, a Lewis acid is a substance that has an electron-deficient site and is capable of accepting electron pairs.
The Lewis acid-base theory, proposed by Gilbert N. Lewis, expanded the understanding of chemical reactions beyond the traditional concept of acid-base reactions. In the Lewis theory, acids and bases are defined based on their electron-pair donating and accepting abilities, respectively, rather than relying solely on the presence of protons (H+ ions) as in the classic Arrhenius and Brønsted-Lowry acid-base theories.
An example of a Lewis base is ammonia (NH3), which donates a lone pair of electrons to form a bond with a Lewis acid. An example of a Lewis acid is boron trifluoride (BF3), which can accept a pair of electrons from a Lewis base to form a covalent bond.
It's worth noting that the Lewis acid-base theory provides a broader perspective on chemical interactions and is applicable not only to traditional acid-base reactions but also to a wide range of chemical processes, including coordination complexes and reactions involving metal ions.