State the Lewis acid-base principle.

(N/A) Definition: $G$.$N$. Lewis in $1923$ defined an acid as a species that accepts an electron pair and a base as a species that donates an electron pair.
Comparison of Bronsted-Lowry and Lewis Base: Regarding bases,there is little difference between the Bronsted-Lowry and Lewis concepts,as the base provides a lone pair in both cases. For example,the Lewis base $NH_{3}$ donates an electron pair to $H^{+}$ to form $NH_{4}^{+}$. Thus,$NH_{3}$ acts as a base in both principles. All Lewis bases are Bronsted-Lowry bases.
Comparison of Bronsted-Lowry and Lewis Acid: In the Bronsted-Lowry concept,an acid must contain a proton $(H^{+})$. However,in the Lewis concept,many acids do not contain a proton. Electron-deficient species like $BF_{3}$,$AlCl_{3}$,$FeCl_{3}$,$NO_{2}^{+}$,$Mg^{2+}$,and $Co^{3+}$ can act as Lewis acids. Thus,not all Lewis acids act as Bronsted-Lowry acids.
Example reaction: $BF_{3} + :NH_{3} \longrightarrow F_{3}B:NH_{3}$
Here,$BF_{3}$ is the Lewis acid and $:NH_{3}$ is the Lewis base.
Species like $H_{2}O$,$NH_{3}$,and $OH^{-}$ which can donate a pair of electrons act as Lewis bases.