Q.What are Lewis acids and bases? Give two examples for each.

Gilbert N. Lewis's 1923 concept is the most general of the three, defined by the electron pair rather than the proton: a Lewis acid is a species that accepts an electron pair, and a Lewis base is a species that donates an electron pair. A Lewis acid is typically a positive ion or an electron-deficient molecule; a Lewis base is typically an anion or a neutral molecule with at least one lone pair.

In $BF_3+NH_3 \rightarrow F_3B{\leftarrow}NH_3$, boron's vacant $2p$ orbital accepts the lone pair donated by nitrogen, forming a new coordinate covalent bond: $BF_3$ is the Lewis acid and $NH_3$ is the Lewis base. In $Cr^{3+}+6H_2O \rightarrow [Cr(H_2O)_6]^{3+}$, each water molecule donates a lone pair to $Cr^{3+}$, so $Cr^{3+}$ is the Lewis acid and $H_2O$ is the Lewis base -- the same logic underlies every coordination compound, where the ligands are Lewis bases and the central metal ion is the Lewis acid.

Families of Lewis acids: electron-deficient molecules ($BF_3, AlCl_3, BeF_2$), all metal ions ($Fe^{2+}, Cr^{3+}, Cu^{2+}$), molecules with a polar double bond ($SO_2, CO_2, SO_3$), molecules whose central atom can expand its octet via empty d orbitals ($SiF_4, SF_4, FeCl_3$), and carbonium ions ($CH_3^+$).

Families of Lewis bases: molecules with one or more lone pairs ($NH_3, H_2O, R{-}O{-}H$), all anions ($F^-, CN^-, SO_4^{2-}$), molecules with a carbon-carbon multiple bond ($CH_2{=}CH_2, CH{\equiv}CH$), all metal oxides ($CaO, MgO, Na_2O$), and carbanions ($CH_3^-$).

Where two possible Lewis bases compete for the same Lewis acid -- as when $NH_3$ displaces $H_2O$ from $[Cu(H_2O)_4]^{2+}$ to form the deep-blue $[Cu(NH_3)_4]^{2+}$ -- the base forming the more stable complex is the stronger Lewis base; here $NH_3$ is the stronger Lewis base than $H_2O$.

"Lewis acid and base theory definition with examples" and "Lewis acid base class 11 important questions" are common search phrases tied to the Equilibrium chapter of the NCERT-aligned CBSE Class 11 Chemistry curriculum, since Lewis theory is the most general of the three acid-base theories tested in JEE Main and NEET. Classifying species into the Lewis-acid and Lewis-base families listed here is a frequent one-mark identification question in board exams.