$(a)$ Describe the chemical properties of alkali metals with air,water,dihydrogen,and halogens.
$(b)$ Explain the reduction potential of alkali metal elements.
$(c)$ Which type of solutions are formed on the reaction of alkali metals with liquid ammonia?

(A) The alkali metals are highly reactive due to their large size and low ionization enthalpy. The reactivity of these metals increases down the group.
$(a)$ Reactivity towards air: Alkali metals tarnish in dry air due to the formation of oxides,which react with moisture to form hydroxides. They burn vigorously in oxygen: $4Li + O_2 \rightarrow 2Li_2O$ (oxide),$2Na + O_2 \rightarrow Na_2O_2$ (peroxide),and $M + O_2 \rightarrow MO_2$ (superoxide,where $M = K, Rb, Cs$). Lithium also forms $Li_3N$ with nitrogen.
Reactivity towards water: They react with water to form hydroxides and dihydrogen: $2M + 2H_2O \rightarrow 2M^+ + 2OH^- + H_2$. Lithium is less vigorous than sodium due to its high hydration energy.
Reactivity towards dihydrogen: They react at high temperatures to form ionic hydrides: $2M + H_2 \rightarrow 2M^+H^-$.
Reactivity towards halogens: They react vigorously to form ionic halides $M^+X^-$,though lithium halides are covalent due to high polarization.
$(b)$ Reduction potential: Alkali metals have high negative standard electrode potentials $(E^{\ominus})$,making them strong reducing agents. Lithium has the most negative $E^{\ominus}$ value due to its high hydration energy.
$(c)$ Reaction with liquid ammonia: Alkali metals dissolve in liquid ammonia to form deep blue,conducting,and paramagnetic solutions containing ammoniated cations and electrons: $M + (x+y)NH_3 \rightarrow [M(NH_3)_x]^+ + [e(NH_3)_y]^-$. The blue color is due to the excitation of ammoniated electrons.