Explain the chemical properties of alkali metal elements (Group $I$).

(N/A) The alkali metals are highly reactive due to their large atomic size and low ionization enthalpy. The reactivity of these metals increases down the group.
Reactivity towards air: The alkali metals tarnish in dry air due to the formation of their oxides,which react with moisture to form hydroxides.
They burn vigorously in oxygen to form oxides. Lithium forms monoxide $(Li_{2}O)$,sodium forms peroxide $(Na_{2}O_{2})$,and other metals form superoxides $(MO_{2})$. The superoxide $(O_{2}^{-})$ ion is stable only in the presence of large cations such as $K^{+}$,$Rb^{+}$,and $Cs^{+}$.
Equations: $4Li + O_{2} \rightarrow 2Li_{2}O$,$2Na + O_{2} \rightarrow Na_{2}O_{2}$,$M + O_{2} \rightarrow MO_{2}$ (where $M = K, Rb, Cs$).
Lithium shows exceptional behavior by reacting directly with nitrogen to form nitride $(Li_{3}N)$.
Reactivity towards water: Alkali metals react with water to form hydroxides and dihydrogen gas: $2M + 2H_{2}O \rightarrow 2M^{+} + 2OH^{-} + H_{2}$.
Although lithium has the most negative $E^{\ominus}$ value,its reaction with water is less vigorous than sodium due to lithium's small size and high hydration energy.
Reactivity towards dihydrogen: Alkali metals react with dihydrogen at high temperatures (e.g.,$673 \ K$) to form ionic hydrides $(M^{+}H^{-})$.
Reactivity towards halogens: They react vigorously with halogens to form ionic halides $(M^{+}X^{-})$. Lithium halides are somewhat covalent due to the high polarizing power of the $Li^{+}$ ion.