Ncert Exercises & Solutions

When heater in air, the alkali metals form various oxides. Mention the oxides formed by Li, Na and K.

The reactivity of alkali metals towards oxygen increases on moving down the group with the increase in atomic size. Thus, Li forms only lithium oxide (Li₂O), sodium forms mainly sodium peroxide Na₂O₂ along with a small amount of sodium oxide while potassium forms only potassium superoxide (KO₂).

4 Li + O_{2} \overset{∆}{\to} 2 {Li}_{2} O

6 Na + 2 O_{2} \overset{∆}{\to} \underset{Sodium peroxide}{{Na}_{2} O_{2}}

(major) + \underset{Monoxide}{2 {Na}_{2} O}

(minor)

K + O_{2} \overset{∆}{\to} \underset{Potassium}{{KO}_{2}}

super oxides + \underset{Peroxide}{K_{2} O_{2}} + \underset{Monoxide}{K_{2} (O)}

The superoxide,

{O_{2}}^{-}

ion is stable only in presence of large cations such as K, Rb etc.

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