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

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Solution

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 $L i_{2} O$ but the sodium forms mainly sodium peroxide $N a_{2} O_{2}$ along with a small amount of sodium oxide while potassium forms only potassium superoxide $(K O_{2})$

$4 L i + O_{2} △ - \to 2 L i_{2} O 6 N a + 2 O_{2} △ - \to N a_{2} O_{2} \begin{matrix} S o d i u m p e r o x i o d e (m a j o r) \end{matrix} + 2 N a_{2} O \begin{matrix} M o n o x i d e (m i n o r) \end{matrix} K + O_{2} △ - \to K O_{2} \begin{matrix} P o t e s s i u m s u p e r o x i d e \end{matrix} + K_{2} O_{2} P e r o x i d e + K_{2} (O) M o n o x i d e$

The superoxede, $O_{2}^{-}$ ion is stable only in presence of large cations such as K, Rb etc.

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