Answer
The $\mathrm{Na}^{+}, \mathrm{K}^{+}, \mathrm{Rb}^{+}$ and $\mathrm{Cs}^{+}$ ions have relatively low charge densities and are better able to stabilize large, polyatomic anions such as $\mathrm{NO}_{2}^{-}$.
$\mathrm{MNO}_{3}(\mathrm{s}) \stackrel{\Delta}{\longrightarrow} \mathrm{MNO}_{2}+\frac{1}{2} \mathrm{O}_{2}(\mathrm{g})$ $(\mathrm{M}=\mathrm{Na}, \mathrm{K}, \mathrm{Rb}, \mathrm{Cs})$
Because the $\mathrm{Li}^{+}$ ion has a very high charge density and high polarizing power, it may kinetically assist the decomposition of polyatomic anions, such as $\mathrm{N}_{3}^{-}$ and $\mathrm{N}_{2}$, to smaller anions such as $\mathrm{O}_{2}^{-}$.
$2 \mathrm{LiNO}_{3}(\mathrm{s}) \stackrel{\Delta}{\longrightarrow} \mathrm{Li}_{2} \mathrm{O}(\mathrm{s})+$$2 \mathrm{NO}_{2}(\mathrm{g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{g})$
Work Step by Step
The $\mathrm{Na}^{+}, \mathrm{K}^{+}, \mathrm{Rb}^{+}$ and $\mathrm{Cs}^{+}$ ions have relatively low charge densities and are better able to stabilize large, polyatomic anions such as $\mathrm{NO}_{2}^{-}$.
$\mathrm{MNO}_{3}(\mathrm{s}) \stackrel{\Delta}{\longrightarrow} \mathrm{MNO}_{2}+\frac{1}{2} \mathrm{O}_{2}(\mathrm{g})$ $(\mathrm{M}=\mathrm{Na}, \mathrm{K}, \mathrm{Rb}, \mathrm{Cs})$
Because the $\mathrm{Li}^{+}$ ion has a very high charge density and high polarizing power, it may kinetically assist the decomposition of polyatomic anions, such as $\mathrm{N}_{3}^{-}$ and $\mathrm{N}_{2}$, to smaller anions such as $\mathrm{O}_{2}^{-}$.
$2 \mathrm{LiNO}_{3}(\mathrm{s}) \stackrel{\Delta}{\longrightarrow} \mathrm{Li}_{2} \mathrm{O}(\mathrm{s})+$$2 \mathrm{NO}_{2}(\mathrm{g})+\frac{1}{2} \mathrm{O}_{2}(\mathrm{g})$
