The decreasing order of thermal stability of hydrides of group$16$ is

Sulphur can form ${\mathrm{S}}_2, {\mathrm{S}}_6, {\mathrm{S}}_8$ and ${\mathrm{S}}_{\mathrm{n}}$ but oxygen forms only ${\mathrm{O}}_2$ and ${\mathrm{O}}_3.$ This is due to

$\left(\mathrm{I}\right) {\mathrm{F}}_2+\mathrm{NaOH}\left(\mathrm{dilute}\right)\to 2\mathrm{NaF}+{\mathrm{H}}_2\mathrm{O}+\mathrm{X}$

$\left(\mathrm{ii}\right) {\mathrm{F}}_2+\mathrm{NaOH} \left(\mathrm{concentrated}\right)\to \mathrm{NaF}+{\mathrm{H}}_2\mathrm{O}+\mathrm{Y}$

$\mathrm{X}$ and $\mathrm{Y}$, respectively are: