Helium is used in balloons in place of hydrogen because it is

Match the compounds given in Column-$I$ with the shapes given in Column-$II$ and mark the correct option.

Column-$I$	Column-$II$
$(a) XeF_6$	$(i) \text{ Distorted Octahedral}$
$(b) XeO_3$	$(ii) \text{ Square planar}$
$(c) XeOF_4$	$(iii) \text{ Pyramidal}$
$(d) XeF_4$	$(iv) \text{ Square pyramidal}$

Explain the variations in the following properties of group $18$ elements:
$(i)$ Atomic radii
$(ii)$ Ionisation enthalpy
$(iii)$ Electron gain enthalpy

$XeF_4 + O_2F_2 \rightarrow X + O_2$
$X + H_2O$ $(1 \ mole)$ $\rightarrow Y + 2 HF$
The shapes of molecules of $X$ and $Y$ respectively are:

Noble gases do not react with other elements because.......

$XeF_4$ on partial hydrolysis produces