Discuss physical properties and chemical properties of group $18$ elements.

(N/A) $(i)$ Physical Properties: All noble gases are monoatomic. They are colourless,odourless,and tasteless.
Noble gases are sparingly soluble in water. They have very low melting and boiling points because the only type of interatomic interaction in these elements is weak dispersion forces. Helium has the lowest boiling point $(4.2 \ K)$ of any known substance. It has an unusual property of diffusing through most commonly used laboratory materials such as rubber,glass,or plastics.
$(ii)$ Chemical Properties: The noble gases in general are least reactive (inert). This is due to the following reasons:
$(a)$ Except helium $(1s^{2})$,the noble gases have completely filled $(ns^{2} np^{6})$ valence shells.
$(b)$ They have high ionisation enthalpy and high positive electron gain enthalpy.
Neil Bartlett first observed the reaction of a noble gas. He first prepared a red compound $O_{2}^{+}[PtF_{6}]^{-}$. He realised that the first ionisation enthalpy of molecular oxygen $(1175 \ kJ \ mol^{-1})$ and that of xenon $(1170 \ kJ \ mol^{-1})$ is almost identical. He made efforts and prepared the same type of red-coloured compound of $Xe$,i.e.,$Xe^{+}[PtF_{6}]^{-}$,by mixing $PtF_{6}$ and $Xe$. Later,many compounds of xenon with fluorine and oxygen were synthesized.
The compounds of krypton are fewer. Only the difluoride $(KrF_{2})$ has been studied in detail. Compounds of radon have not been isolated but only identified $(RnF_{2})$ by radiotracer technique. No true compounds of $Ar$,$Ne$,or $He$ are yet known.