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

(N/A) $(i)$ Atomic radii: The atomic radii of noble gases in their respective periods are exceptionally high because,in the case of noble gases,the atomic radii correspond to Van der Waals radii,which are always larger than covalent radii.
Down the group,the atomic radii increase with the increase in atomic number.
$(ii)$ Ionisation enthalpy: Ionisation enthalpies of noble gases are exceptionally high due to the presence of fully filled valence shells. However,down the group,the ionisation enthalpy decreases.
$(iii)$ Electron gain enthalpy: The values of electron gain enthalpies are large positive as they have fully filled valence shells. Hence,they have no tendency to accept electrons and form anions.