State the occurrence of group-$18$ elements.

All elements of group-$18$ except radon and oganesson occur in the atmosphere. They are gases under standard conditions.
The atmospheric abundance in dry air is $1 \%$ by volume,of which argon is the major constituent.
Helium and sometimes neon are found in minerals of radioactive origin,such as pitchblende,monazite,and cleveite.
The main commercial source of helium is natural gas.
Xenon and radon are the rarest elements of the group. Radon is obtained as a decay product of ${}^{226}Ra$:
${}_{88}^{226}Ra \rightarrow {}_{86}^{222}Rn + {}_{2}^{4}He$
Oganesson has been synthetically produced by the collision of ${}_{98}^{249}Cf$ atoms and ${}_{20}^{48}Ca$ atoms:
${}_{98}^{249}Cf + {}_{20}^{48}Ca \rightarrow {}_{118}^{294}Og + 3{}_{0}^{1}n$
Oganesson has an atomic mass of $294$,an atomic number of $118$,and a half-life of $0.7 \ ms$.