State occurence of group-18 elements.

Explanation

All elements of group-18 except radon and oganesson occur in the atmosphere. They are gases, (except radon and oganesson).
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 e.g., pitchblende, monazite, cleveite.
The main commercial source of helium is natural gas. Xenon and radon are the rarest elements of the group. Radon is obtained as decay product of `""^(226)Ra`
`""_(85)^(226)Ra to ""_(86)^(222)Rn + 4/2He`
Oganesson has been synthetically produced by collision of `""_(98)^(249)Cf + ""_(20)^(48)Ca to ""_(118)^(294)Og + 3(1/0)n`
Oganesson has atomic mass of 294 and half life of 0.7 milliseconds. Its atomic number is 118.