7.23.1 Occurrence
All these gases except radon and oganesson occur in the atmosphere. Their 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 a decay product of 226Ra.
Oganesson has been synthetically produced by collision of 
atoms and 
ions
Example 7.20
Why are the elements of Group 18 known as noble gases ?
Solution
The elements present in Group 18 have their valence shell orbitals completely filled and, therefore, react with a few elements only under certain conditions. Therefore, they are now known as noble gases.
Oganesson has its symbol Og, atomic number 118, atomic mass 294 and electronic configuration [Rn] 5f146d107s27p6. Only very small amount of Og has been produced. Its half life is 0.7 milliseconds. Therefore, mainly predictions about its chemistry have been made.
Here, except for oganesson, important atomic and physical properties of other elements of Group 18 along with their electronic configurations are given in Table 7.12. The trends in some of the atomic, physical and chemical properties of the group are discussed here.
Table 7.12: Atomic and Physical Properties of Group 18 Elements
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