Group 18 consists of elements: helium, neon, argon, krypton, xenon, radon and oganesson. All these are gases and chemically unreactive. They form very few compounds, because of this they are termed as noble gases. All noble gases have general electronic configuration ns²np⁶ except helium which has 1s². Many of the properties of noble gases including their inactive nature are ascribed to their closed shell structures. Due to stable electronic configuration these gases exhibit very high ionization enthalpy. However, it decreases down the group with increase in atomic size. Atomic radii increase down the group with increase in atomic number. Since noble gases have stable electronic configurations, they have no tendency to accept the electron and therefore, have large positive values of electron gain enthalpy.

Which of the following noble gases do not occur in the elemental state in the atmosphere?

Radon

Radon is the last-named member of the 18^th group of elements in the periodic table. It is radioactive in natureand as a result, all other noble gases except radon occur in the elemental state in the atmosphere.