Helium is the first member of group 18 or zero of the periodic table. It consists of six elements helium (He), Neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). Zero group occupies the intermediate position between the elements of VIIA (17th) and IA (1st) groups. These are collectively called as inactive gases or inert gases. However, these are now called noble gases as some compounds of these gases have been obtained under certain specific conditions.

(1) Electronic configuration

(2) Occurrence : Due to the inert nature of noble gases, they always occur in the free state. Except radon, all these gases are present in atmosphere in the atomic state.

He is also present in natural gas to the extent of 2 to 7%.

(3) Isolation

(i) Helium : It is commercially obtained from natural gas. The natural gas contains hydrocarbons (methane etc.), CO₂, H₂S and He as the main constituents.

The natural gas is compressed to about 100 atm and cooled to 73K. He remains unliquefied while other gases get liquefied. About 99% pure He is prepared by this method.

(ii) Argon, Neon, Krypton and Xenon : These gases are prepared by the fractionation distillation of liquid air. Fractional distillation of air gives O₂, N₂ and mixture of noble gases. The individual gases may be obtained by adsorption of air on coconut charcoal. The charcoal adsorbs different gases at different temperatures and can be collected.

(iii) Radon : It can be obtained by radioactive disintegration of radium (226), ₈₈Ra²²⁶ ₈₆Rn²²²+ .