Helium is the second lightest element (Hydrogen is the lightest). It is one-seventh as heavy as air.
Helium is one of the rare gases of the atmosphere, in which it is present in concentration of only 5 ppm). Helium is chemically inert. It has no color, odor, or taste. It is also known nuclear reactor coolant that does not become radioactive.
Helium is nonflammable and has lower solubility. Helium is used as an inert gas shield in arc welding, as a lifting gas for lighter-than-air aircraft, as a gaseous cooling medium in nuclear reactors, as protective atmosphere for growing germanium and silicon crystals for transistors, as a protective atmosphere in the production of such reactive metals as titanium and zirconium, as gas filled for cold-weather fluorescent lamps, to trace leaks in refrigeration systems and other closed systems, and to fill neutron and gas thermometers.
It is used in cryogenic research as superconductivity. In mixtures with oxygen, it has medical applications. Radioactive mixtures of helium with krypton are available to users license by Nuclear Regulatory Commission. Liquid helium is also gaining wide uses for cooling superconductive magnets used in Magnetic Resonance Imaging (MRI).