Chemical element , chemical symbol He, atomic number 2.
A noble gas , it is colourless, odourless, tasteless, completely unreactive, and nontoxic. First found by spectroscopy of the Sun's atmosphere in 1868, it is the second most abundant and second-lightest element in the universe (after hydrogen). Helium makes up a tiny proportion of the atmosphere but as much as 7% of natural gas . It is the product of radioactive decay (see radioactivity ) and is used in helium dating . It is used as an inert gas in welding, rocket propulsion, balloon flight, hyperbaric chamber s, deep-sea diving (see nitrogen narcosis ), cryogenics . Liquid helium, which exists only below -452 kF (-268.9 kC, about 4k C above absolute zero ), is a "quantum fluid" (see fluid mechanics ; quantum mechanics ), with unique properties, including superfluidity , superconductivity , and near-zero viscosity .