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Helium is a chemical element; its atomic symbol is He. It is a colorless, odorless, tasteless, non-toxic, and nearly inert monatomic that heads the noble gas series in the periodic table. Its atomic number is 2 and its boiling and melting points are the lowest among the elements. It exists only as a gas except in extreme conditions. Extreme conditions are also needed to create the small handful of helium compounds, which are all unstable at standard temperature and pressure. Its most abundant stable isotope is helium-4 and it has a rare stable isotope, helium-3. The behavior of liquid helium-4's two different states—helium I and helium II—is important to researchers studying quantum mechanics (in particular the phenomenon of superfluidity) and those looking at the effects that near absolute zero temperatures have on matter (such as superconductivity).
Helium is the second most abundant element in the known Universe and second lightest element in the periodic table. In the modern Universe almost all new helium is created as a result of the nuclear fusion of hydrogen in stars. On Earth it is created by the radioactive decay of much heavier elements (alpha particles are helium-4 nuclei produced by alpha-decay). After its creation, part of it is trapped with natural gas in concentrations up to 7% by volume. It is extracted from the natural gas by a low temperature separation process called fractional distillation.
In 1868 the French astronomer Pierre Janssen first detected helium as an unknown yellow spectral line signature in light from a solar eclipse. (The word helium comes from ancient Greek ἥλιος which is, surprisingly, cognate with the English sun.) Since then large reserves of helium have been found in the natural gas fields of the United States, which is by far the largest supplier of the gas. Helium is used in cryogenics, in deep-sea breathing systems, to cool superconducting magnets, in helium dating, for inflating balloons, for providing lift in airships and as a protective gas for many industrial uses (such as arc welding and growing silicon wafers). Inhaling a small volume of the gas temporarily changes the quality of one's voice.
