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Helium (play /ˈhliəm/ HEE-lee-əm) is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditions. Next to hydrogen, it is the second most abundant element in the universe, and accounts for 24% of the elemental mass of our galaxy.

An unknown yellow spectral line signature in sunlight was first observed from a solar eclipse in 1868 by French astronomer Pierre Janssen. Janssen is jointly credited with the discovery of the element with Norman Lockyer, who observed the same eclipse and was the first to propose that the line was due to a new element which he named helium. In 1903, large reserves of helium were found in the natural gas fields in parts of the United States, which is by far the largest supplier of the gas.

Helium is used in cryogenics (its largest single use, accounting for about a quarter of production), and the cooling of superconducting magnets, particularly the main commercial application in MRI scanners. Helium's other industrial uses as a pressurizing and purge gas, and a protective atmosphere for arc welding and processes (such as growing crystals to make silicon wafers), account for half of its use. Economically minor uses, such as lifting gas in balloons and airships are popularly known.[2] As with any gas with differing density from air, inhaling a small volume of helium temporarily changes the timbre and quality of the human voice. In scientific research, the behavior of two fluid phases of helium-4, helium I and helium II, is important to researchers studying quantum mechanics (in particular the phenomenon of superfluidity) and to those looking at the effects that temperatures near absolute zero have on matter (such as superconductivity).

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