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[Gr. hēlios, the sun + -ium (1)]
SYMB: He A chemical element, one of the noble gases, atomic weight (mass) 4.0026, atomic number 2. A liter of the gas at sea level pressure and 0°C weighs 0.1785 g. The second lightest element known, it is given off by radium and other radioactive elements in the form of charged helium ions known as alpha rays. Because of its low density, it is mixed with air or oxygen and used in the treatment of various respiratory disorders. Because of its low solubility, it is mixed with air supplied to workers laboring under high atmospheric pressure, as in caissons. When so used, it reduces the time required to adjust to increasing or decreasing air pressure and reduces the danger of bends.
SEE: noble gas
CAS # 7440-59-7

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