helium
 氦


 he·li·um /ˈhɪlɪəm/ 名詞
 氦


 He·li·um n.  Chem. An inert, monoatomic, gaseous element occurring in the atmosphere of the sun and stars, and in small quantities in the earth's atmosphere, in several minerals and in certain mineral waters. It is obtained from natural gas in industrial quantities.  Symbol, He; atomic number 2; at. wt., 4.0026 (C=12.011). Helium was first detected spectroscopically in the sun by Lockyer in 1868; it was first prepared by Ramsay in 1895. Helium has a density of 1.98 compared with hydrogen, and is more difficult to liquefy than the latter. Chemically, it is an inert noble gas, belonging to the argon group, and cannot be made to form compounds. The helium nucleus is the charged particle which constitutes alpha rays, and helium is therefore formed as a decomposition product of certain radioactive substances such as radium.  The normal helium nucleus has two protons and two neutrons, but an isotope with only one neutron is also observed in atmospheric helium at an abundance of 0.013 %.  Liquid helium has a boiling point of -268.9° C at atmospheric pressure, and is used for maintaining very low temperatures, both in laboratory experimentation and in commercial applications to maintain superconductivity in low-temperature superconducting devices.  Gaseous helium at normal temperatures is used for buoyancy in blimps, dirigibles, and high-altitude balloons, and also for amusement in party balloons.

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 helium
      n : a very light colorless element that is one of the six inert
          gasses; the most difficult gas to liquefy; occurs in
          economically extractable amounts in certain natural gases
          (as those found in Texas and Kansas) [syn: He, {atomic
          number 2]


 helium
 Symbol: He
 Atomic number: 2
 Atomic weight: 4.0026
 Colourless, odourless gaseous nonmetallic element. Belongs to group 18 of
 the periodic table. Lowest boiling point of all elements and can only be
 solidified under pressure. Chemically inert, no known compounds.
 Discovered in the solar spectrum in 1868 by Lockyer.