It also has extremely high heat conductivity and expands on cooling. It is sometimes called a superfluid because it has extremely low viscosity. Liquid helium-II has a number of unusual properties. When liquid helium-I is cooled below about 2.18K (at atmospheric pressure), liquid helium-II is formed. When it is cooled below 4.22K (its boiling point at atmospheric pressure) it condenses to liquid helium-I, which behaves as an ordinary liquid. Helium-4 is unusual in that it forms two different kinds of liquids. The alpha particles that are emitted from certain radioactive substances are identical to helium-4 nuclei (two protons and two neutrons). The unstable isotopes helium-5, helium-6, and helium-8 have been synthesized. In helium obtained from natural gas about one atom in 10 million is helium-3. Natural helium is a mixture of two stable isotopes, helium-3 and helium-4. Extremely unreactive, it is an inert gas in Group 18 of the periodic table. Helium is less dense than any other known gas except hydrogen and is about one seventh as dense as air. Helium was isolated (1895) from a sample of the uranium mineral cleveite by Sir William Ramsay. ![]() A bright yellow emission line was observed and was later shown to correspond to no known element the new element was named by J. Spectroscopic evidence for the presence of helium in the sun was first obtained during a solar eclipse in 1868. ![]() −268.934℃ at 1 atmosphere pressure density 0.1785 grams per liter at STP valence usually 0. below −272℃ at 26 atmospheres pressure b.p. Helium (hēˈlēəm), gaseous chemical element symbol He at.
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