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Helium (He) is a colorless, odorless, tasteless, non-toxic, nearly Inert Monatomic Chemical Element that heads the Noble Gas series in the Periodic Table and whose Atomic Number is 2. Its Boiling and Melting points are the lowest among the elements and it exists only as a Gas except in extreme conditions. Extreme conditions are also needed to create the small handful of helium Compound s, which are all unstable at Standard Temperature And Pressure . It has a second, rare, Stable Isotope which is called Helium-3 . The behavior of liquid Helium-4 's two fluid phases, 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 ). Helium is the second most Abundant and second lightest element in the Universe and was one of the elements created in the Big Bang . In the modern universe almost all new helium is created as a result of the Nuclear Fusion of hydrogen in Star s. On Earth it is created by the Radioactive Decay of much heavier elements ( Alpha Particle s are helium nuclei). 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 . Since then large reserves of helium have been found in the Natural Gas Field s of the United States , which is by far the largest supplier of the gas. It is used in Cryogenics , in deep-sea breathing systems, to cool Superconducting Magnet s, in Helium Dating , for inflating Balloon s, for providing lift in Airship s and as a protective gas for many industrial uses (such as Arc Welding and growing Silicon wafers). A much less serious use is to temporarily change the timbre and quality of one's voice by inhaling a small Volume of the gas (see Precautions Section below). NOTABLE CHARACTERISTICS Gas and plasma phases Helium is the least reactive member of the Noble Gas elements, and thus also the least reactive of all elements; it is Inert and Monatomic in virtually all conditions. Due to helium's relatively low molar (molecular) mass, in the gas phase it has a Thermal Conductivity , Specific Heat , and Sound Conduction Velocity that are all greater than any gas, except Hydrogen . For similar reasons, and also due to the small size of its molecules, helium's Diffusion rate through Solid s is three times that of air and around 65% that of hydrogen.''The Encyclopedia of the Chemical Elements'', edited by Cifford A. Hampel, "Helium" entry by L. W. Brandt (New York; Reinhold Book Corporation; 1968; page 261) Library of Congress Catalog Card Number: 68-29938 Helium is less water Soluble than any other gas known, and helium's Index Of Refraction is closer to unity than that of any other gas. Helium has a negative Joule-Thomson Coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand. Only below its Joule-Thomson Inversion Temperature (of about 40 K at 1 atmosphere) does it cool upon free expansion. Once precooled below this temperature, helium can be liquefied through expansion cooling. Helium is chemically unreactive under all normal conditions due to its Valence of zero. It is an electrical insulator unless Ion ized. As with the other noble gases, helium has metastable Energy Level s that allow it to remain ionized in an Electrical discharge with a Voltage below its Ionization Potential . Helium can form unstable Compound s with Tungsten , Iodine , Fluorine , Sulfur and Phosphorus when it is subjected to an Electric Glow Discharge , through electron bombardment or is otherwise a Plasma . HeNe, HgHe10, WHe2 and the molecular ions He2+, He2++, HeH+ , and HeD+ have been created this way. This technique has also allowed the production of the neutral molecule He2, which has a large number of Band System s, and HgHe, which is apparently only held together by polarization forces. Theoretically, other compounds, like helium fluorohydride (HHeF), may also be possible. Helium has been put inside the hollow carbon cage molecules (the fullerenes) by heating under high pressure of the gas. The neutral molecules formed are stable up to high temperatures. When chemical derivatives of these fullerenes are formed, the helium stays inside. If Helium-3 is used, it can be readily observed by helium NMR spectroscopy. Many fullerenes containing helium-3 have been reported. These substances fit the definition of compounds in the Handbook of Chemistry and Physics. They are the first stable neutral helium compounds to be formed. Throughout the universe, helium is found mostly in a Plasma state whose properties are quite different from atomic helium. In a plasma, helium's electrons and protons are not bound together, resulting in very high electrical conductivity, even when the gas is only partially ionized. The charged particles are highly influenced by magnetic and electric fields. For example, in the Solar Wind together with ionized hydrogen, they interact with the Earth's Magnetosphere giving rise to Birkeland Current s and the Aurora . Solid and liquid phases Helium solidifies only under great pressure. The resulting colorless, almost invisible of the two phases are nearly the same. The solid has a sharp Melting Point and has a Crystal line structure. Solid helium has a density of 0.214 ±0.006 g/ml (1.15 K, 66 atm) with a mean isothermal compressibility of the solid at 1.15 K between the solidus and 66 atm of 0.0031 ±0.0008/atm. Also, no difference in density was noted between 1.8 K and 1.5 K. This data projects that ''T''=0 solid helium under 25 bar of pressure (the minimum required to freeze helium) has a density of 0.187 ±0.009 g/ml.''Structure of Solid Helium by Neutron Diffraction'', D. G. Henshaw, Physical Review Letters 109, Pg. 328 – 330 (Issue 2 – January 1958) Helium I state Below its Boiling Point of 4.22 Kelvin and above the Lambda Point of 2.1768 kelvin, the Isotope helium-4 exists in a normal colorless Liquid state, called ''helium I''. Like other Cryogenic liquids, helium I boils when it is heated. It also contracts when its temperature is lowered until it reaches the Lambda Point , when it stops boiling and suddenly expands. The rate of expansion decreases below the lambda point until about 1 K is reached; at which point expansion completely stops and helium I starts to contract again. Helium I has a gas-like Index Of Refraction of 1.026 which makes its surface so hard to see that floats of Styrofoam are often used to show where the surface is.''The Encyclopedia of the Chemical Elements'', page 262 This colorless liquid has a very low Viscosity and a Density 1/8th that of Water , which is only 1/4th the value expected from Classical Physics . Quantum Mechanics is needed to explain this property and thus both types of liquid helium are called ''quantum fluids'', meaning they display atomic properties on a macroscopic scale. This is probably due to its boiling point being so close to absolute zero, which prevents random molecular motion ( Heat ) from masking the atomic properties. Helium II state Liquid helium below its lambda point begins to exhibit very unusual characteristics, in a state called ''helium II''. Boiling of helium II is not possible due to its high Thermal Conductivity ; heat input instead causes Evaporation of the liquid directly to gas. The isotope helium-3 also has a superfluid phase, but only at much lower temperatures; as a result, less is known about such properties in the isotope helium-3. also covers the interior of the larger container; if it were not sealed, the helium II would creep out and escape]] Helium II is a Superfluid , a quantum-mechanical state of matter with strange properties. For example, when it flows through even capillaries of 10-7 to 10-8 m width it has no measurable Viscosity . However, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed. Current theory explains this using the ''two-fluid model'' for helium II. In this model, liquid helium below the lambda point is viewed as containing a proportion of helium atoms in a Ground State , which are superfluid and flow with exactly zero viscosity, and a proportion of helium atoms in an excited state, which behave more like an ordinary fluid.Dr. Sidney Yuan, The Two Fluid Model of Superfluid Helium (He II, Superfluidity) , Retrieved 5 January 2007 Helium II also exhibits a "creeping" effect. When a surface extends past the level of helium II, the helium II moves along the surface, seemingly against the force of ''. In the ''fountain effect'', a chamber is constructed which is connected to a reservoir of helium II by a Sintered disc through which superfluid helium leaks easily but through which non-superfluid helium cannot pass. If the interior of the container is heated, the superfluid helium changes to non-superfluid helium. In order to maintain the equilibrium fraction of superfluid helium, superfluid helium leaks through and increases the pressure, causing liquid to fountain out of the container. Liquid Helium , cryowwwebber.gsfc.nasa.gov, Retrieved 5 January 2007 The thermal conductivity of helium II is greater than that of any other known substance, a million times that of helium I and several hundred times that of Copper . This is because heat conduction occurs by an exceptional quantum-mechanical mechanism. Most materials that conduct heat well have a Valence Band of free electrons which serve to transfer the heat. Helium II has no such valence band but nevertheless conducts heat well. The Flow Of Heat is governed by Equation s that are similar to the Wave Equation used to characterize Sound propagation in air. So when heat is introduced, it will move at 20 meters per second at 1.8 K through helium II as waves in a phenomenon called '' Second Sound ''. APPLICATIONS s such as the Goodyear Blimp , as opposed to Hydrogen ]] Helium is used for many purposes that require some of its unique properties, such as its low Boiling Point , low Density , low Solubility , high Thermal Conductivity , or Inert ness. Pressurized helium is commercially available in large quantities.
HISTORY Scientific discoveries Evidence of helium was first detected on August 18 , 1868 as a bright yellow line with a Wavelength of 587.49 nanometres in the Spectrum of the Chromosphere of the Sun , by French astronomer Pierre Janssen during a total Solar Eclipse in Guntur , India . This line was initially assumed to be Sodium . On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line in the solar spectrum, which he named the D3 Line , for it was near the known D1 and D2 lines of sodium,''The Encyclopedia of the Chemical Elements'', page 256 and concluded that it was caused by an element in the Sun unknown on Earth. He and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος (''helios'')''Oxford English Dictionary'' (1989), s.v. "helium". Retrieved December 16, 2006, from Oxford English Dictionary Online. Also, from quotation there: Thomson, W. (1872). ''Rep. Brit. Assoc.'' xcix: "Frankland and Lockyer find the yellow prominences to give a very decided bright line not far from D, but hitherto not identified with any terrestrial flame. It seems to indicate a new substance, which they propose to call Helium." |
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