Helium

otheruses

Helium (He) is a colorless, odorless, tasteless, non-toxic, inert monatomic chemical element that heads the noble gas group 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.

An unknown yellow spectral line signature in light was first observed from a solar eclipse in 1868 by French astronomer Pierre Janssen who is jointly credited with the discovery of the element with Norman Lockyer who observed the same eclipse and was the first to propose this was a new element which he named helium. In 1903, large reserves of helium were found in the natural gas fields of the United States, which is by far the largest supplier of the gas. The substance is used in cryogenics, in deep-sea breathing systems, to cool superconducting magnets, in helium dating, for inflating balloons, for providing lift in airships and as a protective gas for many industrial uses (such as arc welding and growing silicon wafers). Inhaling a small volume of the gas temporarily changes the timbre and quality of the human voice. 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 lightest element and is the second most abundant in the observable Universe. Most helium was formed during the Big Bang, but new helium is being created as a result of the nuclear fusion of hydrogen in stars. On Earth, helium is relatively rare and is created by the natural radioactive decay of some elements, as alpha particles that are emitted consist of helium nuclei. This radiogenic helium is trapped with natural gas in concentrations up to seven percent by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation.

History

cientific discoveries

The first evidence of helium was observed on August 18, 1868 as a bright yellow line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the Sun. The line was detected by French astronomer Pierre Janssen during a total solar eclipse in Guntur, India.cite journal | title = French astronomers in India during the 17th - 19th centuries |journal = Journal of the British Astronomical Association | volume =101 | issue = 2 | pages = pp. 95–100 | url = http://articles.adsabs.harvard.edu//full/1991JBAA..101...95K/0000100.000.html | author = Kochhar, R. K. |accessdate=2008-07-27|year=1991] 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 Fraunhofer line because it was near the known D₁ and D₂ lines of sodium. ["The Encyclopedia of the Chemical Elements", p. 256] He concluded that it was caused by an element in the Sun unknown on Earth. Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος ("helios"). [cite web| title=Helium | publisher = Oxford English Dictionary| year = 2008| url = http://dictionary.oed.com/cgi/entry/50104457?| accessdate = 2008-07-20] [cite book | author=Thomson, W.|year=1872|publisher=Rep. Brit. Assoc. xcix|title=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.]

On March 26, 1895 British chemist Sir William Ramsay isolated helium on Earth by treating the mineral cleveite (a variety of uraninite with at least 10% rare earth elements) with mineral acids. Ramsay was looking for argon but, after separating nitrogen and oxygen from the gas liberated by sulfuric acid, he noticed a bright yellow line that matched the D₃ line observed in the spectrum of the Sun."The Encyclopedia of the Chemical Elements", p. 257] [cite journal | title = On a Gas Showing the Spectrum of Helium, the Reputed Cause of D3 , One of the Lines in the Coronal Spectrum. Preliminary Note | author = Ramsay, William | journal = Proceedings of the Royal Society of London | volume = 58 | pages = pp. 65–67 | year = 1895 | doi = 10.1098/rspl.1895.0006] [cite journal | title = Helium, a Gaseous Constituent of Certain Minerals. Part I | author = Ramsay, William | journal = Proceedings of the Royal Society of London | volume = 58 | pages = pp. 80–89 | year = 1895 | doi = 10.1098/rspl.1895.0010] [cite journal | title = Helium, a Gaseous Constituent of Certain Minerals. Part II-- | author = Ramsay, William | journal = Proceedings of the Royal Society of London | volume = 59 | issue = | pages = pp. 325–330 | year = 1895 | doi = 10.1098/rspl.1895.0097] These samples were identified as helium by Lockyer and British physicist William Crookes. It was independently isolated from cleveite the same year by chemists Per Teodor Cleve and Abraham Langlet in Uppsala, Sweden, who collected enough of the gas to accurately determine its atomic weight. [de icon cite journal | title = Das Atomgewicht des Heliums | author = Langlet, N. A. | journal = Zeitschrift für anorganische Chemie | volume = 10 | issue = 1| pages = pp. 289–292 | year = 1895 | doi =10.1002/zaac.18950100130 | language= German] [cite book| chapter= Bibliography of Helium Literature | author =Weaver, E.R.| title=Industrial & Engineering Chemistry|year=1919] Helium was also isolated by the American geochemist William Francis Hillebrand prior to Ramsay's discovery when he noticed unusual spectral lines while testing a sample of the mineral uraninite. Hillebrand, however, attributed the lines to nitrogen. His letter of congratulations to Ramsay offers an interesting case of discovery and near-discovery in science. [cite book|author=Munday, Pat|year=1999|title=Biographical entry for W.F. Hillebrand (1853–1925), geochemist and US Bureau of Standards administrator in American National Biography|editor=John A. Garraty and Mark C. Carnes|volume=10-11|publisher=Oxford University Press|pages= pp. 808–9; pp. 227-8 ]

In 1907, Ernest Rutherford and Thomas Royds demonstrated that alpha particles are helium nuclei by allowing the particles to penetrate the thin glass wall of an evacuated tube, then creating a discharge in the tube to study the spectra of the new gas inside. In 1908, helium was first liquefied by Dutch physicist Heike Kamerlingh Onnes by cooling the gas to less than one kelvin. [cite journal |title = Little cup of Helium, big Science |author = van Delft, Dirk |journal = Physics today |url = http://www-lorentz.leidenuniv.nl/history/cold/VanDelftHKO_PT.pdf |format=PDF|pages = pp. 36–42 |year = 2008 |accessdate = 2008-07-20] He tried to solidify it by further reducing the temperature but failed because helium does not have a triple point temperature at which the solid, liquid, and gas phases are at equilibrium. Onnes' student Willem Hendrik Keesom was eventually able to solidify 1 cm³ of helium in 1926. [cite news | title = Coldest Cold| publisher = Time Inc.| date = 1929-06-10| url = http://www.time.com/time/magazine/article/0,9171,751945,00.html| accessdate = 2008-07-27]

In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that helium-4 has almost no viscosity at temperatures near absolute zero, a phenomenon now called superfluidity. [cite journal |title = Viscosity of Liquid Helium below the λ-Point |author = Kapitza, P. |journal = Nature |volume = 141 |pages = p. 74 |doi = 10.1038/141074a0 |year = 1938] This phenomenon is related to Bose-Einstein condensation. In 1972, the same phenomenon was observed in helium-3, but at temperatures much closer to absolute zero, by American physicists Douglas D. Osheroff, David M. Lee, and Robert C. Richardson. The phenomenon in helium-3 is thought to be related to pairing of helium-3 fermions to make bosons, in analogy to Cooper pairs of electrons producing superconductivity. [cite journal |title = Evidence for a New Phase of Solid He³ |author = Osheroff, D. D. |coauthors = R. C. Richardson, D. M. Lee |journal = Phys. Rev. Lett. |volume = 28 |issue = 14 |pages = pp. 885–888 |doi = 10.1103/PhysRevLett.28.885 |year = 1972]

Extraction and use

After an oil drilling operation in 1903 in Dexter, Kansas produced a gas geyser that would not burn, Kansas state geologist Erasmus Haworth collected samples of the escaping gas and took them back to the University of Kansas at Lawrence where, with the help of chemists Hamilton Cady and David McFarland, he discovered that the gas consisted of, by volume, 72% nitrogen, 15% methane (insufficient to make the gas combustible), 1% hydrogen, and 12% an unidentifiable gas. [cite journal |author = McFarland, D. F. |title = Composition of Gas from a Well at Dexter, Kan |volume = 19|issue = |pages = pp. 60–62 |url = http://www.jstor.org/stable/3624173 |year = 1903 |accessdate=2008-07-22 |journal = Transactions of the Kansas Academy of Science |doi = 10.2307/3624173 ] With further analysis, Cady and McFarland discovered that 1.84% of the gas sample was helium. [cite web|publisher=American Chemical Society|year=2004|url=http://acswebcontent.acs.org/landmarks/landmarks/helium/helium.html|title=The Discovery of Helium in Natural Gas|accessdate=2008-07-20] [cite journal |author = Cady, H.P. |coauthors = D. F. McFarland |title = Helium in Natural Gas |journal = Science |volume = 14 |issue = |pages = p. 344 |doi = 10.1126/science.24.611.344 |year = 1906 |pmid = 17772798] This showed that despite its overall rarity on Earth, helium was concentrated in large quantities under the American Great Plains, available for extraction from natural gas. [cite journal |author = Cady, H.P. |coauthors = D. F. McFarland |title = Helium in Kansas Natural Gas |journal = Transactions of the Kansas Academy of Science |volume = 20 |issue = |pages = pp. 80–81 |url = http://mc1litvip.jstor.org/stable/3624645 |year = 1906|accessdate=2008-07-20 |doi = 10.2307/3624645 ]

This put the United States in an excellent position to become the world's leading supplier of helium. Following a suggestion by Sir Richard Threlfall, the United States Navy sponsored three small experimental helium production plants during World War I. The goal was to supply barrage balloons with the non-flammable, lighter-than-air gas. A total of 200 thousand cubic feet (5,700 m³) of 92% helium was produced in the program even though only a few cubic feet (less than 100 liters) of the gas had previously been obtained. Some of this gas was used in the world's first helium-filled airship, the U.S. Navy's C-7, which flew its maiden voyage from Hampton Roads, Virginia to Bolling Field in Washington, D.C. on December 1, 1921. [cite book |editor=Emme, Eugene M. comp. |title=Aeronautics and Astronautics: An American Chronology of Science and Technology in the Exploration of Space, 1915–1960 |year=1961 |pages=pp. 11–19 |chapter=Aeronautics and Astronautics Chronology, 1920–1924 |chapterurl=http://www.hq.nasa.gov/office/pao/History/Timeline/1920-24.html |publisher=NASA |location=Washington, D.C. |accessdate=2008-07-20 ]

Although the extraction process, using low-temperature gas liquefaction, was not developed in time to be significant during World War I, production continued. Helium was primarily used as a lifting gas in lighter-than-air craft. This use increased demand during World War II, as well as demands for shielded arc welding. The helium mass spectrometer was also vital in the atomic bomb Manhattan Project. [cite book|chapter=Leak Detection|author=Hilleret, N.|publisher=CERN|title=CERN Accelerator School, vacuum technology: proceedings: Scanticon Conference Centre, Snekersten, Denmark, 28 May – 3 June 1999 |editor=S. Turner |location=Geneva, Switzerland|url=http://doc.cern.ch/yellowrep/1999/99-05/p203.pdf |format=PDF| year=1999 |pages=pp. 203–212 |quote=At the origin of the helium leak detection method was the Manhattan Project and the unprecedented leak-tightness requirements needed by the uranium enrichment plants. The required sensitivity needed for the leak checking led to the choice of a mass spectrometer designed by Dr. A.O.C. Nier tuned on the helium mass.]

The government of the United States set up the National Helium Reserve in 1925 at Amarillo, Texas with the goal of supplying military airships in time of war and commercial airships in peacetime. Due to a US military embargo against Germany that restricted helium supplies, the Hindenburg was forced to use hydrogen as the lift gas. Helium use following World War II was depressed but the reserve was expanded in the 1950s to ensure a supply of liquid helium as a coolant to create oxygen/hydrogen rocket fuel (among other uses) during the Space Race and Cold War. Helium use in the United States in 1965 was more than eight times the peak wartime consumption. [cite journal| author = Williamson, John G.| title = Energy for Kansas| journal = Transactions of the Kansas Academy of Science| volume = 71| issue = 4| pages = 432–438| publisher = Kansas Academy of Science| location = | date = Winter 1968| url = http://www.jstor.org/pss/3627447| accessdate = 2008-07-27]

After the "Helium Acts Amendments of 1960" (Public Law 86–777), the U.S. Bureau of Mines arranged for five private plants to recover helium from natural gas. For this "helium conservation" program, the Bureau built a 425 mile (684 km) pipeline from Bushton, Kansas to connect those plants with the government's partially depleted Cliffside gas field, near Amarillo, Texas. This helium-nitrogen mixture was injected and stored in the Cliffside gas field until needed, when it then was further purified. [cite journal | journal = Federal Register | date = 2005-10-06 | volume = 70 | issue = 193 | pages = p. 58464 | url = http://edocket.access.gpo.gov/2005/pdf/05-20084.pdf | format=PDF| title = Conservation Helium Sale |accessdate=2008-07-20]

By 1995, a billion cubic meters of the gas had been collected and the reserve was US$1.4 billion in debt, prompting the Congress of the United States in 1996 to phase out the reserve.Stwertka, Albert (1998). "Guide to the Elements: Revised Edition". New York; Oxford University Press, p. 24. ISBN 0-19-512708-0] The resulting "Helium Privatization Act of 1996" [Helium Privatization Act of 1996 USPL|104|273] (Public Law 104–273) directed the United States Department of the Interior to start liquidating the reserve by 2005. [cite web| url = http://www.nap.edu/openbook/0309070384/html/index.html | title = Executive Summary |publisher = nap.edu |accessdate=2008-07-20 ]

Helium produced between 1930 and 1945 was about 98.3% pure (2% nitrogen), which was adequate for airships. In 1945, a small amount of 99.9% helium was produced for welding use. By 1949, commercial quantities of Grade A 99.95% helium were available. [cite book|publisher=Bureau of Mines / Minerals yearbook 1949|year=1951|author=Mullins, P.V. |coauthors = R. M. Goodling| title = Helium | pages = pp. 599–602 |url = http://digicoll.library.wisc.edu/cgi-bin/EcoNatRes/EcoNatRes-idx?type=div&did=ECONATRES.MINYB1949.PVMULLINS&isize=text|accessdate=2008-07-20]

For many years the United States produced over 90% of commercially usable helium in the world, while extraction plants in Canada, Poland, Russia, and other nations produced the remainder. In the mid-1990s, a new plant in Arzew, Algeria producing 600 million cubic feet (1.7e|7 m³) began operation, with enough production to cover all of Europe's demand. Meanwhile, by 2000, the consumption of helium within the US had risen to above 15,000 metric tons. [cite web|url=http://minerals.usgs.gov/ds/2005/140/helium-use.pdf|format=PDF| title= Helium End User Statistic | accessdate = 2008-07-12 | publisher = U.S. Geological Survey|accessdate=2008-07-20] In 2004–2006, two additional plants, one in Ras Laffen, Qatar and the other in Skikda, Algeria were built, but as of early 2007, Ras Laffen is functioning at 50%, and Skikda has yet to start up. Algeria quickly became the second leading producer of helium.cite journal
title=Challenges to the Worldwide Supply of Helium in the Next Decade |author=Smith, E.M.
coauthors=T.W. Goodwin, J. Schillinger |journal=Advances in Cryogenic Engineering |volume=49 A |issue=710 |pages=119–138
year=2003 |doi=10.1063/1.1774674 |format=PDF |accessdate=2008-07-20
url=https://www.airproducts.com/NR/rdonlyres/E44F8293-1CEE-4D80-86EA-F9815927BE7E/0/ChallengestoHeliumSupply111003.pdf] Through this time, both helium consumption and the costs of producing helium increased.Citation
last=Kaplan |first=Karen H. |date=June 2007 |title=Helium shortage hampers research and industry
periodical=Physics Today |publisher=American Institute of Physics
volume=60 |issue=6 |pages=31–32 |accessdate=2008-07-20
url=http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_60/iss_6/31_1.shtml] In the 2002 to 2007 period helium prices doubled,Citation
last=Basu |first=Sourish |editor-last=Yam |editor-first=Philip
date=October 2007 |title=Updates: Into Thin Air |accessdate=2008-08-04
periodical=Scientific American |publisher=Scientific American, Inc. |volume=297 |issue=4 |pages=18
url=http://www.sciamdigital.com/index.cfm?fa=Products.ViewIssuePreview&ARTICLEID_CHAR=E0D18FB2-3048-8A5E-104115527CB01ADB] and during 2008 alone the major suppliers raised prices about 50%.Fact|date=August 2008

Characteristics

Gas and plasma phases

Helium is the least reactive noble gas after neon and thus the second least reactive of all elements; it is inert and monatomic in all standard conditions. Due to helium's relatively low molar (atomic) mass, in the gas phase its thermal conductivity, specific heat, and sound speed are all greater than any other gas except hydrogen. For similar reasons, and also due to the small size of helium atoms, helium's diffusion rate through solids is three times that of air and around 65% that of hydrogen."The Encyclopedia of the Chemical Elements", p. 261]

Helium is less water soluble than any other gas known, [cite journal|title = Solubility of helium and neon in water and seawater | author = Weiss, Ray F.| year = 1971| journal = J. Chem. Eng. Data | volume = 16 | issue = 2 | pages = pp. 235–241 |doi = 10.1021/je60049a019 ] and helium's index of refraction is closer to unity than that of any other gas. [cite journal|title = Using helium as a standard of refractive | author = Stone, Jack A. |coauthors = Alois Stejskal| year = 2004| journal = Metrologia | volume = 41 | pages = pp. 189–197 |doi =10.1088/0026-1394/41/3/012] 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 32 to 50 K at 1 atmosphere) does it cool upon free expansion. Once precooled below this temperature, helium can be liquefied through expansion cooling.

Most extraterrestrial helium is found in a plasma state, with properties quite different from those of atomic helium. In a plasma, helium's electrons are not bound to its nucleus, 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, the particles interact with the Earth's magnetosphere giving rise to Birkeland currents and the aurora. [cite journal|title = Helium isotopes in an aurora | author = Buhler, F. |coauthors = W. I. Axford, H. J. A. Chivers, K. Martin| year = 1976 | journal = J. Geophys. Res. | volume = 81 | issue = 1 | pages = pp. 111–115|doi = 10.1029/JA081i001p00111]

olid and liquid phases

Unlike any other element, helium will remain liquid down to absolute zero at normal pressures. This is a direct effect of quantum mechanics: specifically, the zero point energy of the system is too high to allow freezing. Solid helium requires a temperature of 1–1.5 K (about −272 °C or −457 °F) and about 25 bar (2.5 MPa) of pressure. [cite web |date = 2005-10-05 |url = http://www.phys.ualberta.ca/~therman/lowtemp/projects1.htm |title = Solid Helium |publisher = Department of Physics University of Alberta|accessdate=2008-07-20] It is often hard to distinguish solid from liquid helium since the refractive index of the two phases are nearly the same. The solid has a sharp melting point and has a crystalline structure, but it is highly compressible; applying pressure in a laboratory can decrease its volume by more than 30%.cite web |publisher = Los Alamos National Laboratory (LANL.gov): |title = Periodic Table: Helium |url = http://periodic.lanl.gov/elements/2.html |accessdate = 2008-07-23] With a bulk modulus on the order of 5×10⁷ Pa [cite journal |author = Malinowska-Adamska, C. |coauthors = P. Soma, J. Tomaszewski |title = Dynamic and thermodynamic properties of solid helium in the reduced all-neighbours approximation of the self-consistent phonon theory |journal = Physica status solidi (b) |volume = 240 |issue = 1 |pages = pp. 55–67 |doi = 10.1002/pssb.200301871 |year = 2003] it is 50 times more compressible than water. Solid helium has a density of 0.214 ± 0.006 g/ml at 1.15 K and 66 atm; the projected density at 0 K and 25 bar is 0.187 ± 0.009 g/ml. [cite journal |author = Henshaw, D. B. |title = Structure of Solid Helium by Neutron Diffraction |journal = Physical Review Letters |volume = 109 |issue = 2 |pages = pp. 328–330 |doi = 10.1103/PhysRev.109.328 |year = 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 and contracts when its temperature is lowered. Below the lambda point, however, helium doesn't boil, and it expands as the temperature is lowered further.

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", p. 262] This colorless liquid has a very low viscosity and a density one-eighth that of water, which is only one-fourth 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 may be an effect of its boiling point being so close to absolute zero, preventing random molecular motion (thermal energy) 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.

Helium II is a superfluid, a quantum-mechanical state of matter with strange properties. For example, when it flows through capillaries as thin as 10⁻⁷ to 10⁻⁸ m 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. [cite journal |doi = 10.1006/aphy.2000.6019 |title = Microscopic Theory of Superfluid Helium |journal = Annals of Physics |volume = 281 |issue = 1–2 |year = 2000 |month = October |pages = pp. 636–705 12091211 |author = Hohenberg, P. C.|coauthors = P. C. Martin]

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 gravity. Helium II will escape from a vessel that is not sealed by creeping along the sides until it reaches a warmer region where it evaporates. It moves in a 30 nm-thick film regardless of surface material. This film is called a Rollin film and is named after the man who first characterized this trait, Bernard V. Rollin."The Encyclopedia of the Chemical Elements", p. 263] [cite journal |doi = 10.1103/PhysRev.76.1209 |title = Rollin Film Rates in Liquid Helium |journal = Physical Review |volume = 76 |issue = 8 |pages = pp. 1209–1211 |month = October | year = 1949 |author = Fairbank, H. A. |coauthors = C. T. Lane] [cite journal |doi = 10.1016/S0031-8914(39)80013-1 |title = On the "film" phenomenon of liquid helium II |journal = Physica |volume = 6 |issue = 2 |year = 1949 |pages = pp. 219–230 |year = 1939 |author = Rollin, B. V. |coauthors = F. Simon] As a result of this creeping behavior and helium II's ability to leak rapidly through tiny openings, it is very difficult to confine liquid helium. Unless the container is carefully constructed, the helium II will creep along the surfaces and through valves until it reaches somewhere warmer, where it will evaporate. Waves propagating across a Rollin film are governed by the same equation as gravity waves in shallow water, but rather than gravity, the restoring force is the Van der Waals force. [cite web |author = Ellis, Fred M. |url = http://fellis.web.wesleyan.edu/research/thrdsnd.html |title = Third sound |publisher = Wesleyan Quantum Fluids Laboratory |month = September | year = 2005|accessdate = 2008-07-23] These waves are known as "third sound". [cite journal |doi = 10.1103/PhysRev.188.370 |title = Hydrodynamics and Third Sound in Thin He II Films |journal = Physical Review |volume = 188 |issue = 1 |year = 1969 |pages = pp. 370–384 |month = October | year = 1949 |author = Bergman, D.]

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. [cite web |author=Warner, Brent|url=http://cryowwwebber.gsfc.nasa.gov/introduction/liquid_helium.html |title=Introduction to Liquid Helium |publisher=NASA|accessdate=2007-01-05 |archiveurl=http://web.archive.org/web/20050901062951/http://cryowwwebber.gsfc.nasa.gov/introduction/liquid_helium.html |archivedate=2005-09-01]

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 equations that are similar to the wave equation used to characterize sound propagation in air. When heat is introduced, it moves at 20 meters per second at 1.8 K through helium II as waves in a phenomenon known as "second sound".

Isotopes

There are eight known isotopes of helium, but only helium-3 and helium-4 are stable. In the Earth's atmosphere, there is one He-3 atom for every million He-4 atoms.cite book| author = Emsley, John| title = Nature's Building Blocks| publisher = Oxford University Press| year = 2001| location = Oxford| pages = pp. 175–179| isbn = 0-19-850341-5 ] Unlike most elements, helium's isotopic abundance varies greatly by origin, due to the different formation processes. The most common isotope, helium-4, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis.

Helium-3 is present on Earth only in trace amounts; most of it since Earth's formation, though some falls to Earth trapped in cosmic dust.cite web |url = http://www.mantleplumes.org/HeliumFundamentals.html |title = Helium Fundamentals |author = Anderson, Don L. |coauthors = G. R. Foulger, Anders Meibom |date = 2006-09-02 |accessdate = 2008-07-20 |publisher = MantlePlumes.org] Trace amounts are also produced by the beta decay of tritium. [cite journal | title= Half-Life of Tritium| journal=Physical Review | volume= 72| issue= | year= 1947| pages= pp. 972–972 | url=http://link.aps.org/abstract/PR/v72/p972/s2 |accessdate=2008-07-20 |author= Novick, Aaron| doi=10.1103/PhysRev.72.972.2] Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten, and these ratios can be used to investigate the origin of rocks and the composition of the Earth's mantle. He-3 is much more abundant in stars, as a product of nuclear fusion. Thus in the interstellar medium, the proportion of He-3 to He-4 is around 100 times higher than on Earth. [cite journal | title=Isotopic Composition and Abundance of Interstellar Neutral Helium Based on Direct Measurements| journal=Astrophysics| volume=45| issue=2| month=April | year=2002| pages=pp. 131–142| url=http://www.ingentaconnect.com/content/klu/asys/2002/00000045/00000002/00378626 |accessdate=2008-07-20 |author=Zastenker G. N. |coauthors = E. Salerno, F. Buehler, P. Bochsler, M. Bassi, Y. N. Agafonov, N. A. Eismont, V. V. Khrapchenkov, H. Busemann| doi=10.1023/A:1016057812964] Extraplanetary material, such as lunar and asteroid regolith, have trace amounts of helium-3 from being bombarded by solar winds. The Moon's surface contains helium-3 at concentrations on the order of 0.01 ppm. [cite web |url = http://fti.neep.wisc.edu/Research/he3_pubs.html | title = Lunar Mining of Helium-3 |date = 2007-10-19| accessdate = 2008-07-09| publisher = Fusion Technology Institute of the University of Wisconsin-Madison] [ cite web | url= http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2175.pdf | format=PDF| title = The estimation of helium-3 probable reserves in lunar regolith| author= Slyuta, E. N. |coauthors = A. M. Abdrakhimov, E. M. Galimov| work= Lunar and Planetary Science XXXVIII| year=2007|accessdate=2008-07-20] A number of people, starting with Gerald Kulcinski in 1986, [cite news | url = http://www.thespacereview.com/article/536/1 | title = A fascinating hour with Gerald Kulcinski | author=Hedman, Eric R. | date = 2006-01-16 | work = The Space Review|accessdate=2008-07-20] have proposed to explore the moon, mine lunar regolith and use the helium-3 for fusion.

Liquid helium-4 can be cooled to about 1 kelvin using evaporative cooling in a 1-K pot. Similar cooling of helium-3, which has a lower boiling point, can achieve about 0.2 kelvin in a helium-3 refrigerator. Equal mixtures of liquid He-3 and He-4 below 0.8 K separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: helium-4 atoms are bosons while helium-3 atoms are fermions)."The Encyclopedia of the Chemical Elements", p. 264] Dilution refrigerators use this immiscibility to achieve temperatures of a few millikelvins.

It is possible to produce exotic helium isotopes, which rapidly decay into other substances. The shortest-lived heavy helium isotope is helium-5 with a half-life of 7.6×10⁻²² seconds. Helium-6 decays by emitting a beta particle and has a half life of 0.8 seconds. Helium-7 also emits a beta particle as well as a gamma ray. Helium-7 and helium-8 are created in certain nuclear reactions. ["The Encyclopedia of the Chemical Elements", p. 260] Helium-6 and helium-8 are known to exhibit a nuclear halo. Helium-2 (two protons, no neutrons) is a radioisotope that decays by proton emission into protium, with a half-life of 3x10⁻²⁷ seconds.

Compounds

Helium is chemically unreactive under all normal conditions due to its valence of zero. It is an electrical insulator unless ionized. As with the other noble gases, helium has metastable energy levels that allow it to remain ionized in an electrical discharge with a voltage below its ionization potential. Helium can form unstable compounds, known as excimers, 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⁺, He₂²⁺, HeH⁺, and HeD⁺ have been created this way. [cite journal |title = Massenspektrographische Untersuchungen an Wasserstoff- und Heliumkanalstrahlen (H₃⁺, H₂^-, HeH⁺, HeD⁺, He^-) |author = Hiby, Julius W. |journal = Annalen der Physik |volume = 426 |issue = 5 |pages = 473–487 |year = 1939 |doi = 10.1002/andp.19394260506 |accessdate=2008-07-20] This technique has also allowed the production of the neutral molecule He₂, which has a large number of band systems, and HgHe, which is apparently only held together by polarization forces. Theoretically, other compounds may also be possible, such as helium fluorohydride (HHeF) which would be analogous to HArF, discovered in 2000. [cite journal |title = Prediction of a Metastable Helium Compound: HHeF |author = Ming Wah Wong |journal = Journal of the American Chemical Society |volume = 122 |issue = 26 |pages = pp. 6289–6290 |year = 2000 |doi = 10.1021/ja9938175]

Helium has been put inside the hollow carbon cage molecules (the fullerenes) by heating under high pressure. The endohedral fullerene molecules formed are stable up to high temperatures. When chemical derivatives of these fullerenes are formed, the helium stays inside. [cite journal |title = Stable Compounds of Helium and Neon: He@C₆₀ and Ne@C₆₀ |author = Saunders, Martin Hugo |coauthors = A. Jiménez-Vázquez, R. James Cross, Robert J. Poreda |journal = Science |volume = 259 |issue = 5100 |pages = 1428–1430 |year = 1993 |doi = 10.1126/science.259.5100.1428 |pmid = 17801275|accessdate=2008-07-20] If helium-3 is used, it can be readily observed by helium nuclear magnetic resonance spectroscopy. [cite journal |title = Probing the interior of fullerenes by ³He NMR spectroscopy of endohedral ³He@C₆₀ and ³He@C₇₀ |author = Saunders, M.|coauthors = H. A. Jiménez-Vázquez, R. J. Cross, S. Mroczkowski, D. I. Freedberg, F. A. L. Anet|journal = Nature |volume = 367 |issue = |pages = 256–258 |year = 1994 |doi = 10.1038/367256a0 |accessdate=2008-07-20] Many fullerenes containing helium-3 have been reported. Although the helium atoms are not attached by covalent or ionic bonds, these substances have distinct properties and a definite composition, like all stoichiometric chemical compounds.

Occurrence and production

Natural abundance

Helium is the second most abundant element in the known Universe (after hydrogen), constituting 23% of the baryonic mass of the Universe. The vast majority of helium was formed by Big Bang nucleosynthesis from one to three minutes after the Big Bang. As such, measurements of its abundance contribute to cosmological models. In stars, it is formed by the nuclear fusion of hydrogen in proton-proton chain reactions and the CNO cycle, part of stellar nucleosynthesis.cite web|author=Weiss, Achim|title=Elements of the past: Big Bang Nucleosynthesis and observation|url=http://www.einstein-online.info/en/spotlights/BBN_obs/index.html|publisher=Max Planck Institute for Gravitational Physics|accessdate=2008-06-23; cite journal|author=Coc, A. |coauthors = et al.|title=Updated Big Bang Nucleosynthesis confronted to WMAP observations and to the Abundance of Light Elements|journal=Astrophysical Journal|volume=600|year=2004|pages=p. 544|doi=10.1086/380121]

In the Earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million. [cite journal |author=Oliver, B. M. |coauthors = James G. Bradley, Harry Farrar IV |year=1984 |title= Helium concentration in the Earth's lower atmosphere |journal=Geochimica et Cosmochimica Acta |volume=48 |issue=9 |pages=pp. 1759–1767 |doi=10.1016/0016-7037(84)90030-9] [cite web |url=http://www.srh.weather.gov/jetstream/atmos/atmos_intro.htm |title=The Atmosphere: Introduction |work=JetStream - Online School for Weather |publisher=National Weather Service |date = 2007-08-29 |accessdate = 2008-07-12] The concentration is low and fairly constant despite the continuous production of new helium because most helium in the Earth's atmosphere escapes into space by several processes. [cite journal |author=Lie-Svendsen, Ø. |coauthors = M. H. Rees |year=1996 |title=Helium escape from the terrestrial atmosphere: The ion outflow mechanism |journal=Journal of Geophysical Research |volume=101 |issue=A2 |pages=pp. 2435–2444 |doi=10.1029/95JA02208|accessdate=2008-07-20] [cite web|url=http://www.astronomynotes.com/solarsys/s3.htm|chapter=Atmospheres|title=Nick Strobel's Astronomy Notes|year=2007|accessdate=2007-09-25|author=Strobel, Nick] In the Earth's heterosphere, a part of the upper atmosphere, helium and other lighter gases are the most abundant elements.

Nearly all helium on Earth is a result of radioactive decay. The decay product is primarily found in minerals of uranium and thorium, including cleveites, pitchblende, carnotite and monazite, because they emit alpha particles, which consist of helium nuclei (He²⁺) to which electrons readily combine. In this way an estimated 3000 tonnes of helium are generated per year throughout the lithosphere.cite journal |author=Cook, Melvine A. |year=1957 |title=Where is the Earth's Radiogenic Helium? |journal= Nature |volume=179 |issue= |pages=p. 213 |doi=10.1038/179213a0] [cite journal |author= Aldrich, L. T. |coauthors = Alfred O. Nier |year=1948 |title=The Occurrence of He³ in Natural Sources of Helium |journal = Phys. Rev. |volume=74 |issue= |pages= 1590–1594 |doi=10.1103/PhysRev.74.1590|accessdate=2008-07-20] [cite journal |author=Morrison, P. |coauthors = J. Pine |year=1955 |title= Radiogenic Origin of the Helium Isotopes in Rock |journal = Annals of the New York Academy of Sciences |volume=62 |issue=3 |pages=pp. 71–92 |doi=10.1111/j.1749-6632.1955.tb35366.x] In the Earth's crust, the concentration of helium is 8 parts per billion. In seawater, the concentration is only 4 parts per trillion. There are also small amounts in mineral springs, volcanic gas, and meteoric iron. Because helium is trapped in a similar way by non-permeable layer of rock like natural gas the greatest concentrations on the planet are found in natural gas, from which most commercial helium is derived. The concentration varies in a broad range from a few ppm up to over 7% in a small gas field in San Juan County, New Mexico. [cite journal |author=Zartman, R. E. |year=1961 |title= Helium Argon and Carbon in Natural Gases |journal = Journal of Geophysical Research |volume=66 |issue=1 |pages=pp. 277–306 |url=http://www.agu.org/journals/jz/v066/i001/JZ066i001p00277/|accessdate=2008-07-21 |doi=10.1029/JZ066i001p00277] [cite journal |author=Broadhead, Ronald F. |year=2005 |title= Helium in New Mexico – geology distribution resource demand and exploration possibilities |journal = New Mexico Geology |volume=27 |issue=4 |pages=pp. 93–101 |url=http://geoinfo.nmt.edu/publications/periodicals/nmg/27/n4/helium.pdf |format=PDF|accessdate=2008-07-21]

Modern extraction

For large-scale use, helium is extracted by fractional distillation from natural gas, which contains up to 7% helium. [cite web| author = Winter, Mark| title = Helium: the essentials| publisher = Univeristy of Sheffield | year = 2008| url = http://www.webelements.com/helium/| accessdate = 2008-07-14] Since helium has a lower boiling point than any other element, low temperature and high pressure are used to liquefy nearly all the other gases (mostly nitrogen and methane). The resulting crude helium gas is purified by successive exposures to lowering temperatures, in which almost all of the remaining nitrogen and other gases are precipitated out of the gaseous mixture. Activated charcoal is used as a final purification step, usually resulting in 99.995% pure Grade-A helium. ["The Encyclopedia of the Chemical Elements", p. 258] The principal impurity in Grade-A helium is neon. In a final production step, most of the helium that is produced is liquefied via a cryogenic process. This is necessary for applications requiring liquid helium and also allows helium suppliers to reduce the cost of long distance transportation, as the largest liquid helium containers have more than five times the capacity of the largest gaseous helium tube trailers. [cite conference| author = Z. Cai |coauthors = R. Clarke, N. Ward, W. J. Nuttall, B. A. Glowacki | title = Modelling Helium Markets| publisher = University of Cambridge| year = 2007| url = http://www.jbs.cam.ac.uk/programmes/phd/downloads/conference_spring2007/papers/cai.pdf| format=PDF| accessdate = 2008-07-14] In 2005, approximately one hundred and sixty million cubic meters of helium were extracted from natural gas or withdrawn from helium reserves, with approximately 83% from the United States, 11% from Algeria, and most of the remainder from Russia and Poland. [cite conference| title = Helium| booktitle = Mineral Commodity Summaries| pages = pp. 78–79| publisher = U.S. Geological Survey| month = January | year = 2004| url = http://minerals.usgs.gov/minerals/pubs/commodity/helium/heliumcs04.pdf| format = PDF| accessdate = 2008-07-14] In the United States, most helium is extracted from natural gas in Kansas, Oklahoma, and Texas. Diffusion of crude natural gas through special semipermeable membranes and other barriers is another method to recover and purify helium. [cite journal |title = Membrane technology — A new trend in industrial gas separation |author = Belyakov, V.P. |coauthors= S. G. Durgar'yan, B. A. Mirzoyan, et al. |journal = Chemical and Petroleum Engineering |volume = 17 |issue = 1 |pages = pp. 19–21 |year = 1981 |doi = 10.1007/BF01245721] Helium can be synthesized by bombardment of lithium or boron with high-velocity protons, but this is not an economically viable method of production. [cite journal |title = A Photographic Investigation of the Transmutation of Lithium and Boron by Protons and of Lithium by Ions of the Heavy Isotope of Hydrogen |author = Dee, P. I. |coauthors = E. T. S. Walton |journal = Proceedings of the Royal Society of London |volume = 141 |issue = 845 |pages = pp. 733–742 |year = 1933 |doi = 10.1098/rspa.1933.0151]

Applications

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 inertness. Helium is commercially available in either liquid or gaseous form. As a liquid, it can be supplied in small containers called Dewars which hold up to 1,000 liters of helium, or in large ISO containers which have nominal capacities as large as 11,000 gallons (41,637 liters). In gaseous form, small quantities of helium are supplied in high pressure cylinders holding up to 300 standard cubic feet, while large quantities of high pressure gas are supplied in tube trailers which have capacities of up to 180,000 standard cubic feet.

; Airships, balloons and rocketryBecause it is lighter than air, airships and balloons are inflated with helium for lift. While hydrogen gas is approximately 7% more buoyant, helium has the advantage of being non-flammable. In rocketry, helium is used as an ullage medium to displace fuel and oxidizers in storage tanks and to condense hydrogen and oxygen to make rocket fuel. It is also used to purge fuel and oxidizer from ground support equipment prior to launch and to pre-cool liquid hydrogen in space vehicles. For example, the Saturn V booster used in the Apollo program needed about 13 million cubic feet (370,000 m³) of helium to launch.

;Commercial and recreationalHelium alone is less dense than atmospheric air, so it will change the timbre (not pitchcite web | url = http://www.phys.unsw.edu.au/PHYSICS_!/SPEECH_HELIUM/speech.html | title = Physics in speech | publisher = phys.unsw.edu.au. |accessdate=2008-07-20] ) of a person's voice when inhaled. However, inhaling it from a typical commercial source, such as that used to fill balloons, can be dangerous due to the risk of asphyxiation from lack of oxygen, and the number of contaminants that may be present. These could include trace amounts of other gases, in addition to aerosolized lubricating oil.

For its low solubility in nervous tissue, helium mixtures such as trimix, heliox and heliair are used for deep diving to reduce the effects of narcosis. [cite journal |last=Fowler |first=B |coauthors=Ackles KN, Porlier G |year=1985 |title=Effects of inert gas narcosis on behavior—a critical review |journal=Undersea Biomedical Research Journal |pmid=4082343 |url=http://archive.rubicon-foundation.org/3019 |accessdate=2008-06-27] cite journal |author= Thomas, J. R. |year=1976 |title=Reversal of nitrogen narcosis in rats by helium pressure |journal=Undersea Biomed Res. |volume=3 |issue=3 |pages=pp. 249–59 |pmid=969027 |url=http://archive.rubicon-foundation.org/2771 |accessdate=2008-08-06 ] At depths below convert|150|m|ft small amounts of hydrogen are added to a helium-oxygen mixture to counter the effects of high pressure nervous syndrome. [cite journal |author=Rostain, J. C. |coauthors=M. C. Gardette-Chauffour, C. Lemaire, R. Naquet|title=Effects of a H₂-He-O₂ mixture on the HPNS up to 450 msw |journal=Undersea Biomed. Res. |volume=15 |issue=4 |pages=257–70 |year=1988 |issn=0093-5387 |oclc=2068005 |pmid=3212843 |url=http://archive.rubicon-foundation.org/2487 |accessdate=2008-06-24 ] At these depths the low density of helium is found to considerably reduce the effort of breathing. [cite journal| author = Butcher, Scott J.| coauthors = Richard L. Jones, Jonathan R. Mayne, Timothy C. Hartley, Stewart R. Petersen| title = Impaired exercise ventilatory mechanics with the self-contained breathing apparatus are improved with heliox| journal = European Journal of Applied Physiology| volume = 101| issue = 6| pages = p. 659(11)| publisher = Springer| location = Netherlands| month = December | year = 2007| doi = 10.1007/s00421-007-0541-5 ]

Helium-neon lasers have various applications, including barcode readers.

;IndustrialFor its inertness and high thermal conductivity, neutron transparency, and because it does not form radioactive isotopes under reactor conditions, helium is used as a heat-transfer medium in some gas-cooled nuclear reactors. Helium is used as a shielding gas in arc welding processes on materials that are contaminated easily by air.

Helium is used as a protective gas in growing silicon and germanium crystals, in titanium and zirconium production, and in gas chromatography, because it is inert. This property also makes it useful in supersonic wind tunnels. [cite journal |author = Beckwith, I.E. |coauthors = C. G. Miller III |title = Aerothermodynamics and Transition in High-Speed Wind Tunnels at Nasa Langley |journal = Annual Review of Fluid Mechanics |volume = 22 |pages = pp. 419–439 |year= 1990 |doi = 10.1146/annurev.fl.22.010190.002223 ]

Because it diffuses through solids at three times the rate of air, helium is used as a tracer gas to detect leaks in high-vacuum equipment and high-pressure containers.cite encyclopedia| title = Helium| editor = Considine, Glenn D.| encyclopedia = Van Nostrand's Encyclopedia of Chemistry| pages = pp. 764–765| publisher = Wylie-Interscience| year = 2005| isbn = 0-471-61525-0]

Helium, mixed with a heavier gas such as xenon, is useful for thermoacoustic refrigeration due to the resulting high heat capacity ratio and low Prandtl number. [cite journal |title=Working gases in thermoacoustic engines |journal=The Journal of the Acoustical Society of America |year=1999 |volume=105 |issue=5 |pages=pp. 2677–2684 |doi=10.1121/1.426884 | author = Belcher, James R. |coauthors = William V. Slaton, Richard Raspet, Henry E. Bass, Jay Lightfoot] The inertness of helium has environmental advantages over conventional refrigeration systems which contribute to ozone depletion or global warming. [cite book |title=Mending the Ozone Hole: Science, Technology, and Policy |author=Makhijani, Arjun |coauthors = Kevin Gurney |publisher=MIT Press |year=1995 |isbn=0262133083]

;ScientificThe use of helium reduces the distorting effects of temperature variations in the space between lenses in some telescopes, due to its extremely low index of refraction. This method is especially used in solar telescopes where a vacuum tight telescope tube would be too heavy. [cite journal |author = Jakobsson, H. |title = Simulations of the dynamics of the Large Earth-based Solar Telescope |journal = Astronomical & Astrophysical Transactions |volume = 13 |issue = 1 |pages = pp. 35–46 |year= 1997 |doi = 10.1080/10556799708208113] [cite journal | url = http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1983ApOpt..22...10E&db_key=AST | title = Tests of vacuum VS helium in a solar telescope | author = Engvold, O. |coauthors = R.B. Dunn, R. N. Smartt, W. C. Livingston| journal = Applied Optics | year = 1983 | pages = pp. 10–12 | volume = 22|accessdate=2008-07-27]

The age of rocks and minerals that contain uranium and thorium can be estimated by measuring the level of helium with a process known as helium dating.cite book|title=The Encyclopedia of the Chemical Elements|author=Brandt, L. W.|publisher=Reinhold Book Corporation|location=New York|year=1968|pages=pp. 256–268|editor=Clifford A. Hampel|chapter=Helium|id=LCCN 68-29938]

Liquid helium is used to cool certain metals to the extremely low temperatures required for superconductivity, such as in superconducting magnets for magnetic resonance imaging. The Large Hadron Collider at CERN uses 96 tonnes of liquid helium to maintain the temperature at 1.9 Kelvin.cite web|url=http://visits.web.cern.ch/visits/guides/tools/presentation/LHC_booklet-2.pdf LHC Guide booklet|title=CERN - LHC: Facts and Figures|publisher=CERN|accessdate=2008-04-30] Helium at low temperatures is also used in cryogenics.

afety

Neutral helium at standard conditions is non-toxic, plays no biological role and is found in trace amounts in human blood. If enough helium is inhaled that oxygen needed for normal respiration is replaced asphyxia is possible. The safety issues for cryogenic helium are similar to those of liquid nitrogen. The deep temperatures can result in cold burns and the liquid to gas expansion ratio can cause explosions if no pressure-relief devices are installed.

Containers of helium gas at 5 to 10 K should be handled as if they contain liquid helium due to the rapid and significant thermal expansion that occurs when helium gas at less than 10 K is warmed to room temperature.

Biological effects

Listen|right|filename=Helium article read with helium.ogg|title=Effect of helium on a human voice|description=The effect of helium on a human voice|format=Ogg

The voice of a person who has inhaled helium temporarily changes in timbre in a way that makes it sound high-pitched. The speed of sound in helium is nearly three times the speed of sound in air; because the fundamental frequency of a gas-filled cavity is proportional to the speed of sound in the gas, when helium is inhaled there is a corresponding increase in the resonant frequencies of the vocal tract. [cite journal |author=Ackerman MJ, Maitland G |title=Calculation of the relative speed of sound in a gas mixture |journal=Undersea Biomed Res |volume=2 |issue=4 |pages=305–10 |year=1975 |month=December |pmid=1226588 |doi= |url=http://archive.rubicon-foundation.org/2738 |accessdate=2008-08-09] (The opposite effect, lowering frequencies, can be obtained by inhaling a dense gas such as sulfur hexafluoride.)

Inhaling helium can be dangerous if done to excess, since helium is a simple asphyxiant and so displaces oxygen needed for normal respiration.de icon cite journal | title = Suicidal asphyxiation with helium: Report of three cases Suizid mit Helium Gas: Bericht über drei Fälle | journal = Wiener Klinische Wochenschrift| volume = 119 | issue =9–10 | year = 2007 | doi = 10.1007/s00508-007-0785-4 | author = Grassberger, Martin |coauthors = Astrid Krauskopf | pages = 323–325 |language=German & English] Breathing pure helium continuously causes death by asphyxiation within minutes. Inhaling helium directly from pressurized cylinders is extremely dangerous, as the high flow rate can result in barotrauma, fatally rupturing lung tissue.cite news | author = Engber, Daniel| title = Stay Out of That Balloon! | publisher = Slate.com| date = 2006-06-13| url = http://www.slate.com/id/2143631/| accessdate = 2008-07-14] However, death caused by helium is quite rare, with only two fatalities reported between 2000 and 2004 in the United States.

At high pressures (more than about 20 atm or two MPa), a mixture of helium and oxygen (heliox) can lead to high pressure nervous syndrome, a sort of reverse-anesthetic effect; adding a small amount of nitrogen to the mixture can alleviate the problem. [cite journal |author=Rostain JC, Lemaire C, Gardette-Chauffour MC, Doucet J, Naquet R |title=Estimation of human susceptibility to the high-pressure nervous syndrome |journal=J Appl Physiol |volume=54 |issue=4 |pages=1063–70 |year=1983 |month=April |pmid=6853282 |doi= |url=http://jap.physiology.org/cgi/pmidlookup?view=long&pmid=6853282 |accessdate=2008-08-09] [cite journal |last=Hunger Jr |first=W. L. |coauthors=P. B. Bennett. |title=The causes, mechanisms and prevention of the high pressure nervous syndrome |journal=Undersea Biomed. Res. |volume=1 |issue=1 |pages=1–28 |date=1974 |issn=0093-5387 |oclc=2068005 |pmid=4619860 |url=http://archive.rubicon-foundation.org/2661 |accessdate=2008-08-09 ]

ee also

* Abiogenic petroleum origin
* Helium-3 propulsion
* Leidenfrost effect
* Superfluid
* Tracer-gas leak testing method

Notes

References

*cite book|author=Brandt, L. W. |title= The Encyclopedia of the Chemical Elements|editor= Hampel, Clifford A. |chapter=Helium | location=New York |publisher=Reinhold |year=1968 |pages= 261 |LCCCN=Library of Congress Catalog Card Number: 68-29938 |oclc= 449569
*cite book | author = Bureau of Mines | title = Minerals yearbook mineral fuels Year 1965, Volume II (1967) | publisher = U. S. Government Printing Office | year = 1967
*cite web | url=http://chartofthenuclides.com/default.html|title=Chart of the Nuclides: Fourteenth Edition |publisher= General Electric Company |year=1989
*cite book|author=Emsley, John|title=The Elements |edition= 3rd ed. |location= New York |publisher= Oxford University Press |year=1998 |isbn= 978-0198558187
*cite web|publisher=United States Geological Survey (usgs.gov)|url=http://minerals.usgs.gov/minerals/pubs/commodity/helium/heliumcs07.pdf |title=Mineral Information for Helium |format=PDF |accessdate=2007-01-05
*cite web | url = http://web.archive.org/web/20050101090349/www.oma.be/BIRA-IASB/Public/Research/Thermo/Thermotxt.en.html | title = The thermosphere: a part of the heterosphere | author = Vercheval, J. |month= January | year= 2003| accessdate = 2008-07-12 | publisher = Belgian Institute for Space Aeronomy
*cite journal |title = Isotopic Composition and Abundance of Interstellar Neutral Helium Based on Direct Measurements | author = Zastenker, G. N. |coauthors = E. Salerno, F. Buehler, P. Bochsler, M. Bassi, Y. N. Agafonov, N. A. Eismont, V. V. Khrapchenkov, H. Busemann|url= http://www.ingentaconnect.com/content/klu/asys/2002/00000045/00000002/00378626 |journal= Astrophysics |month= April | year= 2002 |volume= 45 |issue= 2 |pages= pp. 131–142 |doi= 10.1023/A:1016057812964

External links

;General
* [http://uk.youtube.com/watch?v=a8FJEiI5e6Q The Periodic Table of Videos - Helium]
* [http://www.blm.gov/wo/st/en/info/newsroom/2007/january/NR0701_2.html US Government' Bureau of Land Management: Sources, Refinement, and Shortage.] With some History of Helium.
* [http://minerals.usgs.gov/minerals/pubs/commodity/helium/ U.S. Geological Survey Publicationson Helium] beginning 1996
* [http://education.jlab.org/itselemental/ele002.html It's Elemental – Helium]

;More detail
* [http://boojum.hut.fi/research/theory/helium.html Helium] at the Helsinki University of Technology; includes pressure-temperature phase diagrams for helium-3 and helium-4
* [http://www.lancs.ac.uk/depts/physics/research/condmatt/ult/index.html Lancaster University, Ultra Low Temperature Physics] - includes a summary of some low temperature techniques

;Miscellaneous
* [http://www.phys.unsw.edu.au/PHYSICS_!/SPEECH_HELIUM/speech.html Physics in Speech] with audio samples that demonstrate the unchanged voice pitch
* [http://www.du.edu/~jcalvert/phys/helium.htm Article about helium and other noble gases]
* [http://www.ebyte.it/stan/blog.html#08Feb29 Ebyte article] on helium scarcity and potential effects on NMR and MRI communities