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Helium

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                 2               hydrogen ← helium → lithium
                 -
                ↑
                He
                ↓
                Ne

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                        Name, Symbol, Number helium, He, 2
                                               Chemical series noble gases
                                             Group, Period, Block 18, 1, s
                                                      Appearance colorless
                                            Atomic mass 4.002602 (2) g/mol
                                               Electron configuration 1s^2
                                                     Electrons per shell 2
                                                       Physical properties
                                                                 Phase gas
                                              Density (0 °C, 101.325 kPa)
                                                                0.1786 g/L
                                        Melting point (at 2.5 MPa) 0.95  K
                                                (-272.2 ° C, -458.0 ° F)
                                                      Boiling point 4.22 K
                                              (-268.93 ° C, -452.07 ° F)
                                          Critical point 5.19 K, 0.227 MPa
                                        Heat of fusion 0.0138 kJ·mol^−1
                                  Heat of vaporization 0.0829 kJ·mol^−1
                         Heat capacity (25 °C) 20.786 J·mol^−1·K^−1

   CAPTION: Vapor pressure

                                             P/Pa  1 10 100 1 k 10 k 100 k
                                            at T/K               3     4

                                                         Atomic properties
                                        Crystal structure hexagonal or bcc
                                    Ionization energies 1st: 2372.3 kJ/mol
                                                        2nd: 5250.5 kJ/mol
                                               Atomic radius (calc.) 31 pm
                                                     Covalent radius 32 pm
                                               Van der Waals radius 140 pm
                                                             Miscellaneous
                     Thermal conductivity (300 K) 151.3 mW·m^−1·K^−1
                                             CAS registry number 7440-59-7
                                                         Selected isotopes

                  CAPTION: Main article: Isotopes of helium

                       iso      NA      half-life DM DE ( MeV)     DP
                       ^3He 0.000137%*  He is stable with 1 neutron
                       ^4He 99.999863%* He is stable with 2 neutrons
                       *Atmospheric value, abundance may differ elsewhere.

                                                                References

   Helium is a colorless, odorless, tasteless chemical element. It is the
   most unreactive of the noble gases and therefore the least
   chemically-active chemical element on the periodic table. Its boiling
   and melting points are the lowest among the elements; except in extreme
   conditions, it exists only as a gas. At temperatures near absolute
   zero, it is a superfluid, a nearly frictionless phase of matter with
   unusual properties.

   After hydrogen, helium is the second lightest element and the second
   most abundant element in the universe, created during big bang
   nucleosynthesis and to a lesser extent from nuclear fusion of hydrogen
   in stars. On Earth, helium is primarily a product of the radioactive
   decay of much heavier elements, which emit helium nuclei called alpha
   particles; it is found in significant amounts only in natural gas, from
   which it is extracted at low temperatures by fractional distillation.

   First detected in 1868 by French astronomer Pierre Janssen as an
   unknown yellow spectral line signature in the light of a solar eclipse,
   helium was separately identified as a new element later that year by
   English astronomer Norman Lockyer. Its presence in natural gas in
   large, useable amounts was identified in 1905. Helium is used in
   cryogenics, as a deep-sea breathing gas, for inflating balloons and
   airships, and as a protective gas for many industrial purposes, such as
   arc welding. Inhaling a small amount of the gas temporarily changes the
   frequency of a person's voice; however, caution must be exercised as
   helium is an asphyxiant.

Notable characteristics

Gas and plasma phases

   Helium is a colorless, odorless, and non-toxic gas. It is the least
   reactive member of group 18 (the noble gases) of the periodic table and
   therefore also the least reactive of all elements; it is inert and
   monatomic in virtually all conditions. It has a thermal conductivity
   that is greater than any gas except hydrogen and its specific heat is
   unusually high. Helium is also less water soluble than any other gas
   known and its diffusion rate through solids is three times that of air
   and around 65% that of hydrogen. Helium's index of refraction is closer
   to unity than 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 discharge tube shaped like the element's atomic symbol
   Enlarge
   Helium discharge tube shaped like the element's atomic symbol

   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 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, HgHe[10], WHe[2] and the molecular ions
   He[2]^+, He[2]^++, HeH^+, and HeD^+ have been created this way. This
   technique has also allowed the production of the neutral molecule
   He[2], which has a large number of band systems, and HgHe, which is
   apparently only held together by polarization forces. Theoretically,
   other compounds, like helium fluorohydride (HHeF), may also be
   possible.

   Throughout the universe, helium is found mostly in a plasma state whose
   properties are quite different to molecular helium. As 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 currents and the aurora.

Solid and liquid phases

   Helium solidifies only under great pressure. The resulting colorless,
   almost invisible solid is highly compressible; applying pressure in the
   laboratory can decrease its volume by more than 30%. With a bulk
   modulus on the order of 5×10^7 Pa it is 50 times more compressible than
   water. Unlike any other element, helium will fail to solidify and
   remain a liquid down to absolute zero at normal pressures. Solid helium
   requires a temperature of 1–1.5 K (about −272 °C or −457 °F) and about
   26 standard atmospheres (2.6 MPa) of pressure. 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.

Helium I state

   Below its boiling point of 4.22 kelvins and above the lambda point of
   2.1768 kelvins, the isotope helium-4 exists in a normal colorless
   liquid state, called helium I. Like other cryogenic liquids, helium I
   boils when heat is added to it. 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. 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.

   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.

   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. As a result of this creeping behaviour 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.

   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.

   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. 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

   Because of its low density, helium is the gas of choice to fill
   airships such as the Holden Airship
   Enlarge
   Because of its low density, helium is the gas of choice to fill
   airships such as the Holden Airship

   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. Pressurized helium is
   commercially available in large quantities.
     * Because it is lighter than air, airships and balloons are inflated
       with helium for lift. In airships, helium is preferred over
       hydrogen because it is not flammable and has 92.64% of the lifting
       power of the alternative hydrogen.
     * For its low solubility in water, the major part of human blood, air
       mixtures of helium with oxygen and nitrogen ( Trimix), with oxygen
       only ( Heliox), with common air ( heliair), and with hydrogen and
       oxygen ( hydreliox), are used in deep-sea breathing systems to
       reduce the high-pressure risk of nitrogen narcosis, decompression
       sickness, and oxygen toxicity.
     * At extremely low temperatures, liquid helium is used to cool
       certain metals to produce superconductivity, such as in
       superconducting magnets used in magnetic resonance imaging. Helium
       at low temperatures is also used in cryogenics.
     * For its inertness and high thermal conductivity, helium is used as
       a coolant in some nuclear reactors, such as pebble-bed reactors,
       and in arc welding air-sensitive metals.
     * Because it is inert, helium is used as a protective gas in growing
       silicon and germanium crystals, in titanium and zirconium
       production, in gas chromatography, and as an atmosphere for
       protecting historical documents. This property also makes it useful
       in supersonic wind tunnels.
     * 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.
     * The gain medium of the helium-neon laser is a mixture of helium and
       neon.
     * Because it diffuses through solids at a rate three times that of
       air, helium is used to detect leaks in high-vacuum equipment and
       high-pressure containers.
     * Because of its extremely low index of refraction, the use of helium
       reduces the distorting effects of temperature variations in the
       space between lenses in some telescopes.
     * The age of rocks and minerals that contain uranium and thorium,
       radioactive elements that emit helium nuclei called alpha
       particles, can be discovered by the level of helium there.
     * Because helium alone is less dense than atmospheric air, it will
       change the timbre (not pitch ) 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 number of
       contaminants that may be present. These could include trace amount
       of other gases, in addition to aerosolized lubricating oil.
     * The high thermal conductivity and sound velocity of helium is also
       desirable in thermoacoustic refrigeration. The inertness of helium
       adds to the environmental advantage of this technology over
       conventional refrigeration systems which may contribute to ozone
       depleting and global warming effects.

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 D[3] line, for it was near the known D[1] and D[2] lines of sodium,
   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).

   On March 26, 1895, British chemist William Ramsay isolated helium on
   Earth by treating the mineral cleveite with mineral acids. Ramsay was
   looking for argon but, after separating nitrogen and oxygen from the
   gas liberated by sulfuric acid, noticed a bright-yellow line that
   matched the D[3] line observed in the spectrum of the Sun. 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.

   In 1907, Ernest Rutherford and Thomas Royds demonstrated that an alpha
   particle is a helium nucleus. In 1908, helium was first liquefied by
   Dutch physicist Heike Kamerlingh Onnes by cooling the gas to less than
   one kelvin. He tried to solidify it by further reducing the temperature
   but failed because helium does not have a triple point temperature
   where the solid, liquid, and gas phases are at equilibrium. It was
   first solidified in 1926 by his student Willem Hendrik Keesom by
   subjecting helium to 25 atmospheres of pressure.

   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. In 1972, the same phenomenon was
   observed in helium-3 by American physicists Douglas D. Osheroff, David
   M. Lee, and Robert C. Richardson.

Extraction and use

   After an oil drilling operation in 1903 in Dexter, Kansas, USA 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
   contained, by volume, 72% nitrogen, 15% methane—insufficient to make
   the gas combustible, 1% hydrogen, and 12% of an unidentifiable gas.
   With further analysis, Cady and McFarland discovered that 1.84% of the
   gas sample was helium. Far from being a rare element, helium was
   present in vast quantities under the American Great Plains, available
   for extraction from natural gas.

   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 lifting gas. A total
   of 200,000 cubic feet (5700 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.

   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. Helium was also vital
   in the atomic bomb Manhattan Project.

   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. Helium use
   following World War II was depressed but the reserve was expanded in
   the 1950s to ensure a supply 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.

   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.

   By 1995, a billion cubic metres 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. The resulting "Helium
   Privatization Act of 1996" (Public Law 104–273) directed the United
   States Department of the Interior to start liquidating the reserve by
   2005.

   Helium produced before 1945 was about 98% 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
   99.995% helium were available.

   For many years the United States produced over 90% of commercially
   usable helium in the world. Extraction plants created in Canada,
   Poland, Russia, and other nations produced the remaining helium. In the
   early 2000s, Algeria and Qatar were added as well. Algeria quickly
   became the second leading producer of helium. Through this time, both
   helium consumption and the costs of producing helium increased.

Occurrence and production

Natural abundance

   Helium is the second most abundant element in the known Universe after
   hydrogen and constitutes 23% of the elemental mass of the universe. It
   is concentrated in stars, where it is formed from hydrogen by the
   nuclear fusion of the proton-proton chain reaction and CNO cycle.
   According to the Big Bang model of the early development of the
   universe, the vast majority of helium was formed during Big Bang
   nucleosynthesis, from one to three minutes after the Big Bang. As such,
   measurements of its abundance contribute to cosmological models.

   In the Earth's atmosphere, the concentration of helium by volume is
   only 5.2 parts per million, largely because most helium in the Earth's
   atmosphere escapes into space due to its inertness and low mass. 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, monazite and beryl,
   because they emit alpha particles, which consist of helium nuclei
   (He^2+) to which electrons readily combine. In this way an estimated
   3.4 litres of helium per year are generated per cubic kilometer of the
   Earth's crust. 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. The greatest concentrations on the planet are
   in natural gas, from which most commercial helium is derived.

Extraction

   For large-scale use, helium is extracted by fractional distillation
   from natural gas, which contains up to 7% helium. 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 principal
   impurity in Grade-A helium is neon.

   As of 2004, over one hundred and fifty million cubic metres of helium
   were extracted from natural gas or withdrawn from helium reserves,
   annually, with approximately 84% of production from the United States,
   10% from Algeria, and most of the remainder from Canada, China, Poland,
   Qatar, and Russia. In the United States, most helium is produced in
   Kansas and Texas.

   Diffusion of crude natural gas through special semi- permeable
   membranes and other barriers is another method to recover and purify
   helium. Helium can be synthesized by bombardment of lithium or boron
   with high-velocity protons, but this is not an economically viable
   method of production.

Isotopes

   Although there are eight known isotopes of helium, 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. However, helium is unusual in that its
   isotopic abundance varies greatly depending on its origin. In the
   interstellar medium, the proportion of He-3 is around a hundred times
   higher. Rocks from the Earth's crust have isotope ratios varying by as
   much as a factor of ten; this is used in geology to study the origin of
   such rocks.

   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.

   Equal mixtures of liquid helium-3 and helium-4 below 0.8 K will
   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). Dilution refrigerators take advantage of
   the immiscibility of these two isotopes to achieve temperatures of a
   few millikelvins. There is only a trace amount of helium-3 on Earth,
   primarily present since the formation of the Earth, although some falls
   to Earth trapped in cosmic dust. Trace amounts are also produced by the
   beta decay of tritium. In stars, however, helium-3 is more abundant, a
   product of nuclear fusion. Extraplanetary material, such as lunar and
   asteroid regolith, have trace amounts of helium-3 from being bombarded
   by solar winds.

   The different formation processes of the two stable isotopes of helium
   produce the differing isotope abundances. These differing isotope
   abundances can be used to investigate the origin of rocks and the
   composition of the Earth's mantle.

   It is possible to produce exotic helium isotopes, which rapidly decay
   into other substances. The shortest-lived isotope is helium-5 with a
   half-life of 7.6×10^−22 second. Helium-6 decays by emitting a beta
   particle and has a half life of 0.8 second. Helium-7 also emits a beta
   particle as well as a gamma ray. Helium-7 and helium-8 are
   hyperfragments that are created in certain nuclear reactions.

Precautions

   The voice of a person who has inhaled helium temporarily sounds
   high-pitched. This is because the speed of sound in helium is nearly
   three times that 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.

   Although the vocal effect of inhaling helium may be amusing, it can be
   dangerous if done to excess since helium is a simple asphyxiant, thus
   it displaces oxygen needed for normal respiration. Death by
   asphyxiation will result within minutes if pure helium is breathed
   continuously. In Mammals (with the notable exception of seals) the
   breathing reflex is triggered by excess of carbon dioxide rather than
   lack of oxygen, so asphyxiation by helium progresses without the victim
   experiencing air hunger. Inhaling helium directly from pressurized
   cylinders is extremely dangerous as the high flow rate can result in
   barotrauma, fatally rupturing lung tissue.

   Neutral helium at standard conditions is non-toxic, plays no biological
   role and is found in trace amounts in human blood. At high pressures, a
   mixture of helium and oxygen ( heliox) can lead to high pressure
   nervous syndrome, however, increasing the proportion of nitrogen can
   alleviate the problem.

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