Helium (fromGreek:ἥλιος,romanized: helios,lit. 'sun') is achemical element; it hassymbolHe andatomic number 2. It is a colorless, odorless, non-toxic,inert,monatomicgas and the first in thenoble gas group in theperiodic table.[a] Itsboiling point is the lowest among all theelements, and it does not have amelting point at standard pressures. It is the second-lightest and second-mostabundant element in the observableuniverse, afterhydrogen. It is present at about 24% of the total elemental mass, which is more than 12 times the mass of all the heavier elements combined. Its abundance is similar to this in both theSun andJupiter, because of the very highnuclear binding energy (pernucleon) ofhelium-4 with respect to the next three elements after helium. This helium-4 binding energy also accounts for why it is a product of bothnuclear fusion andradioactive decay. The most common isotope of helium in the universe is helium-4, the vast majority of which was formed during theBig Bang. Large amounts of new helium are created by nuclear fusion of hydrogen instars.
Helium was first detected as an unknown, yellowspectral line signature in sunlight during asolar eclipse in 1868 byGeorges Rayet,[15] Captain C. T. Haig,[16]Norman R. Pogson,[17] and Lieutenant John Herschel,[18] and was subsequently confirmed by French astronomerJules Janssen.[19] Janssen is often jointly credited with detecting the element, along withNorman Lockyer. Janssen recorded the helium spectral line during the solar eclipse of 1868, while Lockyer observed it from Britain. However, only Lockyer proposed that the line was due to a new element, which he named after the Sun. The formaldiscovery of the element was made in1895 by chemistsSir William Ramsay,Per Teodor Cleve, andNils Abraham Langlet, who found helium emanating from theuranium orecleveite, which is now not regarded as a separate mineral species, but as a variety ofuraninite.[20][21] In 1903, large reserves of helium were found innatural gas fields in parts of the United States, by far the largest supplier of the gas today.
Liquid helium is used incryogenics (its largest single use, consuming about a quarter of production), and in thecooling ofsuperconducting magnets, with its main commercial application inMRI scanners. Helium's other industrial uses—as a pressurizing and purge gas, as a protective atmosphere forarc welding, and in processes such as growingcrystals to makesilicon wafers—account for half of the gas produced. A small but well-known use is as alifting gas inballoons andairships.[22] As with any gas whose density differs from that of air, inhaling a small volume of helium temporarily changes the timbre and quality of thehuman voice. In scientific research, the behavior of the two fluid phases of helium-4 (helium I and helium II) is important to researchers studyingquantum mechanics (in particular the property ofsuperfluidity) and to those looking at the phenomena, such assuperconductivity, produced inmatter nearabsolute zero.
On Earth, it is relatively rare—5.2ppm by volume in theatmosphere. Most terrestrial helium present today is created by the naturalradioactive decay of heavy radioactive elements (thorium and uranium, although there are other examples), as thealpha particles emitted by such decays consist of helium-4nuclei. Thisradiogenic helium is trapped withnatural gas in concentrations as great as 7% by volume, from which it is extracted commercially by a low-temperature separation process calledfractional distillation. Terrestrial helium is a non-renewable resource because once released into the atmosphere, it promptlyescapes into space. Its supply is thought to be rapidly diminishing.[23][24] However, some studies suggest that helium produced deep in the Earth by radioactive decay can collect in natural gas reserves in larger-than-expected quantities,[25] in some cases having been released by volcanic activity.[26]
History
Scientific discoveries
The first evidence of helium was observed on August 18, 1868, as a bright yellow line with awavelength of 587.49 nanometers in thespectrum of thechromosphere of theSun. The line was detected by French astronomerJules Janssen duringa total solar eclipse inGuntur, India.[27][28] This line was initially assumed to besodium. On October 20 of the same year, English astronomerNorman Lockyer observed a yellow line in the solar spectrum, which he named the D3 because it was near the known D1 and D2Fraunhofer lines of sodium.[29][30] He concluded that it was caused by an element in the Sun unknown on Earth. Lockyer named the element with the Greek word for the Sun, ἥλιος (helios).[31][32] It is sometimes said that English chemistEdward Frankland was also involved in the naming, but this is unlikely as he doubted the existence of this new element. The ending "-ium" is unusual, as it normally applies only to metallic elements; probably Lockyer, being an astronomer, was unaware of the chemical conventions.[33]
Spectral lines of helium
In 1881, Italian physicistLuigi Palmieri detected helium on Earth for the first time through its D3 spectral line, when he analyzed a material that had beensublimated during a recent eruption ofMount Vesuvius.[34]
SirWilliam Ramsay, the discoverer of terrestrial heliumThe cleveite sample from which Ramsay first purified helium[35]
On March 26, 1895, Scottish chemistSir William Ramsay isolated helium on Earth by treating the mineral cleveite (a variety of uraninite with at least 10%rare-earth elements) with mineralacids. Ramsay was looking forargon but, after separatingnitrogen andoxygen from the gas, liberated bysulfuric acid, he noticed a bright yellow line that matched the D3 line observed in the spectrum of the Sun.[30][36][37][38] These samples were identified as helium by Lockyer and British physicistWilliam Crookes.[39][40] It was independently isolated from cleveite in the same year by chemistsPer Teodor Cleve andAbraham Langlet inUppsala, Sweden, who collected enough of the gas to accurately determine itsatomic weight.[41][42][28][43] Helium was also isolated by American geochemistWilliam 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 tonitrogen.[44] His letter of congratulations to Ramsay offers an interesting case of discovery, and near-discovery, in science.[45]
In 1907,Ernest Rutherford andThomas Royds demonstrated thatalpha particles are heliumnuclei by allowing the particles to penetrate the thin glass wall of anevacuated tube, then creating a discharge in the tube, to study the spectrum of the new gas inside.[46] In 1908, helium was first liquefied by Dutch physicistHeike Kamerlingh Onnes by cooling the gas to less than 5 K (−268.15 °C; −450.67 °F).[47][48] He tried to solidify it by further reducing the temperature but failed, because helium does not solidify at atmospheric pressure. Onnes' studentWillem Hendrik Keesom was eventually able to solidify 1 cm3 of helium in 1926 by applying additional external pressure.[49][50]
In 1913,Niels Bohr published his "trilogy"[51][52] on atomic structure that included a reconsideration of thePickering–Fowler series as central evidence in support of hismodel of the atom.[53][54] This series is named forEdward Charles Pickering, who in 1896 published observations of previously unknown lines in the spectrum of the starζ Puppis[55] (these are now known to occur withWolf–Rayet and other hot stars).[56] Pickering attributed the observation (lines at 4551, 5411, and 10123 Å) to a new form of hydrogen with half-integer transition levels.[57][58] In 1912,Alfred Fowler[59] managed to produce similar lines from a hydrogen-helium mixture, and supported Pickering's conclusion as to their origin.[60] Bohr's model does not allow for half-integer transitions (nor does quantum mechanics) and Bohr concluded that Pickering and Fowler were wrong, and instead assigned these spectral lines to ionised helium, He+.[61] Fowler was initially skeptical[62] but was ultimately convinced[63] that Bohr was correct,[51] and by 1915 "spectroscopists had transferred [the Pickering–Fowler series] definitively [from hydrogen] to helium."[54][64] Bohr's theoretical work on the Pickering series had demonstrated the need for "a re-examination of problems that seemed already to have been solved within classical theories" and provided important confirmation for his atomic theory.[54]
In 1961, Vignos and Fairbank reported the existence of a different phase of solid helium-4, designated the gamma-phase. It exists for a narrow range of pressure between 1.45 and 1.78 K.[67]
Historical marker, denoting a massive helium find nearDexter, Kansas
After an oil drilling operation in 1903 inDexter, Kansas produced a gas geyser that would not burn, Kansas state geologistErasmus Haworth collected samples of the escaping gas and took them back to theUniversity of Kansas at Lawrence where, with the help of chemistsHamilton Cady and David McFarland, he discovered that the gas consisted of, by volume, 72% nitrogen, 15%methane (acombustible percentage only with sufficient oxygen), 1% hydrogen, and 12% an unidentifiable gas.[28][68] With further analysis, Cady and McFarland discovered that 1.84% of the gas sample was helium.[69][70] This showed that despite its overall rarity on Earth, helium was concentrated in large quantities under theAmerican Great Plains, available for extraction as a byproduct ofnatural gas.[71]
Following a suggestion by SirRichard Threlfall, theUnited States Navy sponsored three small experimental helium plants during World War I. The goal was to supplybarrage balloons with the non-flammable, lighter-than-air gas. A total of 5,700 m3 (200,000 cu ft) of 92% helium was produced in the program even though less than a cubic meter of the gas had previously been obtained.[30] Some of this gas was used in the world's first helium-filled airship, the U.S. Navy'sC-class blimp C-7, which flew its maiden voyage fromHampton Roads, Virginia, toBolling Field in Washington, D.C., on December 1, 1921,[72] nearly two years before the Navy's firstrigid helium-filled airship, theNaval Aircraft Factory-builtUSSShenandoah, flew in September 1923.
Although the extraction process using low-temperaturegas liquefaction was not developed in time to be significant during World War I, production continued. Helium was primarily used as alifting gas in lighter-than-air craft. During World War II, the demand increased for helium for lifting gas and for shielded arcwelding. Thehelium mass spectrometer was also vital in the atomic bombManhattan Project.[73]
Thegovernment of the United States set up theNational Helium Reserve in 1925 atAmarillo, Texas, with the goal of supplying militaryairships in time of war and commercial airships in peacetime.[30] Because of theHelium Act of 1925, which banned the export of scarce helium on which the US then had a production monopoly, together with the prohibitive cost of the gas, GermanZeppelins were forced to use hydrogen as lifting gas, which would gain infamy in theHindenburg disaster. The helium market after World War II was depressed but the reserve was expanded in the 1950s to ensure a supply ofliquid helium as a coolant to create oxygen/hydrogenrocket fuel (among other uses) during theSpace Race andCold War. Helium use in the United States in 1965 was more than eight times the peak wartime consumption.[74]
After the Helium Acts Amendments of 1960 (Public Law 86–777), theU.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 fromBushton, 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, at which time it was further purified.[75]
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.[79]
For many years, the United States produced more than 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 inArzew, Algeria, producing 17 million cubic metres (600 million cubic feet) began operation, with enough production to cover all of Europe's demand. Meanwhile, by 2000, the consumption of helium within the U.S. had risen to more than 15 million kg per year.[80] In 2004–2006, additional plants inRas Laffan,Qatar, andSkikda, Algeria were built. Algeria quickly became the second leading producer of helium.[81] Through this time, both helium consumption and the costs of producing helium increased.[82] From 2002 to 2007 helium prices doubled.[83]
As of 2012[update], theUnited States National Helium Reserve accounted for 30 percent of the world's helium.[84] The reserve was expected to run out of helium in 2018.[84] Despite that, a proposed bill in theUnited States Senate would allow the reserve to continue to sell the gas. Other large reserves were in theHugoton inKansas, United States, and nearby gas fields of Kansas and thepanhandles ofTexas andOklahoma. New helium plants were scheduled to open in 2012 inQatar, Russia, and the US state ofWyoming, but they were not expected to ease the shortage.[84]
In 2013, Qatar started up the world's largest helium unit,[85] although the2017 Qatar diplomatic crisis severely affected helium production there.[86] 2014 was widely acknowledged to be a year of over-supply in the helium business, following years of renowned shortages.[87] Nasdaq reported (2015) that forAir Products, an international corporation that sells gases for industrial use, helium volumes remain under economic pressure due to feedstock supply constraints.[88]
The helium atom. Depicted are thenucleus (pink) and theelectron cloud distribution (black). The nucleus (upper right) in helium-4 is in reality spherically symmetric and closely resembles the electron cloud, although for more complicated nuclei this is not always the case.
In quantum mechanics
In the perspective ofquantum mechanics, helium is the second simplestatom to model, following thehydrogen atom. Helium is composed of two electrons inatomic orbitals surrounding a nucleus containing two protons and (usually) two neutrons. As in Newtonian mechanics, no system that consists of more than two particles can be solved with an exact analytical mathematical approach (see3-body problem) and helium is no exception. Thus, numerical mathematical methods are required, even to solve the system of one nucleus and two electrons. Suchcomputational chemistry methods have been used to create a quantum mechanical picture of helium electron binding which is accurate to within < 2% of the correct value, in a few computational steps.[89] Such models show that each electron in helium partly screens the nucleus from the other, so that theeffective nuclear chargeZeff which each electron sees is about 1.69 units, not the 2 charges of a classic "bare" helium nucleus.
Related stability of the helium-4 nucleus and electron shell
The nucleus of the helium-4 atom is identical with analpha particle. High-energy electron-scattering experiments show its charge to decrease exponentially from a maximum at a central point, exactly as does the charge density of helium's ownelectron cloud. This symmetry reflects similar underlying physics: the pair of neutrons and the pair of protons in helium's nucleus obey the same quantum mechanical rules as do helium's pair of electrons (although the nuclear particles are subject to a different nuclear binding potential), so that all thesefermions fully occupy 1s orbitals in pairs, none of them possessing orbital angular momentum, and each cancelling the other's intrinsic spin. This arrangement is thus energetically extremely stable for all these particles and hasastrophysical implications.[90] Namely, adding another particle – proton, neutron, or alpha particle – would consume rather than release energy; all systems withmass number 5, as well asberyllium-8 (comprising two alpha particles), are unbound.[91]
For example, the stability and low energy of the electron cloud state in helium accounts for the element's chemical inertness, and also the lack of interaction of helium atoms with each other, producing the lowest melting and boiling points of all the elements. In a similar way, the particular energetic stability of the helium-4 nucleus, produced by similar effects, accounts for the ease of helium-4 production in atomic reactions that involve either heavy-particle emission or fusion. Some stable helium-3 (two protons and one neutron) is produced in fusion reactions from hydrogen, though its estimated abundance in the universe is about10−5 relative to helium-4.[92]
Binding energy per nucleon of common isotopes. The binding energy per particle of helium-4 is significantly larger than all nearby nuclides.
The unusual stability of the helium-4 nucleus is also importantcosmologically: it explains the fact that in the first few minutes after theBig Bang, as the "soup" of free protons and neutrons which had initially been created in about 6:1 ratio cooled to the point that nuclear binding was possible, almost all first compound atomic nuclei to form were helium-4 nuclei. Owing to the relatively tight binding of helium-4 nuclei, its production consumed nearly all of the free neutrons in a few minutes, before they could beta-decay, and thus few neutrons were available to form heavier atoms such as lithium, beryllium, or boron. Helium-4 nuclear binding per nucleon is stronger than in any of these elements (seenucleogenesis andbinding energy) and thus, once helium had been formed, no energetic drive was available to make elements 3, 4 and 5.[93] It is barely energetically favorable for helium to fuse into the next element with a lower energy pernucleon, carbon. However, due to the short lifetime of the intermediate beryllium-8, this process requires three helium nuclei striking each other nearly simultaneously (seetriple-alpha process).[91] There was thus no time for significant carbon to be formed in the few minutes after the Big Bang, before the early expanding universe cooled to the temperature and pressure point where helium fusion to carbon was no longer possible. This left the early universe with a very similar ratio of hydrogen/helium as is observed today (3 parts hydrogen to 1 part helium-4 by mass), with nearly all the neutrons in the universe trapped in helium-4.
All heavier elements (including those necessary for rocky planets like the Earth, and for carbon-based or other life) have thus been created since the Big Bang in stars which were hot enough to fuse helium itself. All elements other than hydrogen and helium today account for only 2% of the mass of atomic matter in the universe. Helium-4, by contrast, comprises about 24% of the mass of the universe's ordinary matter—nearly all the ordinary matter that is not hydrogen.[92][94]
Gas and plasma phases
Helium discharge tube shaped into 'He', the element's symbol.
Helium is the second least reactive noble gas afterneon, and thus the second least reactive of all elements.[95] It ischemically inert and monatomic in all standard conditions. Because of helium's relatively low molar (atomic) mass, itsthermal conductivity,specific heat, andsound speed in the gas phase are all greater than any other gas excepthydrogen. For these reasons and the small size of helium monatomic molecules, heliumdiffuses through solids at a rate three times that of air and around 65% that of hydrogen.[30]
Helium is the least water-soluble monatomic gas,[96] and one of the least water-soluble of any gas (CF4,SF6, andC4F8 have lower mole fraction solubilities: 0.3802, 0.4394, and 0.2372 x2/10−5, respectively, versus helium's 0.70797 x2/10−5),[97] and helium'sindex of refraction is closer to unity than that of any other gas.[98] Helium has a negativeJoule–Thomson coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand. Only below itsJoule–Thomson inversion temperature (of about 32 to 50 K at 1 atmosphere) does it cool upon free expansion.[30] Once precooled below this temperature, helium can be liquefied through expansion cooling.
Most extraterrestrial helium isplasma in stars, 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 thesolar wind together with ionized hydrogen, the particles interact with the Earth'smagnetosphere, giving rise toBirkeland currents and theaurora.[99]
Phase diagram of helium-4. (Atmospheric pressure is about 0.1 MPa)Liquefied helium. This helium is not only liquid, but has been cooled to the point ofsuperfluidity. The drop of liquid at the bottom of the glass represents helium spontaneously escaping from the container over the side, to empty out of the container. The energy to drive this process is supplied by the potential energy of the falling helium.
Helium liquifies when cooled below 4.2 K at atmospheric pressure. Unlike any other element, however, helium remains liquid down to a temperature ofabsolute zero. This is a direct effect of quantum mechanics: specifically, thezero point energy of the system is too high to allow freezing. Pressures above about 25 atmospheres are required to freeze it. There are two liquid phases: Helium I is a conventional liquid, and Helium II, which occurs at a lower temperature, is asuperfluid.
Helium I
Below itsboiling point of 4.22 K (−268.93 °C; −452.07 °F) and above thelambda point of 2.1768 K (−270.9732 °C; −455.7518 °F), theisotope helium-4 exists in a normal colorless liquid state, calledhelium I.[30] Like othercryogenic liquids, helium I boils when it is heated and contracts when its temperature is lowered. Below the lambda point, however, helium does not boil, and it expands as the temperature is lowered further.
Helium I has a gas-likeindex of refraction of 1.026 which makes its surface so hard to see that floats ofStyrofoam are often used to show where the surface is.[30] This colorless liquid has a very lowviscosity and a density of 0.145–0.125 g/mL (between about 0 and 4 K),[100] which is only one-fourth the value expected fromclassical physics.[30]Quantum mechanics is needed to explain this property and thus both states of liquid helium (helium I and helium II) are calledquantum 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.[30]
Liquid helium below its lambda point (calledhelium II) exhibits very unusual characteristics. Due to its highthermal conductivity, when it boils, it does not bubble but rather evaporates directly from its surface.Helium-3 also has asuperfluid phase, but only at much lower temperatures; as a result, less is known about the properties of the isotope.[30]
Unlike ordinary liquids, helium II will creep along surfaces in order to reach an equal level; after a short while, the levels in the two containers will equalize. TheRollin film also covers the interior of the larger container; if it were not sealed, the helium II would creep out and escape.[30]
Helium II is a superfluid, aquantum mechanical state of matter with strange properties. For example, when it flows through capillaries as thin as 10 to 100nm it has no measurableviscosity.[28] However, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed. Existing theory explains this using thetwo-fluid model for helium II. In this model, liquid helium below the lambda point is viewed as containing a proportion of helium atoms in aground 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.[101]
In thefountain effect, a chamber is constructed which is connected to a reservoir of helium II by asintered 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.[102]
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 ofcopper.[30] This is because heat conduction occurs by an exceptional quantum mechanism. Most materials that conduct heat well have avalence band of free electrons which serve to transfer the heat. Helium II has no such valence band but nevertheless conducts heat well. Theflow of heat is governed by equations that are similar to thewave 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 assecond sound.[30]
Helium II also exhibits a creeping effect. When a surface extends past the level of helium II, the helium II moves along the surface, against the force ofgravity. 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 aRollin film and is named after the man who first characterized this trait,Bernard V. Rollin.[30][103][104] As a result of this creeping behavior and helium II's ability to leak rapidly through tiny openings, it is very difficult to confine. 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 asgravity waves in shallow water, but rather than gravity, the restoring force is thevan der Waals force.[105] These waves are known asthird sound.[106]
Solid phases
Helium remains liquid down toabsolute zero at atmospheric pressure, but it freezes at high pressure. Solid helium requires a temperature of 1–1.5 K (about −272 °C or −457 °F) at about 25 bar (2.5 MPa) of pressure.[107] It is often hard to distinguish solid from liquid helium since therefractive index of the two phases are nearly the same. The solid has a sharpmelting point and has acrystalline structure, but it is highlycompressible; applying pressure in a laboratory can decrease its volume by more than 30%.[108] With abulk modulus of about 27MPa[109] it is ~100 times more compressible than water. Solid helium has a density of0.214±0.006 g/cm3 at 1.15 K and 66 atm; the projected density at 0 K and 25 bar (2.5 MPa) is0.187±0.009 g/cm3.[110] At higher temperatures, helium will solidify with sufficient pressure. At room temperature, this requires about 114,000 atm.[111]
Helium-4 and helium-3 both form several crystalline solid phases, all requiring at least 25 bar. They both form an α phase, which has ahexagonal close-packed (hcp) crystal structure, a β phase, which isface-centered cubic (fcc), and a γ phase, which isbody-centered cubic (bcc).[112]
There are nine knownisotopes of helium of which two,helium-3 andhelium-4, arestable. In the Earth's atmosphere, one atom is3 He for every million that are4 He.[28] 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 byalpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because itsnucleons are arranged intocomplete shells. It was also formed in enormous quantities duringBig Bang nucleosynthesis.[113]
Helium-3 is present on Earth only in trace amounts. Most of it has been present since Earth's formation, though some falls to Earth trapped incosmic dust.[114] Trace amounts are also produced by thebeta decay oftritium.[115] 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'smantle.[114]3 He is much more abundant in stars as a product of nuclear fusion. Thus in theinterstellar medium, the proportion of3 He to4 He is about 100 times higher than on Earth.[116] Extraplanetary material, such aslunar andasteroidregolith, have trace amounts of helium-3 from being bombarded bysolar winds. TheMoon's surface contains helium-3 at concentrations on the order of 10ppb, much higher than the approximately 5ppt found in the Earth's atmosphere.[117][118] A number of people, starting with Gerald Kulcinski in 1986,[119] have proposed to explore the Moon, mine lunar regolith, and use the helium-3 forfusion.
Liquid helium-4 can be cooled to about 1 K (−272.15 °C; −457.87 °F) usingevaporative cooling in a1-K pot. Similar cooling of helium-3, which has a lower boiling point, can achieve about0.2 kelvin in ahelium-3 refrigerator. Equal mixtures of liquid3 He and4 He below0.8 K separate into two immiscible phases due to their dissimilarity (they follow differentquantum statistics: helium-4 atoms arebosons while helium-3 atoms arefermions).[30]Dilution refrigerators use this immiscibility to achieve temperatures of a few millikelvins.[120]
It is possible to produceexotic helium isotopes, which rapidly decay into other substances. The shortest-lived heavy helium isotope is theunbound helium-10 with ahalf-life of2.6(4)×10−22 s.[7] Helium-6 decays by emitting abeta particle and has a half-life of 0.8 second. Helium-7 and helium-8 are created in certainnuclear reactions.[30] Helium-6 and helium-8 are known to exhibit anuclear halo.[30]
Properties
Table of thermal and physical properties of helium gas at atmospheric pressure:[121][122]
Structure of thehelium hydride ion,[123] HeH+Structure of the suspected fluoroheliate anion, OHeF−
Helium has avalence of zero and is chemically unreactive under all normal conditions.[108] It is an electrical insulator unlessionized. As with the other noble gases, helium has metastableenergy levels that allow it to remain ionized in an electrical discharge with avoltage below itsionization potential.[30] Helium can form unstablecompounds, known asexcimers, with tungsten, iodine, fluorine, sulfur, and phosphorus when it is subjected to aglow discharge, to electron bombardment, or reduced toplasma by other means. The molecular compounds HeNe, HgHe10, and WHe2, and the molecular ionsHe+ 2,He2+ 2,HeH+ , andHeD+ have been created this way.[124] HeH+ is also stable in its ground state but is extremely reactive—it is the strongestBrønsted acid known, and therefore can exist only in isolation, as it will protonate any molecule or counteranion it contacts. This technique has also produced the neutral molecule He2, which has a large number ofband systems, and HgHe, which is apparently held together only by polarization forces.[30]
Theoretically, other true compounds may be possible, such as helium fluorohydride (HHeF), which would be analogous toHArF, discovered in 2000.[126] Calculations show that two new compounds containing a helium-oxygen bond could be stable.[127] Two new molecular species, predicted using theory, CsFHeO and N(CH3)4FHeO, are derivatives of a metastable FHeO− anion first theorized in 2005 by a group from Taiwan.[128]
Helium atoms have been inserted into the hollow carbon cage molecules (thefullerenes) by heating under high pressure. Theendohedral fullerene molecules formed are stable at high temperatures. When chemical derivatives of these fullerenes are formed, the helium stays inside.[129] Ifhelium-3 is used, it can be readily observed by heliumnuclear magnetic resonance spectroscopy.[130] 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.
Under high pressures helium can form compounds with various other elements. Helium-nitrogenclathrate (He(N2)11) crystals have been grown at room temperature at pressures ca. 10 GPa in adiamond anvil cell.[131] TheinsulatingelectrideNa2He has been shown to be thermodynamically stable at pressures above 113 GPa. It has afluorite structure.[132]
Occurrence and production
Natural abundance
Although it is rare on Earth, helium is the second most abundant element in the known Universe, constituting 23% of itsbaryonic mass. Only hydrogen is more abundant.[28] The vast majority of helium was formed byBig Bang nucleosynthesis one to three minutes after the Big Bang. As such, measurements of its abundance contribute to cosmological models. The firstmolecular bonds when formed when primordial helium atoms combined with protons to form helium hydride ions,[133] HeH+. Instars, helium is formed by thenuclear fusion of hydrogen inproton–proton chain reactions and theCNO cycle, part ofstellar nucleosynthesis.[113]
In theEarth's atmosphere, the concentration of helium by volume is only 5.2 parts per million.[134][135] The concentration is low and fairly constant despite the continuous production of new helium because most helium in the Earth's atmosphereescapes into space by several processes.[136][137][138] In the Earth'sheterosphere, a part of the upper atmosphere, helium and hydrogen are the most abundant elements.
Most helium on Earth is a result ofradioactive decay. Helium is found in large amounts in minerals ofuranium andthorium, such asuraninite and its varietiescleveite and pitchblende,[20][139] as well ascarnotite andmonazite (a group name; "monazite" usually refers tomonazite-(Ce)),[140][141] because they emit alpha particles (helium nuclei, He2+) to which electrons immediately combine as soon as the particle is stopped by the rock. In this way an estimated 3000 metric tons of helium are generated per year throughout thelithosphere.[142][143][144] 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 mineralsprings, volcanic gas, andmeteoric iron. Because helium is trapped in the subsurface under conditions that also trap natural gas, the greatest natural concentrations of helium on the planet are found in natural gas, from which most commercial helium is extracted. The concentration varies in a broad range from a few ppm to more than 7% in a small gas field inSan Juan County, New Mexico.[145][146]
As of 2021[update], the world's helium reserves were estimated at 31 billion cubic meters, with a third of that being inQatar.[147] In 2015 and 2016 additional probable reserves were announced to be under the Rocky Mountains in North America[148] and in theEast African Rift.[26]
TheBureau of Land Management (BLM) has proposed an October 2024 plan for managing natural resources in western Colorado. The plan involves closing 543,000 acres to oil and gas leasing while keeping 692,300 acres open. Among the open areas, 165,700 acres have been identified as suitable for helium recovery. The United States possesses an estimated 306 billion cubic feet of recoverable helium, sufficient to meet current consumption rates of 2.15 billion cubic feet per year for approximately 150 years.[149]
Extracting helium from air is not economical.[150] For large-scale use, helium is extracted byfractional distillation from natural gas, which can contain as much as 7% helium.[151] Since helium has a lowerboiling point than any other element, low temperatures and high pressure are used to liquefy nearly all the other gases (mostlynitrogen andmethane). 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.[30] The principal impurity in Grade-A helium isneon. In a final production step, most of the helium that is produced is liquefied via acryogenic 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.[81][152]
In 2008, approximately 169 millionstandard cubic meters (SCM) of helium were extracted from natural gas or withdrawn from helium reserves, with approximately 78% from the United States, 10% from Algeria, and most of the remainder from Russia, Poland, and Qatar.[153] By 2013, increases in helium production in Qatar (under the companyQatargas managed byAir Liquide) had increased Qatar's fraction of world helium production to 25%, making it the second largest exporter after the United States.[154] In 2024, the United States surpassed Qatar as the world's largest producer of the gas, having extracted 68 million SCM of helium that year compared to Qatar's 64 million SCM.[155]An estimated 54 billion cubic feet (1.5×109 m3) deposit of helium was found in Tanzania in 2016,[156] and a large-scale helium plant was opened inNingxia,China in 2020.[157]
In the United States, most helium is extracted from the natural gas of theHugoton and nearby gas fields in Kansas, Oklahoma, and the Panhandle Field in Texas.[81][158] Much of this gas was once sent by pipeline to theNational Helium Reserve, but since 2005, this reserve has been depleted and sold off, and it was expected to be largely depleted by 2021[154] under the October 2013Responsible Helium Administration and Stewardship Act (H.R. 527).[159] Despite efforts to sell the remaining reserve in 2021, the remnants of the National Helium Reserve were auctioned off by theBureau of Land Management over the course of 3 years.[160][161][162] It was finally sold to Messer Group[163] on June 27, 2024.[155] The helium fields of the western United States are emerging as an alternate source of helium supply, particularly those of the "Four Corners" region (the states of Arizona, Colorado, New Mexico and Utah).[164]
Diffusion of crude natural gas through specialsemipermeable membranes and other barriers is another method to recover and purify helium.[165] In 1996, the U.S. hadproven helium reserves in such gas well complexes of about 147 billionstandard cubic feet (4.2 billion SCM).[166] At rates of use at that time (72 million SCM per year in the U.S.; see pie chart below) this would have been enough helium for about 58 years of U.S. use, and less than this (perhaps 80% of the time) at world use rates, although factors in saving and processing impact effective reserve numbers.
Helium is commercially available in either liquid or gaseous form. As a liquid, it can be supplied in small insulated containers calleddewars which hold as much as 1,000 liters of helium, or in large ISO containers, which have nominal capacities as large as 42 m3 (around 11,000 U.S.gallons). In gaseous form, small quantities of helium are supplied in high-pressure cylinders holding as much as 8 m3 (approximately . 282 standard cubic feet), while large quantities of high-pressure gas are supplied in tube trailers, which have capacities of as much as 4,860 m3 (approx. 172,000 standard cubic feet).
According to helium conservationists like Nobel laureate physicistRobert Coleman Richardson, writing in 2010, the free market price of helium has contributed to "wasteful" usage (e.g. forhelium balloons). Prices in the 2000s had been lowered by the decision of the U.S. Congress to sell off the country's large helium stockpile by 2015.[23] Richardson posited that the price of helium needed to be multiplied by 20 to eliminate excessive wasting of the gas. In a 2012 paper by Nuttall and colleagues titled "Stop squandering helium", they proposed to create an International Helium Agency that would build a sustainable helium market.[167]
Applications
The largest single use of liquid helium is to cool the superconducting magnets in modernMRI scanners.
Estimated 2014 U.S. fractional helium use by category. Total use is 34 million cubic meters.[168]
Cryogenics (32.0%)
Pressurizing and purging (18.0%)
Welding (13.0%)
Controlled atmospheres (18.0%)
Leak detection (4.00%)
Breathing mixtures (2.00%)
Other (13.0%)
While balloons are perhaps the best-known use of helium, they are a minor part of all helium use.[76] Helium is used for many purposes that require some of its unique properties, such as its lowboiling point, lowdensity, lowsolubility, highthermal conductivity, orinertness. Of the 2014 world helium total production of about 32 million kg (180 million standard cubic meters) helium per year, the largest use (about 32% of the total in 2014) is in cryogenic applications, most of which involves cooling the superconducting magnets in medicalMRI scanners andNMR spectrometers.[169] Other major uses were pressurizing and purging systems, welding, maintenance of controlled atmospheres, and leak detection. Other uses by category were relatively minor fractions.[168]
Helium is used as ashielding gas inarc welding processes on materials that are contaminated and weakened by air or nitrogen at welding temperatures.[28] A number of inert shielding gases are used in gas tungsten arc welding, but helium is used instead of cheaperargon especially for welding materials that have higherheat conductivity, likealuminium orcopper.
Minor uses
Industrial leak detection
A dual chamber helium leak detection machine
One industrial application for helium isleak detection. Because heliumdiffuses through solids three times faster than air, it is used as a tracer gas to detectleaks in high-vacuum equipment (such as cryogenic tanks) and high-pressure containers.[172] The tested object is placed in a chamber, which is then evacuated and filled with helium. The helium that escapes through the leaks is detected by a sensitive device (helium mass spectrometer), even at the leak rates as small as 10−9 mbar·L/s (10−10 Pa·m3/s). The measurement procedure is normally automatic and is called helium integral test. A simpler procedure is to fill the tested object with helium and to manually search for leaks with a hand-held device.[173]
Helium leaks through cracks should not be confused with gas permeation through a bulk material. While helium has documented permeation constants (thus a calculable permeation rate) through glasses, ceramics, and synthetic materials, inert gases such as helium will not permeate most bulk metals.[174]
Flight
Because of its low density and incombustibility, helium is the gas of choice to fill airships such as theGoodyear blimp.
Because it islighter than air,airships and balloons are inflated with helium forlift. While hydrogen gas is more buoyant and escapes permeating through a membrane at a lower rate, helium has the advantage of being non-flammable, and indeedfire-retardant. Another minor use is inrocketry, where helium is used as anullage medium to backfill rocket propellant tanks in flight and to condense hydrogen and oxygen to makerocket fuel. It is also used to purge fuel and oxidizer from ground support equipment prior to launch and to pre-cool liquid hydrogen inspace vehicles. For example, theSaturn V rocket used in theApollo program needed about 370,000 cubic metres (13 million cubic feet) of helium to launch.[108]
Minor commercial and recreational uses
Helium as a breathing gas has nonarcotic properties, so helium mixtures such astrimix,heliox andheliair are used fordeep diving to reduce the effects of narcosis, which worsen with increasing depth.[175][176] As pressure increases with depth, the density of the breathing gas also increases, and the low molecular weight of helium is found to considerably reduce the effort of breathing by lowering the density of the mixture. This reduces theReynolds number of flow, leading to a reduction ofturbulent flow and an increase inlaminar flow, which requires less breathing.[177][178] At depths below 150 metres (490 ft) divers breathing helium-oxygen mixtures begin to experience tremors and a decrease in psychomotor function, symptoms ofhigh-pressure nervous syndrome.[179] This effect may be countered to some extent by adding an amount of narcotic gas such as hydrogen or nitrogen to a helium–oxygen mixture.[180]
For its inertness and highthermal conductivity, neutron transparency, and because it does not form radioactive isotopes under reactor conditions, helium is used as a heat-transfer medium in somegas-cooled nuclear reactors.[172]
Helium, mixed with a heavier gas such as xenon, is useful forthermoacoustic refrigeration due to the resulting highheat capacity ratio and lowPrandtl number.[181] The inertness of helium has environmental advantages over conventional refrigeration systems which contribute to ozone depletion or global warming.[182]
The use of helium reduces the distorting effects of temperature variations in the space betweenlenses in sometelescopes due to its extremely lowindex of refraction.[30] This method is especially used in solar telescopes where a vacuum tight telescope tube would be too heavy.[184][185]
The age of rocks and minerals that contain uranium and thorium can be estimated by measuring the level of helium with a process known ashelium dating.[28][30]
Thespeed of sound in helium is nearly three times the speed of sound in air. Because thenatural resonance frequency of a gas-filled cavity is proportional to the speed of sound in the gas, when helium is inhaled, a corresponding increase occurs in theresonant frequencies of thevocal tract, which is the amplifier of vocal sound.[28][190] This increase in the resonant frequency of the amplifier (the vocal tract) gives increased amplification to the high-frequency components of the sound wave produced by the direct vibration of the vocal folds, compared to the case when the voice box is filled with air. When a person speaks after inhaling helium gas, the muscles that control the voice box still move in the same way as when the voice box is filled with air; therefore thefundamental frequency (sometimes calledpitch) produced by direct vibration of the vocal folds does not change.[191] However, the high-frequency-preferred amplification causes a change intimbre of the amplified sound, resulting in a reedy, duck-like vocal quality. The opposite effect, lowering resonant frequencies, can be obtained by inhaling a dense gas such assulfur hexafluoride orxenon.
Inhaling helium can be dangerous if done to excess, since helium is a simpleasphyxiant and so displaces oxygen needed for normal respiration.[28][193] Fatalities have been recorded, including a youth who suffocated in Vancouver in 2003 and two adults who suffocated in South Florida in 2006.[194][195] In 1998, an Australian girl from Victoria fell unconscious and temporarilyturned blue after inhaling the entire contents of a party balloon.[196][197][198]Inhaling helium directly from pressurized cylinders or even balloon filling valves is extremely dangerous, as high flow rate and pressure can result inbarotrauma, fatally rupturing lung tissue.[193][199]
Death caused by helium is rare. The first media-recorded case was that of a 15-year-old girl from Texas who died in 1998 from helium inhalation at a friend's party; the exact type of helium death is unidentified.[196][197][198]
In the United States, only two fatalities were reported between 2000 and 2004, including a man who died in North Carolina of barotrauma in 2002.[194][199] A youth asphyxiated in Vancouver during 2003, and a 27-year-old man in Australia had an embolism after breathing from a cylinder in 2000.[194] Since then, two adults asphyxiated in South Florida in 2006,[194][195][200] and there were cases in 2009 and 2010, one of whom was a Californian youth who was found with a bag over his head, attached to a helium tank,[201] and another teenager in Northern Ireland died of asphyxiation.[202] AtEagle Point, Oregon a teenage girl died in 2012 from barotrauma at a party.[203][204][205] A girl from Michigan died from hypoxia later in the year.[206]
On February 4, 2015, it was revealed that, during the recording of their main TV show on January 28, a 12-year-old member (name withheld) of Japanese all-girl singing group3B Junior suffered fromair embolism, losing consciousness and falling into acoma as a result of air bubbles blocking the flow of blood to the brain after inhaling huge quantities of helium as part of a game. The incident was not made public until a week later.[207][208] The staff ofTV Asahi held an emergency press conference to communicate that the member had been taken to the hospital and is showing signs of rehabilitation such as moving eyes and limbs, but her consciousness has not yet been sufficiently recovered. Police have launched an investigation due to a neglect of safety measures.[209][210]
The safety issues for cryogenic helium are similar to those ofliquid nitrogen; its extremely low temperatures can result incold 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 significantthermal expansion that occurs when helium gas at less than 10 K is warmed toroom temperature.[108]
At high pressures (more than about 20 atm or two MPa), a mixture of helium and oxygen (heliox) can lead tohigh-pressure nervous syndrome, a sort of reverse-anesthetic effect; adding a small amount of nitrogen to the mixture can alleviate the problem.[211][179]
^A few authors dispute the placement of helium in the noble gas column, preferring to place it aboveberyllium with thealkaline earth metals. They do so on the grounds of helium's 1s2 electron configuration, which is analogous to the ns2 valence configurations of the alkaline earth metals, and furthermore point to some specific trends that are more regular if helium is placed in group 2.[8][9][10][11][12] These tend to relate tokainosymmetry and the first-row anomaly: the first orbital of any type is unusually small, since unlike its higher analogues, it does not experience interelectronic repulsion from a smaller orbital of the same type. Because of this trend in the sizes of orbitals, a large difference in atomic radii between the first and second members of each main group is seen in groups 1 and 13–17: it exists between neon and argon, and between helium and beryllium, but not between helium and neon. This similarly affects the noble gases' boiling points and solubilities in water, where helium is too close to neon, and the large difference characteristic between the first two elements of a group appears only between neon and argon. Moving helium to group 2 makes this trend consistent in groups 2 and 18 as well, by making helium the first group 2 element and neon the first group 18 element: both exhibit the characteristic properties of a kainosymmetric first element of a group.[13] However, the classification of helium with the other noble gases remains near-universal, as its extraordinary inertness is extremely close to that of the other light noble gases neon and argon.[14]
^Shuen-Chen Hwang, Robert D. Lein, Daniel A. Morgan (2005). "Noble Gases".Kirk Othmer Encyclopedia of Chemical Technology. Wiley. pp. 343–383.doi:10.1002/0471238961.0701190508230114.a01.
^Rayet, G. (1868)"Analyse spectral des protubérances observées, pendant l'éclipse totale de Soleil visible le 18 août 1868, à la presqu'île de Malacca" (Spectral analysis of the protuberances observed during the total solar eclipse, seen on 18 August 1868, from the Malacca peninsula),Comptes rendus ... ,67 : 757–759. From p. 758:" ... je vis immédiatement une série de neuf lignes brillantes qui ... me semblent devoir être assimilées aux lignes principales du spectre solaire, B, D, E, b, une ligne inconnue, F, et deux lignes du groupe G." ( ... I saw immediately a series of nine bright lines that ... seemed to me should be classed as the principal lines of the solar spectrum, B, D, E, b, an unknown line, F, and two lines of the group G.)
^Captain C. T. Haig (1868)"Account of spectroscopic observations of the eclipse of the sun, August 18th, 1868"Proceedings of the Royal Society of London,17 : 74–80. From p. 74: "I may state at once that I observed the spectra of two red flames close to each other, and in their spectra two broad bright bands quite sharply defined, one rose-madder and the other light golden."
^Pogson filed his observations of the 1868 eclipse with the local Indian government, but his report wasn't published. (Biman B. Nath,The Story of Helium and the Birth of Astrophysics (New York, New York: Springer, 2013),p. 8.) Nevertheless, Lockyer quoted from his report.From p. 320Archived 17 August 2018 at theWayback Machine of Lockyer, J. Norman (1896) "The story of helium. Prologue,"Nature,53 : 319–322 : "Pogson, in referring to the eclipse of 1868, said that the yellow line was "at D, or near D." "
^Lieutenant John Herschel (1868)"Account of the solar eclipse of 1868, as seen at Jamkandi in the Bombay Presidency,"Proceedings of the Royal Society of London,17 : 104–120. From p. 113: As the moment of the total solar eclipse approached, " ... I recorded an increasing brilliancy in the spectrum in the neighborhood of D, so great in fact as to prevent any measurement of that line till an opportune cloud moderated the light. I am not prepared to offer any explanation of this." From p. 117: "I also consider that there can be no question that the ORANGE LINE was identical with D, so far as the capacity of the instrument to establish any such identity is concerned."
^In his initial report to the French Academy of Sciences about the 1868 eclipse, Janssen made no mention of a yellow line in the solar spectrum. See:
Wheeler M. Sears,Helium: The Disappearing Element (Heidelberg, Germany: Springer, 2015),p. 44.
Françoise Launay with Storm Dunlop, trans.,The Astronomer Jules Janssen: A Globetrotter of Celestial Physics (Heidelberg, Germany: Springer, 2012),p. 45.
However, subsequently, in an unpublished letter of 19 December 1868 to Charles Sainte-Claire Deville, Janssen asked Deville to inform the French Academy of Sciences that : "Several observers have claimed the bright D line as forming part of the spectrum of the prominences on 18 August. The bright yellow line did indeed lie very close to D, but the light was more refrangible [i.e., of shorter wavelength] than those of the D lines. My subsequent studies of the Sun have shown the accuracy of what I state here." (See: (Launay, 2012), p. 45.)
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^Thomson, William (August 3, 1871)."Inaugural Address of Sir William Thomson".Nature.4 (92): 261–278 [268].Bibcode:1871Natur...4..261..doi:10.1038/004261a0.PMC2070380.Archived from the original on December 2, 2016. RetrievedFebruary 22, 2016.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
^Palmieri, Luigi (1881)."La riga dell'Helium apparsa in una recente sublimazione vesuviana" [The line of helium appeared in a recently sublimated material [from Mt.] Vesuvius.].Rendiconto dell'Accademia delle Scienze Fisiche e Matematiche (Naples, Italy).20: 223.Archived from the original on 1 September 2018. Retrieved1 May 2017.Raccolsi alcun tempo fa una sostanza amorfa di consistenza butirracea e di colore giallo sbiadato sublimata sull'orlo di una fumarola prossima alla bocca di eruzione. Saggiata questa sublimazione allo spettroscopio, ho ravvisato le righe del sodio e del potassio ed una lineare ben distinta che corrisponde esattamente alla D3 che è quella dell'Helium. Do per ora il semplice annunzio del fatto, proponendomi di ritornare sopra questo argomento, dopo di aver sottoposta la sublimazione ad una analisi chimica. (I collected some time ago an amorphous substance having a buttery consistency and a faded yellow color which had sublimated on the rim of a fumarole near the mouth of the eruption. Having analyzed this sublimated substance with a spectroscope, I recognized the lines of sodium and potassium and a very distinct linear line which corresponds exactly to D3, which is that of helium. For the present, I'm making a mere announcement of the fact, proposing to return to this subject after having subjected the sublimate to a chemical analysis.)
Clève, P.T. (1895)."Sur la présence de l'hélium dans le clévéite" [On the presence of helium in cleveite].Comptes rendus hebdomadaires des séances de l'Académie des sciences (in French).120: 834.
Clève (1895)."Sur la densité de l'hélium" [On the density of helium].Comptes rendus hebdomadaires des séances de l'Académie des sciences (in French).120: 1212.
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