Contenido

Aunque el helio tiene una configuración electrónica 1s², no se coloca en el grupo 2 de la tabla periódica junto al hidrógeno y otros elementos del bloque s. En su lugar, se ubica en el grupo 18, correspondiente a los gases nobles, debido a sus propiedades químicas únicas. Este elemento, al poseer su nivel de energía completamente lleno, es químicamente inerte y no forma compuestos fácilmente, características típicas de los gases nobles. Aunque su configuración lo asociaría al bloque s, su comportamiento químico lo justifica como parte del bloque p. Esto hace que el helio sea una excepción importante en la disposición de los elementos en la tabla periódica.

En condiciones normales de presión y temperatura (25 °C y 1 atm), el helio es un gas monoatómico, no inflamable y extremadamente ligero. Es notablemente difícil de licuar debido a su baja temperatura crítica, que es de apenas 5.20K (−267.96 °C). Esto significa que, por encima de esa temperatura, el helio no puede pasar al estado líquido, sin importar cuánta presión se le aplique. A temperaturas inferiores a la crítica, el helio solo puede licuarse mediante una combinación de enfriamiento extremo y alta presión. Este comportamiento lo convierte en un caso único entre todos los elementos químicos.

Otro aspecto fascinante del helio es su elevado calor específico, especialmente en estado gaseoso. Este valor, más alto que el de la mayoría de los gases, hace que el helio se expanda rápidamente cuando se calienta, incluso a temperatura ambiente. En condiciones de bajas temperaturas, el helio líquido también exhibe propiedades únicas, como la superfluidez, una fase en la que fluye sin viscosidad, superando obstáculos sin resistencia.

El helio posee el punto de solidificación más bajo conocido. A presiones normales, incluso en el cero absoluto (0 K o −273.15 °C), el helio-4 permanece en estado líquido debido a su energía de punto cero, un efecto cuántico que impide que sus átomos se estabilicen en un estado sólido. Sin embargo, al someterlo a presiones superiores a 25 atm "Atmósfera (unidad)"), el helio puede solidificarse. El helio sólido"), a temperaturas cercanas a 1 K (−272.15 °C), adopta una estructura cristalina hexagonal compacta"). A medida que la presión aumenta, se han observado cambios a estructuras cristalinas diferentes, aunque con densidades y energías muy similares. Estos cambios reflejan las complejas interacciones entre los átomos del helio en condiciones extremas.[5]​.

The helium atom

Helium is a chemical element whose atom is the simplest to solve using the rules of quantum mechanics after the hydrogen atom. It consists of two electrons orbiting a nucleus containing two protons along with one or two neutrons, depending on the isotope. However, as in Newtonian mechanics, no system consisting of more than two particles can be solved with an exact mathematical analysis approach (see three-body problem) and helium is no exception. Thus, mathematical methods are necessary, even to solve the system of one nucleus and two electrons. However, such computational chemistry methods have been used to create a quantum mechanical picture of helium electron bindings with an accuracy within 2% of the correct value, with a few computational calculation steps. In these models it is observed that each electron partially prevents the other from sensing interaction with the nucleus, such that the effective nuclear charge Z is approximately 1.69 units, and not the 2 charges of a classical "bare nucleus" of helium.

The hydrogen atom is widely used to help solve the helium atom. Bohr's atomic model gave a very precise explanation of the spectrum of the hydrogen atom, but when an attempt was made to use it on helium the model failed. Werner Heisenberg developed a modification of Bohr's analysis, in which he used semi-integrated values ​​of the quantum numbers. Density functional theory is used to obtain the ground state energy levels of the helium atom, together with the Hartree-Fock method.

The nucleus of the helium-4 atom, which is exactly the same as an alpha particle,[7]​ is particularly interesting. The reason for this is because high-energy electron scattering experiments have shown that its charge decreases exponentially from a maximum at its central point, in exactly the same way that the charge density in its own electron cloud decreases. This symmetry reflects similar physical principles: the pair of neutrons and protons in the nucleus of helium obey the same quantum mechanical rules as the two electrons that orbit it - although the union of the particles in the nucleus is due to a different potential "Potential (physics)" than that which keeps the electrons in the cloud around the atom. In this way, these fermions (that is, both protons, electrons and neutrons) completely occupy the 1s orbitals in pairs, none of them possesses orbital angular momentum and each of them cancels the intrinsic spin of the other. Adding another of any of these particles would require angular momentum and release substantially less energy (in fact, no nucleus with five nucleons is stable). For this reason, this arrangement for these particles is extremely energetically stable, and this stability gives rise to many crucial phenomena inherent to helium in nature.

As an example of these facts due to the high stability of the electronic configuration of helium is the low chemical reactivity of this element (the lowest in the entire periodic table), as well as the lack of interaction of its atoms among themselves. This produces the lowest melting and boiling points of all the elements. In the same way, the energetic stability of the helium-4 nucleus gives rise to its easy production in atomic reactions that involve both the emission of heavy particles and nuclear fusion. Some stable helium-3 is produced in fusion reactions from hydrogen, but it is a much smaller fraction compared to helium-4. The stability of helium-4 is the reason why hydrogen is converted to this form of helium in the Sun, instead of helium-3, deuterium or other heavier elements. It is also partially responsible for the fact that the alpha particle is by far the type of baryon particle most commonly ejected from atomic nuclei. In other words, alpha decay is much more common than decay into heavier nuclei.[8]​.

The unusual stability of helium-4 is also important in cosmology. In the first minutes after the Big Bang, the universe was composed of a mixture of free nucleons (protons and neutrons). This "soup" originally had a ratio of six protons to each neutron, and after a time it cooled to the point where nuclear fusion could occur.[9] The stability of helium meant that almost all of the nucleon aggregations formed at that time were helium-4 nuclei. The union of protons and neutrons to form helium-4 is so strong that, in fact, the production of this element consumed almost all the free neutrons in a matter of minutes, before these nuclei could decay through beta decay. This left a very small amount of these particles for lithium, beryllium or boron to form. The nuclear bond per nucleon in helium-4 is stronger than in any of these three elements (see nucleogenesis and binding energy). Therefore, there was no energetic mechanism available, once helium had been formed, to create the elements of atomic number 3, 4 and 5. In terms of energy, the fusion of helium to form the next element in the periodic table with the lowest energy per nucleon was also favorable: carbon. However, due to the lack of intermediate elements, this process required the almost simultaneous collision of three helium-4 nuclei (see triple-alpha process), so there was not enough time for carbon to form in the Big Bang: within minutes, the early universe cooled to a temperature and pressure at which the fusion of helium to carbon was no longer possible. This caused the early universe to have a hydrogen/helium ratio very similar to that observed today (by mass, three parts hydrogen to one part helium-4), with almost all of the neutrons in the universe—as is the case today—trapped within helium-4 nuclei.

All of the heavier elements—including those needed to form rocky planets like Earth and for carbon-based life to exist—had to be created later, in stars hot enough to burn not just hydrogen—since this only produces more helium—but helium itself. Such stars are massive and therefore rare. The above gives rise to the fact that all chemical elements, apart from hydrogen and helium, make up only 2% of the mass in the form of atoms in the universe. Helium-4, for its part, constitutes about 23% of all ordinary matter in the universe, that is, practically all ordinary matter that is not hydrogen.[10]​.

Gas and plasma phases

Helium is the least reactive noble gas after neon and therefore, the second least reactive element of all of them. It is inert and monatomic under normal conditions. Due to its low atomic mass, in the gas phase, the thermal conductivity, specific heat, and speed of sound are greater than in any other gas, except hydrogen. For similar reasons, and also due to the small size of its atoms, its diffusion rate through solids is three times that of air, and about 65% that of hydrogen.[11]​.

It is also less soluble in water than any other known gas,[12]​ and its refractive index is the closest to unity of all gases.[13]​ This element has a negative Joule-Thomson coefficient at normal room temperature, meaning that it heats up when allowed to expand freely. Only below its Joule–Thomson inversion temperature (32 to 50 K at 1 atmosphere "Atmosphere (unit)") does it cool in free expansion.[11] Once precooled below this temperature, helium can be liquefied by cooling due to its expansion.

Most extraterrestrial helium is in a plasma state "Plasma (state of matter)"), with very different properties than atomic helium. In plasma, helium's electrons are not bound to the nucleus, making its electrical conductivity very high, even when the gas is only partially ionized. Charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind, along with ionized hydrogen, particles interact with the Earth's magnetosphere, giving rise to the Birkeland current and auroras.[14]​.

Liquid and solid phases

Unlike any other element, liquid helium will stay that way 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 to 1.5 K (about -272 °C or -457 °F) and about 25 bar (2.5 MPa) pressure.[15] It is often difficult to distinguish solid helium from liquid since the refractive index of the two phases is almost the same. The solid has a marked melting point and crystalline structure, but is very compressible. Applying pressure in a laboratory can reduce its volume by more than 30%.[16]​ With a compressibility modulus of the order of 50 MPa,[16]​ 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 (2.5 MPa) is 0.187 ± 0.009 g/ml.[17]​.

Below its boiling point of 4.22 K, and above the lambda point of 2.1768 K, the isotope helium-4 exists in a normal colorless liquid state, called helium I.[11] Like other cryogenic liquids, helium I boils when heated and contracts when its temperature is lowered. Below the lambda point, however, this phase does not boil and expands as the temperature drops further.

Helium has a refractive index similar to that of a gas, 1.026, which makes its surface very difficult to see, so extruded polystyrene floats are often used to see where the surface is.[11] This colorless liquid has a very low viscosity and a density of 0.145 g/mL, which is only a quarter of the value predicted by classical physics.[11] It is necessary make use of quantum mechanics to explain this property and, therefore, both types of liquid helium are called quantum fluids, which means that they show atomic properties on a macroscopic scale. This may be an effect of the fact that its boiling point is very close to absolute zero, which prevents random molecular motion (thermal energy) from obscuring its atomic properties.[11]​.

Liquid helium below its lambda point shows extremely unusual characteristics, in a state called helium II. Boiling of helium II is not possible due to its high thermal conductivity; Heat input causes evaporation of 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 the properties of this phase in that isotope.[11]​.

Helium II is a superfluid, a quantum state of matter with strange properties. For example, when it flows through capillaries as thin as 10 to 10 m, it has no measurable viscosity. However, when measurements are made between two moving disks, a viscosity comparable to that of gaseous helium is observed. Current theory explains this phenomenon using a two-fluid model for helium II. In this model, liquid helium below the lambda point is considered to contain a proportion of ground-state helium atoms, which make up the superfluid, and which flow with a viscosity exactly equal to zero; and a proportion of helium atoms in an excited state, which behave more like an ordinary fluid.[18]​.

Since helium is a noble gas, in practice it does not participate in chemical reactions, although under the influence of electrical discharges or bombarded with electrons it forms compounds.

Helium has a valence of zero and is not chemically reactive under normal conditions.[24] It is an electrical insulator unless it is ionized. Like other noble gases, it has metastable energy levels, allowing it to remain ionized in an electrical discharge at a voltage below its ionization potential. Helium can form unstable compounds, known as excimers, with tungsten, iodine, fluorine, and phosphorus, when subjected to a luminescent electrical discharge, electron bombardment, or is otherwise a plasma. The molecular compounds HeNe, HgHe, and WHe, and the molecular ions He, He, HeH, and HeD can be created in this way.[25] This technique has also enabled the production of the neutral molecule He, which has a large number of spectral band systems, and the molecule HgHe, which is apparently only held together by polarization forces.[11] In theory, other real compounds are also possible, such as helium fluorohydride. (HHeF), which would be analogous to argon fluorohydride"), discovered in 2000.[26] Calculations indicate that two new compounds containing a helium-oxygen bond could be stable.[27] Two new molecular species, predicted theoretically, CsFHeO and N(CH)FHeO, are derivatives of a metastable anion [F-HeO], anticipated in 2005 theoretically by a group of Taiwan. If confirmed experimentally, these compounds would end the chemical inertness of helium, and the only completely inert element would be neon.[28]​.

The helium has been placed in carbon molecular cages (the fullerenes) by heating at high pressure. The endohedral fullerene molecules formed are stable up to high temperatures. When the chemical derivatives of these fullerenes are formed, the helium remains inside them.[29]​ If helium-3 is used, it can be easily observed by nuclear magnetic resonance spectroscopy.[30]​ A large number of fullerenes containing helium-3 have been reported. Although the said atoms are not linked by means of covalent or ionic bonds, these substances have different properties and a defined composition, like all stoichiometric chemical compounds.

There are eight known isotopes of helium, but only He and He are stable. In the Earth's atmosphere there is one He atom for every million He atoms.[31] Unlike other elements, the isotopic abundance of helium varies greatly depending on its origin, due to the different formation processes. The most common isotope, He, is produced on Earth through the alpha decay of heavier radioactive elements; The alpha particles that appear are completely ionized He atoms. He has an unusually stable nucleus because its nucleons are arranged in complete shells. Furthermore, this isotope was formed in large quantities during primordial nucleosynthesis in the Big Bang.[32]​.

He is present on Earth today in only trace amounts (most dating back to the formation of the Earth), although some of it falls to Earth when trapped in cosmic dust.[33] Some traces are also produced through the beta decay of tritium.[34] Some rocks in the Earth's crust have different isotope ratios that vary by up to a factor of ten. These proportions can be used to investigate the origin of rocks as well as the composition of the Earth's mantle.[33] He is much more abundant in stars as a product of nuclear fusion. Consequently, in the interstellar medium, the ratio of He to He is about 100 times greater than that on Earth.[35] Extraplanetary material, such as asteroid and lunar regolith, has traces of He from the bombardment of solar winds against them. The surface of the Moon has He concentrations of around 0.01 ppm.[36][37] Some people, most notably Gerald Kulcinski in 1986,[38]​ have proposed exploring the Moon, excavating the lunar regolith, and using the He for nuclear fusion.

Liquid He can be cooled down to 1 kelvin using evaporative cooling in containers in which these temperatures can be reached and maintained. A similar cooling for helium-3, which has a lower boiling point, can be achieved around 0.2 K in a helium-3" refrigerator. Mixtures containing the same proportion of helium-3 and helium-4 at a temperature below 0.8 K separate into two immiscible phases due to their incompatibility (each obeys a different quantum statistic): the helium-4 atoms are bosons while the of helium-3 are fermions).[11] Dilution refrigerators") use this impossibility of mixing to reach temperatures of a few millikelvin.

It is possible to produce exotic isotopes of helium, which quickly decay into other substances. The shortest-lived heavy isotope is He, with a half-life of 7.6×10 seconds. He decomposes by emitting a beta particle and its disintegration period is 0.8 seconds. He also emits beta particles as well as gamma rays. Both He and He are created through some nuclear reactions.[11]​ He and He are known to have a nuclear halo. ²He (consisting of two protons and no neutrons) is a radioisotope that decays into protium (hydrogen) through the emission of protons, with a decay period of 3×10 seconds.[11]​.

natural abundance

Helium is the second most abundant element in the known universe after hydrogen and makes up about 23% of the baryon mass of the universe.[31] Most of the helium was formed during the nucleosynthesis of the Big Bang, in the first three minutes after it. In this way, measuring their abundance contributes to cosmological models. In stars, helium is formed by the nuclear fusion of hydrogen in proton-proton chain reactions and in the CNO cycle, which are part of stellar nucleosynthesis.[32]​.

In the Earth's atmosphere the concentration of helium by volume is only 5.2 parts per million.[39][40]​ The concentration is low and practically constant despite the continuous production of new helium, because most of the helium in the atmosphere escapes into space due to different processes.[41][42]​ In the Earth's heterosphere, a part of the upper atmosphere, helium and other light gases are the most abundant elements.

Almost all of the helium present on Earth is the result of radioactive decay, and therefore a terrestrial helium balloon is, in essence, a bag of alpha particles expelled by this process. Helium is found in large quantities in uranium and thorium minerals, including cleveite, pitchblende, carnotite and monazite, as these emit alpha particles (helium nuclei, He) and electrons immediately combine with them as soon as the particles are stopped by the rock. In this way, it is estimated that about 3,000 tons of helium are generated per year throughout the lithosphere.[43]​[44]​[45]​ In the Earth's crust, the concentration of helium is 8 parts per billion. In the sea, the concentration is only 4 parts per billion. There are also small amounts in mineral springs, volcanic gas, and meteoric iron. Because helium is trapped similarly to natural gas by an impermeable layer of rock, the highest concentrations of this element on the planet are found in natural gas, from which most commercial helium is extracted. The concentration varies over a wide range from a few ppm to more than 7% in a small gas field in San Juan County "San Juan County (New Mexico)"), New Mexico.[46]​[47]​.

In 2016 British scientists discovered a large field of helium gas in Africa.[48]​ This field, located in the United Republic of Tanzania, measures approximately 54 million cubic feet in volume and is the largest helium field in the world.[49]​.

Modern extraction

For large-scale use, it is extracted by fractional distillation from natural gas, which contains up to 7% helium.[50] Having a lower boiling point than any other element, low temperatures and high pressures are used to liquefy almost all other gases (mainly nitrogen and methane). The resulting raw helium is purified by successive exposures to low temperatures, in which almost all of the nitrogen and other remaining gases are precipitated out of the gas mixture.[11]​ As a final purification step, activated carbon is used, resulting in grade A helium, with a purity of 99.995%.[11]​ The main impurity in grade A helium is neon. In the final phase of production, the majority of the helium produced is liquefied through a cryogenic process. This is necessary for applications requiring liquid helium and also allows helium suppliers to reduce the cost of long-distance transportation, since most liquid helium containers have a capacity five times that of tank trucks transporting gaseous helium.[51][52]​.

In 2008, about 169 million standard cubic meters (SCM, defined as one cubic meter at a pressure of 1 atm "Atmosphere (unit)") and a temperature of 15 °C) of helium were extracted from natural gas or helium reserves. Of these, approximately 78% came from the United States, 10% from Algeria, and the remainder was mostly extracted in Russia, Poland, and Qatar. 4.2× SCM. This amount is enough for about 25 years of global use, or 35 years of U.S. consumption, although savings and processing factors are expected to impact effective reserve numbers. It is estimated that the basic reserves of helium not yet proven that could be obtained from natural gas in the United States are 3.1 to 5.3 × 10 SCM, or approximately four orders of magnitude greater than the proven reserves. extractions.[56]​.

Helium must be extracted mainly from natural gas, because its presence in the air is only a fraction compared to that of neon, and yet its demand is much greater. It is estimated that if all neon production were retooled to save helium, 0.1% of global helium demands would be met. Likewise, only 1% of global helium demands could be met by retooling all air distillation plants. Helium can be synthesized by bombarding lithium or boron using high-speed protons. However, this production method is totally economically unfeasible.[57]​.

Current helium reserves are being used up much faster than this element can be replenished. Given this situation, there are major concerns that the helium supply could soon run out. In the world's largest reserves, in Amarillo (Texas), Texas, this gas is expected to be depleted in eight years from 2008. This could be prevented if current users captured and recycled the gas and if oil and natural gas companies used helium capture techniques when extracting it.[58]​[59]​.

Due to a reduction in the volume of helium exports by the United States, giving priority to institutional consumers within the country over private companies, the duration of reserves has been extending. In 2019, the price of helium at the auction of the US national helium reserve increased by 135%.[60] Since then, other natural gas exporting countries, such as Algeria and Qatar, have developed helium production plants. However, the largest plant under construction, owned by Gazprom and located in Russia, has suffered delays in its start-up due to the economic sanctions that the country has been receiving since the invasion of Ukraine "Russian invasion of Ukraine (2022-present)").[61]​.

Helium is lighter than air and, unlike hydrogen, it is not flammable. Its lifting power is also 8% less than air, which is why it is used as filling gas in "Balloon (toy)" balloons and zeppelins for advertising, atmospheric research and even for military reconnaissance.

Even though the previous one is the main one, helium has more applications:

• - Helium-oxygen mixtures are used in deep diving, since helium is inert, less soluble in the blood than nitrogen and diffuses "Diffusion (physics)") 2.5 times faster than nitrogen, all of which reduces the time required for decompression. However, the latter must begin at greater depth, reducing the risk of narcosis ("deep drunkenness").

• - Due to its low liquefaction and evaporation point, it can be used as a refrigerant in extremely low temperature applications, such as in superconducting magnets and cryogenic research at temperatures close to absolute zero.

• - In gas chromatography it is used as an inert carrier gas.

• - The inert atmosphere of helium is used in arc welding and in the manufacture of silicon and germanium crystals, as well as to pressurize liquid rocket fuels.

• - In supersonic wind tunnels.

• - As a cooling agent in nuclear reactors.

• - Liquid helium is finding increasing use in medical applications of magnetic resonance imaging (MRI).

• - It is used in laser equipment as one of the most common gases, mainly the helium-neon mixture.

Of the total global helium production in 2008 of 32 million kg, its largest use (about 22% of the total in 2008) was in cryogenic applications. Of these, the majority were in medicine in the cooling of superconducting magnets in magnetic resonance scanners.[62] Other important uses (a total of about 78% of its use in 1996) were in pressurization and sanitation systems, maintenance of controlled atmospheres, and welding.[63]​.

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 its low chemical reactivity. Likewise, it is commercially available in both liquid and gaseous form. As a liquid, it can be supplied in small containers called Dewar flasks that allow up to 1000 liters of helium to be stored, or in large ISO containers that have a nominal capacity of up to 42 m³. In gaseous form, small quantities are supplied in high-pressure cylinders that can contain a volume equivalent to 8 standard m³, while large quantities of high-pressure gas are supplied in tank trucks that have a capacity equivalent to 4,860 standard m³. This is because the volume of the gas is greatly reduced when subjected to high pressures.

Because helium is lighter than air, blimps and balloons are inflated with this gas to lift them. While hydrogen experiences approximately 7% greater thrust force, helium has the advantage of being non-flammable (as well as being fire retardant). In the space industry, it is used as a filling medium to displace fuels and oxidizers in storage tanks, and to condense hydrogen and oxygen to produce rocket fuel. It is also used to purify fuel and oxidizer for ground support equipment before launch, as well as to precool liquid hydrogen in space vehicles. For example, the Saturn V booster used in the Apollo Program required about 370,000 m³ of helium to launch the rocket.

Helium is less dense than atmospheric air, so the timbre (but not the pitch "Pitch (music)") of a person's voice changes when it is inhaled.[64] This is because, since helium is a fairly light gas, it moves faster through spaces, causing the vocal folds to move at greater speed, causing a faster, and therefore higher-pitched, sound wave. However, inhalation from a typical commercial source, such as those used to fill balloons, can be dangerous due to the risk of suffocation from lack of oxygen and the number of contaminants that may be present. These may include traces of other gases, as well as aerosol lubricating oil. However, since these are children's products, there are mechanisms that require guaranteeing the non-toxicity of the gas, such as exceeding the "European Conformity" (CE marking) required for toys and similar derivatives in the European market, to guarantee the safety of children.

Because of their low solubility in nervous tissue, helium mixtures such as trimix, heliox and Heliair are used for deep diving to reduce the effects of narcosis. considerably the effort in breathing.

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

One of the industrial applications of helium is leak detection. Because it diffuses through solids at a rate three times that of air, it is used as an indicator gas to detect leaks in high vacuum equipment and high pressure vessels.[68]​.

The leak rate in industrial containers (generally vacuum chambers and cryogenic tanks) is measured using helium, due to its small molecular diameter and its inert gas condition. There is still no known inert substance that can leak through microcracks or micropores in the wall of a container at a rate greater than helium. A helium leak detector (see mass spectrometer) is used to find leaks in containers. Helium leaks through cracks should not be confused with gas penetration through a massive material. Although permeability constants for helium have been documented through glasses, ceramics, and synthetic materials, inert gases such as helium cannot permeate through most massive metals.[69] If the total leak rate of the product being tested needs to be known (for example in heat pumps or air conditioning systems), the object is placed in a test chamber, the air inside it is removed with vacuum pumps, and the product is filled with helium at a specific pressure. Helium escaping through leaks is detected by a mass spectrometer even at leak rates up to 10 Pa·m³/s. The measurement procedure is normally automatic, and is known as "comprehensive helium testing." In a simpler test, the product is filled with helium and an operator manually looks for the leak with a device called a sniffer.[70]

Due to its lack of reactivity and high thermal conductivity, its transparency to neutrons, and because it does not form radioactive isotopes under reactor conditions, it is used as a heat transfer medium in some gas-cooled nuclear reactors.[68] Another of its uses is to use it as a shielding gas in arc welding processes in materials that are easily contaminated by air.

Because it is inert, it is used as a shielding gas in the growth of silicon and germanium crystals in the production of titanium and zirconium, as well as in gas chromatography. For this same reason, due to its thermal conductivity and the high speed of sound within it, its nature as an ideal gas and the high value of its adiabatic expansion coefficient, it is also useful in supersonic wind tunnels and in test facilities where a sudden release of gas energy is required.[71][72]​.

Helium, mixed with a heavier gas such as xenon, is useful for thermoacoustic cooling due to the resulting high coefficient of adiabatic expansion and its low Prandtl number.[73] The inert behavior of helium has environmental advantages over conventional cooling systems, which contribute to ozone depletion or global warming.[74]

The use of helium reduces the distortion effects caused by temperature variations in space in the lenses of some telescopes, due to its low refractive index. This method is used especially in solar telescopes, in which a tightly sealed vacuum tube would be too heavy.[75][76]​.

Through a process known as helium dating), the age of rocks and minerals containing Uranium and Thorium can be estimated.

Liquid helium is used to cool certain metals—for example, superconducting magnets used in magnetic resonance tomography—to extremely low temperatures, which are necessary for superconductivity. CERN's Large Hadron Collider uses 96 tons of liquid helium to maintain the temperature at 1.9 K.[77] Low-temperature helium is also used in cryogenics.

Helium is a carrier gas commonly used in gas chromatography.

Scientific discovery

The first evidence of the existence of helium was observed on August 18, 1868 as a bright yellow line with a length of 587.49 nanometers in the spectrum of the Sun's chromosphere. The line was detected by French astronomer Pierre Janssen during a total solar eclipse in Guntur, India.[78] This line was initially thought to be produced by sodium. On October 20 of the same year, the English astronomer Joseph Norman Lockyer observed a yellow line in the solar spectrum, which he named the Fraunhofer D line because it was close to the already known sodium lines D and D. Lockyer concluded that this line was caused by an element existing in the Sun but unknown on Earth. Eduard Frankland confirmed Janssen's results and proposed the name helium for the new element, in honor of the Greek sun god (Ἥλιος, Helios), with the suffix -ium since the new element was expected to be metallic.[79][80][81]​.

In 1882, the Italian physicist Luigi Palmieri detected helium on Earth for the first time, through its D spectral line, when he analyzed the lava from Mount Vesuvius.[82]​.

On March 26, 1895 Sir William Ramsay isolated helium by treating cleveite (a variety of uranite containing at least 10% rare earth elements) with mineral acids. Ramsey was actually searching for argon, but after separating the nitrogen and oxygen from the gas released by the sulfuric acid, he noticed a bright yellow line that matched the D line observed in the solar spectrum.[11][83][84][85] The samples were identified as helium by Lockyer and British physicist William Crookes. It was also isolated from cleveite the same year independently by chemists Per Teodor Cleve and Abraham Langlet in Uppsala (Sweden), who were able to obtain enough of the gas to accurately determine its atomic weight. Helium was also isolated by American geochemist William Francis Hillebrand, although he attributed the lines to nitrogen.

In 1907 Ernest Rutherford and Thomas Royds demonstrated that alpha particles are helium nuclei by allowing the particles to penetrate a thin wall of a vacuum glass tube and then creating an electrical discharge inside the tube to study the spectrum of the gas. In 1908, the Dutch physicist Heike Kamerlingh Onnes produced liquid helium for the first time by cooling the gas to 0.9 K,[88]​ which earned him the Nobel Prize. He also tried to solidify helium by reducing its temperature, although he did not succeed because this element lacks a triple point, the temperature at which the solid, liquid and gas phases exist in equilibrium. In 1926 his disciple Willem Hendrik Keesom managed to solidify 1 cm³ helium for the first time.[89]​.

In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that helium-4 has almost no viscosity at temperatures close to absolute zero, a phenomenon now called superfluidity.[90] 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. It is believed that in helium-3 the phenomenon is related to the creation of pairs of fermions of this isotope, in such a way that bosons are formed, in analogy to the Cooper pairs that produce superconductivity.[91]​.

Extraction and use

After a 1903 oil drilling operation in Dexter, Kansas, produced a geyser of unburnable gas, geologist Erasmus Haworth collected samples of the emanating gases and took them to the University of Kansas at Lawrence, where, with the help of chemists Hamilton Cady and David McFarland, he discovered that the gas consisted, by volume, of 72% nitrogen, 15 % methane (a percentage that can only be burned with sufficient oxygen), 1% hydrogen, and 12% an unidentified gas.[92] In further analysis, Cady and McFarland found that 1.84% of the gas sample was helium.[93][94] This demonstrated that despite its global rarity on Earth, helium was concentrated in large quantities beneath the Great Plains of the United States, available for extraction as a byproduct of natural gas.[95] The largest reserves of helium were in the gas fields of southwestern Kansas, Texas, and Oklahoma.

This allowed the United States to become the leading producer of helium in the world. Following a suggestion by Sir Richard Threlfall, the US navy sponsored three small experimental helium production plants during the First World War. The goal was to provide defense balloons with a non-flammable gas lighter than air. With this program, a total of 5,700 m³ of 92% helium was produced, despite the fact that previously only less than one cubic meter of gas had been obtained. Some of it was used in the US Navy's first helium-inflated aircraft, which made its first trip from Hampton Roads, Virginia, to Bolling Field in Washington, D.C., on December 1, 1921.[96]

Although the extraction process using low-temperature gas liquefaction was not developed in time to be relevant during World War I, production continued. Helium was primarily used as a lifting gas in lighter-than-air aircraft. The demand for this use, as well as for arc welding, was greatest during World War II. The helium mass spectrometer was also vital in the atomic bomb developed by the Manhattan Project.[97]​.

The United States government created the National Helium Reserve in 1925 in Amarillo, Texas, with the goal of supplying it to military aircraft in wartime and commercial aircraft in peacetime. Due to a United States military embargo against Germany in which the supply of helium was restricted, the LZ-129 Hindenburg was forced to use hydrogen as a booster gas. Use of helium after World War II was reduced, but reserves were expanded in the 1950s to ensure its supply in liquid form as a coolant to create hydrogen and oxygen fuel (among other uses) for rockets during the space race and the Cold War. Helium use in the United States in 1965 was more than eight times the maximum wartime consumption.[98]​.

The United States Bureau of Mines had five private plants to recover helium from natural gas. [99]

By 1995, nearly one billion cubic meters of gas had been stored, and the reserves constituted a debt of 1.4 billion dollars, which in 1996 forced the United States Congress to eliminate them. The helium produced between 1930 and 1945 was approximately 98.3% pure (with 2% nitrogen), which was enough to fill airships. In 1945, a small amount of 99.9% helium was used to make welds.[100] By 1949, commercial quantities of 99.9% grade A helium were available.

Neutral helium under normal conditions is non-toxic, does not play any biological role and is found in trace amounts in human blood. If enough helium is inhaled to replace the oxygen needed for breathing, it can cause asphyxiation. The precautions that must be taken for helium used in cryogenics are similar to those for liquid nitrogen. Its extremely low temperature can cause freezer burn and the liquid-to-gas expansion rate can cause explosions if pressure release mechanisms are not used.

Tanks of gaseous helium at temperatures of 5 to 10 K should be stored as if they contained liquid helium due to the large increase in pressure and significant thermal expansion that occurs when heating the gas from a temperature below 10 K to room temperature.[24]​.

The speed of sound in helium is almost 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. If helium is inhaled, there is a corresponding increase in the heights of the resonance frequencies of the vocal folds.[105]​ (The opposite effect, lowering frequencies, can be obtained by inhalation of a dense gas such as sulfur hexafluoride).

Its inhalation can be dangerous if done in excess, since it is an asphyxiating gas and displaces the oxygen necessary for normal breathing.[106] Breathing pure helium continuously causes death by asphyxiation in a few minutes. Inhaling helium directly from pressurized cylinders is extremely dangerous, as the high flow velocity can result in rupture of lung tissues.[106][107] However, death caused by helium is very rare, with only two deaths reported in the United States between 2000 and 2004.[107]​.

At high pressures (more than 20 atm or two MPa), a mixture of helium and oxygen (heliox) can lead to high nerve pressure syndrome; a kind of reverse anesthetic effect. Adding a small amount of nitrogen to the mixture can solve the problem.[108]​[109]​.

• - Leidenfrost effect.

• - Noble gas.

• - Inorganic origin of petroleum.

• - Periodic table of the elements.

• - Superfluidity.

• - Goo.

• - Wikimedia Commons hosts a multimedia gallery on Helium.

• - WebElements.com .

• - EnvironmentalChemistry.com .

• - It is Elementary.

• - Chemical Elements: Helium.

• - National Institute of Safety and Hygiene at Work of Spain: International chemical safety sheet for helium.

Contenido

Aunque el helio tiene una configuración electrónica 1s², no se coloca en el grupo 2 de la tabla periódica junto al hidrógeno y otros elementos del bloque s. En su lugar, se ubica en el grupo 18, correspondiente a los gases nobles, debido a sus propiedades químicas únicas. Este elemento, al poseer su nivel de energía completamente lleno, es químicamente inerte y no forma compuestos fácilmente, características típicas de los gases nobles. Aunque su configuración lo asociaría al bloque s, su comportamiento químico lo justifica como parte del bloque p. Esto hace que el helio sea una excepción importante en la disposición de los elementos en la tabla periódica.

En condiciones normales de presión y temperatura (25 °C y 1 atm), el helio es un gas monoatómico, no inflamable y extremadamente ligero. Es notablemente difícil de licuar debido a su baja temperatura crítica, que es de apenas 5.20K (−267.96 °C). Esto significa que, por encima de esa temperatura, el helio no puede pasar al estado líquido, sin importar cuánta presión se le aplique. A temperaturas inferiores a la crítica, el helio solo puede licuarse mediante una combinación de enfriamiento extremo y alta presión. Este comportamiento lo convierte en un caso único entre todos los elementos químicos.

Otro aspecto fascinante del helio es su elevado calor específico, especialmente en estado gaseoso. Este valor, más alto que el de la mayoría de los gases, hace que el helio se expanda rápidamente cuando se calienta, incluso a temperatura ambiente. En condiciones de bajas temperaturas, el helio líquido también exhibe propiedades únicas, como la superfluidez, una fase en la que fluye sin viscosidad, superando obstáculos sin resistencia.

El helio posee el punto de solidificación más bajo conocido. A presiones normales, incluso en el cero absoluto (0 K o −273.15 °C), el helio-4 permanece en estado líquido debido a su energía de punto cero, un efecto cuántico que impide que sus átomos se estabilicen en un estado sólido. Sin embargo, al someterlo a presiones superiores a 25 atm "Atmósfera (unidad)"), el helio puede solidificarse. El helio sólido"), a temperaturas cercanas a 1 K (−272.15 °C), adopta una estructura cristalina hexagonal compacta"). A medida que la presión aumenta, se han observado cambios a estructuras cristalinas diferentes, aunque con densidades y energías muy similares. Estos cambios reflejan las complejas interacciones entre los átomos del helio en condiciones extremas.[5]​.

The helium atom

Helium is a chemical element whose atom is the simplest to solve using the rules of quantum mechanics after the hydrogen atom. It consists of two electrons orbiting a nucleus containing two protons along with one or two neutrons, depending on the isotope. However, as in Newtonian mechanics, no system consisting of more than two particles can be solved with an exact mathematical analysis approach (see three-body problem) and helium is no exception. Thus, mathematical methods are necessary, even to solve the system of one nucleus and two electrons. However, such computational chemistry methods have been used to create a quantum mechanical picture of helium electron bindings with an accuracy within 2% of the correct value, with a few computational calculation steps. In these models it is observed that each electron partially prevents the other from sensing interaction with the nucleus, such that the effective nuclear charge Z is approximately 1.69 units, and not the 2 charges of a classical "bare nucleus" of helium.

The hydrogen atom is widely used to help solve the helium atom. Bohr's atomic model gave a very precise explanation of the spectrum of the hydrogen atom, but when an attempt was made to use it on helium the model failed. Werner Heisenberg developed a modification of Bohr's analysis, in which he used semi-integrated values ​​of the quantum numbers. Density functional theory is used to obtain the ground state energy levels of the helium atom, together with the Hartree-Fock method.

The nucleus of the helium-4 atom, which is exactly the same as an alpha particle,[7]​ is particularly interesting. The reason for this is because high-energy electron scattering experiments have shown that its charge decreases exponentially from a maximum at its central point, in exactly the same way that the charge density in its own electron cloud decreases. This symmetry reflects similar physical principles: the pair of neutrons and protons in the nucleus of helium obey the same quantum mechanical rules as the two electrons that orbit it - although the union of the particles in the nucleus is due to a different potential "Potential (physics)" than that which keeps the electrons in the cloud around the atom. In this way, these fermions (that is, both protons, electrons and neutrons) completely occupy the 1s orbitals in pairs, none of them possesses orbital angular momentum and each of them cancels the intrinsic spin of the other. Adding another of any of these particles would require angular momentum and release substantially less energy (in fact, no nucleus with five nucleons is stable). For this reason, this arrangement for these particles is extremely energetically stable, and this stability gives rise to many crucial phenomena inherent to helium in nature.

As an example of these facts due to the high stability of the electronic configuration of helium is the low chemical reactivity of this element (the lowest in the entire periodic table), as well as the lack of interaction of its atoms among themselves. This produces the lowest melting and boiling points of all the elements. In the same way, the energetic stability of the helium-4 nucleus gives rise to its easy production in atomic reactions that involve both the emission of heavy particles and nuclear fusion. Some stable helium-3 is produced in fusion reactions from hydrogen, but it is a much smaller fraction compared to helium-4. The stability of helium-4 is the reason why hydrogen is converted to this form of helium in the Sun, instead of helium-3, deuterium or other heavier elements. It is also partially responsible for the fact that the alpha particle is by far the type of baryon particle most commonly ejected from atomic nuclei. In other words, alpha decay is much more common than decay into heavier nuclei.[8]​.

The unusual stability of helium-4 is also important in cosmology. In the first minutes after the Big Bang, the universe was composed of a mixture of free nucleons (protons and neutrons). This "soup" originally had a ratio of six protons to each neutron, and after a time it cooled to the point where nuclear fusion could occur.[9] The stability of helium meant that almost all of the nucleon aggregations formed at that time were helium-4 nuclei. The union of protons and neutrons to form helium-4 is so strong that, in fact, the production of this element consumed almost all the free neutrons in a matter of minutes, before these nuclei could decay through beta decay. This left a very small amount of these particles for lithium, beryllium or boron to form. The nuclear bond per nucleon in helium-4 is stronger than in any of these three elements (see nucleogenesis and binding energy). Therefore, there was no energetic mechanism available, once helium had been formed, to create the elements of atomic number 3, 4 and 5. In terms of energy, the fusion of helium to form the next element in the periodic table with the lowest energy per nucleon was also favorable: carbon. However, due to the lack of intermediate elements, this process required the almost simultaneous collision of three helium-4 nuclei (see triple-alpha process), so there was not enough time for carbon to form in the Big Bang: within minutes, the early universe cooled to a temperature and pressure at which the fusion of helium to carbon was no longer possible. This caused the early universe to have a hydrogen/helium ratio very similar to that observed today (by mass, three parts hydrogen to one part helium-4), with almost all of the neutrons in the universe—as is the case today—trapped within helium-4 nuclei.

All of the heavier elements—including those needed to form rocky planets like Earth and for carbon-based life to exist—had to be created later, in stars hot enough to burn not just hydrogen—since this only produces more helium—but helium itself. Such stars are massive and therefore rare. The above gives rise to the fact that all chemical elements, apart from hydrogen and helium, make up only 2% of the mass in the form of atoms in the universe. Helium-4, for its part, constitutes about 23% of all ordinary matter in the universe, that is, practically all ordinary matter that is not hydrogen.[10]​.

Gas and plasma phases

Helium is the least reactive noble gas after neon and therefore, the second least reactive element of all of them. It is inert and monatomic under normal conditions. Due to its low atomic mass, in the gas phase, the thermal conductivity, specific heat, and speed of sound are greater than in any other gas, except hydrogen. For similar reasons, and also due to the small size of its atoms, its diffusion rate through solids is three times that of air, and about 65% that of hydrogen.[11]​.

It is also less soluble in water than any other known gas,[12]​ and its refractive index is the closest to unity of all gases.[13]​ This element has a negative Joule-Thomson coefficient at normal room temperature, meaning that it heats up when allowed to expand freely. Only below its Joule–Thomson inversion temperature (32 to 50 K at 1 atmosphere "Atmosphere (unit)") does it cool in free expansion.[11] Once precooled below this temperature, helium can be liquefied by cooling due to its expansion.

Most extraterrestrial helium is in a plasma state "Plasma (state of matter)"), with very different properties than atomic helium. In plasma, helium's electrons are not bound to the nucleus, making its electrical conductivity very high, even when the gas is only partially ionized. Charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind, along with ionized hydrogen, particles interact with the Earth's magnetosphere, giving rise to the Birkeland current and auroras.[14]​.

Liquid and solid phases

Unlike any other element, liquid helium will stay that way 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 to 1.5 K (about -272 °C or -457 °F) and about 25 bar (2.5 MPa) pressure.[15] It is often difficult to distinguish solid helium from liquid since the refractive index of the two phases is almost the same. The solid has a marked melting point and crystalline structure, but is very compressible. Applying pressure in a laboratory can reduce its volume by more than 30%.[16]​ With a compressibility modulus of the order of 50 MPa,[16]​ 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 (2.5 MPa) is 0.187 ± 0.009 g/ml.[17]​.

Below its boiling point of 4.22 K, and above the lambda point of 2.1768 K, the isotope helium-4 exists in a normal colorless liquid state, called helium I.[11] Like other cryogenic liquids, helium I boils when heated and contracts when its temperature is lowered. Below the lambda point, however, this phase does not boil and expands as the temperature drops further.

Helium has a refractive index similar to that of a gas, 1.026, which makes its surface very difficult to see, so extruded polystyrene floats are often used to see where the surface is.[11] This colorless liquid has a very low viscosity and a density of 0.145 g/mL, which is only a quarter of the value predicted by classical physics.[11] It is necessary make use of quantum mechanics to explain this property and, therefore, both types of liquid helium are called quantum fluids, which means that they show atomic properties on a macroscopic scale. This may be an effect of the fact that its boiling point is very close to absolute zero, which prevents random molecular motion (thermal energy) from obscuring its atomic properties.[11]​.

Liquid helium below its lambda point shows extremely unusual characteristics, in a state called helium II. Boiling of helium II is not possible due to its high thermal conductivity; Heat input causes evaporation of 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 the properties of this phase in that isotope.[11]​.

Helium II is a superfluid, a quantum state of matter with strange properties. For example, when it flows through capillaries as thin as 10 to 10 m, it has no measurable viscosity. However, when measurements are made between two moving disks, a viscosity comparable to that of gaseous helium is observed. Current theory explains this phenomenon using a two-fluid model for helium II. In this model, liquid helium below the lambda point is considered to contain a proportion of ground-state helium atoms, which make up the superfluid, and which flow with a viscosity exactly equal to zero; and a proportion of helium atoms in an excited state, which behave more like an ordinary fluid.[18]​.

Since helium is a noble gas, in practice it does not participate in chemical reactions, although under the influence of electrical discharges or bombarded with electrons it forms compounds.

Helium has a valence of zero and is not chemically reactive under normal conditions.[24] It is an electrical insulator unless it is ionized. Like other noble gases, it has metastable energy levels, allowing it to remain ionized in an electrical discharge at a voltage below its ionization potential. Helium can form unstable compounds, known as excimers, with tungsten, iodine, fluorine, and phosphorus, when subjected to a luminescent electrical discharge, electron bombardment, or is otherwise a plasma. The molecular compounds HeNe, HgHe, and WHe, and the molecular ions He, He, HeH, and HeD can be created in this way.[25] This technique has also enabled the production of the neutral molecule He, which has a large number of spectral band systems, and the molecule HgHe, which is apparently only held together by polarization forces.[11] In theory, other real compounds are also possible, such as helium fluorohydride. (HHeF), which would be analogous to argon fluorohydride"), discovered in 2000.[26] Calculations indicate that two new compounds containing a helium-oxygen bond could be stable.[27] Two new molecular species, predicted theoretically, CsFHeO and N(CH)FHeO, are derivatives of a metastable anion [F-HeO], anticipated in 2005 theoretically by a group of Taiwan. If confirmed experimentally, these compounds would end the chemical inertness of helium, and the only completely inert element would be neon.[28]​.

The helium has been placed in carbon molecular cages (the fullerenes) by heating at high pressure. The endohedral fullerene molecules formed are stable up to high temperatures. When the chemical derivatives of these fullerenes are formed, the helium remains inside them.[29]​ If helium-3 is used, it can be easily observed by nuclear magnetic resonance spectroscopy.[30]​ A large number of fullerenes containing helium-3 have been reported. Although the said atoms are not linked by means of covalent or ionic bonds, these substances have different properties and a defined composition, like all stoichiometric chemical compounds.

There are eight known isotopes of helium, but only He and He are stable. In the Earth's atmosphere there is one He atom for every million He atoms.[31] Unlike other elements, the isotopic abundance of helium varies greatly depending on its origin, due to the different formation processes. The most common isotope, He, is produced on Earth through the alpha decay of heavier radioactive elements; The alpha particles that appear are completely ionized He atoms. He has an unusually stable nucleus because its nucleons are arranged in complete shells. Furthermore, this isotope was formed in large quantities during primordial nucleosynthesis in the Big Bang.[32]​.

He is present on Earth today in only trace amounts (most dating back to the formation of the Earth), although some of it falls to Earth when trapped in cosmic dust.[33] Some traces are also produced through the beta decay of tritium.[34] Some rocks in the Earth's crust have different isotope ratios that vary by up to a factor of ten. These proportions can be used to investigate the origin of rocks as well as the composition of the Earth's mantle.[33] He is much more abundant in stars as a product of nuclear fusion. Consequently, in the interstellar medium, the ratio of He to He is about 100 times greater than that on Earth.[35] Extraplanetary material, such as asteroid and lunar regolith, has traces of He from the bombardment of solar winds against them. The surface of the Moon has He concentrations of around 0.01 ppm.[36][37] Some people, most notably Gerald Kulcinski in 1986,[38]​ have proposed exploring the Moon, excavating the lunar regolith, and using the He for nuclear fusion.

Liquid He can be cooled down to 1 kelvin using evaporative cooling in containers in which these temperatures can be reached and maintained. A similar cooling for helium-3, which has a lower boiling point, can be achieved around 0.2 K in a helium-3" refrigerator. Mixtures containing the same proportion of helium-3 and helium-4 at a temperature below 0.8 K separate into two immiscible phases due to their incompatibility (each obeys a different quantum statistic): the helium-4 atoms are bosons while the of helium-3 are fermions).[11] Dilution refrigerators") use this impossibility of mixing to reach temperatures of a few millikelvin.

It is possible to produce exotic isotopes of helium, which quickly decay into other substances. The shortest-lived heavy isotope is He, with a half-life of 7.6×10 seconds. He decomposes by emitting a beta particle and its disintegration period is 0.8 seconds. He also emits beta particles as well as gamma rays. Both He and He are created through some nuclear reactions.[11]​ He and He are known to have a nuclear halo. ²He (consisting of two protons and no neutrons) is a radioisotope that decays into protium (hydrogen) through the emission of protons, with a decay period of 3×10 seconds.[11]​.

natural abundance

Helium is the second most abundant element in the known universe after hydrogen and makes up about 23% of the baryon mass of the universe.[31] Most of the helium was formed during the nucleosynthesis of the Big Bang, in the first three minutes after it. In this way, measuring their abundance contributes to cosmological models. In stars, helium is formed by the nuclear fusion of hydrogen in proton-proton chain reactions and in the CNO cycle, which are part of stellar nucleosynthesis.[32]​.

In the Earth's atmosphere the concentration of helium by volume is only 5.2 parts per million.[39][40]​ The concentration is low and practically constant despite the continuous production of new helium, because most of the helium in the atmosphere escapes into space due to different processes.[41][42]​ In the Earth's heterosphere, a part of the upper atmosphere, helium and other light gases are the most abundant elements.

Almost all of the helium present on Earth is the result of radioactive decay, and therefore a terrestrial helium balloon is, in essence, a bag of alpha particles expelled by this process. Helium is found in large quantities in uranium and thorium minerals, including cleveite, pitchblende, carnotite and monazite, as these emit alpha particles (helium nuclei, He) and electrons immediately combine with them as soon as the particles are stopped by the rock. In this way, it is estimated that about 3,000 tons of helium are generated per year throughout the lithosphere.[43]​[44]​[45]​ In the Earth's crust, the concentration of helium is 8 parts per billion. In the sea, the concentration is only 4 parts per billion. There are also small amounts in mineral springs, volcanic gas, and meteoric iron. Because helium is trapped similarly to natural gas by an impermeable layer of rock, the highest concentrations of this element on the planet are found in natural gas, from which most commercial helium is extracted. The concentration varies over a wide range from a few ppm to more than 7% in a small gas field in San Juan County "San Juan County (New Mexico)"), New Mexico.[46]​[47]​.

In 2016 British scientists discovered a large field of helium gas in Africa.[48]​ This field, located in the United Republic of Tanzania, measures approximately 54 million cubic feet in volume and is the largest helium field in the world.[49]​.

Modern extraction

For large-scale use, it is extracted by fractional distillation from natural gas, which contains up to 7% helium.[50] Having a lower boiling point than any other element, low temperatures and high pressures are used to liquefy almost all other gases (mainly nitrogen and methane). The resulting raw helium is purified by successive exposures to low temperatures, in which almost all of the nitrogen and other remaining gases are precipitated out of the gas mixture.[11]​ As a final purification step, activated carbon is used, resulting in grade A helium, with a purity of 99.995%.[11]​ The main impurity in grade A helium is neon. In the final phase of production, the majority of the helium produced is liquefied through a cryogenic process. This is necessary for applications requiring liquid helium and also allows helium suppliers to reduce the cost of long-distance transportation, since most liquid helium containers have a capacity five times that of tank trucks transporting gaseous helium.[51][52]​.

In 2008, about 169 million standard cubic meters (SCM, defined as one cubic meter at a pressure of 1 atm "Atmosphere (unit)") and a temperature of 15 °C) of helium were extracted from natural gas or helium reserves. Of these, approximately 78% came from the United States, 10% from Algeria, and the remainder was mostly extracted in Russia, Poland, and Qatar. 4.2× SCM. This amount is enough for about 25 years of global use, or 35 years of U.S. consumption, although savings and processing factors are expected to impact effective reserve numbers. It is estimated that the basic reserves of helium not yet proven that could be obtained from natural gas in the United States are 3.1 to 5.3 × 10 SCM, or approximately four orders of magnitude greater than the proven reserves. extractions.[56]​.

Helium must be extracted mainly from natural gas, because its presence in the air is only a fraction compared to that of neon, and yet its demand is much greater. It is estimated that if all neon production were retooled to save helium, 0.1% of global helium demands would be met. Likewise, only 1% of global helium demands could be met by retooling all air distillation plants. Helium can be synthesized by bombarding lithium or boron using high-speed protons. However, this production method is totally economically unfeasible.[57]​.

Current helium reserves are being used up much faster than this element can be replenished. Given this situation, there are major concerns that the helium supply could soon run out. In the world's largest reserves, in Amarillo (Texas), Texas, this gas is expected to be depleted in eight years from 2008. This could be prevented if current users captured and recycled the gas and if oil and natural gas companies used helium capture techniques when extracting it.[58]​[59]​.

Due to a reduction in the volume of helium exports by the United States, giving priority to institutional consumers within the country over private companies, the duration of reserves has been extending. In 2019, the price of helium at the auction of the US national helium reserve increased by 135%.[60] Since then, other natural gas exporting countries, such as Algeria and Qatar, have developed helium production plants. However, the largest plant under construction, owned by Gazprom and located in Russia, has suffered delays in its start-up due to the economic sanctions that the country has been receiving since the invasion of Ukraine "Russian invasion of Ukraine (2022-present)").[61]​.

Helium is lighter than air and, unlike hydrogen, it is not flammable. Its lifting power is also 8% less than air, which is why it is used as filling gas in "Balloon (toy)" balloons and zeppelins for advertising, atmospheric research and even for military reconnaissance.

Even though the previous one is the main one, helium has more applications:

• - Helium-oxygen mixtures are used in deep diving, since helium is inert, less soluble in the blood than nitrogen and diffuses "Diffusion (physics)") 2.5 times faster than nitrogen, all of which reduces the time required for decompression. However, the latter must begin at greater depth, reducing the risk of narcosis ("deep drunkenness").

• - Due to its low liquefaction and evaporation point, it can be used as a refrigerant in extremely low temperature applications, such as in superconducting magnets and cryogenic research at temperatures close to absolute zero.

• - In gas chromatography it is used as an inert carrier gas.

• - The inert atmosphere of helium is used in arc welding and in the manufacture of silicon and germanium crystals, as well as to pressurize liquid rocket fuels.

• - In supersonic wind tunnels.

• - As a cooling agent in nuclear reactors.

• - Liquid helium is finding increasing use in medical applications of magnetic resonance imaging (MRI).

• - It is used in laser equipment as one of the most common gases, mainly the helium-neon mixture.

Of the total global helium production in 2008 of 32 million kg, its largest use (about 22% of the total in 2008) was in cryogenic applications. Of these, the majority were in medicine in the cooling of superconducting magnets in magnetic resonance scanners.[62] Other important uses (a total of about 78% of its use in 1996) were in pressurization and sanitation systems, maintenance of controlled atmospheres, and welding.[63]​.

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 its low chemical reactivity. Likewise, it is commercially available in both liquid and gaseous form. As a liquid, it can be supplied in small containers called Dewar flasks that allow up to 1000 liters of helium to be stored, or in large ISO containers that have a nominal capacity of up to 42 m³. In gaseous form, small quantities are supplied in high-pressure cylinders that can contain a volume equivalent to 8 standard m³, while large quantities of high-pressure gas are supplied in tank trucks that have a capacity equivalent to 4,860 standard m³. This is because the volume of the gas is greatly reduced when subjected to high pressures.

Because helium is lighter than air, blimps and balloons are inflated with this gas to lift them. While hydrogen experiences approximately 7% greater thrust force, helium has the advantage of being non-flammable (as well as being fire retardant). In the space industry, it is used as a filling medium to displace fuels and oxidizers in storage tanks, and to condense hydrogen and oxygen to produce rocket fuel. It is also used to purify fuel and oxidizer for ground support equipment before launch, as well as to precool liquid hydrogen in space vehicles. For example, the Saturn V booster used in the Apollo Program required about 370,000 m³ of helium to launch the rocket.

Helium is less dense than atmospheric air, so the timbre (but not the pitch "Pitch (music)") of a person's voice changes when it is inhaled.[64] This is because, since helium is a fairly light gas, it moves faster through spaces, causing the vocal folds to move at greater speed, causing a faster, and therefore higher-pitched, sound wave. However, inhalation from a typical commercial source, such as those used to fill balloons, can be dangerous due to the risk of suffocation from lack of oxygen and the number of contaminants that may be present. These may include traces of other gases, as well as aerosol lubricating oil. However, since these are children's products, there are mechanisms that require guaranteeing the non-toxicity of the gas, such as exceeding the "European Conformity" (CE marking) required for toys and similar derivatives in the European market, to guarantee the safety of children.

Because of their low solubility in nervous tissue, helium mixtures such as trimix, heliox and Heliair are used for deep diving to reduce the effects of narcosis. considerably the effort in breathing.

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

One of the industrial applications of helium is leak detection. Because it diffuses through solids at a rate three times that of air, it is used as an indicator gas to detect leaks in high vacuum equipment and high pressure vessels.[68]​.

The leak rate in industrial containers (generally vacuum chambers and cryogenic tanks) is measured using helium, due to its small molecular diameter and its inert gas condition. There is still no known inert substance that can leak through microcracks or micropores in the wall of a container at a rate greater than helium. A helium leak detector (see mass spectrometer) is used to find leaks in containers. Helium leaks through cracks should not be confused with gas penetration through a massive material. Although permeability constants for helium have been documented through glasses, ceramics, and synthetic materials, inert gases such as helium cannot permeate through most massive metals.[69] If the total leak rate of the product being tested needs to be known (for example in heat pumps or air conditioning systems), the object is placed in a test chamber, the air inside it is removed with vacuum pumps, and the product is filled with helium at a specific pressure. Helium escaping through leaks is detected by a mass spectrometer even at leak rates up to 10 Pa·m³/s. The measurement procedure is normally automatic, and is known as "comprehensive helium testing." In a simpler test, the product is filled with helium and an operator manually looks for the leak with a device called a sniffer.[70]

Due to its lack of reactivity and high thermal conductivity, its transparency to neutrons, and because it does not form radioactive isotopes under reactor conditions, it is used as a heat transfer medium in some gas-cooled nuclear reactors.[68] Another of its uses is to use it as a shielding gas in arc welding processes in materials that are easily contaminated by air.

Because it is inert, it is used as a shielding gas in the growth of silicon and germanium crystals in the production of titanium and zirconium, as well as in gas chromatography. For this same reason, due to its thermal conductivity and the high speed of sound within it, its nature as an ideal gas and the high value of its adiabatic expansion coefficient, it is also useful in supersonic wind tunnels and in test facilities where a sudden release of gas energy is required.[71][72]​.

Helium, mixed with a heavier gas such as xenon, is useful for thermoacoustic cooling due to the resulting high coefficient of adiabatic expansion and its low Prandtl number.[73] The inert behavior of helium has environmental advantages over conventional cooling systems, which contribute to ozone depletion or global warming.[74]

The use of helium reduces the distortion effects caused by temperature variations in space in the lenses of some telescopes, due to its low refractive index. This method is used especially in solar telescopes, in which a tightly sealed vacuum tube would be too heavy.[75][76]​.

Through a process known as helium dating), the age of rocks and minerals containing Uranium and Thorium can be estimated.

Liquid helium is used to cool certain metals—for example, superconducting magnets used in magnetic resonance tomography—to extremely low temperatures, which are necessary for superconductivity. CERN's Large Hadron Collider uses 96 tons of liquid helium to maintain the temperature at 1.9 K.[77] Low-temperature helium is also used in cryogenics.

Helium is a carrier gas commonly used in gas chromatography.

Scientific discovery

The first evidence of the existence of helium was observed on August 18, 1868 as a bright yellow line with a length of 587.49 nanometers in the spectrum of the Sun's chromosphere. The line was detected by French astronomer Pierre Janssen during a total solar eclipse in Guntur, India.[78] This line was initially thought to be produced by sodium. On October 20 of the same year, the English astronomer Joseph Norman Lockyer observed a yellow line in the solar spectrum, which he named the Fraunhofer D line because it was close to the already known sodium lines D and D. Lockyer concluded that this line was caused by an element existing in the Sun but unknown on Earth. Eduard Frankland confirmed Janssen's results and proposed the name helium for the new element, in honor of the Greek sun god (Ἥλιος, Helios), with the suffix -ium since the new element was expected to be metallic.[79][80][81]​.

In 1882, the Italian physicist Luigi Palmieri detected helium on Earth for the first time, through its D spectral line, when he analyzed the lava from Mount Vesuvius.[82]​.

On March 26, 1895 Sir William Ramsay isolated helium by treating cleveite (a variety of uranite containing at least 10% rare earth elements) with mineral acids. Ramsey was actually searching for argon, but after separating the nitrogen and oxygen from the gas released by the sulfuric acid, he noticed a bright yellow line that matched the D line observed in the solar spectrum.[11][83][84][85] The samples were identified as helium by Lockyer and British physicist William Crookes. It was also isolated from cleveite the same year independently by chemists Per Teodor Cleve and Abraham Langlet in Uppsala (Sweden), who were able to obtain enough of the gas to accurately determine its atomic weight. Helium was also isolated by American geochemist William Francis Hillebrand, although he attributed the lines to nitrogen.

In 1907 Ernest Rutherford and Thomas Royds demonstrated that alpha particles are helium nuclei by allowing the particles to penetrate a thin wall of a vacuum glass tube and then creating an electrical discharge inside the tube to study the spectrum of the gas. In 1908, the Dutch physicist Heike Kamerlingh Onnes produced liquid helium for the first time by cooling the gas to 0.9 K,[88]​ which earned him the Nobel Prize. He also tried to solidify helium by reducing its temperature, although he did not succeed because this element lacks a triple point, the temperature at which the solid, liquid and gas phases exist in equilibrium. In 1926 his disciple Willem Hendrik Keesom managed to solidify 1 cm³ helium for the first time.[89]​.

In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that helium-4 has almost no viscosity at temperatures close to absolute zero, a phenomenon now called superfluidity.[90] 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. It is believed that in helium-3 the phenomenon is related to the creation of pairs of fermions of this isotope, in such a way that bosons are formed, in analogy to the Cooper pairs that produce superconductivity.[91]​.

Extraction and use

After a 1903 oil drilling operation in Dexter, Kansas, produced a geyser of unburnable gas, geologist Erasmus Haworth collected samples of the emanating gases and took them to the University of Kansas at Lawrence, where, with the help of chemists Hamilton Cady and David McFarland, he discovered that the gas consisted, by volume, of 72% nitrogen, 15 % methane (a percentage that can only be burned with sufficient oxygen), 1% hydrogen, and 12% an unidentified gas.[92] In further analysis, Cady and McFarland found that 1.84% of the gas sample was helium.[93][94] This demonstrated that despite its global rarity on Earth, helium was concentrated in large quantities beneath the Great Plains of the United States, available for extraction as a byproduct of natural gas.[95] The largest reserves of helium were in the gas fields of southwestern Kansas, Texas, and Oklahoma.

This allowed the United States to become the leading producer of helium in the world. Following a suggestion by Sir Richard Threlfall, the US navy sponsored three small experimental helium production plants during the First World War. The goal was to provide defense balloons with a non-flammable gas lighter than air. With this program, a total of 5,700 m³ of 92% helium was produced, despite the fact that previously only less than one cubic meter of gas had been obtained. Some of it was used in the US Navy's first helium-inflated aircraft, which made its first trip from Hampton Roads, Virginia, to Bolling Field in Washington, D.C., on December 1, 1921.[96]

Although the extraction process using low-temperature gas liquefaction was not developed in time to be relevant during World War I, production continued. Helium was primarily used as a lifting gas in lighter-than-air aircraft. The demand for this use, as well as for arc welding, was greatest during World War II. The helium mass spectrometer was also vital in the atomic bomb developed by the Manhattan Project.[97]​.

The United States government created the National Helium Reserve in 1925 in Amarillo, Texas, with the goal of supplying it to military aircraft in wartime and commercial aircraft in peacetime. Due to a United States military embargo against Germany in which the supply of helium was restricted, the LZ-129 Hindenburg was forced to use hydrogen as a booster gas. Use of helium after World War II was reduced, but reserves were expanded in the 1950s to ensure its supply in liquid form as a coolant to create hydrogen and oxygen fuel (among other uses) for rockets during the space race and the Cold War. Helium use in the United States in 1965 was more than eight times the maximum wartime consumption.[98]​.

The United States Bureau of Mines had five private plants to recover helium from natural gas. [99]

By 1995, nearly one billion cubic meters of gas had been stored, and the reserves constituted a debt of 1.4 billion dollars, which in 1996 forced the United States Congress to eliminate them. The helium produced between 1930 and 1945 was approximately 98.3% pure (with 2% nitrogen), which was enough to fill airships. In 1945, a small amount of 99.9% helium was used to make welds.[100] By 1949, commercial quantities of 99.9% grade A helium were available.

Neutral helium under normal conditions is non-toxic, does not play any biological role and is found in trace amounts in human blood. If enough helium is inhaled to replace the oxygen needed for breathing, it can cause asphyxiation. The precautions that must be taken for helium used in cryogenics are similar to those for liquid nitrogen. Its extremely low temperature can cause freezer burn and the liquid-to-gas expansion rate can cause explosions if pressure release mechanisms are not used.

Tanks of gaseous helium at temperatures of 5 to 10 K should be stored as if they contained liquid helium due to the large increase in pressure and significant thermal expansion that occurs when heating the gas from a temperature below 10 K to room temperature.[24]​.

The speed of sound in helium is almost 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. If helium is inhaled, there is a corresponding increase in the heights of the resonance frequencies of the vocal folds.[105]​ (The opposite effect, lowering frequencies, can be obtained by inhalation of a dense gas such as sulfur hexafluoride).

Its inhalation can be dangerous if done in excess, since it is an asphyxiating gas and displaces the oxygen necessary for normal breathing.[106] Breathing pure helium continuously causes death by asphyxiation in a few minutes. Inhaling helium directly from pressurized cylinders is extremely dangerous, as the high flow velocity can result in rupture of lung tissues.[106][107] However, death caused by helium is very rare, with only two deaths reported in the United States between 2000 and 2004.[107]​.

At high pressures (more than 20 atm or two MPa), a mixture of helium and oxygen (heliox) can lead to high nerve pressure syndrome; a kind of reverse anesthetic effect. Adding a small amount of nitrogen to the mixture can solve the problem.[108]​[109]​.

• - Leidenfrost effect.

• - Noble gas.

• - Inorganic origin of petroleum.

• - Periodic table of the elements.

• - Superfluidity.

• - Goo.

• - Wikimedia Commons hosts a multimedia gallery on Helium.

• - WebElements.com .

• - EnvironmentalChemistry.com .

• - It is Elementary.

• - Chemical Elements: Helium.

• - National Institute of Safety and Hygiene at Work of Spain: International chemical safety sheet for helium.