# helium

properties
1 s 2
2 He
General
Name , symbol , atomic number Helium, He, 2
Element category Noble gases
Group , period , block 18 , 1 , p
Appearance Colorless gas
CAS number 7440-59-7
EC number 231-168-5
ECHA InfoCard 100.028.334
ATC code
Mass fraction of the earth's envelope 0.004  ppm
Atomic
Atomic mass 4.002602 (2) and
Van der Waals radius 140 pm
Electron configuration 1 s 2
1. Ionization energy 24.587 388 80 (15) eV 2 372.32 kJ / mol
2. Ionization energy 54.417 765 0 (3) eV 5 250.51 kJ / mol
Physically
Physical state gaseous
density 0.1785 kg m −3
magnetism diamagnetic ( = −1.1 10 −9 ) ${\ displaystyle \ chi _ {m}}$
Melting point 0.95 K (−272.2 ° C, at 2.5 MPa)
boiling point 4.15 K (−269 ° C)
Molar volume (solid) 21.00 · 10 −6 m 3 · mol −1
Heat of evaporation 0.0840 kJ / mol
Heat of fusion 0.02 kJ mol −1
Speed ​​of sound 970 m s −1 at 273.15 K.
Specific heat capacity 5193 J kg −1 K −1
Thermal conductivity 0.1513 W m −1 K −1
Chemically
Oxidation states 0
Electronegativity 5.50 (Allred & Rochow); 4.86 (Mulliken); 5.2 (average);
no information on ( Pauling scale )
Isotopes
isotope NH t 1/2 ZA ZE (M eV ) ZP
3 He 0.000137% Stable
4 He 99.999863  % Stable
5 He {syn.} 7.618 · 10 −22  s n 0.60 4 He
6 He {syn.} 806.7 ms β - 3.508 6 li
For other isotopes see list of isotopes
NMR properties
Spin
quantum
number I
γ in
rad · T −1 · s −1
E r  ( 1 H) f L at
B = 4.7 T
in MHz
3 He 1/2 −20.38016 10 7 0.442 152.45
safety instructions
GHS labeling of hazardous substances

Caution

H and P phrases H: 280
P: 403
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .

Helium ( ancient Greek ἥλιος hélios , German ' sun ' ) is a chemical element and has the atomic number 2. Its element symbol is He. In the periodic table it is in the 18th  IUPAC group , the former VIII main group , and is therefore one of the noble gases . It is a colorless, odorless, tasteless and non-toxic gas .

Helium remains gaseous down to very low temperatures; it only becomes liquid when it is close to absolute zero. It is the only substance that does not solidify even at absolute zero (0  K or −273.15  ° C ) under normal pressure . Besides neon, helium is the only element for which, even under extreme conditions, no compounds could be detected that did not decay immediately after formation. Helium only occurs atomically . The most common stable isotope is 4 He; Another stable isotope is 3 He, which is extremely rare on earth .

The behavior of the two liquid phases helium I and helium II (i.e. helium-I and helium-II ) (especially the phenomenon of superfluidity ) of 4 He is the subject of current research in the field of quantum mechanics . Furthermore, liquid helium is an indispensable tool for achieving extremely low temperatures. These are required , among other things, to cool infrared detectors in space telescopes and to investigate properties such as the superconductivity of matter at temperatures close to absolute zero.

After hydrogen , helium is the second most abundant element in the universe. According to accepted theory, ten seconds after the Big Bang, protons and neutrons combined to form the first atomic nuclei through nuclear fusion . From their total mass, 25% 4 He and 0.001% deuterium and traces of 3 He were formed. Thus, most of the helium was created during the Big Bang . Most of the helium formed inside stars by the fusion of hydrogen continued to fuse to form heavier elements.

On earth, 4 He is formed in the form of alpha particles during the alpha decay of various radioactive elements such as uranium or radium . Helium is created when the alpha particle snatches two electrons from other atoms. The majority of the helium present on earth is therefore of non-stellar origin. The resulting helium is found in natural gas in concentrations of up to 16 percent by volume. Therefore, helium can be obtained from natural gas by fractional distillation .

The first evidence of helium was discovered in 1868 by the French astronomer Jules Janssen while investigating the light spectrum of the sun's chromosphere , where he found the hitherto unknown yellow spectral line of helium.

Helium is used in low-temperature technology , especially as a coolant for superconducting magnets , in deep-sea breathing apparatus , for determining the age of rocks, as a filling gas for balloons, as a lifting gas for airships and as a protective gas for various industrial applications (for example in gas-shielded metal welding , as a carrier gas for the capillary and in the production of silicon wafers ). After inhaling helium, the voice changes briefly (“ Mickey Mouse voice ”) due to the higher speed of sound compared to air .

## history

The full spectrum of the sun
The spectral lines of helium. The yellow spectral line is particularly noticeable.
Jules Janssen , first discoverer of helium
Spectrum of a helium gas discharge

Evidence of the element helium was first obtained from a bright yellow spectral line at a wavelength of 587.49 nanometers in the spectrum of the sun's chromosphere . The French astronomer Jules Janssen made this observation in India during the total solar eclipse on August 18, 1868 . When he made his discovery known, no one believed him at first, as a new element had never been found in space before it could be proven on earth. On October 20 of the same year, the Englishman Norman Lockyer confirmed that the yellow line was indeed present in the solar spectrum and concluded that it was caused by a previously unknown element. Since this spectral line was very close (1.8 nm from the center) to the Fraunhofer double-D line (D2 = 589.00 nm, D1 = 589.60 nm) of the metal sodium, he named the line D3 to it to be able to distinguish precisely from these D1 and D2 lines of sodium. He and his English colleague Edward Frankland suggested the new element Heli to call, what as much sun as metal is. At the time, no one even suspected that it was not a metal.

14 years later, in 1882, Luigi Palmieri succeeded for the first time in detecting the element helium on earth using the spectral analysis of Vesuvius lava.

Sir William Ramsay

On March 23, 1895, the British chemist William Ramsay won helium by adding mineral acids to the uranium mineral Cleveit , a variety of uraninite , and isolating the gas that escaped. He was looking for argon but was able to observe the yellow D3 line after separating nitrogen and oxygen from the isolated gas. The same discovery was made almost simultaneously by the British physicist William Crookes and the Swedish chemists Per Teodor Cleve and Nicolas Langlet in Uppsala , Sweden . These collected sufficient quantities of the gas to be able to determine its atomic mass .

During an oil well in Dexter , Kansas , a natural gas well was found whose natural gas contained 12 percent by volume of an unknown gas. The American chemists Hamilton Cady and David McFarland of the University of Kansas found out in 1905 that it was helium. They published a report that helium can be obtained from natural gas. In the same year Ernest Rutherford and Thomas Royds discovered that alpha particles are helium nuclei.

The first liquefaction of helium was carried out in 1908 by the Dutch physicist Heike Kamerlingh Onnes by cooling the gas to a temperature below 1 K. He could not obtain solid helium even after further cooling; this was only achieved in 1926 by Willem Hendrik Keesom , a student of Onnes', by compressing the helium to 25 bar at an analogous temperature. Onnes first described the phenomenon of superfluid fluids now known as the Onnes Effect .

In the early 20th century, large amounts of helium were found in natural gas fields on the American Great Plains , making the United States the world's leading supplier of helium. Following a suggestion from Sir Richard Threlfall , the US Navy sponsored three small experimental helium production companies during the First World War to use helium as a filling gas for barrier balloons . A total of 5,700 cubic meters of gas with a helium content of 92% was extracted from these companies. This helium was used in the first helium-filled airship, the US Navy's C-7 , in 1921 .

The US government had the National Helium Reserve built in Amarillo , Texas , in 1925 to ensure supplies to military airships in wartime and airships in peacetime. The camp is located in a natural rock formation 20 km northwest of Amarillo. Although demand fell after World War II , the Amarillo production facility was expanded to include liquid helium as a coolant for oxygen-hydrogen rocket fuel and other items to be cooled. US helium consumption rose eight times its peak wartime consumption in 1965.

After the Helium Acts Amendments of 1960 (Public Law 86-777) was passed in the USA , another five private helium production plants were built. The US Department of Mines had a 685-kilometer pipeline built from Bushton in Kansas to Amarillo in Texas; In 1995 this store contained around one billion cubic meters of helium and in 2004 around ten times the annual global demand for helium. The warehouse is to be empty and closed by 2015 ( Helium Privatization Act ).

The purity of the helium obtained increased rapidly after the Second World War. In 1945 a mixture of 98% helium and 2% nitrogen was still used for airships, in 1949 helium with a purity of 99.995% was already commercially available. In order to achieve this degree of purity , activated carbon is necessary in order to remove remaining impurities - mostly consisting of neon  - by means of pressure swing adsorption .

## Occurrence

### In space

Helium makes up approximately 19% of Neptune's outer gas layers. Neptune's main component is hydrogen; the blue-green color is caused by methane .

According to the Big Bang theory , most of the helium present in space today was created in the first three minutes after the Big Bang. After hydrogen, helium is the second most common element. 23% of the mass of visible matter is made up of helium, although hydrogen atoms are eight times as abundant. In addition, helium is produced in stars by nuclear fusion . This so-called hydrogen burning provides the energy that makes the stars on the main sequence , i.e. the majority of all stars, shine. This process provides the stars with energy for most of their lives. When most of the hydrogen in the core is used up at the end of a star's life, the core contracts and increases its temperature. This means that helium can now be burned to carbon ( helium flash , helium burning ). Hydrogen burning continues to take place in a shell around this core. Carbon can also be burned further to form other elements. This process usually continues down to the iron if a supernova explosion does not occur. A supernova explosion also synthesizes elements heavier than iron, which are dispersed in space by the explosion. Over time, the interstellar matter becomes enriched with helium and heavier elements, so that later stars also have a larger proportion of helium and heavier elements.

On star surfaces and in nebulae , helium is preferentially neutral or simply ionized. Unlike in physics and chemistry, the notation with superscript “+” (He + ) is not used in astronomy , as other elements can be so highly ionized that this notation becomes impractical, for example sixteen-fold ionized iron in the solar corona . Therefore, ionization levels in astronomy are designated with Roman numerals, with neutral helium being designated as He I, singly ionized as He II and completely (= twice) ionized as Helium III (Helium III).

Helium is also present in various planetary atmospheres (near the ground or, in the case of gas planets, near the surface):

 Neptune 19% ± 3.2% Uranus 15.2% ± 3.3% Jupiter 10.2% Saturn 03.25% Venus 00.0012% (12  ppm ) earth 00.00052% (5.2 ppm)

### In meteorites and on the moon

Helium can also be generated in meteorites and superficial lunar rocks through interaction ( spallation ) with cosmic rays . 3 He in particular can therefore be used to determine the so-called irradiation age , which usually corresponds to the period from when the meteorite hit the mother body until it arrived on earth. In addition, 4 He is formed in meteorites through the decay of heavy radioactive elements. There are also other helium components in meteorites, which originate from the time the solar system was formed, but were also partly captured from the solar wind .

### On earth

The earth produces helium through radioactive processes in the earth's interior.

4 He is created in the earth's body during radioactive decay ( alpha decay ) of heavy elements such as uranium or thorium , with helium nuclei being emitted as alpha particles and then trapping electrons. It can be found in various uranium and thorium-containing minerals such as pitchblende .

A proportion of 3 He in the earth's mantle, which is far above the atmospheric value, the so-called mantle helium, comes from the time the earth was formed ; the 4 He / 3 He ratio in the upper mantle, which is largely degassed and whose helium stock is therefore essentially replenished by 4 He from alpha decays, is around 86,000. If the convection system of the lower mantle is largely separated from that of the upper mantle and the mass exchange between the two is correspondingly low, the ratio in the lower, barely degassed mantle is between 2500 and 26,000, i.e. the proportion of 3 He is higher. This is of particular geodynamic interest with regard to the causes of hotspot volcanism : while 4 He / 3 He = 86,000 is typical for basalts from mid-ocean ridges, which are formed by melting processes of material from the upper mantle, basalts from some hotspots, for Example oceanic volcanic islands like Hawaii and Iceland, around three to four times 3 He richer. This is commonly explained by the fact that this volcanism is caused by mantle plumes whose origin lies at the core-mantle boundary and which therefore consist at least partially of material from the lower mantle.

Helium occurs - through the same mechanism of accumulation - in natural gas (with up to 16 percent by volume) and in small amounts in crude oil (0.4%). European natural gas reserves only contain proportions of 0.12 (North Sea) to 0.4 percent by volume (Poland), while Siberian, North American (Canada, Texas, Kansas and Oklahoma) and Algerian natural gas reserves of up to 16 percent by volume are possible.

In the lower layers of the earth's atmosphere , especially in the troposphere mixed by the weather , the helium content is about 5.2 ppm. At very high altitudes, gases tend to segregate according to their different densities, also against the mixing effect of the undirected molecular heat movement. Above 100 km altitude ( homosphere ) the atmosphere is increasingly segregated, so at altitudes> 400 km, helium becomes the predominant gas (in terms of number of particles). Helium atoms escape into space at these heights - in the stationary case, as much as is supplied from the earth's surface through diffusion, extraction and volcanism.

## Extraction

Natural gas with a helium content of 0.2% or more is the largest and economically most important helium supplier. Since helium has a very low boiling point, it is possible by cooling the natural gas to separate the helium from the other substances contained in natural gas, such as hydrocarbons and nitrogen compounds .

For many years the US recovered over 90% of the world's commercially usable helium. In 1995, a total of one billion cubic meters of helium were produced in the USA. The remainder was supplied by production facilities in Canada , Poland , Russia (with large quantities in the inaccessible areas of Siberia ) and other countries. After the turn of the millennium, Algeria and Qatar were added. Algeria quickly became the second most important helium supplier. In 2002 Algeria produced 16% of the helium distributed in the world. The helium is obtained there in the course of liquefying natural gas.

In 2004, Amarillo in Texas had around ten times the annual global demand for helium. However, this former strategic reserve of the US government has to be sold to the private sector due to the Helium Privatization Act of the Clinton administration from 1996 to 2015.

This initially caused a glut of helium with very low prices, which led to wasteful use and for a long time did not result in any measures to be taken to be economical. However, because consumption is constantly increasing, helium threatens to become scarce, and facilities for the recovery of helium have increasingly been put into operation by large consumers. Experts even warn of a helium shortage, since helium can only be obtained from some natural gases. In 2016, however, a huge helium deposit was discovered in Tanzania, so that the helium crisis is considered averted for the time being. Since the geological conditions under which helium is formed could also be determined, further discoveries are hoped for in the future. In September 2019, an impending global helium crisis was again pointed out.

The 3 He isotope is only contained in natural helium at around 1.4  ppm and is therefore many times more expensive than the natural isotope mixture.

### generation

In principle, helium can also be obtained in nuclear reactions . Helium 4 He is produced by neutron bombardment of Lithium 6 Li in a nuclear reactor ; tritium 3 H (superheavy hydrogen) is formed as a by-product :

${\ displaystyle \ mathrm {{} ^ {6} Li + n \ \ rightarrow \ {} ^ {3} H + {} ^ {4} He}}$

Tritium breaks down to 3 He through beta decay with a half-life of 12.33 years.

Very small amounts of helium 3 He are also incubated in reactors moderated with water when the hydrogen atoms in the water capture neutrons. Heavy hydrogen ( deuterium ) is formed from normal hydrogen and tritium is formed from it through further neutron capture , which in turn becomes helium 3 He through beta decay . With normal hydrogen the capture rate is higher than in the next step of neutron capture by heavy hydrogen (therefore nuclear power plants that use heavy water as moderator can also be operated with natural uranium):

${\ displaystyle \ mathrm {{} _ {1} ^ {1} H \ {\ xrightarrow {(n, \ gamma)}} \ _ {1} ^ {2} H \ {\ xrightarrow {(n, \ gamma )}} \ _ {1} ^ {3} H \ {\ xrightarrow [{12 {,} 33 \ a}] {\ beta ^ {-}}} \ _ {2} ^ {3} He}}$

The times given are half-lives .

## properties

### Physical Properties

After hydrogen, helium is the chemical element with the second lowest density and has the lowest melting and boiling points of all elements. Therefore it only exists as a liquid or solid at very low temperatures. At temperatures below 2.17 K, 4 He is in a superfluid phase. At normal pressure, helium does not solidify even at a temperature close to 0 K. Only at a pressure above 2.5 MPa (around 25 times atmospheric pressure) does helium change into a solid phase at sufficiently low temperatures.

#### In the gaseous state

Helium is a colorless, odorless, tasteless and non-toxic gas. Under standard conditions , helium behaves almost like an ideal gas . Helium is atomic in practically all conditions. Under standard conditions, one cubic meter of helium has a mass of 179 g, while air is about seven times as heavy. After hydrogen, helium has the greatest thermal conductivity of all gases and its specific heat capacity is extraordinarily large. Helium is a good electrical insulator . The solubility of helium in water is 1.5 mg / l (9.3 ml / l) at 20 ° C and 101,325 kPa, lower than that of any other gas. Its diffusion rate through solids is three times that of air and about 65 percent of hydrogen. Under standard conditions, helium has a negative Joule-Thomson coefficient , which means that this gas heats up when it expands. Only below the Joule-Thomson inversion temperature (approx. 40 K at atmospheric pressure) does it cool down during expansion. Therefore helium has to be pre-cooled below this temperature before it can be liquefied by expansion cooling. Its critical data are a pressure of 2.27 bar, a temperature of −267.95 ° C (5.2 K) and a density of 0.0696 g / cm 3 .

Helium gas discharge tubes of various designs

#### In the liquid state

##### Helium I.

At normal pressure, helium forms a colorless liquid between the lambda point at 2.1768 K and the boiling point at 4.15 K.

##### Helium II

Liquid 4 He develops very unusual properties below its lambda point. Helium with these properties is called Helium II .

Helium II is a superfluid substance. It flows through the smallest openings in the order of magnitude of 10 −7  to 10 −8  m and has no measurable viscosity . However, measurements between two moving disks showed a viscosity similar to that of gaseous helium. This phenomenon is explained with the two-fluid model (or two-fluid model) according to László Tisza . According to this theory, Helium II is like a mixture of 4 He particles in the normal fluid as well as in the superfluid state, accordingly Helium II behaves as if there were a proportion of helium atoms with and one without a measurable viscosity. On the basis of this theory, many phenomena in low temperature physics such as the “thermomechanical effect” can be explained relatively simply and clearly. However, one must clearly point out that the two liquids are neither theoretically nor practically separable. In Helium II, the rotons postulated by Lew Landau could be detected as collective excitations.

Onnes effect of helium II

Helium II shows the Onnes effect : if a surface protrudes from the helium, the helium on this surface also moves against gravity. In this way, Helium II escapes from a container that is not sealed. When it reaches a warmer area, it evaporates. Because of this creep behavior and the ability of Helium II to leak through even the smallest openings, it is very difficult to keep liquid helium in a confined space. A very carefully constructed container is required to store Helium II without it escaping or evaporating.

The thermal conductivity of helium II cannot be compared with classic thermal conduction, it rather shows parallels to heat transport by means of convection. This enables faster and more effective heat transport over long distances, which is not possible with conventional heat conduction even with very good heat conductors. This type of conduction is also referred to as second sound , as it can be described in the same way as sound by a longitudinal wave equation : Helium II at 1.8 K conducts heat as an impulse with a speed of 20 m / s.

In 1971, David M. Lee , Douglas D. Osheroff and Robert C. Richardson succeeded in converting the helium isotope 3 He into a superfluid state by cooling the isotope below the temperature of 2.6 milli-Kelvin. It is assumed that two 3 He atoms form a pair, similar to a Cooper pair . This pair has a magnetic moment and an angular momentum . The three scientists received the 1996 Nobel Prize in Physics for this discovery .

#### In solid condition

Helium is the only substance that cannot be solidified under normal pressure. This only works under increased pressure (around 2.5 MPa / 0 K for helium-4, 2.93 MPa / 0.315 K for helium-3) and at a very low temperature (less than 1.5 K). The almost completely transparent solid formed during the phase transition is very compressible. In the laboratory, its volume can be reduced by up to 30%; Helium is more than 50 times more compressible than water. In the solid state it forms crystalline structures. Solid and liquid helium can hardly be distinguished from one another optically, as their refractive indices are almost the same.

In another case, if the temperature falls below about 200 mK and centrifugation is carried out at the same time, a state can be reached which is called suprasolide or suprafest. Here part of the solid stops its own rotation and penetrates the remaining parts of the matter. There are still no known theses or theories about this partially controversial effect.

### Atomic properties

The two electrons of the helium atom form the closed, spherically symmetrical electron shell of the 1s atomic orbital . This electron configuration is energetically extremely stable; there is no other element with a higher ionization energy and a lower electron affinity . Despite its larger number of electrons, helium is smaller than hydrogen and therefore the smallest atom of all.

Depending on the spin orientation of the two electrons of the helium atom, one speaks of parahelium in the case of two opposing spins (S = 0) and of orthohelium in the case of two parallel spins (S = 1). In the orthohelium one of the electrons is not in the 1s orbital, as this would violate the Pauli prohibition .

The naming of these states goes back to an earlier error: Since the electromagnetic transition between the ground state of the orthohelium and the ground state of the parahelium (i.e. the helium ground state) is prohibited , the two "variants" of helium appear spectroscopically like two different atoms. This led Carl Runge and Louis Paschen to postulate that helium consists of two separate gases, orthohelium ("real helium") and parahelium (for which they suggested the name asterium).

In addition to the electron configuration of the orthohelium, the electrons can assume other excited states, for example when they are bombarded with electrons. These long-lived excited states are known as metastable energy levels .

### Chemical properties

Helium is an inert gas . The only electron shell is fully occupied with two electrons. Due to the close proximity to the atomic nucleus, both electrons are very strongly bound to it. Not least because of this, helium itself is extremely inert compared to other noble gases. This can also be seen in the high ionization energies of the helium atom.

#### Helium dimer

As can be seen from the molecular orbital diagram, helium atoms do not form a chemical bond with one another . In the case of helium, the 1s orbital is occupied by a pair of electrons. When two of these fully occupied atomic orbitals (a) and (b) are combined, both the binding and the antibonding molecular orbital are each occupied by a pair of electrons. In the case of the (hypothetically) developing binding orbitals, the energetically more favorable, so-called binding state is compensated by the antibonding which is also occupied, but energetically less favorable. The overall system is energetically not lower, and there is no bond .

 Occupation of the orbitals of a hypothetical He 2 molecule.

Because of the Van der Waals interaction , which is effective for all atoms and molecules, there is also a dimer for helium , but with an extremely small binding energy of around 1.1 mK (= 9.5 · 10 −26  J) and a correspondingly large one Bond distance of about 52 Å.

#### Ionic bonds

Under extreme conditions it is possible to create a quasi-chemical compound of helium with a proton (HeH + ). This compound is very unstable under normal conditions and cannot be isolated in the form of a salt such as HeH + X - .

${\ displaystyle \ mathrm {He + H ^ {+} \ longrightarrow HeH ^ {+}}}$
A helium hydride ion is formed in a mixture of helium and hydrogen during an electrical discharge

A corresponding reaction can take place between two helium atoms if the energy necessary for ionization is supplied.

${\ displaystyle \ mathrm {He + He \ longrightarrow HeHe ^ {+} + e ^ {-}}}$

However, these compounds cannot be described as real chemical compounds, but rather as ionic agglomerations that arise under exceptional conditions, exist only for a very short time and disintegrate again very quickly.

## Isotopes

Different properties of the helium isotopes
3 He 4 He
Rest energy in MeV 2809 3728
Density in kg / m 3 0.134 0.178
Critical temperature in K 3.32 5.20
Lambda point in K 0.0025 2.1768
Melt pressure at T = 0 K in MPa 3,439 2.536
Boiling point in K 3.19 4.21

Of the eight known isotopes of helium, only 3 He and 4 He are stable. In the earth's atmosphere there is only one 3 He atom per million 4 He atoms . However, the proportion of the two isotopes varies depending on the place of origin of the examined helium sample. In the interstellar medium, 3 He atoms are a hundred times as common. In rocks of the earth's crust and mantle, the proportion is also well above the atmospheric value and varies by a factor of 10 depending on the origin. These variations are used in geology to clarify the origin of the rock (see also section On Earth ).

Due to their different symmetry properties ( 3 He atoms are fermions , 4 He atoms are bosons ) 3 He and 4 He have some different physical properties, which are particularly evident at low temperatures. Equal proportions of liquid 3 He and 4 He below 0.8 Kelvin separate into two immiscible liquids, similar to oil and water, due to their different quantum properties. A phase of pure 3 He floats on a phase that mainly consists of 4 He. Furthermore, the two isotopes clearly differ in their superfluid phases (see section Helium II ).

### Nuclear fusion

In announcements of new space missions from the USA, Russia and China, as well as Europe, India and Japan to the moon , the proportionally larger deposits of 3 He there were mentioned several times as a worthwhile source to enable nuclear fusion reactors based on this isotope on earth. In contrast to the deuterium - tritium fusion reaction, the deuterium 3 He reaction does not produce free neutrons but protons with a similarly high energy gain . This would dramatically reduce the radioactivity problems of fusion power generation. On the other hand, bringing about this reaction is an as yet unsolved technical challenge because of the much higher plasma temperature required.

### Hypothetical diproton

A special, fictitious isotope of helium is 2 He, the nucleus of which, the diproton , would only consist of two protons if it existed. However, there is no bound state for a system made up of two protons, because due to the Pauli principle  - in contrast to the proton and neutron in the deuteron  - they may only be in a singlet state with antiparallel spins. Due to the strong spin dependence of the nucleon-nucleon interaction, however, this is energetically increased and therefore not bound.

## Use and forms of trade

Schematic representation of a helium-neon laser

The helium offered in wholesaling comes from large plants in five countries ( USA , Russia , Poland , Qatar and Algeria ), and helium is extracted from natural gas.

In the technical gases division , helium is supplied in the form of compressed gas in pressure bottles with a pressure of 200 bar and degrees of purity from helium 4.6 (99.996% helium content) to ultra-pure helium 7.0 (99.99999% helium content). Steel bottles with a typical volume of 10-50 liters contain only 1.8 to 9.1 standard cubic meters of helium at 200 bar, as it behaves clearly not ideally at 200 bar . Larger quantities are delivered in pallets of twelve bottles each or bottle bundles of twelve 50-liter bottles each. Even larger quantities come in cryogenic liquid in cryogenic semi-trailers or tube trailers with typically ten 12 m long tubes filled with around 200 bar of helium, a total of 5,000 standard cubic meters.

Helium is also transported in cryogenic liquefied form, for example from a production facility in Africa to a port to the west near Marseille . Helium for end users is offered in the trade with a low purity of approx. 98% to over 99% primarily in the form of one-way gas cylinders as so-called "balloon gas" so that smaller quantities of balloons can be inflated and ascended easily and safely at events and celebrations. Balloon gas can in principle also be used as a lifting gas for larger balloons such as weather balloons , but is more expensive in this application compared to hydrogen .

Helium is used in many ways:

• A helium- oxygen mixture (80:20) is used as breathing gas in intensive care medicine. The mixture flows through constrictions with less resistance and is therefore easier to breathe.
• In professional diving , various mixtures of helium such as trimix (consisting of oxygen, nitrogen and helium), hydreliox (hydrogen, helium and oxygen) and heliox (helium and oxygen) are used as breathing gas . The high heat capacity of the helium has a disadvantageous effect, which (in a cold environment) leads to the cooling of the lungs and thus the diver.
• It is used as a propellant or packing gas in the food industry and is approved as a food additive E 939 .
• Helium is a preferred carrier gas for balloons and airships because it has a very low density compared to air, does not burn and can therefore be safely mixed with air. Helium has therefore largely displaced combustible hydrogen , which forms explosively flammable mixtures with air, even if the density of helium is higher and thus its carrying capacity is somewhat lower than that of hydrogen. However, due to the high diffusion rate, the impermeability requirements of the envelope are higher than for all other gases.
• In welding technology , helium is used in its pure form or as an admixture as an inert gas to protect the welding point from oxygen . In addition, helium can be used to increase the burn-in depth and the welding speed and to reduce the formation of spatter, particularly in robot welding and when processing aluminum and stainless steels.
• Technically, liquefied helium (the isotopes 4 He and 3 He) is used as a coolant to achieve very low temperatures ( see: cryostat ). With 4 He, temperatures of up to about 1 K can be achieved by evaporative cooling, with the isotope 3 He up to about 240 mK. With the method of the 3 He- 4 He mixture cooling be achieved mK to about 5, said method much less expensive than a pure 3 is He-cooling. When using superconducting magnets , helium is used as a coolant to keep the superconductor below its critical temperature . Practical applications here are particularly magnetic resonance imaging (MRT) for medical applications as well as magnetic resonance spectroscopy (NMR) and the operation of particle accelerators in research. In space travel, liquid helium cools infrared telescopes and the highly sensitive infrared cameras in space telescopes, which can only work near absolute zero without excessive intrinsic heat. Examples are: IRAS , ISO , the Spitzer and the Herschel space telescope . Another area of ​​application is the production of optical glass fibers in helium-cooled drop towers.
• Compressed helium gas can be used as a coolant, especially where a coolant that is chemically and physically particularly inert is required. In terms of nuclear physics, however, only the main component 4 He is inert , while 3 He is easily converted into radioactive tritium by thermal neutrons . An example is the thorium high-temperature reactor (short: THTR), where the helium was used at very high temperatures. It should be noted that helium has a high specific but a low molar heat capacity . This is particularly problematic in the case of closed apparatus, since in the event of a temperature rise (for example in the event of a power failure) a massive pressure increase quickly occurs. When used as a coolant, the viscosity of helium, which increases with increasing temperature (as with all gases) has proven to be disadvantageous, since this can worsen the cooling of hot areas.
• The search for leaks in pressurized gas fittings is made easier by filling with helium. A leak detection spray is applied to the outside of the pressure fitting . Helium penetrates through leaks particularly easily and creates more distinct foam bubbles than the operating gas.
• In vacuum systems, helium is used as the most diffusion-friendly leak detection gas by evacuating the vacuum apparatus with a pump and hanging a mass spectrometer behind the pump. If the apparatus is blown with helium - outside, only locally in order to find leaks - the mass spectrometer can be used to detect a possible entry of helium into the apparatus and the leak rate can be measured. This fast and sensitive leak detection method is also used in chemical plants and in the manufacture of heat exchangers for air conditioning systems or gasoline tanks for cars.
• Helium is used in gas form in rocket technology to replace the fuel used in pumped liquid fuel rockets so that the thin-walled fuel tanks of the rockets do not implode when the fuel is sucked out of the tanks by the fuel pumps of the engines. In the case of liquid fuel rockets supported by pressurized gas, helium pushes the fuel into the engines. Helium is used here because of its low weight and low boiling temperature. Since it cannot react with the fuel as a noble gas, aggressive hypergolic fuels are also not a problem.
• Helium is used as an auxiliary gas in various types of lasers, for example the helium-neon laser , the helium-cadmium laser and some types of carbon dioxide lasers . It serves as a collision partner to excite or de-excite the laser levels of the actual active laser media.
• Pure helium is used as a carrier gas in gas chromatography (analysis).
• In gas discharge tubes , helium glows yellowish / white.
• Due to its thermodynamic properties, helium is a very good working medium for Stirling engines .
• Hyperpolarized 3 He is used experimentally in diagnostics as a contrast medium for magnetic resonance imaging of the lungs.
• Instead of compressed air to drive impact wrenches when changing a wheel in Formula 1 automotive racing , it could be operated 30% faster at a certain pressure. In order to avoid costs, prohibited by regulations from 2012.
• In the case of hard disk drives , filling with helium instead of air reduces flow effects and vibrations during operation and thus enables the distance between the individual magnetic disks to be reduced. With the same size, more magnetic disks can be accommodated, thereby increasing the storage capacity of the hard disk.

## hazards

Helium is an inert gas and is non-toxic. When handling larger amounts of gaseous helium, safety measures must be taken if the amount of gas and the spatial situation mean that there is a risk that breathing air may be displaced. The number of accidents caused by asphyxiation is lower with helium in contrast to other gases (e.g. nitrogen ) that are often used as inert gas , because gaseous helium rises immediately due to its low density and therefore only rarely occurs in poorly ventilated rooms in the lower area a complete displacement of oxygen from the surrounding air and thus the risk of suffocation . Potential danger areas can be the accumulation of helium gas in building structures that are impermeable to the top, for example roof trusses, under which a “helium bubble” can form.

When handling liquid helium ( UN number UN 1977) - it is 73 K colder than liquid nitrogen, which is also referred to as "cryogenic" - it is necessary to wear protective clothing to prevent frostbite through contact. The danger mainly arises from deep-frozen containers, apparatus and fittings or from pre-cooling with LN 2 , since liquid helium itself only has an extremely low cooling capacity (220 ml LHe has the cooling capacity of 1 ml LN 2 ). A safety goggles to protect your eyes or sight the whole face, thick gloves and a certain thickness with cuff hands. Open pockets or boot shafts are entry points for splashes and should therefore be avoided. Further dangers arise from icing and the associated clogging and exploding of lines and vessels.

Compressed helium gas containers - mostly seamless steel cylinders for 200 bar high pressure or welded (often: disposable) bottles - are under high pressure. It is strictly to avoid heating it above the standard value of 60 ° C or contact with fire. On the one hand, the internal pressure increases with the temperature and, on the other hand, the strength of the steel wall decreases, so that there is a risk of the vessel bursting very forcefully. Even tearing off the valve, for example if a bottle falls without a protective cap, or breaking a rupture disc, triggers a gas jet with dangerous consequences.

## Others

After inhaling helium, as long as the airways contain a relevant high proportion of helium, the human voice sounds considerably higher. (This effect is popularly called " Mickey Mouse voice", but it was achieved by playing the tape faster, i.e. increasing all frequencies (and the tempo) by a certain factor.) The timbre of a voice depends on the position of the formants in the mouth, which are influenced by factors such as tongue and lip position. (Formants are those frequency ranges that are most strongly amplified by the effect of resonance .) These formants also depend on the speed of sound c in the corresponding medium (c air = 350 m / s, c helium = 1030 m / s). For example, if the position of the first three formants in air is 220, 2270 and 3270 Hz, this changes in (pure) helium to 320, 3900 and 5500 Hz. This results in a different vocal pattern and the voice appears overall higher, even if the pitch of the pitch would remain unchanged by the noble gas.

There is a similar effect when a wind instrument (initially only air-filled) is blown with helium.

## literature

Wiktionary: Helium  - explanations of meanings, word origins, synonyms, translations
Commons : Helium  album with pictures, videos and audio files
Wikibooks: Wikijunior The elements / elements / helium  - learning and teaching materials

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