Everything about Helium totally explained
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For other uses of this term, see Helium (disambiguation).
Helium (
He) is a colorless, odorless, tasteless, non-toxic,
inert monatomic chemical element that heads the
noble gas series in the
periodic table and whose
atomic number is 2. Its
boiling and
melting points are the lowest among the elements and it exists only as a
gas except in extreme conditions. Extreme conditions are also needed to create the small handful of helium
compounds, which are all unstable at
standard temperature and pressure. In its most common form,
helium-4, it has two neutrons in its nucleus, while a second, rarer,
stable isotope called
helium-3 contains just one neutron. The behavior of liquid helium-4's two fluid phases, helium I and helium II, is important to researchers studying
quantum mechanics (in particular the phenomenon of
superfluidity) and to those looking at the effects that temperatures near
absolute zero have on
matter (such as
superconductivity).
In 1868, the French astronomer
Pierre Janssen first detected helium as an unknown yellow
spectral line signature in light from a
solar eclipse. Since then large reserves of helium have been found in the
natural gas fields of the
United States, which is by far the largest supplier of the gas. It is used in
cryogenics, in deep-sea breathing systems, to cool
superconducting magnets, in
helium dating, for inflating
balloons, for providing lift in
airships and as a protective gas for many industrial uses (such as
arc welding and growing
silicon wafers). A much less serious use is to temporarily change the timbre and quality of one's voice by inhaling a small
volume of the gas (for dangers see
Biological effects section below).
Helium is the second most
abundant and second lightest element in the known
universe and is one of the elements believed to have been created in the
Big Bang. In the modern universe almost all new helium is created as a result of the
nuclear fusion of hydrogen in
stars. On
Earth helium is rare, and almost all of that which exists was created by the
radioactive decay of much heavier elements (
alpha particles are helium nuclei). After its creation, part of it was trapped with
natural gas in concentrations up to 7% by volume, from which it's extracted commercially by
fractional distillation. Large reserves of helium have been found in the
natural gas fields of the
United States (the largest supplier) but helium is known in gas reserves of a few other countries.
Notable characteristics
Gas and plasma phases
Helium is the least reactive member of the
noble gas elements, and thus also the least reactive of all elements; it's
inert and
monatomic in virtually all conditions. Due to helium's relatively low molar (atomic) mass, in the gas phase its
thermal conductivity,
specific heat, and
sound conduction velocity are all greater than for any other gas except
hydrogen. For similar reasons, and also due to the small size of helium atoms, helium's
diffusion rate through
solids is three times that of air and around 65% that of hydrogen.
Helium is less water
soluble than any other gas known and helium's
index of refraction is closer to unity than that of any other gas. Helium has a negative
Joule-Thomson coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand. Only below its
Joule-Thomson inversion temperature (of about 40
K at 1 atmosphere) does it cool upon free expansion. Once precooled below this temperature, helium can be liquefied through expansion cooling.
Throughout the universe, helium is found mostly in a
plasma state whose properties are quite different from atomic helium. In a plasma, helium's electrons and protons are not bound together, resulting in very high electrical conductivity, even when the gas is only partially ionized. The charged particles are highly influenced by magnetic and electric fields. For example, in the
solar wind together with ionized hydrogen, they interact with the Earth's
magnetosphere giving rise to
Birkeland currents and the
aurora.
Solid and liquid phases
Helium solidifies only under great pressure. The resulting colorless, almost invisible
solid is highly
compressible; applying pressure in a laboratory can decrease its volume by more than 30%. With a
bulk modulus on the order of 5×10
7 Pa it's 50 times more compressible than water. Unlike any other element, helium will fail to solidify and remain a liquid down to
absolute zero at normal pressures. This is a direct effect of quantum mechanics: specifically, the
zero point energy of the system is too high to allow freezing. Solid helium requires a temperature of 1–1.5 K (about −272 °C or −457 °F) and about 25 bar (2.5 MPa) of pressure. It is often hard to distinguish solid from liquid helium since the
refractive index of the two phases are nearly the same. The solid has a sharp
melting point and has a
crystalline structure.
Solid helium has a density of 0.214 ±0.006 g/ml (1.15 K, 66 atm) with a mean isothermal compressibility of the solid at 1.15 K between the solidus and 66 atm of 0.0031 ±0.0008/atm. Also, no difference in density was noted between 1.8 K and 1.5 K. This data projects that
T=0 solid helium under 25 bar of pressure (the minimum required to freeze helium) has a density of 0.187 ±0.009 g/ml.
Helium I state
Below its
boiling point of 4.22
kelvin and above the
lambda point of 2.1768 kelvin, the
isotope helium-4 exists in a normal colorless
liquid state, called
helium I. Like other
cryogenic liquids, helium I boils when it's heated. It also contracts when its temperature is lowered until it reaches the
lambda point, when it stops boiling and suddenly expands. The rate of expansion decreases below the lambda point until about 1 K is reached; at which point expansion completely stops and helium I starts to contract again.
Helium I has a gas-like
index of refraction of 1.026 which makes its surface so hard to see that floats of
styrofoam are often used to show where the surface is. This colorless liquid has a very low
viscosity and a
density one-eighth that of water, which is only one-fourth the value expected from
classical physics.
Helium II also exhibits a creeping effect. When a surface extends past the level of helium II, the helium II moves along the surface, seemingly against the force of
gravity. Helium II will escape from a vessel that isn't sealed by creeping along the sides until it reaches a warmer region where it evaporates. It moves in a 30
nm-thick film regardless of surface material. This film is called a
Rollin film and is named after the man who first characterized this trait, Bernard V. Rollin. As a result of this creeping behavior and helium II's ability to leak rapidly through tiny openings, it's very difficult to confine liquid helium. Unless the container is carefully constructed, the helium II will creep along the surfaces and through valves until it reaches somewhere warmer, where it'll evaporate. Waves propagating across a Rollin film are governed by the same equation as
gravity waves in shallow water, but rather than gravity, the restoring force is the
Van der Waals force. These waves are known as
third sound.
In the
fountain effect, a chamber is constructed which is connected to a reservoir of helium II by a
sintered disc through which superfluid helium leaks easily but through which non-superfluid helium can't pass. If the interior of the container is heated, the superfluid helium changes to non-superfluid helium. In order to maintain the equilibrium fraction of superfluid helium, superfluid helium leaks through and increases the pressure, causing liquid to fountain out of the container.
The thermal conductivity of helium II is greater than that of any other known substance, a million times that of helium I and several hundred times that of
copper. This is because heat conduction occurs by an exceptional quantum-mechanical mechanism. Most materials that conduct heat well have a
valence band of free electrons which serve to transfer the heat. Helium II has no such valence band but nevertheless conducts heat well. The
flow of heat is governed by equations that are similar to the
wave equation used to characterize sound propagation in air. So when heat is introduced, it'll move at 20 meters per second at 1.8 K through helium II as waves in a phenomenon called
second sound. Helium at low temperatures is also used in
cryogenics.
For its inertness and high thermal conductivity, neutron transparency, and because it doesn't form radioactive isotopes under reactor conditions, helium is used as a coolant in some nuclear reactors, such as pebble-bed reactors.
Helium is used as a shielding gas in arc welding processes on materials that are contaminated easily by air. It is especially useful in overhead welding, because it's lighter than air and thus floats, whereas other shielding gases sink.
Because it's inert, helium is used as a protective gas in growing silicon and germanium crystals, in titanium and zirconium production, in gas chromatography, and as an atmosphere for protecting historical documents. This property also makes it useful in supersonic wind tunnels.
In rocketry, helium is used as an ullage medium to displace fuel and oxidizers in storage tanks and to condense hydrogen and oxygen to make rocket fuel. It is also used to purge fuel and oxidizer from ground support equipment prior to launch and to pre-cool liquid hydrogen in space vehicles. For example, the Saturn V booster used in the Apollo program needed about 13 million cubic feet (370,000 m³) of helium to launch.) of a person's voice when inhaled. However, inhaling it from a typical commercial source, such as that used to fill balloons, can be dangerous due to the risk of asphyxiation from lack of oxygen, and the number of contaminants that may be present. These could include trace amounts of other gases, in addition to aerosolized lubricating oil.
History
Scientific discoveries
Evidence of helium was first detected on August 18, 1868 as a bright yellow line with a wavelength of 587.49 nanometres in the spectrum of the chromosphere of the Sun, by French astronomer Pierre Janssen during a total solar eclipse in Guntur, India. This line was initially assumed to be sodium. On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line in the solar spectrum, which he named the D3 line, for it was near the known D1 and D2 lines of sodium, and concluded that it was caused by an element in the Sun unknown on Earth. He and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος (helios)
On 26 March 1895 British chemist William Ramsay isolated helium on Earth by treating the mineral cleveite (a variety of uraninite with at least 10% rare earth elements) with mineral acids. Ramsay was looking for argon but, after separating nitrogen and oxygen from the gas liberated by sulfuric acid, noticed a bright-yellow line that matched the D3 line observed in the spectrum of the Sun. Helium was also isolated by the American geochemist William Francis Hillebrand prior to Ramsay's discovery when he noticed unusual spectral lines while testing a sample of the mineral uraninite. Hillebrand, however, attributed the lines to nitrogen. His letter of congratulations to Ramsay offers an interesting case of discovery and near-discovery in science.
In 1907, Ernest Rutherford and Thomas Royds demonstrated that alpha particles are helium nuclei, by allowing them to penetrate the thin glass wall of a evacuated tube, then creating a discharge in the tube to study the spectra of the new gas inside. In 1908, helium was first liquefied by Dutch physicist Heike Kamerlingh Onnes by cooling the gas to less than one kelvin. He tried to solidify it by further reducing the temperature but failed, because helium doesn't have a triple point temperature where the solid, liquid, and gas phases are at equilibrium. It was first solidified in 1926 by his student Willem Hendrik Keesom by subjecting helium to 25 atmospheres of pressure.
In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that helium-4 (a boson) has almost no viscosity at temperatures near absolute zero, a phenomenon now called superfluidity. This phenomenon is related to Bose-Einstein condensation. In 1972, the same phenomenon was observed in helium-3, but at temperatures much closer to absolute zero, by American physicists Douglas D. Osheroff, David M. Lee, and Robert C. Richardson. The phenomenon in helium-3 is thought to be related to pairing of helium-3 fermions to make bosons, in analogy to Cooper pairs of electrons producing superconductivity.
Extraction and uses
After an oil drilling operation in 1903 in Dexter, Kansas, U.S. produced a gas geyser that wouldn't burn, Kansas state geologist Erasmus Haworth collected samples of the escaping gas and took them back to the University of Kansas at Lawrence where, with the help of chemists Hamilton Cady and David McFarland, he discovered that the gas contained, by volume, 72% nitrogen, 15% methane—insufficient to make the gas combustible, 1% hydrogen, and 12% of an unidentifiable gas. With further analysis, Cady and McFarland discovered that 1.84% of the gas sample was helium. Far from being a rare element, helium was present in vast quantities under the American Great Plains, available for extraction from natural gas.
This put the United States in an excellent position to become the world's leading supplier of helium. Following a suggestion by Sir Richard Threlfall, the United States Navy sponsored three small experimental helium production plants during World War I. The goal was to supply barrage balloons with the non-flammable lifting gas. A total of 200 thousand cubic feet (5,700 m3) of 92% helium was produced in the program even though only a few cubic feet (less than 100 liters) of the gas had previously been obtained.
Although the extraction process, using low-temperature gas liquefaction, wasn't developed in time to be significant during World War I, production continued. Helium was primarily used as a lifting gas in lighter-than-air craft. This use increased demand during World War II, as well as demands for shielded arc welding. Helium was also vital in the atomic bomb Manhattan Project.
The government of the United States set up the National Helium Reserve in 1925 at Amarillo, Texas with the goal of supplying military airships in time of war and commercial airships in peacetime. Due to a US military embargo against Germany that restricted helium supplies, the Hindenburg was forced to use hydrogen as the lift gas. Helium use following World War II was depressed but the reserve was expanded in the 1950s to ensure a supply of liquid helium as a coolant to create oxygen/hydrogen rocket fuel (among other uses) during the Space Race and Cold War. Helium use in the United States in 1965 was more than eight times the peak wartime consumption.
After the "Helium Acts Amendments of 1960" (Public Law 86–777), the U.S. Bureau of Mines arranged for five private plants to recover helium from natural gas. For this helium conservation program, the Bureau built a 425-mile (684 km) pipeline from Bushton, Kansas to connect those plants with the government's partially depleted Cliffside gas field, near Amarillo, Texas. This helium-nitrogen mixture was injected and stored in the Cliffside gas field until needed, when it then was further purified.
By 1995, a billion cubic metres of the gas had been collected and the reserve was US$1.4 billion in debt, prompting the Congress of the United States in 1996 to phase out the reserve. The resulting "Helium Privatization Act of 1996" (Public Law 104–273) directed the United States Department of the Interior to start liquidating the reserve by 2005.
Helium produced before 1945 was about 98% pure (2% nitrogen), which was adequate for airships. In 1945 a small amount of 99.9% helium was produced for welding use. By 1949 commercial quantities of Grade A 99.995% helium were available.
For many years the United States produced over 90% of commercially usable helium in the world. Extraction plants created in Canada, Poland, Russia, and other nations produced the remaining helium. In the mid 1990s, A new plant in Arzew, Algeria producing 600 million cubic feet (1.7 m3) came on stream, with enough production to cover all of Europe's demand. Subsequently, in 2004–2006 two additional plants, one in Ras Laffen, Qatar and the other in Skikda, Algeria were built, but as of early 2007, Ras Laffen is functioning at 50%, and Skikda has yet to start up. Algeria quickly became the second leading producer of helium. Through this time, both helium consumption and the costs of producing helium increased and during 2007 the major suppliers, Air Liquide, Airgas and Praxair all raised prices from 10 to 30%.
Occurrence and production
Natural abundance
Helium is the second most abundant element in the known Universe after hydrogen and constitutes 23% of the elemental mass of the universe. It is concentrated in stars, where it's formed from hydrogen by the nuclear fusion of the proton-proton chain reaction and CNO cycle. According to the Big Bang model of the early development of the universe, the vast majority of helium was formed during Big Bang nucleosynthesis, from one to three minutes after the Big Bang. As such, measurements of its abundance contribute to cosmological models.
In the Earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million. The concentration is low and fairly constant despite the continuous production of new helium because most helium in the Earth's atmosphere escapes into space by several processes.
In the Earth's heterosphere, a part of the upper atmosphere, helium and other lighter gases are the most abundant elements.
Nearly all helium on Earth is a result of radioactive decay. The decay product is primarily found in minerals of uranium and thorium, including cleveites, pitchblende, carnotite and monazite, because they emit alpha particles, which consist of helium nuclei (He2+) to which electrons readily combine. In this way an estimated 3.4 litres of helium per year are generated per cubic kilometer of the Earth's crust. In the Earth's crust, the concentration of helium is 8 parts per billion. In seawater, the concentration is only 4 parts per trillion. There are also small amounts in mineral springs, volcanic gas, and meteoric iron. The greatest concentrations on the planet are in natural gas, from which most commercial helium is derived.
The world's helium supply may be in danger, according to Washington University in St. Louis chemist Lee Sobotka. The largest reserve is in Texas and would run out in eight years if consumed at the current pace. Helium is non-renewable and irreplaceable by conventional methods. In total, there's currently 25,000 million m³ of helium in reserve bases of various countries.
Modern extraction
For large-scale use, helium is extracted by fractional distillation from natural gas, which contains up to 7% helium. Since helium has a lower boiling point than any other element, low temperature and high pressure are used to liquefy nearly all the other gases (mostly nitrogen and methane). The resulting crude helium gas is purified by successive exposures to lowering temperatures, in which almost all of the remaining nitrogen and other gases are precipitated out of the gaseous mixture. Activated charcoal is used as a final purification step, usually resulting in 99.995% pure, Grade-A, helium. The principal impurity in Grade-A helium is neon. In a final production step, most of the helium that's produced is liquefied via a cryogenic process. This is necessary for applications requiring liquid helium and also allows helium suppliers to reduce the cost of long distance transportation, as the largest liquid helium containers have more than five times the capacity of the largest gaseous helium tube trailers.
In 2005, approximately 160 million m³ of helium were extracted from natural gas or withdrawn from helium reserves, with approximately 83% from the United States, 11% from Algeria, and most of the remainder from Russia and Poland. In the United States, most helium is extracted from natural gas in Kansas and Texas.
Diffusion of crude natural gas through special semipermeable membranes and other barriers is another method to recover and purify helium. Helium can be synthesized by bombardment of lithium or boron with high-velocity protons, but this isn't an economically viable method of production.
Isotopes
There are eight known isotopes of helium, but only helium-3 and helium-4 are stable. In the Earth's atmosphere, there's one He-3 atom for every million He-4 atoms. Unlike most elements, helium's isotopic abundance varies greatly by origin, due to the different formation processes.
The most common isotope, helium-4, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis.
Helium-3 is present on Earth only in trace amounts, most of it since Earth's formation thought some falls to Earth trapped in cosmic dust. Trace amounts are also produced by the beta decay of tritium. Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten, and these ratios can be used to investigate the origin of rocks and the composition of the Earth's mantle. Extraplanetary material, such as lunar and asteroid regolith, have trace amounts of helium-3 from being bombarded by solar winds. The Moon's surface contains helium-3 at concentrations on the order of 0.01 ppm. A number of people, starting with Gerald Kulcinski in 1986, have proposed to explore the moon, mine lunar regolith and use the helium-3 for fusion.
Liquid helium-4 can be cooled to about 1 kelvin using evaporative cooling in a 1-K pot. Similar cooling of helium-3, which has a lower boiling point, can achieve about 0.2 kelvin in a helium-3 refrigerator. Equal mixtures of liquid He-3 and He-4 below 0.8 K separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: helium-4 atoms are bosons while helium-3 atoms are fermions). Dilution refrigerators use this immiscibility to achieve temperatures of a few millikelvins.
It is possible to produce exotic helium isotopes, which rapidly decay into other substances. The shortest-lived heavy helium isotope is helium-5 with a half-life of 7.6×10−22 seconds. Helium-6 decays by emitting a beta particle and has a half life of 0.8 seconds. Helium-7 also emits a beta particle as well as a gamma ray. Helium-7 and helium-8 are hyperfragments that are created in certain nuclear reactions. Helium-6 and helium-8 are known to exhibit a nuclear halo. Helium-2 (two protons, no neutrons) is a radioisotope that decays by proton emission into protium (hydrogen), with a half-life of 3x10−27 second.
Neutral helium at standard conditions is non-toxic, plays no biological role and is found in trace amounts in human blood. At high pressures (more than about 20 atm or two MPa), a mixture of helium and oxygen (heliox) can lead to high pressure nervous syndrome, a sort of reverse-anesthetic effect; adding a small amount of nitrogen to the mixture can alleviate the problem.
Containers of helium gas at 5 to 10 K should be handled as if they contain liquid helium due to the rapid and significant thermal expansion that occurs when helium gas at less than 10 K is warmed to room temperature. It is an electrical insulator unless ionized. As with the other noble gases, helium has metastable energy levels that allow it to remain ionized in an electrical discharge with a voltage below its ionization potential. Helium can form unstable compounds with tungsten, iodine, fluorine, sulfur and phosphorus when it's subjected to an electric glow discharge, through electron bombardment or is otherwise a plasma. HeNe, HgHe10, WHe2 and the molecular ions He2+, He22+, HeH+, and HeD+ have been created this way. This technique has also allowed the production of the neutral molecule He2, which has a large number of band systems, and HgHe, which is apparently only held together by polarization forces.
Helium has been put inside the hollow carbon cage molecules (the fullerenes) by heating under high pressure of the gas. The neutral molecules formed are stable up to high temperatures. When chemical derivatives of these fullerenes are formed, the helium stays inside. If helium-3 is used, it can be readily observed by helium NMR spectroscopy. Many fullerenes containing helium-3 have been reported. Although the helium atoms are not attached by covalent or ionic bonds, these substances fit the definition of compounds in the Handbook of Chemistry and Physics. They are the first stable neutral helium compounds to be formed.
Further Information
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