Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditionsSony VAIO VGN-CS33H battery.
Helium is the second lightest element and is the second most abundant element in the observable universe, being present at about 24% of the total elemental mass, which is more than 12 times the mass of all the heavier elements combined. Its abundance is similar to this figure in our own Sun and in JupiterSony VAIO VGN-CS33H/Z battery. This is due to the very high binding energy (per nucleon) of helium-4 with respect to the next three elements after helium. This helium-4 binding energy also accounts for its commonality as a product in both nuclear fusion and radioactive decay. Most helium in the universe is helium-4, and is believed to have been formed during the Big Bang. Some new helium is being created currently as a result of the nuclear fusion of hydrogen in starsSony VAIO VGN-CS33H/B battery.
Helium is named for the Greek God of the Sun, Helios. It was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with detecting the element along with Norman Lockyer during the solar eclipse of 1868, and Lockyer was the first to propose that the line was due to a new element, which he namedSony VAIO VGN-CS31Z/Q battery. The formal discovery of the element was made in 1895 by two Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, who found helium emanating from the uranium ore cleveite. In 1903, large reserves of helium were found in natural gas fields in parts of the United States, which is by far the largest supplier of the gas todaySony VAIO VGN-CS31S/W battery.
Helium is used in cryogenics (its largest single use, absorbing about a quarter of production), particularly in the cooling of superconducting magnets, with the main commercial application being in MRI scanners. Helium's other industrial uses—as a pressurizing and purge gas, as a protective atmosphere for arc welding and in processes such as growing crystals to make silicon wafers—account for half of the gas producedSony VAIO VGN-CS31S/V battery. A well-known but minor use is as a lifting gas in balloons and airships. As with any gas with differing density from air, inhaling a small volume of helium temporarily changes the timbre and quality of the human voice. In scientific research, the behavior of the two fluid phases of helium-4 (helium I and helium II), is important to researchers studying quantum mechanics Sony VAIO VGN-CS31S/T battery (in particular the property of superfluidity) and to those looking at the phenomena, such as superconductivity, that temperatures near absolute zero produce in matter.
On Earth it is thus relatively rare—0.00052% by volume in the atmosphere. Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements (thorium and uranium) Sony VAIO VGN-CS31S/R battery, as the alpha particles emitted by such decays consist of helium-4 nuclei. This radiogenic helium is trapped with natural gas in concentrations up to 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation.
The first evidence of helium was observed on August 18, 1868 as a bright yellow line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the SunSony VAIO VGN-CS31S/P battery. The line was detected by French astronomer Jules 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 Fraunhofer line because it was near the known D1 and D2 lines of sodiumSony VAIO VGN-CS28 battery. He concluded that it was caused by an element in the Sun unknown on Earth. Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος (helios).
Spectral lines of helium
In 1882, Italian physicist Luigi Palmieri detected helium on Earth, for the first time, through its D3 spectral line, when he analyzed the lava of Mount VesuviusSony VAIO VGN-CS28/Q battery.
Sir William Ramsay, the discoverer of terrestrial helium
On March 26, 1895, Scottish chemist Sir 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, he noticed a bright yellow line that matched the D3 line observed in the spectrum of the Sun. Sony VAIO VGN-CS27 batteryThese samples were identified as helium by Lockyer and British physicist William Crookes. It was independently isolated from cleveite in the same year by chemists Per Teodor Cleve and Abraham Langlet in Uppsala, Sweden, who collected enough of the gas to accurately determine its atomic weight. 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 nitrogenSony VAIO VGN-CS27/W battery. 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 the particles to penetrate the thin glass wall of an evacuated tube, then creating a discharge in the tube to study the spectra of the new gas insideSony VAIO VGN-CS27/R battery. In 1908, helium was first liquefied by Dutch physicist Heike Kamerlingh Onnes by cooling the gas to less than one kelvin. He tried to solidify it by further reducing the temperature but failed because helium does not have a triple point temperature at which the solid, liquid, and gas phases are at equilibrium. Onnes' student Willem Hendrik Keesom was eventually able to solidify 1 cm3 of helium in 1926 by applying additional external pressureSony VAIO VGN-CS27/P battery.
In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that helium-4 has almost no viscosity at temperatures near absolute zero, a phenomenon now called superfluidity. 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 zeroSony VAIO VGN-CS27/C battery, 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 use
After an oil drilling operation in 1903 in Dexter, Kansas, produced a gas geyser that would not burn, Kansas state geologist Erasmus Haworth collected samples of the escaping gas and took them back to the University of Kansas at Lawrence whereSony VAIO VGN-CS26T/W battery, with the help of chemists Hamilton Cady and David McFarland, he discovered that the gas consisted of, by volume, 72% nitrogen, 15% methane (a combustible percentage only with sufficient oxygen), 1% hydrogen, and 12% an unidentifiable gas. With further analysis, Cady and McFarland discovered that 1.84% of the gas sample was helium. Sony VAIO VGN-CS26T/V battery This showed that despite its overall rarity on Earth, helium was concentrated in large quantities under the American Great Plains, available for extraction as a byproduct of natural gas. The greatest reserves of helium were in the Hugoton and nearby gas fields in southwest Kansas and the panhandles of Texas and Oklahoma.
This enabled the United States to become the world's leading supplier of helium. Following a suggestion by Sir Richard ThrelfallSony VAIO VGN-CS26T/T battery, the United States Navy sponsored three small experimental helium plants during World War I. The goal was to supply barrage balloons with the non-flammable, lighter-than-air gas. A total of 5,700 m3 (200,000 cubic feet) of 92% helium was produced in the program even though less than a cubic meter of the gas had previously been obtained. Some of this gas was used in the world's first helium-filled airshipSony VAIO VGN-CS26T/R battery, the U.S. Navy's C-7, which flew its maiden voyage from Hampton Roads, Virginia, to Bolling Field in Washington, D.C., on December 1, 1921.
Although the extraction process, using low-temperature gas liquefaction, was not developed in time to be significant during World War I, production continued. Helium was primarily used as a lifting gas in lighter-than-air craftSony VAIO VGN-CS26T/Q battery. This use increased demand during World War II, as well as demands for shielded arc welding. The helium mass spectrometer 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 peacetimeSony VAIO VGN-CS26T/P battery. Because of a US military embargo against Germany that restricted helium supplies, the Hindenburg, like all German Zeppelins, 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 WarSony VAIO VGN-CS26T/C battery. 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 fieldSony VAIO VGN-CS25H battery, 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 meters 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 emptying the reserve by 2005Sony VAIO VGN-CS25H/W battery.
Helium produced between 1930 and 1945 was about 98.3% pure (2% nitrogen), which was adequate for airships. In 1945, a small amount of 99.9% helium was produced for welding use. By 1949, commercial quantities of Grade A 99.95% helium were available.
For many years the United States produced over 90% of commercially usable helium in the world, while extraction plants in CanadaSony VAIO VGN-CS25H/R battery, Poland, Russia, and other nations produced the remainder. In the mid-1990s, a new plant in Arzew, Algeria, producing 17 million cubic meters (600 million cubic feet) began operation, with enough production to cover all of Europe's demand. Meanwhile, by 2000, the consumption of helium within the U.S. had risen to above 15 million kg per yearSony VAIO VGN-CS25H/Q battery. In 2004–2006, two additional plants, one in Ras Laffan, Qatar, and the other in Skikda, Algeria, were built, but as of early 2007, Ras Laffan 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. In the 2002 to 2007 period helium prices doubledSony VAIO VGN-CS25H/P battery.
The helium atom
Main article: Helium atom
The helium atom. Depicted are the nucleus (pink) and the electron cloud distribution (black). The nucleus (upper right) in helium-4 is in reality spherically symmetric and closely resembles the electron cloud, although for more complicated nuclei this is not always the case.
Helium in quantum mechanics
In the perspective of quantum mechanics, helium is the second simplest atom to model, following the hydrogen atomSony VAIO VGN-CS25H/C battery. Helium is composed of two electrons in atomic orbitals surrounding a nucleus containing two protons along with some neutrons. As in Newtonian mechanics, no system consisting of more than two particles can be solved with an exact analytical mathematical approach (see 3-body problem) and helium is no exception. Thus, numerical mathematical methods are required, even to solve the system of one nucleus and two electronsSony VAIO VGN-CS23T/W battery. Such computational chemistry methods have been used to create a quantum mechanical picture of helium electron binding which is accurate to within < 2% of the correct value, in a few computational steps. In such models it is found that each electron in helium partly screens the nucleus from the other, so that the effective nuclear charge Z which each electron sees, is about 1.69 units, not the 2 charges of a classic "bare" helium nucleusSony VAIO VGN-CS23T/Q battery.
The related stability of the helium-4 nucleus and electron shell
The nucleus of the helium-4 atom is identical with an alpha particle. High energy electron-scattering experiments show its charge to decrease exponentially from a maximum at a central point, exactly as does the charge density of helium's own electron cloud. This symmetry reflects similar underlying physicsSony VAIO VGN-CS23H battery: the pair of neutrons and the pair of protons in helium's nucleus obey the same quantum mechanical rules as do helium's pair of electrons (although the nuclear particles are subject to a different nuclear binding potential), so that all these fermions fully occupy 1s orbitals in pairs, none of them possessing orbital angular momentum, and each cancelling the other's intrinsic spinSony VAIO VGN-CS23H/S battery. Adding another of any of these particles would require angular momentum and would release substantially less energy (in fact, no nucleus with five nucleons is stable). This arrangement is thus energetically extremely stable for all these particles, and this stability accounts for many crucial facts regarding helium in nature.
For example, the stability and low energy of the electron cloud state in helium accounts for the element's chemical inertnessSony VAIO VGN-CS23H/B battery, and also the lack of interaction of helium atoms with each other, producing the lowest melting and boiling points of all the elements.
In a similar way, the particular energetic stability of the helium-4 nucleus, produced by similar effects, accounts for the ease of helium-4 production in atomic reactions involving both heavy-particle emission, and fusion. Some stable helium-3 is produced in fusion reactions from hydrogen, but it is a very small fractionSony VAIO VGN-CS23G battery, compared with the highly favorable helium-4. The stability of helium-4 is the reason hydrogen is converted to helium-4 (not deuterium or helium-3 or heavier elements) in the Sun. It is also partly responsible for the fact that the alpha particle is by far the most common type of baryonic particle to be ejected from atomic nuclei; in other words, alpha decay is far more common than cluster decaySony VAIO VGN-CS23G/W battery.
The unusual stability of the helium-4 nucleus is also important cosmologically: it explains the fact that in the first few minutes after the Big Bang, as the "soup" of free protons and neutrons which had initially been created in about 6:1 ratio cooled to the point that nuclear binding was possible, almost all first compound atomic nuclei to form were helium-4 nucleiSony VAIO VGN-CS23G/Q battery. So tight was helium-4 binding that helium-4 production consumed nearly all of the free neutrons in a few minutes, before they could beta-decay, and also leaving few to form heavier atoms such as lithium, beryllium, or boron. Helium-4 nuclear binding per nucleon is stronger than in any of these elements (see nucleogenesis and binding energy) and thus no energetic drive was availableSony VAIO VGN-CS23G/P battery, once helium had been formed, to make elements 3, 4 and 5. It was barely energetically favorable for helium to fuse into the next element with a lower energy per nucleon, carbon. However, due to lack of intermediate elements, this process requires three helium nuclei striking each other nearly simultaneously (see triple alpha process) Sony VAIO VGN-CS21Z/Q battery. There was thus no time for significant carbon to be formed in the few minutes after the Big Bang, before the early expanding universe cooled to the temperature and pressure point where helium fusion to carbon was no longer possible. This left the early universe with a very similar ratio of hydrogen/helium as is observed today (3 parts hydrogen to 1 part helium-4 by mass), with nearly all the neutrons in the universe trapped in helium-4Sony VAIO VGN-CS21S/W battery.
All heavier elements (including those necessary for rocky planets like the Earth, and for carbon-based or other life) have thus been created since the Big Bang in stars which were hot enough to fuse helium itself. All elements other than hydrogen and helium today account for only 2% of the mass of atomic matter in the universe. Helium-4, by contrast, makes up about 23% of the universe's ordinary matter—nearly all the ordinary matter that is not hydrogenSony VAIO VGN-CS21S/V battery.
Gas and plasma phases
Helium discharge tube shaped like the element's atomic symbol
Helium is the least reactive noble gas after neon and thus the second least reactive of all elements; it is inert and monatomic in all standard conditions. Because of helium's relatively low molar (atomic) mass, its thermal conductivity, specific heat, and sound speed in the gas phase are all greater than any other gas except hydrogen. For similar reasonsSony VAIO VGN-CS21S/T battery, 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 the least water soluble monatomic gas, and one of the least water soluble of any gas (CF4, SF6, and C4F8 have lower mole fraction solubilities: 0.3802, 0.4394, and 0.2372 x2/10−5, respectively, versus helium's 0.70797 x2/10−5), and helium's index of refraction is closer to unity than that of any other gasSony VAIO VGN-CS21S/R battery. 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 32 to 50 K at 1 atmosphere) does it cool upon free expansion. Once precooled below this temperature, helium can be liquefied through expansion coolingSony VAIO VGN-CS21S/P battery.
Most extraterrestrial helium is found in a plasma state, with properties quite different from those of atomic helium. In a plasma, helium's electrons are not bound to its nucleus, resulting in very high electrical conductivity, even when the gas is only partially ionized. The charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind together with ionized hydrogenSony VAIO VGN-CS215J/R battery, the particles interact with the Earth's magnetosphere giving rise to Birkeland currents and the aurora.
Solid and liquid phases
Liquefied helium. This helium is not only liquid, but has been cooled to the point of superfluidity. The drop of liquid at the bottom of the glass represents helium spontaneously escaping from the container over the side, to empty out of the container. The energy to drive this process is supplied by the potential energy of the falling helium. See superfluidSony VAIO VGN-CS215J/Q battery.
Main article: Liquid helium
Unlike any other element, helium will remain 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 sameSony VAIO VGN-CS19/W battery. The solid has a sharp melting point and has a crystalline structure, but it is highly compressible; applying pressure in a laboratory can decrease its volume by more than 30%. With a bulk modulus of about 27 MPa it is ~100 times more compressible than water. Solid helium has a density of 0.214 ± 0.006 g/cm3 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/cm3. Sony VAIO VGN-CS19/R battery
Helium I state
Below its boiling point of 4.22 kelvins and above the lambda point of 2.1768 kelvins, the isotope helium-4 exists in a normal colorless liquid state, called helium I. Like other cryogenic liquids, helium I boils when it is heated and contracts when its temperature is lowered. Below the lambda point, however, helium does not boil, and it expands as the temperature is lowered furtherSony VAIO VGN-CS19/Q battery.
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 of 0.145–0.125 g/mL (between about 0 and 4 K, respectively), which is only one-fourth the value expected from classical physics. Sony VAIO VGN-CS19/P battery Quantum mechanics is needed to explain this property and thus both types of liquid helium are called quantum fluids, meaning they display atomic properties on a macroscopic scale. This may be an effect of its boiling point being so close to absolute zero, preventing random molecular motion (thermal energy) from masking the atomic propertiesSony VAIO VGN-CS17H/W battery.
Helium II state
Liquid helium below its lambda point begins to exhibit very unusual characteristics, in a state called helium II. Boiling of helium II is not possible due to its high thermal conductivity; heat input instead causes evaporation of the liquid directly to gas. Helium-3 also has a superfluid phase, but only at much lower temperatures; as a result, less is known about such properties in the isotopeSony VAIO VGN-CS17H/Q battery.
Unlike ordinary liquids, helium II will creep along surfaces in order to reach an equal level; after a short while, the levels in the two containers will equalize. The Rollin film also covers the interior of the larger container; if it were not sealed, the helium II would creep out and escape.
Helium II is a superfluid, a quantum mechanical state (see: macroscopic quantum phenomena) of matter with strange propertiesSony VAIO VGN-CS16T/W battery . For example, when it flows through capillaries as thin as 10−7 to 10−8 m it has no measurable viscosity. However, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed. Current theory explains this using the two-fluid model for helium II. In this model, liquid helium below the lambda point is viewed as containing a proportion of helium atoms in a ground stateSony VAIO VGN-CS16T/T battery, which are superfluid and flow with exactly zero viscosity, and a proportion of helium atoms in an excited state, which behave more like an ordinary fluid.
In the fountain effect, a chamber is constructed which is connected to a reservoir of helium II by a sintered disc through which superfluid helium leaks easily but through which non-superfluid helium cannot pass. If the interior of the container is heated, the superfluid helium changes to non-superfluid helium. In order to maintain the equilibrium fraction of superfluid heliumSony VAIO VGN-CS16T/R battery, 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 mechanism. Most materials that conduct heat well have a valence band of free electrons which serve to transfer the heatSony VAIO VGN-CS16T/Q battery. 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. When heat is introduced, it moves at 20 meters per second at 1.8 K through helium II as waves in a phenomenon known as second soundSony VAIO VGN-CS16T/P battery.
Helium II also exhibits a creeping effect. When a surface extends past the level of helium II, the helium II moves along the surface, against the force of gravity. Helium II will escape from a vessel that is not sealed by creeping along the sides until it reaches a warmer region where it evaporates. It moves in a 30 nm-thick film regardless of surface materialSony VAIO VGN-CS13T/W battery. 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 is very difficult to confine liquid helium. Unless the container is carefully constructed, the helium II will creep along the surfaces and through valves until it reaches somewhere warmer, where it will evaporateSony VAIO VGN-CS13H/W battery. 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.
Main article: Isotopes of helium
There are eight known isotopes of helium, but only helium-3 and helium-4 are stable. In the Earth's atmosphere, there is one 3He atom for every million 4He atoms. Unlike most elements, helium's isotopic abundance varies greatly by origin, due to the different formation processesSony VAIO VGN-CS13H/R battery. 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 nucleosynthesisSony VAIO VGN-CS13H/Q battery.
Helium-3 is present on Earth only in trace amounts; most of it since Earth's formation, though 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. 3He is much more abundant in stars, as a product of nuclear fusionSony VAIO VGN-CS13H/P battery. Thus in the interstellar medium, the proportion of 3He to 4He is around 100 times higher than on Earth. 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, much higher than the ca. 5 ppt found in the Earth's atmosphere. Sony VAIO VGN-CS11Z/T battery 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 3He and 4He below 0.8 K separate into two immiscible phases due to their dissimilarity (they follow different quantum statisticsSony VAIO VGN-CS11Z/R battery: 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 s. Helium-6 decays by emitting a beta particle and has a half-life of 0.8 secondSony VAIO VGN-CS11S/W battery. Helium-7 also emits a beta particle as well as a gamma ray. Helium-7 and helium-8 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 3×10−27 sSony VAIO VGN-CS11S/Q battery.
See also: Noble gas compound
Helium has a valence of zero and is chemically unreactive under all normal conditions. 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, known as excimers, with tungstenSony VAIO VGN-CS11S/P battery, iodine, fluorine, sulfur and phosphorus when it is subjected to an electric glow discharge, to electron bombardment, or else is a plasma for another reason. The molecular compounds HeNe, HgHe10, and WHe2, and the molecular ions He+
2, 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 held together only by polarization forces(Sony VAIO VGN-AW11M/H battery). Theoretically, other true compounds may also be possible, such as helium fluorohydride (HHeF) which would be analogous to HArF, discovered in 2000. Calculations show that two new compounds containing a helium-oxygen bond could be stable. Two new molecular species, predicted using theory, CsFHeO and N(CH3)4FHeO, are derivatives of a metastable (Sony VAIO VGN-AW11S/B battery) anion first theorized in 2005 by a group from Taiwan. If confirmed by experiment, such compounds will end helium's chemical inertness, and the only remaining inert element will be neon.
Helium has been put inside the hollow carbon cage molecules (the fullerenes) by heating under high pressure. The endohedral fullerene molecules formed are stable up to high temperatures. When chemical derivatives of these fullerenes are formed, the helium stays inside(Sony VAIO VGN-AW11Z/B battery). If helium-3 is used, it can be readily observed by helium nuclear magnetic resonance spectroscopy. Many fullerenes containing helium-3 have been reported. Although the helium atoms are not attached by covalent or ionic bonds, these substances have distinct properties and a definite composition, like all stoichiometric chemical compounds(Sony VAIO VGN-AW170C battery).
Occurrence and production
Helium is the second most abundant element in the known Universe (after hydrogen), constituting 23% of its baryonic mass. The vast majority of helium was formed by Big Bang nucleosynthesis one to three minutes after the Big Bang. As such, measurements of its abundance contribute to cosmological models(Sony VAIO VGN-AW19/Q battery). In stars, it is formed by the nuclear fusion of hydrogen in proton-proton chain reactions and the CNO cycle, part of stellar nucleosynthesis.
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(Sony VAIO VGN-AW19 battery).
Most helium on Earth is a result of radioactive decay. Helium is found in large amounts in minerals of uranium and thorium, including cleveite, pitchblende, carnotite and monazite, because they emit alpha particles (helium nuclei, He2+) to which electrons immediately combine as soon as the particle is stopped by the rock. In this way an estimated 3000 metric tons of helium are generated per year throughout the lithosphere. (Sony VAIO VGN-AW21M/H battery) 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. Because helium is trapped in the subsurface under conditions that also trap natural gas, the greatest natural concentrations of helium on the planet are found in natural gas, from which most commercial helium is extracted(Sony VAIO VGN-AW21S/B battery). The concentration varies in a broad range from a few ppm up to over 7% in a small gas field in San Juan County, New Mexico.
Modern extraction and distribution
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) (Sony VAIO VGN-AW21VY/Q battery). 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 is produced is liquefied via a cryogenic process(Sony VAIO VGN-AW21XY/Q battery). 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 2008, approximately 169 million standard cubic meters (SCM) of helium were extracted from natural gas or withdrawn from helium reserves with approximately 78% from the United States, 10% from Algeria, and most of the remainder from Russia, Poland and Qatar(Sony VAIO VGN-AW21Z/B battery). In the United States, most helium is extracted from natural gas of the Hugoton and nearby gas fields in Kansas, Oklahoma, and Texas. Much of this gas was once sent by pipeline to the National Helium Reserve, but since 2005 this reserve is presently being depleted and sold off.
Diffusion of crude natural gas through special semipermeable membranes and other barriers is another method to recover and purify helium. In 1996, the U.S. had proven helium reserves, in such gas well complexes(Sony VAIO VGN-AW230J/H battery), of about 147 billion standard cubic feet (4.2 billion SCM). At rates of use at that time (72 million SCM per year in the U.S.; see pie chart below) this is enough helium for about 58 years of U.S. use, and less than this (perhaps 80% of the time) at world use rates, although factors in saving and processing impact effective reserve numbers. It is estimated that the resource base for yet-unproven helium in natural gas in the U.S. is 31–53 trillion SCM, about 1000 times the proven reserves(Sony VAIO VGN-AW31M/H battery).
Helium must be extracted from natural gas because it is present in air at only a fraction of that of neon, yet the demand for it is far higher. It is estimated that if all neon production were retooled to save helium, that 0.1% of the world's helium demands would be satisfied. Similarly, only 1% of the world's helium demands could be satisfied by re-tooling all air distillation plants(Sony VAIO VGN-AW31S/B battery). Helium can be synthesized by bombardment of lithium or boron with high-velocity protons, but economically, this is a completely non-viable method of production.
Helium is commercially available in either liquid or gaseous form. As a liquid, it can be supplied in small containers called Dewars which hold up to 1,000 liters of helium, or in large ISO containers which have nominal capacities as large as 42 m3 (around 11,000 U.S. gallons). In gaseous form, small quantities of helium are supplied in high pressure cylinders holding up to 8 m3 (Sony VAIO VGN-AW41JF/H battery) (approx. 282 standard cubic feet), while large quantities of high pressure gas are supplied in tube trailers which have capacities of up to 4,860 m3 (approx. 172,000 standard cubic feet).
According to helium conservationists like Robert Coleman Richardson, the free market price of helium has contributed to "wasteful" usage (e.g. for helium balloons). Prices in the 2000s have been lowered by U.S. Congress' decision to sell off the country's large helium stockpile by 2015. According to Richardson, the current price needs to be multiplied by 20 to eliminate the excessive wasting of helium(Sony VAIO VGN-AW41JF battery).
Helium is used for many purposes that require some of its unique properties, such as its low boiling point, low density, low solubility, high thermal conductivity, or inertness. Of the 2008 world helium total production of about 32 million kg (193 million standard cubic meters) helium per year, the largest use (about 22% of the total in 2008) is in cryogenic applications(Sony VAIO VGN-AW41MF/H battery), most of which involves cooling the superconducting magnets in medical MRI scanners. Other major uses (totalling to about 78% of use in 1996) were pressurizing and purging systems, maintenance of controlled atmospheres, and welding. Other uses by category were relatively minor fractions(Sony VAIO VGN-AW41MF battery).
Helium is used as a protective gas in growing silicon and germanium crystals, in titanium and zirconium production, and in gas chromatography, because it is inert. Because of its inertness, thermally and calorically perfect nature, high speed of sound, and high value of the heat capacity ratio, it is also useful in supersonic wind tunnels and impulse facilities(Sony VAIO VGN-AW41XH/Q battery).
Gas tungsten arc welding
Main article: gas tungsten arc welding
Helium is used as a shielding gas in arc welding processes on materials that at welding temperatures are contaminated and weakened by air or nitrogen. A number of inert shelding gases are used in gas tungsten arc welding, but helium is used instead of cheaper argon especially for welding materials that have higher heat conductivity, like aluminium or copper(Sony VAIO VGN-AW41XH battery).
Industrial leak detection
A dual chamber helium leak detection machine
One industrial application for helium is leak detection. Because helium diffuses through solids three times faster than air, it is used as a tracer gas to detect leaks in high-vacuum equipment (such as cryogenic tanks) and high-pressure containers. The tested object is placed in a chamber, which is then evacuated and filled with helium(Sony VAIO VGN-AW41ZF/B battery). The helium that escapes through the leaks is detected by a sensitive device (helium mass spectrometer), even at the leak rates as small as 10−9 mbar·L/s (10−10 Pa·m3/s). The measurement procedure is normally automatic and is called helium integral test. A simpler procedure is to fill the tested object with helium and to manually search for leaks with a hand-held device(Sony VAIO VGN-AW41ZF battery).
Helium leaks through cracks should not be confused with gas permeation through a bulk material. While helium has documented permeation constants (thus a calculable permeation rate) through glasses, ceramics, and synthetic materials, inert gases such as helium will not permeate most bulk metals.
Because of its low density and incombustibility, helium is the gas of choice to fill airships such as the Goodyear blimp(SONY Vaio VGN-NS38M Battery).
Because it is lighter than air, airships and balloons are inflated with helium for lift. While hydrogen gas is approximately 7% more buoyant, helium has the advantage of being non-flammable (in addition to being fire retardant). While balloons are perhaps the most well-known use of helium, they are a minor part of all helium use(SONY Vaio VGN-NS31S Battery). Another minor use is in rocketry, where 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 370,000 m3 (13 million cubic feet) of helium to launch(SONY Vaio VGN-NS31M Battery).
Minor commercial and recreational uses
For its low solubility in nervous tissue, helium mixtures such as trimix, heliox and heliair are used for deep diving to reduce the effects of narcosis. At depths below 150 metres (490 ft) small amounts of hydrogen are added to a helium-oxygen mixture to counter the effects of high pressure nervous syndrome. At these depths the low density of helium is found to considerably reduce the effort of breathing(SONY Vaio VGN-NS31Z Battery).
Helium-neon lasers, a type of low-powered gas laser producing a red beam, had various practical applications which included barcode readers and laser pointers, before they were almost universally replaced by cheaper diode lasers.
For its inertness and high thermal conductivity, neutron transparency, and because it does not form radioactive isotopes under reactor conditions, helium is used as a heat-transfer medium in some gas-cooled nuclear reactors(SONY Vaio VGN-NS21Z Battery).
Helium, mixed with a heavier gas such as xenon, is useful for thermoacoustic refrigeration due to the resulting high heat capacity ratio and low Prandtl number. The inertness of helium has environmental advantages over conventional refrigeration systems which contribute to ozone depletion or global warming(SONY Vaio VGN-NS21M Battery).
The use of helium reduces the distorting effects of temperature variations in the space between lenses in some telescopes, due to its extremely low index of refraction. This method is especially used in solar telescopes where a vacuum tight telescope tube would be too heavy. (SONY Vaio VGN-NS21S Battery)
Helium is a commonly used carrier gas for gas chromatography.
The age of rocks and minerals that contain uranium and thorium can be estimated by measuring the level of helium with a process known as helium dating.
Helium at low temperatures is used in cryogenics, and in certain crygenics applications. As examples of applications, liquid helium is used to cool certain metals to the extremely low temperatures required for superconductivity, such as in superconducting magnets for magnetic resonance imaging(SONY Vaio VGN-NS12S Battery). The Large Hadron Collider at CERN uses 96 metric tons of liquid helium to maintain the temperature at 1.9 kelvin.
Neutral helium at standard conditions is non-toxic, plays no biological role and is found in trace amounts in human blood. If enough helium is inhaled that oxygen needed for normal respiration is replaced, asphyxia is possible. The safety issues for cryogenic helium are similar to those of liquid nitrogen(SONY Vaio VGN-NS12M Battery); its extremely low temperatures can result in cold burns and the liquid-to-gas expansion ratio can cause explosions if no pressure-relief devices are installed.
Containers of helium gas at 5 to 10 K should be handled as if they contain liquid helium due to the rapid and significant thermal expansion that occurs when helium gas at less than 10 K is warmed to room temperature(SONY Vaio VGN-NS11Z Battery).
Effect of helium on a human voice
The effect of helium on a human voice
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The speed of sound in helium is nearly 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, when helium is inhaled there is a corresponding increase in the pitches of the resonant frequencies of the vocal tract. This causes a reedy, duck-like vocal quality(SONY Vaio VGN-NS11M Battery). (The opposite effect, lowering frequencies, can be obtained by inhaling a dense gas such as sulfur hexafluoride or xenon.)
Inhaling helium can be dangerous if done to excess, since helium is a simple asphyxiant and so displaces oxygen needed for normal respiration. Breathing pure helium continuously causes death by asphyxiation within minutes. Inhaling helium directly from pressurized cylinders is extremely dangerous(SONY Vaio VGN-NS11L Battery), as the high flow rate can result in barotrauma, fatally rupturing lung tissue. However, death caused by helium is rare, with only two fatalities reported between 2000 and 2004 in the United States. However, there were two cases in 2010, one in the USA in January and another in Northern Ireland in November(SONY Vaio VGN-NS11J Battery).
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.
Nitrogen ( /ˈnaɪtrɵdʒən/ ny-trə-jən) is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless(SONY Vaio VGN-NS11E Battery), odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.09% by volume of Earth's atmosphere. The element nitrogen was discovered as a separable component of air, by Scottish physician Daniel Rutherford, in 1772.
Nitrogen is a common element in the universe, estimated at about seventh in total abundance in our galaxy and the solar system(SONY Vaio VGN-NS10L Battery). Its occurrence there is thought to be entirely due to synthesis by fusion from carbon and hydrogen in supernovas. Due to the volatility of elemental nitrogen and its common compounds with hydrogen and oxygen, nitrogen is far less common on the rocky planets of the inner solar system, and it is a relatively rare element on Earth as a whole. However, as is the case on Earth, nitrogen and its compounds occur commonly as gases in the atmospheres of planets and moons that have atmospheres(SONY Vaio VGN-NS10J Battery).
Many industrially important compounds, such as ammonia, nitric acid, organic nitrates (propellants and explosives), and cyanides, contain nitrogen. The extremely strong bond in elemental nitrogen dominates nitrogen chemistry, causing difficulty for both organisms and industry in breaking the bond to convert the N2 into useful compounds(SONY Vaio VGN-NS10E Battery), but at the same time causing release of large amounts of often useful energy when the compounds burn, explode, or decay back into nitrogen gas.
Nitrogen occurs in all living organisms, primarily in amino acids and thus proteins and in the nucleic acids (DNA and RNA). The human body contains about 3% by weight of nitrogen, the fourth most abundant element after oxygen, carbon(SONY Vaio VGN-NS38M/W Battery), and hydrogen. Nitrogen resides in the chemical structure of almost all neurotransmitters, and is a defining component of alkaloids, biological molecules produced as secondary metabolites by many organisms. The nitrogen cycle describes movement of the element from the air into the biosphere and organic compounds, then back into the atmosphere. Synthetically produced nitrates are key ingredients of industrial fertilizers(SONY Vaio VGN-NS38M/P Battery), and key pollutants in causing the eutrophication of water systems.
History and etymology
Nitrogen is formally considered to have been discovered by Scottish physician Daniel Rutherford in 1772, who called it noxious air or fixed air. The fact that there was an element of air that does not support combustion was clear to Rutherford. Nitrogen was also studied at about the same time by Carl Wilhelm Scheele, Henry Cavendish, and Joseph Priestley(SONY Vaio VGN-NS31Z/W Battery), who referred to it as burnt air or phlogisticated air. Nitrogen gas was inert enough that Antoine Lavoisier referred to it as "mephitic air" or azote, from the Greek word ἄζωτος (azotos) meaning "lifeless". In it, animals died and flames were extinguished. Lavoisier's name for nitrogen is used in many languages (French, Polish, Russian, etc.) and still remains in English in the common names of many compounds, such as hydrazine and compounds of the azide ion(y-vaio-vgn-ns31z-s-battery.html">SONY Vaio VGN-NS31Z/S Battery).
The English word nitrogen (1794) entered the language  from the French nitrogène, coined in 1790 by French chemist Jean-Antoine Chaptal (1756–1832), from "nitre" + Fr. gène "producing" (from Gk. -γενής means "forming" or "giving birth to."). The gas had been found in nitric acid. Chaptal's meaning was that nitrogen gas is the essential part of nitric acid(SONY Vaio VGN-NS31Z/P Battery), in turn formed from saltpetre (potassium nitrate), then known as nitre. This word in the more ancient world originally described sodium salts that did not contain nitrate, and is a cognate of natron.
Nitrogen compounds were well known during the Middle Ages. Alchemists knew nitric acid as aqua fortis (strong water). The mixture of nitric and hydrochloric acids was known as aqua regia (royal water), celebrated for its ability to dissolve gold (the king of metals). The earliest military, industrial, and agricultural applications of nitrogen compounds used saltpetre (SONY Vaio VGN-NS31S/S Battery) (sodium nitrate or potassium nitrate), most notably in gunpowder, and later as fertilizer. In 1910, Lord Rayleigh discovered that an electrical discharge in nitrogen gas produced "active nitrogen", an allotrope considered to be monatomic. The "whirling cloud of brilliant yellow light" produced by his apparatus reacted with quicksilver to produce explosive mercury nitride(SONY Vaio VGN-NS31M/W Battery).
Nitrogen gas is an industrial gas produced by the fractional distillation of liquid air, or by mechanical means using gaseous air (i.e., pressurized reverse osmosis membrane or Pressure swing adsorption). Commercial nitrogen is often a byproduct of air-processing for industrial concentration of oxygen for steelmaking and other purposes. When supplied compressed in cylinders it is often called OFN (oxygen-free nitrogen) (SONY Vaio VGN-NS31M/P Battery).
In a chemical laboratory it is prepared by treating an aqueous solution of ammonium chloride with sodium nitrite.
NH4Cl(aq) + NaNO2(aq) → N2(g) + NaCl(aq) + 2 H2O (l)
Small amounts of impurities NO and HNO3 are also formed in this reaction. The impurities can be removed by passing the gas through aqueous sulfuric acid containing potassium dichromate. Very pure nitrogen can be prepared by the thermal decomposition of barium or sodium azide(SONY Vaio VGN-NS21Z/S Battery).
2 NaN3 → 2 Na + 3 N2
Nitrogen is a nonmetal, with an electronegativity of 3.04. It has five electrons in its outer shell and is, therefore, trivalent in most compounds. The triple bond in molecular nitrogen (N2) is one of the strongest. The resulting difficulty of converting N2 into other compounds, and the ease (and associated high energy release) of converting nitrogen compounds into elemental N2, have dominated the role of nitrogen in both nature and human economic activities(SONY Vaio VGN-NS21S/W Battery).
At atmospheric pressure molecular nitrogen condenses (liquefies) at 77 K (−195.79 °C) and freezes at 63 K (−210.01 °C) into the beta hexagonal close-packed crystal allotropic form. Below 35.4 K (−237.6 °C) nitrogen assumes the cubic crystal allotropic form (called the alpha-phase). Liquid nitrogen, a fluid resembling water in appearance, but with 80.8% of the density (the density of liquid nitrogen at its boiling point is 0.808 g/mL), is a common cryogen(SONY Vaio VGN-NS21S/S Battery).
Unstable allotropes of nitrogen consisting of more than two nitrogen atoms have been produced in the laboratory, like N3 and N4. Under extremely high pressures (1.1 million atm) and high temperatures (2000 K), as produced using a diamond anvil cell, nitrogen polymerizes into the single-bonded cubic gauche crystal structure. This structure is similar to that of diamond, and both have extremely strong covalent bonds. N4 is nicknamed "nitrogen diamond." (SONY Vaio VGN-NS21M/W Battery) 
Other (as yet unsynthesized) allotropes include hexazine (N6, a benzene analog) and octaazacubane (N8, a cubane analog). The former is predicted to be highly unstable, while the latter is predicted to be kinetically stable, for reasons of orbital symmetry.
See also: Isotopes of nitrogen
There are two stable isotopes of nitrogen: 14N and 15N. By far the most common is 14N (99.634%), which is produced in the CNO cycle in stars(SONY Vaio VGN-NS21M/P Battery). Of the ten isotopes produced synthetically, 13N has a half-life of ten minutes and the remaining isotopes have half-lives on the order of seconds or less. Biologically mediated reactions (e.g., assimilation, nitrification, and denitrification) strongly control nitrogen dynamics in the soil. These reactions typically result in 15N enrichment of the substrate and depletion of the product(SONY Vaio VGN-NS12S/S Battery).
A small part (0.73%) of the molecular nitrogen in Earth's atmosphere is the isotopologue 14N15N, and almost all the rest is 14N2.
Radioisotope 16N is the dominant radionuclide in the coolant of pressurized water reactors or boiling water reactors during normal operation. It is produced from 16O (in water) via (n,p) reaction. It has a short half-life of about 7.1 s, but during its decay back to 16O produces high-energy gamma radiation (5 to 7 MeV) (SONY Vaio VGN-NS12M/W Battery).
Because of this, the access to the primary coolant piping in a pressurized water reactor must be restricted during reactor power operation. 16N is one of the main means used to immediately detect even small leaks from the primary coolant to the secondary steam cycle.
In similar fashion, access to any of the steam cycle components in a boiling water reactor nuclear power plant must be restricted during operation(SONY Vaio VGN-NS12M/S Battery). Condensate from the condenser is typically retained for 10 minutes to allow for decay of the 16N. This eliminates the need to shield and restrict access to any of the feed water piping or pumps.
Nitrogen discharge (spectrum) tube
Molecular nitrogen (14N2) is largely transparent to infrared and visible radiation because it is a homonuclear molecule and, thus, has no dipole moment to couple to electromagnetic radiation at these wavelengths. Significant absorption occurs at extreme ultraviolet wavelengths(SONY Vaio VGN-NS11Z/S Battery), beginning around 100 nanometers. This is associated with electronic transitions in the molecule to states in which charge is not distributed evenly between nitrogen atoms. Nitrogen absorption leads to significant absorption of ultraviolet radiation in the Earth's upper atmosphere and the atmospheres of other planetary bodies. For similar reasons, pure molecular nitrogen lasers typically emit light in the ultraviolet range(SONY Vaio VGN-NS11ZR/S Battery).
Nitrogen also makes a contribution to visible air glow from the Earth's upper atmosphere, through electron impact excitation followed by emission. This visible blue air glow (seen in the polar aurora and in the re-entry glow of returning spacecraft) typically results not from molecular nitrogen but rather from free nitrogen atoms combining with oxygen to form nitric oxide (NO) (SONY Vaio VGN-NS11S/S Battery).
Nitrogen gas also exhibits scintillation.
Structure of dinitrogen, N2
Structure of [Ru(NH3)5(N2)]2+
In general, nitrogen is unreactive at standard temperature and pressure. N2 reacts spontaneously with few reagents, being resilient to acids and bases as well as oxidants and most reductants. When nitrogen reacts spontaneously with a reagent, the net transformation is often called nitrogen fixation(SONY Vaio VGN-NS11SR/S Battery).
Nitrogen reacts with elemental lithium. Lithium burns in an atmosphere of N2 to give lithium nitride:
6 Li + N2 → 2 Li3N
Magnesium also burns in nitrogen, forming magnesium nitride.
3 Mg + N2 → Mg3N2
N2 forms a variety of adducts with transition metals. The first example of a dinitrogen complex is [Ru(NH3)5(N2)]2+ (see figure at right). However, it is interesting to note that the N2 ligand was obtained by the decomposition of hydrazine, and not coordination of free dinitrogen. Such compounds are now numerous, other examples include IrCl(N2)(PPh3)2(SONY Vaio VGN-NS11M/S Battery), W(N2)2(Ph2PCH2CH2PPh2)2, and [(η5-C5Me4H)2Zr]2(μ2, η2,η2-N2). These complexes illustrate how N2 might bind to the metal(s) in nitrogenase and the catalyst for the Haber process. A catalytic process to reduce N2 to ammonia with the use of a molybdenum complex in the presence of a proton source was published in 2005(SONY Vaio VGN-NS11MR/S Battery).
The starting point for industrial production of nitrogen compounds is the Haber process, in which nitrogen is fixed by reacting N2 and H2 over an iron(II, III) oxide (Fe3O4) catalyst at about 500 °C and 200 atmospheres pressure. Biological nitrogen fixation in free-living cyanobacteria and in the root nodules of plants also produces ammonia from molecular nitrogen(SONY Vaio VGN-NS11L/S Battery). The reaction, which is the source of the bulk of nitrogen in the biosphere, is catalyzed by the nitrogenase enzyme complex that contains Fe and Mo atoms, using energy derived from hydrolysis of adenosine triphosphate (ATP) into adenosine diphosphate and inorganic phosphate (−20.5 kJ/mol).
See also Category: Nitrate minerals
See also Category: Ammonium minerals
Nitrogen is the largest constituent of the Earth's atmosphere (78.082% by volume of dry air, 75.3% by weight in dry air). However, this high concentration does not reflect nitrogen's overall low abundance in the makeup of the Earth, from which most of the element escaped by solar evaporation, early in the planet's formation(SONY Vaio VGN-NS11J/S Battery).
Nitrogen is a common element in the universe, and is estimated to be approximately seventh most abundant chemical element by mass in the universe, our galaxy and the solar system. Its occurrence there is thought to be entirely due to synthesis by fusion from carbon and hydrogen in supernovas. In these places it was originally created by fusion processes from carbon and hydrogen in supernovas(SONY Vaio VGN-NS11E/S Battery).Molecular nitrogen and nitrogen compounds have been detected in interstellar space by astronomers using the Far Ultraviolet Spectroscopic Explorer.
Due to the volatility of elemental nitrogen and also its common compounds with hydrogen and oxygen, nitrogen and its compounds were driven out of the planetesimals in the early solar system by the heat of the Sun, and in the form of gases, were lost to the rocky planets of the inner solar system(SONY Vaio VGN-NS115N/S Battery). Nitrogen is therefore a relatively rare element on these inner planets, including Earth, as a whole. In this, nitrogen resembles neon, which has a similar abundance in the universe, but is also rare in the inner solar system. Nitrogen is estimated at 30th of the elements in crustal abundance. There exist some relatively uncommon nitrogen minerals, such as saltpetre (potassium nitrate), Chile saltpetre (sodium nitrate) and sal ammoniac (ammonium chloride) (SONY Vaio VGN-NS110E/W Battery). Even these are known mainly as concentrated from evaporative ocean beds, on account of their ready solubility of most naturally-occurring nitrogen compounds in water. A similar pattern occurs with the water solubility of the uncommon light element boron.
However, nitrogen and its compounds occur far more commonly as gases in the atmospheres of planets and moons that are large enough to have atmospheres(SONY Vaio VGN-NS110E/S Battery). For example, molecular nitrogen is a major constituent of not only Earth's atmosphere, but also the Saturnian moon Titan's thick atmosphere. Also, due to retension by gravity at colder temperatures, nitrogen and its compounds occur in appreciable to trace amounts in planetary atmospheres of the gas giant planets.
Nitrogen is present in all living organisms, in proteins, nucleic acids, and other molecules. It typically makes up around 4% of the dry weight of plant matter(SONY Vaio VGN-NS110E/L Battery), and around 3% of the weight of the human body. It is a large component of animal waste (for example, guano), usually in the form of urea, uric acid, ammonium compounds, and derivatives of these nitrogenous products, which are essential nutrients for all plants that cannot fix atmospheric nitrogen.
See also Category: Nitrogen compounds
The main neutral hydride of nitrogen is ammonia (NH3), although hydrazine (N2H4) is also commonly used. Ammonia is more basic than water by 6 orders of magnitude. In solution ammonia forms the ammonium ion (NH+(SONY Vaio VGN-NS10L/S Battery)
4). Liquid ammonia (boiling point 240 K) is amphiprotic (displaying either Brønsted-Lowry acidic or basic character) and forms ammonium and the less common amide ions (NH−
2); both amides and nitride (N3−) salts are known, but decompose in water. Singly, doubly, triply and quadruply substituted alkyl compounds of ammonia are called amines (four substitutions, to form commercially and biologically important quaternary amines, results in a positively charged nitrogen, and thus a water-soluble, or at least amphiphilic, compound) (SONY Vaio VGN-NS10J/S Battery). Larger chains, rings and structures of nitrogen hydrides are also known, but are generally unstable.
Other classes of nitrogen anions (negatively charged ions) are the poisonous azides (N−
3), which are linear and isoelectronic to carbon dioxide, but which bind to important iron-containing enzymes in the body in a manner more resembling cyanide. Another molecule of the same structure is the colorless and relatively inert anesthetic gas Nitrous oxide (dinitrogen monoxide, N2O), also known as laughing gas(SONY Vaio VGN-NS10E/S Battery). This is one of a variety of nitrogen oxides that form a family often abbreviated as NOx. Nitric oxide (nitrogen monoxide, NO), is a natural free radical used in signal transduction in both plants and animals, for example, in vasodilation by causing the smooth muscle of blood vessels to relax. The reddish and poisonous nitrogen dioxide NO2 contains an unpaired electron and is an important component of smog(Sony VAIO VGN-SR59VG battery). Nitrogen molecules containing unpaired electrons show a tendency to dimerize (thus pairing the electrons), and are, in general, highly reactive. The corresponding acids are nitrous HNO2 and nitric acid HNO3, with the corresponding salts called nitrites and nitrates.
The higher oxides dinitrogen trioxide N2O3, dinitrogen tetroxide N2O4 and dinitrogen pentoxide N2O5, are unstable and explosive, a consequence of the chemical stability of N2. Nearly every hypergolic rocket engine uses N2O4 as the oxidizer(Sony VAIO VGN-SR59VG/H battery); their fuels, various forms of hydrazine, are also nitrogen compounds. These engines are extensively used on spacecraft such as the space shuttle and those of the Apollo Program because their propellants are liquids at room temperature and ignition occurs on contact without an ignition system, allowing many precisely controlled burns. Some launch vehicles such as the Titan II and Ariane 1 through 4 also use hypergolic fuels(Sony VAIO VGN-SR55TF/B battery), although the trend is away from such engines for cost and safety reasons. N2O4 is an intermediate in the manufacture of nitric acid HNO3, one of the few acids stronger than hydronium and a fairly strong oxidizing agent.
Nitrogen is notable for the range of explosively unstable compounds that it can produce. Nitrogen triiodide NI3 is an extremely sensitive contact explosive. Nitrocellulose, produced by nitration of cellulose with nitric acid, is also known as guncotton(Sony VAIO VGN-SR51MF battery). Nitroglycerin, made by nitration of glycerin, is the dangerously unstable explosive ingredient of dynamite. The comparatively stable, but less powerful explosive trinitrotoluene (TNT) is the standard explosive against which the power of nuclear explosions are measured.
Nitrogen can also be found in organic compounds. Common nitrogen functional groups include: amines, amides, nitro groups, imines, and enamines(Sony VAIO VGN-SR51MF/W battery). The amount of nitrogen in a chemical substance can be determined by the Kjeldahl method.
Nitrogen gas has a variety of applications, including serving as an inert replacement for air where oxidation is undesirable;
As a modified atmosphere, pure or mixed with carbon dioxide, to preserve the freshness of packaged or bulk foods (by delaying rancidity and other forms of oxidative damage)
In ordinary incandescent light bulbs as an inexpensive alternative to argon(Sony VAIO VGN-SR51B battery).
The production of electronic parts such as transistors, diodes, and integrated circuits
Dried and pressurized, as a dielectric gas for high-voltage equipment
The manufacturing of stainless steel
Used in military aircraft fuel systems to reduce fire hazard, (see inerting system)
On top of liquid explosives as a safety measure
Filling automotive and aircraft tires due to its inertness and lack of moisture or oxidative qualities, as opposed to air(Sony VAIO VGN-SR51B/S battery). The difference in N2 content between air and pure N2 is 20%
Used as a propellant for draft wine, and as an alternative to or together with carbon dioxide for other beverages.
Nitrogen is commonly used during sample preparation procedures for chemical analysis. It is used to concentrate and reduce the volume of liquid samples. Directing a pressurized stream of nitrogen gas perpendicular to the surface of the liquid allows the solvent to evaporate while leaving the solute(s) and un-evaporated solvent behind(Sony VAIO VGN-SR51B/P battery).
Nitrogen tanks are also replacing carbon dioxide as the main power source for paintball guns. Nitrogen must be kept at higher pressure than CO2, making N2 tanks heavier and more expensive.
Nitrogen can be used instead of carbon dioxide to pressurize kegs of some beers, in particular, stouts and British ales, due to the smaller bubbles it produces, which makes the dispensed beer smoother and headier(Sony VAIO VGN-SR49VN/H battery). A pressure sensitive nitrogen capsule known commonly as a "widget" allows nitrogen charged beers to be packaged in cans and bottles.
A mixture of nitrogen and carbon dioxide can be used for this purpose as well, to maintain the saturation of beer with carbon dioxide.
Air balloon submerged in liquid nitrogen
Main article: Liquid nitrogen
Liquid nitrogen is a cryogenic liquid. At atmospheric pressure, it boils at −195.8 °C. When insulated in proper containers such as Dewar flasks, it can be transported without much evaporative loss(Sony VAIO VGN-SR49D battery).
Like dry ice, the main use of liquid nitrogen is as a refrigerant. Among other things, it is used in the cryopreservation of blood, reproductive cells (sperm and egg), and other biological samples and materials. It is used in the clinical setting in cryotherapy to remove cysts and warts on the skin. It is used in cold traps for certain laboratory equipment and to cool infrared detectors or X-ray detectors(Sony VAIO VGN-SR49D/Q battery). It has also been used to cool central processing units and other devices in computers that are overclocked, and that produce more heat than during normal operation.
Applications of nitrogen compounds
Molecular nitrogen (N2) in the atmosphere is relatively non-reactive due to its strong bond, and N2 plays an inert role in the human body, being neither produced nor destroyed. In nature, nitrogen is converted into biologically (and industrially) useful compounds by lightning, and by some living organisms(Sony VAIO VGN-SR49D/J battery), notably certain bacteria (i.e., nitrogen fixing bacteria—see Biological role below). Molecular nitrogen is released into the atmosphere in the process of decay, in dead plant and animal tissues.
The ability to combine, or fix, molecular nitrogen is a key feature of modern industrial chemistry, where nitrogen and natural gas are converted into ammonia via the Haber process. Ammonia, in turn, can be used directly (primarily as a fertilizer, and in the synthesis of nitrated fertilizers) (Sony VAIO VGN-SR48J battery), or as a precursor of many other important materials including explosives, largely via the production of nitric acid by the Ostwald process.
The organic and inorganic salts of nitric acid have been important historically as convenient stores of chemical energy. They include important compounds such as potassium nitrate (or saltpeter used in gunpowder) and ammonium nitrate(Sony VAIO VGN-SR48J/J battery), an important fertilizer and explosive (see ANFO). Various other nitrated organic compounds, such as nitroglycerin, trinitrotoluene, and nitrocellulose, are used as explosives and propellants for modern firearms. Nitric acid is used as an oxidizing agent in liquid fueled rockets. Hydrazine and hydrazine derivatives find use as rocket fuels and monopropellants. In most of these compounds(Sony VAIO VGN-SR48J/B battery), the basic instability and tendency to burn or explode is derived from the fact that nitrogen is present as an oxide, and not as the far more stable nitrogen molecule (N2), which is a product of the compounds' thermal decomposition. When nitrates burn or explode, the formation of the powerful triple bond in the N2 produces most of the energy of the reaction(Sony VAIO VGN-SR46TD/B battery).
Nitrogen is a constituent of molecules in every major drug class in pharmacology and medicine. Nitrous oxide (N2O) was discovered early in the 19th century to be a partial anesthetic, though it was not used as a surgical anesthetic until later. Called "laughing gas", it was found capable of inducing a state of social disinhibition resembling drunkenness. Other notable nitrogen-containing drugs are drugs derived from plant alkaloids, such as morphine (Sony VAIO VGN-SR46MD/B battery) (there exist many alkaloids known to have pharmacological effects; in some cases, they appear as natural chemical defenses of plants against predation). Drugs that contain nitrogen include all major classes of antibiotics and organic nitrate drugs like nitroglycerin and nitroprusside that regulate blood pressure and heart action by mimicking the action of nitric oxide(Sony VAIO VGN-SR46GD battery).
See also: Nitrogen cycle and Human impacts on the nitrogen cycle
Nitrogen is an essential building block of amino and nucleic acids, essential to life on Earth.
Elemental nitrogen in the atmosphere cannot be used directly by either plants or animals, and must be converted to a reduced (or 'fixed') state to be useful for higher plants and animals. Precipitation often contains substantial quantities of ammonium and nitrate, thought to result from nitrogen fixation by lightning and other atmospheric electric phenomena(Sony VAIO VGN-SR45T/W battery). This was first proposed by Liebig in 1827 and later confirmed. However, because ammonium is preferentially retained by the forest canopy relative to atmospheric nitrate, most fixed nitrogen reaches the soil surface under trees as nitrate. Soil nitrate is preferentially assimilated by tree roots relative to soil ammonium(Sony VAIO VGN-SR45T/P battery).
Specific bacteria (e.g., Rhizobium trifolium) possess nitrogenase enzymes that can fix atmospheric nitrogen (see nitrogen fixation) into a form (ammonium ion) that is chemically useful to higher organisms. This process requires a large amount of energy and anoxic conditions. Such bacteria may live freely in soil (e.g., Azotobacter) but normally exist in a symbiotic relationship in the root nodules of leguminous plants (Sony VAIO VGN-SR45T/B battery) (e.g. clover, Trifolium, or soybean plant, Glycine max). Nitrogen-fixing bacteria are also symbiotic with a number of unrelated plant species such as alders (Alnus) spp., lichens, Casuarina, Myrica, liverworts, and Gunnera.
As part of the symbiotic relationship, the plant converts the 'fixed' ammonium ion to nitrogen oxides and amino acids to form proteins and other molecules, (e.g., alkaloids). In return for the 'fixed' nitrogen, the plant secretes sugars to the symbiotic bacteria(Sony VAIO VGN-SR45H battery). Legumes maintain an anaerobic (oxygen free) environment for their nitrogen-fixing bacteria.
Plants are able to assimilate nitrogen directly in the form of nitrates that may be present in soil from natural mineral deposits, artificial fertilizers, animal waste, or organic decay (as the product of bacteria, but not bacteria specifically associated with the plant). Nitrates absorbed in this fashion are converted to nitrites by the enzyme nitrate reductase, and then converted to ammonia by another enzyme called nitrite reductase(Sony VAIO VGN-SR45H/P battery).
Nitrogen compounds are basic building blocks in animal biology as well. Animals use nitrogen-containing amino acids from plant sources as starting materials for all nitrogen-compound animal biochemistry, including the manufacture of proteins and nucleic acids. Plant-feeding insects are dependent on nitrogen in their diet, such that varying the amount of nitrogen fertilizer applied to a plant can affect the reproduction rate of insects feeding on fertilized plants(Sony VAIO VGN-SR45H/N battery).
Soluble nitrate is an important limiting factor in the growth of certain bacteria in ocean waters. In many places in the world, artificial fertilizers applied to crop-lands to increase yields result in run-off delivery of soluble nitrogen to oceans at river mouths. This process can result in eutrophication of the water, as nitrogen-driven bacterial growth depletes water oxygen to the point that all higher organisms die(Sony VAIO VGN-SR45H/B battery). Well-known "dead zone" areas in the U.S. Gulf Coast and the Black Sea are due to this important polluting process.
Many saltwater fish manufacture large amounts of trimethylamine oxide to protect them from the high osmotic effects of their environment; conversion of this compound to dimethylamine is responsible for the early odor in unfresh saltwater fish. In animals, free radical nitric oxide (NO) (derived from an amino acid), serves as an important regulatory molecule for circulation(Sony VAIO VGN-SR41M/W battery).
Animal metabolism of NO results in production of nitrite. Animal metabolism of nitrogen in proteins, in general, results in excretion of urea, while animal metabolism of nucleic acids results in excretion of urea and uric acid. The characteristic odor of animal flesh decay is caused by the creation of long-chain, nitrogen-containing amines, such as putrescine and cadaverine(Sony VAIO VGN-SR41M/S battery), which are breakdown products of the amino acids ornithine and lysine, respectively, in decaying proteins.
Decay of organisms and their waste products may produce small amounts of nitrate, but most decay eventually returns nitrogen content to the atmosphere, as molecular nitrogen. The circulation of nitrogen from atmosphere, to organic compounds, then back to the atmosphere, is referred to as the nitrogen cycle(Sony VAIO VGN-SR41M/P battery).
Rapid release of nitrogen gas into an enclosed space can displace oxygen, and therefore represents an asphyxiation hazard. This may happen with few warning symptoms, since the human carotid body is a relatively slow and a poor low-oxygen (hypoxia) sensing system. An example occurred shortly before the launch of the first Space Shuttle mission in 1981(Sony VAIO VGN-SR3CW/B battery), when two technicians lost consciousness (and one of them died) after they walked into a space located in the Shuttle's Mobile Launcher Platform that was pressurized with pure nitrogen as a precaution against fire. The technicians would have been able to exit the room if they had experienced early symptoms from nitrogen-breathing(Sony VAIO VGN-SR39XN/S battery).
When inhaled at high partial pressures (more than about 4 bar, encountered at depths below about 30 m in scuba diving), nitrogen begins to act as an anesthetic agent. It can cause nitrogen narcosis, a temporary semi-anesthetized state of mental impairment similar to that caused by nitrous oxide(Sony VAIO VGN-SR39VN/S battery).
Nitrogen also dissolves in the bloodstream and body fats. Rapid decompression (in particular, in the case of divers ascending too quickly, or astronauts decompressing too quickly from cabin pressure to spacesuit pressure) can lead to a potentially fatal condition called decompression sickness (formerly known as caisson sickness or the bends), when nitrogen bubbles form in the bloodstream, nerves, joints, and other sensitive or vital areas. (Sony VAIO VGN-SR39D battery)Other "inert" gases (those gases other than carbon dioxide and oxygen) cause the same effects from bubbles composed of them, so replacement of nitrogen in breathing gases may prevent nitrogen narcosis, but does not prevent decompression sickness.
Direct skin contact with liquid nitrogen will cause severe frostbite (cryogenic "burns"). This may happen almost instantly on contact(Sony VAIO VGN-SR39D/Q battery), or after a second or more, depending on the form of liquid nitrogen. Bulk liquid nitrogen causes less rapid freezing than a spray of nitrogen mist (such as is used to freeze certain skin growths in the practice of dermatology). The extra surface area provided by nitrogen-soaked materials is also important, with soaked clothing or cotton causing far more rapid damage than a spill of direct liquid to skin(Sony VAIO VGN-SR39D/J battery). Full "contact" between naked skin and large collected-droplets or pools of liquid nitrogen may be prevented for a second or two, by a layer of insulating gas from the Leidenfrost effect. This may give the skin a second of protection from nitrogen bulk liquid. However, liquid nitrogen applied to skin in mists, and on fabrics, bypasses this effect, and causes local frostbite immediately(Sony VAIO VGN-SR38/Q battery).
Oxygen sensors are sometimes used as a safety precaution when working with liquid nitrogen to alert workers of gas spills into a confined space.