ArXiv is a repository of electronic preprints approved for posting after moderation, but not full peer review. It consists of scientific papers in the fields of mathematics, astronomy, electrical engineering, computer science, quantitative biology, mathematical finance and economics, which can be accessed online. In many fields of mathematics and physics all scientific papers are self-archived on the arXiv repository. Begun on August 14, 1991, arXiv.org passed the half-million-article milestone on October 3, 2008, had hit a million by the end of 2014. By October 2016 the submission rate had grown to more than 10,000 per month. ArXiv was made possible by the compact TeX file format, which allowed scientific papers to be transmitted over the Internet and rendered client-side. Around 1990, Joanne Cohn began emailing physics preprints to colleagues as TeX files, but the number of papers being sent soon filled mailboxes to capacity. Paul Ginsparg recognized the need for central storage, in August 1991 he created a central repository mailbox stored at the Los Alamos National Laboratory which could be accessed from any computer.
Additional modes of access were soon added: FTP in 1991, Gopher in 1992, the World Wide Web in 1993. The term e-print was adopted to describe the articles, it began as a physics archive, called the LANL preprint archive, but soon expanded to include astronomy, computer science, quantitative biology and, most statistics. Its original domain name was xxx.lanl.gov. Due to LANL's lack of interest in the expanding technology, in 2001 Ginsparg changed institutions to Cornell University and changed the name of the repository to arXiv.org. It is now hosted principally with eight mirrors around the world, its existence was one of the precipitating factors that led to the current movement in scientific publishing known as open access. Mathematicians and scientists upload their papers to arXiv.org for worldwide access and sometimes for reviews before they are published in peer-reviewed journals. Ginsparg was awarded a MacArthur Fellowship in 2002 for his establishment of arXiv; the annual budget for arXiv is $826,000 for 2013 to 2017, funded jointly by Cornell University Library, the Simons Foundation and annual fee income from member institutions.
This model arose in 2010, when Cornell sought to broaden the financial funding of the project by asking institutions to make annual voluntary contributions based on the amount of download usage by each institution. Each member institution pledges a five-year funding commitment to support arXiv. Based on institutional usage ranking, the annual fees are set in four tiers from $1,000 to $4,400. Cornell's goal is to raise at least $504,000 per year through membership fees generated by 220 institutions. In September 2011, Cornell University Library took overall administrative and financial responsibility for arXiv's operation and development. Ginsparg was quoted in the Chronicle of Higher Education as saying it "was supposed to be a three-hour tour, not a life sentence". However, Ginsparg remains on the arXiv Scientific Advisory Board and on the arXiv Physics Advisory Committee. Although arXiv is not peer reviewed, a collection of moderators for each area review the submissions; the lists of moderators for many sections of arXiv are publicly available, but moderators for most of the physics sections remain unlisted.
Additionally, an "endorsement" system was introduced in 2004 as part of an effort to ensure content is relevant and of interest to current research in the specified disciplines. Under the system, for categories that use it, an author must be endorsed by an established arXiv author before being allowed to submit papers to those categories. Endorsers are not asked to review the paper for errors, but to check whether the paper is appropriate for the intended subject area. New authors from recognized academic institutions receive automatic endorsement, which in practice means that they do not need to deal with the endorsement system at all. However, the endorsement system has attracted criticism for restricting scientific inquiry. A majority of the e-prints are submitted to journals for publication, but some work, including some influential papers, remain purely as e-prints and are never published in a peer-reviewed journal. A well-known example of the latter is an outline of a proof of Thurston's geometrization conjecture, including the Poincaré conjecture as a particular case, uploaded by Grigori Perelman in November 2002.
Perelman appears content to forgo the traditional peer-reviewed journal process, stating: "If anybody is interested in my way of solving the problem, it's all there – let them go and read about it". Despite this non-traditional method of publication, other mathematicians recognized this work by offering the Fields Medal and Clay Mathematics Millennium Prizes to Perelman, both of which he refused. Papers can be submitted in any of several formats, including LaTeX, PDF printed from a word processor other than TeX or LaTeX; the submission is rejected by the arXiv software if generating the final PDF file fails, if any image file is too large, or if the total size of the submission is too large. ArXiv now allows one to store and modify an incomplete submission, only finalize the submission when ready; the time stamp on the article is set. The standard access route is through one of several mirrors. Sev
Kitt Peak National Observatory
The Kitt Peak National Observatory is a United States astronomical observatory located on Kitt Peak of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono O'odham Nation, 88 kilometers west-southwest of Tucson, Arizona. With 22 optical and two radio telescopes, it is the largest, most diverse gathering of astronomical instruments in the northern hemisphere; the observatory is administered by the National Optical Astronomy Observatory. Kitt Peak was selected by its first director, Aden B. Meinel, in 1958 as the site for a national observatory under contract with the National Science Foundation and was administered by the Association of Universities for Research in Astronomy; the land was leased from the Tohono O'odham under a perpetual agreement. The second director was Nicholas U. Mayall. In 1982 NOAO was formed to consolidate the management of three optical observatories — Kitt Peak; the observatory sites are under lease from the Tohono O'odham Nation at the amount of a quarter dollar per acre yearly, overwhelmingly approved by the Council in the 1950s.
In 2005, the Tohono O'odham Nation brought suit against the National Science Foundation to stop further construction of gamma ray detectors in the Gardens of the Sacred Tohono O'odham Spirit I'itoi, which are just below the summit. The largest optical instruments at KPNO are the Mayall 4 meter telescope and the WIYN 3.5 meter telescope. The McMath-Pierce Solar Telescope is the largest solar telescope in the world and the largest unobstructed reflector; the ARO 12m Radio Telescope is at the location. Kitt Peak is famous for hosting the first telescope used to search for near-Earth asteroids, calculating the probability of an impact with planet Earth. Kitt Peak hosts an array of programs for the public to take part in, including: Daytime tours, speaking about the history of the observatory as well as touring a major research telescope; the Nightly Observing Program, which allows visitors to arrive in the late afternoon, watch the sunset, use binoculars and telescopes to view the cosmos. Additionally, there is the Overnight Telescope Observing Program.
This program allows for a one-on-one, full night of observing using any of the visitor center's telescopes. Guests may choose to do DSLR imaging, CCD imaging, or take in the sights with their eye to the telescope. Kitt Peak's Southeastern Association for Research and Astronomy Telescope was featured in the WIPB-PBS documentary, "Seeing Stars in Indiana"; the project followed SARA astronomers from Ball State University to the observatory and featured time-lapse images from various points around Kitt Peak. Due to its high elevation, the observatory experiences a much cooler and wetter climate throughout the year than most of the Sonoran desert. List of astronomical observatories List of radio telescopes Richard Green Discover Magazine article about Kitt Peak, May 2005 Kitt Peak docent training book, 2008 Kitt Peak National Observatory – official site Kitt Peak National Observatory Visitor Center – visiting and tour information Kitt Peak Webcam Kitt Peak Clear Sky Chart Forecasts of observing conditions NOAA general forecast for KPNO NOAA detailed forecast for KPNO Observing At Kitt Peak – General Overview for Observers and Staff
Hydrogen is a chemical element with symbol H and atomic number 1. With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table. Hydrogen is the most abundant chemical substance in the Universe, constituting 75% of all baryonic mass. Non-remnant stars are composed of hydrogen in the plasma state; the most common isotope of hydrogen, termed protium, has no neutrons. The universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, tasteless, non-toxic, nonmetallic combustible diatomic gas with the molecular formula H2. Since hydrogen forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays a important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a negative charge when it is known as a hydride, or as a positively charged species denoted by the symbol H+.
The hydrogen cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds are always more complex. As the only neutral atom for which the Schrödinger equation can be solved analytically, study of the energetics and bonding of the hydrogen atom has played a key role in the development of quantum mechanics. Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, that it produces water when burned, the property for which it was named: in Greek, hydrogen means "water-former". Industrial production is from steam reforming natural gas, less from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production for the fertilizer market. Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks.
Hydrogen gas is flammable and will burn in air at a wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol: 2 H2 + O2 → 2 H2O + 572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74% and with chlorine at 5–95%; the explosive reactions may be triggered by heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C. Pure hydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine, compared to the visible plume of a Space Shuttle Solid Rocket Booster, which uses an ammonium perchlorate composite; the detection of a burning hydrogen leak may require a flame detector. Hydrogen flames in other conditions are blue; the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a rich mixture of hydrogen to oxygen combined with carbon compounds from the airship skin.
H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are potentially dangerous acids; the ground state energy level of the electron in a hydrogen atom is −13.6 eV, equivalent to an ultraviolet photon of 91 nm wavelength. The energy levels of hydrogen can be calculated accurately using the Bohr model of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the Sun. However, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity; because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, therefore only certain allowed energies. A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the Schrödinger equation, Dirac equation or the Feynman path integral formulation to calculate the probability density of the electron around the proton.
The most complicated treatments allow for the small effects of special relativity and vacuum polarization. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum at all—illustrating how the "planetary orbit" differs from electron motion. There exist two different spin isomers of hydrogen diatomic molecules that differ by the relative spin of their nuclei. In the orthohydrogen form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1. At standard temperature and pressure, hydrogen gas contains about 25% of the para form and 75% of the ortho form known as the "normal form"; the equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but because the ortho form is an excited state and has a higher energy
A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed by the reception of a proton by a base—in other words, it is a base with a hydrogen ion added to it. On the other hand, a conjugate base is what is left over after an acid has donated a proton during a chemical reaction. Hence, a conjugate base is a species formed by the removal of a proton from an acid; because some acids are capable of releasing multiple protons, the conjugate base of an acid may itself be acidic. In summary, this can be represented as the following chemical reaction: Acid + Base ⇌ Conjugate Base + Conjugate Acid Johannes Nicolaus Brønsted and Martin Lowry introduced the Brønsted–Lowry theory, which proposed that any compound that can transfer a proton to any other compound is an acid, the compound that accepts the proton is a base. A proton is a nuclear particle with a unit positive electrical charge. A cation can be a conjugate acid, an anion can be a conjugate base, depending on which substance is involved and which acid–base theory is the viewpoint.
The simplest anion which can be a conjugate base is the solvated electron whose conjugate acid is the atomic hydrogen. In an acid-base reaction, an acid plus a base reacts to form a conjugate base plus a conjugate acid: Conjugates are formed when an acid loses a hydrogen proton or a base gains a hydrogen proton. Refer to the following figure: We say that the water molecule is the conjugate acid of the hydroxide ion after the latter received the hydrogen proton donated by ammonium. On the other hand, ammonia is the conjugate base for the acid ammonium after ammonium has donated a hydrogen ion towards the production of the water molecule. We can refer to OH- as a conjugate base of H2O, since the water molecule donates a proton towards the production of NH+4 in the reverse reaction, the predominating process in nature due to the strength of the base NH3 over the hydroxide ion. Based on this information, it is clear that the terms "Acid", "Base", "conjugate acid", "conjugate base" are not fixed for a certain chemical species.
The strength of a conjugate acid is directly proportional to its dissociation constant. If a conjugate acid is strong, its dissociation will have a higher equilibrium constant and the products of the reaction will be favored; the strength of a conjugate base can be seen as the tendency of the species to "pull" hydrogen protons towards itself. If a conjugate base is classified as strong, it will "hold on" to the hydrogen proton when in solution and its acid will not dissociate. On the other hand, if a species is classified as a strong acid, its conjugate base will be weak in nature. An example of this case would be the dissociation of Hydrochloric acid HCl in water. Since HCl is a strong acid, its conjugate base will be a weak conjugate base. Therefore, in this system, most H+ will be in the form of a Hydronium ion H3O+ instead of attached to a Cl anion and the conjugate base will be weaker than a water molecule. If an acid is weak, its conjugate base will be strong; when considering the fact that the Kw is equal to the product of the concentrations of H+ and OH.
A weak acid will have a low concentration of H+. The Kw divided by a low H+ concentration will result in a low OH- concentration as well. Therefore, weak acids will have weak conjugate bases, unlike the misconception that they have strong conjugate bases; the acid and conjugate base as well as the base and conjugate acid are known as conjugate pairs. When finding a conjugate acid or base, it is important to look at the reactants of the chemical equation. In this case, the reactants are the acids and bases, the acid corresponds to the conjugate base on the product side of the chemical equation. To identify the conjugate acid, look for the pair of compounds that are related; the acid–base reaction can be viewed in a before and after sense. The before is the reactant side of the after is the product side of the equation; the conjugate acid in the after side of an equation gains a hydrogen ion, so in the before side of the equation the compound that has one less hydrogen ion of the conjugate acid is the base.
The conjugate base in the after side of the equation lost a hydrogen ion, so in the before side of the equation, the compound that has one more hydrogen ion of the conjugate base is the acid. Consider the following acid–base reaction: HNO3 + H2O → H3O+ + NO−3Nitric acid is an acid because it donates a proton to the water molecule and its conjugate base is nitrate; the water molecule acts as a base because it receives the Hydrogen Proton and its conjugate acid is the hydronium ion. One use of conjugate acids and bases lies in buffering systems. In a buffer, a weak acid and its conjugate base, or a weak base and its conjugate acid, are used in order to limit the pH change during a titration process. Buffers have both non-organic chemical applications. For example, besides buffers being used in lab processes, our blood acts as a buffer to maintain pH; the most important buffer in our bloodstream is the carbonic acid-bicarbonate buffer, which prevents drastic pH changes when CO2 is introduced. This functions as such: CO 2 + H 2 O ↽ − − ⇀ H 2 CO 3 ↽
In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs in outer space, can be an unwelcome source of wear in precision components. However, the fact that it can be made to act on fine layers of material is exploited in science and industry -- there, it is used to perform precise etching, carry out analytical techniques, deposit thin film layers in the manufacture of optical coatings, semiconductor devices and nanotechnology products; when energetic ions collide with atoms of a target material, an exchange of momentum takes place between them. These ions, known as "incident ions", set off collision cascades in the target; such cascades can take many paths. If a collision cascade reaches the surface of the target, its remaining energy is greater than the target's surface binding energy, an atom will be ejected; this process is known as "sputtering".
If the target is thin, the collision cascade can reach through to its back side. The average number of atoms ejected from the target per incident ion is called the "sputter yield"; the sputter yield depends on several things: the angle at which ions collide with the surface of the material, how much energy they strike it with, their masses, the masses of the target atoms, the target's surface binding energy. If the target possesses a crystal structure, the orientation of its axes with respect to the surface is an important factor; the ions that cause sputtering come from a variety of sources -- they can come from plasma, specially constructed ion sources, particle accelerators, outer space, or radioactive materials. A model for describing sputtering in the cascade regime for amorphous flat targets is Thompson's analytical model. An algorithm that simulates sputtering based on a quantum mechanical treatment including electrons stripping at high energy is implemented in the program TRIM. Another mechanism of physical sputtering is called "heat spike sputtering".
This can occur when the solid is dense enough, the incoming ion heavy enough, that collisions occur close to each other. In this case, the binary collision approximation is no longer valid, the collisional process should be understood as a many-body process; the dense collisions induce a heat spike, which melts a small portion of the crystal. If that portion is close enough to its surface, large numbers of atoms may be ejected, due to liquid flowing to the surface and/or microexplosions. Heat spike sputtering is most important for heavy ions with energies in the keV–MeV range bombarding dense but soft metals with a low melting point; the heat spike sputtering increases nonlinearly with energy, can for small cluster ions lead to dramatic sputtering yields per cluster of the order of 10,000. For animations of such a process see "Re: Displacement Cascade 1" in the external links section. Physical sputtering has a well-defined minimum energy threshold, equal to or larger than the ion energy at which the maximum energy transfer from the ion to a target atom equals the binding energy of a surface atom.
That is to say, it can only happen when an ion is capable of transferring more energy into the target than is required for an atom to break free from its surface. This threshold is somewhere in the range of ten to a hundred eV. Preferential sputtering can occur at the start when a multicomponent solid target is bombarded and there is no solid state diffusion. If the energy transfer is more efficient to one of the target components, or it is less bound to the solid, it will sputter more efficiently than the other. If in an AB alloy the component A is sputtered preferentially, the surface of the solid will, during prolonged bombardment, become enriched in the B component, thereby increasing the probability that B is sputtered such that the composition of the sputtered material will return to AB; the term electronic sputtering can mean either sputtering induced by energetic electrons, or sputtering due to high-energy or charged heavy ions that lose energy to the solid by electronic stopping power, where the electronic excitations cause sputtering.
Electronic sputtering produces high sputtering yields from insulators, as the electronic excitations that cause sputtering are not quenched, as they would be in a conductor. One example of this is Jupiter's ice-covered moon Europa, where a MeV sulfur ion from Jupiter's magnetosphere can eject up to 10,000 H2O molecules. In the case of multiple charged projectile ions a particular form of electronic sputtering can take place, termed potential sputtering. In these cases the potential energy stored in multiply charged ions is liberated when the ions recombine during impact on a solid surface; this sputtering process is characterized by a strong dependence of the observed sputtering yields on the charge state of the impinging ion and can take place at ion impact energies well below the physical sputtering threshold. Potential sputtering has only been observed for certain target species and requires a minimum potential energy. Removing atoms by sputtering with an inert gas is called ion ion etching.
Sputtering can play a role in reactive-ion etching (R
Helium is a chemical element with symbol He and atomic number 2. It is a colorless, tasteless, non-toxic, monatomic gas, the first in the noble gas group in the periodic table, its boiling point is the lowest among all the elements. After hydrogen, helium is the second lightest and second most abundant element in the observable universe, being present at about 24% of the total elemental mass, more than 12 times the mass of all the heavier elements combined, its abundance is similar in Jupiter. This is due to the high nuclear binding energy of helium-4 with respect to the next three elements after helium; this helium-4 binding energy accounts for why it is a product of both nuclear fusion and radioactive decay. Most helium in the universe is helium-4, the vast majority of, formed during the Big Bang. Large amounts of new helium are being created by nuclear fusion of hydrogen in stars. Helium is named for the Greek Titan of the Sun, Helios, it was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by Georges Rayet, Captain C. T. Haig, Norman R. Pogson, Lieutenant John Herschel, was subsequently confirmed by French astronomer Jules Janssen.
Janssen is jointly credited with detecting the element along with Norman Lockyer. Janssen recorded the helium spectral line during the solar eclipse of 1868 while Lockyer observed it from Britain. Lockyer was the first to propose; 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, by far the largest supplier of the gas today. Liquid helium is used in cryogenics 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 produced. A well-known but minor use is as a lifting gas in airships; as with any gas whose density differs from that of 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 is important to researchers studying quantum mechanics and to those looking at the phenomena, such as superconductivity, produced in matter near absolute zero. On Earth it is rare—5.2 ppm by volume in the atmosphere. Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements, as the alpha particles emitted by such decays consist of helium-4 nuclei; this radiogenic helium is trapped with natural gas in concentrations as great as 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation. Terrestrial helium—a non-renewable resource, because once released into the atmosphere it escapes into space—was thought to be in short supply. However, recent studies suggest that helium produced deep in the earth by radioactive decay can collect in natural gas reserves in larger than expected quantities, in some cases having been released by volcanic activity.
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 Sun. The line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India; this line was 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 because it was near the known D1 and D2 Fraunhofer line lines of sodium, he concluded. Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος. In 1881, Italian physicist Luigi Palmieri detected helium on Earth for the first time through its D3 spectral line, when he analyzed a material, sublimated during a recent eruption of Mount Vesuvius. On March 26, 1895, Scottish chemist Sir William Ramsay isolated helium on Earth by treating the mineral cleveite 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.
These 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, who collected enough of the gas to determine its atomic weight. Helium was 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, 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 the particles to penetrate the thin glass wall of
Argon is a chemical element with symbol Ar and atomic number 18. It is a noble gas. Argon is the third-most abundant gas in the Earth's atmosphere, at 0.934%. It is more than twice as abundant as water vapor, 23 times as abundant as carbon dioxide, more than 500 times as abundant as neon. Argon is the most abundant noble gas in Earth's crust, comprising 0.00015% of the crust. Nearly all of the argon in the Earth's atmosphere is radiogenic argon-40, derived from the decay of potassium-40 in the Earth's crust. In the universe, argon-36 is by far the most common argon isotope, as it is the most produced by stellar nucleosynthesis in supernovas; the name "argon" is derived from the Greek word ἀργόν, neuter singular form of ἀργός meaning "lazy" or "inactive", as a reference to the fact that the element undergoes no chemical reactions. The complete octet in the outer atomic shell makes argon stable and resistant to bonding with other elements, its triple point temperature of 83.8058 K is a defining fixed point in the International Temperature Scale of 1990.
Argon is produced industrially by the fractional distillation of liquid air. Argon is used as an inert shielding gas in welding and other high-temperature industrial processes where ordinarily unreactive substances become reactive. Argon is used in incandescent, fluorescent lighting, other gas-discharge tubes. Argon makes a distinctive blue-green gas laser. Argon is used in fluorescent glow starters. Argon has the same solubility in water as oxygen and is 2.5 times more soluble in water than nitrogen. Argon is colorless, odorless and nontoxic as a solid, liquid or gas. Argon is chemically inert under most conditions and forms no confirmed stable compounds at room temperature. Although argon is a noble gas, it can form some compounds under various extreme conditions. Argon fluorohydride, a compound of argon with fluorine and hydrogen, stable below 17 K, has been demonstrated. Although the neutral ground-state chemical compounds of argon are presently limited to HArF, argon can form clathrates with water when atoms of argon are trapped in a lattice of water molecules.
Ions, such as ArH+, excited-state complexes, such as ArF, have been demonstrated. Theoretical calculation predicts several more argon compounds that should be stable but have not yet been synthesized. Argon, is named in reference to its chemical inactivity; this chemical property of this first noble gas to be discovered impressed the namers. An unreactive gas was suspected to be a component of air by Henry Cavendish in 1785. Argon was first isolated from air in 1894 by Lord Rayleigh and Sir William Ramsay at University College London by removing oxygen, carbon dioxide and nitrogen from a sample of clean air, they had determined that nitrogen produced from chemical compounds was 0.5% lighter than nitrogen from the atmosphere. The difference was slight, they concluded. Argon was encountered in 1882 through independent research of H. F. Newall and W. N. Hartley; each observed new lines in the emission spectrum of air. Until 1957, the symbol for argon was "A", but now is "Ar". Argon constitutes 0.934% by volume and 1.288% by mass of the Earth's atmosphere, air is the primary industrial source of purified argon products.
Argon is isolated from air by fractionation, most by cryogenic fractional distillation, a process that produces purified nitrogen, neon and xenon. The Earth's crust and seawater contain 0.45 ppm of argon, respectively. The main isotopes of argon found on Earth are 40Ar, 36Ar, 38Ar. Occurring 40K, with a half-life of 1.25×109 years, decays to stable 40Ar by electron capture or positron emission, to stable 40Ca by beta decay. These properties and ratios are used to determine the age of rocks by K–Ar dating. In the Earth's atmosphere, 39Ar is made by cosmic ray activity by neutron capture of 40Ar followed by two-neutron emission. In the subsurface environment, it is produced through neutron capture by 39K, followed by proton emission. 37Ar is created from the neutron capture by 40Ca followed by an alpha particle emission as a result of subsurface nuclear explosions. It has a half-life of 35 days. Between locations in the Solar System, the isotopic composition of argon varies greatly. Where the major source of argon is the decay of 40K in rocks, 40Ar will be the dominant isotope, as it is on Earth.
Argon produced directly by stellar nucleosynthesis, is dominated by the alpha-process nuclide 36Ar. Correspondingly, solar argon contains 84.6% 36Ar, the ratio of the three isotopes 36Ar: 38Ar: 40Ar in the atmospheres of the outer planets is 8400: 1600: 1. This contrasts with the low abundance of primordial 36Ar in Earth's atmosphere, only 31.5 ppmv, comparable with that of neon on Earth and with interplanetary gasses, measured by probes. The atmospheres of Mars and Titan contain argon, predominantly as 40Ar, its content may be as high as 1.93%. The predominance of radiogenic 40Ar is the reason the standard atomic weight of terrestrial argon is greater than that of the next element, potassium, a fact that was