Krypton is a chemical element with symbol Kr and atomic number 36. It is a member of group 18 elements. A colorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere and is used with other rare gases in fluorescent lamps. With rare exceptions, krypton is chemically inert. Krypton, like the other noble gases, is used in photography. Krypton light has many spectral lines, krypton plasma is useful in bright, high-powered gas lasers, each of which resonates and amplifies a single spectral line. Krypton fluoride makes a useful laser medium. From 1960 to 1983, the official length of a meter was defined by the 605 nm wavelength of the orange spectral line of krypton-86, because of the high power and relative ease of operation of krypton discharge tubes. Krypton was discovered in Britain in 1898 by Sir William Ramsay, a Scottish chemist, Morris Travers, an English chemist, in residue left from evaporating nearly all components of liquid air. Neon was discovered by a similar procedure by the same workers just a few weeks later.
William Ramsay was awarded the 1904 Nobel Prize in Chemistry for discovery of a series of noble gases, including krypton. In 1960, the International Conference on Weights and Measures defined the meter as 1,650,763.73 wavelengths of light emitted by the krypton-86 isotope. This agreement replaced the 1889 international prototype meter located in Paris, a metal bar made of a platinum-iridium alloy; this obsoleted the 1927 definition of the ångström based on the red cadmium spectral line, replacing it with 1 Å = 10−10 m. The krypton-86 definition lasted until the October 1983 conference, which redefined the meter as the distance that light travels in vacuum during 1/299,792,458 s. Krypton is characterized by several sharp emission lines the strongest being yellow. Krypton is one of the products of uranium fission. Solid krypton is white and has a face-centered cubic crystal structure, a common property of all noble gases. Occurring krypton in Earth's atmosphere is composed of five stable isotopes, plus one isotope with such a long half-life that it can be considered stable..
In addition, about thirty unstable isotopes and isomers are known. Traces of 81Kr, a cosmogenic nuclide produced by the cosmic ray irradiation of 80Kr occur in nature: this isotope is radioactive with a half-life of 230,000 years. Krypton is volatile and does not stay in solution in near-surface water, but 81Kr has been used for dating old groundwater.85Kr is an inert radioactive noble gas with a half-life of 10.76 years. It is produced by the fission of uranium and plutonium, such as in nuclear bomb testing and nuclear reactors. 85Kr is released during the reprocessing of fuel rods from nuclear reactors. Concentrations at the North Pole are 30% higher than at the South Pole due to convective mixing. Like the other noble gases, krypton is chemically unreactive; the rather restricted chemistry of krypton in its only known nonzero oxidation state of +2 parallels that of the neighboring element bromine in the +1 oxidation state. Before the 1960s, no noble gas compounds had been synthesized. However, following the first successful synthesis of xenon compounds in 1962, synthesis of krypton difluoride was reported in 1963.
In the same year, KrF4 was reported by Grosse, et al. but was subsequently shown to be a mistaken identification. Under extreme conditions, krypton reacts with fluorine to form KrF2 according to the following equation: Kr + F2 → KrF2Compounds with krypton bonded to atoms other than fluorine have been discovered. There are unverified reports of a barium salt of a krypton oxoacid. ArKr+ and KrH+ polyatomic ions have been investigated and there is evidence for KrXe or KrXe+; the reaction of KrF2 with B3 produces an unstable compound, Kr2, that contains a krypton-oxygen bond. A krypton-nitrogen bond is found in the cation +, produced by the reaction of KrF2 with + below −50 °C. HKrCN and HKrC≡CH were reported to be stable up to 40 K. Krypton hydride crystals can be grown at pressures above 5 GPa, they have a face-centered cubic structure where krypton octahedra are surrounded by randomly oriented hydrogen molecules. Earth has retained all of the noble gases. Krypton's concentration in the atmosphere is about 1 ppm.
It can be extracted from liquid air by fractional distillation. The amount of krypton in space is uncertain, because measurement is derived from meteoric activity and solar winds; the first measurements suggest an abundance of krypton in space. Krypton's multiple emission lines make ionized krypton gas discharges appear whitish, which in turn makes krypton-based bulbs useful in photography as a brilliant white light source. Krypton is used in some photographic flashes for high speed photography. Krypton gas is combined with other gases to make luminous signs that glow with a bright greenish-yellow light. Krypton is mixed with argon in energy efficient fluorescent lamps, reducing the power consumption, but reducing the light output and raising the c
Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has been classified as a "rare-earth element". Yttrium is always found in combination with lanthanide elements in rare-earth minerals, is never found in nature as a free element. 89Y is the only stable isotope, the only isotope found in the Earth's crust. In 1787, Carl Axel Arrhenius found a new mineral near Ytterby in Sweden and named it ytterbite, after the village. Johan Gadolin discovered yttrium's oxide in Arrhenius' sample in 1789, Anders Gustaf Ekeberg named the new oxide yttria. Elemental yttrium was first isolated in 1828 by Friedrich Wöhler; the most important uses of yttrium are LEDs and phosphors the red phosphors in television set cathode ray tube displays. Yttrium is used in the production of electrodes, electronic filters, superconductors, various medical applications, tracing various materials to enhance their properties. Yttrium has no known biological role.
Exposure to yttrium compounds can cause lung disease in humans. Yttrium is a soft, silver-metallic and crystalline transition metal in group 3; as expected by periodic trends, it is less electronegative than its predecessor in the group and less electronegative than the next member of period 5, zirconium. Yttrium is the first d-block element in the fifth period; the pure element is stable in air in bulk form, due to passivation of a protective oxide film that forms on the surface. This film can reach a thickness of 10 µm; when finely divided, yttrium is unstable in air. Yttrium nitride is formed; the similarities of yttrium to the lanthanides are so strong that the element has been grouped with them as a rare-earth element, is always found in nature together with them in rare-earth minerals. Chemically, yttrium resembles those elements more than its neighbor in the periodic table, if physical properties were plotted against atomic number, it would have an apparent number of 64.5 to 67.5, placing it between the lanthanides gadolinium and erbium.
It also falls in the same range for reaction order, resembling terbium and dysprosium in its chemical reactivity. Yttrium is so close in size to the so-called'yttrium group' of heavy lanthanide ions that in solution, it behaves as if it were one of them. Though the lanthanides are one row farther down the periodic table than yttrium, the similarity in atomic radius may be attributed to the lanthanide contraction. One of the few notable differences between the chemistry of yttrium and that of the lanthanides is that yttrium is exclusively trivalent, whereas about half the lanthanides can have valences other than three; as a trivalent transition metal, yttrium forms various inorganic compounds in the oxidation state of +3, by giving up all three of its valence electrons. A good example is yttrium oxide known as yttria, a six-coordinate white solid. Yttrium forms a water-insoluble fluoride and oxalate, but its bromide, iodide and sulfate are all soluble in water; the Y3+ ion is colorless in solution because of the absence of electrons in the d and f electron shells.
Water reacts with yttrium and its compounds to form Y2O3. Concentrated nitric and hydrofluoric acids do not attack yttrium, but other strong acids do. With halogens, yttrium forms trihalides such as yttrium fluoride, yttrium chloride, yttrium bromide at temperatures above 200 °C. Carbon, selenium and sulfur all form binary compounds with yttrium at elevated temperatures. Organoyttrium chemistry is the study of compounds containing carbon–yttrium bonds. A few of these are known to have yttrium in the oxidation state 0; some trimerization reactions were generated with organoyttrium compounds as catalysts. These syntheses use YCl3 as a starting material, obtained from Y2O3 and concentrated hydrochloric acid and ammonium chloride. Hapticity is a term to describe the coordination of a group of contiguous atoms of a ligand bound to the central atom. Yttrium complexes were the first examples of complexes where carboranyl ligands were bound to a d0-metal center through a η7-hapticity. Vaporization of the graphite intercalation compounds graphite–Y or graphite–Y2O3 leads to the formation of endohedral fullerenes such as Y@C82.
Electron spin resonance studies indicated the formation of 3 − ion pairs. The carbides Y3C, Y2C, YC2 can be hydrolyzed to form hydrocarbons. Yttrium in the Solar System was created through stellar nucleosynthesis by the s-process, but by the r-process; the r-process consists of rapid neutron capture of lighter elements during supernova explosions. The s-process is a slow neutron capture of lighter elements inside pulsating red giant stars. Yttrium isotopes are among the most common products of the nuclear fission of uranium in nuclear explosions and nuclear reactors. In the context of nuclear waste management, the most important isotopes of yttrium
Nitrogen is a chemical element with symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel Rutherford in 1772. Although Carl Wilhelm Scheele and Henry Cavendish had independently done so at about the same time, Rutherford is accorded the credit because his work was published first; the name nitrogène was suggested by French chemist Jean-Antoine-Claude Chaptal in 1790, when it was found that nitrogen was present in nitric acid and nitrates. Antoine Lavoisier suggested instead the name azote, from the Greek ἀζωτικός "no life", as it is an asphyxiant gas. Nitrogen is the lightest member of group 15 of the periodic table called the pnictogens; the name comes from the Greek πνίγειν "to choke", directly referencing nitrogen's asphyxiating properties. It is a common element in the universe, estimated at about seventh in total abundance in the Milky Way and the Solar System. At standard temperature and pressure, two atoms of the element bind to form dinitrogen, a colourless and odorless diatomic gas with the formula N2.
Dinitrogen forms about 78 % of Earth's atmosphere. Nitrogen occurs in all organisms in amino acids, in the nucleic acids and in the energy transfer molecule adenosine triphosphate; the human body contains about 3% nitrogen by mass, the fourth most abundant element in the body after oxygen and hydrogen. The nitrogen cycle describes movement of the element from the air, into the biosphere and organic compounds back into the atmosphere. Many industrially important compounds, such as ammonia, nitric acid, organic nitrates, cyanides, contain nitrogen; the strong triple bond in elemental nitrogen, the second strongest bond in any diatomic molecule after carbon monoxide, dominates nitrogen chemistry. This causes difficulty for both organisms and industry in converting N2 into useful compounds, but at the same time means that burning, exploding, or decomposing nitrogen compounds to form nitrogen gas releases large amounts of useful energy. Synthetically produced ammonia and nitrates are key industrial fertilisers, fertiliser nitrates are key pollutants in the eutrophication of water systems.
Apart from its use in fertilisers and energy-stores, nitrogen is a constituent of organic compounds as diverse as Kevlar used in high-strength fabric and cyanoacrylate used in superglue. Nitrogen is a constituent including antibiotics. Many drugs are mimics or prodrugs of natural nitrogen-containing signal molecules: for example, the organic nitrates nitroglycerin and nitroprusside control blood pressure by metabolizing into nitric oxide. Many notable nitrogen-containing drugs, such as the natural caffeine and morphine or the synthetic amphetamines, act on receptors of animal neurotransmitters. Nitrogen compounds have a long history, ammonium chloride having been known to Herodotus, they were well known by the Middle Ages. Alchemists knew nitric acid as aqua fortis, as well as other nitrogen compounds such as ammonium salts and nitrate salts; the mixture of nitric and hydrochloric acids was known as aqua regia, celebrated for its ability to dissolve gold, the king of metals. The discovery of nitrogen is attributed to the Scottish physician Daniel Rutherford in 1772, who called it noxious air.
Though he did not recognise it as an different chemical substance, he distinguished it from Joseph Black's "fixed air", or carbon dioxide. The fact that there was a component of air that does not support combustion was clear to Rutherford, although he was not aware that it was an element. Nitrogen was studied at about the same time by Carl Wilhelm Scheele, Henry Cavendish, Joseph Priestley, 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 άζωτικός, "no life". In an atmosphere of pure nitrogen, animals died and flames were extinguished. Though Lavoisier's name was not accepted in English, since it was pointed out that all gases are mephitic, it is used in many languages and still remains in English in the common names of many nitrogen compounds, such as hydrazine and compounds of the azide ion, it led to the name "pnictogens" for the group headed by nitrogen, from the Greek πνίγειν "to choke".
The English word nitrogen entered the language from the French nitrogène, coined in 1790 by French chemist Jean-Antoine Chaptal, from the French nitre and the French suffix -gène, "producing", from the Greek -γενής. Chaptal's meaning was that nitrogen is the essential part of nitric acid, which in turn was produced from nitre. In earlier times, niter had been confused with Egyptian "natron" – called νίτρον in Greek – which, despite the name, contained no nitrate; the earliest military and agricultural applications of nitrogen compounds used saltpeter, most notably in gunpowder, as fertiliser. In 1910, Lord Rayleigh discovered that an electrical discharge in nitrogen gas produced "active nitrogen", a monatomic allotrope of nitrogen; the "whirling cloud of brilliant yellow light
The micrometre or micrometer commonly known by the previous name micron, is an SI derived unit of length equalling 1×10−6 metre. The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria, for grading wool by the diameter of the fibres; the width of a single human hair ranges from 10 to 200 μm. The longest human chromosome is 10 μm in length. Between 1 μm and 10 μm: 1–10 μm – length of a typical bacterium 10 μm – Size of fungal hyphae 5 μm – length of a typical human spermatozoon's head 3–8 μm – width of strand of spider web silk about 10 μm – size of a fog, mist, or cloud water droplet Between 10 μm and 100 μm about 10–12 μm – thickness of plastic wrap 10 to 55 μm – width of wool fibre 17 to 181 μm – diameter of human hair 70 to 180 μm – thickness of paper The term micron and the symbol μ were accepted for use in isolation to denote the micrometre in 1879, but revoked by the International System of Units in 1967; this became necessary because the older usage was incompatible with the official adoption of the unit prefix micro-, denoted μ, during the creation of the SI in 1960.
In the SI, the systematic name micrometre became the official name of the unit, μm became the official unit symbol. In practice, "micron" remains a used term in preference to "micrometre" in many English-speaking countries, both in academic science and in applied science and industry. Additionally, in American English, the use of "micron" helps differentiate the unit from the micrometer, a measuring device, because the unit's name in mainstream American spelling is a homograph of the device's name. In spoken English, they may be distinguished by pronunciation, as the name of the measuring device is invariably stressed on the second syllable, whereas the systematic pronunciation of the unit name, in accordance with the convention for pronouncing SI units in English, places the stress on the first syllable; the plural of micron is "microns", though "micra" was used before 1950. The official symbol for the SI prefix micro- is a Greek lowercase mu. In Unicode, there is a micro sign with the code point U+00B5, distinct from the code point U+03BC of the Greek letter lowercase mu.
According to the Unicode Consortium, the Greek letter character is preferred, but implementations must recognize the micro sign as well. Most fonts use the same glyph for the two characters. Metric prefix Metric system Orders of magnitude Wool measurement The dictionary definition of micrometre at Wiktionary
A fiber laser or fibre laser is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, neodymium, praseodymium and holmium. They are related to doped fiber amplifiers. Fiber nonlinearities, such as stimulated Raman scattering or four-wave mixing can provide gain and thus serve as gain media for a fiber laser; the advantages of fiber lasers over other types include: Light is coupled into a flexible fiber: The fact that the light is in a fiber allows it to be delivered to a movable focusing element. This is important for laser cutting and folding of metals and polymers. High output power: Fiber lasers can have active regions several kilometers long, so can provide high optical gain, they can support kilowatt levels of continuous output power because of the fiber's high surface area to volume ratio, which allows efficient cooling. High optical quality: The fiber's waveguiding properties reduce or eliminate thermal distortion of the optical path producing a diffraction-limited, high-quality optical beam.
Compact size: Fiber lasers are compact compared to rod or gas lasers of comparable power, because the fiber can be bent and coiled to save space. Reliability: Fiber lasers exhibit high temperature and vibrational stability, extended lifetime, maintenance-free turnkey operation. High peak power and nanosecond pulses enable effective engraving; the additional power and better beam quality provide faster cutting speeds. Lower cost of ownership. Fiber lasers are now being used to make high-performance surface-acoustic wave devices; these lasers raise throughput and lower cost of ownership in comparison to older solid-state laser technology. Fiber laser can refer to the machine tool that includes the fiber resonator. Applications of fiber lasers include material processing telecommunications, spectroscopy and directed energy weapons. Unlike most other types of lasers, the laser cavity in fiber lasers is constructed monolithically by fusion splicing different types of fiber. Another type is the single longitudinal mode operation of ultra narrow distributed feedback lasers where a phase-shifted Bragg grating overlaps the gain medium.
Fiber lasers are pumped by other fiber lasers. Q-switched pulsed fiber lasers offer a compact, electrically efficient alternative to Nd:YAG technology. Many high-power fiber lasers are based on double-clad fiber; the gain medium forms the core of the fiber, surrounded by two layers of cladding. The lasing mode propagates in the core, while a multimode pump beam propagates in the inner cladding layer; the outer cladding keeps this pump light confined. This arrangement allows the core to be pumped with a much higher-power beam than could otherwise be made to propagate in it, allows the conversion of pump light with low brightness into a much higher-brightness signal; as a result, fiber lasers and amplifiers are referred to as "brightness converters." There is an important question about the shape of the double-clad fiber. The design should allow the core to be small enough to support only a few modes, it should provide sufficient cladding to confine the core and optical pump section over a short piece of the fiber.
Recent developments in fiber laser technology have led to a rapid and large rise in achieved diffraction-limited beam powers from diode-pumped solid-state lasers. Due to the introduction of large mode area fibers as well as continuing advances in high power and high brightness diodes, continuous-wave single-transverse-mode powers from Yb-doped fiber lasers have increased from 100 W in 2001 to >20 kW. Commercial single-mode lasers have reached 10 kW in CW power. In 2014 a combined beam fiber laser demonstrated power of 30 kW; when linearly polarized light is incident to a piece of weakly birefringent fiber, the polarization of the light will become elliptically polarized in the fiber. The orientation and ellipticity of the final light polarization is determined by the fiber length and its birefringence. However, if the intensity of the light is strong, the non-linear optical Kerr effect in the fiber must be considered, which introduces extra changes to the light polarization; as the polarization change introduced by the optical Kerr effect depends on the light intensity, if a polarizer is put behind the fiber, the light intensity transmission through the polarizer will become light intensity dependent.
Through appropriately selecting the orientation of the polarizer or the length of the fiber, an artificial saturable absorber effect with ultra-fast response could be achieved in such a system, where light of higher intensity experiences less absorption loss on the polarizer. The NPR technique makes use of this artificial saturable absorption to achieve the passive mode locking in a fiber laser. Once a mode-locked pulse is formed, the non-linearity of the fiber further shapes the pulse into an optical soliton and the ultrashort soliton operation is obtained in the laser. Soliton operation is a generic feature of the fiber lasers mode-locked by this technique and has been intensively investigated. Semiconductor saturable absorbers were used for laser mode-locking as early as 1974 when p-type germanium is used to mode lock a CO2 laser which generated pulses ~500 ps. Modern SESAMs are III-V semiconductor single quantum well or multiple quantum wells grown on semiconductor distr
Neodymium is a chemical element with symbol Nd and atomic number 60. It is a soft silvery metal. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach, it is present in significant quantities in the ore minerals bastnäsite. Neodymium is not found in metallic form or unmixed with other lanthanides, it is refined for general use. Although neodymium is classed as a rare earth, it is a common element, no rarer than cobalt, nickel, or copper, is distributed in the Earth's crust. Most of the world's commercial neodymium is mined in China. Neodymium compounds were first commercially used as glass dyes in 1927, they remain a popular additive in glasses; the color of neodymium compounds—due to the Nd3+ ion—is a reddish-purple but it changes with the type of lighting, due to the interaction of the sharp light absorption bands of neodymium with ambient light enriched with the sharp visible emission bands of mercury, trivalent europium or terbium. Some neodymium-doped glasses are used in lasers that emit infrared with wavelengths between 1047 and 1062 nanometers.
These have been used in extremely-high-power applications, such as experiments in inertial confinement fusion. Neodymium is used with various other substrate crystals, such as yttrium aluminium garnet in the Nd:YAG laser; this laser emits infrared at a wavelength of about 1064 nanometers. The Nd:YAG laser is one of the most used solid-state lasers. Another important use of neodymium is as a component in the alloys used to make high-strength neodymium magnets—powerful permanent magnets; these magnets are used in such products as microphones, professional loudspeakers, in-ear headphones, high performance hobby DC electric motors, computer hard disks, where low magnet mass or strong magnetic fields are required. Larger neodymium magnets are used in generators. Neodymium, a rare-earth metal, was present in the classical mischmetal at a concentration of about 18%. Metallic neodymium has a bright, silvery metallic luster, but as one of the more reactive lanthanide rare-earth metals, it oxidizes in ordinary air.
The oxide layer that forms peels off, exposing the metal to further oxidation. Thus, a centimeter-sized sample of neodymium oxidizes within a year. Neodymium exists in two allotropic forms, with a transformation from a double hexagonal to a body-centered cubic structure taking place at about 863 °C. Neodymium metal tarnishes in air and it burns at about 150 °C to form neodymium oxide: 4 Nd + 3 O2 → 2 Nd2O3Neodymium is a quite electropositive element, it reacts with cold water, but quite with hot water to form neodymium hydroxide: 2 Nd + 6 H2O → 2 Nd3 + 3 H2 Neodymium metal reacts vigorously with all the halogens: 2 Nd + 3 F2 → 2 NdF3 2 Nd + 3 Cl2 → 2 NdCl3 2 Nd + 3 Br2 → 2 NdBr3 2 Nd + 3 I2 → 2 NdI3 Neodymium dissolves in dilute sulfuric acid to form solutions that contain the lilac Nd ion; these exist as a 3+ complexes: 2 Nd + 3 H2SO4 → 2 Nd3+ + 3 SO2−4 + 3 H2 Neodymium compounds include halides: neodymium fluoride. Occurring neodymium is a mixture of five stable isotopes, 142Nd, 143Nd, 145Nd, 146Nd and 148Nd, with 142Nd being the most abundant, two radioisotopes, 144Nd and 150Nd.
In all, 31 radioisotopes of neodymium have been detected as of 2010, with the most stable radioisotopes being the occurring ones: 144Nd and 150Nd. All of the remaining radioactive isotopes have half-lives that are shorter than eleven days, the majority of these have half-lives that are shorter than 70 seconds. Neodymium has 13 known meta states, with the most stable one being 139mNd, 135mNd and 133m1Nd; the primary decay modes before the most abundant stable isotope, 142Nd, are electron capture and positron decay, the primary mode after is beta minus decay. The primary decay products before 142Nd are element Pr isotopes and the primary products after are element Pm isotopes. Neodymium was discovered by Baron Carl Auer von Welsbach, an Austrian chemist, in Vienna in 1885, he separated neodymium, as well as the element praseodymium, from a material known as didymium by means of fractional crystallization of the double ammonium nitrate tetrahydrates from nitric acid, while following the separation by spectroscopic analysis.
The name neodymium is derived from the Greek words neos and didymos, twin. Double nitrate crystallization was the means of commercial neodymium purification until the 1950s. Lindsay Chemical Division was the first to commercialize large-scale ion-exchange purification of neodymium. Starting in the 1950s, high purity
Aluminium or aluminum is a chemical element with symbol Al and atomic number 13. It is a silvery-white, soft and ductile metal in the boron group. By mass, aluminium makes up about 8% of the Earth's crust; the chief ore of aluminium is bauxite. Aluminium metal is so chemically reactive that native specimens are rare and limited to extreme reducing environments. Instead, it is found combined in over 270 different minerals. Aluminium is remarkable for its low density and its ability to resist corrosion through the phenomenon of passivation. Aluminium and its alloys are vital to the aerospace industry and important in transportation and building industries, such as building facades and window frames; the oxides and sulfates are the most useful compounds of aluminium. Despite its prevalence in the environment, no known form of life uses aluminium salts metabolically, but aluminium is well tolerated by plants and animals; because of these salts' abundance, the potential for a biological role for them is of continuing interest, studies continue.
Of aluminium isotopes, only 27Al is stable. This is consistent with aluminium having an odd atomic number, it is the only aluminium isotope that has existed on Earth in its current form since the creation of the planet. Nearly all the element on Earth is present as this isotope, which makes aluminium a mononuclidic element and means that its standard atomic weight equates to that of the isotope; the standard atomic weight of aluminium is low in comparison with many other metals, which has consequences for the element's properties. All other isotopes of aluminium are radioactive; the most stable of these is 26Al and therefore could not have survived since the formation of the planet. However, 26Al is produced from argon in the atmosphere by spallation caused by cosmic ray protons; the ratio of 26Al to 10Be has been used for radiodating of geological processes over 105 to 106 year time scales, in particular transport, sediment storage, burial times, erosion. Most meteorite scientists believe that the energy released by the decay of 26Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.
The remaining isotopes of aluminium, with mass numbers ranging from 21 to 43, all have half-lives well under an hour. Three metastable states are known, all with half-lives under a minute. An aluminium atom has 13 electrons, arranged in an electron configuration of 3s23p1, with three electrons beyond a stable noble gas configuration. Accordingly, the combined first three ionization energies of aluminium are far lower than the fourth ionization energy alone. Aluminium can easily surrender its three outermost electrons in many chemical reactions; the electronegativity of aluminium is 1.61. A free aluminium atom has a radius of 143 pm. With the three outermost electrons removed, the radius shrinks to 39 pm for a 4-coordinated atom or 53.5 pm for a 6-coordinated atom. At standard temperature and pressure, aluminium atoms form a face-centered cubic crystal system bound by metallic bonding provided by atoms' outermost electrons; this crystal system is shared by some other metals, such as copper. Aluminium metal, when in quantity, is shiny and resembles silver because it preferentially absorbs far ultraviolet radiation while reflecting all visible light so it does not impart any color to reflected light, unlike the reflectance spectra of copper and gold.
Another important characteristic of aluminium is its low density, 2.70 g/cm3. Aluminium is a soft, lightweight and malleable with appearance ranging from silvery to dull gray, depending on the surface roughness, it is nonmagnetic and does not ignite. A fresh film of aluminium serves as a good reflector of visible light and an excellent reflector of medium and far infrared radiation; the yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa. Aluminium has stiffness of steel, it is machined, cast and extruded. Aluminium atoms are arranged in a face-centered cubic structure. Aluminium has a stacking-fault energy of 200 mJ/m2. Aluminium is a good thermal and electrical conductor, having 59% the conductivity of copper, both thermal and electrical, while having only 30% of copper's density. Aluminium is capable of superconductivity, with a superconducting critical temperature of 1.2 kelvin and a critical magnetic field of about 100 gauss.
Aluminium is the most common material for the fabrication of superconducting qubits. Aluminium's corrosion resistance can be excellent due to a thin surface layer of aluminium oxide that forms when the bare metal is exposed to air preventing further oxidation, in a process termed passivation; the strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper. This corrosion resistance is reduced by aqueous salts in the presence of dissimilar metals. In acidic solutions, aluminium reacts with water to form hydrogen, in alkaline ones to form aluminates—protective passivation under these conditions is negligible; because it is corroded by dissolved chlorides, such as common sodium chloride, household plumbing is never made from aluminium. However, because