The halide mineral class include those minerals with a dominant halide anion. Complex halide minerals may have polyatomic anions in addition to or that include halides. Examples include the following: Atacamite Cu2Cl3 Avogadrite BF Bararite 2SiF6 Bischofite Brüggenite Ca2 Calomel HgCl Carnallite KMgCl3·6H2O Carnallite KMgCl·6H2O Cerargyite/Horn Silver AgCl Chlorargyrite AgCl, bromargyrite AgBr, iodargyrite AgI Cryolite Na3AlF6 Cryptohalite 2SiF6 Dietzeite Ca22CrO4 Eglestonite Hg4OCl2 Embolite AgCl+AgBr Eriochalcite CuCl2·2H2O Fluorite CaF2 Halite NaCl Lautarite Ca2 Marshite CuI Miersite AgI Nantokite CuCl Sal Ammoniac NH4Cl Sylvite KCl Terlinguaite Hg2OCl Tolbachite CuCl2 Villaumite NaF Yttrocerite F2 Yttrofluorite F2 Many of these minerals are water-soluble and are found in arid areas in crusts and other deposits as are various borates, iodates and the like. Some, such as the fluorite group, are not water-soluble. All or most of simple halides of fluorine through iodine of all of the natural alkali metals and alkaline earth metals as well as numerous other metals and cations are found in some quantity at one or more locations.
More complex minerals as shown below are found. Two commercially important halide minerals are fluorite; the former is a major source of sodium chloride, in parallel with sodium chloride extracted from sea water or brine wells. Fluorite is a major source of hydrogen fluoride, complementing the supply obtained as a byproduct of the production of fertilizer. Carnallite and bischofite are important sources of magnesium. Natural cryolite was required for the production of aluminium, however most cryolite used is produced synthetically. Many of the halide minerals occur in marine evaporite deposits; the Atacama Desert has large quantities of halide minerals as well as chlorates, iodates and the like as well as nitrates and other water-soluble minerals—not only underground but it crusts on the surface due to the low rainfall—the Atacama is the world's driest desert as well as one of the oldest IMA-CNMNC proposes a new hierarchical scheme. This list uses the Classification of Nickel–Strunz. Abbreviations: "*" – discredited.
"?" – questionable/doubtful. "REE" – Rare-earth element "PGE" – Platinum-group element 03. C Aluminofluorides, 06 Borates, 08 Vanadates, 09 Silicates: Neso: insular Soro: grouping Cyclo: ring Ino: chain Phyllo: sheet Tekto: three-dimensional framework Nickel–Strunz code scheme: NN. XY.##x NN: Nickel–Strunz mineral class number X: Nickel–Strunz mineral division letter Y: Nickel–Strunz mineral family letter ##x: Nickel–Strunz mineral/group number, x add-on letter 03. A Simple halides, without H2O 03. AA M:X = 1:1, 2:3, 3:5, etc.: Panichiite. AB M:X = 1:2: 05 Tolbachite, 10 Coccinite, 15 Sellaite. AC M:X = 1:3: 05 Zharchikhite, 10 Molysite. B Simple Halides, with H2O 03. BA M:X = 1:1 and 2:3: 05 Hydrohalite, 10 Carnallite 03. BB M:X = 1:2: 05 Eriochalcite, 10 Rokuhnite, 15 Bischofite, 20 Nickelbischofite, 25 Sinjarite, 30 Antarcticite, 35 Tachyhydrite 03. BC M:X = 1:3: 05 Chloraluminite 03. BD Simple Halides with H2O and additional OH: 05 Cadwaladerite, 10 Lesukite, 15 Korshunovskite, 20 Nepskoeite, 25 Koenenite 03.
C Complex Halides 03. C: Steropesite, IMA2008-032, IMA2008-039 03. CA Borofluorides: 05 Ferruccite. CB Neso-aluminofluorides: 05 Cryolithionite. CC Soro-aluminofluorides: 05 Gearksutite. CD Ino-aluminofluorides: 05 Rosenbergite, 10 Prosopite 03. CE Phyllo-aluminofluorides: 05 Chiolite 03. CF Tekto-aluminofluorides: 05 Ralstonite, 10 Boldyrevite?, 15 Bogvadite 03. CG Aluminofluorides with CO3, SO4, PO4: 05 Stenonite. CH: 05 Malladrite, 10 Bararite. CJ With MX6 complexes. D Oxyhalides and Related Double Halides 03. DA With Cu, etc. without Pb: 05 Melanothallite. DB With Pb, Cu, etc.: 05 Diaboleite, 10 Pseudobole
Mercury is a chemical element with symbol Hg and atomic number 80. It is known as quicksilver and was named hydrargyrum. A heavy, silvery d-block element, mercury is the only metallic element, liquid at standard conditions for temperature and pressure. Mercury occurs in deposits throughout the world as cinnabar; the red pigment vermilion is obtained by synthetic mercuric sulfide. Mercury is used in thermometers, manometers, sphygmomanometers, float valves, mercury switches, mercury relays, fluorescent lamps and other devices, though concerns about the element's toxicity have led to mercury thermometers and sphygmomanometers being phased out in clinical environments in favor of alternatives such as alcohol- or galinstan-filled glass thermometers and thermistor- or infrared-based electronic instruments. Mechanical pressure gauges and electronic strain gauge sensors have replaced mercury sphygmomanometers. Mercury remains in use in scientific research applications and in amalgam for dental restoration in some locales.
It is used in fluorescent lighting. Electricity passed through mercury vapor in a fluorescent lamp produces short-wave ultraviolet light, which causes the phosphor in the tube to fluoresce, making visible light. Mercury poisoning can result from exposure to water-soluble forms of mercury, by inhalation of mercury vapor, or by ingesting any form of mercury. Mercury is a silvery-white liquid metal. Compared to other metals, it is a fair conductor of electricity, it has a freezing point of −38.83 °C and a boiling point of 356.73 °C, both the lowest of any stable metal, although preliminary experiments on copernicium and flerovium have indicated that they have lower boiling points. Upon freezing, the volume of mercury decreases by 3.59% and its density changes from 13.69 g/cm3 when liquid to 14.184 g/cm3 when solid. The coefficient of volume expansion is 181.59 × 10−6 at 0 °C, 181.71 × 10−6 at 20 °C and 182.50 × 10−6 at 100 °C. Solid mercury can be cut with a knife. A complete explanation of mercury's extreme volatility delves deep into the realm of quantum physics, but it can be summarized as follows: mercury has a unique electron configuration where electrons fill up all the available 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 5s, 5p, 5d, 6s subshells.
Because this configuration resists removal of an electron, mercury behaves to noble gases, which form weak bonds and hence melt at low temperatures. The stability of the 6s shell is due to the presence of a filled 4f shell. An f shell poorly screens the nuclear charge that increases the attractive Coulomb interaction of the 6s shell and the nucleus; the absence of a filled inner f shell is the reason for the somewhat higher melting temperature of cadmium and zinc, although both these metals still melt and, in addition, have unusually low boiling points. Mercury does not react with most acids, such as dilute sulfuric acid, although oxidizing acids such as concentrated sulfuric acid and nitric acid or aqua regia dissolve it to give sulfate and chloride. Like silver, mercury reacts with atmospheric hydrogen sulfide. Mercury reacts with solid sulfur flakes. Mercury dissolves many metals such as silver to form amalgams. Iron is an exception, iron flasks have traditionally been used to trade mercury.
Several other first row transition metals with the exception of manganese and zinc are resistant in forming amalgams. Other elements that do not form amalgams with mercury include platinum. Sodium amalgam is a common reducing agent in organic synthesis, is used in high-pressure sodium lamps. Mercury combines with aluminium to form a mercury-aluminium amalgam when the two pure metals come into contact. Since the amalgam destroys the aluminium oxide layer which protects metallic aluminium from oxidizing in-depth small amounts of mercury can corrode aluminium. For this reason, mercury is not allowed aboard an aircraft under most circumstances because of the risk of it forming an amalgam with exposed aluminium parts in the aircraft. Mercury embrittlement is the most common type of liquid metal embrittlement. There are seven stable isotopes of mercury, with 202Hg being the most abundant; the longest-lived radioisotopes are 194Hg with a half-life of 444 years, 203Hg with a half-life of 46.612 days. Most of the remaining radioisotopes have half-lives.
199Hg and 201Hg are the most studied NMR-active nuclei, having spins of 1⁄2 and 3⁄2 respectively. Hg is the modern chemical symbol for mercury, it comes from hydrargyrum, a Latinized form of the Greek word ὑδράργυρος, a compound word meaning "water-silver" – since it is liquid like water and shiny like silver. The element was named after the Roman god Mercury, known for his mobility, it is associated with the planet Mercury. Mercury is the only metal for which the al
Amino acids are organic compounds containing amine and carboxyl functional groups, along with a side chain specific to each amino acid. The key elements of an amino acid are carbon, hydrogen and nitrogen, although other elements are found in the side chains of certain amino acids. About 500 occurring amino acids are known and can be classified in many ways, they can be classified according to the core structural functional groups' locations as alpha-, beta-, gamma- or delta- amino acids. In the form of proteins, amino acid residues form the second-largest component of human muscles and other tissues. Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis. In biochemistry, amino acids having both the amine and the carboxylic acid groups attached to the first carbon atom have particular importance, they are known as α-amino acids. They include the 22 proteinogenic amino acids, which combine into peptide chains to form the building-blocks of a vast array of proteins.
These are all L-stereoisomers, although a few D-amino acids occur in bacterial envelopes, as a neuromodulator, in some antibiotics. Twenty of the proteinogenic amino acids are encoded directly by triplet codons in the genetic code and are known as "standard" amino acids; the other two are selenocysteine, pyrrolysine. Pyrrolysine and selenocysteine are encoded via variant codons. N-formylmethionine is considered as a form of methionine rather than as a separate proteinogenic amino acid. Codon–tRNA combinations not found in nature can be used to "expand" the genetic code and form novel proteins known as alloproteins incorporating non-proteinogenic amino acids. Many important proteinogenic and non-proteinogenic amino acids have biological functions. For example, in the human brain and gamma-amino-butyric acid are the main excitatory and inhibitory neurotransmitters. Hydroxyproline, a major component of the connective tissue collagen, is synthesised from proline. Glycine is a biosynthetic precursor to porphyrins used in red blood cells.
Carnitine is used in lipid transport. Nine proteinogenic amino acids are called "essential" for humans because they cannot be produced from other compounds by the human body and so must be taken in as food. Others may be conditionally essential for medical conditions. Essential amino acids may differ between species; because of their biological significance, amino acids are important in nutrition and are used in nutritional supplements, fertilizers and food technology. Industrial uses include the production of drugs, biodegradable plastics, chiral catalysts; the first few amino acids were discovered in the early 19th century. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound in asparagus, subsequently named asparagine, the first amino acid to be discovered. Cystine was discovered in 1810, although its monomer, remained undiscovered until 1884. Glycine and leucine were discovered in 1820; the last of the 20 common amino acids to be discovered was threonine in 1935 by William Cumming Rose, who determined the essential amino acids and established the minimum daily requirements of all amino acids for optimal growth.
The unity of the chemical category was recognized by Wurtz in 1865, but he gave no particular name to it. Usage of the term "amino acid" in the English language is from 1898, while the German term, Aminosäure, was used earlier. Proteins were found to yield amino acids after enzymatic acid hydrolysis. In 1902, Emil Fischer and Franz Hofmeister independently proposed that proteins are formed from many amino acids, whereby bonds are formed between the amino group of one amino acid with the carboxyl group of another, resulting in a linear structure that Fischer termed "peptide". In the structure shown at the top of the page, R represents a side chain specific to each amino acid; the carbon atom next to the carboxyl group is called the α–carbon. Amino acids containing an amino group bonded directly to the alpha carbon are referred to as alpha amino acids; these include amino acids such as proline which contain secondary amines, which used to be referred to as "imino acids". The alpha amino acids are the most common form found in nature, but only when occurring in the L-isomer.
The alpha carbon is a chiral carbon atom, with the exception of glycine which has two indistinguishable hydrogen atoms on the alpha carbon. Therefore, all alpha amino acids but glycine can exist in either of two enantiomers, called L or D amino acids, which are mirror images of each other. While L-amino acids represent all of the amino acids found in proteins during translation in the ribosome, D-amin
In crystallography, the terms crystal system, crystal family, lattice system each refer to one of several classes of space groups, point groups, or crystals. Informally, two crystals are in the same crystal system if they have similar symmetries, although there are many exceptions to this. Crystal systems, crystal families and lattice systems are similar but different, there is widespread confusion between them: in particular the trigonal crystal system is confused with the rhombohedral lattice system, the term "crystal system" is sometimes used to mean "lattice system" or "crystal family". Space groups and crystals are divided into seven crystal systems according to their point groups, into seven lattice systems according to their Bravais lattices. Five of the crystal systems are the same as five of the lattice systems, but the hexagonal and trigonal crystal systems differ from the hexagonal and rhombohedral lattice systems; the six crystal families are formed by combining the hexagonal and trigonal crystal systems into one hexagonal family, in order to eliminate this confusion.
A lattice system is a class of lattices with the same set of lattice point groups, which are subgroups of the arithmetic crystal classes. The 14 Bravais lattices are grouped into seven lattice systems: triclinic, orthorhombic, rhombohedral and cubic. In a crystal system, a set of point groups and their corresponding space groups are assigned to a lattice system. Of the 32 point groups that exist in three dimensions, most are assigned to only one lattice system, in which case both the crystal and lattice systems have the same name. However, five point groups are assigned to two lattice systems and hexagonal, because both exhibit threefold rotational symmetry; these point groups are assigned to the trigonal crystal system. In total there are seven crystal systems: triclinic, orthorhombic, trigonal and cubic. A crystal family is determined by lattices and point groups, it is formed by combining crystal systems which have space groups assigned to a common lattice system. In three dimensions, the crystal families and systems are identical, except the hexagonal and trigonal crystal systems, which are combined into one hexagonal crystal family.
In total there are six crystal families: triclinic, orthorhombic, tetragonal and cubic. Spaces with less than three dimensions have the same number of crystal systems, crystal families and lattice systems. In one-dimensional space, there is one crystal system. In 2D space, there are four crystal systems: oblique, rectangular and hexagonal; the relation between three-dimensional crystal families, crystal systems and lattice systems is shown in the following table: Note: there is no "trigonal" lattice system. To avoid confusion of terminology, the term "trigonal lattice" is not used; the 7 crystal systems consist of 32 crystal classes as shown in the following table: The point symmetry of a structure can be further described as follows. Consider the points that make up the structure, reflect them all through a single point, so that becomes; this is the'inverted structure'. If the original structure and inverted structure are identical the structure is centrosymmetric. Otherwise it is non-centrosymmetric.
Still in the non-centrosymmetric case, the inverted structure can in some cases be rotated to align with the original structure. This is a non-centrosymmetric achiral structure. If the inverted structure cannot be rotated to align with the original structure the structure is chiral or enantiomorphic and its symmetry group is enantiomorphic. A direction is called polar if its two directional senses are physically different. A symmetry direction of a crystal, polar is called a polar axis. Groups containing a polar axis are called polar. A polar crystal possesses a unique polar axis; some geometrical or physical property is different at the two ends of this axis: for example, there might develop a dielectric polarization as in pyroelectric crystals. A polar axis can occur only in non-centrosymmetric structures. There cannot be a mirror plane or twofold axis perpendicular to the polar axis, because they would make the two directions of the axis equivalent; the crystal structures of chiral biological molecules can only occur in the 65 enantiomorphic space groups.
The distribution of the 14 Bravais lattices into lattice systems and crystal families is given in the following table. In geometry and crystallography, a Bravais lattice is a category of translative symmetry groups in three directions; such symmetry groups consist of translations by vectors of the form R = n1a1 + n2a2 + n3a3,where n1, n2, n3 are integers and a1, a2, a3 are three non-coplanar vectors, called primitive vectors. These lattices are classified by the space group of the lattice itself, viewed as a collection of points, they represent the maximum symmetry. All crystalline materials must, by definition, fit into one of these arrangements. For convenience a Bravais lattice is depicted by a unit cell, a factor 1, 2, 3 or 4 larger than the primitive cell. Depending on the symmetry of a crystal or other pattern, the fundamental domain is again smaller, up to a factor 48; the Bravais lattices were studied by Moritz Ludwig Frankenheim in 1842, who found that there we
Sublimation (phase transition)
Sublimation is the transition of a substance directly from the solid to the gas phase, without passing through the intermediate liquid phase. Sublimation is an endothermic process that occurs at temperatures and pressures below a substance's triple point in its phase diagram, which corresponds to the lowest pressure at which the substance can exist as a liquid; the reverse process of sublimation is deposition or desublimation, in which a substance passes directly from a gas to a solid phase. Sublimation has been used as a generic term to describe a solid-to-gas transition followed by a gas-to-solid transition. While a transition from liquid to gas is described as evaporation if it occurs below the boiling point of the liquid, as boiling if it occurs at the boiling point, there is no such distinction within the solid-to-gas transition, always described as sublimation. At normal pressures, most chemical compounds and elements possess three different states at different temperatures. In these cases, the transition from the solid to the gaseous state requires an intermediate liquid state.
The pressure referred to is the partial pressure of the substance, not the total pressure of the entire system. So, all solids that possess an appreciable vapour pressure at a certain temperature can sublime in air. For some substances, such as carbon and arsenic, sublimation is much easier than evaporation from the melt, because the pressure of their triple point is high, it is difficult to obtain them as liquids; the term sublimation refers to a physical change of state and is not used to describe the transformation of a solid to a gas in a chemical reaction. For example, the dissociation on heating of solid ammonium chloride into hydrogen chloride and ammonia is not sublimation but a chemical reaction; the combustion of candles, containing paraffin wax, to carbon dioxide and water vapor is not sublimation but a chemical reaction with oxygen. Sublimation is caused by the absorption of heat which provides enough energy for some molecules to overcome the attractive forces of their neighbors and escape into the vapor phase.
Since the process requires additional energy, it is an endothermic change. The enthalpy of sublimation can be calculated by adding the enthalpy of fusion and the enthalpy of vaporization. Solid carbon dioxide sublimes everywhere along the line below the triple point (e.g. at the temperature of −78.5 °C at atmospheric pressure, whereas its melting into liquid CO2 can occur only along the line at pressures and temperatures above the triple point. Snow and ice sublime, although more at temperatures below the freezing/melting point temperature line at 0 °C for most pressures. In freeze-drying, the material to be dehydrated is frozen and its water is allowed to sublime under reduced pressure or vacuum; the loss of snow from a snowfield during a cold spell is caused by sunshine acting directly on the upper layers of the snow. Ablation is a process that includes erosive wear of glacier ice. Naphthalene, an organic compound found in pesticides such as mothballs, sublimes because it is made of non-polar molecules that are held together only by van der Waals intermolecular forces.
Naphthalene is a solid that sublimes at standard atmospheric temperature with the sublimation point at around 80 °C or 176 °F. At low temperature, its vapour pressure is high enough, 1 mmHg at 53 °C, to make the solid form of naphthalene evaporate into gas. On cool surfaces, the naphthalene vapours will solidify to form needle-like crystals. Iodine produces fumes on gentle heating, it is possible to obtain liquid iodine at atmospheric pressure by controlling the temperature at just above the melting point of iodine. In forensic science, iodine vapor can reveal latent fingerprints on paper. Arsenic can sublime at high temperatures. Cadmium and zinc are not suitable materials for use in vacuum because they sublimate much more than other common materials. Sublimation is a technique used by chemists to purify compounds. A solid is placed in a sublimation apparatus and heated under vacuum. Under this reduced pressure, the solid volatilizes and condenses as a purified compound on a cooled surface, leaving a non-volatile residue of impurities behind.
Once heating ceases and the vacuum is removed, the purified compound may be collected from the cooling surface. For higher purification efficiencies, a temperature gradient is applied, which allows for the separation of different fractions. Typical setups use an evacuated glass tube, heated in a controlled manner; the material flow is from the hot end, where the initial material is placed, to the cold end, connected to a pump stand. By controlling temperatures along the length of the tube, the operator can control the zones of re-condensation, with volatile compounds being pumped out of the system moderately volatile compounds re-condensing along the tube according to their different volatilities, non-volatile compounds remaining in the hot end. Vacuum sublimation of this type is the method of choice for purification of organic compounds for use in the organic electronics industry, where high purities are needed to satisfy the standards for consumer electronics and other applications. In ancient alchemy, a protoscience that contributed to the development of modern chemistry and medicine, alchemists developed a structure of basic laboratory techniques, theory and experimental methods.
Sublimation was used to refer to the process in which a
The chloride ion is the anion Cl−. It is formed when the element chlorine gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts such as sodium chloride are very soluble in water, it is an essential electrolyte located in all body fluids responsible for maintaining acid/base balance, transmitting nerve impulses and regulating fluid in and out of cells. Less the word chloride may form part of the "common" name of chemical compounds in which one or more chlorine atoms are covalently bonded. For example, methyl chloride, with the standard name chloromethane is an organic compound with a covalent C−Cl bond in which the chlorine is not an anion. A chloride ion is much larger than a chlorine atom, 99 pm, respectively; the ion is diamagnetic. In aqueous solution, it is soluble in most cases. In aqueous solution, chloride is bound by the protic end of the water molecules. Sea water contains 1.94% chloride. Some chloride-containing minerals include the chlorides of sodium and magnesium, hydrated MgCl2.
The concentration of chloride in the blood is called serum chloride, this concentration is regulated by the kidneys. A chloride ion is a structural component of e.g. it is present in the amylase enzyme. The chlor-alkali industry is a major consumer of the world's energy budget; this process converts sodium chloride into chlorine and sodium hydroxide, which are used to make many other materials and chemicals. The process involves two parallel reactions: 2 Cl− → Cl2 + 2 e− 2 H2O + 2 e− → H2 + 2 OH− Another major application involving chloride is desalination, which involves the energy intensive removal of chloride salts to give potable water. In the petroleum industry, the chlorides are a monitored constituent of the mud system. An increase of the chlorides in the mud system may be an indication of drilling into a high-pressure saltwater formation, its increase can indicate the poor quality of a target sand. Chloride is a useful and reliable chemical indicator of river / groundwater fecal contamination, as chloride is a non-reactive solute and ubiquitous to sewage & potable water.
Many water regulating companies around the world utilize chloride to check the contamination levels of the rivers and potable water sources. Chloride salts such as sodium chloride are used to preserve food; the presence of chlorides, e.g. in seawater aggravates the conditions for pitting corrosion of most metals by enhancing the formation and growth of the pits through an autocatalytic process. Chloride is an essential electrolyte, trafficking in and out of cells through chloride channels and playing a key role in maintaining cell homeostasis and transmitting action potentials in neurons. Characteristic concentrations of chloride in model organisms are: in both E. coli and budding yeast are 10-200mM, in mammalian cell 5-100mM and in blood plasma 100mM. Chloride can be oxidized but not reduced; the first oxidation, as employed in the chlor-alkali process, is conversion to chlorine gas. Chlorine can be further oxidized to other oxides and oxyanions including hypochlorite, chlorine dioxide and perchlorate.
In terms of its acid–base properties, chloride is a weak base as indicated by the negative value of the pKa of hydrochloric acid. Chloride can be protonated by strong acids, such as sulfuric acid: NaCl + H2SO4 → NaHSO4 + HClIonic chloride salts reaction with other salts to exchange anions; the presence of chloride is detected by its formation of an insoluble silver chloride upon treatment with silver ion: Cl− + Ag+ → AgClThe concentration of chloride in an assay can be determined using a chloridometer, which detects silver ions once all chloride in the assay has precipitated via this reaction. Chlorided silver electrodes are used in ex vivo electrophysiology. An example is table salt, sodium chloride with the chemical formula NaCl. In water, it dissociates into Na Cl − ions. Salts such as calcium chloride, magnesium chloride, potassium chloride have varied uses ranging from medical treatments to cement formation. Calcium chloride is a salt, marketed in pellet form for removing dampness from rooms.
Calcium chloride is used for maintaining unpaved roads and for fortifying roadbases for new construction. In addition, calcium chloride is used as a de-icer, since it is effective in lowering the melting point when applied to ice. Examples of covalently bonded chlorides are phosphorus trichloride, phosphorus pentachloride, thionyl chloride, all three of which are reactive chlorinating reagents that have been used in a laboratory. Chlorine can assume oxidation states of −1, +1, +3, +5, or +7. Several neutral chlorine oxides are known. Halide Renal chloride reabsorption
Ultraviolet designates a band of the electromagnetic spectrum with wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays. UV radiation is present in sunlight, contributes about 10% of the total light output of the Sun, it is produced by electric arcs and specialized lights, such as mercury-vapor lamps, tanning lamps, black lights. Although long-wavelength ultraviolet is not considered an ionizing radiation because its photons lack the energy to ionize atoms, it can cause chemical reactions and causes many substances to glow or fluoresce; the chemical and biological effects of UV are greater than simple heating effects, many practical applications of UV radiation derive from its interactions with organic molecules. Suntan and sunburn are familiar effects of over-exposure of the skin to UV, along with higher risk of skin cancer. Living things on dry land would be damaged by ultraviolet radiation from the Sun if most of it were not filtered out by the Earth's atmosphere.
More energetic, shorter-wavelength "extreme" UV below 121 nm ionizes air so that it is absorbed before it reaches the ground. Ultraviolet is responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans; the UV spectrum thus has effects both harmful to human health. The lower wavelength limit of human vision is conventionally taken as 400 nm, so ultraviolet rays are invisible to humans, although some people can perceive light at shorter wavelengths than this. Insects and some mammals can see near-UV. Ultraviolet rays are invisible to most humans; the lens of the human eye blocks most radiation in the wavelength range of 300–400 nm. Humans lack color receptor adaptations for ultraviolet rays; the photoreceptors of the retina are sensitive to near-UV, people lacking a lens perceive near-UV as whitish-blue or whitish-violet. Under some conditions and young adults can see ultraviolet down to wavelengths of about 310 nm. Near-UV radiation is visible to insects, some mammals, birds.
Small birds have a fourth color receptor for ultraviolet rays. "Ultraviolet" means "beyond violet", violet being the color of the highest frequencies of visible light. Ultraviolet has a higher frequency than violet light. UV radiation was discovered in 1801 when the German physicist Johann Wilhelm Ritter observed that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more than violet light itself, he called them "oxidizing rays" to emphasize chemical reactivity and to distinguish them from "heat rays", discovered the previous year at the other end of the visible spectrum. The simpler term "chemical rays" was adopted soon afterwards, remained popular throughout the 19th century, although some said that this radiation was different from light; the terms "chemical rays" and "heat rays" were dropped in favor of ultraviolet and infrared radiation, respectively. In 1878 the sterilizing effect of short-wavelength light by killing bacteria was discovered.
By 1903 it was known. In 1960, the effect of ultraviolet radiation on DNA was established; the discovery of the ultraviolet radiation with wavelengths below 200 nm, named "vacuum ultraviolet" because it is absorbed by the oxygen in air, was made in 1893 by the German physicist Victor Schumann. The electromagnetic spectrum of ultraviolet radiation, defined most broadly as 10–400 nanometers, can be subdivided into a number of ranges recommended by the ISO standard ISO-21348: A variety of solid-state and vacuum devices have been explored for use in different parts of the UV spectrum. Many approaches seek to adapt visible light-sensing devices, but these can suffer from unwanted response to visible light and various instabilities. Ultraviolet can be detected by suitable photodiodes and photocathodes, which can be tailored to be sensitive to different parts of the UV spectrum. Sensitive ultraviolet photomultipliers are available. Spectrometers and radiometers are made for measurement of UV radiation.
Silicon detectors are used across the spectrum. Vacuum UV, or VUV, wavelengths are absorbed by molecular oxygen in the air, though the longer wavelengths of about 150–200 nm can propagate through nitrogen. Scientific instruments can therefore utilize this spectral range by operating in an oxygen-free atmosphere, without the need for costly vacuum chambers. Significant examples include 193 nm photolithography equipment and circular dichroism spectrometers. Technology for VUV instrumentation was driven by solar astronomy for many decades. While optics can be used to remove unwanted visible light that contaminates the VUV, in general, detectors can be limited by their response to non-VUV radiation, the development of "solar-blind" devices has been an important area of research. Wide-gap solid-state devices or vacuum devices with high-cutoff photocathodes can be attractive compared to silicon diodes. Extreme UV is characterized by a transition in the physics of interaction with matter. Wavelengths longer than about 30 nm interact with the outer valence electrons of atoms, while wavelengths shorter than that interact with inner-shell electrons and nuclei.
The long end of the EUV spectrum is set by a prominent He+ spectr