An epoxide is a cyclic ether with a three-atom ring. This ring approximates an equilateral triangle, which makes it strained, hence reactive, more so than other ethers, they are produced on a large scale for many applications. In general, low molecular weight epoxides are colourless and nonpolar, volatile. A compound containing the epoxide functional group can be called an epoxy, epoxide and ethoxyline. Simple epoxides are referred to as oxides. Thus, the epoxide of ethylene is ethylene oxide. Many compounds have trivial names, ethylene oxide is called "oxirane"; some names emphasize the presence of the epoxide functional group, as in the compound 1,2-epoxyheptane, which can be called 1,2-heptene oxide. A polymer formed from epoxide precursors is called an epoxy, but such materials do not contain epoxide groups; the dominant epoxides industrially are ethylene oxide and propylene oxide, which are produced on the scales of 15 and 3 million tonnes/year. The epoxidation of ethylene involves its reaction of oxygen according to the following stoichiometry: 7 H2C=CH2 + 6 O2 → 6 C2H4O + 2 CO2 + 2 H2OThe direct reaction of oxygen with alkenes is useful only for this epoxide.
Modified heterogeneous silver catalysts are employed. Other alkenes fail to react usefully propylene, though TS-1 supported Au catalysts can perform propylene epoxidation selectively. Aside from ethylene oxide, most epoxides are generated by treating alkenes with peroxide-containing reagents, which donate a single oxygen atom. Safety considerations weigh on these reactions because organic peroxides are prone to spontaneous decomposition or combustion. Metal complexes are useful catalysts for epoxidations involving hydrogen peroxide and alkyl hydroperoxides. Peroxycarboxylic acids, which are more electrophilic, convert alkenes to epoxides without the intervention of metal catalysts. In specialized applications, other peroxide-containing reagents are employed, such as dimethyldioxirane. Depending on the mechanism of the reaction and the geometry of the alkene starting material, cis and/or trans epoxide diastereomers may be formed. In addition, if there are other stereocenters present in the starting material, they can influence the stereochemistry of the epoxidation.
Metal-catalyzed epoxidations were first explored using tert-butyl hydroperoxide. Association of TBHP with the metal generates the active metal peroxy complex containing the MOOR group, which transfers an O center to the alkene. Organic peroxides are used for the production of propylene oxide from propylene. Catalysts are required as well. Both t-butyl hydroperoxide and ethylbenzene hydroperoxide can be used as oxygen sources. More for laboratory operations, the Prilezhaev reaction is employed; this approach involves the oxidation of the alkene with a peroxyacid such as m-CPBA. Illustrative is the epoxidation of styrene with perbenzoic acid to styrene oxide: The reaction proceeds via what is known as the "Butterfly Mechanism"; the peroxide is viewed as an electrophile, the alkene a nucleophile. The reaction is considered to be concerted; the butterfly mechanism allows ideal positioning of the O-O sigma star orbital for C-C Pi electrons to attack. Because two bonds are broken and formed to the epoxide oxygen, this is formally an example of a coarctate transition state.
Hydroperoxides are employed in catalytic enantioselective epoxidations, such as the Sharpless epoxidation and the Jacobsen epoxidation. Together with the Shi epoxidation, these reactions are useful for the enantioselective synthesis of chiral epoxides. Oxaziridine reagents may be used to generate epoxides from alkenes. Arene oxides are intermediates in the oxidation of arenes by cytochrome P450. For prochiral arenes, the epoxides are obtained in high enantioselectivity. Chiral epoxides can be derived enantioselectively from prochiral alkenes. Many metal complexes give active catalysts, but the most important involve titanium and molybdenum; the Sharpless epoxidation reaction is one of the premier enantioselective chemical reactions. It is used to prepare 2,3-epoxyalcohols from secondary allylic alcohols; this method involves dehydrohalogenation. It is a variant of the Williamson ether synthesis. In this case, an alkoxide ion intramolecularly displaces chloride; the precursor compounds are called halohydrins.
Starting with propylene chlorohydrin, most of the world's supply of propylene oxide arises via this route. An intramolecular epoxide formation reaction is one of the key steps in the Darzens reaction. In the Johnson–Corey–Chaykovsky reaction epoxides are generated from carbonyl groups and sulfonium ylides. In this reaction, a sulfonium is the leaving group instead of chloride. Electron-deficient olefins, such as enones and acryl derivatives can be epoxidized using nucleophilic oxygen compounds such as peroxides; the reaction is a two-step mechanism. First the oxygen performs a nucleophilic conjugate addition to give a stabilized carbanion; this carbanion attacks the same oxygen atom, displacing a leaving group from it, to close the epoxide ring. Epoxides are uncommon in nature, they arise via oxygenation of alkenes by the action of cytochrome P450. Ring-opening reactions dominate the reactivity of epoxides. Alcohols, amines and many other reagents add to epoxides; this reaction is the basi
Tetrahydrofuran is an organic compound with the formula 4O. The compound is classified as heterocyclic compound a cyclic ether, it is a water-miscible organic liquid with low viscosity. It is used as a precursor to polymers. Being polar and having a wide liquid range, THF is a versatile solvent. About 200,000 tonnes of tetrahydrofuran are produced annually; the most used industrial process involves the acid-catalyzed dehydration of 1,4-butanediol. Ashland/ISP is one the biggest producers of this chemical route; the method is similar to the production of diethyl ether from ethanol. The butanediol is derived from condensation of acetylene with formaldehyde followed by hydrogenation. DuPont developed a process for producing THF by oxidizing n-butane to crude maleic anhydride, followed by catalytic hydrogenation. A third major industrial route entails hydroformylation of allyl alcohol followed by hydrogenation to 1,4-butanediol. THF can be synthesized by catalytic hydrogenation of furan. Certain sugars can be converted to THF, although this method is not practiced.
Furan is thus derivable from renewable resources. In the presence of strong acids, THF converts to a linear polymer called poly glycol known as polytetramethylene oxide: n C4H8O → −n−This polymer is used to make elastomeric polyurethane fibers like Spandex; the other main application of THF is as an industrial solvent for polyvinyl chloride and in varnishes. It is an aprotic solvent with a dielectric constant of 7.6. It is a moderately polar solvent and can dissolve a wide range of nonpolar and polar chemical compounds. THF is water-miscible and can form solid clathrate hydrate structures with water at low temperatures. THF has been explored as a miscible co-solvent in aqueous solution to aid in the liquefaction and delignification of plant lignocellulosic biomass for production of renewable platform chemicals and sugars as potential precursors to biofuels. Aqueous THF augments the hydrolysis of glycans from biomass and dissolves the majority of biomass lignin making it a suitable solvent for biomass pretreatment.
THF is used in polymer science. For example, it can be used to dissolve polymers prior to determining their molecular mass using gel permeation chromatography. THF dissolves PVC as well, thus it is the main ingredient in PVC adhesives, it can be used to liquefy old PVC cement and is used industrially to degrease metal parts. THF is used as a component in mobile phases for reversed-phase liquid chromatography, it has a greater elution strength than methanol or acetonitrile, but is less used than these solvents. THF is used as a solvent in 3D printing, it can be used to clean clogged 3D printer parts, as well as when finishing prints to remove extruder lines and add a shine to the finished product. In the laboratory, THF is a popular solvent, it is more basic than diethyl ether and forms stronger complexes with Li+, Mg2+, boranes. It is a popular solvent for hydroboration reactions and for organometallic compounds such as organolithium and Grignard reagents. Although similar to diethyl ether, THF is a stronger base.
Thus, while diethyl ether remains the solvent of choice for some reactions, THF fills that role in many others, where strong coordination is desirable and the precise properties of ethereal solvents such as these allows fine-tuning modern chemical reactions. Commercial THF contains substantial water that must be removed for sensitive operations, e.g. those involving organometallic compounds. Although THF is traditionally dried by distillation from an aggressive desiccant, molecular sieves are superior. THF is a weak Lewis base. Typical complexes are of the stoichiometry MCl33; such compounds are used reagents. In the presence of a solid acid catalyst, THF reacts with hydrogen sulfide to give tetrahydrothiophene. THF is a nontoxic solvent, with the median lethal dose comparable to that for acetone. Reflecting its remarkable solvent properties, it penetrates the skin. THF dissolves latex and is handled with nitrile or neoprene rubber gloves, it is flammable. One danger posed by THF follows from its tendency to form explosive peroxides on storage in air.
To minimize this problem, commercial samples of THF are inhibited with butylated hydroxytoluene. Distillation of THF to dryness is avoided because the explosive peroxides concentrate in the residue. Polytetrahydrofuran 2-Methyltetrahydrofuran Trapp mixture Other cyclic ethers: oxirane, oxane Loudon, G. Mark. Organic Chemistry. New York: Oxford University Press. P. 318. ISBN 9780981519432. International Chemical Safety Card 0578 NIOSH Pocket Guide to Chemical Hazards U. S. OSHA info on THF "2-Methyltetrahydrofuran, An alternative to Tetrahydrofuran and Dichloromethane". Sigma-Aldrich. Retrieved 2007-05-23
Tin is a chemical element with the symbol Sn and atomic number 50. It is a post-transition metal in group 14 of the periodic table of elements, it is obtained chiefly from the mineral cassiterite, which contains stannic oxide, SnO2. Tin shows a chemical similarity to both of its neighbors in group 14, germanium and lead, has two main oxidation states, +2 and the more stable +4. Tin is the 49th most abundant element and has, with 10 stable isotopes, the largest number of stable isotopes in the periodic table, thanks to its magic number of protons, it has two main allotropes: at room temperature, the stable allotrope is β-tin, a silvery-white, malleable metal, but at low temperatures it transforms into the less dense grey α-tin, which has the diamond cubic structure. Metallic tin does not oxidize in air; the first tin alloy used on a large scale was bronze, made of 1/8 tin and 7/8 copper, from as early as 3000 BC. After 600 BC, pure metallic tin was produced. Pewter, an alloy of 85–90% tin with the remainder consisting of copper and lead, was used for flatware from the Bronze Age until the 20th century.
In modern times, tin is used in many alloys, most notably tin/lead soft solders, which are 60% or more tin, in the manufacture of transparent, electrically conducting films of indium tin oxide in optoelectronic applications. Another large application for tin is corrosion-resistant tin plating of steel; because of the low toxicity of inorganic tin, tin-plated steel is used for food packaging as tin cans. However, some organotin compounds can be as toxic as cyanide. Tin is a soft, malleable and crystalline silvery-white metal; when a bar of tin is bent, a crackling sound known as the "tin cry" can be heard from the twinning of the crystals. Tin melts at low temperatures of about 232 °C, the lowest in group 14; the melting point is further lowered to 177.3 °C for 11 nm particles. Β-tin, stable at and above room temperature, is malleable. In contrast, α-tin, stable below 13.2 °C, is brittle. Α-tin has a diamond cubic crystal structure, similar to silicon or germanium. Α-tin has no metallic properties at all because its atoms form a covalent structure in which electrons cannot move freely.
It is a dull-gray powdery material with no common uses other than a few specialized semiconductor applications. These two allotropes, α-tin and β-tin, are more known as gray tin and white tin, respectively. Two more allotropes, γ and σ, exist at temperatures above 161 pressures above several GPa. In cold conditions, β-tin tends to transform spontaneously into α-tin, a phenomenon known as "tin pest". Although the α-β transformation temperature is nominally 13.2 °C, impurities lower the transition temperature well below 0 °C and, on the addition of antimony or bismuth, the transformation might not occur at all, increasing the durability of the tin. Commercial grades of tin resist transformation because of the inhibiting effect of the small amounts of bismuth, antimony and silver present as impurities. Alloying elements such as copper, bismuth and silver increase its hardness. Tin tends rather to form hard, brittle intermetallic phases, which are undesirable, it does not form wide solid solution ranges in other metals in general, few elements have appreciable solid solubility in tin.
Simple eutectic systems, occur with bismuth, lead and zinc. Tin was one of the first superconductors to be studied. Tin can be attacked by acids and alkalis. Tin can be polished and is used as a protective coat for other metals. A protective oxide layer prevents further oxidation, the same that forms on pewter and other tin alloys. Tin helps to accelerate the chemical reaction. Tin has ten stable isotopes, with atomic masses of 112, 114 through 120, 122 and 124, the greatest number of any element. Of these, the most abundant are 120Sn, 118Sn, 116Sn, while the least abundant is 115Sn; the isotopes with mass numbers have no nuclear spin, while those with odd have a spin of +1/2. Tin, with its three common isotopes 116Sn, 118Sn and 120Sn, is among the easiest elements to detect and analyze by NMR spectroscopy, its chemical shifts are referenced against SnMe4; this large number of stable isotopes is thought to be a direct result of the atomic number 50, a "magic number" in nuclear physics. Tin occurs in 29 unstable isotopes, encompassing all the remaining atomic masses from 99 to 137.
Apart from 126Sn, with a half-life of 230,000 years, all the radioisotopes have a half-life of less than a year. The radioactive 100Sn, discovered in 1994, 132Sn are one of the few nuclides with a "doubly magic" nucleus: despite being unstable, having lopsided proton–neutron ratios, they represent endpoints beyond which stability drops off rapidly. Another 30 metastable isomers have been characterized for isotopes between 111 and 131, the most stable being 121mSn with a half-life of 43.9 years. The relative differences in the abundances of tin's stable isotopes can be explained by their different modes of formation in stellar nucleosynthesis. 116Sn through 120Sn inclusive are formed in the s-process in most stars and hence they are the most common isotopes, while 122Sn and 124Sn are only formed in the r-process (rapid neutr
Silicon is a chemical element with symbol Si and atomic number 14. It is a brittle crystalline solid with a blue-grey metallic lustre, it is a member of group 14 in the periodic table: carbon is above it. It is unreactive; because of its high chemical affinity for oxygen, it was not until 1823 that Jöns Jakob Berzelius was first able to prepare it and characterize it in pure form. Its melting and boiling points of 1414 °C and 3265 °C are the second-highest among all the metalloids and nonmetals, being only surpassed by boron. Silicon is the eighth most common element in the universe by mass, but rarely occurs as the pure element in the Earth's crust, it is most distributed in dusts, sands and planets as various forms of silicon dioxide or silicates. More than 90% of the Earth's crust is composed of silicate minerals, making silicon the second most abundant element in the Earth's crust after oxygen. Most silicon is used commercially without being separated, with little processing of the natural minerals.
Such use includes industrial construction with clays, silica sand, stone. Silicates are used in Portland cement for mortar and stucco, mixed with silica sand and gravel to make concrete for walkways and roads, they are used in whiteware ceramics such as porcelain, in traditional quartz-based soda-lime glass and many other specialty glasses. Silicon compounds such as silicon carbide are used as abrasives and components of high-strength ceramics. Silicon is the basis of the used synthetic polymers called silicones. Elemental silicon has a large impact on the modern world economy. Most free silicon is used in the steel refining, aluminium-casting, fine chemical industries. More visibly, the small portion of highly purified elemental silicon used in semiconductor electronics is essential to integrated circuits – most computers, cell phones, modern technology depend on it. Silicon is an essential element in biology. However, various sea sponges and microorganisms, such as diatoms and radiolaria, secrete skeletal structures made of silica.
Silica is deposited in many plant tissues. In 1787 Antoine Lavoisier suspected that silica might be an oxide of a fundamental chemical element, but the chemical affinity of silicon for oxygen is high enough that he had no means to reduce the oxide and isolate the element. After an attempt to isolate silicon in 1808, Sir Humphry Davy proposed the name "silicium" for silicon, from the Latin silex, silicis for flint, adding the "-ium" ending because he believed it to be a metal. Most other languages use transliterated forms of Davy's name, sometimes adapted to local phonology. A few others use instead a calque of the Latin root. Gay-Lussac and Thénard are thought to have prepared impure amorphous silicon in 1811, through the heating of isolated potassium metal with silicon tetrafluoride, but they did not purify and characterize the product, nor identify it as a new element. Silicon was given its present name in 1817 by Scottish chemist Thomas Thomson, he retained part of Davy's name but added "-on" because he believed that silicon was a nonmetal similar to boron and carbon.
In 1823, Jöns Jacob Berzelius prepared amorphous silicon using the same method as Gay-Lussac, but purifying the product to a brown powder by washing it. As a result, he is given credit for the element's discovery; the same year, Berzelius became the first to prepare silicon tetrachloride. Silicon in its more common crystalline form was not prepared until 31 years by Deville. By electrolyzing a mixture of sodium chloride and aluminium chloride containing 10% silicon, he was able to obtain a impure allotrope of silicon in 1854. More cost-effective methods have been developed to isolate several allotrope forms, the most recent being silicene in 2010. Meanwhile, research on the chemistry of silicon continued; the first organosilicon compound, was synthesised by Charles Friedel and James Crafts in 1863, but detailed characterisation of organosilicon chemistry was only done in the early 20th century by Frederic Kipping. Starting in the 1920s, the work of William Lawrence Bragg on X-ray crystallography elucidated the compositions of the silicates, known from analytical chemistry but had not yet been understood, together with Linus Pauling's development of crystal chemistry and Victor Goldschmidt's development of geochemistry.
The middle of the 20th century saw the development of the chemistry and industrial use of siloxanes and the growing use of silicone polymers and resins. In the late 20th century, the complexity of the crystal chemistry of silicides was mapped, along with the solid-state chemistry of doped semiconductors; because silicon is an important element in high-technology semiconductor devi
Diazirines are a class of organic molecules consisting of a carbon bound to two nitrogen atoms, which are double-bonded to each other, forming a cyclopropene-like ring, 3H-diazirene. Upon irradiation with ultraviolet light, diazirines form reactive carbenes, which can insert into C-H, N-H, O-H bonds. Hence, diazirines have grown in popularity as small photo-reactive crosslinking reagents, they are used in photoaffinity labeling studies to observe a variety of interactions, including ligand-receptor, ligand-enzyme, protein-protein, protein-nucleic acid interactions. A number of methods exist in the literature for the preparation of diazirines, which begin from a variety of reagents. Synthetic schemes that begin with ketones involve conversion of the ketone with the desired substituents to diaziridines; these diaziridenes are subsequently oxidized to form the desired diazirines. Diaziridines can be prepared from ketones by oximation, followed by tosylation, finally by treatment with ammonia. Oximation reactions are performed by reacting the ketone with hydroxylammonium chloride under heat in the presence of a base such as pyridine.
Subsequent tosylation or mesylation of the alpha substituted oxygen with tosyl or mesyl chloride in the presence of base yields the tosyl or mesyl oxime. The final treatment of the tosyl or mesyl oxime with ammonia produces the diaziridine. Diaziridines can be produced directly by the reaction of ketones with ammonia in the presence of an aminating agent such as a chloramine or hydroxyl amine O-sulfonic acid. Diaziridines can be oxidized to diazirines by a number of methods; these include oxidation by chromium based reagents such as the Jones oxidation, oxidation by iodine and triethylamine, oxidation by silver oxide, oxidation by oxalyl chloride, or electrochemical oxidation on a platinum-titanium anode. Diazirines can be alternatively formed in a one-pot process using the Graham reaction. In these schemes, amidines can be directly converted to diazirines by hypohalite oxidation; this reaction yields a halogenated diazirine. The resulting aforementioned halodiazirine can undergo an exchange reaction to further functionalize the diazirene.
In these reactions, anion nucleophiles, such as tetra-n-butylammonium fluoride or methoxytetra-n-butylammonium, can replace the halogen substituents yielding a fluorodiazirine or methoxydiazirine respectively. Upon irradiation with UV light, diazirines form reactive carbene species; the carbene may exist in the singlet form, in which the two free electrons occupy the same orbital, or the triplet form, with two unpaired electrons in different orbitals. The substituents on the diazirine affect which carbene species is generated upon irradiation and subsequent photolytic cleavage. Diazirine substituents that are electron donating in nature can donate electron density to the empty p-orbital of the carbene that will be formed, hence can stabilize the singlet state. For example, phenyldiazirine produces phenylcarbene in the singlet carbene state whereas p-nitrophenylchlorodiazirine or trifluorophenyldiazirine produce the respective triplet carbene products. Electron donating substituents can encourage photoisomerization to the linear diazo compound, rather than the singlet carbene, hence these compounds are unfavorable for use in biological assays.
On the other hand, trifluoroaryldiazirines in particular show favorable stability and photolytic qualities and are most used in biological applications. Carbenes produced from diazirines are quenched by reaction with water molecules, hence yields for photoreactive crosslinking assays are low. Yet, as this feature minimizes unspecific labeling, it is an advantage of using diazirines. Diazirines are used as photoreactive crosslinking reagents, as the reactive carbenes they form upon irradiation with UV light can insert into C-H, N-H, O-H bonds; this results in proximity dependent labeling of other species with the diazirine containing compound. Diazirines are preferred to other photoreactive crosslinking reagents due to their smaller size, longer irradiation wavelength, short period of irradiation required, stability in the presence of various nucleophiles, in both acidic and basic conditions. Benzophenones, which form reactive triplet carbonyl species upon irradiation require long periods of irradiation which can result in non-specific labeling, moreover are inert to various polar solvents.
Aryl azides require a low wavelength of irradiation, which can damage the biological macromolecules under investigation. Diazirines are used in receptor labeling studies; this is because diazirine-containing analogs of various ligands can be synthesized and incubated with their respective receptors, subsequently exposed to light to produce reactive carbenes. The carbene will covalently bond to residues in the binding site of the receptor; the carbene compound may include a bioorthogonal tag or handle by which the protein of interest can be isolated. The protein can be digested and sequenced by mass spectrometry in order to the identity which residues the carbene containing ligand is bound to, hence the identity of the binding site in the receptor. Examples of diazirines used in receptor labeling studies include: The discovery of a brassinosteroid receptor for brassinosteroid plant hormones by Kinoshita et al. Researchers used a plant hormone analog with a diazirine crosslinking moiety and a biotin tag for isolation to identity the new receptor.
This study led to a number of similar studies conducted with regards to other plant hormones. The discovery of novel non-CB1/CB2 cannabinoid receptors using anandamide analog probes containing a diazirine group by Balas et al; the binding cavity
A chemical element is a species of atom having the same number of protons in their atomic nuclei. For example, the atomic number of oxygen is 8, so the element oxygen consists of all atoms which have 8 protons. 118 elements have been identified, of which the first 94 occur on Earth with the remaining 24 being synthetic elements. There are 80 elements that have at least one stable isotope and 38 that have radionuclides, which decay over time into other elements. Iron is the most abundant element making up Earth, while oxygen is the most common element in the Earth's crust. Chemical elements constitute all of the ordinary matter of the universe; however astronomical observations suggest that ordinary observable matter makes up only about 15% of the matter in the universe: the remainder is dark matter. The two lightest elements and helium, were formed in the Big Bang and are the most common elements in the universe; the next three elements were formed by cosmic ray spallation, are thus rarer than heavier elements.
Formation of elements with from 6 to 26 protons occurred and continues to occur in main sequence stars via stellar nucleosynthesis. The high abundance of oxygen and iron on Earth reflects their common production in such stars. Elements with greater than 26 protons are formed by supernova nucleosynthesis in supernovae, when they explode, blast these elements as supernova remnants far into space, where they may become incorporated into planets when they are formed; the term "element" is used for atoms with a given number of protons as well as for a pure chemical substance consisting of a single element. For the second meaning, the terms "elementary substance" and "simple substance" have been suggested, but they have not gained much acceptance in English chemical literature, whereas in some other languages their equivalent is used. A single element can form multiple substances differing in their structure; when different elements are chemically combined, with the atoms held together by chemical bonds, they form chemical compounds.
Only a minority of elements are found uncombined as pure minerals. Among the more common of such native elements are copper, gold and sulfur. All but a few of the most inert elements, such as noble gases and noble metals, are found on Earth in chemically combined form, as chemical compounds. While about 32 of the chemical elements occur on Earth in native uncombined forms, most of these occur as mixtures. For example, atmospheric air is a mixture of nitrogen and argon, native solid elements occur in alloys, such as that of iron and nickel; the history of the discovery and use of the elements began with primitive human societies that found native elements like carbon, sulfur and gold. Civilizations extracted elemental copper, tin and iron from their ores by smelting, using charcoal. Alchemists and chemists subsequently identified many more; the properties of the chemical elements are summarized in the periodic table, which organizes the elements by increasing atomic number into rows in which the columns share recurring physical and chemical properties.
Save for unstable radioactive elements with short half-lives, all of the elements are available industrially, most of them in low degrees of impurities. The lightest chemical elements are hydrogen and helium, both created by Big Bang nucleosynthesis during the first 20 minutes of the universe in a ratio of around 3:1 by mass, along with tiny traces of the next two elements and beryllium. All other elements found in nature were made by various natural methods of nucleosynthesis. On Earth, small amounts of new atoms are produced in nucleogenic reactions, or in cosmogenic processes, such as cosmic ray spallation. New atoms are naturally produced on Earth as radiogenic daughter isotopes of ongoing radioactive decay processes such as alpha decay, beta decay, spontaneous fission, cluster decay, other rarer modes of decay. Of the 94 occurring elements, those with atomic numbers 1 through 82 each have at least one stable isotope. Isotopes considered stable are those. Elements with atomic numbers 83 through 94 are unstable to the point that radioactive decay of all isotopes can be detected.
Some of these elements, notably bismuth and uranium, have one or more isotopes with half-lives long enough to survive as remnants of the explosive stellar nucleosynthesis that produced the heavy metals before the formation of our Solar System. At over 1.9×1019 years, over a billion times longer than the current estimated age of the universe, bismuth-209 has the longest known alpha decay half-life of any occurring element, is always considered on par with the 80 stable elements. The heaviest elements undergo radioactive decay with half-lives so short that they are not found in nature and must be synthesized; as of 2010, there are 118 known elements (in this context, "known" means observed well enough from just a few de
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table, a reactive nonmetal, an oxidizing agent that forms oxides with most elements as well as with other compounds. By mass, oxygen is the third-most abundant element in the universe, after helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. As compounds including oxides, the element makes up half of the Earth's crust. Dioxygen is used in cellular respiration and many major classes of organic molecules in living organisms contain oxygen, such as proteins, nucleic acids and fats, as do the major constituent inorganic compounds of animal shells and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide.
Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation. However, ozone present at the surface is a byproduct of thus a pollutant. Oxygen was isolated by Michael Sendivogius before 1604, but it is believed that the element was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, Joseph Priestley in Wiltshire, in 1774. Priority is given for Priestley because his work was published first. Priestley, called oxygen "dephlogisticated air", did not recognize it as a chemical element; the name oxygen was coined in 1777 by Antoine Lavoisier, who first recognized oxygen as a chemical element and characterized the role it plays in combustion. Common uses of oxygen include production of steel and textiles, brazing and cutting of steels and other metals, rocket propellant, oxygen therapy, life support systems in aircraft, submarines and diving.
One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle and surrounding the vessel's neck with water resulted in some water rising into the neck. Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries Leonardo da Vinci built on Philo's work by observing that a portion of air is consumed during combustion and respiration. In the late 17th century, Robert Boyle proved. English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. In one experiment, he found that placing either a mouse or a lit candle in a closed container over water caused the water to rise and replace one-fourteenth of the air's volume before extinguishing the subjects.
From this he surmised that nitroaereus is consumed in both combustion. Mayow observed that antimony increased in weight when heated, inferred that the nitroaereus must have combined with it, he thought that the lungs separate nitroaereus from air and pass it into the blood and that animal heat and muscle movement result from the reaction of nitroaereus with certain substances in the body. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract "De respiratione". Robert Hooke, Ole Borch, Mikhail Lomonosov, Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element; this may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, modified by the chemist Georg Ernst Stahl by 1731, phlogiston theory stated that all combustible materials were made of two parts.
One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. Combustible materials that leave little residue, such as wood or coal, were thought to be made of phlogiston. Air did not play a role in phlogiston theory, nor were any initial quantitative experiments conducted to test the idea. Polish alchemist and physician Michael Sendivogius in his work De Lapide Philosophorum Tractatus duodecim e naturae fonte et manuali experientia depromti described a substance contained in air, referring to it as'cibus vitae', this substance is identical with oxygen. Sendivogius, during his experiments performed between 1598 and 1604, properly recognized that the substance is equivalent to the gaseous byproduct released by the thermal decomposition of potassium nitrate. In Bugaj’s view, the isolation of oxygen and the proper association of the substance to that part of air, required for life, lends sufficient weight to the discovery of oxygen by Sendivogius.