Community and Environment
Web Designers in Marrakech. No Fault Accident Claim. Cela montre qu'il n'est pas utile d'établir des catégories rigides pour distinguer ce qui est nouveau de ce qui ne l'est pas. Size 0 00 empty capsules. Lose Weight Fast Tips.
You and Your Family
Some poorly crystalline forms of quartz are also known, such as chalcedony , chrysoprase , carnelian , agate , onyx , jasper , heliotrope , and flint. Other modifications of silicon dioxide are known in some other minerals such as tridymite and cristobalite , as well as the much less common coesite and stishovite.
Biologically generated forms are also known as kieselguhr and diatomaceous earth. Vitreous silicon dioxide is known as tektites , and obsidian , and rarely as lechatelierite. Some synthetic forms are known as keatite and W-silica. Opals are composed of complicated crystalline aggregates of partially hydrated silicon dioxide.
Other high-pressure forms of silica are known, such as coesite and stishovite: Similar melting and cooling of silica occurs following lightning strikes, forming glassy lechatelierite. Silica is rather inert chemically. It is not attacked by any acids other than hydrofluoric acid. However, it slowly dissolves in hot concentrated alkalis, and does so rather quickly in fused metal hydroxides or carbonates to give metal silicates.
Among the elements, it is attacked only by fluorine at room temperature to form silicon tetrafluoride: Silica nevertheless reacts with many metal and metalloid oxides to form a wide variety of compounds important in the glass and ceramic industries above all, but also have many other uses: Increasing water concentration results in the formation of hydrated silica gels and colloidal silica dispersions.
Hence, although some simple silicic acids have been identified in dilute solutions, such as orthosilicic acid Si OH 4 and metasilicic acid SiO OH 2 , none of these are likely to exist in the solid state.
Silicate and aluminosilicate minerals have many different structures and varying stoichiometry, but they may be classified following some general principles. The lattice of oxygen atoms that results is usually close-packed or close to it, with the charge being balanced by other cations in various different polyhedral sites according to size.
Be 2 SiO 4 phenacite is rather unusual as both Be II and Si IV occupy tetrahedral four-coordinated sites; the other divalent cations instead occupy six-coordinated octahedral sites and often isomorphously replace each other as in olivine , Mg,Fe,Mn 2 SiO 4. Ca, Mg, Fe are eight-coordinated and the trivalent ones are six-coordinated e.
Regular coordination is not always present: Soro -silicates, involving discrete double or triple tetrahedral units, are quite rare: Many differences arise due to the differing repeat distances of conformation across the line of tetrahedra. A repeat distance of two is most common, as in most pyroxene minerals, but repeat distances of one, three, four, five, six, seven, nine, and twelve are also known. These chains can then link across each other to form double chains and ribbons, as in the asbestos minerals, involving repeated chains of cyclic tetrahedron rings.
Layer silicates, such as the clay minerals and the micas , are very common, and are often formed by horizontal cross-linking of metasilicate chains or planar condensation of smaller units. Three-dimensional framework aluminosilicates are structurally very complex; they may be conceived of as starting from the SiO 2 structure, but having replaced up to one-half of the Si IV atoms with Al III they require more cations to be included in the structure to balance charge.
Examples include feldspars the most abundant minerals on the Earth , zeolites , and ultramarines. Zeolites have many polyhedral cavities in their frameworks truncated cuboctahedra being most common, but other polyhedra are also known as zeolite cavities , allowing them to include loosely bound molecules such as water in their structure.
However, SiS 2 lacks the variety of structures of SiO 2 , and quickly hydrolyses to silica and hydrogen sulfide. It is also ammonoloysed quickly and completely by liquid ammonia as follows to form an imide: It reacts with the sulfides of sodium, magnesium, aluminium, and iron to form metal thiosilicates: Ethylsilicate is useful as its controlled hydrolysis produces adhesive or film-like forms of silica.
It would make a promising ceramic if not for the difficulty of working with and sintering it: Reacting silyl halides with ammonia or alkylammonia derivatives in the gaseous phase or in ethanolic solution produces various volatile silylamides, which are silicon analogues of the amines: Many such compounds have been prepared, the only known restriction being that the nitrogen is always tertiary, and species containing the SiH—NH group are unstable at room temperature.
Similarly, trisilylamines are weaker as ligands than their carbon analogues, the tertiary amines , although substitution of some SiH 3 groups by CH 3 groups mitigates this weakness.
Silicon carbide SiC was first made by Edward Goodrich Acheson in , who named it carborundum to reference its intermediate hardness and abrasive power between diamond an allotrope of carbon and corundum aluminium oxide. He soon founded a company to manufacture it, and today about one million tonnes are produced each year. They are variations of the same chemical compound that are identical in two dimensions and differ in the third.
Thus, they can be viewed as layers stacked in a certain sequence. It is resistant to most aqueous acids, phosphoric acid being an exception. It is mostly used as an abrasive and a refractory materia, as it is chemically stable and very strong, and it fractures to form a very sharp cutting edge. It is also useful as an intrinsic semiconductor, as well as an extrinsic semiconductor upon being doped.
Because the Si—C bond is close in strength to the C—C bond, organosilicon compounds tend to be markedly thermally and chemically stable. Furthermore, since carbon and silicon are chemical congeners, organosilicon chemistry shows some significant similarities with carbon chemistry, for example in the propensity of such compounds for catenation and forming multiple bonds. Thus the Si—F bond is significantly stronger than even the C—F bond and is one of the strongest single bonds, while the Si—H bond is much weaker than the C—H bond and is readily broken.
Furthermore, the ability of silicon to expand its octet is not shared by carbon, and hence some organosilicon reactions have no organic analogues. For example, nucleophilic attack on silicon does not proceed by the S N 2 or S N 1 processes, but instead goes through a negatively charged true pentacoordinate intermediate and appears like a substitution at a hindered tertiary atom.
Nevertheless, despite these differences, the mechanism is still often called "S N 2 at silicon" for simplicity. One of the most useful silicon-containing groups is trimethylsilyl , Me 3 Si—. The Si—C bond connecting it to the rest of the molecule is reasonably strong, allowing it to remain while the rest of the molecule undergoes reactions, but is not so strong that it cannot be removed specifically when needed, for example by the fluoride ion, which is a very weak nucleophile for carbon compounds but a very strong one for organosilicon compounds.
It may be compared to acidic protons ; while trisilylmethyl is removed by hard nucleophiles instead of bases, both removals usually promote elimination. As a general rule, while saturated carbon is best attacked by nucleophiles that are neutral compounds, those based on nonmetals far down on the periodic table e. For example, enolates react at the carbon in haloalkanes , but at the oxygen in silyl chlorides; and when trimethylsilyl is removed from an organic molecule using hydroxide as a nucleophile, the product of the reaction is not the silanol as one would expect from using carbon chemistry as an analogy, because the siloxide is strongly nucleophilic and attacks the original molecule to yield the silyl ether hexamethyldisiloxane , Me 3 Si 2 O.
Thus, for example, the silyl triflates are so electrophilic that they react 10 8 to 10 9 times faster than silyl chlorides with oxygen-containing nucleophiles.
Trimethylsilyl triflate is in particular a very good Lewis acid and is used to convert carbonyl compounds to acetals and silyl enol ethers , reacting them together analogously to the aldol reaction. Si—C bonds are commonly formed in three ways. The second route has the drawback of not being applicable to the most important silanes, the methyl and phenyl silanes. Standard organic reactions suffice to produce many derivatives; the resulting organosilanes are often significantly more reactive than their carbon congeners, readily undergoing hydrolysis, ammonolysis, alcoholysis, and condensation to form cyclic oligomers or linear polymers.
The word "silicone" was first used by Frederic Kipping in He invented the word to illustrate the similarity of chemical formulae between Ph 2 SiO and benzophenone , Ph 2 CO, although he also stressed the lack of chemical resemblance due to the polymeric structure of Ph 2 SiO, which is not shared by Ph 2 CO.
Furthermore, they are resistant over long periods of time to ultraviolet radiation and weathering, and are inert physiologically. They are fairly unreactive, but do react with concentrated solutions bearing the hydroxide ion and fluorinating agents, and occasionally can be even used as mild reagents for selective syntheses. In the universe, silicon is the seventh most abundant element, coming after hydrogen , helium , carbon , nitrogen , oxygen , and neon.
These abundances are not replicated well on Earth due to substantial separation of the elements taking place during the formation of the Solar System. Silicon makes up Further fractionation took place in the formation of the Earth by planetary differentiation: Earth's core , which makes up The crystallisation of igneous rocks from magma depends on a number of factors; among them are the chemical composition of the magma, the cooling rate, and some properties of the individual minerals to be formed, such as lattice energy , melting point, and complexity of their crystal structure.
As magma is cooled, olivine appears first, followed by pyroxene , amphibole , biotite mica, orthoclase feldspar , muscovite mica , quartz , zeolites , and finally hydrothermal minerals. This sequence shows a trend towards increasingly complex silicate units with cooling, and the introduction of hydroxide and fluoride anions in addition to oxides. Many metals can substitute for silicon. After these igneous rocks undergo weathering , transport, and deposition, sedimentary rocks like clay, shale, and sandstone are formed.
Metamorphism also can occur at high temperatures and pressures, creating an even vaster variety of minerals. The reduction is carried out in an electric arc furnace , with an excess of SiO 2 used to stop silicon carbide SiC from accumulating: This reaction, known as carbothermal reduction of silicon dioxide, is usually conducted in the presence of scrap iron with low amounts of phosphorus and sulfur , produing ferrosilicon.
It is followed by Russia , t , Norway , t , Brazil , t and the United States , t. However, even greater purity is needed for semiconductor applications, and this is produced from the reduction of tetrachlorosilane or trichlorosilane.
The former is made by chlorinating scrap silicon and the latter is a byproduct of silicone production. These compounds are volatile and hence can be purified by repeated fractional distillation , followed by reduction to elemental silicon with very pure zinc metal as the reducing agent. The spongy pieces of silicon thus produced are melted and then grown to form cylindrical single crystals, before being purified by zone refining. Other routes use the thermal decomposition of silane or tetraiodosilane.
Another process used is the reduction of sodium hexafluorosilicate , a common waste product of the phosphate fertiliser industry, by metallic sodium: Hyperfine silicon is made at a higher purity than almost every other material: Most silicon is used industrially without being purified, and indeed often with comparatively little processing from its natural form. Many of these have direct commercial uses, such as clays, silica sand and most kinds of building stone.
Thus, the vast majority of uses for silicon are as structural compounds, either as the silicate minerals or silica crude silicon dioxide. Silicates are used in making Portland cement made mostly of calcium silicates which is used in building mortar and modern stucco , but more importantly, combined with silica sand, and gravel usually containing silicate minerals like granite , to make the concrete that is the basis of most of the very largest industrial building projects of the modern world.
Silica is used to make fire brick , a type of ceramic. Silicate minerals are also in whiteware ceramics , an important class of products usually containing various types of fired clay minerals natural aluminium phyllosilicates.
An example is porcelain which is based on the silicate mineral kaolinite. Traditional glass silica-based soda-lime glass also functions in many of the same ways, and is also used for windows and containers. In addition, specialty silica based glass fibers are used for optical fiber , as well as to produce fiberglass for structural support and glass wool for thermal insulation.
Silicones are often used in waterproofing treatments, molding compounds, mold- release agents , mechanical seals, high temperature greases and waxes, and caulking compounds. Silicone is also sometimes used in breast implants , contact lenses, explosives and pyrotechnics. Silicon is a component of some superalloys. Elemental silicon is added to molten cast iron as ferrosilicon or silicocalcium alloys to improve performance in casting thin sections and to prevent the formation of cementite where exposed to outside air.
The presence of elemental silicon in molten iron acts as a sink for oxygen, so that the steel carbon content, which must be kept within narrow limits for each type of steel, can be more closely controlled. Silicon is an important constituent of electrical steel , modifying its resistivity and ferromagnetic properties.
The properties of silicon can be used to modify alloys with metals other than iron. This greatly reduces tearing and cracks formed from stress as casting alloys cool to solidity. Silicon also significantly improves the hardness and thus wear-resistance of aluminium. Monocrystalline silicon of such purity is usually produced by the Czochralski process , is used to produce silicon wafers used in the semiconductor industry , in electronics, and in some high-cost and high-efficiency photovoltaic applications.
Pure silicon has too low a conductivity i. In practice, pure silicon is doped with small concentrations of certain other elements, which greatly increase its conductivity and adjust its electrical response by controlling the number and charge positive or negative of activated carriers. Such control is necessary for transistors , solar cells , semiconductor detectors , and other semiconductor devices used in the computer industry and other technical applications. In common integrated circuits , a wafer of monocrystalline silicon serves as a mechanical support for the circuits, which are created by doping and insulated from each other by thin layers of silicon oxide , an insulator that is easily produced on Si surfaces by processes of Thermal Oxidation or Local Oxidation LOCOS , which involve exposing the element to oxygen under the proper conditions that can be predicted by the Deal—Grove model.
Silicon has become the most popular material for both high power semiconductors and integrated circuits because it can withstand the highest temperatures and greatest electrical activity without suffering avalanche breakdown an electron avalanche is created when heat produces free electrons and holes, which in turn pass more current, which produces more heat. In addition, the insulating oxide of silicon is not soluble in water, which gives it an advantage over germanium an element with similar properties which can also be used in semiconductor devices in certain fabrication techniques.
Monocrystalline silicon is expensive to produce, and is usually justified only in production of integrated circuits, where tiny crystal imperfections can interfere with tiny circuit paths. For other uses, other types of pure silicon may be employed. These include hydrogenated amorphous silicon and upgraded metallurgical-grade silicon UMG-Si used in the production of low-cost, large-area electronics in applications such as liquid crystal displays and of large-area, low-cost, thin-film solar cells.
Such semiconductor grades of silicon are either slightly less pure or polycrystalline rather than monocrystalline, and are produced in comparable quantities as the monocrystalline silicon: The market for the lesser grade is growing more quickly than for monocrystalline silicon. Although silicon is readily available in the form of silicates , very few organisms use it directly.
Diatoms , radiolaria and siliceous sponges use biogenic silica as a structural material for skeletons. In more advanced plants, the silica phytoliths opal phytoliths are rigid microscopic bodies occurring in the cell; some plants, for example rice , need silicon for their growth. There is some evidence that silicon is important to nail, hair, bone and skin health in humans,  for example in studies that show that premenopausal women with higher dietary silicon intake have higher bone density , and that silicon supplementation can increase bone volume and density in patients with osteoporosis.
People can be exposed to elemental silicon in the workplace by breathing it in, swallowing it, skin contact, and eye contact. In the latter two cases, silicon poses a slight hazard as an irritant; it is hazardous if inhaled.
From Wikipedia, the free encyclopedia. This article is about the chemical element. For other uses, see Silicon disambiguation. Not to be confused with the silicon-containing synthetic polymer silicone. For other uses, see Element 14 disambiguation. Silicide and Binary compounds of silicon. Chemical Rubber Company Publishing. Journal of Microelectromechanical Systems. Other elements isolated with the aid of potassium and sodium: Journal of Chemical Education.
Russian Journal of Applied Chemistry. Part 1 Cells and Tissues 2nd ed. About these "elements", Lavoisier speculates: It is extremely probable that barytes, which we have just now arranged with earths, is in this situation; for in many experiments it exhibits properties nearly approaching to those of metallic bodies. It is even possible that all the substances we call earths may be only metallic oxyds, irreducible by any hitherto known process.
Lavoisier with Robert Kerr, trans. The original passage appears in: Cuchet, , vol. Deterville, , vol. Baldwin, Cradock, and Joy, , vol. But as there is not the smallest evidence for its metallic nature, and as it bears a close resemblance to boron and carbon, it is better to class it along with these bodies, and to give it the name of silicon. Berzelius announced his discovery of silicon "silicium" in: The isolation of silicon and its characterization are detailed in the section titled "Flussspatssyrad kisseljords sönderdelning med kalium," pp.
The above article was reprinted in German in: Berzelius " II. Untersuchungen über Flussspathsäure und deren merkwürdigsten Verbindungen " II. Investigations of hydrofluoric acid and its most noteworthy compounds , Annalen der Physik , The isolation of silicon is detailed in the section titled: Kieselerde durch Kalium" Decomposition of silicate fluoride by potassium , pp.
The above article was reprinted in French in: Berzelius "Décomposition du fluate de silice par le potassium" Decomposition of silica fluoride by potassium , Annales de Chimie et de Physique , Reprinted in English in: Frequently Asked Questions for Practitioners. Diabetes Prevention and Management. Ease of Use Project. Education Videos for Providers. Blood Lead Testing and Reporting.
Blood Lead Test Reporting. Colon Cancer Screening Resources. Heart Disease, Stroke, and Diabetes. Washington Recovery and Monitoring Program. Public Health Meaningful Use. Public Health Registry — Cancer. Public Health Registry — Prescription Review. Sexual and Domestic Violence. Suicide Prevention Training for Health Professionals. Types of training Six-hour training for suicide assessment, treatment and management Three-hour training for suicide screening and referral Three-hour training for pharmacists and dentists suicide screening, referral and imminent harm via lethal means Six-hour training for suicide assessment, treatment and management Please note: Online Act Against Suicide: In-person Youth and Adult Suicide: Online All Patients Safe: Suicide Prevention for Medical Providers.
Assessment and Management of the Suicidal Patient. Everyone Can Save a Life: HO Suicide Prevention Training. Recognizing and Responding to Suicide Risk: Essential Skills for Clinicians.
Suicide Assessment, Intervention and Treatment. Suicide Assessment, Management, and Treatment. Suicide Assessment, Treatment and Management. Dimensions of Prevention and Intervention. New Interventions That Work. Suicide Prevention Training for Healthcare Professionals.