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Adapted from Design News. Max Gerson in this particular piece. En quinze ans, le marché des TIC a été bouleversé, d'abord avec l'essor phénoménal de la téléphonie mobile et de l'Internet fixe à haut débit, puis récemment avec l'Internet mobile à haut débit, ce qui caractérise l'étape majeure de la convergence des technologies du traitement de l'information et de la communication. Crystalline bulk silicon is rather inert, but becomes more reactive at high temperatures. Online Act Against Suicide:
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Following periodic trends , its single-bond covalent radius of At standard temperature and pressure, silicon is a shiny semiconductor with a bluish-grey metallic lustre; as typical for semiconductors, its resistivity drops as temperature rises. This arises because silicon has a small energy gap between its highest occupied energy levels the valence band and the lowest unoccupied ones the conduction band. The Fermi level is about halfway between the valence and conduction bands and is the energy at which a state is as likely to be occupied by an electron as not.
Hence pure silicon is an insulator at room temperature. However, doping silicon with a pnictogen such as phosphorus , arsenic , or antimony introduces one extra electron per dopant and these may then be excited into the conduction band either thermally or photolytically, creating an n-type semiconductor. Similarly, doping silicon with a group 13 element such as boron , aluminium , or gallium results in the introduction of acceptor levels that trap electrons that may be excited from the filled valence band, creating a p-type semiconductor.
Joining n-type silicon to p-type silicon creates a p-n junction with a common Fermi level; electrons flow from n to p, while holes flow from p to n, creating a voltage drop. This p-n junction thus acts as a diode that can rectify alternating current that allows current to pass more easily one way than the other. A transistor is an n-p-n junction, with a thin layer of weakly p-type silicon between two n-type regions.
Biasing the emitter through a small forward voltage and the collector through a large reverse voltage allows the transistor to act as a triode amplifier. Silicon crystallises in a giant covalent structure at standard conditions, specifically in a diamond cubic lattice. It is not known to have any allotropes at standard pressure, but several other crystal structures are known at higher pressures.
Naturally occurring silicon is composed of three stable isotopes , 28 Si The fusion of 28 Si with alpha particles by photodisintegration rearrangement in stars is known as the silicon-burning process ; it is the last stage of stellar nucleosynthesis before the rapid collapse and violent explosion of the star in question in a type II supernova.
The known isotopes of silicon range in mass number from 22 to Crystalline bulk silicon is rather inert, but becomes more reactive at high temperatures. Like its neighbour aluminium, silicon forms a thin, continuous surface layer of silicon dioxide SiO 2 that protects the metal from oxidation. Silicon does not react with most aqueous acids, but is oxidised and fluorinated by a mixture of concentrated nitric acid and hydrofluoric acid ; it readily dissolves in hot aqueous alkali to form silicates.
At high temperatures, silicon also reacts with alkyl halides ; this reaction can be catalysed by copper to directly synthesise organosilicon chlorides as precursors to silicone polymers. Upon melting, silicon becomes extremely reactive, alloying with most metals to form silicides , and reducing most metal oxides because the heat of formation of silicon dioxide is so large.
As a result, containers for liquid silicon must be made of refractory , unreactive materials such as zirconium dioxide or group 4, 5, and 6 borides. Tetrahedral coordination is a major structural motif in silicon chemistry just as it is for carbon chemistry. However, the 3p subshell is rather more diffuse than the 2p subshell and does not hybridise as well with the 3s subshell. As a result, the chemistry of silicon and its heavier congeners shows significant differences from that of carbon,  and thus octahedral coordination is also significant.
The poor overlap of 3p orbitals also results in a much lower tendency towards catenation formation of Si—Si bonds for silicon than for carbon due to the concomitant weakening of the Si—Si bond compared to the C—C bond: Silicon already shows some incipient metallic behaviour, particularly in the behaviour of its oxide compounds and its reaction with acids as well as bases though this takes some effort , and is hence often referred to as a metalloid rather than a nonmetal.
Silicon shows clear differences with carbon. For example, organic chemistry has very few analogies with silicon chemistry, while silicate minerals have a structural complexity unseen in oxocarbons. Additionally, the lower Ge—O bond strength compared to the Si—O bond strength results in the absence of "germanone" polymers that would be analogous to silicone polymers.
Many metal silicides are known, most of which have formulae that cannot be explained through simple appeals to valence: They are structurally more similar to the borides than the carbides , in keeping with the diagonal relationship between boron and silicon, although the larger size of silicon than boron means that exact structural analogies are few and far between. The heats of formation of the silicides are usually similar to those of the borides and carbides of the same elements, but they usually melt at lower temperatures.
Except for copper , the metals in groups 11—15 do not form silicides. Most instead form eutectic mixtures , although the heaviest post-transition metals mercury , thallium , lead , and bismuth are completely immiscible with liquid silicon. Silicides are usually prepared by direct reaction of the elements. For example, the alkali metals and alkaline earth metals react with silicon or silicon oxide to give silicides.
Nevertheless, even with these highly electropositive elements true silicon anions are not obtainable, and most of these compounds are semiconductors. Cu 5 Si ; with increasing silicon content, catenation increases, resulting in isolated clusters of two e.
U 3 Si 2 or four silicon atoms e. CaSi , layers e. CaSi 2 , or three-dimensional networks of silicon atoms spanning space e. The silicides of the group 1 and 2 metals are usually more reactive than the transition metal silicides. The latter usually do not react with aqueous reagents, except for hydrofluoric acid ; however, they do react with much more aggressive reagents like liquid potassium hydroxide , or gaseous fluorine or chlorine when red-hot.
The pre-transition metal silicides instead readily react with water and aqueous acids, usually producing hydrogen or silanes: Products often vary with the stoichiometry of the silicide reactant.
For example, Ca 2 Si is polar and non-conducting and has the anti-PbCl 2 structure with single isolated silicon atoms, and reacts with water to produce calcium hydroxide , hydrated silicon dioxide, and hydrogen gas.
CaSi with its zigzag chains of silicon atoms instead reacts to give silanes and polymeric SiH 2 , while CaSi 2 with its puckered layers of silicon atoms does not react with water, but will react with dilute hydrochloric acid: Speculation on silicon hydride chemistry started from the s, contemporary with the development of synthetic organic chemistry.
Silane itself, as well as trichlorosilane , were first synthesised by Friedrich Wöhler and Heinrich Buff in by reacting aluminium—silicon alloys with hydrochloric acid , and characterised as SiH 4 and SiHCl 3 by Charles Friedel and Albert Ladenburg in Disilane Si 2 H 6 followed in , when it was first made by Henri Moissan and Samuel Smiles by the protonolysis of magnesium silicides. Further investigation had to wait until because of the great reactivity and thermal instability of the silanes; it was then that Alfred Stock began the study of silicon hydrides in earnest with new greaseless vacuum techniques, as they were found as contaminants of his focus, the boron hydrides.
The names silanes and boranes are due to him, based on analogy with the alkanes. Direct reaction of HX or RX with silicon, possibly with a catalyst such as copper, is also a viable method to produce substituted silanes.
They are all strong reducing agents. The first two, silane and disilane, are colourless gases; the heavier members of the series are volatile liquids. All silanes are very reactive and catch fire or explode spontaneously in air. They become less thermally stable with room temperature, so that only silane is indefinitely stable at room temperature, although disilane does not decompose very quickly only 2.
They are much more reactive than the corresponding alkanes, because the larger radius of silicon compared to carbon facilitates nucleophilic attack at the silicon, the greater polarity of the Si—H bond compared to the C—H bond, and the ability of silicon to expand its octet and hence form adducts and lower the reaction's activation energy. Silane pyrolysis gives polymeric species and finally elemental silicon and hydrogen; indeed ultrapure silicon is commercially produced by the pyrolysis of silane.
While the thermal decomposition of alkanes starts by the breaking of a C—H or C—C bond and the formation of radical intermediates, polysilanes decompose by eliminating silenes: While pure silanes do not react with pure water or dilute acids, traces of alkali catalyse immediate hydrolysis to hydrated silicon dioxide. The Si—H bond also adds to alkenes , a reaction which proceeds slowly and speeds up with increasing substitution of the silane involved.
The monohalosilanes may be formed by reacting silane with the appropriate hydrogen halide with an Al 2 X 6 catalyst, or by reacting silane with a solid silver halide in a heated flow reactor: Among the derivatives of silane, iodosilane SiH 3 I and potassium silanide KSiH 3 are very useful synthetic intermediates in the production of more complicated silicon-containing compounds: Silicon and silicon carbide readily react with all four stable halogens, forming the colourless, reactive and volatile silicon tetrahalides.
The melting and boiling points of these species usually rise with increasing atomic weight, though there are many exceptions: While catenation in carbon compounds is maximised in the hydrogen compounds rather than the halides, the opposite is true for silicon, so that the halopolysilanes are known up to at least Si 14 F 30 , Si 6 Cl 14 , and Si 4 Br These halopolysilanes may be produced by comproportionation of silicon tetrahalides with elemental silicon, or by condensation of lighter halopolysilanes trimethylammonium being a useful catalyst for this reaction.
Silicon dioxide SiO 2 , also known as silica, is one of the most well-studied compounds, second only to water. It is also known to occur pure as rock crystal ; impure forms are known as rose quartz , smoky quartz , morion , amethyst , and citrine. 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. We'll continue to add approved programs to this model list. Some approved in-person trainings are on the suicide prevention events page. Only trainings that meet the minimum standards PDF will be included. See the program approval process for more information.
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