# Silicon

properties
[ Ne ] 3 s 2 3 p 2
14 Si
General
Name , symbol , atomic number Silicon, Si, 14
Element category Semi-metals
Group , period , block 14 , 3 , p
Appearance dark gray,
bluish hue
CAS number 7440-21-3
EC number 231-130-8
ECHA InfoCard 100.028.300
Mass fraction of the earth's envelope 25.8%
Atomic
Atomic mass 28,085 (28,084-28,086) u
Atomic radius (calculated) 110 (111) pm
Van der Waals radius 210 pm
Electron configuration [ Ne ] 3 s 2 3 p 2
1. Ionization energy 8th.15168 (3) eV786.52 kJ / mol
2. Ionization energy 16.34585 (4) eV1 577.13 kJ / mol
3. Ionization energy 33.49300 (9) eV3 231.58 kJ / mol
4. Ionization energy 45.14179 (7) eV4 355.52 kJ / mol
5. Ionization energy 166.767 (3) eV16 091 kJ / mol
Physically
Physical state firmly
Crystal structure Diamond structure
density 2.336 g / cm 3 (20 ° C )
Mohs hardness 6.5
magnetism diamagnetic ( Χ m = −4.1 10 −6 )
Melting point 1683 K (1410 ° C)
boiling point 3533 K (3260 ° C)
Molar volume 12.06 · 10 −6 m 3 · mol −1
Heat of evaporation 383 kJ / mol
Heat of fusion 50.66 kJ mol −1
Speed ​​of sound 8433 m s −1 at 293 K
Specific heat capacity 703 J kg −1 K −1 at 298 K
Electric conductivity ( Intrinsic conduction ) 5 · 10 −4 A · V −1 · m −1 at 300 K.
Thermal conductivity 150 W m −1 K −1
Chemically
Oxidation states −4, (2) +4
Electronegativity 1.90 ( Pauling scale )
Isotopes
isotope NH t 1/2 ZA ZE (M eV ) ZP
26 Si {syn.} 2.234 s ε 5.066 26 Al
27 Si {syn.} 4.16 s ε 4.812 27 Al
28 Si 92.23  % Stable
29 Si 4.67% Stable
30 Si 3.1% Stable
31 Si {syn.} 157.3 min β - 1.492 31 P.
32 Si {syn.} 153 a β - 0.224 32 P
33 Si {syn.} 6.18 s β - 5.845 33 P
34 Si {syn.} 2.77 s β - 4.601 34 P
For other isotopes see list of isotopes
NMR properties
Spin
quantum
number I
γ in
rad · T −1 · s −1
E r  ( 1 H) f L at
B = 4.7 T
in MHz
29 Si 1/2 0−5.319 · 10 7 7.86 · 10 −3 19.864 (2.3488 T)
safety instructions
GHS labeling of hazardous substances
 no GHS pictograms
H and P phrases H: no H-phrases
P: no P-phrases
MAK

Switzerland: 3 mg m −3 (measured as respirable dust )

As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .

Silicon , also called silicon , is a chemical element with the symbol Si and the atomic number 14. It is in the 4th main group ( carbon group ), or the 14th  IUPAC group , and the 3rd period of the periodic table of the elements . In the earth sheath , it is, on the mass fraction ( ppmw based), after oxygen , the second most abundant element .

Silicon is a classic semi-metal , therefore has properties of both metals and non-metals and is an element semiconductor . Pure, elemental silicon is gray-black in color and has a typically metallic, often bronze to bluish sheen.

Silicon is extremely important for electronics and has also been used in isotopically pure form to define the kilogram since 2018 . Elemental silicon is non-toxic for the human body, silicon in its bound silicate form is important for humans. The human body contains about 20 mg / kg body mass silicon; the amount decreases with age.

## Spelling and etymology

The element "silicon" is written in the standard language. The spelling with "c" is mainly used in chemical terminology. Both spellings come from the Latin expression silicia " silica ", linked with the Latin silex "pebble", "rock".

The English word for silicon is silicon . For example, it is included in the name Silicon Valley . The occasional translation silicone is a wrong friend , because silicones are a class of chemical compounds of silicon.

## history

### Use in pre-industrial times

Silicon-containing compounds , especially rocks, have traditionally played an important role as a building material in human history. A typical example of an early stone structure is Stonehenge . Another important silicate-containing material that has been used as building material for a long time is loam , which was used as rammed earth, reinforced with twigs or straw in loam construction , or used in mud brick constructions, later also burnt as brick . Cement , which also contains silicate, was first developed by the Romans.

Due to their sharp cutting edges, rocks containing silicon were also used as tools in the Stone Age . Obsidian , for example , was already mined as a particularly suitable tool material in prehistoric times and was widely distributed through trade. Also Feuerstein was in chalk areas such Belgium and Denmark , mined. In metal extraction, especially in steel production, silicate slag is used to protect stoves and ovens from the ingress of oxygen and as a mold made of clay or sand; glass production may have been discovered in the process.

### Discovery as an element

Antoine Lavoisier , postulated the existence of silicon

It was first predicted in 1789 by Antoine Lavoisier that silex was the oxide of a metal. In 1807, after electrochemical experiments , Humphry Davy postulated the existence of the metals silicon, aluminum , zirconium and glucinium ( beryllium ).

"Had I been so fortunate as to have obtained more certain evidence on this subject, and to have procured the metallic substances I was in search of, I should have proposed for them the names of silicon, alumium, zirconium and glucium."

"Had I been so fortunate to have more reliable evidence on the subject and to have taught the metallic substances I was looking for, I would have suggested the names silicon, aluminum, zirconium, and glucium for them."

- Humphry Davy

In 1811 the chemists Joseph Louis Gay-Lussac and Louis Jacques Thénard (see Thénards Blau ) produced impure and amorphous silicon (a-Si, the non-crystalline, allotropic form of silicon). To do this, they convert silicon tetrafluoride with elemental potassium . A similar approach was taken in 1824 by Jöns Jakob Berzelius in Sweden by converting a hexafluorosilicate with elemental potassium. Berzelius cleaned the amorphous silicon thus obtained by washing. He was the first to recognize the elementary nature of silicon and gave it his name.

The term silicon is derived from the Latin word silex (pebble, flint ). It expresses that silicon is a common component of many minerals .

The English term silicon was proposed in 1817 by the Scottish chemist Thomas Thomson (1773-1852). The ending -on is intended to indicate the chemical relationship to the non-metals carbon ( carbon ) and boron ( boron ).

The first production of pure, crystalline silicon was achieved in 1854 by the French chemist Henri Etienne Sainte-Claire Deville using electrolysis .

## Occurrence

### Silicon in inanimate nature

The entire earth consists of about 15 percent by mass of silicon; in particular the earth's mantle is made up to a considerable extent of silicate rock melts. The earth's crust consists of about 25.8 percent by weight silicon; this makes it the second most common chemical element after oxygen . Here silicon occurs mainly in the form of silicate minerals or as pure silicon dioxide .

Sand consists mainly of silicon dioxide. Quartz is pure silicon dioxide. Many gemstones consist of silicon dioxide and more or less admixtures of other substances, such as amethyst , rose and smoky quartz , agate , jasper and opal . With many metals, silicon forms silicates. Examples of rocks containing silicate are mica , asbestos , clay , slate , feldspar and sandstone . The oceans also represent an enormous reservoir of silicon: in the form of monomeric silica , it is dissolved in considerable quantities in all oceans. A total of 1437 silicon minerals are known to date (as of 2011), the rare moissanite with a content of up to 70% having the highest silicon content (for comparison: mineral quartz has a silicon content of up to 46.7%).

Since silicon also occurs naturally in a dignified , i.e. elementary, form, it is recognized as a mineral by the International Mineralogical Association (IMA) and is listed in Strunz's mineral system (9th edition) under system no. 1.CB.15 ( 8th edition: I / B.05-10) in the department of semi-metals and non-metals. In the systematics of minerals according to Dana , which is mainly known in the English-speaking area , the element mineral has the system no. 01.03.07.01.

So far (as of 2011), solid silicon has been found at 15 sites, including the first in the Nuevo Potosí deposit in Cuba . Other locations are in the People's Republic of China , Russia , Turkey and the United States .

### Silicate cycle

Silicate minerals are permanently broken down by reaction with the carbonic acid of the water to form metasilicic acid and carbonates, as can be shown using the example of calcium silicate:

${\ displaystyle \ mathrm {CaSiO_ {3} + H_ {2} CO_ {3} \ longrightarrow CaCO_ {3} + SiO (OH) _ {2}}}$

The insoluble metosilicic acid reacts further with carbonic acid to form soluble orthosilicic acid:

${\ displaystyle \ mathrm {SiO (OH) _ {2} + H_ {2} CO_ {3} \ rightleftharpoons Si (OH) _ {4} + CO_ {2}}}$

However, orthosilicic acid reacts with itself again relatively quickly to (amorphous) silicon dioxide and water, provided that the pH value is ≥ 3. The absolute concentration of orthosilicic acid is relatively low (e.g. <approx. 7 mmol in sea water).

Through the incorporation of silica or water-soluble silicates in marine organisms (1.), which sediment on the seabed after dying, or through volcanism and the escape of magma on the seabed, the silicate minerals are re-formed (2.), and the cycle is closed:

${\ displaystyle 1.) \ \ mathrm {Si (OH) _ {4} \ longrightarrow SiO_ {2} +2 \ H_ {2} O}}$
${\ displaystyle 2.) \ \ mathrm {CaO + SiO_ {2} \ longrightarrow CaSiO_ {3}}}$

The time horizon in which this process takes place is several million years, which is considerably longer than in the case of the carbon cycle in living nature.

### Silicon in living nature

In addition to the already mentioned essential nature of silicon, there are a number of living things that produce structures containing silicon dioxide. The best known are the diatoms , sponges (Porifera, Spongiaria) and radiolarians , which build up an exoskeleton from silicon dioxide through the enzyme-catalyzed condensation of orthosilicic acid Si (OH) 4 . Many plants also contain silicon dioxide in their stems and leaves. Well-known examples are the horsetail and the bamboo plant . The built-up silicon dioxide framework gives them additional stability.

## Physiological importance for humans

Silicon appears to be needed for bone formation and maturation. In calves, the administration of orthosilicate led to an increase in collagen in the skin and cartilage. The desirable intake derived from animal experiments is 30 mg / d. Deficiencies in humans are not yet known.

Oral preparations are offered as silica or Silicea terra . They essentially contain silicic acid anhydrides (silicon dioxide) and are supposed to strengthen the skin, nails, bones and connective tissue and keep them healthy. An effect has not been scientifically proven.

An excess of silicon can lead to hemolysis of erythrocytes and, as a direct consequence, cause cell changes.

## Extraction in the laboratory

Metallurgical silicon powder

Elemental silicon can be obtained on a laboratory scale by reduction , starting with silicon dioxide or silicon tetrafluoride, with base metals. Reaction 2) is an aluminothermic process that only works with the addition of elemental sulfur, the third route corresponds to element discovery:

${\ displaystyle 1.) \ \ mathrm {SiO_ {2} +2 \ Mg \ longrightarrow Si + 2 \ MgO}}$
${\ displaystyle 2.) \ \ mathrm {3 \ SiO_ {2} +4 \ Al \ longrightarrow 3 \ Si + 2 \ Al_ {2} O_ {3}}}$
${\ displaystyle 3.) \ \ mathrm {3 \ SiF_ {4} +4 \ Al \ longrightarrow 3 \ Si + 4 \ AlF_ {3}}}$

Highly reactive amorphous silicon can be obtained by reduction with sodium or acidolysis of silicides :

${\ displaystyle 1.) \ \ mathrm {SiCl_ {4} +4 \ Na \ longrightarrow Si + 4 \ NaCl}}$
${\ displaystyle 2.) \ \ mathrm {CaSi_ {2} +2 \ HCl \ longrightarrow 2 \ Si + CaCl_ {2} + H_ {2}}}$

## Extraction in industry

Elemental silicon is used in different degrees of purity in metallurgy ( ferrosilicon ), photovoltaics ( solar cells ) and in microelectronics (semiconductors, computer chips ). Accordingly, it is common in business to classify elemental silicon based on different degrees of purity. A distinction is made between Si mg ( metallurgical grade , raw silicon, 98–99% purity), Si sg ( solar grade , solar silicon, impurities less than 0.01%) and Si eg ( electronic grade , semiconductor silicon, impurities less than 10 −9 ). For solar cells, the purity of the material in its entire thickness is important in order to ensure the longest possible charge carrier life. For many applications in microelectronics, only the upper layers of around 20 to 30 µm need to be extremely pure.

The Siemens process is traditionally used, in which the silicon is first converted to trichlorosilane (silicochloroform) with gaseous hydrogen chloride at 300-350 ° C in a fluidized bed reactor .

${\ displaystyle \ mathrm {Si + 3 \ HCl \ longrightarrow H_ {2} + HSiCl_ {3}}}$

After several distillation steps, the trichlorosilane is thermally decomposed again in the presence of hydrogen in a reverse of the above reaction on heated high-purity silicon rods at 1000-1200 ° C. The elemental silicon grows on the bars. The hydrogen chloride released in the process is returned to the cycle. Silicon tetrachloride is produced as a by-product , which is either converted to trichlorosilane and fed back into the process or burned in the oxygen flame to produce pyrogenic silica . With the Siemens process, 19 kg of waste and by-products are produced per kg of ultra-pure silicon.

### Raw silicon

On an industrial scale, elemental silicon is obtained by reducing silicon dioxide with carbon in a smelting reduction furnace at temperatures of around 2000 ° C. The starting material is quartz sand or quartz gravel.

${\ displaystyle \ mathrm {SiO_ {2} +2 \ C \ longrightarrow Si + 2 \ CO}}$

About 4.1 million tons of this industrial raw silicon (Si mg ) were produced in 2002. It is sufficiently clean for metallurgical purposes and is used as an alloy component and deoxidant for steels (improvement of corrosion resistance , suppression of cementite ) and as a starting material for the production of silane using the Müller-Rochow process , which is ultimately used primarily for the production of silicones . To produce ferrosilicon for the steel industry (deoxidizer in the blast furnace process ), the following reaction is expediently carried out in the presence of elemental iron .

${\ displaystyle \ mathrm {SiO_ {2} +2 \ C + Fe \ longrightarrow FeSi + 2 \ CO}}$

Further digestion options for SiO 2 are:

The soda digestion at approx. 1600 ° C in the melting tank :

${\ displaystyle \ mathrm {4 \ SiO_ {2} + Na_ {2} CO_ {3} \ longrightarrow Na_ {2} O \ cdot 4 \ SiO_ {2} \ + \ CO_ {2}}}$

The hydrothermal digestion at approx. 200 ° C with water in an autoclave :

${\ displaystyle \ mathrm {2 \ SiO_ {2} +2 \ NaOH \ longrightarrow Na_ {2} O \ cdot 2 \ SiO_ {2} + \ H_ {2} O}}$

### Solar silicon

Purified polycrystalline silicon

For the production of solar cells , the raw silicon is further purified into so-called solar silicon (Si sg ) (purity> 99.99%). The various processes used for this have many complex intermediate steps and are the most energy-intensive part of the production of solar cells. In the course of government subsidies and lower energy costs through the use of cheap electricity from coal to generate electricity, Chinese providers of solar silicon have recently been able to significantly increase their market share (as of the end of 2019).

A common process is the production of trichlorosilane from metallurgical silicon and the subsequent distillation of the trichlorosilane. The pure trichlorosilane is then pyrolyzed and the silicon formed is deposited on silicon that has already been introduced.

Examples are also the UMG process (Upgraded Metallurgical Grade) and the FBR process (Fluidized Bed Reactor).

A chlorine-free alternative is the decomposition of monosilane , which disintegrates again after a cleaning step on heated surfaces or when it is passed through fluidized bed reactors .

${\ displaystyle \ mathrm {SiH_ {4} \ longrightarrow 2 \ H_ {2} + Si}}$

In the case of polycrystalline solar cells, a higher purity does not make sense, because the impurities reduce the quantum yield and increase the leakage current - but both are influenced in this way by the crystal defects at the grain boundaries. The efficiency of monocrystalline solar cells made of high-purity silicon is higher, especially when there is little lighting, but since they are much more expensive, they are rarely used.

One method used by the DuPont company is only of historical interest . It was based on the reduction of tetrachlorosilane with elemental zinc vapor at temperatures of 950 ° C.

${\ displaystyle \ mathrm {SiCl_ {4} +2 \ Zn \ longrightarrow Si + 2 \ ZnCl_ {2}}}$

However, due to technical problems and the large quantities of zinc chloride which is produced as waste , this process is no longer used today.

### Semiconductor silicon

Monocrystalline semiconductor silicon

For applications in microelectronics , high-purity, monocrystalline silicon (Si eg ) is required. In particular, impurities with elements that are also suitable as doping elements must be brought to concentrations below certain critical values by crucible pulling or zone melting . The manufacturer Shin-Etsu advertised an “11N” purity (= 99.999 999 999%) of its ingots .

In crucible pulling ( Czochralski process ), the solar silicon obtained in the Siemens process is melted in quartz crucibles . A seed crystal of high-purity, monocrystalline silicon is brought into this melt and slowly pulled out of the melt while rotating, with high-purity silicon crystallizing out on the crystal in monocrystalline form, leaving almost all impurities in the melt. The physical background of this cleaning process is the lowering of the melting point and the tendency of substances to crystallize as pure as possible.

Alternatively, in zone melting, a melting zone is moved through a silicon rod with the aid of (ring-shaped) electrical induction heating , whereby a large part of the impurities in the melt dissolve and migrate with it.

Highly pure crystalline silicon is currently the most suitable base material for microelectronics; less in terms of its electrical properties than in terms of the chemical, physical and technically useful properties of silicon and its compounds (silicon dioxide, silicon nitride, etc.). All common computer chips , memories, transistors etc. use high-purity silicon as a starting material. These applications are based on the fact that silicon is a semiconductor. The targeted incorporation of foreign atoms ( doping ), such as indium , antimony , arsenic , boron or phosphorus , can change the electrical properties of silicon over a wide range. A wide variety of electronic circuits can be implemented primarily by means of the PN transition effects that can be generated in this way . Because of the increasing importance of electronic circuits, one speaks of the silicon age. The designation Silicon Valley (“silicon valley”) for the high-tech region in California also indicates the enormous importance of silicon in the semiconductor and computer industries.

Amorphous silicon can be converted into polycrystalline silicon with the help of excimer lasers . This is of increasing importance for the production of thin -film transistors (TFT) for flat screens .

Silicon wafer

### Manufacturer

Silicon is available commercially both as a fine-grain powder and in larger pieces. High-purity silicon for use in solar modules or in semiconductor components is usually produced in the form of thin disks from single crystals , so-called silicon wafers (see illustration). Due to the high initial investments and long construction times for the necessary furnaces, however, only a few companies in the world produce raw silicon.

The largest producers of metallurgical silicon are:

1. Elkem (N, USA)
2. Invensil (F, USA)
3. Globe Metallurgical (USA)
4. Rima Metal (Br)

There are about 15 other large producers. There are a number of smaller plants in the People's Republic of China , making it the largest producer in a country comparison.

The market for polysilicon or hyperpure silicon has been in a state of upheaval since the mid-2000s. Due to the high demand in the solar industry, there was a silicon shortage in 2006.

Leading manufacturer of polysilicon in 2016
Manufacturer Production in 2016 Head office
GCL poly 69.1 ct China (Hong Kong)
Wacker Chemie 67.5 ct Germany
OCI 49.4 ct South Korea
Xinte Energy 22.8 ct China
Daqo New Energy 13.1 ct China

## properties

### Physical Properties

Extended zone scheme for silicon (unoccupied areas colored)

Like the neighboring germanium , gallium , phosphorus and antimony in the periodic table, silicon is an elemental semiconductor . According to the band model, the energetic distance between the valence band and the conduction band is 1.107 eV (at room temperature). The conductivity can be increased by a factor of 10 6 by doping with suitable doping elements such as boron or arsenic . In silicon doped in this way, the impurity conduction caused by foreign atoms and lattice defects is significantly greater than that of the intrinsic conduction, which is why such materials are referred to as impurity semiconductors . The lattice parameter is 543 pm.

Spectrum of the complex index of refraction ( N = n + i k ) of silicon

The complex refractive index , which depends on the wavelength of the light, is shown in the adjacent picture. Here, too, information about the band structure can be read. The strongly increasing course of the extinction coefficient k shows a direct band transition at 370 nm ( E Γ1  = 3.4 eV). Another direct band transition can be observed at ≈ 300 nm ( E Γ2  = 4.2 eV). The indirect band transition of silicon ( E g  = 1.1 eV) can only be guessed at. The fact that there are further indirect band transitions can be seen from the wide curve of k for wavelengths> 400 nm.

Like water and a few other substances, silicon has a density anomaly : its density in liquid form (at T m = 1685 K) is 10–11% higher than in solid, crystalline form (c-Si) at 300 K.

### Chemical properties

In all naturally occurring compounds and in the majority of synthetically produced compounds, silicon only forms single bonds . The stability of the Si-O single bond in contrast to the CO double bond is due to its partial double bond character, which is created by the overlapping of the lone electron pairs of oxygen with the empty d orbitals of silicon. The double bond rule , which has long been considered valid , according to which silicon as an element of the 3rd period does not form multiple bonds, must now be regarded as outdated, since a large number of synthetically produced compounds with Si-Si double bonds are now known. In 2004 the first connection with a formal Si-Si triple bond was structurally characterized.

With the exception of hydrofluoric acid containing nitric acid (in which hexafluorosilicate is formed), silicon is insoluble in acids because passivation occurs through the formation of a solid silicon dioxide layer . On the other hand, it dissolves easily in hot alkaline solutions with formation of hydrogen. Despite its negative normal potential (−0.81 V), it is relatively inert in compact form, as it is covered with a protective oxide layer in air.

### Mechanical properties

The mechanical properties of silicon are anisotropic (direction-dependent). Depending on the chosen crystal orientation , the modulus of elasticity takes on values ​​between 130 GPa and 189 GPa. A general description of the elastic behavior is given in Voigt notation as for all cubic crystals via the three independent elastic constants C 11 , C 12 and C 44 . The elasticity matrix for silicon is:

${\ displaystyle C = {\ begin {pmatrix} C_ {11} & C_ {12} & C_ {12} & 0 & 0 & 0 \\ C_ {12} & C_ {11} & C_ {12} & 0 & 0 & 0 \\ C_ {12} & C_ {12} & C_ {11} & 0 & 0 & 0 \\ 0 & 0 & 0 & C_ {44} & 0 & 0 \\ 0 & 0 & 0 & 0 & C_ {44} & 0 \\ 0 & 0 & 0 & 0 & 0 & C_ {44} \\\ end {pmatrix}}}$

The elastic constants have the following values:

${\ displaystyle C_ {11} = 165 {,} 7 \, \ mathrm {GPa}, \ qquad C_ {12} = 63 {,} 9 \, \ mathrm {GPa}, \ qquad C_ {44} = 79 { ,} 6 \, \ mathrm {GPa}}$

From the elastic constants the respective moduli of elasticity can be calculated for the individual main crystal directions of silicon (100, 110 and 111):

${\ displaystyle E_ {100} = C_ {11} -2 {\ frac {C_ {12}} {C_ {11} + C_ {12}}} C_ {12} \ approx 130 \, \ mathrm {GPa}}$
${\ displaystyle E_ {110} = 4 {\ frac {(C_ {11} ^ {2} + C_ {12} C_ {11} -2C_ {12} ^ {2}) C_ {44}} {2C_ {44 } C_ {11} + C_ {11} ^ {2} + C_ {12} C_ {11} -2C_ {12} ^ {2}}} \ approx 170 \, \ mathrm {GPa}}$
${\ displaystyle E_ {111} = 3 {\ frac {C_ {44} (C_ {11} + 2C_ {12})} {C_ {11} + 2C_ {12} + C_ {44}}} \ approx 189 \ , \ mathrm {GPa}}$

## Isotopes

A total of 23 isotopes between 22 Si and 45 Si of silicon are known. Of these, three, the isotopes 28 Si, 29 Si and 30 Si, are stable and occur in nature. The isotope with the largest proportion of the natural isotopic composition is 28 Si with 92.223%, 29 Si has a proportion of 4.685% and 30 Si of 3.092%. The longest-lived unstable isotopes are 32 Si, which converts to 32 P ( phosphorus ) with a half-life of 153 years under beta decay , and 31 Si, which also decays to 31 P with a half-life of 157.36 minutes under beta decay. All other isotopes only have short half-lives of seconds or milliseconds.

28 Si is formed in large quantities in heavy stars towards the end of their evolution ( oxygen burning ). This is the reason for the high proportion of 28 Si in total silicon (92.23%) and also in the abundance of silicon compared to other elements. Since 2009 attempts have been made to redefine the SI base unit kilogram as a certain amount of 28 Si atoms; these attempts led to a corresponding redefinition in November 2018. The isotopes 29 Si (4.67% of the total silicon) and 30 Si (3.1%) are also stable .

The radioactive isotope 31 Si decays rapidly ( half-life 157.3 minutes) through beta radiation to form stable phosphorus . This fact can be used to produce very homogeneously n-doped silicon. For this purpose, silicon is irradiated with neutrons, and through neutron capture , 31 Si and consequently 31 P. A neutron source suitable for this process is the Heinz Maier-Leibnitz research neutron source . 32 Si with a half-life of 172 years is more durable . Traces of this isotope are created in the earth's atmosphere by the spallation of argon by cosmic rays . 32 Si decays to 32 P, which is also radioactive (half-life 14.3 days), and then further to stable 32 S ( sulfur ). All other isotopes decay within a few seconds (see list of isotopes ).

## safety

Like many elements, silicon is flammable as a powder. As a powder and granules, it is irritating. Compact silicon is harmless.

Hydrogenated , i.e. porous silicon superficially covered with hydrogen , can be highly explosive when exposed to laser radiation and an increase in oxygen , as researchers at the Technical University of Munich discovered by chance. Explosions in the micrometer range are possible. The detonation speed and detonation energy are higher than with TNT and dynamite .

## Use in technology

1947 discovered John Bardeen , Walter Brattain and William Shockley the adjustable electrical resistance, the transistor , first at a germanium - crystal . The compound-friendly silicon could only later be isolated in the purity required for electronic purposes. In 1958 Robert Noyce at Fairchild and Jack S. Kilby at Texas Instruments independently developed the integrated circuit (IC) on a silicon chip. Since around 1970 silicon has been the basic material of most products in the semiconductor industry and the basic material for many sensors and other micromechanical systems (e.g. lever arm in an atomic force microscope). Silicon is also the elementary component of most solar cells .

In November 2005, the first promising test results with silicon lasers were reported.

Silicon is used as a high-energy fuel in many explosives.

Since silicon expands when it solidifies, while most substances contract, it is added to many casting alloys . For example, cast iron always contains around 2% Si. Aluminum-silicon alloys , in which the Si content can be up to 20%, are of particular importance . This is the most important of all cast aluminum materials.

In chemical compounds, silicon is almost always tetravalent . Accordingly, the silicon atom in compounds is usually four-coordinate. In addition, there are now a number of compounds in which silicon has five or sixfold coordination. In addition to tetravalent silicon, synthetically produced compounds of divalent silicon ( silylenes ) are known, but these are usually very unstable. Of greater significance is only the silicon monoxide , as a material for the compensation of optical lenses is used. In addition, a three-coordinate connection of the one-dimensional structure of 2012 has been similar to the graph shown experimentally, called silicene .

The entire chemistry of silicon is essentially characterized by the high affinity of silicon for oxygen . As a rule, silicon is the electropositive partner of a chemical compound, although compounds with formally negative silicon also exist. These are mostly silicides, in which silicon can also form true anions .

Binding polarity inversion

Particularly noteworthy is the inversion of the bond polarity of element-hydrogen bonds at the transition from carbon to silicon. Here the electronegativity difference changes from +0.45 (carbon-hydrogen) to −0.2, which is why silicon-hydrogen compounds have a completely different reactivity than hydrocarbons .

The most important compounds of silicon can be divided into the following classes, some of which are named:

Binary connections

Silicates

Silicon halides

Silicon hydrides

Organic silicon compounds

Polymeric silicon compounds

• Silicones (silicones, polyorganosiloxanes) are formed by polymerization and are among the most important industrial plastics. Polymeric silicon-oxygen compounds are used in many areas; they serve as lubricants and sealants in the cosmetics and construction industries.
• Polysilanes, -carbosilanes, -carbosilazanes, -carbosiloxanes

## Others

To this day it happens again and again that the English word “ silicon ” (for silicon) is incorrectly translated or pronounced as “ silicon ” in popular scientific articles or in film dubbing. This happened, for example, in the science fiction series Star Trek , the James Bond agent thriller In the Face of Death or in the animated series The Simpsons . Example: "Is the life form made of carbon or silicon?"

Even in untranslated texts such as the 1980s hit Monopoli , songwriter and interpreter Klaus Lage fell for the wrong use, because he wrote: "[...] your job is now done by a piece of silicone [...]".

## literature

• Tracy L. Simpson, Benjamin E. Volcani: Silicon and siliceous structures in biological systems . Springer-Verlag, New York 1981, ISBN 3-540-90592-8 .
• Thomas Thomson: On the Daltonian Theory of Definite Proportions in Chemical Combinations . In: Annals of Philosophy . tape 2 , 1813, p. 32 .

Wiktionary: Silicium  - explanations of meanings, word origins, synonyms, translations
Commons : Silicium  album with pictures, videos and audio files
Wikibooks: Internship Inorganic Chemistry / Silicon  - Learning and teaching materials
Wikibooks: Silicon Processing  - Learning and Teaching Materials

## Individual evidence

1. a b Harry H. Binder: Lexicon of the chemical elements. S. Hirzel, Stuttgart 1999, ISBN 3-7776-0736-3 .
2. The values ​​for the properties (info box) are taken from www.webelements.com (silicon) , unless otherwise stated .
3. The standard value recommended by IUPAC is given, since the isotopic composition of this element can vary locally, the mass range given in brackets results for the mean atomic weight. See: Michael E. Wieser, Tyler B. Coplen: Atomic weights of the elements 2009 (IUPAC Technical Report). In: Pure and Applied Chemistry. 2010, p. 1, doi: 10.1351 / PAC-REP-10-09-14 .
4. ^
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48. Dr. Hibbert in: The Springfield Files. (Engl. The Springfield Files ) Season 8, Episode 1, German first broadcast October 24, 1997.