Silicon reacts with sulfuric acid with hydrogen. Silicon compounds and their properties. Questions for control
in the synthesis and creation of technical products, dishes, glass, equipment, construction and finishing materials, jewelry and other industries.
General characteristics of silicon If we consider the position of silicon in periodic table
- , then we can say this:
- Located in group IV of the main subgroup.
- Serial number 14.
- Atomic mass 28.086.
- Chemical symbol Si.
- The name is silicon, or in Latin - silicium.
The electronic configuration of the outer layer is 4e:2e:8e.
The crystal lattice of silicon is similar to that of diamond. Atoms are located at the nodes; its type is face-centered cubic. However, due to the longer bond length, the physical properties of silicon are very different from the properties of the allotropic modification of carbon.
Physical and chemical properties
- A few more variations of silicon dioxide:
- quartz;
- river and;
- flint;
feldspars. The use of silicon in such forms is realized in construction work, technology, radio electronics, chemical industry
, metallurgy. All the listed oxides together belong to a single substance - silica.
Silicon carbide and its applications Silicon and its compounds are real. One such material is carborundum or carbide of this element. Chemical formula
SiC. Occurs in nature as the mineral moissanite. IN pure form
a compound of carbon and silicon - these are beautiful transparent crystals resembling diamond structures. However, for technical purposes, substances colored green and black are used.
- The main characteristics of this substance, allowing its use in metallurgy, technology, and the chemical industry, are as follows:
- wide-gap semiconductor;
- very high degree of strength (7 on the Mohs scale);
- resistant to high temperatures;
All this allows the use of carborundum as an abrasive material in metallurgy and chemical synthesis. And also on its basis to produce wide-spectrum LEDs, parts for glass furnaces, nozzles, torches, jewelry (moissanite is valued higher than cubic zirconia).
Silan and its meaning
The hydrogen compound of silicon is called silane and cannot be obtained by direct synthesis from starting materials. To obtain it, silicides of various metals are used, which are treated with acids. As a result, silane gas is released and a metal salt is formed.
The interesting thing is that the compound in question never forms alone. The reaction always results in a mixture of mono-, di- and trisilane, in which the silicon atoms are connected to each other in chains.
By their properties, these compounds are strong reducing agents. At the same time, they themselves are easily oxidized by oxygen, sometimes with an explosion. Reactions with halogens are always violent, with a large release of energy.
The areas of application of silanes are as follows:
- Organic synthesis reactions that result in the formation of important organosilicon compounds - silicones, rubbers, sealants, lubricants, emulsions and others.
- Microelectronics (liquid crystal monitors, integrated technical circuits, etc.).
- Obtaining ultra-pure polysilicon.
- Dentistry for prosthetics.
Thus, the value of silanes in modern world high.
Silicic acid and silicates
The hydroxide of the element in question is different silicic acids. Highlight:
- meta;
- ortho;
- polysilicic and other acids.
They all unite general properties- extreme instability in a free state. They decompose easily under the influence of temperature. Under normal conditions, they do not exist for long, turning first into a sol and then into a gel. After drying, such structures are called silica gels. They are used as adsorbents in filters.
Important, from an industrial point of view, are salts of silicic acids - silicates. They underlie the production of substances such as:
- glass;
- concrete;
- cement;
- zeolite;
- kaolin;
- porcelain;
- faience;
- crystal;
- ceramics.
Alkali metal silicates are soluble, all others are not. Therefore, sodium and potassium silicate is called liquid glass. Regular office glue is the sodium salt of silicic acid.
But the most interesting compounds are still glass. What variants of this substance have they come up with! Today they receive color, optical, matte options. Glassware amazes with its splendor and diversity. By adding certain metal and non-metal oxides to the mixture, the most different types glass Sometimes even the same composition, but different percentage components leads to differences in the properties of the substance. An example is porcelain and earthenware, the formula of which is SiO 2 *AL 2 O 3 *K 2 O.
This is a form of highly pure product whose composition is described as silicon dioxide.
Discoveries in the field of silicon compounds
Over the past few years of research, it has been proven that silicon and its compounds are the most important participants in the normal state of living organisms. Diseases such as:
- tuberculosis;
- arthritis;
- cataract;
- leprosy;
- dysentery;
- rheumatism;
- hepatitis and others.
The aging processes of the body themselves are also associated with the quantitative content of silicon. Numerous experiments on mammals have proven that with a deficiency of the element, heart attacks, strokes, cancer occur and the hepatitis virus is activated.
The most outstanding feature of silicon chemistry is its predominance of very persistent oxygen compounds. All its other compounds are not only unstable, but also rare in terrestrial conditions; in general, they are formed and are stable only when completely special conditions: in the absence of oxygen and water. So far, no more than several hundred such silicon compounds have been obtained in laboratories, much less than the number of natural silicates. Like carbon, silicon forms two compounds with oxygen: SiO and. Monoxide SiO does not occur in nature. The thermodynamic stability region of this compound lies at high temperatures when it is in a vapor state. SiO can be obtained by reduction at 1350-1500°C:
Rapid cooling (quenching) of vapors SiO obtain it in a solid state. With slow cooling SiO disproportion.
Hard oxide SiO is a dark yellow powder. It does not conduct electricity and is an excellent insulating material. SiO is slowly oxidized by atmospheric oxygen and easily dissolves in alkalis:
those. exhibits restorative properties. Dioxide is the most characteristic and stable oxygen compound of silicon. It forms three crystalline modifications: quartz, tridymite and cristobalite. A quartz crystal is a giant polymer molecule consisting of individual tetrahedra, in which each silicon atom is surrounded by four oxygen atoms, and each oxygen atom bridges a three-center bond, being a common corner atom for the two tetrahedra. Schematically in a planar image it can be represented as:
Along with the usual -bonds between atoms Si And O non-localized -bonds also arise, which are formed according to the donor-acceptor mechanism due to free 3 d-orbitals of silicon atoms, lone 2 p-electron pairs of oxygen atoms.
Recently, new modifications have been obtained - stishovite and cousite. The latter are obtained only under high pressure, and under normal conditions in a metastable state they can exist indefinitely (like diamond). A commonly found variety of quartz in nature is rock crystal. Colored varieties of quartz: marion (black), topaz (smoky), amethyst (purple), citrine (yellow). Fibrous modifications (chalcedony and quartzine) are also described. In addition, at the bottom of the seas and oceans, amorphous is formed from algae and ciliates. In general, silicon dioxide is the most common oxide in earth's crust. Quartz, tridymite and cristobalite can transform into each other, but these transitions are greatly inhibited. As a result, tridymite and cristobalite, despite their thermodynamic instability, can be preserved indefinitely at room temperature and exist in nature in the form of independent minerals. Each of these crystal modifications, in turn, can be in the form of two or more mutually transforming forms, of which the b-form is stable at room temperature, and the c-form is stable at higher temperatures. A modification stable at high temperatures - β-cristobalite - melts at 1723°C. When molten silica is cooled rapidly, glass forms.
Various crystalline modifications, like anhydrous amorphous silica, are inorganic heterochain polymers. In all forms (except for steshovite), the structural motif is a silicon-oxygen tetrahedron. Despite the identical way of articulating structural motifs, their spatial arrangement for various modifications various. Therefore, for example, β-cristobalite has a cubic lattice, and β-tridymite has a hexagonal lattice. The difference between these structures is the same as between sphalerite and wurtzite. The densest modification (stishovite) is characterized by an unusual coordination of atoms for oxygen compounds of silicon. Here, each silicon atom is surrounded by six oxygen atoms. Therefore, the stishovite structure is formed by a combination of silicon-oxygen octahedra.
The chemical activity of the modifications increases from quartz to cristobalite and especially to silica obtained by dehydrating silicic acid gel. Fluorine, HF gas and hydrofluoric acid react vigorously with:
In the first reaction, fluorine displaces oxygen from silicon oxide. Both reactions occur because silicon tetrafluoride is a stronger compound than dioxide. The enthalpy of formation of the latter is -910.9, and for D = -1614.9 kJ/mol.
In addition, these processes are accompanied by an increase in entropy (on the left - a solid and a gas, and on the right - two gases). Therefore, the Gibbs free energy as a result of these interactions is greatly reduced.
It is practically insoluble in water. Acids and aqua regia. In alkaline solutions, especially when heated, it dissolves easily:
Typically, the reaction to produce silicates is carried out not in solution, but by sintering with alkalis, carbonates and metal oxides:
All these reactions prove the acidic nature of silicon dioxide. The chemical properties of quartz glass are almost the same as crystal glass.
Since it is insoluble in water, silicic acid is obtained indirectly:
The resulting silicic acid is released from the solution in the form of a gelatinous precipitate or remains in the solution in a colloidal state. Its composition corresponds to values and that vary depending on conditions. Acid with =1 and =1 is called metasilicon, and orthosilicon has =2. All acids for which >1 are called polysilicon. These acids cannot be isolated in the free state. Their composition is determined by salts - silicates. All silicic acids are very weak. So, it has 10. Therefore, water-soluble silicates are highly hydrolyzed:
Partially dehydrated gelatinous silicic acid is a solid, white, highly porous mass called silica gel. It has a high adsorption capacity and vigorously absorbs water, oils, ethers, etc.
Hydrogen compounds of silicon - hydrogen silicates or silanes - are obtained by the action of acids on silicides active metals, For example
Along with monosilane, hydrogen and polysilanes are released, up to hexasilane. The content of other hydrogen silicates in the decomposition products of magnesium silicide naturally increases as their molecular weight decreases.
By structure and physical properties silanes are similar to hydrocarbons of the homologous series of methane. All homologues of monosilane are known, up to octasilane. To obtain the practically most important monosilane, reduction reactions of silicon halides with hydrogen or lithium aluminum hydride are used:
All silanes have a characteristic unpleasant odor and are toxic. Compared to hydrocarbons, silanes are characterized by higher density and higher melting and boiling points, but are thermally less stable. Their chemical properties are very different from representatives of the homologous methane series and resemble boranes (diagonally similar to boron).
They are easily oxidized in air, i.e. are reducing agents:
Silanes are reduced to Fe(+3) derivatives to Fe(+2). In addition, silanes are characterized by hydrolysis. In the presence of traces of acids and especially alkalis, silanes are destroyed:
The formation of silica or silicates during hydrolysis of silanes indicates the acidic nature of the silanes.
For silicon, a few representatives of unsaturated hydrosilicates such as polysilene and polysilines are also known. All of them are solids, unstable to heat and extremely reactive. They spontaneously ignite in air and are completely decomposed by water.
Silicon halides can be prepared by synthesis from simple substances. All of them interact vigorously with water:
For fluoride the reaction is reversible (therefore dissolves in HF), but for the remaining halides it is almost completely shifted to the right. When halides with silicon are heated above 1000°C, a reaction occurs to form dihalides: which, when cooled, disproportionately release silicon. This reaction can be used as a transport reaction to produce high-purity silicon.
Of the silicon halides, the largest matter, and. Silicon tetrachloride is obtained by chlorinating a mixture of coal and quartz sand (600-700°C):
Significant quantities are captured as a by-product in superphosphate plants operating on apatite raw materials. Alternatively, it can be prepared by heating a mixture of quartz sand, calcium fluoride and sulfuric acid:
Silicon tetrafluoride, adding two formula units of HF, transforms into hydrofluorosilicic (hexafluorosilicic) acid:
In the individual state it is not isolated, its strength is close to sulfuric acid. Its salts - hexafluorosilicates - when heated, decompose into metal fluorides. In the octahedral structure of ions, silicon is in a state of -hybridization and its coordination number is 6. For other halogens, compounds of similar composition are unknown.
Trichlorosilane (or silicochloroform) is obtained by passing a current of dry hydrogen chloride over silicon (400-500°C). It does not ignite in air, but burns when ignited. Silicon compounds similar to trichlorosilane are also known for other halogens. By reducing trichlorosilane, high-purity silicon is obtained.
Compounds with other non-metals
Silicon disulfide is obtained by direct interaction of the components. Disulfide is also formed by the displacement of hydrogen from silicon in the absence of air at 1300°C:
Silicon disulfide - white silky crystals. Silicon disulfide decomposes with water into i. Silicon monosulfide SiS is also known. It is obtained by reducing the disulfide in vacuum at 900°C. Monosulfide is a polymer needle-shaped crystals that decomposes with water:
Silicon nitride is obtained either by the interaction of components (at temperatures above 1300°C) or from i. In the latter case, silicon imide is formed as an intermediate product, which is converted into nitride during thermal decomposition:
Colorless crystals are characterized by great chemical resistance. Up to 1000°C it is not affected by oxygen, hydrogen and water vapor. It does not dissolve in acids and alkali solutions. Only melts of alkalis and hot concentrated hydrofluoric acid slowly decompose it.
Of the silicon-phosphorus compounds, the most famous are mono- and diphosphide: SiP and. They are obtained by direct interaction of components in the required stoichiometric quantities and are characterized by chemical resistance. Silicon arsenides have a similar composition.
Silicon(IV) oxide
Silicon compounds with hydrogen and halogens
The action of hydrochloric acid on magnesium silicide Mg 2 Si produces hydrogen silica (silane) SiH 4, similar to methane:
Mg 2 Si + 4 HC1 = 2 MgCl 2 + SiH 4
Silan SiH 4 is a colorless gas that ignites spontaneously in air and burns to form silicon dioxide and water:
SiH 4 + 2 O 2 = SiO 2 + 2 H 2 O
Silane is easily hydrolyzed, especially in an alkaline environment:
SiH 4 + 2H 2 O = SiO 2 + 4H 2
SiH 4 + 2NaOH + H 2 O = Na 2 SiO 3 + 4H 2
Silicon chloride SiCl 4 is obtained by heating a mixture of silicon dioxide and carbon in a stream of chlorine:
SiO 2 + 2 C + 2 C1 2 = SiCl 4 + 2 CO
or chlorination of technical silicon. It is a liquid boiling at 57 °C.
When exposed to water, silicon chloride undergoes complete hydrolysis with the formation of silicic and hydrochloric acids;
SiCl 4 + 3 H 2 O = H 2 SiO 3 + 4 HCl
Silicon chloride is used for the synthesis of organosilicon compounds.
Silicon fluoride SiF 4 is formed by the interaction of hydrogen fluoride with silicon dioxide:
SiO 2 + 4 НF = SiF 4 + 2 Н 2 О
It is a colorless gas with a pungent odor.
Like silicon chloride, SiF 4 hydrolyzes in aqueous solutions:
SiF 4 + 3 H 2 O = H 2 SiO 3 + 4 HF
The resulting hydrogen fluoride interacts with SiF 4 . In this case it turns out hexafluorosilicon(or hydrofluorosilicic acid H2SiF6:
3 SiF 4 + 3 H 2 O ═ 2 H 2 SiF 6 + H 2 SiO 3
The strength of hexafluorosilicic acid is close to sulfuric acid. Its salts - silicofluorides, or fluorosilicates, are mostly soluble in water; sodium, potassium, rubidium, cesium salts are slightly soluble; barium salt is practically insoluble. The acid itself and all fluorosilicates are poisonous.
The most stable silicon compound is silica, or silica, SiO2. It occurs in both crystalline and amorphous forms.
Crystalline silicon dioxide occurs in nature primarily as a mineral. quartz.
Crystalline silicon dioxide is very hard, insoluble in water and melts at about 1610°C, turning into a colorless liquid. Upon cooling this liquid, a transparent glassy mass of amorphous silicon dioxide is obtained, similar in appearance to glass.
Amorphous silicon dioxide is much less common in nature than crystalline silicon dioxide. At the bottom of the seas there are deposits of fine, porous amorphous silica called trembling or diatomaceous earth. These deposits were formed from SiO 2, which was part of the organisms of diatoms and some ciliates.
1) SiO 2 is an acidic oxide, therefore amorphous silica slowly dissolves in aqueous solutions of alkalis, forming the corresponding salts of silicic acid (silicates):
SiO 2 + 2 NaOH ═ Na 2 SiO 3 + H 2 O
2) SiO 2 also interacts with basic oxides when heated:
SiO 2 + K 2 O = K 2 SiO 3
SiO 2 + CaO = CaSiO 3
3) Being a non-volatile oxide, SiO 2 displaces carbon dioxide from Na 2 CO 3 (when fused):
SiO 2 + Na 2 CO 3 = Na 2 SiO 3 + CO 2
4) Acids, with the exception of hydrofluoric acid, do not act on silicon dioxide. Hydrofluoric acid easily reacts with it, forming silicon fluoride and water:
SiO 2 + 4 HF ═ SiF 4 + 2 H 2 O
5) At temperature, SiO 2 interacts with gaseous HF and F 2, forming tetrafluorosilane (silicon tetrafluoride):
SiO 2 + 4HF = SiF 4 + 2H 2 O
Silicon does not interact directly with hydrogen. Only when water or acid acts on silicides, a mixture of hydrogen silicates is formed, called silanes: gaseous and liquid, resembling saturated hydrocarbons, Si n H 2 n +2. Unlike hydrocarbons, silanes are more reactive in redox reactions, in which they always exhibit the properties of reducing agents due to Si-4:
SiH 4 + O 2 = SiO 2 + 2H 2 O
Silicon compounds with oxygen.
Silicon forms one oxide, SiO 2, with oxygen. A refractory solid, insoluble in water and acids, with the exception of hydrofluoric acid (HF), in which silicon oxide forms gaseous silicon tetrafluoride:
SiO 2 + 4HF = SiF 4 + 2H 2 O
Silicon(IV) oxide, which is insoluble in water, has weak acidic properties, so it reacts with basic oxides and bases, but only at high temperatures:
Na 2 O + SiO 2 = Na 2 SiO 3 2NaOH + SiO 2 = Na 2 SiO 3 + H 2 O
The silicic acid corresponding to SiO 2 oxide can be obtained in an aqueous solution only from silicates under the action of stronger acids:
Na 2 SiO 3 + 2HCl = H 2 SiO 3 ↓ + 2NaCl
Na 2 SiO 3 + CO 2 + H 2 O = H 2 SiO 3 ↓ + Na 2 CO 3
Silicic acid is a very weak acid, and therefore water-soluble alkaline Me silicates undergo strong hydrolysis at the anion, forming an alkaline environment:
SiO 3 2- + H-OH ↔ HSiO 3 - + OH -
HSiO 3 - + H-OH ↔ H 2 SiO 3 + OH -
In redox reactions, silicon (IV) oxide is a very weak oxidizing agent and can only be reduced by strong reducing agents, such as Mg or Ca, and then at high temperatures (>1000 0 C):
2Mg + SiO 2 = Si + 2MgO
Questions for control
1. Describe the position of silicon in the periodic table of D.I. Mendeleev. what oxidation states are characteristic of silicon.
2. What compounds of silicon with halogens, hydrogen, oxygen, nitrogen, and metals do you know? Write the formulas of these compounds. How can they be obtained?
3. How does silicon(IV) oxide differ from other acidic oxides?
Tasks and exercises for independent work
1. Ultrapure silicon for transistors is produced by converting chemically pure silicon into silicon(IV) bromide, which is then reduced with hydrogen. Write the equations for the corresponding reactions.
2. Why does silicon, which does not dissolve in concentrated nitric acid, dissolve in a mixture of HNO 3 and HF? What role does HF play in such a mixture?
3. Make up reaction equations between: a) silica and soda; b) silica and barium hydroxide; c) potassium silicate and hydrofluoric acid.
4. What is the nature of the environment aqueous solution potassium silicate? Support your answer with molecular and ionic equations for the reaction occurring in this solution.
5. Create reaction equations that can be used to carry out the following transformations:
a) SiO 2 → Si → Mg 2 Si → SiH 4 → SiO 2 → Na 2 SiO 3 → H 2 SiO 3 ;
b) SiCl 4 → Si → K 2 SiO 3 → H 2 SiO 3 → SiO 2 → glass.
6. Three test tubes contain solutions of sodium silicate, sodium carbonate and potassium sulfide. Which one reagent can be used to determine each of these solutions? Give equations for the corresponding reactions.
7. There is a mixture of silicon, graphite and calcium carbonate. Find its quantitative composition if it is known that when treating 34 g of a mixture with a NaOH solution, 22.4 liters of gas (no.s.) were obtained, and when treating the same portion of the mixture with hydrochloric acid, 2.24 liters of gas (no.s.) were obtained. .
8. There is a mixture of silicon, aluminum and calcium carbonate. What is its quantitative composition if it is known that when treating the mixture with an excess of alkali solution, 17.92 liters of gas (n.s.) are released, and when processing the same portion of the mixture with hydrochloric acid, also 17.92 liters of gas are released, passing through the hydroxide solution calcium was formed 16.2 g Ca(HCO 3) 2?
ANSWERS TO PROBLEMS
Si is one of the most common elements in the earth's crust. The most common after O2. In nature, Si occurs only in the form of a compound: SiO2. The most important element of the plant and animal kingdom.
Preparation: Technical: SiO2 + 2C ==== Si + 2CO. Pure: SiCl4 + 2H2 = Si + 4HCl. SiH4 =(t) Si + 2H2. Used in metallurgy and semiconductor technology. To remove O2 from molten Me and serves integral part alloys For the manufacture of photocells, amplifiers, rectifiers.
Physical properties aza. Silicon - gray-steel color. fragile, only when heated above 800 °C does it become plastic substance. Transparent to infrared radiation, semiconductor. The crystal lattice is cubic like diamond, but due to the longer bond length between Si-Si atoms compared to the length S-S connections The hardness of silicon is significantly less than diamond. Allotropic gray Si powder.
Chemical properties: When n. u. Si is slightly active and reacts only with fluorine gas: Si + 2F2 = SiF4
Amorphous Si is more reactive, molten Si is very active.
When heated to a temperature of 400-500 °C, silicon reacts with O2, Cl2, Br2, S: Si + O2 = SiO2 . Si + 2 Cl2 = SiCl4
With nitrogen, silicon at a temperature of about 1000 °C forms nitride Si3N4,
with boron - thermally and chemically stable borides SiB3, SiB6 and SiB12.,
with carbon - silicon carbide SiC (carborundum).
When silicon is heated with metals, silicides can form.
Si does not react with acids; it only oxidizes it with a mixture of HNO3 and HF to hexafluorosilicic acid: 3Si+8HNO3+18HF=3H2+4NO+8H2O
In alkali solutions, it dissolves vigorously in the cold (non-metallic properties): Si + 2NaOH + H2O = Na2SiO3 + 2 H2
At high temperatures it slowly reacts with water: Si + 3H2O = H2SiO3 + 2H2
Hydrogen compoundsSi.Silicon does not react directly with hydrogen; silicon compounds with hydrogen do silanes with the general formula SinH2n+2 is obtained indirectly. Monosilane SiH4 Ca2Si + 4HCl → 2CaCl2 + SiH4 is an admixture of other silanes, disilane Si2H6 and trisilane Si3H8.
Polysilanes are toxic, have an unpleasant odor, are less thermally stable compared to СnH2n+2Reducing agentsSiH4 + O2 = SiO2 + 2 H2O
Hydrolyze in water: SiH4 + 2H2O = SiO2 + 4H2
Silicon compounds with metals – SILICIDES
I.Ionic-covalent: silicides of alkali, alkaline earth metals and magnesium Ca2Si, Mg2Si
Easily destroyed by water: Na2Si + 3H2O = Na2SiO3 + 3 H2
Decomposes under the influence of acids: Ca2Si + 2H2SO4 = 2CaSO4 + SiH4
II. Metal-like: transition metal silicides. Chemically stable and do not decompose under the influence of acids, resistant to oxygen even at high temperatures. They have high melting point (up to 2000 °C). Many have metallic conductivity. The most common are MeSi, Me3Si2, Me2Si3, Me5Si3 and MeSi2.
Silicides of d-elements are used to produce heat-resistant and acid-resistant alloys. Lanthanide silicides are used in nuclear energy as neutron absorbers.
SiC – carborundum Solid, refractory substance. The crystal lattice is similar to that of a diamond. Is a semiconductor. Used to make artificial gemstones
Silica easily reacts with F2 and HF: SiO2 + 4HF = SiF4 + 2 H2O. SiO2 + F2 = SiF4 + O2 Insoluble in water.
Dissolves in alkali solutions when heated: SiO2 + 2NaOH = Na2SiO3 + H2O
Sintered with salts: SiO2 + Na2CO3 = Na2SiO3 + CO2. SiO2 + PbO = PbSiO3
Silicic acids Very weak, slightly soluble acids in water. In water, silicic acids form colloidal solutions.
Salts of silicic acids are called silicates. SiO2 corresponds to silicic acid, which can be obtained by the action of a strong acid on silicateNa2SiO3 + HCl = H2SiO3 + NaCl
H2SiO3 is metasilicon or silicic acid. H4SiO4 - orthosilicic acid exists only in solution and is irreversibly converted to SiO2 if water is evaporated.
Silicates-salts of silicic acids, each Si atom surrounds an O2 atom tetrahedrally located around it. Close connection between Si and O2.