Silicon history, properties, structure, obtaining, uses

The silicon It is a non-metallic and at the same time metalloid element that is represented by the chemical symbol Si. It is a semiconductor, which is an essential part of computers, calculators, cell phones, solar cells, diodes, etc .; It is practically the main component that has allowed the establishment of the Digital Age.

Silicon has always been present in quartz and silicates, both minerals making up about 28% by mass of the entire earth's crust. It is thus the second most abundant element on the surface of the Earth, and the vastness of the deserts and beaches offer a perspective of how abundant it is..

Silicon belongs to group 14 of the periodic table, the same as carbon, located below it. That is why this element is considered a tetravalent metalloid; It has four valence electrons and in theory it can lose all of them to form the cation Si⁴⁺.

One property it shares with coal is its ability to link together; that is, their atoms are covalently linked to define molecular chains. Likewise, silicon can form its own “hydrocarbons”, called silanes..

The predominant compounds of silicon in nature are the famous silicates. In its pure form it can appear as a monocrystalline, polycrystalline or amorphous solid. It is a relatively inert solid, so it does not represent considerable risks.

Article index

• 1 History
  • 1.1 The silicon stone
  • 1.2 Insulation
  • 1.3 Crystalline silicon
• 2 Physical and chemical properties
  • 2.1 Physical appearance
  • 2.2 Molar mass
  • 2.3 Atomic number (Z)
  • 2.4 Melting point
  • 2.5 Boiling point
  • 2.6 Density
  • 2.7 Heat of fusion
  • 2.8 Heat of vaporization
  • 2.9 Molar heat capacity
  • 2.10 Electronegativity
  • 2.11 Ionization energies
  • 2.12 Atomic radius
  • 2.13 Thermal conductivity
  • 2.14 Electrical resistivity
  • 2.15 Mohs hardness
  • 2.16 Concatenation
  • 2.17 oxidation numbers
  • 2.18 Reactivity
• 3 Structure and electronic configuration
  • 3.1 Covalent character
  • 3.2 Electrical conductivity
  • 3.3 Amorphous silicon
• 4 Where to find and obtaining
  • 4.1 -Reduction of silica
• 5 Isotopes
• 6 Risks
• 7 Uses
  • 7.1 Construction industry
  • 7.2 Glass and ceramics
  • 7.3 Alloys
  • 7.4 Desiccants
  • 7.5 Electronic industry
  • 7.6 Silicone polymers
• 8 References

Story

Silicon stone

Silicon is perhaps one of the elements that has had the most influence in the history of mankind.

This element is the protagonist of the Stone Age, and also of the Digital Age. Its origins date back to when civilizations once worked with quartz and made their own glasses; and currently, it is the main component of computers, laptops and smartphones.

Silicon has practically been the stone of two clearly defined eras in our history.

Isolation

Since silica is so abundant, a name born of flint rock, it must have contained an extremely rich element in the earth's crust; this was the right suspicion of Antoine Lavoisier, who in 1787 failed in his attempts to reduce it from its rust.

Some time later, in 1808 Humphry Davy made his own attempts and gave the element its first name: 'silicium', which translated would come to be as 'flint metal'. That is, silicon was then considered a metal due to its lack of characterization..

Then in 1811 the French chemists Joseph L. Gay-Lussac and Louis Jacques Thénard succeeded in preparing amorphous silicon for the first time. For this they reacted the silicon tetrafluoride with metallic potassium. However, they did not purify or characterize the product obtained, so they did not conclude that it was the new element silicium.

It was not until 1823 that the Swedish chemist Jacob Berzelius obtained an amorphous silicon of sufficient purity to recognize it as silicon; name given in 1817 by the Scottish chemist Thomas Thomson when considering it a non-metallic element. Berzelius carried out the reaction between potassium fluorosilicate and molten potassium to produce this silicon.

Crystalline silicon

Crystalline silicon was first prepared in 1854 by the French chemist Henry Deville. To achieve this, Deville performed an electrolysis of a mixture of aluminum and sodium chlorides, thus obtaining silicon crystals covered by a layer of aluminum silicide, which he removed (apparently) by washing them with water..

Physical and chemical properties

Physical appearance

Silicon in its pure or elemental form consists of a grayish or bluish-black solid (top image), which although not a metal, has shiny faces as if it really were.

It is a hard but brittle solid, which also exhibits a flaky surface if it is made up of polycrystals. Amorphous silicon, on the other hand, looks like a dark brown powdered solid. Thanks to this, it is easy to identify and differentiate one type of silicon (crystalline or polycrystalline) from another (amorphous).

Molar mass

28.085 g / mol

Atomic number (Z)

14 (₁₄Yes)

Melting point

1414 ºC

Boiling point

3265 ºC

Density

-At room temperature: 2.33 g / mL

-Right at the melting point: 2.57 g / mL

Note that liquid silicon is denser than solid silicon; which means that its crystals will float on a liquid phase of it, as happens with the ice-water system. The explanation is due to the fact that the interatomic space between the Si atoms in their crystal is larger (less dense) than the corresponding one in the liquid (more dense).

Heat of fusion

50.21 kJ / mol

Heat of vaporization

383 kJ / mol

Molar heat capacity

19.789 J / (mol K)

Electronegativity

1.90 on the Pauling scale

Ionization energies

-First: 786.5 kJ / mol

-Second: 1577.1 kJ / mol

-Third: 3231.6 kJ / mol

Atomic radio

111 pm (measured on their respective diamond crystals)

Thermal conductivity

149 W / (m K)

Electrical resistivity

2.3 10³ Ω · m at 20 ºC

Mohs hardness

6.5

Concatenation

Silicon atoms have the ability to form simple Si-Si bonds, which end up defining a chain (Si-Si-Si…).

This property is also manifested by carbon and sulfur; however, sp hybridizations³ silicon are more deficient compared to that of the other two elements and, in addition, their 3p orbitals are more diffuse, so the overlap of the sp orbitals³ resulting is weaker.

The average energies of the Si-Si and C-C covalent bonds are 226 kJ / mol and 356 kJ / mol, respectively. Therefore, the Si-Si bonds are weaker. Because of this, silicon is not the cornerstone of life (and neither is sulfur). In fact, the longest chain or skeleton that silicon can form is usually four-membered (Si₄).

Oxidation numbers

Silicon can have any of the following oxidation numbers, assuming in each of them the existence of ions with their respective charges: -4 (Si^4-), -3 (Yes^3-), -2 (Yes^two-), -1 (Yes^-), +1 (Yes⁺), +2 (Yes^two+), +3 (Yes³⁺) and +4 (Yes⁴⁺). Of all of them, -4 and +4 are the most important.

For example, -4 is assumed in silicides (Mg_twoYes or Mg_two^two+Yes^4-); while the +4 corresponds to that of silica (SiO_two or if⁴⁺OR_two^two-).

Reactivity

Silicon is completely insoluble in water, as well as strong acids or bases. However, it dissolves in a concentrated mixture of nitric and hydrofluoric acids (HNO₃-HF). Likewise, it dissolves in a hot alkaline solution, with the following chemical reaction occurring:

Si (s) + 2NaOH (aq) + H_twoO (l) => Na_twoYes₃(aq) + 2H_two(g)

The sodium metasilicate salt, Na_twoYes₃, It is also formed when silicon dissolves in molten sodium carbonate:

Si (s) + Na_twoCO₃(l) => Na_twoYes₃(l) + C (s)

At room temperature it does not react at all with oxygen, not even at 900 ºC, when a protective vitreous layer of SiO begins to form_two; and then, at 1400 ºC, the silicon reacts with the nitrogen in the air to form a mixture of nitrides, SiN and Si₃N₄.

Silicon also reacts at high temperatures with metals to form metal silicides:

2Mg (s) + Si (s) => Mg_twoYes (s)

2Cu (s) + Si (s) => Cu_twoYes (s)

At room temperature it reacts explosively and directly with halogens (there is no SiO layer_two to protect you from this). For example, we have the reaction of formation of SiF₄:

Yes (s) + 2F_two(g) => SiF₄(g)

And although silicon is insoluble in water, it reacts red hot with a stream of vapor:

Yes (s) + H_twoO (g) => SiO_two(s) + 2H_two(g)

Structure and electronic configuration

The image above shows the face-centered cubic structure (fcc), the same as that of diamond, for silicon crystal. The greyish spheres correspond to the Si atoms, which, as can be seen, are covalently linked to each other; in addition, they have tetrahedral environments that are reproduced along the crystal.

The silicon crystal is fcc because an Si atom is observed located on each of the faces of the cube (6 × 1/2). Likewise, there are eight Si atoms at the vertices of the cube (8 × 1/8), and four located inside it (those that show a well-defined tetrahedron around them, 4 × 1).

That said, each unit cell has a total of eight silicon atoms (3 + 1 + 4, numbers indicated in the paragraph above); characteristic that helps to explain its high hardness and rigidity, since pure silicon is a covalent crystal like diamond.

Covalent character

This covalent character is due to the fact that, like carbon, silicon has four valence electrons according to its electronic configuration:

[Ne] 3s^two 3p^two

For bonding, the pure 3s and 2p orbitals are useless. That is why the atom creates four hybrid sp orbitals³, with which it can form four Si-Si covalent bonds and, in this way, complete the valence octet for the two silicon atoms.

The silicon crystal is then visualized as a covalent, three-dimensional network composed of interconnected tetrahedra..

However, this network is not perfect, since it has defects and grain boundaries, which separate and define one crystal from another; and when such crystals are very small and numerous, we speak of a polycrystalline solid, identified by its heterogeneous luster (similar to a silver mosaic or scaly surface).

Electric conductivity

The Si-Si bonds, with their well-located electrons, in principle disagree with what is expected of a metal: a sea of ​​electrons "wetting" its atoms; at least this is so at room temperature.

When the temperature increases, however, the silicon begins to conduct electricity and therefore behaves like a metal; that is, it is a semiconductor metalloid element.

Amorphous silicon

Silicon tetrahedra do not always adopt a structural pattern, but can be arranged in a disorderly way; and even with silicon atoms whose hybridizations seem not to be sp³ but sp^two, which contributes to further increasing the degree of disorder. Therefore, we speak of an amorphous and non-crystalline silicon.

In amorphous silicon there are electronic vacancies, where some of its atoms have an orbital with an unpaired electron. Thanks to this, its solid can be hydrogenated, giving rise to the formation of hydrogenated amorphous silicon; that is, it has Si-H bonds, with which the tetrahedra are completed in disordered and arbitrary positions.

This section concludes then by saying that silicon can be presented in three types of solids (without mentioning its degree of purity): crystalline, polycrystalline and amorphous..

Each of them has its own production method or process, as well as its applications and trade-offs when deciding which of the three to use, knowing its advantages and disadvantages..

Where to find and obtaining

Silicon is the seventh most abundant element in the Universe, and the second in the Earth's crust, also enriching the Earth's mantle with its vast family of minerals. This element associates extremely well with oxygen, forming a wide range of oxides; among them, silica, SO_two, and silicates (of diverse chemical composition).

Silica can be seen with the naked eye in deserts and beaches, as sand is mainly composed of SiO_two. In turn, this oxide can manifest itself in a few polymorphs, the most common being: quartz, amethyst, agate, cristobalite, tripoli, coesite, stishovite and tridymite. In addition, it can be found in amorphous solids such as opals and diatomaceous earth..

Silicates, meanwhile, are even richer structurally and chemically. Some of the silicate minerals include: asbestos (white, brown and bluish), feldspar, clays, micas, olivines, aluminosilicates, zeolites, amphiboles and pyroxenes.

Virtually all rocks are composed of silicon and oxygen, with their stable Si-O bonds, and their silicas and silicates mixed with metal oxides and inorganic species..

-Silica reduction

The problem of obtaining silicon is breaking said Si-O bond, for which special furnaces and a good reduction strategy are needed. The raw material for this process is silica in the form of quartz, which is previously ground until it is a fine powder..

From this ground silica, either amorphous or polycrystalline silicon can be prepared..

Amorphous silicon

On a small scale, carried out in a laboratory and with the appropriate measures, the silica is mixed with magnesium powder in a crucible and incinerated in the absence of air. The following reaction then takes place:

Yes_two(s) + Mg (s) => 2MgO (s) + Si (s)

Magnesium and its oxide are removed with a dilute hydrochloric acid solution. Then, the remaining solid is treated with hydrofluoric acid, so that the SiO finishes reacting_two in excess; otherwise, the excess of magnesium favors the formation of its respective silicide, Mg_twoYes, undesirable compound for the process.

The SiO_two becomes the volatile gas SiF₄, which is recovered for other chemical syntheses. Finally, the amorphous silicon mass is dried under a stream of hydrogen gas..

Another similar method to obtain amorphous silicon is to use the same SiF₄ previously produced, or the SiCl₄ (previously purchased). The vapors of these silicon halides are passed over liquid sodium in an inert atmosphere, so that the reduction of the gas can take place without the presence of oxygen:

SiCl₄(g) + 4Na (l) => Si (s) + 4NaCl (l)

Interestingly, amorphous silicon is used to make high-energy-efficient solar panels.

Crystalline silicon

Starting again from the pulverized silica or quartz, they are taken to an electric arc furnace, where they react with coke. In this way, the reducing agent is no longer a metal but a carbonaceous material of high purity:

Yes_two(s) + 2C (s) => Si (s) + 2CO (g)

The reaction also produces silicon carbide, SiC, which is neutralized with an excess of SiO_two (again the quartz is in excess):

2SiC (s) + SiO_two(s) => 3Si (s) + 2CO (g)

Another method to prepare crystalline silicon is to use aluminum as a reducing agent:

3YES_two(s) + 4Al (l) => 3Si (s) + 2Al_twoOR₃(s)

And starting from the potassium hexafluorurosilicate salt, K_two[SiF₆], it is also reacted with metallic aluminum or potassium to produce the same product:

K_two[SiF₆] (l) + 4Al (l) => 3Si (s) + 6KF (l) + 4AlF₃(g)

Silicon immediately dissolves in molten aluminum, and when the system is cooled, the first crystallizes and separates from the second; that is, silicon crystals are formed, which appear grayish colors.

Polycrystalline silicon

Unlike the other syntheses or productions, to obtain polycrystalline silicon one starts with a silane gas phase, SiH₄. This gas is subjected to a pyrolysis above 500 ºC, in such a way that a thermal decomposition occurs and thus, from its initial vapors, polycrystals of silicon end up being deposited on a semiconductor surface.

The following chemical equation exemplifies the reaction that takes place:

Yes H₄(g) => Si (s) + H_two(g)

Obviously, there should be no oxygen in the chamber, as it would react with the SiH₄:

Yes H₄(g) + 2O_two(g) => SiO_two(s) + 2H_twoO (g)

And such is the spontaneity of the combustion reaction that it occurs rapidly at room temperature with minimal exposure of the silane to air..

Another synthetic route to produce this type of silicon starts from crystalline silicon as a raw material. They make it react with hydrogen chloride at a temperature around 300 ºC, so that trichlorosilane is thus formed:

Si (s) + 3HCl (g) => SiCl₃H (g) + H_two(g)

And the SiCl₃H reacts at 1100 ºC to regenerate the silicon, but now polycrystalline:

4SiCl₃H (g) => Si (s) + 3SiCl₄(g) + 2H_two(g)

Just look at the equations to get an idea of ​​the work and rigorous production parameters that must be considered..

Isotopes

Silicon occurs naturally and mainly as the isotope ²⁸Yes, with an abundance of 92.23%.

In addition to this, there are two other isotopes that are stable and therefore do not undergo radioactive decay: ²⁹Yes, with an abundance of 4.67%; Y ³⁰Yes, with an abundance of 3.10%. Being so abundant the ²⁸Yes, it is not surprising that the atomic weight of silicon is 28.084 u.

Silicon can also be found in several radioisotopes, among which are the ³¹Yes (t_1/2= 2.62 hours) and ³²Yes (t_1/2= 153 years). Others (²²Yes - ⁴⁴If they have t_1/2 very short or brief (less than hundredths of a second).

Risks

Pure silicon is a relatively inert substance, so it does not usually accumulate in any organ or tissue as long as the exposure to it is low. In powder form it can irritate the eyes, causing tearing or redness, while touching it can cause skin discomfort, itching and peeling.

When the exposure is very high, silicon can damage the lungs; but without after-effects, unless the amount is sufficient to cause suffocation. However, this is not the case with quartz, which is associated with lung cancer and diseases such as bronchitis and emphysema..

Likewise, pure silicon is very rare in nature, and its compounds, so abundant in the earth's crust, do not pose any risk to the environment..

Now, with regard to organosilicon, these could be toxic; But since there are many of them, it depends on which one is being considered, as well as other factors (reactivity, pH, mechanism of action, etc.).

Applications

Construction Industry

Silicon minerals make up the "stone" with which buildings, houses, or monuments are built. For example, cements, concretes, stuccoes and refractory bricks, consist of solid mixtures based on silicates. From this approach, one can imagine the usefulness of this element in cities and in architecture..

Glass and ceramics

Crystals used in optical devices can be made from silica, whether as insulators, sample cells, spectrophotometers, piezoelectric crystals or mere lenses..

Likewise, when the material is prepared with multiple additives, it ends up transforming into an amorphous solid, well known as glass; and mountains of sand are usually the source of the silica or quartz necessary for its production. On the other hand, with silicates ceramic materials and porcelains are manufactured.

Intertwining ideas, silicon is also present in crafts and ornamentation.

Alloys

Silicon atoms can coalesce and be miscible with a metallic matrix, making it an additive for many alloys or metals; for example, steel, to make magnetic cores; bronzes, for the manufacture of telephone cables; and aluminum, in the production of the aluminum-silicon alloy destined for light automotive parts.

Therefore, it can not only be found in the "stone" of buildings, but also in the metals of their columns..

Desiccants

Silica, in gel or amorphous form, makes it possible to manufacture solids that act as desiccants by trapping the water molecules that enter the container and keeping its interior dry..

Electronic industry

Silicon layers of different thicknesses and colors are part of computer chips, as with their solid (crystalline or amorphous), integrated circuits and solar cells have been designed.

Being a semiconductor, it incorporates atoms with less (Al, B, Ga) or more electrons (P, As, Sb) to transform it into semiconductors of the type p or n, respectively. With the junctions of two silicones, one n and the other p, light emitting diodes are manufactured.

Silicone polymers

The famous silicone glue consists of an organic polymer supported by the stability of the chains of Si-O-Si bonds ... If these chains are very long, short or cross-linked, the properties of the silicone polymer change, as well as their final applications..

Among its uses, listed below, the following can be mentioned:

-Glues or adhesives, not only to join papers, but building blocks, rubbers, glass panels, rocks, etc..

-Lubricants in hydraulic braking systems

-Strengthens paints and improves the brightness and intensity of their colors, while allowing them to withstand changes in temperature without cracking or eating away.

-They are used as water repellent sprays, which keeps some surfaces or objects dry

-They give personal hygiene products (toothpastes, shampoos, gels, shaving creams, etc.) the feeling of being silky

-Its coatings protect the electronic components of delicate devices, such as microprocessors, from heat and moisture

-With silicone polymers, several of the rubber balls have been made that bounce as soon as they are dropped to the floor..

References

1. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
2. Wikipedia. (2019). Silicon. Recovered from: en.wikipedia.org
3. MicroChemicals. (s.f.). Crystallography of silicon. Recovered from: microchemicals.com
4. Lenntech B.V. (2019). Periodic table: silicon. Recovered from: lenntech.com
5. Marques Miguel. (s.f.). Silicon Occurrence. Recovered from: nautilus.fis.uc.pt
6. More Hemant. (November 5, 2017). Silicon. Recovered from: hemantmore.org.in
7. Pilgaard Michael. (August 22, 2018). Silicon: Occurrence, isolation & synthesis. Recovered from: pilgaardelements.com
8. Dr. Doug Stewart. (2019). Silicon Element Facts. Chemicool. Recovered from: chemicool.com
9. Christiana Honsberg and Stuart Bowden. (2019). A collection of resources for the photovoltaic educator. PVeducation. Recovered from: pveducation.org
10. American Chemistry Council, Inc. (2019). Silicones in Everyday Life. Recovered from: sehsc.americanchemistry.com