SILICON: HISTORY, PROPERTIES, STRUCTURE, OBTAINING, USES - CHEMISTRY - 2025

The silicon is a non - metallic and metalloid the same time element 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.

Deserts are an abundant natural source of silica particles or granites along with other minerals. Source: Pxhere.

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 Si ⁴⁺ cation.

One property it shares with coal is its ability to link together; that is, their atoms are covalently linked to define molecular chains. Also, 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 pose considerable risks.

History

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 today, 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.

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 considered a metal by then due to its lack of characterization.

Then, in 1811, 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

Elemental silicon, which has a metallic luster, but is actually a metalloid. Source: Hi-Res Images of Chemical Elements

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 (₁₄ Yes)

Melting point

1414 ºC

Boiling point

3265 ºC

Density

-At room temperature: 2.33 g / mL

-Right at 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 the same, as it happens with the ice-water system. The explanation is due to the fact that the interatomic space between the Si atoms in its 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 · 10 ³ 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, the sp ³ hybridizations of silicon are poorer compared to that of the other two elements and, furthermore, their 3p orbitals are more diffuse, so the overlap of the resulting sp ³ orbitals is weaker.

The average energies of the Si-Si and CC 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 (Si ₄).

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 (Si ^3-), -2 (Si ^2-), -1 (Si ^-), +1 (Si ⁺), +2 (Si ²⁺), +3 (Si ³⁺) and +4 (Si ⁴⁺). Of all of them, the -4 and +4 are the most important.

For example, -4 is assumed in silicides (Mg ₂ Si or Mg ₂ ²⁺ Si ^4-); while the +4 corresponds to that of silica (SiO ₂ or Si ⁴⁺ O ₂ ^2-).

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 (HNO ₃ -HF). Likewise, it dissolves in a hot alkaline solution, the following chemical reaction occurs:

Si (s) + 2NaOH (aq) + H ₂ O (l) => Na ₂ SiO ₃ (aq) + 2H ₂ (g)

The sodium metasilicate salt, Na ₂ SiO ₃, is also formed when silicon is dissolved in molten sodium carbonate:

Si (s) + Na ₂ CO ₃ (l) => Na ₂ SiO ₃ (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 ₂ begins to form; and then, at 1400 ºC, the silicon reacts with the nitrogen in the air to form a mixture of nitrides, SiN and Si ₃ N ₄.

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

2Mg (s) + Si (s) => Mg ₂ Si (s)

2Cu (s) + Si (s) => Cu ₂ Si (s)

At room temperature it reacts explosively and directly with halogens (there is no SiO ₂ layer to protect it from this). For example, we have the formation reaction of SiF ₄:

Si (s) + 2F ₂ (g) => SiF ₄ (g)

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

Si (s) + H ₂ O (g) => SiO ₂ (s) + 2H ₂ (g)

Structure and electronic configuration

Crystalline structure or unit cell of silicon represented by a spheres and rods model. Source: Benjah-bmm27

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 bound to each other; in addition, they in turn 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 it, 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, as 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:

3s ² 3p ²

For bonding, the pure 3s and 2p orbitals are useless. That is why the atom creates four sp ³ hybrid orbitals, 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 three-dimensional, covalent lattice 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 silvery mosaic or scaly surface).

Electric conductivity

Si-Si bonds, with their well-located electrons, in principle differ from 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 thus 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 to be not sp ³ but sp ², 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

Quartz (silica) crystals are one of the main and most extraordinary minerals where silicon is found. Source: James St. John (https://www.flickr.com/photos/jsjgeology/22437758830)

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 ₂, 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 ₂. 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.

-Reduction of silica

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, silica is mixed with magnesium powder in a crucible and incinerated in the absence of air. The following reaction then takes place:

SiO ₂ (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 excess SiO ₂ finishes reacting; otherwise, the excess of magnesium favors the formation of its respective silicide, Mg ₂ Si, an undesirable compound for the process.

SiO ₂ is transformed into the volatile gas SiF ₄, 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 consists of using the same SiF ₄ produced previously, or the SiCl ₄ (previously acquired). 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:

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

Interestingly, amorphous silicon is used to make 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:

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

The reaction also produces silicon carbide, SiC, which is neutralized with an excess of SiO ₂ (again the quartz is in excess):

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

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

3SiO ₂ (s) + 4Al (l) => 3Si (s) + 2Al ₂ O ₃ (s)

And starting from the potassium hexafluorurosilicate salt, K ₂, it is also reacted with metallic aluminum or potassium to produce the same product:

K ₂ (l) + 4Al (l) => 3Si (s) + 6KF (l) + 4AlF ₃ (g)

Silicon immediately dissolves in molten aluminum, and when the system is cooled, the first crystallizes and separates from the second; that is to say, 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, SiH ₄. This gas is subjected to pyrolysis above 500 ºC, in such a way that thermal decomposition occurs and thus, from its initial vapors, polycrystals of silicon end up depositing on a semiconductor surface.

The following chemical equation exemplifies the reaction that takes place:

SiH ₄ (g) => Si (s) + H ₂ (g)

Obviously, there should be no oxygen in the chamber, as it would react with SiH ₄:

SiH ₄ (g) + 2O ₂ (g) => SiO ₂ (s) + 2H ₂ O (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) => SiCl ₃ H (g) + H ₂ (g)

And the SiCl ₃ H reacts at 1100 ºC to regenerate the silicon, but now polycrystalline:

4SiCl ₃ H (g) => Si (s) + 3SiCl ₄ (g) + 2H ₂ (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 ²⁸ Si isotope, 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: ²⁹ Si, with an abundance of 4.67%; and ³⁰ Yes, with an abundance of 3.10%. ²⁸ Si being so abundant, it is not surprising that the atomic weight of silicon is 28.084 u.

Silicon can also be found in various radioisotopes, among which are ³¹ Si (t _1/2 = 2.62 hours) and ³² Si (t _1/2 = 153 years). The others (²² Si - ⁴⁴ Si) have very short or brief t _1/2 (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 watering 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 represent any risk to the environment.

Now, with respect 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 firebricks consist of solid mixtures based on silicates. From this approach, one can imagine the utility 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.

Also, 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 cohesion 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

Gelatinous silica balls, used as desiccants. Source: 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

Polycrystalline and amorphous silicon are used to make solar panels. Source: Pxhere.

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 fewer (Al, B, Ga) or more electrons (P, As, Sb) to transform it into pon-type semiconductors, respectively. With the junctions of two silicones, one n and the other p, light-emitting diodes are made.

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 may be mentioned:

-Glue or adhesive, 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 resist temperature changes 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 humidity

-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. (sf). Crystallography of silicon. Recovered from: microchemicals.com
4. Lenntech BV (2019). Periodic table: silicon. Recovered from: lenntech.com
5. Marques Miguel. (sf). Silicon Occurrence. Recovered from: nautilus.fis.uc.pt
6. More Hemant. (November 05, 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