SELENIUM: HISTORY, PROPERTIES, STRUCTURE, OBTAINING, USES - CHEMISTRY - 2024

The Selenium is a nonmetallic chemical element belonging to the group 16 of the periodic table and which is represented by the symbol is. This element has intermediate properties between sulfur and tellurium, which are members of the same group.

Selenium was discovered in 1817 by Jöhs J. Berzelius and John G. Gahn, who when vaporizing the pyrite observed a red residue (lower image). At first they confused it with tellurium, but later they realized that they were dealing with a new element.

A vial of amorphous red selenium, the best-known allotrope for this element. Source: W. Oelen

Berzelius named the new element selenium, based on the name "selene" which means "goddess of the moon." Selenium is an essential trace element for plants and animals, although in high concentrations it is a toxic element.

Selenium has three main allotropic forms: red, black, and gray. The latter has the property of modifying its electrical conductance depending on the intensity of the light that radiates it (photoconductor), for which it has had many applications.

Selenium is widely distributed in the earth's crust, however the minerals that contain it are not abundant, so there is no mining of selenium.

It is obtained mainly as a by-product of the copper electrolysis refining process. Selenium accumulates in the silt found at the anode of electrolysis cells.

Human beings possess around 25 selenoproteins, some of which have an antioxidant action and control the generation of free radicals. Also, there are amino acids of selenium, such as selenomethionine and selenocysteine.

History

First observation

The alchemist Arnold de Villanova may have observed selenium in 1230. He trained in medicine at the Sorbonne in Paris, and was even the physician of Pope Clement V.

Villanova in his book Rosarium Philosophorum describes a red sulfur or "sulfur rebeum" that had been left in a furnace after vaporizing the sulfur. This red sulfur may have been an allotrope of selenium.

Discovery

In 1817, Jöhs Jakob Berzelius and John Gottlieb Gahn discovered selenium in a chemical plant for the production of sulfuric acid, near Gripsholm, Sweden. The raw material for making the acid was pyrite, which was extracted from a Falun mine.

Berzelius was struck by the existence of a red residue that remained in the lead container after the sulfur had burned.

Also, Berzelius and Gahn observed that the red residue had a strong horseradish odor, similar to that of tellurium. Which is why he wrote to his friend Marect that they believed that the observed deposit was a tellurium compound.

However, Berselius continued to analyze the material deposited when the pyrite was incinerated and reconsidered that no tellurium had been found at the Falun mine. He concluded in February 1818 that he had discovered a new element.

Origin of its name

Berzelius pointed out that the new element was a combination of sulfur and tellurium, and that the similarity of tellurium to the new element had given him the opportunity to name the new substance selenium.

Berzelius explained that "tellus" means goddess of the earth. Martin Klaport in 1799 gave this name to tellurium and wrote: “No single element is called that. It had to be done! "

Because of the similarity of tellurium to the new substance, Berzelius named it with the word selenium, derived from the Greek word "selene" which means "goddess of the moon."

Development of your applications

In 1873 Willoughby Smith discovered that the electrical conductivity of selenium depended on the light that radiated it. This property allowed selenium to have numerous applications.

Alexander Graham Bell in 1979 used selenium in his photophone. Selenium produces an electric current proportional to the intensity of the light that illuminates it, being used in light meters, security mechanisms for opening and closing doors, etc.

The use of selenium rectifiers in electronics began in the 1930s, with numerous commercial applications. In the 1970s it was supplanted in rectifiers by silicon.

In 1957 it was discovered that selenium was an essential element for mammalian life, as it was present in enzymes that protect from reactive oxygen and free radicals. Furthermore, the existence of amino acids such as selenomethionine was discovered.

Physical and chemical properties

Appearance

As there are several allotropes for selenium, its physical appearance varies. It usually appears as a reddish solid in powder form.

Standard atomic weight

78.971 u

Atomic number (Z)

3. 4

Melting point

221 ºC

Boiling point

685 ºC

Density

The density of selenium varies depending on which allotrope or polymorph is considered. Some of its densities determined at room temperature are:

Gray: 4.819 g / cm ³

Alpha: 4.39 g / cm ³

Vitreous: 4.28 g / cm ³

Liquid state (melting point): 3.99 g / cm ³

Heat of fusion

Gray: 6.69 kJ / mol

Heat of vaporization

95.48 kJ / mol

Molar caloric capacity

25.363 J / (mol K)

Oxidation numbers

Selenium can bind in its compounds manifesting the following numbers or oxidation states: -2, -1, +1, +2, +3, +4, +5, +6. Among all of them, the most important are -2 (Se ^2-), +4 (Se ⁴⁺) and +6 (Se ⁶⁺).

For example, in SeO ₂, selenium has an oxidation number of +4; that is, the existence of the Se ⁴⁺ cation (Se ⁴⁺ O ₂ ^2-) is assumed. Similarly with SeO ₃, selenium has an oxidation number of +6 (Se ⁶⁺ O ₃ ^2-).

In hydrogen selenide, H ₂ Se, selenium has an oxidation number of -2; that is, again, the existence of the ion or anion Se ^2- (H ₂ ⁺ Se ^2-) is assumed. This is so because selenium is more electronegative than hydrogen.

Electronegativity

2.55 on the Pauling scale.

Ionization energy

-First: 941 kJ / mol.

-Second: 2,045 kJ / mol.

-Third: 2,973.7 kJ / mol.

Magnetic Order

Diamagnetic.

Hardness

2.0 on the Mohs scale.

Isotopes

There are five natural and stable isotopes of selenium, which are shown below with their respective abundances:

- ⁷⁴ Se (0.86%)

- ⁷⁶ Se (9.23%)

- ⁷⁷ Se (7.6%)

- ⁷⁸ Se (23.69%)

- ⁸⁰ Se (49.8%)

Allotropy

Bottle with black selenium coated with a thin film of gray selenium. Source: W. Oelen

Selenium prepared in chemical reactions is a brick-red amorphous powder, which when rapidly melted gives rise to the glassy black form, similar to rosary beads (top image). Black Selenium is a brittle and lustrous solid.

Also, black selenium is slightly soluble in carbon sulfide. When this solution is heated to 180 ºC, gray selenium, its most stable and dense allotrope, precipitates.

Gray selenium is resistant to oxidation and is inert to the action of non-oxidizing acids. The main property of this selenium is its photoconductivity. When illuminated, its electrical conductivity increases by a factor of 10 to 15 times.

Reactivity

Selenium in its compounds exists in the oxidation states -2, +4 and +6. It shows a clear tendency to form acids in the higher oxidation states. Compounds that have selenium with oxidation state -2, are called selenides (Se ^2-).

Reaction with hydrogen

Selenium reacts with hydrogen to form hydrogen selenide (H ₂ Se), a colorless, flammable and malodorous gas.

Reaction with oxygen

Selenium burns emitting a blue flame and forming selenium dioxide:

Se ₈ (s) + 8 O ₂ => 8 SeO ₂ (s)

Selenium oxide is a solid, white, polymeric substance. Its hydration produces selenious acid (H ₂ SeO ₃). Selenium also forms selenium trioxide (SeO ₃), analogous to sulfur (SO ₃).

Reaction with halogens

Selenium reacts with fluorine to form selenium hexafluoride:

Se ₈ (s) + 24 F ₂ (g) => 8 SeF ₆ (l)

Selenium reacts with chlorine and bromine to form disilenium dichloride and dibromide, respectively:

Se ₈ (s) + 4 Cl ₂ => 4 Se ₂ Cl ₂

Se ₈ (s) + 4 Br ₂ => 4 Se ₂ Br ₂

Selenium can also form SeF ₄ and SeCl ₄.

On the other hand, selenium forms compounds in which a selenium atom joins with one of halogen and another of oxygen. An important example is selenium oxychloride (SeO ₂ Cl ₂), with selenium in the +6 oxidation state, an extremely powerful solvent.

Reaction with metals

Selenium reacts with metals to form selenides of aluminum, cadmium, and sodium. The chemical equation below corresponds to that of the formation of aluminum selenide:

3 Se ₈ + 16 Al => 8 Al ₂ Se ₃

Selenites

Selenium forms salts known as selenites; for example: silver selenite (Ag ₂ SeO ₃) and sodium selenite (Na ₂ SeO ₃). This name has been used, in a literary context, to refer to inhabitants of the Moon: the Selenites.

Acids

The most important acid in selenium is selenic acid (H ₂ SeO ₄). It is as strong as sulfuric acid and is more easily reduced.

Structure and electronic configuration

- Selenium and its links

Selenium has six valence electrons, which is why it is located in group 16, the same as oxygen and sulfur. These six electrons are in the 4s and 4p orbitals, according to their electronic configuration:

3d ¹⁰ 4s ² 4p ⁴

It therefore needs, like sulfur, to form two covalent bonds to complete its octet of valence; although it has availability of its 4d orbitals to bond with more than two atoms. Thus, three selenium atoms come together and form two covalent bonds: Se-Se-Se.

Selenium with its highest atomic mass has a natural tendency to form structures governed by covalent bonds; instead of being arranged as diatomic molecules Se ₂, Se = Se, analogous to O ₂, O = O.

- Rings or chains

Among the molecular structures that selenium atoms adopt, two can be mentioned in general terms: rings or chains. Note that in the Se ₃ hypothetical case, the extreme Se atoms still require electrons; therefore, they must be bonded to other atoms, in succession, until the chain can be closed into a ring.

The most common rings are the eight-membered rings or atoms of selenium: Se ₈ (a selenite crown). Why eight? Because the smaller the ring, the more stress it will suffer; that is, the angles of their bonds deviate from the natural values ​​set by their sp ³ hybridizations (similar as occurs with cycloalkanes).

As there are eight atoms, the separation between the Se-Se atoms is sufficient so that their bonds are "relaxed" and not "bent"; although the angle of its links is 105.7º and not 109.5º. On the other hand, there may be smaller rings: Se ₆ and Se ₇.

Ring units of selenium represented by a model of spheres and bars. Source: Benjah-bmm27.

The Se ₈ ring units are shown in the image above. Note the resemblance they have to the sulfur crowns; only they are bigger and heavier.

In addition to rings, selenium atoms can also be arranged in helical chains (think spiral staircases):

Helical selenium chains. Source: Materialscientist at English Wikipedia

At its ends there may be terminal double bonds (-Se = Se), or Se ₈ rings.

- Allotropes

Taking into account that there may be helical rings or chains of selenium, and that their dimensions can also vary depending on the number of atoms they contain, then it is evident that there is more than one allotrope for this element; that is, pure selenium solids but with different molecular structures.

Red selenium

Among the most prominent allotropes of selenium we have red, which can appear as an amorphous powder, or as monoclinic and polymorphic crystals (see image of Se ₈ rings).

In amorphous red selenium the structures are disordered, with no apparent patterns; whereas in the lens, the rings establish a monoclinic structure. Red crystalline selenium is polymorphic, having three phases: α, β and γ, which differ in their densities.

Black selenium

The structure of black selenium also consists of rings; but not with eight members, but with many more, reaching up to rings of a thousand atoms (Se ₁₀₀₀). It is said then that its structure is complex and consists of polymeric rings; some larger or smaller than others.

Since there are polymer rings of different sizes, it is difficult to expect them to establish a structural order; so the black selenium is also amorphous, but in contrast to the reddish powder mentioned above, it has glassy textures, although it is brittle.

Gray selenium

And finally, of the simplest allotropes of selenium is gray, which stands out above the others as it is the most stable under normal conditions, and also has a metallic appearance.

Its crystals can be hexagonal or trigonal, established by the London dispersion forces between its polymeric helical chains (top image). The angle of their bonds is 130.1º, which indicates a positive deviation from the tetrahedral surroundings (with angles of 109.5º).

That is why selenium helical chains give the impression of being "open." By way of elucubration, in this structure the Se atoms are facing each other, so in theory there must be a greater overlap of their orbitals to create conduction bands.

The heat with the increase of the molecular vibrations damages these bands when the chains become disordered; while the energy of a photon directly affects the electrons, exciting them and promoting their transactions. From this viewpoint, it is "easy" to imagine the photoconductivity for gray selenium.

Where to find and production

Although it is widely distributed, selenium is a rare element. It is found in its native state associated with sulfur and minerals such as eucairite (CuAgSe), claustalite (PbSe), naumanite (Ag ₂ Se), and crookesite.

Selenium is found as an impurity that replaces sulfur in a small portion of the sulfurous minerals of metals; such as copper, lead, silver, etc.

There are soils in which selenium exists in the soluble form of selenates. These are carried by the rainwater to the rivers and from there, to the ocean.

Some plants are able to absorb and concentrate selenium. For example, a cup of Brazil nuts contains 544 µg of selenium, an amount that is equivalent to 777% of the daily recommended amount of selenium.

In living beings, selenium is found in some amino acids, such as: selenomethionine, selenocysteine ​​and methylselenocysteine. Selenocysteine ​​and selenite are reduced to hydrogen selenide.

Electrolysis of copper

There is no mining for selenium. Most of it is obtained as a by-product of the copper electrolysis refining process, found in the silt that accumulates at the anode.

The first step is the production of selenium dioxide. For this, the anodic silt is treated with sodium carbonate to produce its oxidation. Then water is added to selenium oxide and acidified to form selenium acid.

Finally, selenious acid is treated with sulfur dioxide to reduce it and obtain elemental selenium.

In another method in the mixture of the silt and sludge formed in the production of sulfuric acid, an impure red selenium is obtained that dissolves in sulfuric acid.

Selenious acid and selenic acid are then formed. This selenium acid receives the same treatment as the previous method.

Chlorine can also be used, which acts on metal selenides to produce volatile chlorinated selenium compounds; such as: Se ₂ Cl ₂, SeCl ₄, SeCl ₂ and SeOCl ₂.

These compounds, in a process carried out in water, are converted into selenium acid, which is treated with sulfur dioxide to release the selenium.

Biological role

Deficiency

Selenium is an essential trace element for plants and animals, whose deficiency in humans has caused serious disorders such as Keshan's disease; a disease characterized by damage to the myocardium.

Additionally, selenium deficiency is associated with male infertility and may play a role in Kashin-Beck disease, a type of osteoarthritis. Also, a selenium deficiency has been observed in rheumatoid arthritis.

Enzyme cofactor

Selenium is a component of enzymes with antioxidant action, such as glutathione peroxidase and thioredoxin reductase that act in the elimination of substances with reactive oxygen.

Additionally, selenium is a cofactor of thyroid hormone deiodinases. These enzymes are important in regulating the functioning of thyroid hormones.

The use of selenium has been reported in the treatment of Hasimoto's disease, an autoimmune disease with the formation of antibodies against thyroid cells.

Selenium has also been used to reduce the toxic effects of mercury, as some of its actions are exerted on selenium-dependent antioxidant enzymes.

Proteins and amino acids

Man has about 25 selenoproteins that exert an antioxidant action for protection against oxidative stress, initiated by an excess of reactive oxygen species (ROS) and reactive nitrogen species (NOS).

The amino acids selenomethiocin and selenocysteine ​​have been detected in humans. Selenomethionine is used as a dietary supplement in the treatment of selenium deficiency states.

Risks

A high body concentration of selenium can have numerous damaging effects on health, starting with brittle hair and brittle nails, to skin rashes, heat, edema of the skin and severe pain.

When treating selenium in contact with the eyes, people may experience burning, irritation and tearing. Meanwhile, prolonged exposure to smoke high in selenium can cause pulmonary edema, garlic breath, and bronchitis.

Additionally, the person may experience pneumonitis, nausea, chills, fever, sore throat, diarrhea, and hepatomegaly.

Selenium can interact with other medicines and dietary supplements, such as antacids, antineoplastic drugs, corticosteroids, niacin, and birth control pills.

Selenium has been associated with an increased risk of developing skin cancer. A study by the National Cancer Institute found that men with a high body level of selenium were twice as likely to have aggressive prostate cancer.

A study indicates that the daily intake of 200 µg of selenium increases the possibility of developing type II diabetes by 50%.

Applications

Cosmetics

Selenium sulfide is used in the treatment of seborrhea, as well as greasy or dandruff hair.

Doctors

It is used as an alternative medicine in the treatment of Hasimoto's disease, an autoimmune disease of the thyroid.

Selenium reduces the toxicity of mercury, one of its toxic activities is exerted on deoxidizing enzymes, which use selenium as a cofactor.

Manganese Electrolysis

The use of selenium oxide in the electrolysis of manganese considerably reduces the costs of the technique, since it reduces electricity consumption.

Pigment

Selenium is used as a pigment in paints, plastics, ceramics, and glass. Depending on the selenium used, the color of the glass varies from deep red to light orange.

Photoconductive

Due to the property of gray selenium to change its electrical conductivity as a function of the intensity of the light that radiates it, selenium has been used in photocopiers, photocells, photometers and solar cells.

The use of selenium in photocopiers was one of the main applications of selenium; but the appearance of organic photoconductors has been decreasing their use.

Crystals

Selenium is used for discoloration of glasses, as a result of the presence of iron that produces a green or yellow color. In addition, it allows a red coloration of the glass, depending on the use you want to give it.

Vulcanization

Selenium diethyldithiocarbonate is used as a vulcanizing agent for rubber products.

Alloys

Selenium is used in combination with bismuth in brass, in order to replace lead; Very toxic element that has been decreasing its use due to the recommendations of health organizations.

Selenium is added in low concentrations to steel and copper alloys to improve the usability of these metals.

Rectifiers

Selenium rectifiers began to be used in 1933 until the 1970s, when they were replaced by silicon because of its low cost and superior quality.

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