What elements does selenium belong to? Selenium - what is it? The chemical element selenium. Application of selenium

Selenium

SELENIUM-a; m.[from Greek. Selēnē - Moon] A chemical element (Se), a gray-black toxic substance, a semiconductor with photoelectric properties (used in engineering and the glass industry).

◁ Selenium, th, th. C. photocell. C acid. C glass.

selenium

(lat. Selenium), a chemical element of group VI of the periodic system. The name is from the Greek selēnē - moon. Forms several modifications. The most stable gray selenium - crystals, density 4.807 g / cm 3, t pl 221°C. It is scattered in nature, accompanies sulfur, mined from waste (sludge) during the electrolytic purification of copper. A semiconductor that has photoelectric properties. Selenium photocells are used in various devices, such as photoelectric exposure meters. All selenium compounds are poisonous.

SELENIUM

SELENIUM (Latin selenium, from the Greek Selene - Moon), Se, reads "selenium", a chemical element with atomic number 34, atomic mass 78.96. Natural selenium consists of six stable isotopes: 74 Se (0.87% by mass), 76 Se (9.02%), 77 Se (7.58%), 78 Se (23.52%), 80 Se (49 .82%) and 82Se (9.19%). Atomic radius 0.160 nm. Ion radius (coordination number 6) Se 2– - 0.184 nm, Se 4+ - 0.069 nm and Se 6+ - 0.056 nm. Sequential ionization energies 9.752, 21.2, 32.0, 42.9 and 68.3 eV. It is located in the VIA group in the 4th period of the Periodic Table of the Elements. Chalcogen (cm. CHALCOGENES). Outer electron layer configuration 4 s 2 4p 4 . Oxidation states: -2, +2, +4, +6 (valencies II, IV, VI). Electronegativity according to Pauling (cm. PAULING Linus) 2,40.
Discovery history
Selenium was discovered in 1817 by the Swedish chemist J. J. Berzelius (cm. BERZELIUS Jens Jacob) in the sludge of lead chambers of one of the sulfuric acid plants. The properties turned out to be similar to those discovered in 1782 by M. G. Klaproth (cm. KLAPROT Martin Heinrich) tellurium (cm. TELLURIUM).
Being in nature
Selenium is a rare, trace element. The content in the earth's crust is 1.4 10 -5% by weight. Selenides are almost always isomorphic to the corresponding sulfides, they are usually found in nature as an impurity in the corresponding sulfides (in iron pyrite (cm. pyrites) FeS 2 , chalcopyrite (cm. chalcopyrite) CuFeS 2, zinc blende (cm. sphalerite) ZnS).
Selenium minerals are very rare, among them: berzelianite Cu 2 Se, thiemannite HgSe, naumanite Ag 2 Se, chalcomenite CuSeO 3 2H 2 O. Native selenium is rare in nature.
Receipt
The main sources of selenium are dust formed during the firing of selenium-containing sulfides and sludge from lead chambers.
After treatment of sludge with concentrated sulfuric acid containing sodium nitrate, selenium goes into solution, forming selenous acid H 2 SeO 3 and, partially, selenic acid H 2 SeO 4 . Selenic acid, when heated with hydrochloric acid, is reduced to selenous acid. Then sulfur dioxide gas SO 2 is passed through the resulting solution of selenous acid
H 2 SeO 3 + 2SO 2 + H 2 O \u003d Se + 2H 2 SO 4
a red precipitate of elemental selenium precipitates.
For purification, selenium is further burned in oxygen saturated with vapors of fuming nitric acid HNO 3 . In this case, pure selenium dioxide SeO 2 is sublimated. From a solution of SeO 2 in water, after adding hydrochloric acid, selenium is again precipitated by passing sulfur dioxide gas through the solution.
The resulting selenium is remelted, the melt is filtered through fiberglass or activated carbon and subjected to vacuum distillation or rectification in an inert atmosphere.
Physical and chemical properties
Selenium is a gray, brittle non-metal with a metallic sheen.
At atmospheric pressure, there are several dozen modifications of selenium. The most stable is gray selenium, g-Se, with a hexagonal lattice ( a= 0.436388 nm, c= 0.495935 nm). Melting point 221°C, boiling point 685°C, density 4.807 kg/dm 3 . The density of liquid selenium at 221°C is 4.06 kg/dm 3 . Gray selenium is obtained from other forms by prolonged heating and slow cooling of the melt or selenium vapor. Its structure consists of parallel helical chains.
Three modifications of red crystalline selenium with a monoclinic lattice have been isolated from selenium solutions in CS 2 . a-Se orange-red, a= 0.9054 nm, b= 0.9083 nm, c\u003d 1.1601 nm, angle b \u003d 90.81 °, melting point 170 ° C, density 4.46 kg / dm 3.
b-Se dark red, a= 1.285 nm, b= 0.807 nm, c\u003d 0.931 nm, angle b \u003d 93.13 °, melting point 180 ° C, density 4.50 kg / dm 3.
g-se red, a= 1.5018 nm, b= 1.4713 nm, c\u003d 0.8789 nm, angle b \u003d 93.61 °, density 4.33 kg / dm 3. Red selenium contains Se 8 ring molecules.
When selenous acid is reduced or selenium vapor is rapidly cooled, amorphous red selenium is formed. Amorphous red selenium differs from another modification of amorphous vitreous selenium only in the size of its constituent microparticles. The density of red selenium is 4.28 kg/dm 3 .
At 27 MPa, a cubic modification of selenium was obtained. Gray g-Se is a semiconductor with hole conductivity, the band gap is 1.8 eV. Conducts electricity very poorly in the dark. When illuminated, the electrical conductivity increases thousands of times.
Selenium is reactive. When heated in air, it burns to form a colorless crystalline SeO 2:
Se + O 2 \u003d SeO 2.
With fluorine (cm. FLUORINE), chlorine (cm. CHLORINE) and bromine (cm. BROMINE) selenium reacts at room temperature. With iodine (cm. IOD) selenium fuses, but does not form iodides. Above 200°C, selenium reacts with hydrogen (cm. HYDROGEN) with the formation of hydrogen selenide H 2 Se. When heated, it reacts with metals, forming selenides.
It also interacts with water when heated:
3Se + 3H 2 O \u003d 2H 2 Se + H 2 SeO 3.
Selenium does not react with non-oxidizing and dilute acids. With concentrated sulfur (cm. SULPHURIC ACID) Selenium reacts with acid in the cold (the color of solutions containing Se 8 + polymer cations is green). over time, Se 8 + cations pass into Se 4 2+ and the solution turns yellow.
Selenium reacts when heated with nitric acid, with the formation of selenious acid H 2 SeO 3:
3Se + 4HNO 3 + H 2 O = 3H 2 SeO 3 + 4NO.
When boiling in alkaline solutions, selenium disproportionates:
3Se + 6KOH = K 2 SeO 3 + 2K 2 Se + 3H 2 O.
If selenium is boiled in an alkaline solution through which air or oxygen is passed, then red-brown solutions containing polyselenides are formed:
K 2 Se + 3 Se = K 2 Se 4
Selenium reacts with sulfides and polysulfides to form thioselenides. When selenium is heated with solutions of Na 2 SO 3 and KCN, the following reactions occur:
Na 2 SO 3 + Se \u003d Na 2 SSeO 3;
KCN + Se = KSeCN.
Strong oxidizing agents (ozone (cm. OZONE) O 3, fluorine (cm. FLUORINE) F 2) oxidize selenium to Se +6:
Se + O 3 \u003d SeO 3,
Se + 3F 2 = SeF 6 .
Application
Amorphous Se is part of the photosensitive layers in xerography (cm. XEROGRAPHY) and laser printers. Gray Se is used in the manufacture of diodes, photoresistors, etc. Selenium is a pigment for glasses, an additive to steel, an additive to sulfur during vulcanization, to produce catalysts, herbicides, insecticides, and medicines.
Physiological action
Trace element (mass fraction in the body 10–5–10–7%).
Selenium enters the human body with food (55-110 mg per year). Concentrated in the liver and kidneys. At high doses, it primarily accumulates in nails and hair, which are based on sulfur-containing amino acids. Selenium atoms replace sulfur atoms:
R–S–S_–R + 2Se = R–Se–Se_–R + 2S
Small amounts of selenium should be found in the diet of chickens, calves, lambs and rabbits. Selenium is part of the active centers of enzymes: formate dehydrogenase, glutathione reductase and glutathione peroxidase, the active center of which contains an amino acid residue - selenocysteine:
Selenium is able to protect the body from mercury poisoning ( cm.) and cadmium (cm. CADMIUM) because it connects them. There is an association between high dietary selenium and low cancer mortality.
Selenium vapors are poisonous. MPC of amorphous selenium in the air is 2 mg / m 3, SeO 2, Na 2 SeO 3 - 0.1 mg / m 3. Maximum concentration limit for selenium in water is 0.01 mg/m 3 .

encyclopedic Dictionary. 2009 .

Synonyms:

See what "selenium" is in other dictionaries:

- (new lat.) A metalloid akin to sulfur and tellurium. Dictionary of foreign words included in the Russian language. Chudinov, A.N., 1910. SELENIUM, a metalloid similar to sulfur and tellurium, is very common; mined from iron pyrite and blende, in which ... Dictionary of foreign words of the Russian language

Selenium- Selenium, quite similar to sulfur, exists in several varieties: a) amorphous selenium in the form of reddish flakes (selenium color); b) vitreous selenium, a poor conductor of heat and electricity. It has a shiny kink of brown or ... ... Official terminology

SELENE, village husband. a simple chemical principle, whose properties are closer to sulfur than to metals. Selenic acid. Selenium copper containing selenium. Selenite, gypsum spar, fossil. Dahl's Explanatory Dictionary. IN AND. Dal. 1863 1866 ... Dahl's Explanatory Dictionary

- (Salen) a ski resort in the southwest of Sweden (see Sweden). Located near the border with Norway (see Norway). It is a mountain range stretching for 30 km. Distance from Stockholm 400 km, from Oslo 150 km. Ski season... ... Geographic Encyclopedia

- (Selenium), Se, a chemical element of group VI of the periodic system, atomic number 34, atomic mass 78.96; refers to chalcogens; gray or red non-metal, mp 221shC; semiconductor. Selenium is used in photocells, xerography, etc. ... ... Modern Encyclopedia

Do you know, dear readers, that the health and longevity of our body depends on three saving systems: immune, antioxidant and detoxifying?

The activity of each of them is a complex biological mechanism, depending on different components and circumstances.

But the common factor on which the coordinated work of these systems depends is the presence of a universal chemical element in them. What is selenium, useful properties, why does the body need it, what is the result of selenium deficiency and its deficiency, today in the article.

The lack of selenium in the body immediately affects the state of health, even the air that we breathe against the background of a lack of selenium becomes a real poison, its active forms not only turn into a destroyer of vitamins, but also disrupt the system for removing toxins from the body.

What is selenium and why does the body need it

The universal chemical element selenium was discovered and described in the 19th century. According to its chemical properties, it belongs to non-metals, although it has an unusually beautiful iridescent metallic hue. It is a semiconductor in its physical properties.

There are few of them in nature. But in the places of volcanic eruptions, the presence of selenium in the soil is noted in large quantities, so both flora and fauna are rich in it.

This amazing element was considered the strongest poison for a long time, and was treated with caution until it was rediscovered in 1973 and found out how important it is for the life of the human body and animals.

Now selenium is rightfully considered a chemical element of health and longevity, because it was found in almost all enzymes and hormones. The work of all systems of the human body depends on this substance.

In some cells or systems, selenium is present as a substance, in others - as an integral part of enzymes. Experts have calculated that it is present in 200 enzymes, and with a deficiency of selenium in the body, these enzymes cannot be synthesized.

Selenium is concentrated in different organs in different ways, more of it in the brain, kidneys, liver, seminal fluid, cartilage tissue.

Nature assigned selenium an important role - protection from the destructive effects of heavy metals (arsenic, cadmium, mercury, lead, thallium) and control over all the processes of creation in our body.

The role of selenium in the human body

This irreplaceable trace element is considered an element of longevity, due to its unique properties. Selenium is involved in the synthesis of the peptide - glutathione, which plays a major role in the course of redox reactions, holds the shield of the immune system, which saves us from infections, cancer and early aging.

Selenium protects the human body at the intracellular level, it controls genetic disorders, "monitors" the health of chromosomes. It has antibacterial and antiviral properties, antitumor and anti-inflammatory, antistress and antipoptotic effect.

Selenium performs numerous functions in the body:

☀ enhances the body's immunity (stimulates the formation of antibodies, white blood cells, killer cells, macrophages and interferon, participates in the production of red blood cells),

☀ is a strong antioxidant (prevents the development of tumor processes and aging of the body, neutralizes and removes foreign substances, activates vitamin E),

☀ reduces the risk of developing cardiovascular diseases (prevents muscular dystrophy of the heart, neutralizes toxins, stimulates hemoglobin synthesis, participates in the production of red blood cells and coenzyme Q10),

☀ acts as a strong antitumor factor (prevents and stops the development of malignant tumors),

☀ part of most hormones, enzymes and some proteins,

☀ stimulates metabolic processes in the body,

☀ protects the body from toxic manifestations of mercury, cadmium, lead, thallium and silver,

☀ stimulates reproductive function (included in spermatozoa),

☀ stabilizes the work of the nervous system,

☀ normalizes the work of the endocrine system,

☀ reduces the severity of inflammatory processes

☀ has a beneficial effect on the condition of the skin, nails and hair.

Beneficial features

How does selenium work in the body. Scientists note that despite the meager content of selenium in the body, its role in maintaining the vital activity of all systems and organs is great. Everyone knows that a person needs energy for life, movement, work. Now it is precisely established that almost all the energy produced by the mitochondria of cells occurs only with the participation of selenium (ie 85%)! Energy is needed by all organs:

• the heart needs it in order to give the movement of blood, which delivers nutrition to all cells and systems, and takes away waste from them,
• light - to carry out gas exchange,
• digestive organs - to break down and digest food, and then assimilate it,
• kidneys - energy is needed to filter the blood and excrete unnecessary metabolic products in the urine.

All these processes in the body go with the participation of selenium, and if it is not enough, then the activity of any organ will be disrupted, which is accompanied by a mass of unpleasant symptoms, the appearance of chronic fatigue and the emergence of various diseases.

Imagine, in every cell of our body, in just one minute, 1 million chemical reactions take place. And in one second there are thousands of free radicals. These are flawed molecules that lack one electron and they barbarously take it away from other molecules, thereby turning them into the same barbarians - free radicals.

But we remember that for the construction of all cells and tissues, only stable, not defective molecules are needed. But if the process of formation of free radicals will progress, it will cause a response - the destruction of tissues and cells of the body.

It inhibits the growth of radicals due to the action of the body's antioxidant system, which includes vitamin C, A, E and the element selenium. And only in the presence of selenium is the synthesis of enzymes involved in antioxidant protection. Therefore, selenium is one of the main links for the strength of immunity and detoxification.

It protects the body from contaminated water and food, neutralizes tobacco smoke, exhaust gases, all heavy metals that accumulate imperceptibly in the body (lead, arsenic, mercury, cadmium).

Now scientists say with confidence that selenium controls the entire life cycle of the cell, from its inception to death.

What diseases are caused by selenium deficiency in the body?

Scientists have found that in conditions of lack of selenium, the immune system cannot fully perform its functions. Crash after crash:

The process of iodine metabolism and the work of the thyroid gland are disrupted, since its well-coordinated work depends on the presence of iodine. And the thyroid gland regulates metabolic processes, so there is a process of inhibition of growth and development of the body.

Selenium deficiency in the body causes many diseases:

• the number of pathologies during pregnancy and difficult childbirth increases,
• the growth of children born with pathologies increases,
• growing number of sick teenagers,
• develop female and male diseases that cause infertility,
• chronic diseases are severe,
• capillaries become fragile and brittle,
• spermatozoa lose their mobility, become inert,
• increasing incidence of mental illness
• anemia and diabetes, hepatitis and endemic goiter appear,
• stroke and myocardial infarction and some cancers,
• new viral diseases are emerging
• the quality of human life and its duration are reduced.

How Selenium Deficiency Affects the Heart

We found out that selenium is an important element, which is called a biocorrector, since not a single cell is able to work without its presence. The chemical element selenium:

• supports the correct and balanced metabolism of cholesterol and fat, preventing the formation of cholesterol plaques,
• makes sure that the walls of the vessels do not break the elasticity, so that the muscles of the heart are in good shape,
• controls blood viscosity, and inhibits the formation of blood clots,
• resists the occurrence of atherosclerosis and hypertension,
• protects the cells of the circulatory system and the heart from the damaging effects of free radicals.

The heart is very sensitive to the content of selenium in the cells, if its level drops to 45 mcg / kg, then immediately there is a risk of heart failure. Acute heart failure appears if the level of selenium drops to 20 mcg / kg, which leads to cardiac arrest.

Scientists have established a natural relationship between the content of selenium in the blood and the development of coronary heart disease. Practical observations have shown that heart disease is 70 times more likely to occur in people with low levels of selenium.

Coenzyme Q10, which protects the heart muscle from oxygen deficiency and is involved in the recovery of the heart after a heart attack, is also synthesized in the body only in the presence of the element selenium.

How does a deficiency affect the thyroid gland?

The thyroid gland plays a leading role in the human body in the energy metabolism of cells, regulating the work of all organs and systems, and for this it has to synthesize certain hormones.

One of these hormones is T4, which, after the donation of one iodine atom, turns into the active hormone - triiodothyronine, which is involved in energy metabolism. This process proceeds with the participation of selenium.

If there is not enough selenium in the cells of the thyroid gland, then the hormone triiodothyronine is not synthesized. If there is no this hormone, then there is no energy in the cells of the body. And then the human brain sends a signal to the thyroid gland about the unproductiveness of its work. The thyroid gland responds with hard work and increases in size, or forms knots. But selenium is still not enough, and problems appeared in the thyroid gland from its lack.

Recent studies by scientists indicate that often thyroid diseases are caused not by iodine deficiency, but by a lack of selenium. The presence of selenium, which has a pronounced antioxidant property, protects the cells and tissues of the thyroid gland, reduces inflammation, and destroys free radicals.

The chemical element selenium, with the help of the thyroid gland and iodine, controls the vital activity of our body, regulating metabolism, immunity, the functioning of brain cells ...

Lack of selenium in the body causes cancer

The incidence of cancer depends on the level of selenium in the body, this relationship has already been proven by scientists. When the level of selenium is normal, it inhibits the growth of the tumor that has begun.

Atypical or cancerous cells are formed in all people, such cells are born every 10 minutes. But in the presence of selenium, special enzymes are immediately produced in the body that break down cancer cells.

But if there is a lack of selenium in the body, then such enzymes cannot be produced, and atypical cells continue to grow and multiply, forming a tumor. In the process of reproduction of cancer cells, toxic substances accumulate, which are also not neutralized due to a lack of selenium and continue to accumulate in the body.

Studies by American scientists that have been conducted for more than 10 years to study the effect of selenium on the human body have shown that among people with oncology, in the group taking the element at 200 mcg per day, mortality was reduced by 50% compared to those who did not take the drug. Among the studied people, there were patients with cancer of the intestine, lung, prostate.

It is interesting:

"Scientists have recently found that serum selenium concentrations below 45 μg/L lead to the development of both benign and malignant tumors."

Why is selenium deficiency formed?

Daily requirement of selenium. The fact is that the body is not able to synthesize selenium and receives it from the food consumed. On average, up to 100 mcg / per day should enter the human body: adult men need 70 mcg, up to 55 mcg - women, pregnant and lactating women - up to 75 mcg. Given that a person will receive such an amount of selenium daily, then for a lifetime, the amount of selenium eaten will not exceed 1/2 teaspoon.

But unfortunately, the statistics for Russia are completely different. Experts emphasize that, on average, the inhabitants of Russia consume only 30 micrograms of selenium with food, so 80% of the inhabitants experience a selenium deficiency.

Man satisfies the need for selenium by 90% through food and 10% through water. It is believed that the largest amount of selenium is found in brazil nuts, onions and garlic.

It is important to consider whether there is a natural deficiency of selenium? If there is a lack of selenium in the biosphere of your region, then no matter how much you eat garlic and onions grown on your site, there will be no benefit or it will be scanty.

And the use of chemicals further reduces the level of digestible form of selenium in the soil, and hence in plants grown on this land. The amount of selenium in the environment is affected by forest fires, burning grass and dry leaves.

Data from the Research Institute of Nutrition of the Russian Academy of Medical Sciences indicate that almost the entire territory of Russia has a deficiency of selenium in the soil. Especially, here in Transbaikalia and Buryatia, studies have confirmed a low level of biologically active selenium in the soil, up to 10 μg / kg, which causes diseases in humans, animals and plants.

A low level of selenium is noted in Udmurtia, the North-Western region of the country (Leningrad region, Karelia), the Upper Volga region (Ivanovo, Kostroma, Yaroslavl regions).

In people living in areas with a low content of selenium in the environment, diseases of the stomach and intestines, liver and genitourinary system progress much more often, a lack of selenium manifests itself in the condition of the skin, nails, hair, teeth, bones.

What are selenium pits? Scientists identify several periods in a person's life when selenium is very, very necessary for the body:

• “newborn, from birth to 2 years;
• at puberty from 11-20 years;
• "Christ's age" (30 - 33 years), when the aging process of the body already begins;
• the period of pregnancy, especially dangerous if this period coincides with the age of Christ;
• menopause in women and men and beyond, when the content of selenium in the blood plasma and connective tissue becomes low (after 70 years).

These periods, coinciding with selenium deficiency, are a huge risk factor for the onset of various diseases.

Therefore, scientists raise the issue of conducting selenization of the country, as is done in other countries. The general director of the Moscow Cardiology Center, academician E. Chazov, claims that if we launch a selenium deficiency replenishment program in our country, then in just one year, about 600 thousand people suffering from heart disease can be saved.

But in Finland, this program works on a national scale. There, selenium is added during food production and cooking. Statistics show amazing results:

• heart disease in the country decreased by almost 2.5 times,
• diseases of the endocrine system decreased by 77%,
• common diseases began to occur less frequently by 47%,
• and oncology decreased by 1.8 times.

What is dangerous excess selenium in the body

To maintain your health and well-being in good condition, a certain balance of the chemical element selenium in the body is necessary. The fact is that this unique element shows its cunning. Not only a lack of selenium, but also an excess of selenium in the body can lead to poor health.

Therefore, you need to be careful and try to comply with the daily requirement of selenium, which was mentioned above.

An excess of selenium in the body can appear in workers in contact with this element: in the electronics, glass, paint and varnish industries. Those who permanently live in areas of the country with active volcanoes (Kurils, Kamchatka).

There are selenium overdoses when taking dietary supplements, when a person takes several food supplements containing selenium at once.

It must be remembered that long-term consumption of selenium in high doses, up to 200 mcg / day, can cause poisoning, which manifests itself in nausea and abdominal pain, accompanied by diarrhea. Often there is irritability and neuropathy, a characteristic garlic breath and a metallic taste.

An excess of selenium is also reflected on the skin in the form of peeling and dermatitis, hair can fall out, nails exfoliate, and enamel on the teeth is damaged.

Health disorders associated with an excess of selenium in the body are extremely rare. And yet, when taking nutritional supplements, do not forget about a possible overdose, strictly follow the instructions, or consult your doctor.

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Selenium was discovered in 1817 by Jens Jakob Berzelius. The story of Berzelius himself about how this discovery came about has been preserved: “I investigated, in collaboration with Gottlieb Hahn, the method that is used to produce sulfuric acid in Gripsholm. We found a precipitate in sulfuric acid, partly red, partly light brown. ... Curiosity , inspired by the hope of discovering a new rare metal in this brown precipitate, led me to investigate the precipitate ... I found that the mass (i.e., the precipitate) contains a hitherto unknown metal, very similar in its properties to tellurium. In accordance with this analogy I called the new body selenium (Selenium) from the Greek selhnh(moon), since tellurium is named after Tellus - our planet ".

Being in nature, getting:

The content of selenium in the earth's crust is about 500 mg/t. Selenium forms 37 minerals, among which ashavalite FeSe, claustalite PbSe, timannite HgSe, guanahuatite Bi 2 (Se,S) 3 , hastite CoSe 2 , platinite PbBi 2 (S, Se) 3 should be noted first of all. Occasionally, native selenium is found. Sulfide deposits have the main industrial value for selenium. The content of selenium in sulfides ranges from 7 to 110 g/t. The concentration of selenium in sea water is 4*10 -4 mg/l.
Selenium is obtained from waste products of sulfuric acid, pulp and paper production, and also significant amounts are obtained from the sludge of copper-electrolyte production, in which selenium is present in the form of silver selenide. Several methods are used to obtain selenium from sludge: oxidative roasting with sublimation of SeO 2 ; oxidative sintering with soda, conversion of the resulting mixture of selenium compounds to Se(IV) compounds and their reduction to elemental selenium by the action of SO 2 .

Physical properties:

The diversity of the molecular structure determines the existence of selenium in various allotropic modifications: amorphous (powdered, colloidal, vitreous) and crystalline (monoclinic, a- and b-shape and hexagonal g-the form). Amorphous (red) powdered and colloidal selenium is obtained by reduction from a solution of selenous acid by rapid cooling of selenium vapor. Vitreous (black) selenium is obtained by heating any modification of selenium above 220°C, followed by rapid cooling. It has a vitreous luster and is fragile. Thermodynamically the most stable is hexagonal (gray) selenium. It is obtained from other forms of selenium by heating to melting with slow cooling to 180-210°C and holding at this temperature. Its lattice is built from parallel helical chains of atoms.

Chemical properties:

At ordinary temperatures, selenium is resistant to oxygen, water and dilute acids. When heated, selenium interacts with all metals, forming selenides. In oxygen, with additional heating, it slowly burns with a blue flame, turning into SeO 2 dioxide.
With halogens, with the exception of iodine, it reacts at room temperature to form compounds SeF 6 , SeF 4 , SeCl 4 , Se 2 Cl 2 , SeBr 4 , etc. Selenium reacts with chlorine or bromine water according to the equation:
Se + 3Br 2 + 4H 2 O \u003d H 2 SeO 4 + 6 HBr
Hydrogen interacts with selenium at t>200°C, giving H 2 Se.
In conc. H 2 SO 4 in the cold, selenium dissolves, giving a green solution containing polymer cations Se 8 2+.
With water when heated and in conc. solutions of alkalis, selenium disproportionates:
3Se + 3H 2 O = 2H 2 Se + H 2 SeO 3 and 3Se + 6KOH = K 2 SeO 3 + 2K 2 Se + 3H 2 O
forming compounds of selenium (-2) and selenium (+4).
Similarly, selenium sulfur dissolves when heated in solutions of Na 2 SO 3 or KCN, forming, respectively, Na 2 SSeO 3 (analogue of thiosulfate) or KCNSe (analogue of thiocyanate).

The most important connections:

For selenium, the most characteristic oxidation states are -2, +4, +6.
Selenium(IV) oxide SeO 2- white shiny crystals with a polymer molecule (SeOsub> 2) sub> n, tpl. 350°C. The vapors are yellowish-green in color and smell like rotten radish. Easily soluble in water to form H 2 SeO 3 .
Selenic acid, H 2 SeO 3- white rhombic crystals. Possesses high hygroscopicity. Well soluble in water. Unstable, when heated above 70°C, it decomposes into water and selenium(IV) oxide. Salts are selenites.
Sodium selenite, Na 2 SeO 3 are colorless crystals, mp. 711°C. Hygroscopic, soluble in water. When heated in an inert atmosphere, it decomposes into oxides. When heated in air, it oxidizes to selenate: 2Na 2 SeO 3 + O 2 = 2Na 2 SeO 4
Selenium(VI) oxide SeO 3- - colorless crystals, mp. 121°C. It is hygroscopic, reacts with water with high heat release and formation of H 2 SeO 4 . Strong oxidizing agent, reacts violently with organic substances
Selenic acid, H 2 SeO 4- a colorless crystalline substance, highly soluble in water. Poisonous, hygroscopic, is a strong oxidizing agent. Selenic acid is one of the few compounds that, when heated, dissolve gold, forming a red-yellow solution of gold(III) selenate.
2Au + 6H 2 SeO 4 = Au 2 (SeO 4) 3 + 3H 2 SeO 3 + 3H 2 O
selenates- salts of selenic acid. Sodium selenate Na 2 SeO 4 - rhombic crystals; tpl. 730 °C. Prepared by neutralization of an acid with sodium oxide, hydroxide or carbonate or by oxidation of sodium selenite. Slightly soluble in water, below 32 ° C crystallizes from aqueous solutions in the form of decahydrate Na 2 SeO 4 10H 2 O
Hydrogen selenide, H 2 Se It is a colorless flammable gas with an unpleasant odour. The most toxic compound of selenium. In air, it is easily oxidized at ordinary temperature to free selenium. It is also oxidized to free selenium by chlorine, bromine and iodine. When burned in air or oxygen, selenium(IV) oxide and water are formed. Stronger acid than H 2 S.
selenides- compounds of selenium with metals. Crystalline substances, often with a metallic sheen. There are monoselenides of the composition M 2 Se, MSe; polyselenides M 2 Se n (except Li), where n = 2-6; hydroselenides MHSe. Air oxygen is oxidized to selenium: 2Na 2 Se n + O 2 + 2H 2 O \u003d 2n Se + 4NaOH

Application:

Selenium is used in rectifying semiconductor diodes, as well as for photoelectric devices, electrophotographic copiers, as phosphors in television, optical and signal devices, thermistors, etc. Selenium is widely used to decolorize green glass and obtain ruby ​​glasses; in metallurgy - to give steel a fine-grained structure, improve their mechanical properties; in the chemical industry - as a catalyst.
The stable selenium-74 isotope made it possible, on its basis, to create a plasma laser with colossal amplification in the ultraviolet region (about a billion times).
The radioactive isotope selenium-75 is used as a powerful source of gamma radiation for flaw detection.

Biological role and toxicity:

Selenium enters the active sites of some proteins in the form of the amino acid selenocysteine. It has antioxidant properties, increases the perception of light by the retina, affects many enzymatic reactions. The requirement of man and animals for selenium does not exceed 50-100 mcg/kg of diet.

Polkovnikov A.A.
KhF Tyumen State University, 581 groups. 2011

Sources: Wikipedia: http://ru.wikipedia.org/wiki/Selenium
Website "Handbook of a chemist":

This is a metalloid (non-metal), the content of which in the soil depends on the region. This trace element, necessary for vital processes, is present throughout the body, but its highest concentration is in the kidneys, liver, spleen, pancreas and testicles.

Useful properties of selenium

Selenium acts as part of selenoproteins. The most famous of them is glutathione peroxidase. These antioxidant enzymes form the main line of defense against free radical attacks. Those, in turn, are continuously produced by the body itself during cellular respiration and reach particularly high concentrations during acute stress and fatigue. Their excess is fraught with premature aging of all tissues, the development of degenerative pathologies, atherosclerosis and cancer. Adequate selenium intake is necessary to prevent all these troubles. Selenoproteins restore antioxidant activity and E, act against free radicals in conjunction with them, participate in the detoxification of the body, protecting it from certain heavy metals and poisons, and are necessary for the regulation and modulation of inflammatory and immune processes.

The main benefits of selenium

Of particular interest to scientists is the role of selenium in the prevention of malignant neoplasms. Experts from Cornell and Arizona universities in the USA, observing 1,300 people over the course of several years, concluded that a daily intake of 200 micrograms of this trace element reduces the risk of prostate cancer by 63%, colon - by 58, lungs - by 46, and in general of all its incurable types - by 39%. Shocked by the results, the researchers terminated the study early and recommended that participants in the placebo group replace it with selenium supplements. Selenium also shows good prospects in the prevention of other types of cancer, but the data on this subject are only preliminary and require confirmation. In addition, by stimulating the immune system, it enhances antiviral protection. It might be useful in hepatitis and some types of cancer. The potential in the fight against herpes viruses (herpes simplex and shingles) and especially against HIV is being studied.

Additional benefit

As an antioxidant, selenium certainly protects us from cardiovascular disease, so its deficiency is especially dangerous for those who have already been diagnosed with such a diagnosis, as well as for smokers. In conjunction with vitamin E, it has a pronounced anti-inflammatory effect. Their combination is recommended in the treatment of chronic diseases such as psoriasis, lupus and eczema. Finally, selenium helps prevent cataracts and macular degeneration.

Our Needs

Recommended Daily Allowance for Selenium is 75 mcg for men and 60 mcg for women (60-80 mcg starting from 65 years). However, a therapeutic dose of up to 200 mg per day may be required to achieve maximum effectiveness.

Flaw. The poorer the soil in selenium, the less it is in food. Deficiency of this trace element, as follows from all of the above, simply increases the risk of cancer, coronary heart disease, viral and inflammatory diseases. Early symptoms of selenium deficiency include muscle weakness and fatigue.

Excess. If you get selenium only from food, busting is excluded. However, if you use supplements, be aware that doses above 900 mcg/day lead to intoxication. Symptoms include nervousness, depression, nausea and vomiting, garlic breath, hair loss, and nail damage.

Indications and methods of application, food sources of selenium

Indications for the use of selenium

Prevention of cancer and cardiovascular diseases (in combination with).

Prevention of cataracts and macular degeneration of the retina.

Weakness of the immune system.

Viral infections: herpes and shingles; slows down the development of HIV/AIDS.

Lupus symptoms.

Ways to use selenium

Doses

For long-term prophylactic use, nutritionists recommend about 100-200 mcg/day.

How to use

If you are at risk for coronary heart disease, eat foods rich in selenium and vitamin E, which act synergistically.

Release form

Capsules

Tablets

Food Sources of Selenium

The best dietary sources of selenium include American walnuts, seafood, liver, kidneys, poultry, and meat. A lot of selenium is also found in whole grains, especially oats and brown rice, but only if they grew in soil rich in this element.

DEFINITION

Selenium located in the fourth period of group VI of the main (A) subgroup of the Periodic Table.

Relates to elements p-families. Non-metal. Designation - Se. Ordinal number - 34. Relative atomic mass - 78.96 a.m.u.

The electronic structure of the atom of selenium

The atom of selenium consists of a positively charged nucleus (+34), inside which there are 34 protons and 45 neutrons, and 34 electrons move around in three orbits.

Fig.1. Schematic structure of the atom of selenium.

The distribution of electrons in orbitals is as follows:

34Se) 2) 8) 18) 6 ;

1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 4 .

The outer energy level of the selenium atom contains 6 electrons, which are valence. The energy diagram of the ground state takes the following form:

Each valence electron of a selenium atom can be characterized by a set of four quantum numbers: n(chief quantum), l(orbital), m l(magnetic) and s(spin):

sublevel

The presence of two unpaired electrons indicates that the oxidation state of selenium is +2. Since the fourth level has vacant orbitals 4 d-sublevel, then the selenium atom is characterized by the presence of an excited state:

That is why the oxidation state of +4 is also characteristic of selenium.

Examples of problem solving

EXAMPLE 1