Why does the body need selenium, what are the benefits and harms of the substance? Structure of the selenium atom

Poison or medicine: functions of selenium in the human body

The discovery of the vital necessity of selenium for the human body in the 70s of the last century became a sensation. It turned out that this element protects nucleic acids (including DNA and RNA) from damage. These acids play a key role in the biosynthesis of any proteins in the body, in the transmission of its hereditary properties and characteristics. But the protective functions of selenium did not end there.

Selenium is one of the most powerful antioxidants, since it is the main element of glutathione peroxidase, an enzyme responsible for fighting free radicals. Due to its chemical activity as a reducing agent, the microelement protects cells from damage by free radicals, preventing oxidation products from accumulating in the body, normalizes the activity of cell nuclei, prevents damage to chromosomes, and stimulates ribosomal functions (obtaining information about the structure of renewable proteins). This function of selenium, according to many scientific studies, allows you to prolong the life of the body - protect cells from destruction, stimulate their growth and development.

Another important function of selenium as a vital element for humans is oncoprotective. Selenium has antiproliferative properties - that is, it is able to stop the division of damaged cells, including some types of cancer. Researchers have established a direct relationship between the level of selenium in the soils of individual regions and the development of malignant neoplasms in the population of these regions. The protective effect of selenium against the development of prostate cancer has been reliably confirmed by large-scale studies.

A significant function of selenium is that it, in the form of the amino acid seleniumprotein, is part of the sperm capsule and is responsible for its motility, protecting the integrity of its flagella.

Selenium is part of the active nucleus of more than 200 enzymes in the human body, so the list of its functions is quite impressive. Among the most important:

• is part of an enzyme that activates the thyroid gland to produce hormones;
• is part of the proteins of the heart muscle and regulates its work;
• stimulates the activity of immune system cells;
• prevents the development of fatty liver;
• affects the absorption of insulin;
• participates in the absorption of fats, prevents stone formation in the kidneys and gall bladder;
• regulates blood cholesterol levels;
• participates in the restoration of damaged pancreatic cells;
• protects blood vessels from damage.

In addition to the most important protective properties, the benefits of selenium for the body are manifested in the participation of this element in many processes that ensure our life:

• increases the body's defenses;
• normalizes mineral metabolism;
• supports hormonal levels in women, mitigating unpleasant symptoms during PMS and perimenopause;
• prevents the formation of cholesterol plaques in blood vessels;
• able to stop the activity of mold fungi;
• promotes the removal of heavy metal salts (lead, mercury, etc.);
• helps overcome depression and chronic fatigue;
• affects the improvement of cognitive functions.

Selenium is involved in the synthesis of coenzyme Q-10, an essential element for youthful skin and improved condition of hair and nails.

Interaction of selenium in the body and role in disease prevention

In the body, selenium interacts with enzymes and vitamins, reacts with cell membranes, participates in the breakdown of proteins, fats, carbohydrates, in the synthesis reactions of some hormones and proteins, as well as many vital bioactive compounds.

Selenium interactions begin in the body almost immediately after absorption, which occurs in the duodenum. Selenium is immediately picked up by red blood cells and binds with glutathione in them, promoting the formation of hemoglobin, and after 15-20 minutes it leaves the blood cells, interacting and binding with albumin, and then with blood plasma globulins. Then, under the influence of various enzymes, selenium binds with blood hydrogen, forming the H2Se compound - it is quite stable and acts as a kind of storage of the microelement. Since selenium is extremely active chemically, for its safe transportation to organs and tissues, selenoprotein is formed from the microelement stored in hydrogen selenide molecules. Selenium is excreted through the intestines and kidneys, and sometimes - in case of toxicosis - with exhaled air, through accumulation in nails and hair.

Selenium interacts with any component of food eaten and digested that can affect the antioxidant balance in the cell (for example, with dishes that increase the acidic environment in the body). When interacting with vitamins and minerals in the body, selenium supports the activity of vitamins E and, improves the absorption of vitamin K and other fat-soluble vitamins. Selenium acts as a synergist for iodine. Without this trace element, iodine is poorly absorbed.

A lack of selenium in the body is associated with impaired (slow) growth, hair loss and brittle nails, an increased risk of developing hepatitis and cirrhosis of the liver, cataracts, and thyroid diseases. Selenium deficiency is fraught with an increased likelihood of oncological pathologies, diseases of the cardiovascular system (atherosclerosis, cardiomyopathy, heart attack), diseases of the gastrointestinal tract and the development of renal failure.

A sufficient level of selenium prevents urolithiasis and cholelithiasis, reduces the likelihood of developing arterial hypertension, and prevents the development of arthritis and bronchial asthma.

Sources of selenium that can be included in the menu

Nutritionists in many countries around the world are ready to sound the alarm about insufficient selenium levels in the bodies of adults and children. On the one hand, this element is often found in nature, and it can easily be included in the diet along with foods that are rich in it. On the other hand, in modern society a huge amount of sweet dishes is consumed - baked goods, confectionery, just sugar in curds, yoghurts, juices and other products, and in the presence of carbohydrates (especially from flour products and sweets) selenium is almost not absorbed. Excessively fatty foods also interfere with the absorption of selenium, reducing its absorption fourfold.

Organal meats contain the most selenium - but this food option is often unacceptable for many people for ethical or religious reasons. Therefore, such products do not appear on their menu at all. Cereals are rich in selenium, but the microelement is quickly destroyed during heat treatment. In addition, porridge has long ceased to be an obligatory component of the daily menu of a modern person. In addition, often the soils where cereals are grown are poor in selenium, so the grain accumulates less of this element than we expect. Nuts and seeds can help a lot if you have a selenium deficiency, but fruits and vegetables contain very little of this microelement.

The champion in selenium content is the Brazil nut - only six nuts can cover a person’s daily need for this microelement.

Some foods you can get selenium from (in mcg per 100g)

Animals, seafood		Fruits vegetables		Cereals, nuts		Mushrooms, legumes, seeds
Turkey liver	71	Garlic	14	Wheat bran	78	sunflower seed	53
Duck liver	68	Coconut (flesh)	10	Pearl barley	37	Sesame	34
Chicken liver	55	Broccoli	2,5	Rice	28	Corn	30
Pork liver	53	Asparagus	2,3	Barley grits	22	Beans	25
Shrimps	45	Banana	1,0	Pistachios	19	Lentils	20
Octopus	44	Spinach	0,9	Cashew	19	Champignon	19
Beef liver	40	Beet	0,7	Semolina	15	Soya beans	17
Chicken egg	32	Currant	0,6	Buckwheat	13	Peas	13
Pork lard	20	Orange	0,5	Cereals	12	Dried oyster mushrooms	11
Tuna	12	Potato	0,3	Peanut	7	Dried porcini mushrooms	10

Selenium from plant sources is absorbed by the body almost completely, from animal products - by a third, and from synthetic dietary supplements - by about 10%.

A valuable and accessible source of easily digestible selenium is yeast, not baker’s yeast, but brewer’s yeast. Daily consumption of 2 g of such yeast, dissolved in a glass of warm water without sugar, can solve the problem of selenium deficiency in the body. A good source of selenium is wholemeal flour from any grain, since it contains bran.

Selenium is unstable during culinary processing of foods: in cooked food it becomes half as much, and when preserving vegetables or fruits it is practically destroyed. When soaking products before cooking, when boiling in a large amount of water, when defrosting semi-finished products, selenium goes into the water that surrounds the product, therefore, to maximize the preservation of selenium in food, products should be cooked in a small amount of water and cut up immediately before eating, if possible, not frozen.

The need for selenium increases if the menu often contains vegetable oil rich in polyunsaturated fatty acids (flaxseed, corn, sunflower) and fish oil. Their compatibility with selenium is low, since these nutrients compete with the trace element for penetration into tissue cells.

Advice! Selenium goes well with vitamin E (tocopherol) - they are both oxidizing agents, but do not interact, but complement each other. Therefore, it is useful to include foods rich in both of these substances in your menu. For example, buckwheat, chicken eggs, spinach, beef liver and potatoes

Selenium combines well with vitamin C and products containing it, as well as with vitamin A and zinc - these elements are very important for the body and, having antioxidant properties to one degree or another, can enhance each other’s effects. But the ingestion of heavy metals (for example, cadmium or lead from the exhaust gases that we inhale) almost completely blocks the absorption of selenium in the intestines.

Human needs for selenium and the consequences of its deficiency

The amount of selenium that should enter the human body per day (mcg)

The maximum permissible daily dose of selenium is 400 mcg, if it is urgent to restore selenium levels when its content in the body is very low. But in this case, selenium intake and its dose, as well as its content in the diet, must be strictly controlled by a doctor. It is believed that selenium consumption in amounts below 50 mcg/day for an adult is fraught with an increased risk of cardiovascular diseases and cancer.

The daily intake of selenium can be increased beyond the WHO-regulated amount under the following circumstances that provoke a deficiency of this mineral in the body:

• pregnancy (up to 65 mcg/day, but very carefully and under medical supervision, since the element is toxic and in case of overdose can provoke pathologies in the development of the fetus);
• breastfeeding (with a lack of selenium in the mother’s body - up to 75 mcg/day);
• heavy physical and mental stress, severe stress;
• work in conditions containing heavy metal particles in the inhaled air;
• poor environment, polluted air;
• prolonged exposure to active sun.

For adult men and women (from 19 to 70 years old), the selenium norm can be increased to 200 mcg/day without the risk of overdose and intoxication, but at the same time it is necessary to control the content of the trace element in the blood and reduce the dose if there is sufficient accumulation of selenium in the body.

Among the factors that can contribute to the development of selenium deficiency in the body are the following:

• the soil is depleted of selenium, which is why grass-fed cattle (a source of meat and dairy products for humans) will not receive this microelement in the required quantity;
• acidic soil environment, into which nitrogen and other types of fertilizers are regularly added: vegetables and legumes generally cannot obtain selenium from such soil;
• impaired absorption of selenium in the intestines;
• liver and kidney diseases;
• intoxication of the body with salts of heavy metals;
• inadequate physical, mental, and nervous overloads that accelerate the removal of selenium from the body.

The consequences of a deficiency of this mineral depend on how severely the body lacks it, and how long this deficiency has been ignored. It must be remembered that the following symptoms may be associated with a lack of selenium in the body:

• impaired visual acuity;
• a sharp decrease in working capacity: there is no strength for either physical or mental work;
• pustular rash on the skin of unknown origin;
• slow healing of any cuts, scrapes and sores.

Selenium deficiency in the human body can cause focal degeneration and even necrosis of the heart muscle and skeletal muscles (alimentary muscular dystrophy). The consequence of chronic selenium deficiency in the body is often the development of cataracts, male infertility, and the risk of prostate cancer, stomach cancer, and, in smokers, lung cancer.

The question of how to compensate for selenium deficiency in the human body is often resolved in many countries at the state level. So, for example, table salt is enriched with selenium (we use this product a little, and the risk of getting an overdose is minimal). Selenium is added to feed for birds and animals, thereby increasing the content of this microelement in chicken eggs, meat and dairy products.

Advice! Avoiding sweet flour products and carbonated drinks will help prevent the development of selenium deficiency in the body, since the carbohydrates they contain block the absorption of the microelement in the intestines.

Some medicinal plants are capable of accumulating selenium and can become a good source of replenishing this element in the body. Licorice, horsetail, sweet clover, eucalyptus, and ephedra are rich in selenium. Those who decide to use these natural sources of selenium in the form of decoctions or infusions need to remember the strong biological activity of plants and their ability to significantly enhance each other’s effects and influence the effectiveness of medications taken. It is advisable not to mix different sources of herbal materials rich in selenium, and first consult with your doctor about the possibility, dosage and duration of using herbs.

Horsetail decoction: 4 tablespoons of dried herb, poured a glass of boiling water. Keep in a water bath under a lid for half an hour, then cool for 10 minutes, strain, dilute with a glass of hot water. Drink half a glass three times a day, an hour after meals.

Infusion of licorice root: pour 1 tablespoon of finely chopped root into two glasses of cold water and leave for 8 hours. Drink 100 ml twice a day before meals.

Another way to compensate for the lack of selenium in the body is to turn to pharmaceutical drugs and bioactive supplements.

Preparations with selenium: rules of administration and features of choice

The need to take drugs or dietary supplements containing selenium, as well as the dosage and duration of their use, is determined by the doctor based on a laboratory blood test. The normal level of selenium in whole blood is 1.14-1.9 µmol/l. If values ​​are below normal, additional medications may be recommended, which are selected taking into account the depth of manifestations of selenium deficiency, the inability to correct the deficiency with diet, and possible concomitant diseases.

First generation selenium preparations - selenium sulfide and sodium selenite - are poorly absorbed by the body and can cause side effects such as nausea and abdominal pain. Second-generation drugs are a combination of active selenium particles with special molecules - biological ligands, which significantly improve the absorption of the microelement into the blood: these are selenium-cysteine, selenium-methionine, divalent selenium. Most often, doctors recommend the following drugs and bioactive supplements to compensate for selenium deficiency in the body.

Cephasel. The drug is prescribed for the complex treatment of cardiovascular diseases, gastrointestinal pathologies, cancer, and disorders of the thyroid gland. It can be prescribed for heavy physical activity, severe stress, for elderly people and pregnant women with an unbalanced diet, and for intoxication from inhalation of heavy metal particles.

The drug is available in tablets. Take three times a day after meals, without chewing and with water.

Selenase. Selenium preparation in ampoules can be used for intravenous and intramuscular administration, as well as orally - the contents of the ampoule are swallowed and washed down with water. It can be recommended for prolonged parenteral nutrition, in case of impaired absorption of selenium in the intestines, program dialysis, and malnutrition.

The drug is administered three times a day, 1 ampoule, the course of treatment is until the symptoms of selenium deficiency completely disappear.

Selenium active. A bioactive supplement containing selenium and vitamin C. It can be recommended for many indications, including cancer and heart diseases, allergies and male infertility, overwork and unfavorable ecology. Adults and children take 1 tablet per day with food, the course of therapy lasts a month.

Selenium. A dietary supplement based on brewer's yeast, used as a strong antioxidant for the prevention of heart and cancer diseases, to strengthen the immune system. The daily value of selenium is covered by 1 tablet taken with meals.

Triovit. A complex multivitamin containing selenium, beta-carotene, vitamins C and E, yeast complex. All components of the complex belong to the class of antioxidants and create a synergistic effect, enhancing each other’s action. The complex is recommended when working in hazardous industries, when living in an environmentally unfavorable region, when exposed to active sun, and under intense mental and physical stress. The multivitamin is taken 1 capsule twice a day after meals with water, recommended for adults and children over 15 years of age, course of use is 2 months.

Dangers of Selenium Overdose

Selenium is vital for the human body, but in very small doses - it is not for nothing that the level of its daily requirement is estimated in micrograms, while most microelements entering the body with food are calculated in milligrams. Selenium is a very active element that can cause severe harm to the body due to its high toxicity - it is not for nothing that it has long been classified as an element that is toxic to humans. Since selenium is partially absorbed from food, and the foods themselves are not always rich in this microelement, there is no risk of an overdose with regular, nutritious meals. But attempts to independently compensate for the lack of selenium using pharmaceutical preparations or bioactive supplements can result in acute or chronic overdose. There are known cases of acute selenium poisoning when taking a dietary supplement, in which, due to a technological error, the content of this element turned out to be more than 100 times higher.

An excess of selenium can develop if its absorption is impaired, or with diseases of the excretory tract, when the elimination of trace element residues is impaired.

Symptoms of acute overdose are:

• garlicky breath;
• nausea, diarrhea, abdominal pain;
• rapid and severe fatigue;
• periodically a feeling of numbness in the arms and legs.

Signs of chronic excess selenium in the body:

• severe hair loss;
• splitting of nails;
• yellowing of the skin;
• pain in joints and limbs;
• frequent multiple dental caries, since selenium damages tooth enamel;
• exacerbation of dermatitis;
• severe irritability and chronic fatigue.

With a chronic overdose of selenium, peripheral polyneuropathy develops. Excess selenium threatens anemia, arthritis and atherosclerosis, increased blood pressure, inflammatory processes on the skin and mucous membranes.

In case of acute overdose of selenium, gastric lavage is prescribed and then symptomatic assistance is provided. Chronic overdose is treated by complete withdrawal of the drug or, if absolutely necessary, by reducing the dose of the drug and adjusting the diet in favor of reducing selenium-containing products in the menu.

Selenium is an active fighter against skin aging

The powerful antioxidant properties of selenium make it possible to neutralize and partially compensate for the negative changes that accumulate in the skin with age. This element, in combination with sulfur-containing amino acids, can restart DNA repair processes, reduce the likelihood of gene mutations, and counteract any attack by damaging elements, including from the environment. Therefore, selenium is included in face creams and some types of decorative cosmetics, which are applied during the day to protect the skin.

As part of cosmetics, selenium can quickly improve the condition of the skin after it is damaged by active solar ultraviolet radiation. The trace element suppresses inflammation and prevents vascular changes. Even after too much sun exposure, a cream containing selenium will help avoid painful redness and swelling of the skin and reduce the likelihood of pigmentation. Such cosmetics are especially useful for those who like to sunbathe well, and for those whose age spots appear again, even if their treatment was previously successful.

Among manufacturers of cosmetic products for residents of megacities, selenium is considered one of the most promising components of cosmetics. The fact is that regular exposure of the skin to smog, tobacco smoke, exhaust fumes, and insufficient clean water increases stress on cells and accelerates skin aging. Selenium is able to protect the skin from environmental stress and significantly delay the process of premature aging.

To care for mature skin with signs of age-related changes, cosmetic preparations are used in which selenium is enhanced with vitamins C and E. The synergy of these components allows not only to prevent the development of oxidative processes in cells, but also to have a regenerative, anti-inflammatory, and detoxifying effect. Hair care products also include selenium - to prevent hair loss, strengthen, increase density and shine.

About the effect of selenium on human health and prolongation of youth, about the antioxidant properties of this microelement and about products containing selenium, see the video below.

DEFINITION

Selenium- thirty-fourth element of the Periodic Table. Designation - Se from the Latin "selenium". Located in the fourth period, VIA group. Refers to non-metals. The nuclear charge is 34.

Selenium is not widely distributed in nature. In the earth's crust, the selenium content is 0.00006% (mass.). Its compounds are found as impurities in natural sulfur compounds with metals (PbS, FeS 2, etc.). Therefore, selenium is obtained from waste generated during the production of sulfuric acid, during the electrolytic refining of copper and in some other processes.

Atomic and molecular mass of selenium

The relative molecular mass of a substance (M r) is a number showing how many times the mass of a given molecule is greater than 1/12 the mass of a carbon atom, and the relative atomic mass of an element (A r) is how many times the average mass of atoms of a chemical element is greater than 1/12 mass of a carbon atom.

Since in the free state selenium exists in the form of monatomic Se molecules, the values ​​of its atomic and molecular masses coincide. They are equal to 78.96.

Allotropy and allotropic modifications of selenium

In the free state, selenium, like sulfur, forms several allotropic modifications. The most stable is hexagonal or gray selenium. Its crystals are formed by zigzag chains Se ∞. When liquid selenium is rapidly cooled, a red-brown glassy modification is obtained. It is formed by Se ∞ molecules of various lengths. The crystalline species of red selenium are composed of cyclic Se8 molecules similar to S8.

Gray selenium is a semiconductor. Its electrical conductivity increases sharply when illuminated. Liquid selenium exhibits conductive properties.

Rice. Allotropic modifications of selenium. Appearance.

Selenium isotopes

It is known that in nature arsenic exists in the form of six stable isotopes, one of which is radioactive [82 Se (9.19%)]: 74 Se (0.87%), 76 Se (9.02%), 77 Se ( 7.58%), 78 Se (23.52%) and 80 Se (49.82%). Their mass numbers are 74, 76, 77, 78, 80 and 82, respectively. The nucleus of an atom of the selenium isotope 80Se contains thirty-four protons and forty-six neutrons; other isotopes differ from it only in the number of neutrons.

There are artificial unstable isotopes of selenium with mass numbers from 65 to 94, as well as nine isomeric states of nuclei, among which the longest-lived isotope 79 S with a half-life of 6.5 × 10 5 years.

Selenium ions

The outer energy level of the selenium atom has six electrons, which are valence electrons:

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

As a result of chemical interaction, selenium either gives up its valence electrons, i.e. is their donor, and turns into a positively charged ion, or accepts electrons from another atom, i.e. is their acceptor and turns into a negatively charged ion:

Se 0 +2e → Se 2- ;

Se 0 -2e → Se 2+ ;

Se 0 -4e → Se 4+ ;

Se 0 -6e → Se 6+ .

Selenium molecule and atom

In the free state, selenium exists in the form of monoatomic Se molecules. Here are some properties characterizing the selenium atom and molecule:

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Exercise	What mass of selenium (VI) oxide should be added to 100 g of a 15% solution of selenic acid to double its mass fraction?
Solution	Let's write the reaction equation: SeO 3 + H 2 O = H 2 SeO 4. Let's find the mass of selenic acid in the original solution: ω = m solute / m solution × 100%. m solute = ω / 100% ×m solution ; m solute (H 2 SeO 4) = ω (H 2 SeO 4) / 100% ×m solution (H 2 SeO 4); m solute (H 2 SeO 4) = 15 / 100% × 100 = 15g. Let the number of moles of selenium (VI) oxide that must be added to a solution of selenic acid to double its mass fraction be equal to x mole. Then the mass of selenium (VI) oxide (molar mass - 127 g/mol) is equal to: m (SeO 3) = n (SeO 3) × M (SeO 3); m (SeO 3) = x × 127 = 127x. According to the reaction equation n (SeO 3): n (H 2 SeO 4) = 1:1, i.e. n (SeO 3) = n (H 2 SeO 4) = x mol. Then, the mass of selenic acid formed after adding selenium oxide (VI) will be equal (molar mass - 145 g/mol): m 2 (H 2 SeO 4) = n (H 2 SeO 4) × M (H 2 SeO 4); m 2 (H 2 SeO 4) = x × 145 = 145x. Let's find the mass of the new solution and the mass of selenic acid in it: m ’ solution (H 2 SeO 4) = m solution (H 2 SeO 4) + m (SeO 3) = 100 + 127x; m 3 (H 2 SeO 4) = m solute (H 2 SeO 4) + m 2 (H 2 SeO 4) = 15 + 145x. Let's calculate the mass of selenium oxide (VI) that needs to be added to a solution of 15% selenic acid in order to double its mass fraction: ω ' (H 2 SeO 4) = m 3 (H 2 SeO 4) / m ' solution (H 2 SeO 4) × 100%; 0.3 = 15 + 145x/100 +127x; x = 0.14, i.e. n (SeO 3) = 0.14 mol; m (SeO 3) = 0.14 × 127 = 17.8 g.
Answer	The mass of selenium (VI) oxide is 17.8 g

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

Being in nature, receiving:

The selenium content in the earth's crust is about 500 mg/t. Selenium forms 37 minerals, among which the first to be noted are ashavalite FeSe, clausthalite PbSe, timannite HgSe, guanajuatite Bi 2 (Se,S) 3, hastite CoSe 2, platinite PbBi 2 (S,Se) 3. Native selenium is occasionally found. Sulfide deposits are of major industrial importance for selenium. The selenium content 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 from sulfuric acid and pulp and paper production, and also significant amounts are obtained from sludge from 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 different allotropic modifications: amorphous (powdery, colloidal, glassy) and crystalline (monoclinic, a- And b-shapes and hexagonal g-form). Amorphous (red) powdered and colloidal selenium is obtained by reduction from a solution of selenous acid by rapid cooling of selenium vapor. Glassy (black) selenium is obtained by heating any modification of selenium above 220°C, followed by rapid cooling. It has a glassy luster and is fragile. Thermodynamically, hexagonal (gray) selenium is the most stable. It is obtained from other forms of selenium by heating until melting, slowly cooling to 180-210°C and holding at this temperature. Its lattice is built from parallel spiral chains of atoms.

Chemical properties:

At ordinary temperatures, selenium is resistant to oxygen, water and dilute acids. When heated, selenium reacts with all metals, forming selenides. In oxygen, with additional heating, it slowly burns with a blue flame, turning into dioxide SeO 2.
It reacts with halogens, with the exception of iodine, at room temperature to form the compounds SeF 6, SeF 4, SeCl 4, Se 2 Cl 2, SeBr 4, etc. With chlorine or bromine water, selenium reacts according to the equation:
Se + 3Br 2 + 4H 2 O = H 2 SeO 4 + 6 HBr
Hydrogen reacts 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 finally. In alkali solutions, 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 selenium(-2) and selenium(+4) compounds.
Similarly to sulfur, selenium dissolves when heated in solutions of Na 2 SO 3 or KCN, forming, respectively, Na 2 SSeO 3 (an analogue of thiosulfate) or KCNSe (an analogue of thiosulfate).

The most important connections:

The most typical oxidation states for selenium are -2, +4, +6.
Selenium(IV) oxide SeO 2- white shiny crystals with a polymer molecule (SeOsub>2)sub>n, m.p. 350°C. The vapors are yellowish-green in color and have the smell of rotten radish. Easily dissolves in water to form H 2 SeO 3 .
Selenous acid, H 2 SeO 3- white rhombic crystals. Highly hygroscopic. Highly 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– colorless crystals, m.p. 711°C. Hygroscopic, highly 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, m.p. 121°C. It is hygroscopic, reacts with water with large heat release and the 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. It is toxic, hygroscopic, and a strong oxidizing agent. Selenic acid is one of the few compounds that, when heated, dissolves 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 - crystals of orthorhombic system; tmelt 730 °C. Obtained by neutralizing the acid with sodium oxide, hydroxide or carbonate or by oxidizing sodium selenite. Slightly soluble in water, below 32 °C it crystallizes from aqueous solutions in the form of decahydrate Na 2 SeO 4 10H 2 O
Hydrogen selenide, H 2 Se- a colorless flammable gas with an unpleasant odor. The most toxic selenium compound. In air it is easily oxidized at ordinary temperatures 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 H2S.
Selenides- compounds of selenium with metals. Crystalline substances, often with a metallic luster. 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 = 2n Se + 4NaOH

Application:

Selenium is used in rectifying semiconductor diodes, as well as for photovoltaic devices, electrophotographic copying devices, as phosphors in television, optical and signal devices, thermistors, etc. Selenium is widely used for decolorizing green glass and producing ruby ​​glasses; in metallurgy - to give steel a fine-grained structure and improve their mechanical properties; in the chemical industry - as a catalyst.
The stable isotope selenium-74 made it possible 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 is present in the active centers of some proteins in the form of the amino acid selenocysteine. It has antioxidant properties, increases the perception of light by the retina, and affects many enzymatic reactions. The requirement of humans and animals for selenium does not exceed 50-100 mcg/kg of diet.

Polkovnikov A.A.
HF Tyumen State University, 581 group. 2011

Sources: Wikipedia: http://ru.wikipedia.org/wiki/Selenium
Chemist's Handbook website:

Selenium is a chemical element with atomic number 34 in the periodic table of chemical elements D.I. Mendeleev, denoted by the symbol Se (lat. Selenium), a brittle, shiny, black non-metal (stable allotropic form, unstable form - cinnabar-red).

Story

The element was discovered by J. Ya. Berzelius in 1817. The name comes from the Greek. σελήνη - Moon. The element is so named due to the fact that in nature it is a satellite of the chemically similar tellurium (named after the Earth).

Receipt

Significant quantities of selenium are obtained from copper-electrolyte production sludge, in which selenium is present in the form of silver selenide. Several production methods are used: oxidative roasting with sublimation of SeO 2; heating the sludge with concentrated sulfuric acid, oxidizing selenium compounds to SeO 2 with its subsequent sublimation; 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

Solid selenium has several allotropic modifications. The most stable modification is gray selenium. Red selenium is a less stable amorphous modification.
When gray selenium is heated, it gives a gray melt, and upon further heating it evaporates to form brown vapors. When the vapor is cooled sharply, selenium condenses in the form of a red allotropic modification.

Chemical properties

Selenium is an analogue of sulfur and exhibits oxidation states −2 (H 2 Se), +4 (SeO 2) and +6 (H 2 SeO 4). However, unlike sulfur, selenium compounds in the +6 oxidation state are the strongest oxidizing agents, and selenium compounds (-2) are much stronger reducing agents than the corresponding sulfur compounds.
A simple substance - selenium is much less active chemically than sulfur. Thus, unlike sulfur, selenium is not capable of burning in air on its own. Selenium can be oxidized only with additional heating, during which it slowly burns with a blue flame, turning into SeO 2 dioxide. Selenium reacts (very violently) with alkali metals only when molten.
Unlike SO 2, SeO 2 is not a gas, but a crystalline substance, highly soluble in water. Obtaining selenous acid (SeO 2 + H 2 O → H 2 SeO 3) is no more difficult than sulfurous acid. And by acting on it with a strong oxidizing agent (for example, HClO 3), they obtain selenic acid H 2 SeO 4, almost as strong as sulfuric acid.

Global trends are such that people are increasingly thinking about the state of their body and health in general. Research is being conducted and theories of the influence of certain chemical compounds on organs and systems are being proven. Selenium was no exception, since the benefits of this chemical element for living organisms are invaluable.

What is selenium?

Selenium is one of the chemical elements presented in the periodic table of D.I. Mendeleev. Refers to non-metals. Its atomic number is 34. Selenium minerals can be black, red or gray in color.

Natural selenium is found in the earth's crust. It got its name from the Greek word “Selene” (Moon). Found in mineral waters and sea water. Its properties are similar to sulfur, but its chemical activity is less.

Biological role of a substance in living nature

Selenium is one of the elements necessary for life, as it is part of proteins. In proteins it is represented by the amino acid selenocysteine. Selenium deficiency (especially in children) is fatal.

In the human body, this element performs several functions:

• immunomodulatory;
• anticancer;
• promotes the formation of certain hormones;
• is an antioxidant.

Interesting fact

Selenium is also present in the nuclei of living cells. It promotes the metabolism of nucleic acids. Nucleic acids (DNA and RNA) are found in all living organisms and are responsible for the transmission of hereditary information.

Benefits of selenium

For the human body, this element plays an invaluable role. But you should be careful with it, since its deficiency (as well as its excess) can lead to the death of a living organism. The recommended daily dose of the substance for normal human life is 70-100 mcg.

Why is this microelement so necessary for any organism?

1. It has anti-radiation and anti-toxic effects.
2. Reduces the likelihood of mutation risk.
3. Strengthens myocardial muscle fibers and prevents the development of diseases of the cardiovascular system.
4. Normalizes metabolic processes.
5. Promotes the formation of thyroid hormones (triiodotyranine) and the absorption of iodine.
6. Removes heavy metal salts from tissues (lead, mercury, cadmium).
7. Promotes the synthesis of pancreatic hormones. Enhances insulin production. Therapy with selenium-containing drugs is used for type II diabetes mellitus. This treatment also gives good results for pancreatitis.
8. Normalizes the functioning of the gastrointestinal tract (improves mucus secretion and enzyme production).
9. Stimulates the production of interferon, which allows the immune system to successfully fight influenza viruses.
10. Accelerates the synthesis of vitamin E, enhancing its effect as an antioxidant.
11. Reduces the severity of some common diseases (psoriasis, bronchial asthma, dermatosis). For this purpose, selenium is used in combination with vitamins E and C.
12. Helps improve joint mobility. Used for arthritis, arthrosis, rheumatism. In traumatology it is used for fractures, as it promotes the growth of bone tissue.
13. The nervous system responds quite well to selenium intake: memory and neuron activity improve. Sometimes the trace element is used in the fight against multiple sclerosis.
14. Selenium improves mineral metabolism, promotes harmonious functioning of the kidneys and urinary system. Used in the treatment of pyelonephritis.
15. The protective effect of the non-metal allows the restoration of liver cells. Used in hepatoprotective drugs for cirrhosis.
16. Protects humans from the toxic effects of waste products of mold fungi.

On a note

Selenium is also extremely important for men's health: the microelement helps enhance reproductive function, significantly increasing the activity of male germ cells.

This element increases the activity of the immune system, which is extremely important in childhood. Selenium deficiency can lead to endemic cretinism and mental retardation. And in combination with the Coxsackie virus, which has recently been spreading across the globe, a deficiency of the microelement can provoke Keshan disease. This pathology leads to degeneration of myocardial tissue with further necrosis. Without qualified help, it causes death.

Anti-cancer properties of the element

Unfortunately, malignant neoplasms are increasingly affecting people in all corners of the planet. Cancer treatment is expensive and does not always lead to positive results. Taking certain substances helps reduce the risk of developing cancer. One of these substances is selenium.

This trace element helps reduce the risk of tumor development by almost 40%. It seems incredible, but a sufficient amount of selenium-rich foods in your diet will help prevent cancer. And selenium therapy during oncology can almost halve the mortality rate of patients.

Many developed countries have long adopted programs for the “selenization” of the population. The main objectives of such programs are aimed at preventing pathologies among residents. Research in recent years shows that the benefits of selenium in the process of such programs are obvious: the risk of developing cancer is significantly reduced, as well as pathologies of the cardiovascular and immune systems.

Harm to the human body

Despite the positive qualities of selenium, this trace element can cause enormous harm to the body. Doctors strongly recommend using it in doses so as not to cause excess.

The negative effects of the chemical on the skin are as follows.

• When non-metal salts come into contact with the epidermis, a strong reaction occurs. A feeling of numbness and dermatitis are a small part of the consequences. The element can cause severe chemical burns.
• If the substance gets on the mucous membranes (eyes, mouth), irritation, pain, and hyperemia occurs. Causes conjunctivitis.

All selenium compounds are toxic. Hydrogen selenide is especially dangerous in this regard. Gray, or metallic, selenium is the least dangerous, but even 1 gram of the substance ingested can cause poisoning. Symptoms: abdominal pain, nausea, vomiting, diarrhea. After some time, the symptoms go away.

The female body and selenium

For women, a sufficient amount of micronutrients in the diet plays an important role. It provides many vital processes:

1. normalizes metabolism, which allows women to look slim;
2. delays the onset of menopause;
3. improves reproductive function, which allows you to easily get pregnant;
4. improves the condition of skin, hair and nails;
5. prevents the development of inflammatory processes in the female genital organs.

Thanks to a sufficient amount of this microelement, the risk of developing uterine and mammary gland cancer is reduced. If oncology has begun, then the chemical element can successfully cure the tumor. Taking selenium reduces the risk of having to remove the uterus, which allows you to preserve reproductive function.

During pregnancy, the microelement is also very important. It helps to cope with many difficulties of bearing a child:

1. prevents toxic effects on the fetus;
2. reduces the risk of developing fetal mutations;
3. improves the condition of toxicosis;
4. allows the female body to quickly recover in the postpartum period.

It is very important to consume enough selenium while breastfeeding. It ensures good health of the mother and good health of the child. But you should not choose vitamin complexes with a high selenium content - give preference to natural sources of this substance.

Symptoms of selenium deficiency and excess

How does a substance deficiency manifest itself? Its deficiency can be understood by the following symptoms:

• weakness and aches throughout the body;
• constant drowsiness;
• deterioration of skin and hair condition;
• vision problems arise (temporary myopia);
• there is a constant feeling of aching in the muscles;
• Rashes may appear on various areas of the skin.
• memory deteriorates;
• body weight may increase;
• immunity decreases, colds often occur.

But the following symptoms indicate an overabundance of the substance:

• abdominal pain, nausea, vomiting;
• bowel movements may become more frequent;
• alopecia (hair loss) develops;
• the breath smells like garlic (therefore, sometimes excess selenium can be confused with arsenic poisoning);
• dizziness and headache appear;
• eyes hurt and turn red.

Available Sources of Selenium

To understand how much of a substance is contained in certain products, just look at the comparison table. It contains plant products that are record holders for selenium content.

The product's name	Microelement content in mcg per 100 g of raw product
Bertholetia (Brazil nut) grains	1500
Dates	400
Oyster mushroom (mushrooms)	130
Coconut pulp	70-100
Sunflower seeds	50
Corn	30
Broccoli	2,5

The following table shows animal foods rich in selenium.

The pharmaceutical industry presents vitamin complexes enriched with selenium and preparations with a high content of this substance. Such medications should not be taken without medical prescription: they have a fairly large number of side effects and contraindications. Children, pregnant and nursing mothers especially should not experiment with such complexes.

To get the required amount of microelement, you should not stuff yourself with chemicals. It is enough to review your diet and enrich it with the necessary products. And resort to pharmaceutical products only as a last resort. Take care of your health!