Selenium chemical element interesting facts. What is selenium? Selenium atomic and molecular weight

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. Selenium what is it beneficial features, for what it is needed by the body, what leads to a deficiency of selenium and its lack, today in the article.

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

What is selenium and why does the body need it

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

There is not much of it in nature. But in places of volcanic eruptions, the presence of selenium in the soil is noted in large quantities, therefore, both flora and fauna are rich in them.

For a long time this amazing element was considered the strongest poison, and they treated it with caution, until, in 1973, they rediscovered it and found out how important it is for life. 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 estimate 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.

Selenium has been assigned an important role by nature - protection from the destructive effects of heavy metals (arsenic, cadmium, mercury, lead, thallium) and control over all 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 takes part in the synthesis of the peptide - glutathione, which plays a major role in the course of redox reactions, keeps the shield of the immune system that 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, anti-tumor and anti-inflammatory, anti-stress and anti-optic effects.

Selenium has numerous functions in the body:

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

☀ 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 anti-tumor factor (prevents and stops the development of malignant tumors),

☀ is 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 (it is a part of sperm),

☀ stabilizes the functioning of the nervous system,

☀ normalizes 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 selenium works in the body. Scientists note that despite the scanty 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 well established that almost all the energy produced by the mitochondria of cells occurs only with the participation of selenium (i.e. 85%)! Energy is necessary for all organs:

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

All these processes in the body take place with the participation of selenium, and if it is not enough, 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, thousands of free radicals appear. These are defective 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, and not defective, molecules are needed. But if the process of formation of free radicals progresses, this 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 vitamins 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 the immune system and detoxification.

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

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

What diseases causes selenium deficiency in the body?

Scientists have found that in conditions of a lack of selenium, the immune system cannot fully perform its functions. Failure after failure starts:

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, therefore, the process of inhibition of the growth and development of the body occurs.

Selenium deficiency in the body causes the appearance of many diseases:

• the number of pathologies during pregnancy and difficult childbirth increases,
• the growth of children born with pathologies increases,
• the number of sick teenagers is growing,
• develops female and male diseases that cause infertility,
• chronic diseases are severe,
• capillaries become brittle and brittle,
• sperm lose their mobility, become inert,
• the number of mental illnesses is growing,
• anemia and diabetes, hepatitis and endemic goiter appear,
• stroke and myocardial infarction and some cancers,
• new viral diseases are born,
• 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 no cell can work without its presence. Selenium chemical element:

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

The heart is very sensitive to the content of selenium in cells, if its level drops to 45 μg / kg, then there is an immediate risk of heart failure. Acute heart failure occurs when selenium levels drop to 20 mcg / kg, leading to cardiac arrest.

Scientists have established a natural relationship between the selenium content 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 selenium levels.

Coenzyme Q10, which protects the heart muscle from oxygen deficiency and is involved in the restoration 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, regulation of the work of all organs and systems, and for this it has to synthesize some hormones.

One of these hormones is T4, which, after the release of one iodine atom, turns into an active hormone - triiodothyronine, which is involved in energy metabolism. This process takes place 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 this hormone is absent, 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 strenuous work and increases in size, or forms nodes. But there is still not enough selenium, and problems have appeared in the thyroid gland from its lack.

Recent research by scientists indicates 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 the inflammatory process, 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 work 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 selenium levels are normal, it inhibits the growth of the incipient tumor.

Atypical or cancerous cells form in all people, such cells are born every 10 minutes. But in the presence of selenium, the body immediately produces special enzymes 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 multiplication of cancer cells, toxic substances accumulate, which are also not neutralized due to a lack of selenium and continue to accumulate in the body.

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

It is interesting:

"Scientists have recently discovered that serum selenium concentrations below 45 μg / L lead to tumors, both benign and malignant."

Why is selenium deficiency formed?

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

Unfortunately, the statistics for Russia is completely different. Experts emphasize that on average, residents of Russia consume only 30 mcg of selenium with food, so 80% of residents experience selenium deficiency.

The human needs for selenium meets 90% through food and 10% through water. It is believed that the largest amounts of selenium are found in Brazilian nuts, onions and garlic.

It is important to consider whether there is a natural selenium deficiency? 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 the assimilable form of selenium in the soil, and therefore in the plants grown on this land. The amount of selenium in the environment is affected by forest fires, burning of grass and dry foliage.

The data of the Research Institute of Nutrition of the Russian Academy of Medical Sciences indicate that there is a deficiency of selenium in the soil almost throughout Russia. Especially in Transbaikalia and Buryatia, studies have confirmed the 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-West region of the country (Leningrad region, Karelia), the Upper Volga region (Ivanovo, Kostroma, Yaroslavl regions).

In people living in areas with a low selenium content in the environment, diseases of the stomach and intestines, liver and genitourinary system progress much more often, a lack of selenium is manifested 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:

• “Newborns, from birth to 2 years;
• at puberty from 11-20 years;
• “Christ's age” (30 - 33 years old), when the aging process of the organism has already begun;
• the period of pregnancy is especially dangerous if this period coincides with the age of Christ;
• climacteric periods in women and men and further, when the selenium content in 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 are raising the issue of 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 program in our country, then in just one year, about 600 thousand people with heart diseases can be saved.

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

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

Why is excess selenium in the body dangerous?

To maintain your health and wellness in good condition, you need a certain balance of the chemical element selenium in the body. 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 health problems.

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 among workers in contact with this element: in the electronic, glass, paint and varnish industries. Those who permanently reside in areas of the country with active volcanoes (Kuriles, Kamchatka).

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

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

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

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

History

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 tellurium, which is chemically similar to it (named after the Earth).

Receiving

Significant amounts of selenium are obtained from the sludge of copper electrolyte production, in which selenium is present in the form of silver selenide. Several production methods are used: oxidative calcining with sublimation of SeO 2; heating sludge with concentrated sulfuric acid, oxidation of selenium compounds to SeO 2, followed by 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. Selenium red is a less stable amorphous modification.
When gray selenium is heated, it gives a gray melt, and upon further heating it evaporates with the formation of brown vapors. When the vapor is rapidly cooled, selenium condenses in the form of a red allotropic modification.

Chemical properties

Selenium is an analogue of sulfur and exhibits the 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. So, unlike sulfur, selenium is not able to burn in air on its own. Selenium can be oxidized only with additional heating, at which it slowly burns with a blue flame, turning into SeO 2 dioxide. Selenium reacts (very violently) with alkali metals only when melted.
Unlike SO 2, SeO 2 is not a gas, but a crystalline substance that is highly soluble in water. To obtain selenous acid (SeO 2 + H 2 O → H 2 SeO 3) is not at all more difficult than sulfurous acid. And acting on it with a strong oxidizing agent (for example, HClO 3), selenic acid H 2 SeO 4 is obtained, almost as strong as sulfuric acid.

Selenium (lat.selenium) - an element of non-metallic origin. In its pure form, it is a solid gray-black substance with a metallic luster (see photo), which is quite fragile. In another modification (unstable), its color becomes reddish. Under normal conditions, it is quite stable, interacts with halogens (chlorine and fluorine), and at elevated temperatures - with oxygen.

The name has Greek roots and comes from the word "Moon". The reason for this was that in natural conditions this element is a satellite of tellurium, and that is named after the planet Earth. Accordingly, by analogy, he was given the name of the satellite of the planet.

The element was discovered in 1817 by Berzelius from sulfuric acid sediments.

Selenium is widely used in industry due to its electrical conductivity properties. Although it is quite rare in nature, almost never in the form of minerals.

Selenium action and its biological role

The effect of the macronutrient on the human body has been recognized and has been studied since 1973. Since that time, the whole world has embraced a real "selenomania". Then it seemed that the use of this element would become a panacea for all diseases and began to prescribe it in large doses to everyone. But as it turned out, selenium can be both a miracle cure and a poison, if you don't know when to stop.

Selenium has a very powerful effect on almost all the activities of our body and plays the following biological role:

In general, there is no area in the body that is not affected by selenium. It is part of most enzymes and proteins, which in turn carry out redox reactions, neutralize free radicals that harm at all levels of the body's activity. Selenium binds and removes salts of heavy metals (mercury, manganese, cadmium, lead) from the body.

The most important action of the element is, of course, the fight against cancer and the body's immune defense against a variety of viruses: hepatitis, Ebola, HIV / AIDS. The latter disease is still incurable, but the element is quite capable of keeping it in a latent state. Selenium retains the virus within the cell, when the reserves of the element are depleted, the virus spreads to healthy cells. Its effect on the production of thyroxine, a thyroid hormone, is also important, and when it is lacking, this organ grows. Taking selenium in combination with iodine, with an already developed disease, can cause at least a suspension, or even regression of the disease. The latter is more likely in children.

Selenium also acts on the growth of pancreatic cells and insulin-producing cells. This accelerates the consumption of glucose, which is a good prevention or treatment for the development of diabetes. In the gastrointestinal tract, with the participation of a macronutrient, microflora normalizes, and damaged tissues and mucous membranes are regenerated. Bile formation is stabilized in the liver, which improves its functioning.

Don't forget about selenium during pregnancy. After all, it will help fight toxicosis and give a good mood, removing all the symptoms of anxiety. During feeding, it is also necessary, and in a double amount, because the baby receives all the elements with mother's milk.

The element itself is not a guaranteed medicine, but an integrated qualified approach can help in most cases. Preventive use will significantly reduce all the risks of acquiring the above diseases. But, in any case, self-medication is not recommended.

Daily rate - what is the body's need for a chemical element?

The daily norm of a macronutrient is determined by the World Health Organization within the following limits:

• adults (woman / man) - 55-70 mcg;
• infants under 1 year old - 10-15 mcg;
• preschool children - 20 mcg;
• children of primary school age and adolescents - 30-50 mcg;
• pregnant and lactating women - 65-200 mcg;
• athletes, on the other hand, need up to 200 μg of the substance.

The maximum dose is considered to be the use of 400 mcg per day. Shock doses are prescribed only when a significant deficiency is diagnosed under the close supervision of doctors.

Selenium deficiency - symptoms of deficiency and characteristics of its harm to health

The lack of a macronutrient can cause the so-called "diseases of civilization", despite the fact that for a long time it was considered a real poison. The consequences of the deficiency can be extremely disappointing and harmful to health, in particular, they manifest themselves in the following symptoms:

The propensity to develop deficiency increases with age, and accordingly, the propensity to disease, too. Recent times it was noticed that there is a sharp increase in the number of the above-mentioned diseases, and they become more chronic and aggressive. These processes are associated with a deterioration in lifestyle and environment... Our food today is too poor in useful elements and vitamins, because grown in impoverished lands. Fertilizers, poisoning of soils and grounds with industrial waste affect chemical composition products.

Excess selenium in the body

As mentioned, selenium can also be poisonous. Although its effect on the body is invaluable, too little of it is required - about one hundred thousandth of one gram.

Excess doses will quickly affect your health and appearance, causing the following symptoms: weak nails and hair, muscle pain, redness or flaking of the skin, dental problems, dysfunction of the nervous system, kidneys, liver, and decreased immunity.

Most often, an excess of an element is found in people who earn money by mining this element in the development of natural layers, as well as in residents bordering on mining sites, settlements. Also, uncontrolled intake of drugs in pursuit of magic healing from cancer causes excess. Although it has been proven that a large amount of selenium can have the opposite effect, that is, it can accelerate the growth of neoplasms and interfere with the action of chemotherapy.

Sources containing this item

Almost the entire population of the planet suffers from mild selenium deficiency - they receive about 70-80%. And only residents of coastal areas to the seas receive the required amount of the element, eating seafood rich in it.

It is quite possible to maintain the required level of the element in the body with food. The main thing is to know what is! It will be enough to add dairy products, pig lard, veal, legumes and coconuts to the diet. A certain amount of it can be found in rock or sea salt, eggs, bran, tomato, corn, and wholemeal products. But all these products should be eaten fresh, without processing (conservation, concentration), because it completely destroys selenium, and heat treatment reduces the amount of the useful element by half.

There are several products that can be called the leaders in selenium content. These are broccoli, oregano, garlic and yeast. True, everything that follows applies to products grown on ecologically clean lands.

Brewer's and baker's yeast is a wonderful source of selenium, because here it is in a biologically active form and is easily absorbed. In the middle of the 20th century, it was found that the use of yeast prevented or even stopped the development of liver necrosis. It turned out that our macronutrient turned out to be the mysterious component. The only thing is that the yeast must be processed before use, otherwise there will be little benefit from it.

Why? As you know, this substance is a living organism and even in the body it continues to feed on, mainly B vitamins, and specifically biotin. The same, in turn, affects immunity. Therefore, it is necessary to extinguish the activity of the yeast, and for this you need to treat them with hot water (at least 60 degrees Celsius). There is no need to get carried away - two grams a day is more than enough for a healthy person. For all others, the doctor will determine the norm and the course of treatment most often lasts no more than 2 weeks.

You can close the selenium deficiency with garlic. It is the garlic smell that appears with an overdose of the element. In addition to selenium, you will get a lot of useful substances: proteins, carbohydrates, minerals, vitamins B and C. And the smell is nothing more than allicin (a sulfur compound with an unpleasant odor characteristic of garlic), which kills bacteria. Note that preparations with supposedly garlic extracts do not have the properties of a natural product.

Italian and Spanish cuisines are rich in garlic, and it has long been noted that people living in these countries are much less likely to suffer from heart disease, atherosclerosis and senile marasmus. And their use with meat slightly neutralizes the effect of fats and cholesterol.

Brazil nuts can be placed in the rows of selenium-containing products. They grow on selenium-rich tropical soils and one nut can replace the entire daily requirement of the element for an adult. But this amount (up to 100 mcg) is contained only in unpeeled nuts. Kernels without shells contain 4-5 times less element.

The lack of selenium in infants, namely in boys, can most often be explained by artificial nutrition. After all, mother's milk contains twice as much selenium and 5 times more vitamin E than cow's milk. The male sex needs more element, and the deficit is more pronounced. Many doctors associate the mysterious "sudden death" of newborns precisely with the lack of these two substances.

Interaction with other substances

The presence of certain elements and vitamins helps to cope with diseases. For example, vitamins E and C in combination with selenium fight viruses, infections and cancer. Together with cobalt and magnesium, it affects the protection of chromosomes and genetic material in cells.

The enemy, with full confidence, can be called carbohydrates, which are capable of completely destroying all accumulations of selenium in the body. So all the cakes and pastries have once again proven their inappropriateness.

There is one more substance that can be called an enemy of a useful element. It is a mold that has a carcinogenic effect on cells. It can enter the body from food with rotting grain or potatoes stored in damp premises.

Indications for appointment

Indications for the appointment of a macroelement are due to its biological effect on the human body. It serves as a prophylactic agent for many diseases that are caused by its deficiency. Main purposes:

• dysfunction of the thyroid gland;
• multiple sclerosis;
• fetal and pregnancy anomalies;
• pancreatitis treatment;
• skin diseases (psoriasis, lichen, dermatitis) - used for internal and external use.

When choosing drugs, pay close attention to some points. For example, the content of an element in supplements. There was a known case when one of the manufacturers put 100 mg of an element instead of 100 μg into a preparation. It turned out that consumers for some time exceeded the norm by 1000 times, which naturally led to multiple poisoning. Do not forget that you will also receive some amount from food.

There are still drugs that contain inorganic compounds selenium, which can cause side effects like nausea and stomach discomfort. It would be better to use medicines with easily digestible organic compounds.

Black metal selenium crystals formed on a burning coal dump. Image width - 1.8 mm. Germany, North Rhine-Westphalia, Aachen, Alsdorf, Anna mine

Fragile black non-metal, shiny at the fracture (stable allotropic form, unstable form - cinnabar red). Refers to chalcogenes. It is a part of the active centers of some proteins in the form of the amino acid selenocysteine. The human body contains 10-14 mg of selenium, most of it is concentrated in the liver, kidneys, spleen, heart, testes and spermatic cord in men. Selenium is a part of muscle tissue proteins, myocardial proteins.

See also:

STRUCTURE

There are two modifications of selenium:
1. Crystalline (monoclinic selenium of a- and b-forms, hexagonal selenium of g-form).
2. Amorphous (powdery, colloidal and glassy forms of selenium).
The amorphous red selenium modification is one of the unstable modifications of the element. Powdered and colloidal forms of selenium are obtained by reducing the substance from a solution of selenous acid H 2 SeO 3.
Black vitreous selenium can be obtained by heating an element of any modification to a temperature of 220 degrees Celsius with rapid cooling. Hexagonal selenium is gray in color. This modification, the most stable thermodynamically, can also be obtained by heating to the melting temperature with further cooling to a temperature of 180-210 degrees Celsius. It is necessary to maintain such a temperature regime for some time.

PROPERTIES

The melting point of the substance is 217 (α-Se) and 170-180 degrees Celsius (β-Se), and it boils at a temperature of 685 0.

The oxidation states that selenium exhibits in reactions: (-2), (+2), (+4), (+6), it is resistant to air, oxygen, water, hydrochloric acid and dilute sulfuric acid.

It lends itself to dissolution in nitric acid of high concentration, "aqua regia", it dissolves for a longer time in an alkaline medium with oxidation. It is a diamagnet.

RESERVES AND PRODUCTION

Since selenium is mixed with sulfur, the element is extracted from ferrous sulfate. You don't even need to do anything special for this. The 34th metal is accumulated in the dust-cleaning chambers of sulfuric acid plants. Selenium is also taken from copper electrolysis units. After it, anode sludge remains. It is from it that the 34th element is isolated. It is enough to treat the sludge with sodium hydroxide and sulfur dioxide solutions. The resulting selenium must be purified. For this, the distillation method is used. After that, the metal is dried.

ORIGIN

Excellent glossy crystals from burning coal dumps. Image width - 1.8 mm. Germany, North Rhine-Westphalia, Aachen, Alsdorf, Anna mine

The selenium content in the earth's crust is about 500 mg / t. The main features of the geochemistry of selenium in the earth's crust are determined by the proximity of its ionic radius to the ionic radius of sulfur. Selenium forms 37 minerals, among which ashavalite FeSe, claustalite PbSe, thymannite HgSe, guanahuatite Bi 2 (Se, S) 3, chastite CoSe 2, platinum PbBi 2 (S, Se) 3, associated with various sulfides and sometimes also with cassiterite. Native selenium is rarely found. Sulfide deposits are of major industrial importance for selenium. The selenium content in sulfides ranges from 7 to 110 g / t. The selenium concentration in sea water is 4 · 10 -4 mg / l.

APPLICATION

One of the most important areas of its technology, production and consumption is the semiconducting properties of both selenium itself and its numerous compounds (selenides), their alloys with other elements, in which selenium began to play a key role. IN modern technology semiconductors are used selenides of many elements, for example, selenides of tin, lead, bismuth, antimony, lanthanide selenides. The photoelectric and thermoelectric properties of both selenium itself and selenides are especially important.
The stable isotope selenium-74 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.
Potassium selenide together with vanadium pentoxide is used in the thermochemical production of hydrogen and oxygen from water (selenium cycle). The semiconducting properties of selenium in its pure form were widely used in the mid-20th century for the manufacture of rectifiers, especially in military equipment for the following reasons: unlike germanium, silicon, selenium is insensitive to radiation, and, in addition, a selenium rectifier diode has a unique property of self-healing in case of breakdown: the place of breakdown evaporates and does not lead to a short circuit, the permissible diode current is slightly reduced, but the product remains functional. The disadvantages of selenium rectifiers include their significant dimensions.
Selenium is used as a powerful anti-cancer agent, as well as for the prevention of a wide range of diseases. According to studies, taking 200 mcg of selenium per day reduces the risk of cancer of the rectum and colon by 58%, prostate tumors by 63%, lung cancer by 46%, and reduces overall mortality from cancer by 39%.
Low concentrations of selenium suppress histamine and, due to this, have an anti-dystrophic effect and antiallergic effect. Selenium also stimulates tissue proliferation, improves the function of the sex glands, heart, thyroid gland, and the immune system.
In combination with iodine, selenium is used to treat iodine deficiency diseases and thyroid gland pathologies.
Selenium salts help restore low blood pressure in shock and collapse

Selenium - Se

CLASSIFICATION

Hey's CIM Ref1.53

Selenium was discovered in 1817 by Jens Jakob Berzelius. The story of Berzelius himself about how this discovery happened has survived: “I researched, in collaboration with Gottlieb Hahn, the method that is used to produce sulfuric acid in Gripsholm. We found a precipitate in sulfuric acid, part red, part light brown. ... Curiosity , prompted by the hope of discovering a new rare metal in this brown sediment, prompted me to investigate the sediment ... I found that the mass (that is, the sediment) contains a hitherto unknown metal, very similar in properties to tellurium. I named the new body selenium (Selenium) from Greek selhnh (moon), because tellurium is named after Tellus - our planet. "

Being in nature, getting:

The selenium content in the earth's crust is about 500 mg / t. Selenium forms 37 minerals, among which ashavalite FeSe, claustalite PbSe, thymannite HgSe, guanahuatite Bi 2 (Se, S) 3, hastite CoSe 2, platinum PbBi 2 (S, Se) 3 should be noted first. Native selenium is rarely found. Sulfide deposits are of major industrial importance for selenium. The selenium content in sulfides ranges from 7 to 110 g / t. Selenium concentration in seawater 4 * 10 -4 mg / l.
Selenium is obtained from the waste 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 variety of 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-the form). Amorphous (red) powdery 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 glassy luster and is fragile. Hexagonal (gray) selenium is the most stable thermodynamically. 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 of parallel spiral chains of atoms.

Chemical properties:

Selenium is resistant to oxygen, water and dilute acids at ambient temperatures. When heated, selenium interacts with all metals to form selenides. In oxygen, upon additional heating, it slowly burns with a blue flame, turning into dioxide SeO 2.
With halogens, with the exception of iodine, it reacts at room temperature with the formation of 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 reacts with selenium at t\u003e 200 ° C, giving H 2 Se.
At the end. 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. in alkali solutions, selenium disproportionates:
3Se + 3H 2 O \u003d 2H 2 Se + H 2 SeO 3 and 3Se + 6KOH \u003d K 2 SeO 3 + 2K 2 Se + 3H 2 O
forming compounds of selenium (-2) and selenium (+4).
Similarly, selenium dissolves on heating in Na 2 SO 3 or KCN solutions, forming, respectively, Na 2 SSeO 3 (thiosulfate analog) or KCNSe (thiosulfate analog).

The most important connections:

For selenium, the most typical oxidation states are -2, +4, +6.
Selenium (IV) oxide SeO 2 - white shiny crystals with a polymer molecule (SeOsub\u003e 2) sub\u003e n, tmelt. 350 ° C. The vapors are yellowish-green in color and have a rotten radish smell. Easily dissolves in water to form H 2 SeO 3.
Selenous acid, H 2 SeO 3 - white rhombic crystals with high hygroscopicity. Well soluble in water. Unstable, when heated above 70 ° C decomposes into water and selenium (IV) oxide. Salts are selenites.
Sodium selenite, Na 2 SeO 3 - colorless crystals, tpl. 711 ° C. It is hygroscopic, well soluble in water. When heated in an inert atmosphere, decomposes into oxides. When heated in air, it oxidizes to selenate: 2Na 2 SeO 3 + O 2 \u003d 2Na 2 SeO 4
Selenium (VI) oxide SeO 3 - - colorless crystals, tpl. 121 ° C. It is hygroscopic, reacts with water with great 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, readily soluble in water. Poisonous, hygroscopic, strong oxidizing agent. Selenic acid is one of the few compounds that dissolves gold when heated, forming a red-yellow solution of gold (III) selenate.
2Au + 6H 2 SeO 4 \u003d Au 2 (SeO 4) 3 + 3H 2 SeO 3 + 3H 2 O
Selenates - selenic acid salts. Sodium selenate Na 2 SeO 4 - crystals of rhombic system; tmelt. 730 ° C. It is 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 - colorless combustible gas with an unpleasant odor. The most toxic compound of selenium. It readily oxidizes in air at ambient temperatures to free selenium. It is also oxidized to free selenium with chlorine, bromine and iodine. When burning 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 luster. There are monoselenides of the composition M 2 Se, MSe; polyselenides M 2 Se n (except Li), where n \u003d 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 rectifier semiconductor diodes, as well as for photoelectric devices, electrophotographic copiers, as phosphors in television, optical and signaling devices, thermistors, etc. Selenium is widely used to discolor green glass and obtain ruby \u200b\u200bglasses; in metallurgy - to give steel a fine-grained structure, improve their mechanical properties; in chemical industry - as a catalyst.
The stable isotope selenium-74 made it possible, on its basis, to create a plasma laser with a 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 part of 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 influences many enzymatic reactions. The requirement of humans and animals for selenium does not exceed 50-100 μg / kg of the diet.

A.A. Polkovnikov
KhF Tyumen State University, 581 group. 2011 r.