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Chromium - a general characteristic of the element, the chemical properties of chromium and its compounds. Chromium and its compounds Molar mass of chromium

  • Designation - Cr (Chromium);
  • Period - IV;
  • Group - 6 (VIb);
  • Atomic mass - 51.9961;
  • Atomic number - 24;
  • Radius of an atom = 130 pm;
  • Covalent radius = 118 pm;
  • Electron distribution - 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 ;
  • melting point = 1857°C;
  • boiling point = 2672°C;
  • Electronegativity (according to Pauling / according to Alpred and Rochov) = 1.66 / 1.56;
  • Oxidation state: +6, +3, +2, 0;
  • Density (n.a.) \u003d 7.19 g / cm 3;
  • Molar volume = 7.23 cm 3 / mol.

Chromium (color, paint) was first found at the Berezovsky gold deposit (Middle Urals), the first mentions date back to 1763, in his work "The First Foundations of Metallurgy" M.V. Lomonosov calls it "red lead ore".


Rice. The structure of the chromium atom.

The electronic configuration of the chromium atom is 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 (see Electronic structure of atoms). In the formation of chemical bonds with other elements, 1 electron located at the outer 4s level + 5 electrons of the 3d sublevel (6 electrons in total) can participate, therefore, in compounds, chromium can take oxidation states from +6 to +1 (the most common are +6 , +3, +2). Chromium is a chemically inactive metal, it reacts with simple substances only at high temperatures.

Physical properties of chromium:

  • bluish-white metal;
  • very hard metal (in the presence of impurities);
  • fragile at n. y.;
  • plastic (in its pure form).

Chemical properties of chromium

  • at t=300°C it reacts with oxygen:
    4Cr + 3O 2 \u003d 2Cr 2 O 3;
  • at t>300°C it reacts with halogens, forming mixtures of halides;
  • at t>400°C it reacts with sulfur to form sulfides:
    Cr + S = CrS;
  • at t=1000°C, finely ground chromium reacts with nitrogen to form chromium nitride (a semiconductor with high chemical resistance):
    2Cr + N 2 = 2CrN;
  • reacts with dilute hydrochloric and sulfuric acids to release hydrogen:
    Cr + 2HCl \u003d CrCl 2 + H 2;
    Cr + H 2 SO 4 \u003d CrSO 4 + H 2;
  • warm concentrated nitric and sulfuric acids dissolve chromium.

With concentrated sulfuric and nitric acid at n.o. chromium does not interact, chromium also does not dissolve in aqua regia, it is noteworthy that pure chromium does not react even with dilute sulfuric acid, the reason for this phenomenon has not yet been established. During long-term storage in concentrated nitric acid, chromium is covered with a very dense oxide film (passivated), and ceases to react with dilute acids.

Chromium compounds

It has already been said above that the "favorite" oxidation states of chromium are +2 (CrO, Cr (OH) 2), +3 (Cr 2 O 3, Cr (OH) 3), +6 (CrO 3, H 2 CrO 4 ).

Chrome is chromophore, i.e., an element that gives color to the substance in which it is contained. For example, in the +3 oxidation state, chromium gives a lilac-red or green color (ruby, spinel, emerald, garnet); in oxidation state +6 - yellow-orange color (crocoite).

Chromophores, in addition to chromium, are also iron, nickel, titanium, vanadium, manganese, cobalt, copper - all these are d-elements.

The color of common compounds that include chromium:

  • chromium in oxidation state +2:
    • chromium oxide CrO - red;
    • chromium fluoride CrF 2 - blue-green;
    • chromium chloride CrCl 2 - has no color;
    • chromium bromide CrBr 2 - has no color;
    • chromium iodide CrI 2 - red-brown.
  • chromium in oxidation state +3:
    • Cr 2 O 3 - green;
    • CrF 3 - light green;
    • CrCl 3 - violet-red;
    • CrBr 3 - dark green;
    • CrI 3 - black.
  • chromium in oxidation state +6:
    • CrO 3 - red;
    • potassium chromate K 2 CrO 4 - lemon yellow;
    • ammonium chromate (NH 4) 2 CrO 4 - golden yellow;
    • calcium chromate CaCrO 4 - yellow;
    • lead chromate PbCrO 4 - light brown-yellow.

Chromium oxides:

  • Cr +2 O - basic oxide;
  • Cr 2 +3 O 3 - amphoteric oxide;
  • Cr +6 O 3 - acid oxide.

Chromium hydroxides:

  • ".

    Application of chromium

    • as a alloying additive in the smelting of heat-resistant and corrosion-resistant alloys;
    • for chrome plating of metal products in order to give them high corrosion resistance, abrasion resistance and a beautiful appearance;
    • chromium-30 and chromium-90 alloys are used in plasma torch nozzles and in the aviation industry.

Chromium, - a chemical element, a solid silvery metal with atomic number 24. For the bright colors characteristic of salts, chromium received the name - χρώμα (Greek color, paint).

Biological action

Chromium plays a leading role in glucose metabolism:

  • it is necessary for the processing of glucose (it is an active component of GTF - glucose tolerance factor);
  • improves insulin sensitivity;
  • reduces cravings for sweets;
  • lowers blood sugar levels in people with type 1 and type 2 diabetes;
  • is a catalyst for the synthesis of certain proteins, necessary for muscle growth;
  • participating in the metabolism of fats, regulates the level of "bad" cholesterol in the blood;
  • promotes weight loss.

Table 1. Physiological requirements for chromium depending on age

Higher doses of chromium are needed for increased metabolism, such as in athletes.

Chromium Sources

Yeast, liver, meat, brown rice, whole grains, corn, eggs, tomatoes, oatmeal, iceberg lettuce, mushrooms, cheese. These foods are the richest in chromium (in descending order), but it should be borne in mind that it is contained in microdoses and the average diet barely reaches the minimum requirements for this mineral.

Absorption of chromium reduces iron levels.

Chromium deficiency

Chromium deficiency is observed when eating foods depleted of this trace element, and the level of chromium in the body decreases in old age.

It should also be taken into account that the absorption of chromium in the intestine is low, even from modern complexes with chromium, where it is in the most assimilable form (chromium picolinate, an amino acid complex with chromium), absorption is 1.5-3%.

Chromium deficiency results to decrease glucose tolerance, decrease in growth rate, increased risk of developing diabetes mellitus, coronary heart disease, hypercholesterolemia (increased blood cholesterol levels), hyperglycemia and hypoglycemia (changes in sugar levels).

Most suitable for deficiency elimination chromium picolinate, at the same time, it is necessary to reduce the amount of light carbohydrates consumed (sugar, etc.). Chromium chloride (CrCl2) is practically useless for this purpose, due to the very low absorption of chromium from this form.

Long-term use of chromium-containing drugs for prophylactic purposes (in the absence of deficiency) increases the load on the body, which is fraught with activation of mutagenesis.

Excess chromium in the body

An excess of chromium among Russians is a fairly common phenomenon, but it is caused by hexavalent chromium, a known carcinogen used in the metallurgical and textile industries. Hexavalent chromium compounds cause allergic reactions (dermatitis), increase the risk of lung cancer.

Chromium in food has a trivalent form, which is safe for the body.

mineral-details

Trivalent chromium is part of alum, which has long been used in dressing hides, dyeing fabrics, in our time, alum has been used as a cauterizing agent "alum pencil", as a deodorant-antiperspirant, is part of cosmetics, etc.

Getting a normal amount of chromium (in accordance with physiological needs) by dieters allows you to reduce "fat deposits" while maintaining muscle.

History of chromium

The first mention of chromium as an independent element is found in the works of M.V. Lomonosov in 1763, after the metal was discovered at the Berezovsky gold ore deposit. The author called him red lead ore. Chromium compounds have a variety of colors, apparently, therefore, the elements were given the name chromium - from the Greek χρῶμα - paint, color.

Chromium is an element of a side subgroup of the VI group of the IV period in the periodic system of chemical elements of D.I. Mendeleev, has an atomic number of 24 and an atomic mass of 51.966. The accepted designation is Cr (from the Latin Chromium).

Being in nature

Chromium is common in the earth's crust, the most famous compounds are chromite and crocoite. Chromium deposits are located in South Africa, Turkey, Zimbabwe, Armenia, India and in the Middle Urals in Russia.

Chromium is a hard metal (often called black metal), has a white-blue color and one of the highest hardness.

daily requirement for chromium

The required daily dosage in chromium for children ranges from 11 to 35 micrograms depending on age, for women it is necessary to receive 50-70 micrograms of chromium per day, during pregnancy the need rises to 100-120 micrograms. Adult healthy men should receive 60-80 micrograms of chromium per day, with active sports or other physical activity, the daily dosage is 120-200 micrograms.

The main suppliers of chromium to the human body are and, followed in uniform, and, and, wholemeal bread, there is chromium in seafood, cheese, and, fruits and berries, legumes and some cereals - and.

Signs of a Chromium Deficiency

Signs of a lack of chromium in the human body are:

  • insomnia and fatigue,
  • headache and anxiety
  • an increase in the level of "bad" cholesterol,
  • trembling and decreased sensation in the extremities,
  • wasting and hair loss.

Signs of excess chromium

An excess of chromium in the body is characterized by allergic reactions and inflammatory processes, sores on the mucous membranes, nervous disorders and disturbances in the activity of the liver and kidneys.

Chromium plays an important role in human life, takes part in lipid and carbon metabolism, promotes the removal of "bad" cholesterol and is responsible for the processing of body fat, thereby maintaining normal weight. The ability of chromium to replace iodine plays an important role for the thyroid gland, and chromium is also indispensable for the prevention of osteoporosis, strengthening bone tissue. Chromium stimulates the processes of tissue regeneration - it preserves hereditary information in the genes.

Chromium has found its main application in the metallurgical industry, where it is used to increase the hardness and corrosion resistance of alloys, in the process of chromium plating, and is also used in the aerospace industry.

Chromium is a chemical element with atomic number 24. It is a hard, shiny, steel-gray metal that polishes well and does not tarnish. Used in alloys such as stainless steel and as a coating. The human body requires small amounts of trivalent chromium to metabolize sugar, but Cr(VI) is highly toxic.

Various chromium compounds, such as chromium(III) oxide and lead chromate, are brightly colored and are used in paints and pigments. The red color of a ruby ​​is due to the presence of this chemical element. Some substances, especially sodium, are oxidizing agents used to oxidize organic compounds and (along with sulfuric acid) to clean laboratory glassware. In addition, chromium oxide (VI) is used in the production of magnetic tape.

Discovery and etymology

The history of the discovery of the chemical element chromium is as follows. In 1761, Johann Gottlob Lehmann found an orange-red mineral in the Ural Mountains and named it "Siberian red lead". Although it was erroneously identified as a compound of lead with selenium and iron, the material was actually lead chromate with the chemical formula PbCrO 4 . Today it is known as the croconte mineral.

In 1770, Peter Simon Pallas visited the place where Leman found a red lead mineral that had very useful pigment properties in paints. The use of Siberian red lead as a paint developed rapidly. In addition, bright yellow from croconte has become fashionable.

In 1797, Nicolas-Louis Vauquelin obtained samples of red By mixing croconte with hydrochloric acid, he obtained the oxide CrO 3 . Chromium as a chemical element was isolated in 1798. Vauquelin obtained it by heating oxide with charcoal. He was also able to detect traces of chromium in gemstones such as ruby ​​and emerald.

In the 1800s, Cr was mainly used in paints and leather salts. Today, 85% of the metal is used in alloys. The rest is used in the chemical industry, the production of refractory materials and the foundry industry.

The pronunciation of the chemical element chromium corresponds to the Greek χρῶμα, which means "color", because of the many colored compounds that can be obtained from it.

Mining and production

The element is made from chromite (FeCr 2 O 4). Approximately half of this ore in the world is mined in South Africa. In addition, Kazakhstan, India and Turkey are its major producers. There are enough explored deposits of chromite, but geographically they are concentrated in Kazakhstan and southern Africa.

Deposits of native chromium metal are rare, but they do exist. For example, it is mined at the Udachnaya mine in Russia. It is rich in diamonds, and the reducing environment helped form pure chromium and diamonds.

For the industrial production of metal, chromite ores are treated with molten alkali (caustic soda, NaOH). In this case, sodium chromate (Na 2 CrO 4) is formed, which is reduced by carbon to Cr 2 O 3 oxide. The metal is obtained by heating the oxide in the presence of aluminum or silicon.

In 2000, about 15 Mt of chromite ore was mined and processed into 4 Mt of ferrochromium, 70% chromium-iron, with an estimated market value of US$2.5 billion.

Main characteristics

The characteristic of the chemical element chromium is due to the fact that it is a transition metal of the fourth period of the periodic table and is located between vanadium and manganese. Included in the VI group. It melts at a temperature of 1907 °C. In the presence of oxygen, chromium quickly forms a thin layer of oxide, which protects the metal from further interaction with oxygen.

As a transition element, it reacts with substances in various proportions. Thus, it forms compounds in which it has various oxidation states. Chromium is a chemical element with ground states +2, +3 and +6, of which +3 is the most stable. In addition, states +1, +4 and +5 are observed in rare cases. Chromium compounds in the +6 oxidation state are strong oxidizing agents.

What color is chrome? The chemical element imparts a ruby ​​hue. The Cr 2 O 3 used for is also used as a pigment called "chrome green". Its salts color glass in an emerald green color. Chromium is a chemical element whose presence makes a ruby ​​red. Therefore, it is used in the production of synthetic rubies.

isotopes

Isotopes of chromium have atomic weights from 43 to 67. Typically, this chemical element consists of three stable forms: 52 Cr, 53 Cr and 54 Cr. Of these, 52 Cr is the most common (83.8% of all natural chromium). In addition, 19 radioisotopes have been described, of which 50 Cr is the most stable, with a half-life exceeding 1.8 x 10 17 years. 51 Cr has a half-life of 27.7 days, and for all other radioactive isotopes it does not exceed 24 hours, and for most of them it lasts less than one minute. The element also has two metastates.

Chromium isotopes in the earth's crust, as a rule, accompany manganese isotopes, which finds application in geology. 53 Cr is formed during the radioactive decay of 53 Mn. The Mn/Cr isotope ratio reinforces other information about the early history of the solar system. Changes in the ratios of 53 Cr/ 52 Cr and Mn/Cr from different meteorites prove that new atomic nuclei were created just before the formation of the solar system.

Chemical element chromium: properties, formula of compounds

Chromium oxide (III) Cr 2 O 3, also known as sesquioxide, is one of the four oxides of this chemical element. It is obtained from chromite. The green compound is commonly referred to as "chrome green" when used as a pigment for enamel and glass painting. The oxide can dissolve in acids, forming salts, and in molten alkali, chromites.

Potassium bichromate

K 2 Cr 2 O 7 is a powerful oxidizing agent and is preferred as a cleaning agent for laboratory glassware from organic matter. For this, its saturated solution is used. Sometimes, however, it is replaced with sodium dichromate, based on the higher solubility of the latter. In addition, it can regulate the process of oxidation of organic compounds, converting primary alcohol into aldehyde, and then into carbon dioxide.

Potassium dichromate can cause chromium dermatitis. Chromium is probably the cause of the sensitization leading to the development of dermatitis, especially of the hands and forearms, which is chronic and difficult to treat. Like other Cr(VI) compounds, potassium bichromate is carcinogenic. It must be handled with gloves and appropriate protective equipment.

Chromic acid

The compound has the hypothetical structure H 2 CrO 4 . Neither chromic nor dichromic acids are found in nature, but their anions are found in various substances. "Chromic acid", which can be found on sale, is actually its acid anhydride - CrO 3 trioxide.

Lead(II) chromate

PbCrO 4 has a bright yellow color and is practically insoluble in water. For this reason, it has found application as a coloring pigment under the name "yellow crown".

Cr and pentavalent bond

Chromium is distinguished by its ability to form pentavalent bonds. The compound is created by Cr(I) and a hydrocarbon radical. A pentavalent bond is formed between two chromium atoms. Its formula can be written as Ar-Cr-Cr-Ar where Ar is a specific aromatic group.

Application

Chromium is a chemical element whose properties have provided it with many different uses, some of which are listed below.

It gives metals resistance to corrosion and a glossy surface. Therefore, chromium is included in alloys such as stainless steel, used in cutlery, for example. It is also used for chrome plating.

Chromium is a catalyst for various reactions. It is used to make molds for firing bricks. Its salts tan the skin. Potassium bichromate is used to oxidize organic compounds such as alcohols and aldehydes, as well as to clean laboratory glassware. It serves as a fixing agent for dyeing fabric and is also used in photography and photo printing.

CrO 3 is used to make magnetic tapes (for example, for audio recording), which have better characteristics than iron oxide films.

Role in biology

Trivalent chromium is a chemical element essential for the metabolism of sugar in the human body. In contrast, hexavalent Cr is highly toxic.

Precautionary measures

Chromium metal and Cr(III) compounds are not generally considered hazardous to health, but substances containing Cr(VI) can be toxic if ingested or inhaled. Most of these substances are irritating to the eyes, skin and mucous membranes. With chronic exposure, chromium(VI) compounds can cause eye damage if not properly treated. In addition, it is a recognized carcinogen. The lethal dose of this chemical element is about half a teaspoon. According to the recommendations of the World Health Organization, the maximum allowable concentration of Cr (VI) in drinking water is 0.05 mg per liter.

Since chromium compounds are used in dyes and leather tanning, they are often found in the soil and groundwater of abandoned industrial sites that require environmental cleanup and remediation. Primer containing Cr(VI) is still widely used in the aerospace and automotive industries.

Element Properties

The main physical properties of chromium are as follows:

  • Atomic number: 24.
  • Atomic weight: 51.996.
  • Melting point: 1890 °C.
  • Boiling point: 2482 °C.
  • Oxidation state: +2, +3, +6.
  • Electron configuration: 3d 5 4s 1 .

A hard bluish-white metal. Chromium is sometimes referred to as a ferrous metal. This metal is capable of painting compounds in different colors, which is why it was called "chromium", which means "paint". Chromium is a microelement necessary for the normal development and functioning of the human body. Its most important biological role is in the regulation of carbohydrate metabolism and blood glucose levels.

See also:

STRUCTURE

Depending on the types of chemical bond - like all metals, chromium has a metallic type of crystal lattice, that is, there are metal atoms at the lattice nodes.
Depending on the spatial symmetry - cubic, body-centered a = 0.28839 nm. A feature of chromium is a sharp change in its physical properties at a temperature of about 37°C. The crystal lattice of a metal consists of its ions and mobile electrons. Similarly, the chromium atom in the ground state has an electronic configuration. At 1830°C, transformation into a modification with a face-centered lattice is possible, a = 3.69Å.

PROPERTIES

Chromium has a Mohs hardness of 9, one of the hardest pure metals (second only to iridium, beryllium, tungsten and uranium). Very pure chrome can be machined fairly well. Stable in air due to passivation. For the same reason, it does not react with sulfuric and nitric acids. At 2000 °C, it burns out with the formation of green chromium (III) oxide Cr 2 O 3, which has amphoteric properties. When heated, it reacts with many non-metals, often forming compounds of non-stoichiometric composition - carbides, borides, silicides, nitrides, etc. Chromium forms numerous compounds in various oxidation states, mainly +2, +3, +6. Chromium has all the properties characteristic of metals - it conducts heat and electric current well, and has the brilliance inherent in most metals. It is an antiferromagnet and a paramagnet, that is, at a temperature of 39 ° C it changes from a paramagnetic state to an antiferromagnetic state (Néel point).

RESERVES AND PRODUCTION

The largest deposits of chromium are in South Africa (1st place in the world), Kazakhstan, Russia, Zimbabwe, Madagascar. There are also deposits in Turkey, India, Armenia, Brazil, and the Philippines. The main deposits of chromium ores in the Russian Federation are known in the Urals (Donskoye and Saranovskoye). Explored reserves in Kazakhstan are over 350 million tons (2nd place in the world).Chromium occurs in nature mainly in the form of chromium iron ore Fe(CrO 2) 2 (iron chromite). Ferrochromium is obtained from it by reduction in electric furnaces with coke (carbon). To obtain pure chromium, the reaction is carried out as follows:
1) iron chromite is fused with sodium carbonate (soda ash) in air;
2) dissolve sodium chromate and separate it from iron oxide;
3) convert chromate to dichromate by acidifying the solution and crystallizing the dichromate;
4) pure chromium oxide is obtained by reduction of sodium dichromate with charcoal;
5) with the help of aluminothermy, metallic chromium is obtained;
6) using electrolysis, electrolytic chromium is obtained from a solution of chromic anhydride in water containing the addition of sulfuric acid.

ORIGIN

The average content of Chromium in the earth's crust (clarke) is 8.3·10 -3%. This element is probably more characteristic of the Earth's mantle, since ultramafic rocks, which are believed to be closest in composition to the Earth's mantle, are enriched in Chromium (2·10 -4%). Chromium forms massive and disseminated ores in ultramafic rocks; the formation of the largest deposits of Chromium is associated with them. In basic rocks, the content of Chromium reaches only 2 10 -2%, in acidic rocks - 2.5 10 -3%, in sedimentary rocks (sandstones) - 3.5 10 -3%, shale - 9 10 -3 %. Chromium is a relatively weak water migrant; Chromium content in sea water is 0.00005 mg/l.
In general, Chromium is the metal of the deep zones of the Earth; stony meteorites (analogues of the mantle) are also enriched in Chromium (2.7·10 -1%). Over 20 chromium minerals are known. Only chrome spinels (up to 54% Cr) are of industrial importance; in addition, chromium is contained in a number of other minerals that often accompany chromium ores, but are of no practical value in themselves (uvarovite, volkonskoite, kemerite, fuchsite).
There are three main chromium minerals: magnochromite (Mg, Fe)Cr 2 O 4 , chrompicotite (Mg, Fe) (Cr, Al) 2 O 4 and aluminochromite (Fe, Mg) (Cr, Al) 2 O 4 . They are indistinguishable in appearance and are inaccurately referred to as "chromites".

APPLICATION

Chromium is an important component in many alloyed steels (in particular, stainless steels), as well as in a number of other alloys. The addition of chromium significantly increases the hardness and corrosion resistance of the alloys. The use of Chromium is based on its heat resistance, hardness and corrosion resistance. Most of all Chromium is used for smelting chromium steels. Alumino- and silicothermic chromium is used for smelting nichrome, nimonic, other nickel alloys, and stellite.
A significant amount of Chromium is used for decorative corrosion-resistant coatings. Chromium powder has been widely used in the production of metal-ceramic products and materials for welding electrodes. Chromium, in the form of the Cr 3+ ion, is an impurity in ruby, which is used as a gemstone and laser material. Chromium compounds are used to etch fabrics during dyeing. Some Chromium salts are used as an ingredient in tanning solutions in the leather industry; PbCrO 4 , ZnCrO 4 , SrCrO 4 - as art paints. Chromite-magnesite refractory products are made from a mixture of chromite and magnesite.
It is used as wear-resistant and beautiful electroplating (chrome plating).
Chromium is used for the production of alloys: chromium-30 and chromium-90, indispensable for the production of nozzles of powerful plasma torches and in the aerospace industry.

Chromium - Cr