Chromium, chemical element: description, properties, formula and characteristics

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

Various chromium compounds, such as chromium (III) oxide and lead chromate, are brightly colored and used in paints and pigments. The red color of the ruby is due to the presence of this chemical element. Some substances, especially potassium and sodium dichromate , are oxidants used to oxidize organic compounds and (together with sulfuric acid) to clean laboratory dishes. 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 in the Ural mountains an orange-red mineral and called it "Siberian red lead". Although it was mistakenly identified as a compound of lead with selenium and iron, the material was in fact lead chromate with the chemical formula PbCrO ₄ . Today it is known as a mineral crown.

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

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

In the 1800s Cr was mainly used in the composition of paints and tanning 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 multitude of color compounds that can be obtained from it.

Extraction and production

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

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

For the industrial production of metal, chromite ores are treated with molten alkali (caustic soda, NaOH). At the same time, 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 million tons of chromite ore was mined, which was processed into 4 million tons of ferrochromium, 70% of the chromium-iron alloy, with an approximate market value of $ 2.5 billion.

Main characteristics

Characterization of the chemical element of 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 rapidly forms a thin layer of oxide, which protects the metal from further interaction with oxygen.

As a transitional element, it reacts with substances in different ratios. Thus, it forms compounds in which it has different degrees of oxidation. Chromium is a chemical element with the ground states +2, +3 and +6, of which +3 is the most stable. In addition, in rare cases, states +1, +4 and +5 are observed. The chromium compounds in oxidation state +6 are strong oxidants.

What color is Chrome? The chemical element gives the anodized aluminum a ruby tint. Cr ₂ O ₃ , used for polishing the metal, is also used as a pigment called "chrome greens". Its salts color the glass in an emerald green color. Chromium is a chemical element whose presence makes the ruby red. Therefore, it is used in the production of synthetic rubies.

Isotopes

Chromium isotopes have atomic weight from 43 to 67. Usually, 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 total natural chromium). In addition, 19 radioisotopes have been described, of which the most stable is ⁵⁰ Cr with a half-life exceeding 1.8x10 ¹⁷ years. In ⁵¹ Cr, the half-life is 27.7 days, and for all other radioactive isotopes it does not exceed 24 hours, most of which last less than one minute. The element also has two meta-states.

The isotopes of chromium in the earth's crust, as a rule, accompany the isotopes of manganese, which finds application in geology. ⁵³ Cr is formed in the radioactive decay ⁵³ Mn. The ratio of Mn / Cr isotopes supports other information on the early history of the solar system. Changes in ⁵³ Cr / ⁵² Cr and Mn / Cr ratios 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 (III) oxide Cr ₂ O ₃ , also known as sesquioxide, is one of the four oxides of this chemical element. It is obtained from chromite. A green compound is commonly called "chrome greens" when used as a pigment for enamel and glass painting. Oxide can dissolve in acids, forming salts, and in molten alkali - chromites.

Potassium dichromate

K ₂ Cr ₂ O ₇ is a powerful oxidizer and is given preference as a means for cleaning laboratory utensils from organic materials. To do this, use its saturated solution in concentrated sulfuric acid. 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 the primary alcohol to aldehyde, and then to carbon dioxide.

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

Chromic acid

The compound has a hypothetical H ₂ CrO ₄ structure. 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 in fact its acid anhydride - trioxide CrO ₃ .

Lead chromium (II)

PbCrO ₄ has a bright yellow color and is practically insoluble in water. For this reason, he found application as a coloring pigment called "yellow crowns".

Cr and pentavalent bond

Chromium is distinguished by its ability to form pentavalent bonds. The compound is created by Cr (I) and the hydrocarbon radical. A five-valent 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 him with a variety of different applications, some of which are listed below.

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

Chromium is the catalyst for various reactions. From it, forms are made for burning bricks. His salts tan the skin. Potassium dichromate is used for the oxidation of organic compounds, such as alcohols and aldehydes, as well as for cleaning laboratory utensils. It serves as a fixing agent for dyeing fabrics, and is also used in photography and photo printing.

CrO ₃ is used for the manufacture of magnetic tapes (for example, for audio recording), which have better characteristics than films with iron oxide.

Role in Biology

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

Precautionary measures

Metallic chromium and Cr (III) compounds are generally not considered to be 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. When exposed to a constant exposure, chromium (VI) compounds can cause eye damage if left untreated. 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 for tanning of the skin, they are often found in soil and groundwater of abandoned industrial objects that require environmental cleaning and restoration. 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.
• Degree of oxidation: +2, +3, +6.
• The electron configuration: [Ar] 3d ⁵ 4s ¹ .