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 for sugar metabolism, 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 ruby is due to the presence of this chemical element. Some substances, especially potassium and sodium dichromate , are oxidizing agents used to oxidize organic compounds and (together with sulfuric acid) to clean laboratory glassware. In addition, chromium oxide (VI) is used in the manufacture of magnetic tape.
Discovery and etymology
The history of the discovery of the chemical element chromium is as follows. In 1761, Johann Gotlob Lehman found an orange-red mineral in the Ural Mountains and called it "Siberian red lead." Although it was mistakenly 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 Crococone mineral.
In 1770, Peter Simon Pallas visited the place where Lehman found a red lead mineral, which had very useful pigment properties in paints. The use of Siberian red lead as a paint has developed rapidly. In addition, the bright yellow color from Crocont has become fashionable.
In 1797, Nicolas-Louis Vauclin received samples of red lead ore. By mixing Crocont with hydrochloric acid, he obtained oxide CrO 3 . Chrome as a chemical element was isolated in 1798. Vauclin received it by heating oxide with charcoal. He was also able to detect traces of chromium in gems such as ruby and emerald.
In the 1800s, Cr was mainly used in paints and leather salts. Today, 85% of metal is used in alloys. The remainder is used in the chemical industry, the production of refractory materials, and the foundry industry.
The pronunciation of the chemical element chrome corresponds to the Greek χρῶμα, which means “color”, due to the many colored compounds that can be obtained from it.
Mining and production
The element is made from chromite (FeCr 2 O 4 ). About half of the world's ore is mined in South Africa. In addition, Kazakhstan, India and Turkey are its major producers. Explored chromite deposits are sufficient, but geographically they are concentrated in Kazakhstan and southern Africa.
Deposits of native metallic chromium are rare, but they are. For example, it is mined at the Udachnaya mine in Russia. It is a diamond-rich kimberlite pipe, and a reducing medium has helped form pure chromium and diamonds.
For industrial metal production, chromite ores are treated with molten alkali (sodium hydroxide, NaOH). In this case, sodium chromate (Na 2 CrO 4 ) is formed, which is reduced by carbon to the oxide Cr 2 O 3 . 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 ferrochrome, 70% consisting of an alloy of chromium with iron, with an approximate market value of $ 2.5 billion.
Main characteristics
The characteristic 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 group VI. It melts at a temperature of 1907 ° C. In the presence of oxygen, chromium quickly forms a thin oxide layer that 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 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. Chromium compounds in oxidation state +6 are strong oxidizing agents.
What color is chrome? The chemical element gives the anodized aluminum a ruby hue. Cr 2 O 3 used to polish the metal is also used as a pigment called “chrome green”. Its salts stain the glass in emerald green. Chromium is a chemical element whose presence makes ruby red. Therefore, it is used in the production of synthetic rubies.
Isotopes
Chromium isotopes have an atomic weight of 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 abundant (83.8% of all natural chromium). In addition, 19 radioisotopes are described, of which 50 Cr is the most stable with a half-life exceeding 1.8x10 17 years. At 51 Cr, the half-life is 27.7 days, and for all other radioactive isotopes it does not exceed 24 hours, with most of them lasting less than one minute. An element also has two meta states.
Chromium isotopes in the earth's crust, as a rule, accompany manganese isotopes, which is used 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 immediately before the formation of the solar system.
Chemical element chrome: 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. The green compound is usually called “chrome green” when used as a pigment for painting on enamel and glass. The oxide can dissolve in acids to form salts, and chromites in molten alkali.
Potassium dichromate
K 2 Cr 2 O 7 is a powerful oxidizing agent and is preferred as an agent for cleaning laboratory glassware from organics. For this, its saturated solution in concentrated sulfuric acid is used. Sometimes, however, it is replaced with sodium dichromate, based on the higher solubility of the latter. In addition, it can regulate the oxidation of organic compounds by converting the primary alcohol to aldehyde and then to carbon dioxide.
Potassium bichromate can cause chromic dermatitis. Chromium is probably the cause of sensitization, leading to the development of dermatitis, especially of the hands and arms, which is chronic and difficult to treat. Like other Cr (VI) compounds, potassium dichromate is carcinogenic. It must be handled with gloves and appropriate protective equipment.
Chromic acid
The compound has a hypothetical structure of H 2 CrO 4 . Neither chromic nor dichromic acids are found in nature, but their anions are found in various substances. The "chromic acid" that can be found on sale is actually its acid anhydride, CrO 3 trioxide.
Chromate Lead (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 crown."
Cr and pentavalent bond
Chrome 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 represents a specific aromatic group.
Application
Chromium is a chemical element whose properties have provided it with many different applications, some of which are given below.
To metals it gives resistance to corrosion and a glossy surface. Therefore, chromium is part of alloys such as stainless steel, used, for example, in cutlery. It is also used for chrome plating.
Chromium is a catalyst for various reactions. Brick making forms are made of it. Its salts tan skin. Potassium dichromate is used for the oxidation of organic compounds such as alcohols and aldehydes, as well as for cleaning laboratory glassware. 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 hazardous to health, but substances containing Cr (VI) can be toxic if taken orally or inhaled. Most of these substances are irritating to the eyes, skin and mucous membranes. When exposed to chromium (VI) compounds, they can cause eye damage if not treated properly. 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 permissible concentration of Cr (VI) in drinking water is 0.05 mg per liter.
Since chromium compounds are used in dyes and for tanning leather, they are often found in the soil and groundwater of abandoned industrial facilities that require environmental cleaning and restoration. A primer containing Cr (VI) is still widely used in the aerospace and automotive industries.
Item Properties
The basic physical properties of chromium are as follows:
- Atomic number: 24.
- Atomic weight: 51,996.
- Melting point: 1890 ° C.
- Boiling point: 2482 ° C.
- The degree of oxidation: +2, +3, +6.
- Electron Configuration: [Ar] 3d 5 4s 1 .