Chromium, atomic properties, chemical and physical properties

CHROME
– (Chromium) Cr, chemical element 6(VIb) of group of the Periodic table.
Atomic number 24, atomic mass 51.996. There are 24 known isotopes of chromium from 42Cr to 66Cr. The isotopes 52Cr, 53Cr, 54Cr are stable. Isotopic composition of natural chromium: 50Cr (half-life 1.8 1017 years) – 4.345%, 52Cr – 83.489%, 53Cr – 9.501%, 54Cr – 2.365%. The main oxidation states are +3 and +6. Also on topic:
CHEMISTRY

In 1761, chemistry professor at St. Petersburg University Johann Gottlob Lehmann, at the eastern foot of the Ural Mountains at the Berezovsky mine, discovered a wonderful red mineral, which, when crushed into powder, gave a bright yellow color. In 1766 Lehman brought samples of the mineral to St. Petersburg. Having treated the crystals with hydrochloric acid, he obtained a white precipitate, in which he discovered lead. Lehman called the mineral Siberian red lead (plomb rouge de Sibérie); it is now known that it was crocoite (from the Greek “krokos” - saffron) - a natural lead chromate PbCrO4.

The German traveler and naturalist Peter Simon Pallas (1741–1811) led an expedition of the St. Petersburg Academy of Sciences to the central regions of Russia and in 1770 visited the Southern and Middle Urals, including the Berezovsky mine and, like Lehmann, became interested in crocoite. Pallas wrote: “This amazing red lead mineral is not found in any other deposit. When ground into powder it turns yellow and can be used in artistic miniatures.” Despite the rarity and difficulty of delivering crocoite from the Berezovsky mine to Europe (it took almost two years), the use of the mineral as a coloring agent was appreciated. In London and Paris at the end of the 17th century. all noble persons rode in carriages painted with finely ground crocoite; in addition, the best examples of Siberian red lead replenished the collections of many mineralogical cabinets in Europe.

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CHROMATS AND DICHROMATS

In 1796, a sample of crocoite came to the professor of chemistry at the Paris Mineralogical School, Nicolas-Louis Vauquelin (1763–1829), who analyzed the mineral, but found nothing in it except oxides of lead, iron and aluminum. Continuing his research on Siberian red lead, Vaukelin boiled the mineral with a solution of potash and, after separating the white precipitate of lead carbonate, obtained a yellow solution of an unknown salt. When treated with lead salt, a yellow precipitate was formed, with mercury salt, a red one, and when tin chloride was added, the solution became green. By decomposing crocoite with mineral acids, he obtained a solution of “red lead acid,” the evaporation of which gave ruby-red crystals (it is now clear that it was chromic anhydride). Having calcined them with coal in a graphite crucible, after the reaction I discovered many fused gray needle-shaped crystals of a metal unknown to that time. Vaukelin noted the high refractoriness of the metal and its resistance to acids.

Vaukelin named the new element chromium (from the Greek crwma - color, color) due to the many multi-colored compounds it forms. Based on his research, Vauquelin was the first to state that the emerald color of some precious stones is explained by the admixture of chromium compounds in them. For example, natural emerald is a deep green colored beryl in which aluminum is partially replaced by chromium.

Most likely, Vauquelin obtained not pure metal, but its carbides, as evidenced by the needle-shaped shape of the resulting crystals, but the Paris Academy of Sciences nevertheless registered the discovery of a new element, and now Vauquelin is rightly considered the discoverer of element No. 24.

In 1798, Lowitz and Klaproth, independently of Vaukelin, discovered chromium in a sample of a heavy black mineral (it was chromite FeCr2O4), found in the Urals, but much north of the Berezovsky deposit. In 1799, F. Tassaert discovered a new element in the same mineral found in southeastern France. It is believed that it was Tassert who first managed to obtain relatively pure metal chromium.

What is

Chromium is a metal, element of the periodic table No. 24.

International designation and formula – Chromium, Cr.

The silvery-bluish shiny substance is one of the hardest (5.5 Mohs) and refractory metals, but is brittle. Refers to ferrous metals.

Its structure and properties are similar to iron, manganese, titanium, and nickel. These elements are combined into one family.

A unique feature of the element is the bright rainbow color of the compounds: blue, violet, green to emerald, yellow, orange, purple. They determined the name, thanks to them it is easy to distinguish chrome from other metals.

In ancient Greek χρῶμα (chroma) - paint, color.

History of the discovery of Chrome Chromium

Discovery of the Chromium element in the Middle Urals, in the Berezovsky gold deposit. It was first mentioned in the work of M.V. Lomonosov “The First Foundations of Metallurgy” (1763) as red lead ore, PbCrO4. The modern name is crocoite. In 1797, the French chemist L.N. Vauquelin isolated a new refractory metal from it (most likely, Vauquelin obtained chromium carbide). He calcined the green oxide Cr2O3 with coal and isolated a refractory metal (with an admixture of carbides). Vauquelin obtained the oxide Cr2O3 by decomposing “Siberian red lead” - the crocoite mineral PbCrO4.

The modern method of obtaining pure chromium (since 1894) differs from the Vauquelin method only in the type of reducing agent. The process of electrolytic coating of iron with chromium was developed in the 20s of the twentieth century.

Physico-chemical characteristics

The physical and chemical properties of chromium are typical of metals:

• Chemically inactive. Under normal conditions it does not interact with water or alkali solutions. The reaction starts at +600°C.
• Oxygen creates a protective oxide film on its surface.
• In compounds it exhibits three degrees: +2, +3, +6. The most stable are trivalent.

The use of chromium is hampered by disadvantages:

• Obvious deterioration of characteristics due to impurities in the composition.
• The need for additional processing of superhard metal to obtain plasticity.

However, they are compensated by the advantages of the metal: refractoriness, hardness (the fifth among metals), and resistance to corrosion.

Chromium crystal lattice:

Being in nature

Two dozen chromium minerals have been identified in nature. The main ones are chromite and crocoite.

Igneous rocks contain different concentrations of the element (g/t):

• ultrabasic – 2000;
• basalts, other basic ones – 200.

Each ton of the earth's crust contains on average 83 g of chromium.

One class of industrial interest is chrome spinels.

The metal contains precious stones - chrome tourmaline, uvarovite (chrome garnet), and others.

Heavy metals

What are heavy metals

Pollution of the biosphere by a group of pollutants, collectively called “heavy metals,” . These include more than 40 chemical elements of D.I. Mendeleev’s periodic table.

Heavy metals are chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, germanium, molybdenum, cadmium, tin, antimony, tellurium, tungsten, mercury, thallium, lead, bismuth . The sometimes used term “toxic elements” is unfortunate here, since any elements and their compounds can become toxic to living organisms at a certain concentration and environmental conditions.

The main natural sources of heavy metals are rocks (igneous and sedimentary) and rock-forming minerals. Many minerals in the form of highly dispersed particles are included as accessory (microimpurities) in the mass of rocks. Examples of such minerals are titanium minerals (brucite, ilmenite, anatase), chromium (FeCr2O4). Many elements enter the atmosphere with cosmic and meteorite dust, volcanic gases, hot springs, and gas jets.

The entry of heavy metals into the biosphere due to technogenic dispersion occurs in a variety of ways. The most important of them is emissions during high-temperature processes in ferrous and non-ferrous metallurgy, during the firing of cement raw materials, and the combustion of mineral fuels. In addition, irrigation with water with a high content of heavy metals and the introduction of household wastewater sludge into soils as fertilizer can serve as sources of pollution of biocenoses. Secondary pollution also occurs due to the removal of heavy metals from the dumps of mines or metallurgical enterprises by water or air currents, the entry of large quantities of heavy metals with the constant application of high doses of organic, mineral fertilizers and pesticides containing heavy metals.

Some of the man-made emissions of heavy metals entering the atmosphere in the form of aerosols are transported over a considerable distance and cause global pollution. The other part with hydrochemical runoff ends up in drainless reservoirs, where it accumulates in waters and bottom sediments and can become a source of secondary pollution. Heavy metal compounds spread relatively quickly throughout the volume of a water body. They partially precipitate in the form of carbonates and sulfates, and are partially adsorbed on mineral and organic sediments. As a result, the content of heavy metals in sediments is constantly increasing, and when the absorption capacity of sediments is exhausted and heavy metals enter the water, a particularly tense situation arises. This is facilitated by increased water acidity, severe overgrowth of water bodies, and intensified CO2 release as a result of the activity of microorganisms. Significant contamination with heavy metals, especially lead, as well as zinc and cadmium, has been found near highways. The width of roadside lead anomalies in the soil reaches 100 m or more.

Heavy metals entering the soil surface accumulate in the soil column, especially in the upper humus horizons, and are slowly removed through leaching, consumption by plants, and erosion. The first period of half-removal (i.e., removal of half of the initial concentration) of heavy metals varies significantly among different elements and takes a very long period of time: for zinc - from 70 to 510 years; cadmium from 13 to 110 years, copper - from 310 to 1500 years, lead - from 770 to 5900 years.

Heavy metals are capable of forming complex compounds with soil organic matter, so in soils with a high humus content they are less available for leaching. Excess moisture in the soil promotes the transition of heavy metals to lower oxidation states and into soluble forms. Anaerobic conditions increase the availability of heavy metals to plants. Therefore, drainage systems that regulate the water regime contribute to the predominance of oxidized forms of heavy metals and thereby reduce their migration characteristics. Plants can absorb microelements from the soil, including heavy metals, accumulating them in tissues or on the surface of leaves, thus being an intermediate link in the chain “soil - plant - animal - human”.

Different plants concentrate different numbers of microelements: in most cases, selectively. Thus, copper is absorbed by plants of the clove family, and cobalt by peppers. A high coefficient of biological absorption of zinc is characteristic of dwarf birch and lichens, nickel and copper are characteristic of speedwell and lichens. Heavy metals are protoplasmic poisons, the toxicity of which increases with increasing atomic mass. Their toxicity manifests itself in different ways. Many metals at toxic levels inhibit enzyme activity (copper, mercury). Some of them form chelate-like complexes with common metabolites, disrupting normal metabolism (iron). Metals such as cadmium, copper, iron interact with cell membranes, changing their permeability.

Of particular interest is the study of animals, which are sensitive indicators of the initial stages of heavy metal pollution. They accumulate elements in available biologically active forms and reflect the actual level of pollution of ecosystems. Soil animals, especially saprophytic groups, due to their close connection with soil conditions and limited habitat, can be good indicators of chemical pollution of the biosphere. Among animals, such indicators can be the European mole, brown bear, elk, and bank vole. Having information on the content of heavy metals in mammals, it is possible to predict their effect on the human body.

Receiving technology

The traditional raw material for chromium production is chrome spinels.

The main methods of obtaining metal are ore enrichment by electrolysis or reduction.

To increase the purity of the final product, the raw materials are fused in an electric furnace with soda, adding oxygen.

The production of metallic chromium of almost absolute purity is carried out by the electrolysis of concentrated chromium solutions or by the reduction of chromium oxide with aluminum in vacuum furnaces (at 1500°C).

Classification and properties

Elements can be classified based on their physical states (states of matter), such as gas, solid or liquid. Cr is a hard element. It is classified in chemistry as a "transition metal", which is found in groups 3−12 of the periodic table. Elements classified as transition metals are usually described as ductile and capable of conducting electricity and heat.

Chemical properties are characteristics that become apparent when a material undergoes a chemical reaction or chemical change. The actual structure of the material must be changed in order for the chemical properties to be observed .

• Interaction with non-metals. When heated above 600 °C, Cr burns in oxygen. With F the reaction occurs at 350 °C, with Cl - at 300 °C, with Br - at red heat, forming chromium (III) halides. With N - at temperatures above 1000 °C with the formation of nitrides.
• Interaction with acids. In NCl and HCl in high concentrations, Cr can dissolve only with strong heating, forming Cr(III) salts and acid reduction products. The interaction of metal salts Cr and nitric acid produces chromium nitrate Cr (NO3)3.

It is a blue-gray metal that can be polished to achieve a high shine. It is very shiny, and although chrome is relatively hard, it is also very brittle. This is a fairly active metal. Although it does not react with water, it is capable of reacting with most acids as well as oxygen at room temperature. One of the most important properties of Cr is its passivation. Although stable in air, it nevertheless oxidizes, forming a thin layer that acts as a protective coating to prevent further corrosion.

Cr in its elemental form exhibits paramagnetic properties. It was recently discovered that this element can exhibit different magnetic properties depending on how it is heated and cooled, which in turn affects the spin orientation of the electrons. Compounds of the element, such as chromium dioxide, are considered ferromagnetic. The ferromagnetic properties of these compounds make it possible to use them in data tape as a method of storing information.

Cr can be added to other compounds while maintaining its magnetic properties. It depends on the number of other elements in the compound.

For example, some stainless steel joints are magnetic depending on the amount of chromium they contain.

Where is it used?

The metal is used in two ways: as a ligature for other metals and as a coating.

Metallurgy

An industry that takes up three quarters of metal volumes. Steel is alloyed with chromium to improve its quality.

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• stainless;
• wear-resistant;
• heat resistant.

Such advantages of steels led to their use as a material for artillery barrels, submarine hulls, safes, metal-cutting, medical, and chemical instruments. The engines of spaceships and the filling of plasmatrons are made from them.

Even a small amount of chromium in the composition significantly improves the mechanical properties of the material.

The most famous chromium-containing alloys are with nickel (nichrome) and iron (fechral). These are precision materials with increased electrical resistance. Used to work at extreme temperatures.

Other industries

Products made from metal and its alloys are produced for different market segments:

• Bricks are the body of metallurgical furnaces.
• Heating elements (nickel alloy).
• Surgical instruments (alloy with nickel, molybdenum, cobalt).
• Chromium compounds are useful in the production of matches, shoes, clothing (the famous shiny-durable chrome leather), dyeing textiles, and processing furniture wood.
• Green chrome paint is applied to the ceramic before glazing and firing.

Paints made from ground chrome ores were also used by icon painters of Ancient Rus'.

• Trivalent metal oxide is the starting material for growing synthetic rubies for lasers.
• The green fireworks lights are thanks to chrome.

Chrome is purchased by pharmaceutical giants, manufacturers of dietary supplements, and weight loss drugs.

Decor

Chrome plating on a watch case or car parts is not only a status marker. This treatment protects against wear, corrosion, and mechanical damage.

The thickness of the metal coating depends on the purpose of the product: from 2 microns (decorative assortment) to 0.1 mm (parts of bikes, bicycles, cars).

The process of coating with chromium is called chrome plating. It is technologically simple and inexpensive.

The concept of thermal resistance and thermal conductivity coefficient

If thermal conductivity characterizes the ability of metals to transfer the temperature of bodies from one surface to another, then thermal resistance shows an inverse relationship, i.e. the ability of metals to prevent such transfer, in other words, to resist. Air has high thermal resistance. It is he who, most of all, prevents the transfer of heat between bodies.

The quantitative characteristic of the change in temperature of a unit area per unit of time by one degree (K) is called the thermal conductivity coefficient. The international system of units usually measures this parameter in W/m*deg. This characteristic is very important when choosing metal products that must transfer heat from one body to another.

Table 1

Meaning for humans

Chromium is present in the human body initially.

Health

He is a participant in a number of biological processes:

1. Lipid, carbon metabolism.
2. Removing “bad” cholesterol
3. Blood sugar balance.
4. Strengthening bone tissue.
5. Activation of insulin action.
6. Ability to replace iodine.
7. Stimulation of tissue regeneration.

A sufficient level of chromium in the body is critical for people with excess weight, diabetes, diseases of the thyroid gland, heart, and blood vessels.

Nutrition

Products from all main groups are rich in chromium:

• Meat – chicken, beef (and liver);
• Fish – mackerel, tuna, herring.
• Cereals – semolina, pearl barley.
• Vegetables – tomatoes, radishes, green onions.

Cheeses, legumes, corn oil, fruits, coarse bread, and brewer's yeast are saturated with metal.

Dosage

Daily requirement for chromium (mcg):

• Children – 12-34 (depending on age).
• Women – 55-68.
• Men – 59-79.

During pregnancy in women, an active lifestyle, and physical activity in men, the need doubles.

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Application

Metal chromium is very resistant to oxidation in air, since its surface is covered with a thin and dense film of oxides, which prevents corrosion. At the same time, being additionally passivated by reaction with concentrated acids, chromium becomes an extremely corrosion-resistant material that can be used for coating metal products (chrome plating).

BMW M3 covered in shiny chrome

Specific heat capacity of non-ferrous alloys

The table shows the values ​​of the specific (mass) heat capacity of two-component and multi-component non-ferrous alloys that do not contain iron at temperatures from 123 to 1000K. Heat capacity is indicated in kJ/(kg deg). The heat capacity of the following alloys is given: alloys containing aluminum, copper, magnesium, vanadium, zinc, bismuth, gold, lead, tin, cadmium, nickel, iridium, platinum, potassium, sodium, manganese, titanium, bismuth-lead-tin alloy, alloy bismuth-lead, bismuth-lead-cadmium, alumel, lipovitz alloy, nichrome, rose alloy.

There is also a separate table that shows the specific heat capacity of metals at various temperatures.