calcium in nature. Chemical and physical properties of calcium, its interaction with water

Calcium has been known to man since ancient times in the form of alkaline compounds such as chalk or limestone. In its pure form, this element was obtained in the early 19th century. At the same time, it was found that, in terms of its basic properties, calcium belongs to the alkali metals.

Calcium plays an important biological role - it is the main macronutrient of the skeleton (including the outer one) in most species on the planet, is part of hormones, and is a regulator of neuronal and muscle interactions. Chemically pure calcium is used in various reactions, in metallurgy and in many other industries.

general characteristics

Calcium is one of the typical members of the active alkali metal family. In its pure form, in texture and appearance, it resembles iron, with a less pronounced luster. Brittle, breaks with the formation of heterogeneous crystalline granules. Most of all, calcium is known in the form of its compounds (chalk, limestone, silica, and others), where it has the appearance of a whitish crumbling substance.

It is not found in its pure form due to its high reactivity. It is part of most minerals, among which marble, granite, alabaster and some other valuable rocks are of the greatest importance.

Basic physical and chemical properties

It belongs to the second group of the periodic system of elements, showing similar physical properties with other representatives of the alkaline group:

• Relatively low density (1.6g/cm3);
• Melting point limit - 840 0 C under normal conditions;
• The average thermal conductivity is generally noticeably lower than that of most metals;

In general, the physics of calcium presents no particular surprises. Possessing a typical crystal lattice, this element has a rather low strength and almost zero ductility, easily crumbles and breaks with the formation of a characteristic crystal pattern at the fracture boundary.

However, recent studies have shown some very interesting results. It has been established that at high atmospheric pressure, the physical properties of the element begin to change. Semiconductor properties appear, which are absolutely uncharacteristic for any metals. Extreme pressure leads to the appearance of calcium and superconducting properties. These studies have far-reaching implications, but so far the scope of calcium is limited to its usual properties.

In its chemical qualities, calcium does not stand out in any way and is a typical alkaline earth metal:

• High reactivity;
• Willing interaction with the atmosphere and the formation of a characteristic dull film on the surface of the element;
• Actively interacts with water, but, unlike elements such as sodium, an explosive exothermic reaction does not occur;
• Reacts with all active non-metals, including iodine and bromine;

Unlike more active alkali metals, calcium requires a catalyst or strong heating to react with metals and relatively inert elements (for example, with carbon). Calcium is stored in tightly sealed glass containers to prevent spontaneous reactions.

Calcium is one of the five most common substances on the planet, second only to oxygen, silicon and aluminum with iron. Moreover, in nature, this element occurs mainly in the form of solid or loose minerals. The best known calcium compound is limestone. Calcium also forms a wide range of different minerals, from the above-mentioned granite and marble, to the less common barites and spars. According to approximate estimates of researchers, the calcium content in pure equivalent is about 3.4% by weight.

Scope of industrial application

In the industrial sphere, calcium is included in the group of widely demanded materials for the purposes of metallurgy. With its help, refined metals are obtained, including uranium and thorium, as well as some rare earth elements. The addition of calcium to steel melts contributes to the binding and removal of free oxygen, which improves the structural properties of the metal alloy. Calcium is also used as an electrolytic element in accumulators and batteries.

Calcium is located in the fourth large period, the second group, the main subgroup, the element's serial number is 20. According to Mendeleev's periodic table, the atomic weight of calcium is 40.08. The formula of the highest oxide is CaO. Calcium has a Latin name calcium, so the element's atom symbol is Ca.

Characterization of calcium as a simple substance

Under normal conditions, calcium is a silvery-white metal. Having a high chemical activity, the element is able to form many compounds of different classes. The element is of value for technical and industrial chemical syntheses. The metal is widely distributed in the earth's crust: its share is about 1.5%. Calcium belongs to the group of alkaline earth metals: when dissolved in water, it gives alkalis, but in nature it occurs in the form of multiple minerals and. Sea water contains calcium in high concentrations (400 mg/l).

The characteristics of calcium depend on the structure of its crystal lattice. This element has two types of it: cubic face-centric and volume-centric. The type of bond in the molecule is metallic.

Natural sources of calcium:

• apatite;
• alabaster;
• gypsum;
• calcite;
• fluorite;
• dolomite.

Physical properties of calcium and methods for producing metal

Under normal conditions, calcium is in a solid state of aggregation. The metal melts at 842 °C. Calcium is a good electrical and thermal conductor. When heated, it passes first into a liquid, and then into a vapor state and loses its metallic properties. The metal is very soft and can be cut with a knife. Boils at 1484 °C.

Under pressure, calcium loses its metallic properties and electrical conductivity. But then the metallic properties are restored and the properties of a superconductor appear, several times greater than the rest in their performance.

For a long time it was not possible to obtain calcium without impurities: due to its high chemical activity, this element does not occur in nature in its pure form. The element was discovered at the beginning of the 19th century. Calcium as a metal was first synthesized by the British chemist Humphrey Davy. The scientist discovered the features of the interaction of melts of solid minerals and salts with an electric current. Nowadays, the electrolysis of calcium salts (mixtures of calcium and potassium chlorides, mixtures of calcium fluoride and calcium chloride) remains the most relevant method for producing metal. Calcium is also extracted from its oxide using aluminothermy, a method common in metallurgy.

Chemical properties of calcium

Calcium is an active metal that enters into many interactions. Under normal conditions, it easily reacts, forming the corresponding binary compounds: with oxygen, halogens. Click to learn more about calcium compounds. When heated, calcium reacts with nitrogen, hydrogen, carbon, silicon, boron, phosphorus, sulfur and other substances. In the open air, it instantly interacts with oxygen and carbon dioxide, therefore it becomes covered with a gray coating.

Reacts violently with acids, sometimes igniting. In salts, calcium exhibits interesting properties. For example, cave stalactites and stalagmites are calcium carbonate, gradually formed from water, carbon dioxide and bicarbonate as a result of processes inside groundwater.

Due to its high activity in the normal state, calcium is stored in laboratories in dark sealed glassware under a layer of paraffin or kerosene. A qualitative reaction to the calcium ion is the coloring of the flame in a rich brick-red color.

The metal in the composition of compounds can be identified by insoluble precipitates of some salts of the element (fluoride, carbonate, sulfate, silicate, phosphate, sulfite).

The reaction of water with calcium

Calcium is stored in jars under a layer of protective liquid. To conduct, demonstrating how the reaction of water and calcium occurs, you cannot just get the metal and cut off the desired piece from it. Metallic calcium in the laboratory is easier to use in the form of shavings.

If there are no metal shavings, and there are only large pieces of calcium in the bank, pliers or a hammer will be required. The finished piece of calcium of the desired size is placed in a flask or glass of water. Calcium shavings are placed in a dish in a gauze bag.

Calcium sinks to the bottom, and hydrogen evolution begins (first, in the place where the fresh fracture of the metal is located). Gradually, gas is released from the calcium surface. The process resembles rapid boiling, at the same time a precipitate of calcium hydroxide (slaked lime) is formed.

A piece of calcium floats up, picked up by bubbles of hydrogen. After about 30 seconds, the calcium dissolves and the water turns cloudy white due to the formation of hydroxide slurry. If the reaction is carried out not in a beaker, but in a test tube, heat evolution can be observed: the test tube quickly becomes hot. The reaction of calcium with water does not end with a spectacular explosion, but the interaction of the two substances proceeds violently and looks spectacular. The experience is safe.

If the bag with the remaining calcium is removed from the water and held in the air, then after a while, as a result of the ongoing reaction, strong heating will occur and the remaining in the gauze will boil. If part of the cloudy solution is filtered through a funnel into a beaker, then when carbon monoxide CO₂ is passed through the solution, a precipitate will form. This does not require carbon dioxide - you can blow exhaled air into the solution through a glass tube.

Ufa State Petroleum Technical University

Department of General and Analytical Chemistry

Presentation on theme: "The element calcium. Properties, obtaining, application "

Prepared by a student of the group BTS-11-01 Prokaev G.L.

Associate Professor Krasko S.A.

Introduction

History and origin of the name

Being in nature

Receipt

Physical properties

Chemical properties

Applications of metallic calcium

The use of calcium compounds

Biological role

Conclusion

Bibliography

Introduction

Calcium is an element of the main subgroup of the second group, the fourth period of the periodic system of chemical elements of D. I. Mendeleev, with atomic number 20. It is designated by the symbol Ca (lat. Calcium). The simple substance calcium (CAS number: 7440-70-2) is a soft, reactive, silver-white alkaline earth metal.

Calcium is called an alkaline earth metal, it is classified as an S element. At the external electronic level, calcium has two electrons, so it gives compounds: CaO, Ca (OH) 2, CaCl2, CaSO4, CaCO3, etc. Calcium belongs to typical metals - it has a high affinity for oxygen, reduces almost all metals from their oxides, and forms a fairly strong base Ca (OH) 2.

Despite the ubiquity of element #20, even chemists have not seen elemental calcium. But this metal, both externally and in behavior, is not at all similar to alkali metals, contact with which is fraught with the danger of fires and burns. It can be safely stored in air, it does not ignite from water.

Elemental calcium is almost never used as a structural material. He's too active for that. Calcium easily reacts with oxygen, sulfur, halogens. Even with nitrogen and hydrogen, under certain conditions, it reacts. The environment of carbon oxides, inert for most metals, is aggressive for calcium. It burns in an atmosphere of CO and CO2.

History and origin of the name

The name of the element comes from lat. calx (in the genitive case calcis) - "lime", "soft stone". It was proposed by the English chemist Humphrey Davy, who in 1808 isolated calcium metal by the electrolytic method. Davy electrolyzed a mixture of wet slaked lime with mercury oxide HgO on a platinum plate, which was the anode. A platinum wire immersed in liquid mercury served as the cathode. As a result of electrolysis, calcium amalgam was obtained. Having driven away mercury from it, Davy received a metal called calcium.

Calcium compounds - limestone, marble, gypsum (as well as lime - a product of burning limestone) have been used in construction for several millennia ago. Until the end of the 18th century, chemists considered lime to be a simple body. In 1789, A. Lavoisier suggested that lime, magnesia, barite, alumina and silica are complex substances.

Being in nature

Due to the high chemical activity of calcium in the free form in nature is not found.

Calcium accounts for 3.38% of the mass of the earth's crust (5th place in abundance after oxygen, silicon, aluminum and iron).

Isotopes. Calcium occurs in nature as a mixture of six isotopes: 40Ca, 42Ca, 43Ca, 44Ca, 46Ca and 48Ca, among which the most common - 40Ca - is 96.97%.

Of the six naturally occurring calcium isotopes, five are stable. The sixth 48Ca isotope, the heaviest of the six and quite rare (its isotopic abundance is only 0.187%), was recently discovered to undergo double beta decay with a half-life of 5.3 ×1019 years.

in rocks and minerals. Most of the calcium is contained in the composition of silicates and aluminosilicates of various rocks (granites, gneisses, etc.), especially in feldspar - anorthite Ca.

In the form of sedimentary rocks, calcium compounds are represented by chalk and limestone, consisting mainly of the mineral calcite (CaCO3). The crystalline form of calcite - marble - is found in nature much less frequently.

Calcium minerals such as calcite CaCO3, anhydrite CaSO4, alabaster CaSO4 0.5H2O and gypsum CaSO4 2H2O, fluorite CaF2, apatites Ca5(PO4)3(F,Cl,OH), dolomite MgCO3 CaCO3 are quite widespread. The presence of calcium and magnesium salts in natural water determines its hardness.

Calcium, which migrates vigorously in the earth's crust and accumulates in various geochemical systems, forms 385 minerals (fourth in terms of the number of minerals).

Migration in the earth's crust. In the natural migration of calcium, a significant role is played by the “carbonate equilibrium”, associated with the reversible reaction of the interaction of calcium carbonate with water and carbon dioxide with the formation of soluble bicarbonate:

CaCO3 + H2O + CO2 ↔ Ca (HCO3) 2 ↔ Ca2+ + 2HCO3ˉ

(the equilibrium shifts to the left or right depending on the concentration of carbon dioxide).

biogenic migration. In the biosphere, calcium compounds are found in almost all animal and plant tissues (see also below). A significant amount of calcium is part of living organisms. So, hydroxyapatite Ca5(PO4)3OH, or, in another notation, 3Ca3(PO4)2·Ca(OH)2 is the basis of the bone tissue of vertebrates, including humans; shells and shells of many invertebrates, egg shells, etc. are composed of calcium carbonate CaCO3. In living tissues of humans and animals, 1.4-2% Ca (by mass fraction); in a human body weighing 70 kg, the calcium content is about 1.7 kg (mainly in the composition of the intercellular substance of bone tissue).

Receipt

Free metallic calcium is obtained by electrolysis of a melt consisting of CaCl2 (75-80%) and KCl or from CaCl2 and CaF2, as well as by aluminothermic reduction of CaO at 1170-1200 °C:

CaO + 2Al = CaAl2O4 + 3Ca.

A method has also been developed for obtaining calcium by thermal dissociation of calcium carbide CaC2

Physical properties

Calcium metal exists in two allotropic modifications. Resistant up to 443°C α -Ca with cubic lattice, higher stable β-Ca with a cubic body-centered lattice of the type α -Fe. Standard enthalpy ΔH0 transition α → β is 0.93 kJ/mol.

Calcium is a light metal (d = 1.55), silver-white in color. It is harder and melts at a higher temperature (851°C) than sodium, which is next to it in the periodic table. This is because there are two electrons per calcium ion in the metal. Therefore, the chemical bond between ions and electron gas is stronger than that of sodium. In chemical reactions, calcium valence electrons are transferred to atoms of other elements. In this case, doubly charged ions are formed.

Chemical properties

Calcium is a typical alkaline earth metal. The chemical activity of calcium is high, but lower than that of all other alkaline earth metals. It easily reacts with oxygen, carbon dioxide and moisture in the air, due to which the surface of metallic calcium is usually dull gray, therefore, in the laboratory, calcium is usually stored, like other alkaline earth metals, in a tightly closed jar under a layer of kerosene or liquid paraffin.

In the series of standard potentials, calcium is located to the left of hydrogen. The standard electrode potential of the Ca2+/Ca0 pair is −2.84 V, so that calcium actively reacts with water, but without ignition:

2H2O \u003d Ca (OH) 2 + H2 + Q.

With active non-metals (oxygen, chlorine, bromine), calcium reacts under normal conditions:

Ca + O2 = 2CaO, Ca + Br2 = CaBr2.

When heated in air or oxygen, calcium ignites. With less active non-metals (hydrogen, boron, carbon, silicon, nitrogen, phosphorus and others), calcium interacts when heated, for example:

Ca + H2 = CaH2, Ca + 6B = CaB6,

Ca + N2 = Ca3N2, Ca + 2C = CaC2,

Ca + 2P = Ca3P2 (calcium phosphide),

calcium phosphides of CaP and CaP5 compositions are also known;

Ca + Si = Ca2Si (calcium silicide),

Calcium silicides of compositions CaSi, Ca3Si4 and CaSi2 are also known.

The course of the above reactions, as a rule, is accompanied by the release of a large amount of heat (that is, these reactions are exothermic). In all compounds with non-metals, the oxidation state of calcium is +2. Most of the calcium compounds with non-metals are easily decomposed by water, for example:

CaH2 + 2H2O \u003d Ca (OH) 2 + 2H2, N2 + 3H2O \u003d 3Ca (OH) 2 + 2NH3.

The Ca2+ ion is colorless. When soluble calcium salts are added to the flame, the flame turns brick red.

Calcium salts such as CaCl2 chloride, CaBr2 bromide, CaI2 iodide and Ca(NO3)2 nitrate are highly soluble in water. CaF2 fluoride, CaCO3 carbonate, CaSO4 sulfate, Ca3(PO4)2 orthophosphate, CaC2O4 oxalate and some others are insoluble in water.

Important is the fact that, unlike calcium carbonate CaCO3, acidic calcium carbonate (hydrocarbonate) Ca(HCO3) 2 is soluble in water. In nature, this leads to the following processes. When cold rain or river water, saturated with carbon dioxide, penetrates underground and falls on limestones, their dissolution is observed:

CaCO3 + CO2 + H2O \u003d Ca (HCO3) 2.

In the same places where water saturated with calcium bicarbonate comes to the surface of the earth and is heated by the sun's rays, the reverse reaction occurs:

Ca (HCO3) 2 \u003d CaCO3 + CO2 + H2O.

So in nature there is a transfer of large masses of substances. As a result, huge gaps can form underground, and beautiful stone "icicles" - stalactites and stalagmites - form in the caves.

The presence of dissolved calcium bicarbonate in water largely determines the temporary hardness of water. It is called temporary because when water is boiled, the bicarbonate decomposes, and CaCO3 precipitates. This phenomenon leads, for example, to the fact that scale forms in the kettle over time.

calcium metal chemical physical

The main use of calcium metal is as a reducing agent in the production of metals, especially nickel, copper and stainless steel. Calcium and its hydride are also used to obtain hard-to-recover metals such as chromium, thorium and uranium. Alloys of calcium with lead are used in batteries and bearing alloys. Calcium granules are also used to remove traces of air from electrovacuum devices. Soluble calcium and magnesium salts determine the overall hardness of water. If they are present in water in small quantities, then the water is called soft. With a high content of these salts, the water is considered hard. Hardness is eliminated by boiling; water is sometimes distilled to completely eliminate it.

Metalthermy

Pure metallic calcium is widely used in metallothermy to obtain rare metals.

Alloying

Pure calcium is used to alloy lead, which is used for the manufacture of battery plates, maintenance-free starter lead-acid batteries with low self-discharge. Also, metallic calcium is used for the production of high-quality calcium babbits BKA.

Nuclear fusion

The 48Ca isotope is the most efficient and widely used material for the production of superheavy elements and the discovery of new elements in the periodic table. For example, in the case of using 48Ca ions to produce superheavy elements in accelerators, the nuclei of these elements are formed hundreds and thousands of times more efficiently than when using other "projectiles" (ions).

The use of calcium compounds

calcium hydride. By heating calcium in a hydrogen atmosphere, CaH2 (calcium hydride) is obtained, which is used in metallurgy (metallothermy) and in the production of hydrogen in the field.

Optical and laser materials. Calcium fluoride (fluorite) is used in the form of single crystals in optics (astronomical objectives, lenses, prisms) and as a laser material. Calcium tungstate (scheelite) in the form of single crystals is used in laser technology, and also as a scintillator.

calcium carbide. Calcium carbide CaC2 is widely used to obtain acetylene and to reduce metals, as well as in the production of calcium cyanamide (by heating calcium carbide in nitrogen at 1200 ° C, the reaction is exothermic, carried out in cyanamide furnaces).

Chemical current sources. Calcium, as well as its alloys with aluminum and magnesium, are used in reserve thermal electric batteries as an anode (for example, a calcium-chromate element). Calcium chromate is used in such batteries as the cathode. A feature of such batteries is an extremely long shelf life (decades) in a usable condition, the ability to operate in any conditions (space, high pressures), high specific energy by weight and volume. The disadvantage is the short duration. Such batteries are used where it is necessary to create colossal electric power for a short time (ballistic missiles, some spacecraft, etc.).

Refractory materials. Calcium oxide, both in free form and as part of ceramic mixtures, is used in the production of refractory materials.

Medicines. In medicine, Ca drugs eliminate disorders associated with a lack of Ca ions in the body (with tetany, spasmophilia, rickets). Ca preparations reduce hypersensitivity to allergens and are used to treat allergic diseases (serum sickness, sleeping fever, etc.). Ca preparations reduce increased vascular permeability and have an anti-inflammatory effect. They are used for hemorrhagic vasculitis, radiation sickness, inflammatory processes (pneumonia, pleurisy, etc.) and some skin diseases. It is prescribed as a hemostatic agent, to improve the activity of the heart muscle and enhance the effect of digitalis preparations, as an antidote for poisoning with magnesium salts. Together with other drugs, Ca preparations are used to stimulate labor. Ca chloride is administered by mouth and intravenously.

Ca preparations also include gypsum (CaSO4), used in surgery for plaster casts, and chalk (CaCO3), administered orally with increased acidity of gastric juice and for the preparation of tooth powder.

Biological role

Calcium is a common macronutrient in plants, animals and humans. In humans and other vertebrates, most of it is found in the skeleton and teeth in the form of phosphates. The skeletons of most groups of invertebrates (sponges, coral polyps, mollusks, etc.) are composed of various forms of calcium carbonate (lime). Calcium ions are involved in the processes of blood coagulation, as well as in maintaining a constant osmotic pressure of the blood. Calcium ions also serve as one of the universal second messengers and regulate a variety of intracellular processes - muscle contraction, exocytosis, including the secretion of hormones and neurotransmitters, etc. The calcium concentration in the cytoplasm of human cells is about 10−7 mol, in intercellular fluids about 10− 3 mol.

Most of the calcium that enters the human body with food is found in dairy products, the remaining calcium is found in meat, fish, and some plant foods (legumes are especially rich). Absorption occurs in both the large and small intestines and is facilitated by an acidic environment, vitamin D and vitamin C, lactose, and unsaturated fatty acids. The role of magnesium in calcium metabolism is also important, with its deficiency, calcium is “washed out” of the bones and deposited in the kidneys (kidney stones) and muscles.

Assimilation of calcium is prevented by aspirin, oxalic acid, estrogen derivatives. Combining with oxalic acid, calcium gives water-insoluble compounds that are components of kidney stones.

Due to the large number of processes associated with calcium, the content of calcium in the blood is precisely regulated, and with proper nutrition, deficiency does not occur. Prolonged absence from the diet can cause cramps, joint pain, drowsiness, growth defects, and constipation. A deeper deficiency leads to permanent muscle cramps and osteoporosis. Abuse of coffee and alcohol can be the causes of calcium deficiency, as part of it is excreted in the urine.

Excessive doses of calcium and vitamin D can cause hypercalcemia, followed by intense calcification of bones and tissues (mainly affecting the urinary system). A prolonged excess disrupts the functioning of muscle and nerve tissues, increases blood clotting and reduces the absorption of zinc by bone cells. The maximum daily safe dose for an adult is 1500 to 1800 milligrams.

Products Calcium, mg/100 g

Sesame 783

Nettle 713

Plantain big 412

Sardines in oil 330

Budra ivy 289

Dog rosehip 257

Almond 252

Plantain lanceolate. 248

Hazelnut 226

Watercress 214

Soy beans dry 201

Children under 3 years old - 600 mg.

Children from 4 to 10 years old - 800 mg.

Children from 10 to 13 years old - 1000 mg.

Adolescents from 13 to 16 years old - 1200 mg.

Youth 16 and older - 1000 mg.

Adults 25 to 50 years old - 800 to 1200 mg.

Pregnant and breastfeeding women - 1500 to 2000 mg.

Conclusion

Calcium is one of the most abundant elements on earth. There is a lot of it in nature: mountain ranges and clay rocks are formed from calcium salts, it is found in sea and river water, and is part of plant and animal organisms.

Calcium constantly surrounds the townspeople: almost all the main building materials - concrete, glass, brick, cement, lime - contain this element in significant quantities.

Naturally, having such chemical properties, calcium cannot be found in nature in a free state. But calcium compounds - both natural and artificial - have become of paramount importance.

Bibliography

1.Editorial board: Knunyants I. L. (editor-in-chief) Chemical Encyclopedia: in 5 volumes - Moscow: Soviet Encyclopedia, 1990. - T. 2. - S. 293. - 671 p.

2.Doronin. N. A. Kaltsy, Goshimizdat, 1962. 191 pages with illustrations.

.Dotsenko V.A. - Therapeutic and preventive nutrition. - Q. nutrition, 2001 - N1-p.21-25

4.Bilezikian J. P. Calcium and bone metabolism // In: K. L. Becker, ed.

5.M.Kh. Karapetyants, S.I. Drakin - General and Inorganic Chemistry, 2000. 592 pages with illustrations.

DEFINITION

Calcium- the twentieth element of the Periodic table. Designation - Ca from the Latin "calcium". Located in the fourth period, IIA group. Refers to metals. The core charge is 20.

Calcium is one of the most abundant elements in nature. It contains approximately 3% (mass) in the earth's crust. It occurs as numerous deposits of limestone and chalk, as well as marble, which are natural varieties of calcium carbonate CaCO 3 . Gypsum CaSO 4 × 2H 2 O, phosphorite Ca 3 (PO 4) 2 and, finally, various calcium-containing silicates are also found in large quantities.

In the form of a simple substance, calcium is a malleable, rather hard white metal (Fig. 1). In air, it quickly becomes covered with a layer of oxide, and when heated, it burns with a bright reddish flame. Calcium reacts relatively slowly with cold water, but quickly displaces hydrogen from hot water, forming hydroxide.

Rice. 1. Calcium. Appearance.

Atomic and molecular weight of calcium

The relative molecular mass of a substance (M r) is a number showing how many times the mass of a given molecule is greater than 1/12 of the mass of a carbon atom, and the relative atomic mass of an element (Ar r) is how many times the average mass of atoms of a chemical element is greater than 1/12 the mass of a carbon atom.

Since in the free state calcium exists in the form of monatomic Ca molecules, the values ​​of its atomic and molecular masses are the same. They are equal to 40.078.

Isotopes of calcium

It is known that in nature calcium can be found in the form of four stable isotopes 40Ca, 42Ca, 43Ca, 44Ca, 46Ca and 48Ca, with a clear predominance of the 40Ca isotope (99.97%). Their mass numbers are 40, 42, 43, 44, 46 and 48, respectively. The nucleus of the atom of the calcium isotope 40 Ca contains twenty protons and twenty neutrons, and the remaining isotopes differ from it only in the number of neutrons.

There are artificial calcium isotopes with mass numbers from 34 to 57, among which the most stable is 41 Ca with a half-life of 102 thousand years.

Calcium ions

At the outer energy level of the calcium atom, there are two electrons that are valence:

1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 .

As a result of chemical interaction, calcium gives up its valence electrons, i.e. is their donor, and turns into a positively charged ion:

Ca 0 -2e → Ca 2+.

Molecule and atom of calcium

In the free state, calcium exists in the form of monatomic Ca molecules. Here are some properties that characterize the calcium atom and molecule:

calcium alloys

Calcium serves as an alloying component of some lead alloys.

Examples of problem solving

EXAMPLE 1

History and origin of the name

The name of the element comes from lat. calx (in the genitive case calcis) - "lime", "soft stone". It was proposed by the English chemist Humphry Davy, who in 1808 isolated calcium metal by the electrolytic method. Davy electrolyzed a mixture of wet hydrated lime on a platinum plate, which was the anode. A platinum wire immersed in liquid served as the cathode. As a result of electrolysis, calcium amalgam was obtained. Having driven away mercury from it, Davy received a metal called calcium.

isotopes

Calcium occurs in nature as a mixture of six isotopes: 40 Ca, 42 Ca, 43 Ca, 44 Ca, 46 Ca and 48 Ca, among which the most common - 40 Ca - is 96.97%. Calcium nuclei contain the magic number of protons: Z= 20 . isotopes 40
20 Ca20
and 48
20 Ca28
are two of the five doubly magic nuclei that exist in nature.

Of the six naturally occurring calcium isotopes, five are stable. The sixth isotope 48Ca, the heaviest of the six and very rare (its isotopic abundance is only 0.187%), undergoes double beta decay with a half-life of (4.39 ± 0.58)⋅10 19 years.

In rocks and minerals

Calcium, which migrates vigorously in the earth's crust and accumulates in various geochemical systems, forms 385 minerals (fourth in terms of the number of minerals).

Most of the calcium is contained in the composition of silicates and aluminosilicates of various rocks (granites, gneisses, etc.), especially in feldspar - anorthite Ca.

Calcium minerals such as calcite CaCO 3 , anhydrite CaSO 4 , alabaster CaSO 4 0.5H 2 O and gypsum CaSO 4 2H 2 O, fluorite CaF 2 , apatites Ca 5 (PO 4) 3 (F, Cl, OH), dolomite MgCO 3 CaCO 3 . The presence of calcium and magnesium salts in natural water determines its hardness.

Sedimentary rock, consisting mainly of cryptocrystalline calcite - limestone (one of its varieties is chalk). Under the action of regional metamorphism, limestone is transformed into marble.

Migration in the earth's crust

In the natural migration of calcium, a significant role is played by the “carbonate equilibrium”, associated with the reversible reaction of the interaction of calcium carbonate with water and carbon dioxide with the formation of soluble bicarbonate:

(the equilibrium shifts to the left or right depending on the concentration of carbon dioxide).

Biogenic migration plays an important role.

In the biosphere

Calcium compounds are found in almost all animal and plant tissues (see below). A significant amount of calcium is part of living organisms. So, hydroxyapatite Ca 5 (PO 4) 3 OH, or, in another entry, 3Ca 3 (PO 4) 2 Ca (OH) 2 - the basis of the bone tissue of vertebrates, including humans; shells and shells of many invertebrates, egg shells, etc. are composed of calcium carbonate CaCO 3. In living tissues of humans and animals, 1.4-2% Ca (by mass fraction); in a human body weighing 70 kg, the calcium content is about 1.7 kg (mainly in the composition of the intercellular substance of bone tissue).

Receipt

Free metallic calcium is obtained by electrolysis of a melt consisting of CaCl 2 (75-80%) and KCl or from CaCl 2 and CaF 2, as well as aluminothermic reduction of CaO at 1170-1200 ° C 4 C a O + 2 A l → C a A l 2 O 4 + 3 C a (\displaystyle (\mathsf (4CaO+2Al\rightarrow CaAl_(2)O_(4)+3Ca)))

Physical properties

The calcium metal exists in two allotropic modifications. Up to 443 °C resistant α-Ca with a cubic face-centered lattice (parameter a= 0.558 nm), higher stable β-Ca with a cubic body-centered lattice of the type α-Fe(parameter a= 0.448 nm). Standard enthalpy ∆ H 0 (\displaystyle \Delta H^(0)) transition α → β is 0.93 kJ / mol.

With a gradual increase in pressure, it begins to show the properties of a semiconductor, but does not become a semiconductor in the full sense of the word (it is no longer a metal either). With a further increase in pressure, it returns to the metallic state and begins to exhibit superconducting properties (the superconductivity temperature is six times higher than that of mercury, and far exceeds all other elements in conductivity). The unique behavior of calcium is similar in many ways to strontium (that is, the parallels in the periodic system are preserved).

Chemical properties

In the series of standard potentials, calcium is located to the left of hydrogen. The standard electrode potential of the pair Ca 2+ / Ca 0 −2.84 V, so that calcium actively reacts with water, but without ignition:

The presence of dissolved calcium bicarbonate in water largely determines the temporary hardness of water. It is called temporary because when water is boiled, the bicarbonate decomposes, and CaCO 3 precipitates. This phenomenon leads, for example, to the fact that scale forms in the kettle over time.

Application

The main use of calcium metal is as a reducing agent in the production of metals, especially nickel, copper and stainless steel. Calcium and its hydride are also used to produce hard-to-reduce metals such as chromium, thorium and uranium. Calcium-lead alloys are used in some types of batteries and in the manufacture of bearings. Calcium granules are also used to remove traces of air from electrovacuum devices. Pure metallic calcium is widely used in metallothermy to obtain rare earth elements.

Calcium is widely used in metallurgy to deoxidize steel along with aluminum or in combination with it. Out-of-furnace processing with calcium-containing wires occupies a leading position due to the multifactorial effect of calcium on the physico-chemical state of the melt, the macro- and microstructure of the metal, the quality and properties of metal products and is an integral part of steel production technology. In modern metallurgy, an injection wire is used to introduce calcium into the melt, which is calcium (sometimes silicocalcium or aluminum calcium) in the form of a powder or pressed metal in a steel shell. Along with deoxidation (removal of oxygen dissolved in steel), the use of calcium makes it possible to obtain non-metallic inclusions that are favorable in nature, composition and shape, which do not collapse during further technological operations.

The 48 Ca isotope is one of the most effective and useful materials for the production of superheavy elements and the discovery of new elements in the periodic table. This is due to the fact that calcium-48 is a doubly magic nucleus, so its stability allows it to be sufficiently neutron-rich for a light nucleus; the synthesis of superheavy nuclei requires an excess of neutrons.

Biological role

The concentration of calcium in the blood, due to its importance for a large number of vital processes, is precisely regulated, and with proper nutrition and sufficient intake of low-fat dairy products and vitamin D, deficiency does not occur. Prolonged deficiency of calcium and/or vitamin D in the diet leads to an increased risk of osteoporosis and causes rickets in infancy.

Notes

1. Brinell hardness 200-300 MPa
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