General characteristics of elements of group VII. Properties of elements VII (17) of group of the main subgroup Characteristics of group 7 of the periodic system

Group VII p-elements include fluorine ( F), chlorine ( Cl), bromine ( Br), iodine ( I) and astatine ( At). These elements are called halogens (producing salts). All elements of this subgroup are non-metals.

The general electronic formula of the valence band of atoms has the form ns 2 np 5, from which it follows that there are seven electrons on the outer electronic layer of the atoms of the elements under consideration and they can exhibit odd valences 1, 3, 5, 7. The fluorine atom does not have a d-sublevel, therefore there are no excited states and the valency of fluorine is only 1.

Fluorine is the most electronegative element in the periodic table and, accordingly, in compounds with other elements exhibits only a negative oxidation state of –1. The remaining halogens can have oxidation states –1, 0, +1, +3, +5, +7. Each halogen in its period is the most powerful oxidizing agent. With an increase in the atomic number of elements in the series F, C1, Br, I and At, the radii of the atoms increase and the oxidative activity of the elements decreases.

Molecules of simple substances are diatomic: F 2, C1 2, Br 2, I 2. Under normal conditions, fluorine is a pale yellow gas, chlorine is a yellow-green gas, bromine is a red-brown liquid, and iodine is a dark purple crystalline substance. All halogens have a very pungent odor. Inhaling them leads to severe poisoning. When heated, iodine sublimates (sublimes), turning into purple vapor; When cooled, iodine vapor crystallizes, bypassing the liquid state.

Halogens are slightly soluble in water, but much better in organic solvents. Fluorine cannot be dissolved in water, as it decomposes it:

2F 2 + 2H 2 O = 4НF + O 2.

When chlorine is dissolved in water, its partial auto-oxidation-self-reduction occurs according to the reaction

C1 2 + H 2 O ↔ HC1+ HC1O.

The resulting solution is called chlorine water. It has strong acidic and oxidizing properties and is used to disinfect drinking water.

Halogens interact with many simple substances, exhibiting the properties of oxidizing agents. Fluorine reacts explosively with many non-metals:

H 2 + F 2 → 2HF,

Si + 2F 2 → SiF 4,

S + 3F 2 → SF 6.

In a fluorine atmosphere, stable substances such as glass in the form of cotton wool and water burn:

SiO 2 + 2F 2 → SiF 4 + O 2,

2H 2 O + 2F 2 → 4HF + O 2.

Fluorine does not directly interact only with oxygen, nitrogen, helium, neon and argon.

In an atmosphere of chlorine, many metals burn, forming chlorides:

2Na + С1 2 → 2NaCl (bright flash);

Сu + С1 2 → СuС1 2,

2Fe + 3Cl 2 → 2FeCl 3.

Chlorine does not directly interact with N2, O2 and inert gases.

The oxidative activity of halogens decreases from fluorine to astatine, and the reducing activity of halide ions increases in this direction. It follows from this that the more active halogen displaces the less active one from solutions of its salts:

F 2 + 2NaCl → Cl 2 + 2NaF,

Cl 2 + 2NaBr → Br 2 + 2NaCl,

Br 2 + 2NaI → I 2 + 2NaBr.

Hydrogen compounds of halogens are highly soluble in water. Their aqueous solutions are acids:

HF – hydrofluoric (fluoric) acid,

HC1 – hydrochloric acid (aqueous solution – hydrochloric),

НВг – hydrobromic acid,

HI – hydroiodic acid.

HF should be one of the strongest acids, but due to the formation of a hydrogen bond (H–F···H–F) it is a weak acid. Confirmation of the presence of a hydrogen bond between H–F molecules, as in the case of water, is the anomalously high boiling point of H–F.

Hydrofluoric acid reacts with SiO 2, so HF cannot be prepared and stored in glass containers

SiO 2 + 4HF = SiF 4 + 2H 2 O.

The remaining hydrogen halides are strong acids.

Chlorine, bromine and iodine form oxygen-containing acids and their corresponding salts. Below, using chlorine as an example, are the formulas

acids and their corresponding salts:

HClO, HClO 2, HClO 3, HClO 4;

hypochlorous chloride hypochlorous chlorine

strengthening acid properties

KClO, KClO 2, KClO 3, KClO 4.

potassium hypochlorite potassium chlorite potassium chlorate potassium perchlorate

Perchloric and hypochlorous acids are strong, while chloric and hypochlorous acids are weak. Among the salts we can note:

CaOC1 2 - “bleach” is a mixed salt of hydrochloric and hypochlorous acids.

KClO 3 – potassium chlorate, technical name – Berthollet salt.

Fluorine and its compounds are used to produce heat-resistant plastics (Teflon) and refrigerants (freon) for refrigeration machines.

Chlorine is used in large quantities for the production of hydrochloric acid by the synthetic method, organochlorine insecticides, plastics, synthetic fibers, bleach, bleaching fabrics and paper, chlorinating water for disinfection purposes, and chlorinating ores in the production of metals.

Bromine and iodine compounds are used for the production of medicines and photographic materials.

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Introduction

Group VII of the periodic table of elements includes manganese, technetium, rhenium, bohrium, as well as, according to the old nomenclature, fluorine, chlorine, bromine, iodine, astatine - which are halogens.

Group 7 elements have 7 valence electrons. All of them are silvery-white refractory metals. In the series Mn - Tc - Re, chemical activity decreases. The electrical conductivity of rhenium is approximately 4 times less than that of tungsten. In air, compact metallic manganese is covered with a thin film of oxide, which protects it from further oxidation even when heated. On the contrary, in a finely crushed state it oxidizes quite easily.

At the outer energy level, halogens have 7 electrons and are strong oxidizing agents. When interacting with metals, an ionic bond occurs and salts are formed. When interacting with more electronegative elements, halogens (except fluorine) can also exhibit reducing properties up to the highest oxidation state of +7.

Technetium and bohrium are radioactive with a fairly short half-life, which is why they do not occur in nature. Manganese is one of the common elements, making up 0.03% of the total number of atoms in the earth's crust.

As for halogens, they are highly reactive and therefore are usually found in nature in the form of compounds. Their prevalence in the earth's crust decreases with increasing atomic radius from fluorine to iodine.

halogen element astatine manganese

1. Seventh1st group of the periodic table

1.1 Glava undergroup seven group. Halogens

To the main subgroup VII groups includes the elements fluorine, chlorine, bromine, iodine, astatine.

Halogens (from the Greek ?lt - salt and gEnpt - birth, origin; sometimes the outdated name halogens is used) - chemical elements of group VII of the periodic table of chemical elements of D. I. Mendeleev

They react with almost all simple substances, except some non-metals. All halogens are energetic oxidizing agents and therefore are found in nature only in the form of compounds. As the atomic number increases, the chemical activity of halogens decreases; the chemical activity of halide ions F ? ,Cl? ,Br? ,I? ,At? decreases.

All halogens are non-metals. At the outer energy level, 7 electrons are strong oxidizing agents. When interacting with metals, an ionic bond occurs and salts are formed. When interacting with more electronegative elements, halogens (except fluorine) can also exhibit reducing properties up to the highest oxidation state of +7.

As mentioned above, halogens have high reactivity, therefore they are usually found in nature in the form of compounds.

Their prevalence in the earth's crust decreases with increasing atomic radius from fluorine to iodine. The amount of astatine in the earth's crust is measured in grams, and ununseptium is absent in nature. Fluorine, chlorine, bromine and iodine are produced on an industrial scale, with production volumes of chlorine being significantly higher than the other three stable halogens.

In nature, these elements occur primarily as halides (with the exception of iodine, which also occurs as sodium or potassium iodate in alkali metal nitrate deposits). Since many chlorides, bromides and iodides are soluble in water, these anions are present in the ocean and natural brines. The main source of fluorine is calcium fluoride, which is very slightly soluble and is found in sedimentary rocks (as fluorite CaF 2).

The main way to obtain simple substances is the oxidation of halides. High positive standard electrode potentials E o (F 2 /F ?) = +2.87 V and E o (Cl 2 /Cl ?) = +1.36 V show that oxidize F ions? and Cl? only possible with strong oxidizing agents. In industry, only electrolytic oxidation is used. When producing fluorine, an aqueous solution cannot be used, since water oxidizes at a much lower potential (+1.32 V) and the resulting fluorine would quickly react with water. Fluorine was first obtained in 1886 by the French chemist Henri Moissan by electrolysis of a solution of potassium hydrofluoride KHF 2 in anhydrous hydrofluoric acid.

In industry, chlorine is mainly produced by electrolysis of an aqueous solution of sodium chloride in special electrolysers. In this case, the following reactions occur:

half-reaction at the anode:

half-reaction at the cathode:

Oxidation of water at the anode is suppressed by using an electrode material that has a higher overvoltage with respect to O 2 than to Cl 2 (such a material is, in particular, RuO 2).

In modern electrolysers, the cathode and anode spaces are separated by a polymer ion-exchange membrane. The membrane allows Na + cations to move from the anode space to the cathode space. The transition of cations maintains electrical neutrality in both parts of the electrolyzer, since during electrolysis negative ions are removed from the anode (conversion of 2Cl ? into Cl 2) and accumulate at the cathode (formation of OH ?). Moving OH ? in the opposite direction could also maintain electrical neutrality, but the OH ion? would react with Cl 2 and negate the entire result.

Bromine is obtained by chemical oxidation of the bromide ion found in sea water. A similar process is used to obtain iodine from natural brines rich in I? . In both cases, chlorine, which has stronger oxidizing properties, is used as an oxidizing agent, and the resulting Br 2 and I 2 are removed from the solution by a stream of air.

Table 1, Some propertieshalogens.

1.2 Fluorine

Fluorine(lat. Fluorum), F, chemical element of group VII of the periodic system of Mendeleev, belongs to the halogens, atomic number 9, atomic mass 18.998403; under normal conditions (0 °C; 0.1 Mn/m2, or 1 kgf/cm2) - a pale yellow gas with a pungent odor.

Natural Fluorine consists of one stable isotope 19 F. A number of isotopes have been artificially obtained, in particular: 16 F with a half-life T ½< 1 сек, 17 F (T Ѕ = 70 сек) , 18 F (T Ѕ = 111 мин) , 20 F (T Ѕ = 11,4 сек) , 21 F (T Ѕ = 5 сек).

Historical reference. The first fluorine compound - fluorite (fluorspar) CaF 2 - was described at the end of the 15th century under the name "fluor" (from the Latin fluo - flow, due to the property of CaF 2 to make viscous slags of metallurgical production liquid-flowing). In 1771, K. Scheele obtained hydrofluoric acid. Free fluorine was isolated by A. Moissan in 1886 by electrolysis of liquid anhydrous hydrogen fluoride containing an admixture of acid potassium fluoride KHF 2.

Fluorine chemistry began to develop in the 1930s, especially quickly during and after the Second World War of 1939-45 in connection with the needs of the nuclear industry and rocket technology. The name "Fluorine" (from the Greek phthoros - destruction, death), proposed by A. Ampere in 1810, is used only in Russian; In many countries the name "fluor" is accepted.

Distribution of fluorine in nature. The average fluorine content in the earth's crust (clarke) is 6.25·10 -2% by mass; in acidic igneous rocks (granites) it is 8·10 -2%, in basic rocks - 3.7·10 -2%, in ultrabasic rocks - 1·10 -2%. Fluorine is present in volcanic gases and thermal waters. The most important fluorine compounds are fluorite, cryolite and topaz. In total, more than 80 fluorine-containing minerals are known. Fluorine compounds are also found in apatites, phosphorites and others. Fluorine is an important biogenic element. In the history of the Earth, the source of fluorine entering the biosphere was the products of volcanic eruptions (gases, etc.).

Physical properties of Fluorine. Gaseous Fluorine has a density of 1.693 g/l (0°C and 0.1 Mn/m2, or 1 kgf/cm2), liquid - 1.5127 g/cm3 (at boiling point); t pl -219.61 °C; boiling point -188.13 °C. The Fluorine molecule consists of two atoms (F 2); at 1000 °C 50% of the molecules dissociate, the dissociation energy is about 155 kJ/mol (37 kcal/mol). Fluorine is poorly soluble in liquid hydrogen fluoride; solubility 2.5·10 -3 g in 100 g HF at -70 °C and 0.4·10 -3 g at -20 °C; in liquid form, unlimitedly soluble in liquid oxygen and ozone.

Chemical properties Fluoride. The configuration of the outer electrons of the Fluorine atom is 2s 2 2p 5. In compounds it exhibits an oxidation state of -1. The covalent radius of the atom is 0.72E, the ionic radius is 1.3ЗЕ. Electron affinity 3.62 eV, ionization energy (F > F+) 17.418 eV. High values of electron affinity and ionization energy explain the strong electronegativity of the Fluorine atom, the largest among all other elements. The high reactivity of Fluorine determines the exothermic nature of fluorination, which, in turn, is determined by the abnormally low dissociation energy of the Fluorine molecule and the large values of the bond energy of the Fluorine atom with other atoms. Direct fluoridation has a chain mechanism and can easily lead to combustion and explosion. Fluorine reacts with all elements except helium, neon and argon. It interacts with oxygen in a glow discharge, forming oxygen fluorides O 2 F 2, O 3 F 2 and others at low temperatures. Reactions of fluorine with other halogens are exothermic, resulting in the formation of interhalogen compounds. Chlorine interacts with Fluorine when heated to 200-250 "C, giving chlorine monofluoride ClF and chlorine trifluoride ClF 3. ClF 5 is also known, obtained by fluoridation of ClF 3 at high temperature and pressure 25 Mn/m2 (250 kgf/cm2). Bromine and iodine ignite in a fluorine atmosphere at ordinary temperatures, and BrF 3, BrF 5, IF 3, IF 2 can be obtained. Fluorine reacts directly with krypton, xenon and radon to form the corresponding fluorides (for example, XeF 4, XeF 6, KrF 2). Xenon oxyfluorides are also known.

The interaction of fluorine with sulfur is accompanied by the release of heat and leads to the formation of numerous sulfur fluorides. Selenium and tellurium form higher fluorides SeF 6 and TeF 6 . Fluorine and hydrogen react with combustion; this produces hydrogen fluoride. This is a radical chain branching reaction:

HF* + H 2 = HF + H 2 *; H 2 * + F 2 = HF + H + F

(where HF* and H 2 * are molecules in a vibrationally excited state); the reaction is used in chemical lasers. Fluorine reacts with nitrogen only in an electrical discharge. Charcoal, when interacting with fluorine, ignites at ordinary temperatures; graphite reacts with it under strong heating, and the formation of solid graphite fluoride (CF) X or gaseous perfluorocarbons CF 4, C 2 F 6 and others is possible. Fluorine reacts with boron, silicon, phosphorus, and arsenic in the cold, forming the corresponding fluorides.

Fluorine combines vigorously with most metals; alkali and alkaline earth metals ignite in an atmosphere of fluorine in the cold, Bi, Sn, Ti, Mo, W - with slight heating. Hg, Pb, U, V react with Fluorine at room temperature, Pt - at a dark red heat temperature. When metals react with fluorine, higher fluorides are usually formed, for example UF 6, MoF 6, HgF 2. Some metals (Fe, Cu, Al, Ni, Mg, Zn) react with Fluorine to form a protective film of fluorides, preventing further reaction.

When fluorine interacts with metal oxides in the cold, metal fluorides and oxygen are formed; The formation of metal oxyfluorides (for example, MoO 2 F 2) is also possible. Non-metal oxides either add Fluorine, for example SO 2 + F 2 = SO 2 F 2, or the oxygen in them is replaced by Fluorine, for example SiO 2 + 2F 2 = SiF 4 + O 2. Glass reacts very slowly with Fluorine; in the presence of water the reaction proceeds quickly. Water interacts with Fluorine: 2H 2 O + 2F 2 = 4HF + O 2; in this case, OF 2 and hydrogen peroxide H 2 O 2 are also formed. Nitrogen oxides NO and NO 2 easily add fluorine to form nitrosyl fluoride FNO and nitrile fluoride FNO 2 , respectively. Carbon monoxide adds fluorine when heated to form carbonyl fluoride:

CO + F 2 = COF 2.

Metal hydroxides react with Fluorine to form metal fluoride and oxygen, e.g.

2Ba(OH) 2 + 2F 2 = 2BaF 2 + 2H 2 O + O 2.

Aqueous solutions of NaOH and KOH react with Fluorine at 0°C to form OF 2 .

Metal or nonmetal halides react with fluorine in the cold, with fluorine replacing all halogens.

Sulfides, nitrides and carbides are easily fluorinated. Metal hydrides form metal fluoride and HF with fluorine in the cold; ammonia (in vapor) - N 2 and HF. Fluorine replaces hydrogen in acids or metals in their salts, for example HNO 3 (or NaNO 3) + F 2 = FNO 3 + HF (or NaF); under more severe conditions, Fluorine displaces oxygen from these compounds, forming sulfuryl fluoride, for example

Na 2 SO 4 + 2F 2 = 2NaF + SO 2 F 2 + O 2.

Carbonates of alkali and alkaline earth metals react with fluorine at ordinary temperatures; this produces the corresponding fluoride, CO 2 and O 2 .

Fluorine reacts vigorously with organic substances.

Obtaining Fluorine. The source for the production of fluorine is hydrogen fluoride, which is obtained mainly either by the action of sulfuric acid H 2 SO 4 · on fluorite CaF 2, or by processing apatites and phosphorites. Fluorine production is carried out by electrolysis of the melt of acidic potassium fluoride KF-(1.8-2.0)HF, which is formed when the KF-HF melt is saturated with hydrogen fluoride to a content of 40-41% HF. The material for the electrolyzer is usually steel; electrodes - carbon anode and steel cathode. Electrolysis is carried out at 95-100 °C and a voltage of 9-11 V; Fluorine current output reaches 90-95%. The resulting fluorine contains up to 5% HF, which is removed by freezing followed by absorption with sodium fluoride. Fluorine is stored in a gaseous state (under pressure) and in liquid form (when cooled with liquid nitrogen) in devices made of nickel and alloys based on it (Monel metal), copper, aluminum and its alloys, brass, stainless steel.

Application of Fluorine. Gaseous Fluorine is used for the fluorination of UF 4 into UF 6, used for isotope separation of uranium, as well as for the production of chlorine trifluoride ClF 3 (fluorinating agent), sulfur hexafluoride SF 6 (gaseous insulator in the electrical industry), metal fluorides (for example, W and V ). Liquid Fluorine is an oxidizer for rocket fuels.

Numerous Fluorine compounds are widely used - hydrogen fluoride, aluminum fluoride, silicofluorides, fluorosulfonic acid (solvent, catalyst, reagent for the production of organic compounds containing the group - SO 2 F), BF 3 (catalyst), organofluorine compounds and others.

Safety precautions. Fluorine is toxic, its maximum permissible concentration in the air is approximately 2·10 -4 mg/l, and the maximum permissible concentration with exposure for no more than 1 hour is 1.5·10 -3 mg/l.

Fluoride in the body. Fluorine is constantly included in animal and plant tissues; microelement In the form of inorganic compounds it is found mainly in the bones of animals and humans - 100-300 mg/kg; There is especially a lot of fluoride in teeth. The bones of marine animals are richer in fluorine compared to the bones of land animals. It enters the body of animals and humans mainly with drinking water, the optimal fluorine content in which is 1-1.5 mg/l. With a lack of fluoride, a person develops dental caries, and with an increased intake - fluorosis. High concentrations of fluorine ions are dangerous due to their ability to inhibit a number of enzymatic reactions, as well as to bind biologically important elements. (P, Ca, Mg and others), disrupting their balance in the body. Organic fluoride derivatives are found only in some plants (for example, in the South African Dichapetalum cymosum). The main ones are derivatives of fluoroacetic acid, toxic to both other plants and animals. A connection has been established between fluoride metabolism and the formation of skeletal bone tissue and especially teeth.

Fluorine poisoning is possible among workers in the chemical industry, during the synthesis of fluorine-containing compounds and in the production of phosphate fertilizers. Fluoride irritates the respiratory tract and causes skin burns. In acute poisoning, irritation of the mucous membranes of the larynx and bronchi, eyes, salivation, and nosebleeds occur; in severe cases - pulmonary edema, damage to the central nervous system and others; in chronic cases - conjunctivitis, bronchitis, pneumonia, pneumosclerosis, fluorosis. Skin lesions such as eczema are characteristic. First aid: rinsing the eyes with water, for skin burns - irrigation with 70% alcohol; in case of inhalation poisoning - inhalation of oxygen. Prevention: compliance with safety regulations, wearing special clothing, regular medical examinations, inclusion of calcium and vitamins in the diet.

1.3 Chlorine

Chlorine(lat. Chlorum), Cl, chemical element of group VII of the periodic system of Mendeleev, atomic number 17, atomic mass 35.453; belongs to the halogen family. Under normal conditions (0°C, 0.1 Mn/m2, or 1 kgf/cm2) it is a yellow-green gas with a pungent irritating odor. Natural Chlorine consists of two stable isotopes: 35 Cl (75.77%) and 37 Cl (24.23%). Radioactive isotopes with mass numbers 31-47 have been artificially obtained, in particular: 32, 33, 34, 36, 38, 39, 40 with half-lives (T S) of 0.31, respectively; 2.5; 1.56 sec; 3.1·105 years; 37.3, 55.5 and 1.4 min. 36Cl and 38Cl are used as isotopic tracers.

Historical reference. Chlorine was first obtained in 1774 by K. Scheele by reacting hydrochloric acid with pyrolusite MnO 2 . However, only in 1810 G. Davy established that chlorine is an element and named it chlorine (from the Greek chloros - yellow-green). In 1813, J. L. Gay-Lussac proposed the name Chlorine for this element.

Distribution of Chlorine in nature. Chlorine occurs in nature only in the form of compounds. The average content of Chlorine in the earth's crust (clarke) is 1.7·10 -2% by mass, in acidic igneous rocks - granites and others - 2.4·10 -2, in basic and ultrabasic rocks 5·10 -3. The main role in the history of chlorine in the earth's crust is played by water migration. In the form of Cl ion, it is found in the World Ocean (1.93%), underground brines and salt lakes. The number of its own minerals (mainly natural chlorides) is 97, the main one being halite NaCl (Rock salt). Large deposits of potassium and magnesium chlorides and mixed chlorides are also known: sylvinite KCl, sylvinite (Na,K)Cl, carnalite KCl MgCl 2 6H 2 O, kainite KCl MgSO 4 3H 2 O, bischofite MgCl 2 6H 2 O In the history of the Earth, the supply of HCl contained in volcanic gases to the upper parts of the earth's crust was of great importance.

Physical properties of Chlorine. Chlorine has a boiling point of -34.05°C, a melting point of -101°C. The density of chlorine gas under normal conditions is 3.214 g/l; saturated steam at 0°C 12.21 g/l; liquid Chlorine at a boiling point of 1.557 g/cm3; solid Chlorine at - 102°C 1.9 g/cm 3 . Saturated vapor pressure of Chlorine at 0°C 0.369; at 25°C 0.772; at 100°C 3.814 Mn/m 2 or, respectively, 3.69; 7.72; 38.14 kgf/cm2. Heat of fusion 90.3 kJ/kg (21.5 cal/g); heat of evaporation 288 kJ/kg (68.8 cal/g); The heat capacity of gas at constant pressure is 0.48 kJ/(kg K). Critical constants of Chlorine: temperature 144°C, pressure 7.72 Mn/m2 (77.2 kgf/cm2), density 573 g/l, specific volume 1.745·10 -3 l/g. Solubility (in g/l) of Chlorine at a partial pressure of 0.1 Mn/m2, or 1 kgf/cm2, in water 14.8 (0°C), 5.8 (30°C), 2.8 ( 70°C); in a solution of 300 g/l NaCl 1.42 (30°C), 0.64 (70°C). Below 9.6°C, Chlorine hydrates of variable composition Cl 2 ·nH 2 O (where n = 6-8) are formed in aqueous solutions; These are yellow cubic crystals that decompose with increasing temperature into Chlorine and water. Chlorine is highly soluble in TiCl 4, SiCl 4, SnCl 4 and some organic solvents (especially hexane C 6 H 14 and carbon tetrachloride CCl 4). The Chlorine molecule is diatomic (Cl 2). The degree of thermal dissociation of Cl 2 + 243 kJ = 2Cl at 1000 K is 2.07·10 -4%, at 2500 K 0.909%.

Chemical properties of Chlorine. External electronic configuration of the Cl 3s 2 Sp 5 atom. In accordance with this, Chlorine in compounds exhibits oxidation states of -1, +1, +3, +4, +5, +6 and +7. The covalent radius of the atom is 0.99 E, the ionic radius of Cl is 1.82 E, the electron affinity of the Chlorine atom is 3.65 eV, and the ionization energy is 12.97 eV.

Chemically, Chlorine is very active, directly combines with almost all metals (with some only in the presence of moisture or when heated) and with non-metals (except carbon, nitrogen, oxygen, inert gases), forming the corresponding chlorides, reacts with many compounds, replaces hydrogen in saturated hydrocarbons and joins unsaturated compounds. Chlorine displaces bromine and iodine from their compounds with hydrogen and metals; Of the compounds of chlorine with these elements, it is replaced by fluorine. Alkali metals in the presence of traces of moisture react with Chlorine with ignition; most metals react with dry Chlorine only when heated. Steel, as well as some metals, are resistant in an atmosphere of dry Chlorine at low temperatures, so they are used for the manufacture of equipment and storage facilities for dry Chlorine. Phosphorus ignites in an atmosphere of Chlorine, forming PCl 3, and with further chlorination - PCl 5; sulfur with Chlorine when heated gives S 2 Cl 2, SCl 2 and other S n Cl m. Arsenic, antimony, bismuth, strontium, tellurium interact vigorously with Chlorine. A mixture of chlorine and hydrogen burns with a colorless or yellow-green flame with the formation of hydrogen chloride (this is a chain reaction).

The maximum temperature of the hydrogen-chlorine flame is 2200°C. Mixtures of chlorine with hydrogen containing from 5.8 to 88.5% H 2 are explosive.

With oxygen, Chlorine forms oxides: Cl 2 O, ClO 2, Cl 2 O 6, Cl 2 O 7, Cl 2 O 8, as well as hypochlorites (salts of hypochlorous acid), chlorites, chlorates and perchlorates. All oxygen compounds of chlorine form explosive mixtures with easily oxidized substances. Chlorine oxides are weakly stable and can spontaneously explode; hypochlorites slowly decompose during storage; chlorates and perchlorates can explode under the influence of initiators.

Chlorine in water hydrolyzes, forming hypochlorous and hydrochloric acids: Cl 2 + H 2 O = HClO + HCl. When aqueous solutions of alkalis are chlorinated in the cold, hypochlorites and chlorides are formed: 2NaOH + Cl 2 = NaClO + NaCl + H 2 O, and when heated, chlorates are formed. Chlorination of dry calcium hydroxide produces bleach.

When ammonia reacts with chlorine, nitrogen trichloride is formed. When chlorinating organic compounds, chlorine either replaces hydrogen or joins multiple bonds, forming various chlorine-containing organic compounds.

Chlorine forms interhalogen compounds with other halogens. Fluorides ClF, ClF 3, ClF 3 are very reactive; for example, in a ClF 3 atmosphere, glass wool spontaneously ignites. Known compounds of chlorine with oxygen and fluorine are Chlorine oxyfluorides: ClO 3 F, ClO 2 F 3, ClOF, ClOF 3 and fluorine perchlorate FClO 4.

Getting Chlorine. Chlorine began to be produced industrially in 1785 by reacting hydrochloric acid with manganese (II) oxide or pyrolusite. In 1867, the English chemist G. Deacon developed a method for producing chlorine by oxidizing HCl with atmospheric oxygen in the presence of a catalyst. Since the late 19th and early 20th centuries, chlorine has been produced by electrolysis of aqueous solutions of alkali metal chlorides. These methods produce 90-95% of Chlorine in the world. Small amounts of Chlorine are obtained by-product in the production of magnesium, calcium, sodium and lithium by electrolysis of molten chlorides. Two main methods of electrolysis of aqueous solutions of NaCl are used: 1) in electrolyzers with a solid cathode and a porous filter diaphragm; 2) in electrolyzers with a mercury cathode. In both methods, Chlorine gas is released on a graphite or oxide titanium-ruthenium anode. According to the first method, hydrogen is released at the cathode and a solution of NaOH and NaCl is formed, from which commercial caustic soda is separated by subsequent processing. According to the second method, sodium amalgam is formed at the cathode; when it is decomposed with pure water in a separate apparatus, a NaOH solution, hydrogen and pure mercury are obtained, which again goes into production. Both methods give 1.125 t of NaOH per 1 ton of Chlorine.

Electrolysis with a diaphragm requires less capital investment to organize the production of Chlorine and produces cheaper NaOH. The mercury cathode method produces very pure NaOH, but the loss of mercury pollutes the environment.

Use of Chlorine. One of the important branches of the chemical industry is the chlorine industry. The main quantities of Chlorine are processed at the site of its production into chlorine-containing compounds. Chlorine is stored and transported in liquid form in cylinders, barrels, railway tanks or in specially equipped vessels. Industrial countries are characterized by the following approximate consumption of Chlorine: for the production of chlorine-containing organic compounds - 60-75%; inorganic compounds containing Chlorine, -10-20%; for bleaching pulp and fabrics - 5-15%; for sanitary needs and water chlorination - 2-6% of total production.

Chlorine is also used to chlorinate some ores to extract titanium, niobium, zirconium and others.

Chlorine in the body. Chlorine is one of the biogenic elements, a constant component of plant and animal tissues. The Chlorine content in plants (a lot of Chlorine in halophytes) ranges from thousandths of a percent to whole percent, in animals - tenths and hundredths of a percent. The daily requirement of an adult for Chlorine (2-4 g) is covered by food products. Chlorine is usually supplied in excess with food in the form of sodium chloride and potassium chloride. Bread, meat and dairy products are especially rich in Chlorine. In the animal body, Chlorine is the main osmotically active substance in blood plasma, lymph, cerebrospinal fluid and some tissues. Plays a role in water-salt metabolism, promoting tissue retention of water. Regulation of acid-base balance in tissues is carried out along with other processes by changing the distribution of Chlorine between the blood and other tissues. Chlorine is involved in energy metabolism in plants, activating both oxidative phosphorylation and photophosphorylation. Chlorine has a positive effect on the absorption of oxygen by roots. Chlorine is necessary for the production of oxygen during photosynthesis by isolated chloroplasts. Most nutrient media for artificial plant cultivation do not contain chlorine. It is possible that very low concentrations of Chlorine are sufficient for plant development.

Chlorine poisoning is possible in the chemical, pulp and paper, textile, pharmaceutical industries and others. Chlorine irritates the mucous membranes of the eyes and respiratory tract. Primary inflammatory changes are usually accompanied by a secondary infection. Acute poisoning develops almost immediately. When inhaling medium and low concentrations of Chlorine, tightness and pain in the chest, dry cough, rapid breathing, pain in the eyes, lacrimation, increased levels of leukocytes in the blood, body temperature, etc. are observed. Bronchopneumonia, toxic pulmonary edema, depression, convulsions are possible . In mild cases, recovery occurs within 3-7 days. As long-term consequences, catarrh of the upper respiratory tract, recurrent bronchitis, pneumosclerosis and others are observed; possible activation of pulmonary tuberculosis. With prolonged inhalation of small concentrations of Chlorine, similar but slowly developing forms of the disease are observed. Prevention of poisoning: sealing production facilities, equipment, effective ventilation, using a gas mask if necessary. The production of chlorine, bleach and other chlorine-containing compounds is classified as production with hazardous working conditions.

1.4 Bromine

Bromine(lat. Bromum), Br, a chemical element of group VII of the periodic system of Mendeleev, belongs to the halogens; atomic number 35, atomic mass 79.904; red-brown liquid with a strong unpleasant odor. Bromine was discovered in 1826 by the French chemist A. J. Balard while studying the brines of the Mediterranean salt fields; named from Greek. bromos - stench. Natural Bromine consists of 2 stable isotopes 79 Br (50.54%) and 81 Br (49.46%). Of the artificially obtained radioactive isotopes, Bromine is the most interesting 80 Br, on the example of which I. V. Kurchatov discovered the phenomenon of isomerism of atomic nuclei.

Distribution of Bromine in nature. The content of Bromine in the earth's crust (1.6·10 -4% by mass) is estimated at 10 15 -10 16 tons. Bromine is found mostly in a dispersed state in igneous rocks, as well as in widespread halides. Bromine is a constant companion of chlorine. Bromide salts (NaBr, KBr, MgBr 2) are found in deposits of chloride salts (in table salt up to 0.03% Br, in potassium salts - sylvite and carnallite - up to 0.3% Br), as well as in sea water (0.065% Br), salt lake brines (up to 0.2% Br) and underground brines commonly associated with salt and oil deposits (up to 0.1% Br). Due to their good solubility in water, bromide salts accumulate in residual brines of sea and lake water bodies. Bromine migrates in the form of easily soluble compounds, very rarely forming solid mineral forms represented by bromyrite AgBr, embolite Ag (Cl, Br) and iodembolite Ag (Cl, Br, I). The formation of minerals occurs in oxidation zones of sulfide silver deposits that form in arid desert areas.

Physical properties of Bromine. At -7.2°C, liquid Bromine solidifies, turning into red-brown needle-shaped crystals with a faint metallic luster. Bromine vapor is yellow-brown in color, boiling point 58.78°C. The density of liquid Bromine (at 20°C) is 3.1 g/cm 3 . Bromine is soluble in water to a limited extent, but better than other halogens (3.58 g of Bromine in 100 g of H 2 O at 20 ° C). Below 5.84°C, garnet-red crystals of Br 2 8H 2 O precipitate from water. Bromine is especially soluble in many organic solvents, which is used to extract it from aqueous solutions. Bromine in solid, liquid and gaseous states consists of 2-atomic molecules. Noticeable dissociation into atoms begins at a temperature of about 800°C; dissociation is also observed under the influence of light.

Chemical properties of Bromine. The configuration of the outer electrons of the Bromine atom is 4s 2 4p 5. The valence of Bromine in compounds is variable, the oxidation state is -1 (in bromides, for example KBr), +1 (in hypobromites, NaBrO), +3 (in bromites, NaBrO 2), +5 (in bromates, KBrOz) and +7 ( in perbromates, NaBrO 4). Chemically, Bromine is very active, occupying a place in reactivity between chlorine and iodine. The interaction of Bromine with sulfur, selenium, tellurium, phosphorus, arsenic and antimony is accompanied by strong heating, sometimes even the appearance of a flame. Bromine also reacts vigorously with some metals, such as potassium and aluminum. However, many metals react with anhydrous Bromine with difficulty due to the formation of a protective film of bromide, which is insoluble in Bromine, on their surface. Of the metals, the most resistant to the action of Bromine, even at elevated temperatures and in the presence of moisture, are silver, lead, platinum and tantalum (gold, unlike platinum, reacts vigorously with Bromine). Bromine does not directly combine with oxygen, nitrogen and carbon, even at elevated temperatures. Bromine compounds with these elements are obtained indirectly. These are the extremely fragile oxides Br 2 O, Br O 2 and Br 3 O 8 (the latter is obtained, for example, by the action of ozone on Bromine at 80°C). Bromine reacts directly with halogens, forming BrF 3, BrF 5, BrCl, IBr and others.

Bromine is a strong oxidizing agent. Thus, it oxidizes sulfites and thiosulfates in aqueous solutions to sulfates, nitrites to nitrates, ammonia to free nitrogen (3Br 2 + 8NH 3 = N 2 + NH 4 Br). Bromine displaces iodine from its compounds, but is itself displaced by chlorine and fluorine. Free Bromine is released from aqueous solutions of bromides also under the influence of strong oxidizing agents (KMnO 4, K 2 Cr 2 O 7) in an acidic environment. When dissolved in water, Bromine partially reacts with it (Br 2 + H 2 O = HBr + HBrO) to form hydrobromic acid HBr and unstable hypobromous acid HBrO. A solution of Bromine in water is called bromine water. When Bromine dissolves in alkali solutions in the cold, bromide and hypobromite are formed (2NaOH + Br 2 = NaBr + NaBrO + H 2 O), and at elevated temperatures (about 100 ° C) - bromide and bromate (6NaOH + 3Br 2 = 5NaBr + NaBrO 3 + 3H 2 O). Of the reactions of Bromine with organic compounds, the most typical are addition at the C=C double bond, as well as substitution of hydrogen (usually under the action of catalysts or light).

Obtaining Bromine. The starting materials for the production of Bromine are sea water, lake and underground brines and potassium production liquors containing Bromine in the form of bromide ion Br - (from 65 g/m 3 in sea water to 3-4 kg/m 3 and higher in potassium liquors production). Bromine is isolated with the help of chlorine (2Br - + Cl 2 = Br 2 + 2Cl -) and distilled from the solution with water vapor or air. Steam stripping is carried out in columns made of granite, ceramics or other material resistant to bromine. Heated brine is fed into the column from above, and chlorine and water vapor are supplied from below. Bromine vapor leaving the column is condensed in ceramic refrigerators. Next, Bromine is separated from water and purified from chlorine impurities by distillation. Air stripping makes it possible to use brines with a low content of bromine to obtain bromine; it is unprofitable to extract bromine from it by steam as a result of high steam consumption. Bromine is removed from the resulting bromine-air mixture using chemical absorbents. For this, solutions of iron bromide (2FeBr 2 + Br 2 = 2FeBr 3) are used, which, in turn, is obtained by reducing FeBr 3 with iron filings, as well as solutions of sodium hydroxides or carbonates or gaseous sulfur dioxide reacting with Bromine in the presence of water vapor with the formation of hydrobromic and sulfuric acids (Br 2 + SO 2 + 2H 2 O = 2HBr + H 2 SO 4). Bromine is isolated from the resulting intermediates by the action of chlorine (from FeBr 3 and HBr) or acid (5NaBr + NaBrO 3 + 3 H 2 SO 4 = 3Br 2 + 3Na 2 SO 4 + 3H 2 O). If necessary, intermediate products are processed into bromide compounds without releasing elemental Bromine.

Inhalation of Bromine vapors when their content in the air is 1 mg/m3 or more causes cough, runny nose, nosebleeds, dizziness, headache; at higher concentrations - suffocation, bronchitis, and sometimes death. The maximum permissible concentration of Bromine vapor in the air is 2 mg/m3. Liquid Bromine acts on the skin, causing poorly healing burns. Work with Bromine should be carried out in fume hoods. In case of poisoning with Bromine vapor, it is recommended to inhale ammonia, using for this purpose a highly diluted solution of it in water or ethyl alcohol. Sore throat caused by inhaling Bromine vapor is relieved by ingesting hot milk. Bromine that gets on the skin is washed off with plenty of water or blown off with a strong stream of air. Burnt areas are lubricated with lanolin.

Application of Bromine. Bromine is used quite widely. It is the starting product for the production of a number of bromide salts and organic derivatives. Large quantities of Bromine are used to produce ethyl bromide and dibromoethane - components of the ethyl liquid added to gasoline to increase their detonation resistance. Bromine compounds are used in photography, in the production of a number of dyes, methyl bromide and some other Bromine compounds are used as insecticides. Some organic bromine compounds serve as effective fire extinguishing agents. Bromine and bromine water are used in chemical analyzes to determine many substances. In medicine, sodium, potassium, ammonium bromides are used, as well as organic bromine compounds, which are used for neuroses, hysteria, increased irritability, insomnia, hypertension, epilepsy and chorea.

Bromine in the body. Bromine - constant component tissues of animals and plants. Terrestrial plants contain on average 7·10 -4% Bromine in raw matter, animals ~1·10 -4%. Bromine is found in various secretions (tears, saliva, sweat, milk, bile). In blood healthy person Bromine content ranges from 0.11 to 2.00 mg%. Using radioactive Bromine (82 Br), its selective absorption by the thyroid gland, the medulla of the kidneys and the pituitary gland was established. Bromides introduced into the body of animals and humans increase the concentration of inhibitory processes in the cerebral cortex and help normalize the state of the nervous system, which has suffered from overstrain of the inhibitory process. At the same time, lingering in the thyroid gland, Bromine enters into a competitive relationship with iodine, which affects the activity of the gland, and in connection with this, the state of metabolism.

1.5 Iodine

Iodine(lat. Iodum), I, a chemical element of group VII of the periodic system of Mendeleev, belongs to the halogens (the outdated name Iodine and the symbol J are also found in the literature); atomic number 53, atomic mass 126.9045; crystals of black-gray color with a metallic sheen. Natural iodine consists of one stable isotope with a mass number of 127. Iodine was discovered in 1811 by the French chemist B. Courtois. By heating the mother brine of seaweed ash with concentrated sulfuric acid, he observed the release of violet vapor (hence the name Iodine - from the Greek iodes, ioides - violet-like in color, violet), which condensed into dark shiny plate-like crystals. In 1813-1814, the French chemist J. L. Gay-Lussac and the English chemist G. Davy proved the elemental nature of iodine.

Distribution of iodine in nature. The average iodine content in the earth's crust is 4·10 -5% by mass. Iodine compounds are scattered in the mantle and magmas and in the rocks formed from them (granites, basalts and others); deep minerals of Iodine are unknown. The history of iodine in the earth's crust is closely related to living matter and biogenic migration. In the biosphere, processes of its concentration are observed, especially by marine organisms (algae, sponges and others). Eight supergene iodine minerals are known to form in the biosphere, but they are very rare. The main reservoir of iodine for the biosphere is the World Ocean (1 liter contains on average 5·10 -5 g of iodine). From the ocean, iodine compounds dissolved in drops of sea water enter the atmosphere and are carried by winds to the continents. (Areas remote from the ocean or fenced off from sea winds by mountains are depleted in iodine.) Iodine is easily adsorbed by organic matter in soils and marine silts. When these silts become compacted and sedimentary rocks form, desorption occurs and some of the iodine compounds pass into groundwater. This is how iodine-bromine waters used for the extraction of iodine are formed, especially characteristic of oil field areas (in some places, 1 liter of these waters contains over 100 mg of iodine).

Physical properties of Iodine. Density of Iodine 4.94 g/cm3, melting point 113.5°C, boiling point 184.35°C. The molecule of liquid and gaseous iodine consists of two atoms (I 2). A noticeable dissociation of I 2 = 2I is observed above 700 °C, as well as under the influence of light. Already at ordinary temperatures, iodine evaporates, forming a sharp-smelling purple vapor. When heated slightly, iodine sublimes, settling in the form of shiny thin plates; this process serves to purify iodine in laboratories and industry. Iodine is poorly soluble in water (0.33 g/l at 25 °C), well soluble in carbon disulfide and organic solvents (benzene, alcohol and others), as well as in aqueous solutions of iodides.

Chemical properties of Iodine. The configuration of the outer electrons of the Iodine atom is 5s 2 5p 5. In accordance with this, iodine exhibits variable valence (oxidation state) in compounds: -1 (in HI, KI), +1 (in HIO, KIO), +3 (in ICl 3), +5 (in HIO 3, KIO 3 ) and +7 (in HIO 4, KIO 4). Chemically, iodine is quite active, although to a lesser extent than chlorine and bromine. Iodine reacts vigorously with metals when slightly heated, forming iodides (Hg + I 2 = HgI 2). Iodine reacts with hydrogen only when heated and not completely, forming hydrogen iodide. Iodine does not combine directly with carbon, nitrogen, or oxygen. Elemental Iodine is an oxidizing agent, less powerful than chlorine and bromine. Hydrogen sulfide H 2 S, sodium thiosulfate Na 2 S 2 O 3 and other reducing agents reduce it to I - (I 2 + H 2 S = S + 2HI). Chlorine and other strong oxidizing agents in aqueous solutions convert it into IO 3 - (5Cl 2 + I 2 + 6H 2 O = 2HIO 3 H + 10HCl). When dissolved in water, iodine partially reacts with it (I 2 + H 2 O = HI + HIO); in hot aqueous solutions of alkalis, iodide and iodate are formed (3I 2 + 6NaOH = 5NaI + NaIO 3 + 3H 2 O). When adsorbed on starch, iodine turns it dark blue; it is used in iodometry and qualitative analysis for the detection of Iodine.

Iodine vapors are poisonous and irritate mucous membranes. Iodine has a cauterizing and disinfecting effect on the skin. Iodine stains are washed off with solutions of soda or sodium thiosulfate.

Obtaining Iodine. The raw material for the industrial production of iodine is oil drilling water; seaweed, as well as mother solutions of Chilean (sodium) nitrate containing up to 0.4% Iodine in the form of sodium iodate. To extract iodine from oil waters (usually containing 20-40 mg/l Iodine in the form of iodides), they are first treated with chlorine (2 NaI + Cl 2 = 2NaCl + I 2) or nitrous acid (2NaI + 2NaNO 2 + 2H 2 SO 4 = 2Na 2 SO 4 + 2NO + I 2 + 2H 2 O). The released iodine is either adsorbed by active carbon or blown out with air. Iodine adsorbed by coal is treated with caustic alkali or sodium sulfite (I 2 + Na 2 SO 3 + H 2 O = Na 2 SO 4 + 2HI). Free Iodine is isolated from the reaction products by the action of chlorine or sulfuric acid and an oxidizing agent, for example, potassium dichromate (K 2 Cr 2 O 7 + 7H 2 SO 4 + 6NaI = K 2 SO 4 + 3Na 2 SO 4 + Cr 2 (SO 4)S + 3I 2). When blown out with air, iodine is absorbed by a mixture of sulfur oxide (IV) with water vapor (2H 2 O + SO 2 + I 2 = H 2 SO 4 + 2HI) and then Iodine is replaced with chlorine (2HI + Cl 2 = 2HCl + I 2). Crude crystalline iodine is purified by sublimation.

Application of Iodine. Iodine and its compounds are used mainly in medicine and analytical chemistry, as well as in organic synthesis and photography.

Iodine in the body. Iodine is a microelement essential for animals and humans. In soils and plants of taiga-forest non-chernozem, dry steppe, desert and mountain biogeochemical zones, iodine is contained in insufficient quantities or is not balanced with some other microelements (Co, Mn, Cu); This is associated with the spread of endemic goiter in these areas. The average iodine content in soils is about 3·10 -4%, in plants about 2·10 -5%. There is little iodine in surface drinking waters (from 10 -7 to 10 -9%). In coastal areas, the amount of iodine in 1 m 3 of air can reach 50 mcg, in continental and mountainous areas it is 1 or even 0.2 mcg.

The absorption of iodine by plants depends on the content of its compounds in the soil and on the type of plant. Some organisms (so-called iodine concentrators), for example, seaweed - fucus, kelp, phyllophora, accumulate up to 1% Iodine, some sponges - up to 8.5% (in the skeletal substance spongin). Algae that concentrate iodine are used for its industrial production. Iodine enters the animal body with food, water, and air. The main source of iodine is plant products and feed. Iodine absorption occurs in the anterior sections of the small intestine. The human body accumulates from 20 to 50 mg of iodine, including about 10-25 mg in the muscles, and 6-15 mg in the thyroid gland. Using radioactive iodine (131 I and 125 I), it was shown that in the thyroid gland Iodine accumulates in the mitochondria of epithelial cells and is part of the diiodo- and monoiodotyrosines formed in them, which condense into the hormone tetraiodothyronine (thyroxine). Iodine is excreted from the body mainly through the kidneys (up to 70-80%), mammary, salivary and sweat glands, partly with bile.

In different biogeochemical provinces, the iodine content in the daily diet varies (for humans from 20 to 240 mcg, for sheep from 20 to 400 mcg). An animal's need for iodine depends on its physiological state, time of year, temperature, and the body's adaptation to the iodine content in the environment. The daily need for Iodine in humans and animals is about 3 mcg per 1 kg of body weight (increases during pregnancy, increased growth, and cooling). The introduction of Iodine into the body increases basal metabolism, enhances oxidative processes, tones muscles, and stimulates sexual function.

Due to a greater or lesser deficiency of Iodine in food and water, iodization of table salt is used, usually containing 10-25 g of potassium iodide per 1 ton of salt. The use of fertilizers containing iodine can double or triple its content in crops.

Iodine in medicine. Preparations containing iodine have antibacterial and antifungal properties, they also have an anti-inflammatory and distracting effect; They are used externally to disinfect wounds and prepare the surgical field. When taken orally, Iodine preparations affect metabolism and enhance thyroid function. Small doses of Iodine (microiodine) inhibit the function of the thyroid gland, affecting the formation of thyroid-stimulating hormone in the anterior pituitary gland. Since iodine affects protein and fat (lipid) metabolism, it has found application in the treatment of atherosclerosis, as it reduces cholesterol in the blood; also increases the fibrinolytic activity of the blood. For diagnostic purposes, radiopaque agents containing iodine are used.

With prolonged use of Iodine preparations and with increased sensitivity to them, iodism may appear - runny nose, urticaria, Quincke's edema, salivation and lacrimation, acne-like rash (iododerma), etc. Iodine preparations should not be taken in case of pulmonary tuberculosis, pregnancy, kidney disease, chronic pyoderma, hemorrhagic diathesis, urticaria.

Iodine is radioactive. Artificially radioactive isotopes of Iodine - 125 I, 131 I, 132 I and others are widely used in biology and especially in medicine to determine the functional state of the thyroid gland and treat a number of its diseases. The use of radioactive iodine in diagnostics is associated with the ability of iodine to selectively accumulate in the thyroid gland; use for medicinal purposes is based on the ability of beta-radiation of iodine radioisotopes to destroy the secretory cells of the gland. When the environment is contaminated with nuclear fission products, radioactive isotopes of iodine quickly enter the biological cycle, ultimately ending up in milk and, consequently, in the human body. Their penetration into the body of children, whose thyroid gland is 10 times smaller than that of adults and also has greater radiosensitivity, is especially dangerous. In order to reduce the deposition of radioactive isotopes of iodine in the thyroid gland, it is recommended to use stable iodine preparations (100-200 mg per dose). Radioactive iodine is quickly and completely absorbed from the gastrointestinal tract and selectively deposited in the thyroid gland. Its absorption depends on the functional state of the gland. Relatively high concentrations of radioisotopes of Iodine are also found in the salivary and mammary glands and the mucous membrane of the gastrointestinal tract. Radioactive iodine not absorbed by the thyroid gland is almost completely and relatively quickly excreted in the urine.

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Bromine and iodine are trace elements and do not form their own mineral deposits. Significant quantities of bromine and iodine, along with other halogens, are found in sea water in the form of various salts, from where they are actively absorbed by algae.

The structure of halogen atoms, their physical and chemical properties

The atoms of all halogens contain 7 electrons in their outer electron layer, the structure of which can be represented as follows: Fluorine differs from other halogens in that it does not have a d-sublevel on its outer electronic layer. Some

physical characteristics


halogen atoms are presented in Table 4.
Table 4. Comparative characteristics of some physical properties of halogens and the simple substances they form
Orbital radius of an atom, nm
First ionization energy (Г 0 – 1 ē → Г +1), kJ/mol
Electron affinity, kJ/mol
Electronegativity according to Pauling
Binding energy in a molecule of a simple substance G 2, kJ/mol

In the normal state, elements of group VIIA have one unpaired electron on their outer layer, so they can form only one covalent bond with other atoms through the exchange mechanism (valence in this case will be equal to 1). Upon excitation, the number of unpaired electrons in halogens (except F) increases to 3, 5 or 7 due to the pairing of electron pairs.

Accordingly, the possible valence values during bond formation via the exchange mechanism in this case will also be equal to 3, 5 or 7.

Fluorine, unlike all other halogens, usually exhibits a valence of 1, because

he cannot vaporize his electron pairs. Theoretically, fluorine, as an element of the second period, can exhibit a maximum valence of 4, if we take into account, in addition to the exchange mechanism, the donor-acceptor mechanism of covalent bond formation. Indeed, on the outer electron layer of the fluorine atom, along with one unpaired electron, there are also 3 electron pairs. Due to them, acting as a donor, fluorine can additionally form 3 covalent bonds. True, one can assume that fluorine will be “reluctant” to do this, because is the most electronegative element and it is more typical for it to take away electron pairs than to provide it to another atom, albeit for shared use.

The compound BF is known, in which the bond multiplicity is 3. This fact can be explained if we assume that the fluorine atom formed one bond by an exchange mechanism, and the other two by a donor-acceptor mechanism.FHalogens (except

) in compounds can exhibit both positive and negative oxidation states.

They exhibit a positive oxidation state when interacting with atoms of elements that are more electronegative than themselves. In this case, the halogens act as a reducing agent and give away their unpaired electrons to other atoms from the outer layer. The value of the oxidation state will be equal to +1 (in a stationary state), +3, +5, +7 (in an excited state).

Fluorine cannot exhibit a positive oxidation state, since it is the most electronegative element and in chemical reactions always takes electrons from other atoms, acting only as an oxidizing agent, and in all compounds it exhibits an oxidation state of -1. F 2 For this reason, receiving F - from fluorides chemically (using atoms of another element, i.e. oxidation 2 F 2 0 )) cannot be carried out. This can only be done electrically (by electrolysis of a fluoride melt, for example, saltNaF ).

Other halogens exhibit a negative oxidation state when interacting with atoms of elements less electronegative than themselves. In this case, they act as an oxidizing agent and take from other atoms one electron missing to complete their outer layer. The value of the oxidation state is equal to -1.

Halogens form general compounds with hydrogen
.

These are gaseous substances (boiling point HF ≈ 16 o C), highly soluble in H 2 O. Their aqueous solutions have acidic properties, and the strength of these acids in the series HF, HCl, HBr, HI increases from left to right. The weakest acid is HF, the strongest is HI. This is due to the fact that in the group from top to bottom the radii of the halogen atoms increase, which weakens the strength R-H connections

(since its length increases) and H + ions are more easily split off.

Hydrofluoric, or hydrofluoric, acid HF is to a certain extent weaker than all other hydrohalic acids, and due to the ability of its molecules to form associates of type (HF) n (where n varies from 1 to 8) due to the formation of hydrogen bonds:

H – F  H – F;FH – F  H – F  H – F, etc.

With oxygen, halogens (except) can form 4 types of oxides: 2 H, ) can form 4 types of oxides: 2 H 7; (Oxides obtained for chlorineBr 2 HClI 2 H, I 2 H 5 , I 2 H 7 for bromine -; 2 H 3 for iodine –

.
Oxide
R

in free form is not isolated for any halogen). Oxides obtained for chlorine:

And, in which it exhibits uncharacteristic oxidation states of +4 and +6. These are valently unsaturated compounds prone to dimerization. They have paramagnetic properties, because the chlorine atoms contain an unpaired electron. All oxides are obtained not through the direct interaction of simple halogen substances with oxygen, but indirectly. These are acidic oxides. When dissolved in H 2 O, they form acids All oxides are obtained not through the direct interaction of simple halogen substances with oxygen, but indirectly. These are acidic oxides. When dissolved in H 2 O, they form acids general view For each element, as its oxidation state increases, the strength of the acids in that series increases from left to right. The strength of acids in which elements exhibit the same degree of oxidation decreases in the group from top to bottom. For example, in the row:HO,H

O-
Oxide
, in which O exhibits a positive oxidation state of +2 or +1. Therefore, these substances are not oxides.

Like all oxygen-containing halogen compounds, they are also mainly obtained indirectly.F 2 Halogens form simple substances (with the same names), the molecules of which consist of two atoms connected by a single covalent bond.) can form 4 types of oxides: 2 MoreoverBr 2 AndI 2 under normal conditions - gases,

– liquid,

- solid substance.

The bond strength in the molecules of simple substances decreases from chlorine to iodine. F2 falls out of this pattern, the bond strength of which is significantly less than the bond strength in the Cl 2 molecule (Table 4).

Such anomalous properties of fluorine can be explained by the absence of a vacant d-sublevel in the outer electronic layer of its atoms.

The atoms of chlorine and other halogens have free d-orbitals and therefore there is an additional donor-acceptor interaction between them in the molecules of simple substances, which strengthens the bond. This is shown in the following diagram: As follows from Table 4, the ionization energy, electron affinity energy and relative electronegativity of the halogen atoms in the group decrease from top to bottom. In accordance with this, the nonmetallic properties of halogens, and, therefore, the oxidizing ability of their atoms and the simple substances they form in the group from top to bottom will also decrease.

Each upstream halogen can displace downstream ones from their compounds with hydrogen and metals.

For example, Cl 2 can displace Br 2 and I 2.

And Br 2 can only displace I 2:

Сl 2 + 2HBr = Br 2 + 2HCl

Br 2 + 2NaІ = І 2 + 2NaBr

These reactions usually occur in aqueous solutions, so F2 does not participate in them, since it energetically decomposes water:
2 F 2 + 2H 2 O = 4 HF + O 2

The remaining halogens are relatively slightly soluble in H 2 O and, to an even lesser extent, reversibly interact with it according to the following scheme:

G 2 + H 2 O ) can form 4 types of oxides: 2 Halogens form simple substances (with the same names), the molecules of which consist of two atoms connected by a single covalent bond. Br 2 NG + NGO Moreover, when moving from chlorine to iodine, the equilibrium of this reaction shifts more and more to the left, and for I 2 it is practically uncharacteristic.

General characteristics of elements of group VII. Properties of elements VII (17) of group of the main subgroup Characteristics of group 7 of the periodic system

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