Chemical element fluorine: valence, properties, characteristics

Fluorine (F) is the most reactive chemical element and the lightest halogen of group 17 (VIIa) of the periodic table. This characteristic of fluorine is explained by its ability to attract electrons (this is the most electronegative element) and the small size of its atoms.

History of the discovery

Fluorine-containing mineral fluorspar was described in 1529 by a German physician and mineralogist Georgy Agricola. It is probable that hydrofluoric acid was first obtained by an unknown British glass blower in 1720. And in 1771 the Swedish chemist Karl Wilhelm Scheele received unrefined hydrofluoric acid by heating the fluorspar with concentrated sulfuric acid in a glass retort that was largely corroded by the product . Therefore, in subsequent experiments, metal vessels were used. Almost anhydrous acid was obtained in 1809, and two years later the French physicist André-Marie Ampere suggested that this is a hydrogen compound with an unknown element similar to chlorine, for which he proposed the name fluorine from the Greek φθόριος, which "destroys". Fluoride was calcium fluoride.

The release of fluorine remained one of the main unresolved problems of inorganic chemistry until 1886, when the French chemist Henri Moissan obtained the element by electrolysis of a solution of potassium hydrofluoride in hydrogen fluoride. For this in 1906 he received the Nobel Prize. Difficulty in handling this element and the toxic properties of fluorine contributed to the slow progress in the chemistry of this element. Until the Second World War it was a laboratory wonder. Then, however, the use of uranium hexafluoride in the separation of uranium isotopes along with the growth of the industrial value of the organic compounds of this element made it a chemical that is of considerable benefit.

Prevalence

Fluorine-containing fluorite (fluorite, CaF ₂ ) for many centuries has been used as a flux (cleaning agent) in metallurgical processes. The mineral subsequently turned out to be the source of the element, which was also called fluor. Colorless transparent fluorite crystals under illumination have a bluish tinge. This property is known as fluorescence.

Fluorine is an element that occurs in nature only in the form of its chemical compounds, with the exception of extremely small amounts of a free element in fluorspar exposed to radiation from radium. The content of the element in the earth's crust is about 0.065%. The main fluorine-containing minerals are fluorspar, cryolite (Na ₃ AlF ₆ ), fluorapatite (Ca ₅ [PO ₄ ] ₃ [F, Cl]), topaz (Al ₂ SiO ₄ [F, OH] ₂ ) and lepidolite.

Physical and chemical properties of fluorine

At room temperature, fluorine is a pale yellow gas with an irritating odor. Its inhalation is dangerous. When cooled, it becomes a yellow liquid. There is only one stable isotope of this chemical element - fluorine-19.

The first ionization energy of this halogen is very high (402 kcal / mol), which is the standard heat of formation of the cation F ⁺ 420 kcal / mol.

The small size of the element atom allows them to be placed relatively large amounts around the central atom, forming a number of stable complexes, for example, hexafluorosilicate (SiF ₆ ) ^2- and hexafluoroaluminate (AlF ₆ ) ^3- . Fluorine is an element that has the strongest oxidizing properties. No other substance is capable of oxidizing the fluoride anion to become a free element, and for this reason the element is not in a free state in nature. This characteristic of fluorine for more than 150 years did not allow it to be obtained by any chemical method. This was achieved only through the use of electrolysis. Nevertheless, in 1986, an American chemist, Carl Christ, reported the first "chemical" production of fluorine. He used K ₂ MnF ₆ and antimony pentafluoride (SbF ₅ ), which can be obtained from solutions of HF.

Fluorine: valence and oxidation state

The outer shell of halogens contains an unpaired electron. That is why the valence of fluorine in the compounds is unity. However, the atoms of the elements of group VIIa can increase the number of such electrons to 7. The maximum valence of fluorine and its degree of oxidation are -1. The element is not able to expand its valence shell, since its atom does not have a d-orbital. Other halogens, due to its presence, are able to exhibit valence up to 7.

High oxidizing ability of the element allows to reach the maximum possible degree of oxidation in other elements. Fluorine (valency I) can form compounds that do not exist in any other halides: silver difluoride (AgF ₂ ), cobalt trifluoride (CoF ₃ ), rhenium heptafluoride (ReF ₇ ), bromine pentafluoride (BrF ₅ ) and iodine heptafluoride (IF ₇ ).

Connections

The fluorine formula (F ₂ ) is composed of two element atoms. It can join compounds with all other elements, except helium and neon, forming ionic or covalent fluorides. Some metals, such as nickel, are quickly coated with a layer of this halogen, which prevents further bonding of the metal to the element. Some dry metals, such as mild steel, copper, aluminum or monel (66% nickel and 31.5% copper alloy) do not react at normal temperatures with fluorine. For working with the element at temperatures up to 600 ° C, a monel is suitable; Sintered alumina is stable up to 700 ° C.

Fluorocarbon oils are the most suitable lubricants. Element reacts violently with organic substances (for example, rubber, wood and fabrics), therefore controlled fluorination of organic compounds by elemental fluorine is possible only with the adoption of special precautions.

Production

Fluorspar is the main source of fluoride. In the production of hydrogen fluoride (HF), the powdered fluorite is distilled with concentrated sulfuric acid in a lead or cast iron apparatus. During the distillation, calcium sulfate (CaSO ₄ ) is formed, which is insoluble in HF. Hydrogen fluoride is produced in a sufficiently anhydrous state by fractional distillation in copper or steel vessels and stored in steel cylinders. The usual impurities in industrial hydrogen fluoride are sulfuric and sulfuric acids, as well as fluorosilicic acid (H ₂ SiF ₆ ), which are formed due to the presence of silica in the fluorite spar. Moisture traces can be removed by electrolysis using platinum electrodes, treatment with elemental fluorine or storage over a stronger Lewis acid (MF ₅ , where M is a metal) that can form salts (H ₃ O) ⁺ (MF ₆ ) ^- : H ₂ O + SbF ₅ + HF → (H ₃ O) ⁺ (SbF ₆ ) ^- .

Hydrogen fluoride is used in the preparation of a variety of industrial inorganic and organic fluorine compounds, for example aluminum fluoride (Na ₃ AlF ₆ ), used as an electrolyte in the smelting of metallic aluminum. A solution of gaseous hydrogen fluoride in water is called hydrofluoric acid, a large amount of which is used for cleaning metals and for polishing, rendering the glass dull or for etching it.

The free element is obtained by electrolytic procedures in the absence of water. As a rule, they have the form of electrolysis of a melt of potassium fluoride with hydrogen fluoride (in the ratio 1 to 2.5-5) at temperatures of 30-70, 80-120 or 250 ° C. During the process, the content of hydrogen fluoride in the electrolyte decreases, and the melting temperature rises. Therefore, it is necessary that its addition occurs continuously. In a high-temperature chamber, the electrolyte is replaced when the temperature exceeds 300 ° C. Fluoride can be safely stored under pressure in stainless steel cylinders if the cylinder valves are free of traces of organic substances.

Using

The element is used to produce various fluorides, such as chlorine trifluoride (ClF ₃ ), sulfur hexafluoride (SF ₆ ) or cobalt trifluoride (CoF ₃ ). The compounds of chlorine and cobalt are important fluorinating agents of organic compounds. (If appropriate precautions are in place, fluorine can be used directly for this). Sulfur hexafluoride is used as a gaseous dielectric.

Elemental fluorine, often diluted with nitrogen, reacts with hydrocarbons to form the corresponding fluorocarbons in which part or all of the hydrogen is replaced by halogen. The obtained compounds, as a rule, are characterized by high stability, chemical inertness, high electrical resistance, as well as other valuable physicochemical properties.

Fluorination can also be carried out by treating organic compounds with cobalt trifluoride (CoF ₃ ) or by electrolyzing their solutions in anhydrous hydrogen fluoride. Useful plastics with non-stick properties, such as polytetrafluoroethylene [(CF ₂ CF ₂ ) _x ], known under the commercial name Teflon, are derived from unsaturated fluorinated hydrocarbons.

Organic compounds containing chlorine, bromine or iodine are fluorinated to produce substances such as dichlorodifluoromethane (Cl ₂ CF ₂ ), a refrigerant that has been widely used in domestic refrigerators and air conditioners. Since chlorofluorocarbons, such as dichlorodifluoromethane, play an active role in the depletion of the ozone layer, their production and use has been limited, and now the use of refrigerants containing hydrofluorocarbons is preferred.

The element is also used to produce uranium hexafluoride (UF ₆ ), used in the gas diffusion process of separation of uranium-235 from uranium-238 in the production of nuclear fuel. Hydrofluoric acid and boron trifluoride (BF ₃ ) are produced commercially, since they are good catalysts for alkylation reactions used to produce many organic compounds. Sodium fluoride is usually added to drinking water in order to reduce the incidence of tooth decay in children. In recent years, the use of fluoride compounds in the pharmaceutical and agricultural fields has become most important. Selective substitution of fluoride dramatically changes the biological properties of substances.

Analysis

It is difficult to accurately determine the amount of this halogen in the compounds. Free fluorine, whose valence is equal to 1, can be detected by its oxidation of mercury Hg + F ₂ → HgF ₂ , and also by measuring the increase in the weight of mercury and the change in the volume of the gas. The main qualitative tests for the presence of element ions are:

• The evolution of hydrogen fluoride under the influence of sulfuric acid,
• The formation of a precipitate of calcium fluoride with the addition of a solution of calcium chloride,
• Decolorization of a yellow solution of titanium tetraoxide (TiO ₄ ) and hydrogen peroxide in sulfuric acid.

Quantitative methods of analysis:

• Precipitation of calcium fluoride in the presence of sodium carbonate and treatment of the precipitate with acetic acid,
• Precipitation of lead chlorofluoride by addition of sodium chloride and lead nitrate,
• Titration (determination of the concentration of the solute) with a solution of thorium nitrate (Th [NO ₃ ] ₄ ) using sodium alizarine sulfonate as an indicator: Th (NO ₃ ) ₄ + 4KF ↔ ThF ₄ + 4KNO ₃ .

Covalently bound fluorine (valence I), as, for example, in fluorocarbons, it is more difficult to analyze. This requires a compound with metallic sodium followed by analysis of the F ^- ions, as described above.

Element Properties

Finally, we give some properties of fluorine:

• Atomic number: 9.
• Atomic weight: 18,9984.
• Possible fluorine valences: 1.
• Melting point: -219.62 ° C.
• Boiling point: -188 ° C.
• Density (1 atm, 0 ° C): 1.696 g / l.
• Electronic formula of fluorine: 1s ² 2s ² 2p ⁵ _.