From Big Medical Encyclopedia

HALOGENS (grech, hals, hal[os] salt + gennao to create, make; a synonym haloids) — chemical elements of the main subgroup of the VII group of a periodic system of elements of D. I. Mendeleyev. Enter into this subgroup fluorine (see), chlorine (see), bromine (see), iodine (see) and artificially received astatine. Actively participate in a metabolism in a human body, animals and plants. In medical practice are used as medicines and desinfectants.

Owing to high chemical activity of G. in the nature hl meet only in the form of connections. obr. salts of galogenovodorodny acids. Their ground mass is sprayed in insignificant concentration in crust, water of the seas and oceans and in live organisms. The most widespread of G. are fluorine and chlorine; bromine and iodine are spread much less. Compounds of bromine and iodine usually accompany compounds of chlorine; in a lithosphere and in the hydrosphere on the 200th weight. h chlorine falls about 1 h bromine and 1/10 h iodine. In nek-ry places of the globe considerable accumulations of the minerals containing fluorine or chlorine — fluorite, or fluorspar (CaF2), cryolite (Na3AlF6), rock salt (NaCl), sylvinite (KCl-NaCl), carnallite (KCl-MgCl2-6H2O) meet. Significant amounts of fluorine contain in phosphorites and apatites. Sodium chloride and magnesium chloride contain in waters of salty lakes, seas and oceans. On average contains in sea water apprx. 3% of sodium chloride. For bromine and iodine accumulation of separate minerals is uncharacteristic; in the form of sodium, magnesian and potassium salts in very small concentration they are in sea water from where accumulate nek-ry seaweed (iodine) and mollusks (bromine). In quite considerable concentration (10 — 100 mg/l) of compound of iodine and bromine contain in waters of wells in oil fields.

Are an indispensable component of vegetable and animal organisms. The body of the person on average contains 156 mg of % of chlorine and 0,9 mg of % of fluorine; the last is preferential in bones, especially in teeth (as a part of an adamantine substance of tooth). Anions of chlorine are active ions of blood, a lymph, cellular contents, cerebrospinal liquid most important osmotically. Erythrocytes contain 190 mg of % and in blood serum of 370 mg of % of chlorine, hl. obr. in the form of sodium chlorides, potassium, calcium. Chlorine in the form of salt to - you in quantity apprx. 0,5% contains in a gastric juice. Normal the body of the person contains about 25 mg of % of iodine, a half of this quantity collects in the form of complex protein — Thyreoidinum in a thyroid gland. In blood there are only traces of fluorine, 8,5 — 15,5 mg of % of iodine and 0,16 — 1,5 mg of % of bromine.

In pure form fluorine and chlorine of a gazoobrazna, bromine — liquid, iodine and astatine — solids (see the tab.). In total G. in gaseous state form diatomic molecules. Are extremely active in the chemical relation; the first element of this group — fluorine — the activest of all known elements. All of them directly unite to hydrogen, forming halogen hydrides (NG; — halogen atom) — colourless gases, well water soluble. Aqueous solutions of halogen hydrides represent strong acids. Acid properties of halogen hydrides are shown not only in water, but also in spirit and radio solutions. At direct interaction with metals G. form typical salts — halogenides (e.g., CaF2, NaCl, KBr, KI), many from to-rykh well rastvorima in water. Chlorides, bromides and silver iodides and calcium fluoride in water are insoluble. At dissociation in aqueous solutions galogenovodorodny acids and their salts form negatively charged monovalent ions of halogens.

G.'s activity in relation to hydrogen and metals increases with reduction of sequence number of an element. (As well as with nitrogen, carbon, sulfur) G. directly do not connect to oxygen, but an indirect way form quite unstable oxides and oxygen acids, in to-rykh they have variable positive valence (+1, +3, +5 and +7). The maximum positive valence + 7 is characteristic of compounds of chlorine and iodine. G.'s activity in relation to oxygen increases with increase in sequence number. In the practical relation the following acids are the most important: hypochloric (HClO), chloric (HClO3), chloric (HClO4), bromic (HBrO3), iodic (HIO3), iodic (HIO4-2H2O) and their salts. All of them are strong oxidizers, and oxidizing properties increase with reduction of valency G. Ftor forms only one oxide (F2O); its oxygen acids are unknown.

Of a rastvorima in water and partially react with it on the equation:

G2 + H2O = Ng + NGO.

Interaction with alkalis on cold proceeds on the equation:

2 + 2 NaOH = Nar + Naro + H20,

and during the heating:

ZG2 + 6NaOH = 5nag + NAGO3 + 3H2.

At the same time along with halogenides salts of oxygen acids are formed. The exception makes fluorine, to-ry violently decomposes water and solutions of alkalis with release of oxygen (and partially ozone) and formation of HF and fluorides respectively. In saturated hydrocarbons G. easily replace hydrogen (partially or completely), at the same time one of atoms in the molecule G2 connects to hydrogen atom, forming NG halogen hydride, and another unites to carbon atom, e.g.: C2H6+Br2 = HBr + C2H5Br (ethyl bromide).

Join unsaturated hydrocarbons of a molecule G. completely: C2H4+Br2 = C2H4Br2 (ethylene bromide).

Easily replace hydrogen in aromatic compounds, e.g. C6H6+Br2 = HBr+ Sbn5br (brombenzene).

With each other form connections, molecules to-rykh contain that bigger number of atoms, than elements forming them are spaced far apart among G., e.g. ClF, BrF, IF5 etc.

Similarity and G.'s distinction among themselves are explained by structure of their atoms. All G.' atoms in an enveloping electron layer have 7 electrons; to a steady configuration of an enveloping electron layer is not enough for them only on one electron, to-ry they the easier attach, than the radius of atom is less. In this regard G.'s affinity to an electron increases with reduction of sequence number of G. that, e.g., affects in reactions of replacement of one G. by another. So, fluorine easily forces out chlorine, bromine and iodine from their connections with hydrogen and metals, chlorine forces out only bromine and iodine, and bromine — only iodine. G.'s similarity among themselves is explained by similar structure of an enveloping electron layer of their atoms, distinction is connected with increase of quantity of intermediate electron layers between an atomic nucleus and its enveloping electron layer (at atom of fluorine — 1, at chlorine — 2, at bromine — 3, iodine — 4 and at astatine — 5).

Find broad application in practice. All of them, except for astatine, are used for preparation of numerous organic and inorganic compounds; many of them are widely applied in medicine as medicines and disinfecting substances. All of them, especially chlorine and iodine, play a significant role in a metabolism in a human body, animals and plants (see the Metabolism and energy). With increase in sequence number of G. increase in their ability to formation of biologically active organic compounds is observed. About practical application of certain representatives of G. in medicine, and also professional vrednost — see articles devoted to each of halogens.


Bibliography: Voynar A. O. A biological role of microelements in an organism of animals and the person, page 346, M., 1953; Nekrasov B. V. Fundamentals of the general chemistry, t. 1 — 2, M., 1973; P e of m and G. Kurs of inorganic chemistry, the lane with it., page 313, 739, M., 1972.

V. P. Mishin.