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Halogens: physical properties, chemical properties. The use of halogens and their compounds
Halogens in the periodic table are located to the left of the noble gases. These five toxic nonmetallic elements are included in Group 7 of the periodic table. These include fluorine, chlorine, bromine, iodine and astatine. Although the astat is radioactive and has only short-lived isotopes, it behaves like iodine, and it is often reckoned as halogens. Since the halogen elements have seven valence electrons, they need only one additional electron to form a full octet. This characteristic makes them more active than other groups of non-metals.
general characteristics
Halogens form diatomic molecules (type X 2 , where X is a halogen atom) - a stable form of the existence of halogens in the form of free elements. The bonds of these diatomic molecules are nonpolar, covalent, and single. The chemical properties of halogens allow them to easily join with most elements, so they never occur unbound in nature. Fluoride is the most active halogen, and astat is the least active.
All halogens form salts of Group I with similar properties. In these compounds, halogens are present in the form of halide anions with a charge of -1 (for example, Cl-, Br-). The ending -id indicates the presence of halide-anions; For example Cl - is called "chloride".
In addition, the chemical properties of halogens allow them to act as oxidants - oxidize metals. Most of the chemical reactions in which halogens are involved are oxidation-reduction in an aqueous solution. Halogens form single bonds with carbon or nitrogen in organic compounds, where the degree of their oxidation (CO) is -1. When the halogen atom is replaced by a covalently bound hydrogen atom in the organic compound, the halo prefix can be used in a general sense, or the fluoro-, chloro-, bromo-, iodine- prefixes for specific halogens. Halogen elements can have a cross-link with the formation of diatomic molecules with polar covalent single bonds.
Chlorine (Cl 2 ) became the first halogen discovered in 1774, then iodine (I 2 ), bromine (Br 2 ), fluorine (F 2 ) and astat (At, discovered last in 1940) were discovered. The name "halogen" comes from the Greek roots hal- ("salt") and -gen ("form"). Together, these words mean "salt-forming," emphasizing the fact that halogens, reacting with metals, form salts. Galite is the name of rock salt, a natural mineral consisting of sodium chloride (NaCl). And, finally, halogens are used in everyday life - fluoride is found in toothpaste, chlorine disinfects drinking water, and iodine promotes the production of thyroid hormones.
Chemical elements
Fluorine is an element with atomic number 9, denoted by the symbol F. Elemental fluorine was first discovered in 1886 by separation from hydrofluoric acid. In a free state, fluorine exists in the form of a diatomic molecule (F 2 ) and is the most abundant halogen in the earth's crust. Fluorine is the most electronegative element in the periodic table. At room temperature it is a pale yellow gas. Fluorine also has a relatively small atomic radius. Its CO-1, with the exception of an elementary diatomic state, in which its oxidation degree is zero. Fluorine is extremely chemically active and directly interacts with all elements except helium (He), neon (Ne) and argon (Ar). In a solution of H 2 O, hydrofluoric acid (HF) is a weak acid. Although fluorine is strongly electronegative, its electronegativity does not determine acidity; HF is a weak acid due to the fact that the fluorine ion is basic (pH> 7). In addition, fluoride produces very powerful oxidants. For example, fluorine can react with an inert gas with xenon and forms a strong oxidant of xenon difluoride (XeF 2 ). Fluoride has many uses.
Chlorine is an element with atomic number 17 and chemical symbol Cl. Found in 1774 by separating it from hydrochloric acid. In its elementary state, it forms a diatomic molecule, Cl 2 . Chlorine has several CO: -1, +1, 3, 5 and 7. At room temperature it is a light green gas. Since the bond that forms between the two chlorine atoms is weak, the Cl 2 molecule has a very high ability to join. Chlorine reacts with metals to form salts, which are called chlorides. Chlorine ions are the most common ions, they are contained in sea water. Chlorine also has two isotopes: 35 Cl and 37 Cl. Sodium chloride is the most common compound of all the chlorides.
Bromine is a chemical element with atomic number 35 and the symbol Br. It was first discovered in 1826. In its elementary form, bromine is a diatomic Br 2 molecule. At room temperature, it is a reddish-brown liquid. Its CO is -1, + 1, 3, 4 and 5. Bromine is more active than iodine, but less active than chlorine. In addition, bromine has two isotopes: 79 Br and 81 Br. Bromine is found in the form of bromide salts dissolved in sea water. In recent years, world production of bromide has increased significantly due to its availability and long life. Like other halogens, bromine is an oxidizing agent and very toxic.
Iodine is a chemical element with atomic number 53 and symbol I. Iodine has oxidation states: -1, +1, +5 and +7. It exists in the form of a diatomic molecule, I 2 . At room temperature is a solid substance of violet. Iodine has one stable isotope - 127 I. It was first discovered in 1811 with the help of seaweed and sulfuric acid. Currently, iodine ions can be isolated in sea water. Despite the fact that iodine is not very soluble in water, its solubility can increase with the use of individual iodides. Iodine plays an important role in the body, participating in the production of thyroid hormones.
Astat is a radioactive element with atomic number 85 and symbol At. Its possible oxidation states are: -1, +1, 3, 5 and 7. The only halogen that is not a diatomic molecule. In normal conditions it is a metallic solid of black color. Astat is a very rare element, so little is known about it. In addition, the astatine has a very short half-life, not longer than a few hours. Obtained in 1940 as a result of synthesis. It is believed that the astatine is similar to iodine. It differs in its metallic properties.
The table below shows the structure of halogen atoms, the structure of the outer layer of electrons.
Halogen | Electron configuration |
Fluorine | 1s 2 2s 2 2p 5 |
Chlorine | 3s 2 3p 5 |
Bromine | 3d 10 4s 2 4p 5 |
Iodine | 4d 10 5s 2 5p 5 |
Astat | 4f 14 5d 10 6s 2 6p 5 |
A similar structure of the outer layer of electrons determines the physical and chemical properties of halogens. At the same time, when comparing these elements, differences are also observed.
Periodic properties in the halogen group
The physical properties of simple halogen substances vary with the increase in the atomic number of the element. For better understanding and greater clarity, we offer you several tables.
The melting and boiling points in the group increase with the size of the molecule (F Table 1. Halogens. Physical properties: melting point and boiling point Halogen Melting point (˚C) Boiling point (˚C) Fluorine -220 -188 Chlorine -101 -35 Bromine -7.2 58.8 Iodine 114 184 Astat 302 337 The size of the nucleus increases (F Table 2. Halogens. Physical properties: atomic radii Halogen Covalent radius (pm) Ionic (X - ) radius (pm) Fluorine 71 133 Chlorine 99 181 Bromine 114 196 Iodine 133 220 Astat 150 If the external valence electrons are not near the nucleus, then to remove them from it, it does not take much energy. Thus, the energy necessary to push out an external electron is not so high in the lower part of the group of elements, since there are more energy levels here. In addition, the high ionization energy causes the element to exhibit non-metallic qualities. Iodine and the astat display exhibit metallic properties, because the ionization energy decreases (At
Table 3. Halogens. Physical properties: ionization energy Halogen The ionization energy (kJ / mol) fluorine 1681 chlorine 1251 bromine 1140 iodine 1008 Astatine 890 ± 40 The number of valence electrons in an atom increases with increasing energy levels at progressively lower levels. The electrons are progressively farther from the nucleus; Thus, the nucleus and electrons are not attracted to each other. An increase in screening is observed. Therefore, the electronegativity decreases with increasing period (At
Table 4. Halogens. Physical properties: electronegativity Halogen Electronegativity fluorine 4.0 chlorine 3.0 bromine 2.8 Iodine 2.5 Astatine 2.2 Since the size of the atom increases with increasing period, the affinity to the electron, as a rule, decreases (B
Table 5. The affinity of halogens for an electron Halogen Affinity for the electron (kJ / mol) fluorine -328.0 chlorine -349.0 bromine -324.6 Iodine -295.2 Astatine -270.1 The reactivity of halogens decreases with increasing period (At
A halide is formed when the halogen reacts with another, less electronegative element to form a binary compound. Hydrogen reacts with halogens to form halides of the type HX: The hydrogen halides readily dissolve in water with the formation of a hydrohalic (hydrofluoric, hydrochloric, hydrobromic, hydroiodic) acid. The properties of these acids are given below. The acids are formed by the following reaction: HX (aq) + H 2 O (l) → X - (aq) + H 3 O + (aq). All hydrogen halides form strong acids, with the exception of HF. The acidity of hydrohalic acids increases: HF Hydrofluoric acid is capable of etching glass and some inorganic fluorides for a long time. It may seem illogical that HF is the weakest hydrohalic acid, since fluorine has the highest electronegativity. Nevertheless, the H-F bond is very strong, as a result of which the acid is very weak. A strong bond is determined by the short bond length and high dissociation energy. Of all the hydrogen halides, HF has the shortest bond length and the largest dissociation bond energy. Halogen oxo acids are acids with hydrogen, oxygen and halogen atoms. Their acidity can be determined by analysis of the structure. Halogen oxo acids are listed below: In each of these acids, the proton is bonded to the oxygen atom, so comparing the lengths of proton bonds is useless here. The dominant role here is electronegativity. Acid activity increases with increasing number of oxygen atoms, bound to the central atom. The main physical properties of halogens can be summarized in the following table. The state of the substance (at room temperature) Halogen Appearance Solid iodine purple Astatine the black Liquid bromine Reddish-brown Gaseous fluorine Pale yellow-brown chlorine Pale green The color of halogens is the result of the absorption of visible light by molecules, which causes the excitation of electrons. Fluorine absorbs violet light, and, therefore, looks pale yellow. Iodine, on the contrary, absorbs yellow light and looks purple (yellow and violet - complementary colors). The color of halogens becomes darker as the period increases. In closed containers liquid bromine and solid iodine are in equilibrium with their vapors, which can be observed in the form of a colored gas. Although the color of astata is unknown, it is assumed that it should be darker than iodine (i.e., black) in accordance with the observed regularity. Now, if you are asked: "Characterize the physical properties of halogens," you will have something to say. The degree of oxidation is often used in place of the concept of "valency of halogens." As a rule, the degree of oxidation is -1. But if the halogen is bonded to oxygen or another halogen, it can assume other states: CO oxygen-2 has priority. In the case of two different halogen atoms joined together, the more electronegative atom prevails and takes CO-1. For example, in chloride of iodine (ICI), chlorine has CO-1, and iodine +1. Chlorine is more electronegative than iodine, so its CO is -1. In bromic acid (HBrO 4 ), oxygen has CO-8 (-2 x 4 atoms = -8). Hydrogen has a total oxidation state of +1. Adding these values gives CO-7. Since the final CO of the compound must be zero, the CO of bromine is +7. The third exception to the rule is the degree of oxidation of the halogen in the elemental form (X 2 ), where its CO is zero. Halogen CO in compounds fluorine -1 chlorine -1, +1, +3, +5, +7 bromine -1, +1, +3, +4, +5 iodine -1, +1, +5, +7 Astatine -1, +1, +3, +5, +7 Electronegativity increases with the growth of the period. Therefore, fluorine has the highest electronegativity from all elements, which is confirmed by its position in the periodic table. Its electronic configuration is 1s 2 2s 2 2p 5 . If fluorine gets another electron, the extreme p-orbitals are completely filled and make up a full octet. Since fluorine has a high electronegativity, it can easily take an electron from a neighboring atom. Fluorine in this case is isoelectronic to an inert gas (with eight valence electrons), all its external orbitals are filled. In this state, fluoride is much more stable. In nature, halogens are in the state of anions, so free halogens are obtained by oxidation by electrolysis or with the aid of oxidants. For example, chlorine is produced by hydrolysis of a solution of table salt. The use of halogens and their compounds is diverse. Inorganic chemistry. Hydrogen + halogens
Halogen Oxoacids
Appearance and condition of substance
Explanation of appearance
The degree of oxidation of halogens in compounds
Why is CO of fluorine always -1?
Preparation and use of halogens
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