Periodic system Ⅶ A group elements, includingfluorine(F)、chlorine(Cl)、bromine(Br)、iodine(I)、Astatine(At) and "tin" (Ts), referred to as halogen.[4]They all exist in nature as typical salts and are salt forming elements. Halogen means "making salt" in Greek.
The elements of halogen group elements areDiatomic moleculeThe changes of their physical properties are very regular. With the increase of molecular weight, the dispersion force between halogen molecules gradually increases, and the color becomes darker (fluorine is a yellowish green gas; chlorine is a yellowish green gas; bromine is a dark reddish brown liquid; iodine is a purple black solid). Their melting point, boiling point, density, and atomic volume also increase in turn.
Halogens have very similar chemical properties. They all have 7 electrons on the outermost electron layer, and have a tendency to obtain one electron to form a stable octagonal halogen ion. Therefore, halogens have oxidation. The smaller the atomic radius, the stronger the oxidation. Therefore, fluorine is the strongest oxidizer in the single substance.
Valence state
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The oxidation state of halogen is - 1,+1,+3,+5,+7 (F has no positive valence state), formed with typical metalsIonic compoundOther halides areCovalent compound。
Halogen combines with hydrogen to form hydrogen halide, which dissolves in water to form hydrohalide acid[1]。
Compounds formed between halogens are called mutualhalide, e.g. ClFthree、ICl。
Halogens can also form various valence oxygen containing acids, such as HClO（Hypochloric acid）、HClOtwo（Chlorite）、HClOthree（Chlorate）、HClOfour(perchloric acid).
Halogen elements are very stable, except ItwoIn addition, halogen molecules are difficult to decompose at high temperatures.
purpose
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Halogens and their compounds are widely used.For example, the salt we eat every day is mainly the chloride composed of chlorine and sodium.
Physical and chemical properties
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The toxicity of halogen decreases from fluorine.Oxidation: Ftwo> Cltwo> Brtwo> I2。The reducibility is opposite.
fluorine
Fluorine gas
Fluorine is a light yellow gas at room temperature, which is extremely toxic. It reacts with water to generate hydrofluoric acid and oxygen immediately and burns. At the same time, it can break the container, and there is an explosion risk in large quantities.Fluorine, hydrogen fluoride and hydrofluoric acid have strong corrosiveness to glass.Fluorine is the most oxidizing element, which can only assume - 1 valence.The reaction of elemental fluorine with salt solution is to react with water first, and the generated hydrofluoric acid may cause explosion when it reacts with salt and passes into alkali.Hydrofluoric acid in aqueous solution is a weak acid.But it is the most corrosive halogen acid. If the skin sticks to it, it will corrode to the bone marrow.Fluorine can react with all elements (except helium, neon and argon).
chlorine
chlorine
Chlorine is a yellow green gas at room temperature, which is soluble in water. One volume of water can dissolve two volumes of chlorine.Extremely toxic.The aqueous solution of chlorine is called chlorine water. Chlorine is dissolved in water and partially reacts with water to generate hydrochloric acid and hypochlorite. Hypochloric acid is highly oxidizing and can be used for bleaching. Hypochloric acid is unstable and decomposes into hydrochloric acid and oxygen when exposed to light.Hydrochloric acid is a strong acid.Chlorine has many changeable valences.Chlorine is strongly irritating to the lungs.Chlorine reacts with most elements.
bromine
bromine
Bromine is a dark reddish brown liquid at room temperature, soluble in water, and about 3g bromine can be dissolved in 100g water.Extremely volatile, toxic, and steam strongly irritates eyes, mucous membranes, etc.The aqueous solution is called bromine water.Bromine needs to be sealed with water to prevent the escape of vapor from harming human body.It is oxidizing and has a variety of changeable valences. It reacts weakly with water at room temperature to generate hydrobromic acid and hypobromic acid.Heating can accelerate the reaction.Hydrobromic acid is a strong acid, stronger than hydrochloric acid.Bromine is generally used in organic synthesis.
iodine
iodine
Iodine is a dark purple black solid at room temperature, easily soluble in gasoline, ethanol and other solvents, and slightly soluble in water. Iodide can increase the solubility of iodine and accelerate the dissolution rate.100g of water can dissolve about 0.02g of iodine at room temperature.Low toxicity, weak oxidation, and various changeable valences.It is sublimating when heated. The steam is purple red, but it is dark blue when there is no air.Sometimes it is necessary to add water for storage.Hydroiodic acid is the strongest halogenated acid without radioactivity and the strongest anaerobic acid without radioactivity.However, the corrosivity is the weakest of all non radioactive halogenated acids, and it is only irritating to the skin.Reductive.
Iodine is allHalogen group elementBecause fluorine, chlorine and bromine are more toxic and corrosive than iodine, astatine[2]Although less toxic than iodine, it is radioactive[3]。
physical property
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Halogen element
Color and status
density
Melting point/℃
Boiling point/℃
F2
Pale yellow green gas
1.69g/L(15℃)
-219.6
-188.1
Cl2
Yellow green gas
3.214g/L(0℃)
-101
-34.6
Br2
Dark reddish brown liquid
3.119g/cm3(20℃)
-7.2
fifty-eight point seven eight
I2
Purple black solid, purple gas
4.93g/cm3(15°C)
one hundred and thirteen point five
one hundred and eighty-four point four
The oxidizability gradually weakens and the reducibility gradually increases.
