Halogeni

Halogeni, or halogens, are the elements in group 17 of the periodic table: fluorine, chlorine, bromine, iodine, astatine, and tennessine. They are highly reactive nonmetals with seven valence electrons and high electronegativity. In standard conditions, fluorine, chlorine, bromine, and iodine exist as diatomic molecules (F2, Cl2, Br2, and I2); astatine is rare and radioactive, and tennessine is synthetic. Halogens form salts with metals, known as halides, and readily form hydrogen halides, HX, with hydrogen.

Chemical trends include decreasing reactivity and oxidizing power down the group, while atomic size and the number of electron shells increase. They are typically poor conductors of heat and electricity. Physical states change from a highly reactive gas (fluorine) to a poisonous gas (chlorine) to a volatile liquid (bromine) to a solid (iodine) at room temperature; astatine is radioactive and not found in bulk form. Halogens are highly reactive and can be corrosive and toxic.

Halogens occur in minerals as halide minerals such as fluorite (CaF2) and halite (NaCl). Industrially, chlorine is produced mainly by electrolysis of brine (the chlor-alkali process). Fluorine, bromine, and iodine are obtained through specialized chemical processes. In biology, iodine is essential for thyroid hormones; fluoride strengthens teeth. Halogens have wide applications, including disinfection, water treatment, and the synthesis of organic and inorganic compounds. Safety concerns include toxicity and corrosivity, especially for chlorine and fluorine compounds.