Halogens

Halogens are a group of elements in the periodic table known as Group 17. They include fluorine, chlorine, bromine, iodine, astatine, and tennessine. They are nonmetals with seven valence electrons and are highly reactive, readily forming salts with metals and hydrogen halides with hydrogen.

In their elemental form, halogens are diatomic: F2, Cl2, Br2, I2, At2. Their physical states vary: fluorine and chlorine are gases at room temperature; bromine is a liquid; iodine is a solid. Astatine is radioactive and extremely rare, and tennessine is synthetic and not well characterized.

Key chemical properties include high electronegativity, especially fluorine, and strong oxidizing power that generally decreases down the group. They form a wide range of compounds, including metal halides and interhalogen compounds. Hydrogen halides (HF, HCl, HBr, HI) are formed when halogens react with hydrogen; HF is a weak acid in water, while HCl, HBr, and HI are strong acids.

Occurrence and uses: halogens occur in minerals and in seawater. Chlorine is abundant in the oceans as NaCl; fluorine occurs in minerals such as fluorite; iodine is found in seawater and in biological tissues. Applications include fluoride in dental care, chlorine for disinfection, bromine in flame retardants and some pharmaceuticals, and iodine in nutrition and antiseptics. Astatine and tennessine are largely laboratory curiosities with no commercial uses.

History: halogens have been studied since the 18th century, with chlorine isolated by Humphry Davy, iodine by Courtois, bromine by Balard, and fluorine by Moissan. Tennessine was created in the 21st century through nuclear reactions but remains poorly characterized.