Halogens (group 17)

Seven outer electrons

Every halogen has seven outer electrons in an ns² np⁵ configuration, such as fluorine ([He] 2s² 2p⁵) and chlorine ([Ne] 3s² 3p⁵). Gaining one electron gives them the stable arrangement of the next noble gas, so they readily form halide ions with a −1 charge: F⁻, Cl⁻, Br⁻ and I⁻. Two halogen atoms can also share a pair of electrons, so the elements exist as diatomic molecules: F₂, Cl₂, Br₂ and I₂.

Physical properties and trends

Group 17 is the only group with a gas, a liquid and a solid at room temperature. Fluorine is a pale yellow gas, chlorine a yellow-green gas, bromine a dark red-brown liquid, and iodine a shiny gray-black solid that turns into a violet vapor when heated.

Going down the group the molecules get larger and the forces between them stronger, so melting and boiling points rise and the colors darken. Electronegativity and reactivity decrease: fluorine is the most electronegative and most reactive element of all, while iodine is much milder.

Typical reactions

Halogens react directly with most metals to form ionic salts called halides: 2Na + Cl₂ → 2NaCl. A more reactive halogen displaces a less reactive one from its salts, so chlorine turns potassium bromide solution orange: Cl₂ + 2KBr → 2KCl + Br₂.

With hydrogen they form hydrogen halides (H₂ + Cl₂ → 2HCl), which dissolve in water to give acids; hydrofluoric acid is the only weak one. In water, chlorine forms hypochlorous acid, which kills bacteria: Cl₂ + H₂O ⇌ HCl + HOCl.

Uses of the halogens

Chlorine disinfects drinking water and swimming pools, is the basis of household bleach (sodium hypochlorite) and is used to make PVC plastic. Fluoride in toothpaste helps prevent tooth decay, and fluorine is part of the nonstick coating PTFE. Bromine compounds are used as flame retardants, and iodine is an antiseptic and is added to table salt to prevent iodine deficiency.

Astatine and tennessine

All isotopes of astatine are radioactive, the longest-lived with a half-life of about 8 hours, so it is one of the rarest elements on Earth. No visible sample has ever been made, its properties are mostly estimates, and some calculations suggest bulk astatine would be a metal. Tennessine, first made in 2010, sits below astatine, but only a few atoms have ever existed and it is expected to behave less like a typical halogen.