Halogen
A halogen is one of the five elements fluorine, chlorine, bromine, iodine, and astatine. These reactive nonmetals readily form salts with metals.
Periodic table: A tabular arrangement of chemical elements ordered by atomic number and recurring chemical properties. Halogens occupy group 17, where similar outer-electron structures produce recurring properties.
Halide: A negatively charged ion formed when a halogen gains an electron, or a compound containing such an ion. Halides are the ions halogens form when they acquire electrons during reactions.
Sodium chloride: An ionic compound of sodium and chlorine, commonly known as table salt. It is a familiar example of a halogen forming a salt with a metal.
Noble gas: An element in group 18, generally characterized by a filled outer electron shell and low chemical reactivity. Noble gases contrast with halogens, which readily gain electrons and form compounds.
Astatine: A rare, radioactive element with atomic number 85, placed among the halogens. Its scarcity and short-lived isotopes make its chemistry difficult to study directly.
Group 17: The periodic-table column containing fluorine, chlorine, bromine, iodine, astatine, and tennessine. This group defines the halogen family and its position among the elements.
Ionic bond: An electrostatic attraction between oppositely charged ions, typically formed through electron transfer. Metal atoms can transfer electrons to halogens, producing ionic salts.
Fluoride: The anion F⁻ and compounds containing it, including substances used in dental care. Fluoride compounds are used to help prevent tooth decay.
Alkali metal: A group 1 metal with one outer-shell electron and high chemical reactivity. Alkali metals tend to donate the electron halogens tend to accept.
Tennessine: A synthetic element with atomic number 117, placed in group 17 of the periodic table. Its group placement suggests halogen kinship, but only a few atoms have been produced.