any of five related nonmetallic elements (fluorine or chlorine or bromine or iodine or astatine) that are all monovalent and readily form negative ions
"The scientist explained that the new fire extinguisher relies on halogen gases to suppress flames by releasing negatively charged ions."
Any element of group 17, i.e. fluorine, chlorine, bromine, iodine, astatine and tennessine, which form a salt by direct union with a metal.
"The chemist warned that mixing the halogen gas with sodium would instantly create an explosive reaction as they formed a new salt."