Answer is: group 17 on the periodic table.
Group 17 (halogens) elements are: fluorine (F), chlorine (Cl), bromine (Br) and iodine (I).
They are very reactive and easily form many compounds.
Fluorine and chlorine are in gaseous state, bromine is liquid and iodine is in solid state in room temperature.
Halogens need to gain one electron to have electron cofiguration like next to it noble gas.
For example, fluorine has 9 electrons and it gain easily one electron in chemical reaction to have electron configuration like noble gas neon (Ne) with 10 electrons.
