Answer is: fluorine (F).
Group 17 (halogens) elements: fluorine (F), chlorine (Cl), bromine (Br) and iodine (I). They are very reactive and easily form many compounds.
Halogens need to gain one electron to have electron cofiguration like next to it noble gas.
Fluorine has atomic number 9, it means it has 9 protons and 9 electrons.
Fluorine tends to have eight electrons in outer shell like neon (noble gas) and gains one electron in chemical reaction. Fluorine is most reactive nonmetal.
Electron configuration of fluorine: ₉F 1s² 2s² 2p⁵.
Electron configuration of neon: ₁₀Ne 1s² 2s² 2p⁶.