which ion is formed when an atom of fluorine (f) gains one electron?
Fluorine is in Group 7. It has seven electrons in its highest energy level. It gains an electron from another atom in reactions, forming a fluoride ion, F-. Note that the atom is called fluorine, but the ion is called fluoride. Note that a fluoride ion has the same electronic structure as a neon atom (Ne). Once again, a fluoride ion is not a neon atom, because the nucleus of a fluoride ion is the nucleus of a fluorine atom, with 9 protons, and not of a neon atom, with 10.