The fluorine atom has seven valence electrons. The atomic number of fluorine is 9, and its electronic configuration is [He] 2s2 2p5. The atom of fluorine prefers to gain an electron to complete its last orbit.
Fluorine forms a negative ion by gaining an electron during bond formation. The "S" and "P" subshells are filled by electrons in that order and are in the second orbit of the fluorine atom.
Valence electrons are involved in bond formation and in chemical reactions. If the outermost orbit of an atom is complete with eight electrons, it is stable chemically. Such stable elements with eight electrons in the outermost orbit of their atoms are called inert elements, or noble gases.