Each of the seven outer electrons in a fluorine atom experiences an effective nuclear charge of +9 - 2 = +7. There is room for one more electron in the second shell, and if an electron is brought near the F atom, it too will feel the pull from a +7 charge. Energy is given off when the F atom and an electron combine to form the more stable F ion:


fluorine atomfluorine ion

The same amount of energy must be resupplied to pull the electron away again:


fluoride ionfluorine atom

The energy given off when one electron is added to a neutral atom is called the electron affinity of the atom.