Remember, the only way to take an electron away from the outer shell of an atom is to apply work, is to input energy. Because work must be done against the force of the protons in the nucleus attracting those electrons, those outer electrons. So that means the reverse must be the following whenever an electron or whenever an atom gains an electron energy must be released. And that's exactly what happens when Fluorine, for example, gains electrons. When fluorine goes from a neutral molecule, gains an electron to form an anion, it loses energy. The energy level of the outer shell is lower.