So let's look at our final periodic trend called electron affinity. Now. Electron affinity is the amount of energy released when an atom gains an electron. 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.