1. The problem statement, all variables and given/known data If removing electrons from atoms requires energy, and adding electrons to atoms usually liberates energy, then why is energy liberated when the ionic compound NaCl forms from the elements sodium and chlorine? 2. Relevant equations None 3. The attempt at a solution My solutions manual says: "Once Na+ and Cl- have been made, the attraction between them is very strong, resulting in the liberation of energy." I do not understand why the strong attraction between the ions would liberate energy. My book relates it to the emission of a photon when electrons move from higher to lower energy levels, but this just confuses me even more.