I don't get how ionization is an endothermic reaction. In an endothermic Reactions: the reactants have less potential energy than do the products. Energy must be input in order to raise the particles up to the higher energy level. The ionization energy, is the energy required to completely remove a valence electron from a gaseous atom or ion. EX: Na(g) → Na+(g) + e- I1 = 496 kJ/mole The reactant though, Na, has more potential energy (it is further away from the nucleus when compared to its ionic counterpart, which just lost an electron) than the product? Am I thinking wrong? PlEASE HELP!