The concept of mass defect and binding energy Iron has most mass defect and the greatest binding energy. It has the most stable nuclei. Nuclei with higher atomic number than iron undergo fission reactions to form nuclei with atomic number close to iron (middle nuclei) On the other hand, nuclei with lower atomic number than iron undergo fusion reactions to form the same middle nuclei. So isn't it entropy that all the nuclei in the universe aren't of iron or of elements closer to iron(middle nuclei)? What would happen if all these reactions come to completion so that every atom in the universe is of iron? Fusion and fission are non-renewable in the sense. That will be the end?