Finally, iron is the most tightly bound nucleus. This means that it has the least average mass per nucleon (protons and neutrons). Another way to think about this is using something called the binding energy. Iron also has the largest binding energy per nucleon. This means that as the binding energy per nucleon increases from hydrogen to iron the fusion processes are exothermic. Once elements heavier than iron begin to fuse then the binding energy per nucleon decreases and the fusion processes become endothermic. Why do you think this happens