Final answer:
Lighter elements produce more energy per fusion reaction than heavier elements because the fusion of light nuclei forms more stable and tightly bound nuclei, turning mass into energy. Heavy elements like iron, with already high binding energies, consume energy to fuse, making their fusion reactions less energetic.
Step-by-step explanation:
The fusion of lighter elements into heavier ones tends to release more energy per fusion reaction. This is because light nuclei, when combined, form more stable nuclei with higher binding energy per nucleon, converting some mass into energy as dictated by Einstein's famous equation E=mc². In particular, fusion reactions involving very light nuclei, such as hydrogen isotopes fusing into helium, are highly energetic. For instance, the fusion of deuterium (²H) and tritium (³H) to form helium-4 (4He) and a neutron releases about 17.6 MeV of energy, which is considerable.
Heavier elements like iron and cobalt require energy input to fuse because they are already among the most tightly bound nuclei, making the fusion process endothermic for them. The binding energy peaks around iron, so fusing nuclei heavier than that requires more energy than is released. The power output of stars like our Sun comes primarily from fusion reactions, converting mass into energy, which is what makes nuclear fusion an attractive energy source.