Final answer:
Energy is required to fuse medium-mass nuclei into heavier ones due to the necessity of overcoming Coulomb repulsion and the fact that, beyond iron, the nuclear forces are no longer as effective in binding nucleons, making the process endothermic.
Step-by-step explanation:
To understand why energy input is required to fuse medium-mass nuclei, such as iron or cobalt, into more massive nuclei, we must consider the properties and forces at play in nuclear reactions. Medium-mass nuclei have greater binding energy per nucleon, meaning they are more stable. However, as we move to nuclei heavier than iron, we encounter a turning point where the nuclear forces are no longer able to bind the nucleons as efficiently. This is partly due to the Coulomb repulsion between the positively charged protons within the nuclei.
For light nuclei, the fusion process is exothermic, meaning it releases energy. This happens because when light nuclei fuse, the resulting nucleus has a higher binding energy and less mass per nucleon compared to the starting nuclei, and this mass difference is converted into energy, according to Einstein's equation, E=mc². However, for nuclei heavier than iron, fusion becomes endothermic, meaning energy must be added for the process to occur. The major obstacle for fusing these nuclei is overcoming the Coulomb repulsion, which requires a significant amount of kinetic energy to bring the nuclei close enough for the strong nuclear force to take over and bind them together.