Fig 10Binding energy per particlehigher is more tightly boundpeak near iron
How tightly each proton or neutron is held, against the size of the nucleus. The curve rises steeply through the light elements and peaks near iron, then falls slowly as proton repulsion grows. Any change that moves nuclei uphill releases the difference as energy. Fusing light nuclei moves them right and up, which powers the Sun, and splitting heavy ones moves them left and up, which powers nuclear reactors. Iron sits at the top, so neither can release energy from it.