Both fusion of light nuclei and fission of heavy ones release energy, and the binding-energy curve shows why. Slide the mass number A past iron-56 and flip the reaction toggle to watch the direction label and the released energy update through E = Δm·c2.
Every nucleus sits somewhere on the binding-energy-per-nucleon curve, and this simulator lets you trace it directly. Drag the mass-number A slider from 1 toward roughly 240 and watch the readout: binding energy per nucleon climbs steeply through the light elements, crests near iron-56 at about 8.79 MeV, then eases gently downward across the heavy nuclei. That single peak is the pivot of nuclear energy. Iron-56 is among the most tightly bound and most stable nuclides, so the two reactions here release energy by moving nuclei toward it, not away.
Higher binding energy per nucleon means a more tightly bound, more stable nucleus. Set the toggle to D-T fusion and stay below A = 56: light nuclei climbing the steep left slope release energy, which is why the direction label reads "fusion below iron." Switch to U-235 fission above the peak and heavy nuclei split downhill toward iron, again releasing energy, so the label flips to "fission above iron." The energy is real mass that goes missing. Products weigh slightly less than reactants, and the mass defect Δm becomes energy through E = Δm·c2, with 1 u equal to 931.494 MeV/c2 and c = 299,792,458 m/s. No nucleons are destroyed; only the binding rearranges.
The reaction panel shows why choices differ. U-235 fission yields about 200 MeV per event and D-T fusion only 17.6 MeV, yet fusion releases far more per nucleon — the energy density that lights the stars. Explore further with the E = mc2 calculator, the Half-Life Simulator, and the Speed of Light Simulator.
Because both move nuclei toward the peak of the binding-energy-per-nucleon curve near iron-56. Fusing light nuclei and splitting heavy ones both produce more tightly bound products, which releases energy.
From a mass defect. The products of the reaction are slightly less massive than the reactants, and that missing mass becomes energy through E = Δm·c². No nucleons are destroyed; the binding simply rearranges.
The energy needed to remove one nucleon from a nucleus, averaged over all of them. It peaks near iron-56 at about 8.8 MeV, which is why iron-region nuclei are the most stable.
Per reaction, uranium-235 fission releases about 200 MeV and D-T fusion about 17.6 MeV. But per nucleon, fusion releases far more, which is what makes it so energy-dense.