Binding Energy Curve

Iron-56 is the most tightly bound nucleus in nature. Every lighter element would release energy by fusing toward iron. Every heavier element would release energy by splitting toward iron. This single fact explains why both fission and fusion produce energy, and why iron is the endpoint of both processes.

The binding energy per nucleon measures how tightly protons and neutrons are held together in a nucleus. Plotted against atomic mass number, it forms a curve that rises steeply for light elements, peaks at iron-56 (about 8.8 MeV per nucleon), and declines gradually for heavy elements. The curve is the roadmap for nuclear energy.

Fission works at the heavy end. Uranium-235 has about 7.6 MeV/nucleon. Its fission products (elements near mass 90-140) have about 8.5 MeV/nucleon. The difference, roughly 0.9 MeV per nucleon times 235 nucleons, is released as energy: about 200 MeV per fission event.

Fusion works at the light end. Deuterium and tritium (hydrogen isotopes) have about 1-2.8 MeV/nucleon. Their fusion product (helium-4) has about 7.1 MeV/nucleon. The jump is even larger per nucleon than fission, which is why fusion releases more energy per unit mass.

Worked Example

Compare the energy released per nucleon from fissioning uranium versus fusing deuterium-tritium.

• Fission. Products gain about 8.5 - 7.6 = 0.9 MeV per nucleon.
• Fusion. Products gain about 7.1 - 2.8 = 4.3 MeV per nucleon (approximate average).

Fusion releases about how many times more energy per nucleon than fission?

4.3 / 0.9 = about 4.8 times more per nucleon. This is why fusion fuel (hydrogen isotopes) has even higher energy density than fission fuel (uranium), and why achieving controlled fusion is one of the most sought-after goals in energy research.

Every reactor operating today sits on the heavy side of that curve, splitting toward iron. Every fusion program on Earth is racing to build a machine that works the light side. Same curve, opposite direction, same destination.

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