Nuclear Binding Energy Calculator
Why fusion works below iron and fission above it.
Work out Nuclear Binding Energy. Why fusion works below iron and fission above it. Refuses out-of-range input instead of guessing.
Binding energy per nucleon
8.7903 MeV
492.26 MeV total across 56 nucleons
A nucleus weighs less than the sum of its parts, and the missing mass is the binding energy by E = mc². The defect is under 1% of the total mass, which is why chemistry never notices it and why nuclear reactions release millions of times more energy than chemical ones. Binding energy per nucleon peaks near iron-56 at about 8.8 MeV. That peak is why fusion releases energy for light elements and fission releases it for heavy ones — both move toward iron, and neither can go past it. It is also why stellar fusion stops at iron, and why what happens next is a supernova. The comparison here uses hydrogen atoms rather than bare protons, so the electron masses appear on both sides and cancel. Using proton masses against an atomic mass would leave Z electrons unaccounted for and overstate the defect.
How the Nuclear Binding Energy Calculator works
Binding energy and mass defect from a measured atomic mass. Binding energy per nucleon peaks near iron-56, which is why fusion releases energy below it and fission releases it above — both move toward the same peak.
Also known as: mass defect to energy · why does fusion release energy · binding energy per nucleon iron · e equals mc squared nuclear
Frequently asked questions
What is mass defect?
The difference between a nucleus's measured mass and the sum of its parts. The missing mass is the binding energy, by E = mc², and it is what holds the nucleus together.
Why does binding energy peak at iron?
Below iron the strong force gains more from adding nucleons than electrostatic repulsion costs; above it, repulsion between the growing number of protons wins. Iron-56 is the balance point at about 8.8 MeV per nucleon.
Why does fusion release energy?
Because light nuclei are loosely bound. Fusing them moves toward the iron peak, and the difference in binding energy comes out as energy — which is what powers every star.
Why does fission release energy?
The same reason from the other side. Heavy nuclei are also loosely bound per nucleon, so splitting them also moves toward iron and releases the difference.
How big is the mass defect?
Under 1% of the total mass, which is why chemistry never notices it. But c² is enormous, so that fraction of a percent is millions of times more energy than any chemical reaction releases.
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