Where Does Nuclear Energy Come From?

Weigh a nucleus and weigh its parts separately, and the nucleus always comes up short. That shortfall is the mass defect — mass that became binding energy when the protons and neutrons were fused together. This mass defect calculator finds it and converts it to energy.

The Numbers

Protons weigh 1.007276466621 u and neutrons 1.008664916 u. Sum the constituents, subtract the measured atomic mass, and multiply the difference by 931.494 MeV/u — the energy locked in each atomic mass unit. Dividing by A gives the binding energy per nucleon, the curve that peaks at iron-56.

Why It Matters

Nuclei lighter than iron gain energy by fusing (stars), heavier ones by splitting (reactors). The mass defect quantifies both: it is the exact energy released, and it explains why fission and fusion dwarf any chemical reaction.