Splitting a heavy nucleus releases so much energy because the resulting fragments are more tightly bound than the original nucleus. In nuclear physics, stability is measured by binding energy per nucleon—the amount of energy holding each proton and neutron in place. Heavy nuclei, such as uranium or plutonium, have relatively low binding energy per nucleon. When they split, the resulting medium-sized nuclei have higher binding energy per nucleon. This difference in binding energy appears as released energy, following the principle that systems naturally evolve toward states of lower total energy.
The key reason heavy nuclei have lower binding energy is that they contain many protons. Protons repel each other through the electrostatic force, which grows stronger with increasing charge. The strong nuclear force holds nucleons together, but it is short-range; beyond a few femtometers, it becomes ineffective. In very large nuclei, distant protons repel more than the strong force can compensate. This creates a stressed, unstable structure. When the nucleus splits, each fragment has fewer protons, making the strong force relatively more effective in those smaller systems. As a result, the binding energy per nucleon rises.
The energy released during fission is enormous because nuclear forces are far stronger than chemical bonds. A typical fission event converts about 0.1% of the nucleus’s mass into energy through Einstein’s equation, E = mc². Although this fraction seems small, the mass-to-energy conversion is so efficient that even tiny amounts of nuclear fuel yield vast amounts of energy—millions of times more than chemical reactions.
Another contributor to the energy release is the motion of the fission fragments. When the nucleus splits, the repulsive force between the newly formed positively charged fragments accelerates them apart. This produces large kinetic energies, which are later transferred to surrounding matter as heat. Additional energy also comes from emitted neutrons and gamma radiation. These neutrons can trigger further fission events, enabling chain reactions that amplify the total energy output.
Ultimately, splitting a heavy nucleus releases energy because it transforms an unstable, weakly bound configuration into smaller, more stable ones with higher binding energy. The difference in energy is not lost—it is liberated, demonstrating how powerful nuclear rearrangement can be.
Frequently Asked Questions
Why are medium-sized nuclei more stable?
Because the strong nuclear force is more effective relative to proton repulsion in smaller systems, increasing binding energy per nucleon.
Does mass really disappear during fission?
A small amount converts into energy according to E = mc², and this accounts for the large energy release.
Why do fission fragments fly apart so fast?
Because strong electrostatic repulsion between the positively charged fragments accelerates them immediately after the split.
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