Physics · nuclei · NEET
Stability is decided by binding energy PER nucleon, not total binding energy. Fe-56 has about 8.8 MeV per nucleon, near the top of the graph. This means the average energy holding each proton and neutron is maximum here, so the nucleus is the hardest to pull apart per particle. That is why iron is treated as the most stable nucleus in NEET.
For NEET, treat Fe-56 as the most stable nucleus, because that is what NCERT and the binding energy per nucleon graph show as the peak. In strict physics, Ni-62 has a slightly higher binding energy per nucleon, but this level of detail is NOT asked in NEET. Always answer iron (Fe-56) in the exam.
Fusion of light nuclei releases energy because the product has higher binding energy per nucleon. This rise continues only up to iron, the peak of the graph. Beyond iron, joining nuclei would need energy instead of releasing it. So a star cannot get energy by fusing iron into heavier elements, and fusion stops at iron.
Fission gives energy only when a heavy nucleus splits into pieces with higher binding energy per nucleon. Iron is already at the peak, so its fragments would have LOWER binding energy per nucleon. Splitting iron would need energy, not release it. That is why iron cannot power a nucleus by fission or fusion, which is why it is the end point of energy release.
It means highest binding energy per nucleon, which links to a high mass defect per nucleon, not the largest total mass defect. Heavy nuclei like uranium have a larger TOTAL binding energy and total mass defect just because they have more nucleons. Stability is judged per nucleon, so iron wins even though uranium has a bigger total.
About 8.8 MeV per nucleon, which is close to the maximum value on the binding energy per nucleon graph. This peak value is the reason iron is called the most stable nucleus.
Because it has the maximum binding energy per nucleon, so each nucleon is bound most tightly and the nucleus is hardest to break apart per particle.
No. Iron is at the peak of the binding energy per nucleon curve, so both splitting it (fission) and joining it (fusion) would need energy instead of releasing energy.
Uranium has more TOTAL binding energy because it has far more nucleons. But iron has higher binding energy per nucleon, which is what decides stability for NEET.
Iron sits at the top (peak) of the binding energy per nucleon versus mass number graph, around mass number A = 56, where the curve is flat and maximum.
