Binding Energy Per Nucleon

NCERT presents the binding-energy-per-nucleon curve, which plots how tightly each nucleon is held as a function of mass number. The curve rises steeply for light nuclei, reaches a broad maximum of about 8.8 MeV per nucleon near mass number 56, and then declines slowly for heavier nuclei. The peak marks the most stable nuclei, which is why nuclei near iron are the endpoint of stellar fusion. The helium region is wrong because, although helium-4 is unusually stable for a light nucleus, its per-nucleon binding of about 7 MeV is below the iron peak. The uranium region is wrong because heavy nuclei have lower per-nucleon binding and can release energy by fission. The hydrogen region is wrong because a lone proton has zero binding energy per nucleon. As stated in NCERT Class 12, Chapter 13 (Nuclei), the shape of this curve explains why fusion of light nuclei and fission of heavy nuclei both release energy. A plausibility check: energy is released only when reactions move nuclei toward the iron peak, consistent with the curve's maximum lying there.