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MotionLab
Why iron-56 is the most stable nucleus and why both fission and fusion release energy.
Binding energy per nucleon peaks near iron-56 (~8.8 MeV/nucleon) — lighter nuclei release energy by fusing toward iron (fusion, powering stars), while heavier nuclei release energy by splitting apart toward iron (fission, powering nuclear reactors). Both processes move toward greater stability.
The semi-empirical mass formula balances five terms: volume (bulk nuclear attraction), surface (fewer neighbors at the surface), Coulomb (proton-proton repulsion), asymmetry (neutron-proton imbalance penalty), and pairing (even numbers of nucleons are more stable).
Binding energy (MeV)
495.184
Semi-empirical mass formula (Weizsäcker)
What you entered
Volume term (avA)
15.56 × 56= 871.36 MeVSurface term (−asA^⅔)
−17.23 × 56^(2/3)= -252.1993 MeVCoulomb term (−acZ(Z−1)/A^⅓)
−0.7 × 26(26−1) ÷ 56^(1/3)= -118.9274 MeVAsymmetry term (−aa(A−2Z)²/A)
−23.285 × (56−2×26)² ÷ 56= -6.6529 MeVTotal binding energy
sum of all terms + pairing δ=1.6036= 495.1839 MeVBinding energy per nucleon
495.1839 ÷ 56= 8.8426 MeVResult
Binding energy (MeV): 495.184
A nucleus with Z = 26, N = 30 (A = 56) has a total binding energy of 495.18 MeV (8.843 MeV/nucleon).