Calculating Binding Energy per Nucleon
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Calculating binding energy per nucleon means finding a nuclide's mass defect from the difference between its separate-particle mass and its measured mass, converting that mass defect to energy using the mass-energy equivalence relationship, and dividing by the total number of nucleons.
For iron-56, comparing the summed mass of 26 protons, 30 neutrons, and 26 electrons against the nuclide's measured mass of 55.9349 amu gives a mass defect that converts to a binding energy near 492 MeV total, or about 8.8 MeV per nucleon once divided across all 56 nucleons.
Comparing a bare-nucleon sum (protons and neutrons only) against the atomic, electron-inclusive measured mass mismatches the two sides — the summed side must include electrons too, as the iron-56 example does — and reporting total binding energy where per-nucleon was asked reverses stability comparisons between nuclides.