• $summary_{type}$: concept
• $word_{count}$: 180

The mass of any nucleus is less than the sum of its constituent free nucleons. This mass defect $Delta_m$ = Z*$m_p$ + (A-Z)*$m_n$ - $M_{nucleus}$ converts to binding energy: BE = $Delta_m$ x 931.5 MeV. This energy must be supplied to completely disassemble the nucleus. Binding energy per nucleon $\frac{BE}{A}$ is the key stability indicator. The BE/A curve rises steeply from H-2 (1.1 MeV) to He-4 (7.07 MeV), peaks at Fe-56 (8.79 MeV — most stable), then gradually falls to U-238 (7.57 MeV). Local maxima at He-4, C-12, O-16 correspond to magic numbers (closed nuclear shells). The curve's shape explains both fusion (light nuclei combine, moving toward the peak) and fission (heavy nuclei split, also moving toward the peak). Both processes release energy equal to the increase in total binding energy. When atomic masses are given (including electrons), use $M_H$ instead of $m_p$; the electron masses cancel automatically.