Look up mass excess values, compute mass defect, and check stability for any nuclide by entering Z and A.
The mass excess of a nuclide is defined as the difference between its actual atomic mass (in amu) and its mass number A, converted to energy units: Δ = (M − A) × 931.494 MeV. It's a convenient way to express nuclear masses because it directly shows how far a nuclide's mass deviates from the integer approximation.
Nuclides with large negative mass excess values tend to be more tightly bound and more stable. The most tightly bound nuclei (like Fe-56 and Ni-62) have the most negative mass excess per nucleon. This tool uses the semi-empirical mass formula to estimate values — for precise measurements, refer to the Atomic Mass Evaluation (AME) database maintained by the IAEA.