Ydinmassan

Ydinmassan, or the nuclear mass, is the rest mass of an atomic nucleus, consisting of Z protons and N neutrons (A = Z + N). It is a fundamental property of nuclides and is most commonly expressed in atomic mass units (u) or in energy units (MeV/c^2) through E = mc^2. For any nuclide, the nuclear mass reflects the energy that binds the nucleons together inside the nucleus.

The mass of a bound nucleus is smaller than the sum of the masses of its constituent free nucleons. This difference, the mass defect Δm, equals the binding energy divided by c^2: Δm = Z mp + N mn − Mnucleus and B = Δm c^2. The average binding energy per nucleon is about 8 MeV, varying across the chart of nuclides; heavy nuclei bind more tightly per nucleon than very light ones.

Atomic masses include the mass of electrons; the nuclear mass Mnucleus is obtained from the atomic mass Ma by Mnucleus ≈ Ma − Z me − E_e(Z), where me is the electron mass and E_e is the total electronic binding energy. In many nuclear physics contexts Mnucleus is quoted via the mass excess, Δ, defined as Δ = (Mnucleus − A u) c^2, usually expressed in MeV; with 1 u ≈ 931.494 MeV/c^2.

Mass values are obtained through direct and indirect measurements, using Penning-trap mass spectrometry, storage rings, and time-of-flight methods. The best-known compilations are the Atomic Mass Evaluation (AME) tables, which provide recommended masses and uncertainties for nuclides across the chart. Nuclear masses underpin reaction Q-values, decay energies, and predictions of nucleosynthesis in stars.

Beyond testing nuclear models, ydinmassan data inform applications in medicine, energy, and astrophysics. They also drive theoretical developments, such as semi-empirical mass formulas and mass models that describe trends across isotopes.