Massdefekter

Massdefekter, or mass defect, refers to the difference between the total mass of the free nucleons that make up a nucleus (the Z protons and N neutrons) and the actual mass of the nucleus itself. In symbols, Δm = Z mp + N mn − M(nucleus). This deficit arises because energy is released when nucleons bind together via the strong nuclear force, and, according to Einstein’s E = mc^2, some of the mass is converted into binding energy.

The binding energy associated with the mass defect is E_b = Δm c^2. In practice, Δm is often expressed in atomic mass units (u) and E_b in MeV, with 1 u ≈ 931.5 MeV/c^2. The mass defect thus provides a direct measure of how tightly a nucleus is bound: larger Δm (in the right range) corresponds to greater stability and a larger binding energy.

Mass defects vary across isotopes and are a key part of understanding nuclear stability and nuclear reactions. The binding energy per nucleon generally increases for light nuclei, peaks around iron-56, and then slowly decreases for heavier nuclei. This distribution explains why fusion releases energy for light elements and fission releases energy for very heavy elements.

In addition to the nuclear mass defect, there is an atomic mass defect, which considers the masses of the entire neutral atom, including electrons. Electron binding energies are small compared with nuclear binding energies, but they contribute to the total mass difference measured in precise mass determinations.

Mass defects are central to calculations of nuclear binding energies, reaction energetics, and the overall understanding of nuclear structure and stability.