Mass excess

{
  "id": "MLC-T-CHM-006977",
  "term": "Mass excess",
  "field": "Chemistry",
  "definition": "The difference between an atom's actual mass and the value obtained by multiplying its mass number by the atomic mass constant, usually quoted in energy units such as keV or MeV. It is zero for carbon-12 by the definition of the atomic mass unit, positive for hydrogen-1 and helium-4, and negative for many medium-mass nuclides such as iron-56. It differs from the mass defect, which compares an atom's mass with the summed masses of its separate nucleons.",
  "subject": "Physical and Biophysical Chemistry",
  "see_also": [
    "atomic mass constant",
    "mass number"
  ],
  "references": [
    "Green Book, 2nd ed., p. 20 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)"
  ],
  "url": "https://mlchart.com/terminology/chemistry/mass-excess/"
}