Massnummer

Massnummer, in German often referred to as Massenzahl, is the total number of nucleons in the nucleus of an atom. It equals the sum of protons (Z) and neutrons (N), so A = Z + N. The mass number is an integer and is used to distinguish isotopes of the same element, for example carbon-12 (Z = 6, N = 6, A = 12) and carbon-14 (Z = 6, N = 8, A = 14).

The mass number is not the same as the atomic mass. While A counts nucleons, the actual atomic mass is measured in atomic mass units and is affected by the binding energy that holds the nucleus together. The mass of a nucleus is slightly less than A times the atomic mass unit due to this binding energy, a phenomenon known as the mass defect. Precise masses require experimental measurement, whereas the mass number remains a simple, discrete count.

Mass numbers are used to label and categorize nuclei and to describe nuclear properties and reactions. In nuclear reactions, the total mass number is approximately conserved. The stability of an isotope depends on the neutron-to-proton ratio (N/Z); certain A values are associated with more stable nuclei, while others are radioactive and decay through processes such as beta decay or alpha decay.

Usage note: in German, the standard term is Massenzahl; Massnummer is a less common alternative that may appear in some texts. The notation for isotopes typically writes the mass number as a superscript before the chemical symbol, e.g., 14C or 238U, indicating A alongside Z.