atomtömegtl

Atomtömegtal, commonly denoted A, is the atomic mass number of a nuclide. It represents the total number of protons and neutrons in the nucleus, so A = Z + N, where Z is the atomic number (protons) and N is the number of neutrons. Because protons and neutrons contribute most of the nucleus’s mass, A identifies a specific nuclide and is an integer. Different isotopes of the same element have different A values. In isotopic notation, a nuclide is written as X-A, where X is the element symbol and A is the mass number, for example carbon-12 (12C) or carbon-14 (14C).

The mass number is not the same as atomic mass or atomic weight. The atomic mass of a nuclide is its mass measured in unified atomic mass units (u), while the atomic weight (relative atomic mass) is the average mass of all natural isotopes of an element, weighted by their abundances. For light elements, A roughly approximates the nuclide’s mass in atomic mass units, but precise masses differ due to binding energy and electron mass.

A is widely used in nuclear physics, chemistry, and geochronology. It helps describe decay processes, reaction energetics (Q-values), and isotopic identification in spectroscopy and mass spectrometry. While A provides a simple count of nucleons, it does not capture binding energy, mass defects, or electronic structure, which are addressed by more detailed mass measurements.