The mass number A is defined as the sum of the number of protons Z and neutrons N of an atomic nucleus: A = Z + N. In mass spectrometry the mass number is abbreviated as m.
Since the heavy nuclear building blocks neutrons and protons are also referred to as nucleons (nuclear building blocks), we also speak of the nucleon number A or the nucleus size of an atom.
The term mass number comes from because the sum of the nucleons corresponds to the atomic weight of the atomic nucleus, rounded to an integer.
A specific chemical element E is clearly defined by the atomic number Z = proton number: The element 95E is 95Am, i.e. around Americium Am. Now, however, there are atomic nuclei of each element with different numbers of neutrons: isotopes (nuclides).
The mass number is used to designate a specific nuclide or isotope. The most stable of the Americium isotopes, for example, has 148 neutrons in addition to its 95 protons; The mass number is therefore 95 + 148 = 243 and the nucleus is called 243Am, Am-243 or Americium-243.
 - Mass number.
IUPAC Gold Book, DOI 10.1351/goldbook.M03726.
Last update: 2023-12-01
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