Mass number

The mass number, nucleon number, nucleon number, sometimes also nuclear number is the number of nucleons (protons and neutrons) forming the nucleus of a nuclide. It is thus the sum of the atomic number and the neutron number :

The term "mass" number refers to the fact that the electrons of the atomic shell contribute less than 0.1% to the mass of the atom, so the mass number approximates the atomic mass in atomic mass units .

The formula symbol of the mass number is usually , but in the field of mass spectrometry

Tenness-294 and Oganesson-294 have the highest detected mass number to date (as of July 2018).

Use for designating the individual nuclide

The mass number is usually added to the element name or symbol when a particular nuclide (i.e., a particular isotope of the element) is to be designated, e.g., carbon-12 or C-12, uranium-238 or U-238, and so on. In formulas, the mass number is written at the top left of the element symbol. The atomic number (number of protons) can be entered at the bottom left:

The carbon isotope with mass number 14 is thus expressed in formulas as

is written. However, is already specified by the element symbol and is therefore often omitted from the notation unless it is of particular interest, as in nuclear reactions.