# Mass number

The mass number , nucleon , Nukleonenanzahl , sometimes core size is the number of nucleons ( protons and neutrons ) that the nucleus of a nuclide form. It is therefore the sum of the atomic number and the neutron number : ${\ displaystyle A}$ ${\ displaystyle Z}$ ${\ displaystyle N}$

${\ displaystyle A = Z + N \ ,.}$

The term “mass” indicates that the electrons in the atomic shell contribute less than 0.1% to the mass of the atom, so that the mass number approximately indicates the atomic mass in atomic mass units . ${\ displaystyle \ mathrm {u}}$

The symbol of the mass number is usually , in the field of mass spectrometry , however,${\ displaystyle A}$${\ displaystyle m \ ,.}$

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

## Use to denote the individual nuclide

The mass number is usually added to the element name or symbol when a specific nuclide (i.e. a specific isotope of the element) is to be identified, e.g. B. Carbon-12 or C-12, Uranium-238 or U-238, etc. In formulas, the mass number is written at the top left of the element symbol. The atomic number (number of protons) can be specified at the bottom left : ${\ displaystyle Z}$

${\ displaystyle {} _ {Z} ^ {A} {\ text {Symbol}} \ ,.}$

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

${\ displaystyle {} _ {\ 6} ^ {14} \ mathrm {C}}$

written. However, it is already defined by the element symbol and is therefore often omitted from the notation, unless it is of particular interest , as is the case with nuclear reactions . ${\ displaystyle Z}$

## Individual evidence

1. ^ Ulrich Harten: Physics. An introduction for engineers and scientists. 6th edition. Springer Vieweg, Berlin / Heidelberg 2014, ISBN 978-3-642-53854-4 , p. 313.
2. Entry on mass number . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.M03726 Version: 2.3.