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.