Mass number

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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 :

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 .

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

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 :

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

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 .

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.