Mass number

The mass number (symbol: A) of an atom is the sum of the number of protons and the number of neutrons in the nucleus.[1]:20 The mass number is different for each isotope of a chemical element.

We write a mass number after an element's name or as a superscript to the left of an element's symbol. For example, the most common isotope of carbon has 6 protons and 6 neutrons. We write it as carbon-12 or 12C.

Mass number is not the same as:

The difference between the mass number and the atomic number gives the number of neutrons (N) in a given nucleus: N = A Z.[1]:44

The mass number is not shown on the periodic table.[1]:21 For each element, what is shown is the standard atomic weight and the atomic number.


  1. 1.0 1.1 1.2 Moore, John T. (2010). Chemistry Essentials For Dummies. Wiley. ISBN 978-0-470-61836-3.

Categories: Chemistry

Information as of: 29.10.2020 01:23:42 CET

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