Mass number

number of heavy particles in the atomic nucleus

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:

  • atomic number (symbol: Z), which is the number of protons in a nucleus of an atom
  • atomic mass (symbol: ma), which is the mass of a single atom, commonly expressed in unified atomic mass units
  • relative atomic mass (symbol: Ar, also called atomic weight), which is the ratio of the average mass per atom of an element from a given sample to 1/12 the mass of a carbon-12 atom.

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.