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.

References change

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