Why is atomic number called "Z"? Why is mass number called "A"?

Serena 9/16/99

atomic number*
atomic weight*
mass number*
mass spectrometer*
x-ray spectrum*
"Atomic number" in German is "Atomzahl", so the Z symbol for atomic number probably comes from "Zahl" (number). The Encyclopedia of Symbols has a more poetic interpretation:

    "The letter Z is one of the signs for the highest god in Greek mythology, Zeus. In modern physics Z represents the greatest energy, nuclear power, in its potential form, nuclear charge." [1]

M is sometimes used for mass number ("Massenzahl" in German), but A is the symbol recommended in the ACS Style Guide [2]. The symbol may have been chosen so that the positions of the mass and atomic numbers are less likely to be confused in a nuclide symbol:


  1. Symbols.com, HME Media, 1999. URL: http://www.symbols.com/index.html
  2. The ACS Style Guide : A Manual for Authors and Editors , Janet S. Dodd (Editor), American Chemical Society, 1998. ISBN: 0841234620

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! Why is atomic number called "Z"? Why is mass number called "A"?

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/history/faq/print-why-is-atomic-number-Z.shtml