Just Ask Antoine!
Atoms & ions
Energy & change
The quantum theory
Electrons in atoms
The periodic table
Acids & bases
Why is atomic number called "Z"? Why is mass number called "A"?
"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." 
M is sometimes used for mass number ("Massenzahl" in German), but A is the symbol recommended in the ACS
Style Guide . 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:
- Symbols.com, HME Media, 1999. URL: http://www.symbols.com/index.html
The ACS Style Guide : A Manual for Authors and Editors , Janet S. Dodd (Editor), American Chemical Society, 1998.
Author: Fred Senese firstname.lastname@example.org