Stoichiometry, Moles, and Molecular Weight

"A" is the mass number of the nucleus, or the total number of protons and neutrons.  "Z" is the atomic number of atom "X," or the number of protons in atom "X."  For example, a nuclide of sodium might be the following:

(Sodium nuclide) 23/11 Na

In most cases, atoms are electrically neutral, where the number of electrons is the same as the number of protons.  In the aforementioned example, sodium has the same number of electrons and protons, eleven.