Nucleon Number

The nucleon number (mass number), $A$, counts the total number of particles in the nucleus:

$A = Z + N$

where $Z$ is the proton number and $N$ is the number of neutrons.

In nuclide notation, the nucleon number is written as a superscript: $^{A}_{Z}X$.

Isotopes of the same element have the same $Z$ but different $A$ values. The nucleon number determines the approximate relative atomic mass of the isotope.