An atom of a sodium has 11 proton, 11 electrons, and 12 neutrons, what is its mass number?
Here are some of the rules:
(1) the atomic number,
(2) for ground state elements (no charge) the number of protons is equal to the number of electrons
otherwise, the number of electrons is greater than the number of protons if it's an anion (negative charge), and the number of electrons is less than the number of protons if it's a cation (positive charge).
(3) the atomic mass,
Therefore, to answer your question above, based on rule 3: