An atom of sodium has 11 protons, 11 electrons,and 12 neutrons, what is its atomic number and mass number?
The atomic number tells you the number of protons for an atom. Since the atom is neutral, the number of electrons must be the same as the number of protons.
The mass number is found by adding the number of protons and neutrons.
Here is a video which discusses this concept.
Video from: Noel Pauller
Hope this helps!