Yes. The atomic number is the number of protons in an atom. Potassium always has 19 protons, and so its atomic number is 19. As soon as there is a change in the number of protons, then the element is different.
The atomic number of the potassium is 19. So the number of protons and electrons is 19 in the atom of potassium. The number of neutrons can be calculated as mass number minus atomic number. It comes to be 39 minus 19. That is 20.