To determine the number of protons and neutrons, you need to understand what are the meaning of the numbers written with the atom. The number that is written above the atom represents the mass number of the atom. The mass number is the sum of the number of protons and neutrons in the atom. The one written below would represent the atomic number of the atom and this represents the number of protons in the atom. So, for this case, mass number would be 39 and the atomic number would be 19. Therefore, the number of protons would be 19 and the number of neutrons would be 39-19=20.
