Potassium’s atomic number in periodic table is 19, Potassium can only have a mass numbers of 41, 40 and 39. Potassium 39, 40, and 41 are all different isotopes of the element potassium. Each of the isotopes has 19 electrons and 19 protons but the number of neutrons vary. So, in your question Potassium-39 has twenty (20) neutrons, and potassium-40 has 21 neutrons.
