An atom of potassium with a mass number of 40 is known as K-40. Use the periodic table to determine how many neutrons are present in an atom of K-40. Type the correct answer in the box. Use numerals instead of words. One atom of K-40 has neutrons.
Neutral atoms of the same element always have the same number of protons and electrons. Only the neutron count changes with different isotopes. Potassium has 19 protons, so a K-40 atom has 21 neutrons (40 − 19 = 21).