Explanation

• The nucleon number (mass number) of Potassium is 39, and the atomic number (number of protons) is 19.
• To find the number of neutrons, subtract the number of protons from the nucleon number:
• 39 (nucleon number) - 19 (proton number) = 20
• So, the number of neutrons in Potassium-39 is 20.