To determine the mass number of an atom of potassium with 20 neutrons, we need to find the number of protons in the nucleus.
Potassium (K) has an atomic number of 19, which means it has 19 protons. The mass number is the sum of protons and neutrons in the nucleus of an atom. So, if there are 20 neutrons, we add that to the 19 protons:
Mass number = Number of protons + Number of neutrons Mass number = 19 + 20 = 39
Therefore, an atom of potassium with 20 neutrons has a mass number of 39.