All atoms of the element potassium have 19 protons. One of the most stable types of potassium atoms has the mass number 39. How many neutrons make up one of these potassium atoms?

Mass number is the total number of protons and neutrons. Subtract the 19 protons from potassium’s mass number of 39: 39 − 19 = 20 neutrons. Therefore, the correct choice is 20.

AI-written and checked against the verified answer.