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?

A. 19

B. 29

C. 38

D. 58

For atoms in the periodic table, the given mass number is the sum of the number of the protons (also called the atomic number) and the number of the neutrons inside its nucleus. Mathematically,

mass number = atomic number + number of neutrons

Substituting,

39 = 19 + n

n = 39 - 19 = 20

Therefore, the answer is not found in the choices.