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? 19 20 38 58

The correct answer is 20.
All atoms has three sub atomic particles, which are electron, proton and neutron. For a neutral atom, the number of proton and neutron are the same. The mass number of an atom is the summation of the number of proton and neutrons, which are located inside the nucleus of an element. 
For potassium, since it has 19 protons, it will also have 19 electrons. 
Mass number = number of proton + number of neurtron
Mass number = 39
Proton number = 19
Neutron number = mass number - neutron number = 39 - 19 = 20
Thus, the neutron number = 20.

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