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
To find the number of neutrons in an atom, subtract the atomic number (number of protons) from the mass number.
So, the number of neutrons in one of these potassium atoms with a mass number of 39 would be:
39 - 19 = 20.
Therefore, there are 20 neutrons in one of these potassium atoms.