Answer:
Potassium has 20 neutrons.
Explanation:
The atomic number for potassium is 19, so that tells you that there are 19 protons in one atom of potassium.
Once you know the number of protons, you also know the number of electrons. There are the same number of protons and electrons in one atom, so there are also 19 electrons in one atom of potassium.
To find the number of neutrons, you need to work with the 'atomic weight' of the atom. This is also given in the table. The atomic weight for potassium is 39.10. Round this number to the nearest whole number, which would make it 39. This number is called the 'mass number.'
Subtract the number of protons from the mass number. For potassium, your equation would be 39 minus 19.
The resulting answer, 20, tells you the number of neutrons are in one atom.