Neutron Proton Ratio Atomic Number >20
Date: Winter 2012-2013
Why does a nucleus need more neutrons than protons once they reach 20 protons in the nucleus, when a proton's electromagnetic force is not as great as a neutron's Strong Nuclear force? Would it not be the opposite, like once they reach 20 protons, you need more protons to hold the neutrons together because the Strong nuclear force is stronger than the electromagnetic force?
The protons repel each other, because they all have the same charge. The only thing that holds the nucleus together is the strong force.
When very close, the Strong Nuclear force is much stronger. At a distance of two protons away, the Strong Nuclear Force becomes very weak. It is a very short-range force. The electromagnetic force literally extends forever. Only near-neighbors (both neutrons and protons) hold the protons together. All of the protons in the nucleus push each other apart. Neutrons allow the protons to be a little further apart from each other. They act like spacers. They pull on protons and neutrons, but never push.
The universe is set up to keep in balance. Much of quantum physics is about balance and conservation. For a small nucleus, an exact balance between protons and neutrons works best. It is not necessary, but a balanced atom is more relaxed: it has less energy to release. For a larger atom with no extra neutrons, the energy stored in the electric force (per proton or neutron) is larger, but the energy per particle in the strong force is the same. Adding a few neutrons releases some of the electric energy per particle. A large atom with a few extra neutrons is more relaxed.
Then why do atoms not have huge numbers of neutrons? Neutrons are more massive than are protons. More mass means more E=mc^2 energy. Too many neutrons provide too much mass energy per particle and make an atom radioactive. For atomic number greater than 20, the most relaxed atom (the most balanced in energy) has a few extra neutrons.
Dr. Ken Mellendorf
Illinois Central College
Thanks for the question. The nucleus needs more protons than neutrons as the neutrons are needed to shield the protons from each other. Remember that the protons repel each other and having neutrons between the protons will decrease the repulsive forces. The strong force acts over short ranges only and bind protons to other protons and neutrons. However, the electromagnetic force (also called the Coulomb force) causes repulsions between protons. The electromagnetic force acts over short ranges and long ranges too.
I hope this helps. Please let me know if you have any other questions.
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Update: November 2011