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Name: Karl
Status: student
Age: N/A
Location: NY
Country: US
Date: N/A

Why couldn't atoms be made higher than number 138? Is there an upper limit to atomic numbers?

Yes. Although we do not know exactly what the limit is, it is clear that there is an upper limit. There are competing forces within a nucleus: an attractive force and a repelling force (and lesser forces that we can ignore for now). For a large enough nucleus, the repelling force wins. Here is how it works:

The attractive force (called the nuclear force) is very strong but very short range. It attracts nuclear particles (protons and neutrons) to each other only if they are almost touching. The nuclear force does not much care whether the particles are protons or neutrons; it is pretty much the same for both.

The other force is the electromagnetic force. It acts only between charged particles, and the only nuclear particle with a charge is the proton. Protons all have the same charge, and the electromagnetic force causes like charges to repel each other. The electromagnetic force is much weaker than the nuclear force, but it is a long range force; protons in a nucleus are repelled by all other protons in the nucleus, no matter how big the nucleus is.

So you can see where this is going. If protons are attracted only by their near neighbors, but repelled by all the other protons in a nucleus, the larger the nucleus, the stronger will be the repelling force on the outermost protons. But the attractive force on a proton does not increase as a large nucleus gets larger, because it is so short range. For a large enough nucleus, the repelling force on the outermost protons is greater than the attractive force of their near neighbors, and the outermost protons leave.

The larger nuclei take advantage of the fact that the nuclear force does not much care whether a particle is a proton or a neutron, by having more neutrons than protons. This lessens the repulsive force (fewer positively charged protons) without changing the attractive force very much. But there's a limit to how many neutrons can pack together. A neutron will decay (into a proton, an electron, and a neutrino) if it can. Neutrons inside a stable nucleus do not have enough energy to decay, but neutrons that are not near a proton do have enough energy. (I have not told you why this is so, because neutron decay is pretty far from the topic.)

Tim Mooney

Dear Karl,

There certainly is an upper limit to the atomic number of stable atoms and that limit is 92, the atomic number of uranium. Nuclei are made up of Z protons (the atomic number), which are positively charged, and N neutrons (the neutron number), which are electrically neutral. The mass number A = Z + N.

The reason massive nuclei with many protons are unstable is that protons, being positively charged, repel one another. When packed closely in a nucleus, they repel one another strongly. This is why there are more neutrons than protons in heavy nuclei. Uranium has 92 protons and 146 neutrons for a mass number of A = 238.

For maximum stability if protons were neutral, there would be equal numbers of neutrons and protons. (This is explained by the Pauli Exclusion Principle, which I will not go into here.) This is seen in light nuclei where the coulomb forces between the protons are not overwhelming. For example, helium (the most stable nucleus) has 2 protons and 2 neutrons. Carbon has 6 of each.

I hope this is helpful; a full explanation would be quite involved.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


The nucleus of an atom has two major forces: "strong" and "electromagnetic". The strong force holds the nucleus together. All protons and neutrons in the nucleus pull on their neighboring protons and neutrons. The electromagnetic force pushes the nucleus apart. All protons, and only the protons, push on all the protons of the nucleus. Attraction is limited to nearby particles. Repulsion is from the entire nucleus. As a nucleus grows, the attraction that each particle feels stays the same. The number of near neighbors does not change. As a nucleus grows, the repulsion increases. The number of protons increases. If a nucleus gets too big, the repulsion force overcomes the attraction force and the atom breaks into two pieces. This is known as fission. When at the current limit for atomic number, the atom falls apart before another proton can be added.

Dr. Ken Mellendorf

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