Too Many Neutrons: Why Does Nuclear Stability Decrease?

In summary: This is a double beta-decay train, first to Xe-128 (which is stable), then to Ba-128 (ditto). It is so long-lived that it was actually measured to have a half-life, which is amazing in its own right.In summary, when a large nucleus has too few neutrons, the binding energy is not enough to hold the nucleus together. By adding neutrons, the binding energy increases without increasing the repulsion between nucleons, allowing the nucleus to stay together. However, atoms with too many neutrons can also undergo nuclear decay, which is explained by the asymmetry between the strong nuclear force and the Coulomb force acting on adjacent nucleons. Some elements, like Technetium
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gsingh2011
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My teacher explained that when a large nucleus has too few neutrons, the binding energy is not enough to hold the nucleus together. But by adding neutrons the binding energy increases without increasing the repulsion between nucleons, and thus the nucleus can stay together. But atoms that have too many neutrons also undergo nuclear decay. If the previous logic is right, those atoms should be very stable, so why does too many neutrons make the atom unstable?
 
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do you understand why a single neutron is unstable?
 
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I have not heard of that. Can you explain?
 
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gsingh2011 said:
atoms that have too many neutrons also undergo nuclear decay.[...] why does too many neutrons make the atom unstable?

If I recall correctly there are models for the the physics of the nucleon, where there are nested configurations, somewhat analogous to electron configuration as nested structures.

Recapitulating the case of electrons: elements where all atomic orbitals that are occupied by electrons are completely filled are chemically the most stable.

If I recall correctly: just as the electron configuration is reflected in the ordering of the elements in the periodic table, one can use nucleon configuration to order nucleons.

Presumably the instability of Technetium is explained along these lines. (For most elements there are several stable isotopes, but in the case of Technetium (42 protons) there is not even one stable isotope.)

It has also been hypothesized that above atomic number 100 there may be one or more 'islands of stability', for nucleon configurations where all the "shells" are perfectly filled.
 
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FAQ: Why does the line of stability have the average over-all shape it does?

For light nuclei, the line of stability hugs the N=Z line, and this is because of the Pauli exclusion principle. If you have N=8 and Z=8 (16O), you can put the 8 neutrons in the 8 lowest energy states, and the 8 protons in the 8 lowest energy states. With N=10 and Z=6 (16C), the exclusion principle forces you to put those last few neutrons in high-energy states that weren't occupied in 16O.

For heavy nuclei, the mutual electrical repulsion of the protons breaks the symmetry in the way the strong nuclear force treats neutrons and protons. This effect favors higher N/Z ratios, so the line of stability bends away from N=Z.

The line of stability also has little wiggles superimposed on top of its broad over-all curve. These are caused by quantum mechanical shell effects, the nuclear analogs of the ones in atomic physics that make the noble gases so chemically stable. These shell effects have nothing to do with the over-all shape of the line of stability. For example, the nucleus 100Sn (N=50, Z=50) has two closed shells, but it is very far from the line of stability.
 
  • #8


gsingh2011 said:
My teacher explained that when a large nucleus has too few neutrons, the binding energy is not enough to hold the nucleus together.

It dawned on me that the reply about nuclear configuration in shells, partly or completely filled, doesn't address the actual question.

Let me generalize, making the reasoning independent from how the nucleus is modeled (other models treat the nucleus as analogous to a drop of fluid)

The strong nuclear force acts between nucleons (protons and neutrons). The strong nuclear force is extremely short range. The Coulomb force between the protons falls off with distance too, (inversely proportional to the square of the distance) but the strong nuclear force is in effect only active between adjacent nucleons.

So you have the Coulomb force acting significantly over distances that span multiple adjacent nucleons, but the force that provides cohesion of the nucleus, the strong force, acts only between adjacent nucleons.
 
  • #9


Cleonis said:
If I recall correctly there are models for the the physics of the nucleon, where there are nested configurations, somewhat analogous to electron configuration as nested structures.

Recapitulating the case of electrons: elements where all atomic orbitals that are occupied by electrons are completely filled are chemically the most stable.

If I recall correctly: just as the electron configuration is reflected in the ordering of the elements in the periodic table, one can use nucleon configuration to order nucleons.

Presumably the instability of Technetium is explained along these lines. (For most elements there are several stable isotopes, but in the case of Technetium (42 protons) there is not even one stable isotope.)

It has also been hypothesized that above atomic number 100 there may be one or more 'islands of stability', for nucleon configurations where all the "shells" are perfectly filled.
Slight correction, Tc has 43 protons, whereas Mo has Z=42. Tc is the lightest element without a stable isotope.

Promethium, Pm (Z=61) is another element lighter than Bi and without a stable isotope.

Bi-209 (Z=83) is the heaviest nuclide considered stable. All other nuclides of Z>83 are radioactive.

Nuclides with too few neutrons (or too many protons) may decay by positron emission (generally restricted to lighter radionuclides with neutron deficit) or electron capture.
 
  • #10


Bi-209 is very, very slightly radioactive. About 10 years ago it was discovered that it has a half-life in the few x 1019 year ballpark. It had been calculated for quite some time that this was alpha-unstable to Ti-205 with a 3 MeV alpha, but it wasn't observed until then.

Of course, with a half-life this long (much longer than the age of the universe), it is for all intents and purposes stable. The most radioactive thing in a typical bottle of Pepto-Bismol is probably the glass. :wink:

The longest measured half-life that I am aware of is Te-128, at a few x 1024 years.
 

What is nuclear stability?

Nuclear stability refers to the ability of an atom's nucleus to remain unchanged over time. This is determined by the balance between the strong nuclear force, which holds the nucleus together, and the repulsive force between protons, which can cause the nucleus to break apart.

Why does nuclear stability decrease with too many neutrons?

When there are too many neutrons in a nucleus, the repulsive force between them becomes stronger, disrupting the balance with the strong nuclear force. This can lead to the nucleus becoming unstable and undergoing radioactive decay.

How does the number of protons affect nuclear stability?

The number of protons in a nucleus, also known as the atomic number, determines the element and can also affect its stability. Elements with higher atomic numbers tend to have less stable nuclei, as the repulsive force between protons increases with more protons present.

What is the role of neutrons in a nucleus?

Neutrons play a crucial role in maintaining nuclear stability. They add mass to the nucleus, which helps balance the repulsive force between protons. They also help to "dilute" the repulsive force by increasing the overall size of the nucleus.

Can nuclear stability be increased?

Yes, nuclear stability can be increased through processes such as nuclear fusion and neutron capture. These processes can change the balance of forces within a nucleus, making it more stable. Additionally, certain isotopes of elements may have more stable nuclei than others due to their specific arrangement of protons and neutrons.

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