Is anyone aware of the explanation behind why technetium is radioactive while molybdenum and ruthenium, the two elements astride it in the periodic table are perfectly normal?
Searching on google on why certain elements are radioactive are giving results which are descriptive, as in X is radioactive, Y is radioactive, Z is what happens when radioactive decay occurs, etc.
None seem to go into the theories which have been proposed to explain why something is radioactive.
The dynamics of the strong nuclear force are not well understood when high numbers of nucleons are involved. By which I mean, we have some empirical models that kinda-sorta work for various regimes, given some tinkering with the constants, but we have no from-first-principles understanding. You by no means need to go as far as biology before you get into stuff we cannot calculate from the equations; but in this case we don’t even know the equations all that well, because the strong-force constant (ie, the equivalent of G in gravity and alpha in electromagnetism) varies drastically with the energy involved, and we don’t know exactly how it varies. (“So why”, you ask plaintively, “is it called a constant?” By analogy with G and alpha, which genuinely are constants so far as anyone knows.) So while nuclear dynamics are not my particular subfield of physics, I would be unsurprised to learn that the answer to your question is “N. N.’s PhD thesis, submitted 2025”.
One more observation: Nuclear dynamics is the field in which physicists refer unironically to Magic Numbers; that is, some numbers of protons and neutrons are particularly stable compared to their neighbours, and it’s not quite clear why. Presumably there’s some sort of symmetry involved.
For the general question “why are some elements/isotopes less or more stable”—generally an isotope is more stable if it has a balanced number of protons and neutrons .
At low atomic number, isotopes that are more stable tend to be close to a 1:1 ratio of neutrons to protons. At high atomic number, this ratio approaches 3:2. I do not know why this is the case, and I believe it is not entirely understood by anyone. Also, this is not a very good predictor anyway.
The real problem is that unlike electron energy levels in an atom, which are well known and easily approximable by various systems and techniques, the nuclear energy levels are not very well understood, and I think to an extent they are even difficult to measure. I believe it is known that unlike the electrons’ spherical potential well, the nucleons are bound in a well that is a mixture of a spherical and cubic well, and the exact form is unknown, thus we can’t predict the levels very well. I don’t know why this is the case, and I believe it is not entirely understood by anyone else either.
In short, I think that a good theoretical model that predicts these kind of things has yet to come.
I do not know why this is the case, and I believe it is not entirely understood by anyone
We know exactly why the balance tends more towards the neutrons for heavier elements, but the system is messy enough that it’s very hard to predict just how much it does.
I believe it is known that unlike the electrons’ spherical potential well, the nucleons are bound in a well that is a mixture of a spherical and cubic well,
Sphericality and cubicality are orthogonal issues, and not that big a deal in the grand scheme of things. The main issues that make nucleons harder than electrons are:
1) There isn’t an externally imposed force that dominates the system (for the electrons, that’s the nucleus); it’s all internal, and that’s harder. Every time you add a new particle, the new ground state is little like the old ground state. For electrons, a thorough understanding of Hydrogen tells you nearly everything you need to know about, say, Oxygen; at a nuclear level, a thorough understanding of Hydrogen barely tells you anything about Oxygen.
2) The questions you need to answer are much much harder. You aren’t perturbing the system and finding the new ground state, like in chemistry. You need to find barrier heights and transition rates on upheavals to the whole system.
3) Last and least, there are two species (electrons → protons AND neutrons), with differences in how they feel the forces.
Is anyone aware of the explanation behind why technetium is radioactive while molybdenum and ruthenium, the two elements astride it in the periodic table are perfectly normal? Searching on google on why certain elements are radioactive are giving results which are descriptive, as in X is radioactive, Y is radioactive, Z is what happens when radioactive decay occurs, etc. None seem to go into the theories which have been proposed to explain why something is radioactive.
The dynamics of the strong nuclear force are not well understood when high numbers of nucleons are involved. By which I mean, we have some empirical models that kinda-sorta work for various regimes, given some tinkering with the constants, but we have no from-first-principles understanding. You by no means need to go as far as biology before you get into stuff we cannot calculate from the equations; but in this case we don’t even know the equations all that well, because the strong-force constant (ie, the equivalent of G in gravity and alpha in electromagnetism) varies drastically with the energy involved, and we don’t know exactly how it varies. (“So why”, you ask plaintively, “is it called a constant?” By analogy with G and alpha, which genuinely are constants so far as anyone knows.) So while nuclear dynamics are not my particular subfield of physics, I would be unsurprised to learn that the answer to your question is “N. N.’s PhD thesis, submitted 2025”.
One more observation: Nuclear dynamics is the field in which physicists refer unironically to Magic Numbers; that is, some numbers of protons and neutrons are particularly stable compared to their neighbours, and it’s not quite clear why. Presumably there’s some sort of symmetry involved.
The answer to the specific question about technetium is “it’s complicated, and we may not know yet”, according to physics Stack Exchange.
For the general question “why are some elements/isotopes less or more stable”—generally an isotope is more stable if it has a balanced number of protons and neutrons .
Here’s what I know about the matter:
At low atomic number, isotopes that are more stable tend to be close to a 1:1 ratio of neutrons to protons. At high atomic number, this ratio approaches 3:2. I do not know why this is the case, and I believe it is not entirely understood by anyone. Also, this is not a very good predictor anyway.
The real problem is that unlike electron energy levels in an atom, which are well known and easily approximable by various systems and techniques, the nuclear energy levels are not very well understood, and I think to an extent they are even difficult to measure. I believe it is known that unlike the electrons’ spherical potential well, the nucleons are bound in a well that is a mixture of a spherical and cubic well, and the exact form is unknown, thus we can’t predict the levels very well. I don’t know why this is the case, and I believe it is not entirely understood by anyone else either.
In short, I think that a good theoretical model that predicts these kind of things has yet to come.
We know exactly why the balance tends more towards the neutrons for heavier elements, but the system is messy enough that it’s very hard to predict just how much it does.
Sphericality and cubicality are orthogonal issues, and not that big a deal in the grand scheme of things. The main issues that make nucleons harder than electrons are:
1) There isn’t an externally imposed force that dominates the system (for the electrons, that’s the nucleus); it’s all internal, and that’s harder. Every time you add a new particle, the new ground state is little like the old ground state. For electrons, a thorough understanding of Hydrogen tells you nearly everything you need to know about, say, Oxygen; at a nuclear level, a thorough understanding of Hydrogen barely tells you anything about Oxygen.
2) The questions you need to answer are much much harder. You aren’t perturbing the system and finding the new ground state, like in chemistry. You need to find barrier heights and transition rates on upheavals to the whole system.
3) Last and least, there are two species (electrons → protons AND neutrons), with differences in how they feel the forces.
Looking at http://www.frankswebspace.org.uk/ScienceAndMaths/physics/physicsGCE/nuclearImages/islandOfStability.jpg , one of the patterns I see is that even numbers of protons and neutrons are systematically more stable than odd numbers. So that might answer the specific part of your question about its neighbors. (As to why even numbers, I don’t know but I bet it’s related to spins.)
EDIT: Apparently this is enough of a thing that it even has its own Wikipedia page. http://en.wikipedia.org/wiki/Even_and_odd_atomic_nuclei