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Author: Subject: Why technetium and promethium doesn't have a stable isotope?
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[*] posted on 14-9-2021 at 13:41
Why technetium and promethium doesn't have a stable isotope?


Why technetium and promethium doesn't have a stable isotope? Technetium is in the middle of the periodic table, surrounded by stable elements, so why it doesn't have stable isotope? And promethium is only one lanthanoid without stable isotope, which is also weird. Is for this some explanation?

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[*] posted on 14-9-2021 at 15:07

I don't fully understand the nuclear physics behind it, but the nuclear shell model can explain it.

As below, so above.
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[*] posted on 15-9-2021 at 21:31

Promethium is rather a victim of circumstances there was no free position to have at least one stable isotope. It is related also to the Liquid drop model. (
There are few semi-empirical rules, related to proton and neutron fermionic pairing within the nucleus model:
Elements with the odd proton number have maximally 2 stable isotopes.
Isotopes not close enough to the Valley of stability are not stable, as they have too little or too many neutrons. (
The stability of isotopes of similar nucleon number generally decreases in order

- Both proton/neutron numbers are even
- One number is odd, the other is even.
- Both numbers are odd.
For Pm and Tc, both have odd proton numbers what gives them disadvantage.
All their possible stable isotope candidates are already taken by their neighbors as energetically preferred variants. Therefore all isotopes of given 2 elements beta decay to nuclei of adjacent elements with less energy.
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Maurice VD 37
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[*] posted on 17-9-2021 at 10:56

There is a law in nuclear physics stating that if two nuclides are isobar (same mass number) and neighbors in the periodic table, and are they cannot be both stable. One of the isobar must emit a beta particle, to be transformed into the second member of the pair.
Technetium has two neighbors : Molybdenum and ruthenium. These two elements have plenty of stable isotope (seven each).. It is impossible to find an isotope of Technetium whose left or right isobar neighbor is not a stable isotope. So all nuclides from Techn├ętium are radioactive.
The same thing happens for Promethium.
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