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Trend of half life in actinides

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Douglas100:
Hi,
I recently had a question to do with the trends of decreasing half life as the atomic number increased for the actinide group within the periodic table.
I answered by saying that the half life decreases due to the large size of the nuclei as the atomic number increases. This is due to the elements only being stable up until the atomic number of 81, thereafter the elements are less stable as the atomic number increases.
Could anybody tell me if this is correct and the scientific reason for this trend occurring.
Thank you in advance.

Enthalpy:
More protons in a nucleus repel an other more strongly.

With details: more nucleons also attract an other more strongly, but this interaction occurs only at a short distance, so it increases slowly with the number of nucleons, while the repulsion increases more quickly at bigger nuclei.

This "water drop" model is not the whole picture, as for instance 238U and 232Th are more stable than lighter nuclei, so details about the numbers of protons and neutrons matter. Even numbers of protons and of neutrons tend to be more stable, suggesting spin pairing, but this explanation doesn't suffice. Other models have had a limited success too, including with shells as inspired by electrons in atoms, but none is good. As far as I know (and I ignore much) this is still an open topic.

Enthalpy:
Thorium and uranium, or at least some natural isotopes, are abnormally stable, much more so than the nuclides between them and bismuth, so I believe a general trend shouldn't include these two.

I'm not even sure that a solid theory exists that explains the stability of Th and U.

gippgig:
Thorium and uranium aren't abnormally stable; the elements from polonium-actinium are abnormally unstable, particularly the lighter ones, because they are just above the closed shell at lead. A good analogy is that the closer you are to a sinkhole the more likely you are to fall even tho someone a long way away may actually be higher up than you are. The elements from thorium-rutherfordium are about normal stability. From dubnium on elements are abnormally stable (as they approach the partially closed shell at hassium & closed shell somewhere in the 114-126 region).

Enthalpy:
The shell theory has serious difficulties - pity, it was simple. For instance, the doubly magic 56Ni is unstable while 56Fe has the highest binding energy per nucleon.

One may also notice that (the stablest isotopes of) polonium is more stable than radon
https://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html
https://en.wikipedia.org/wiki/Table_of_nuclides_%28complete%29
despite being nearer to lead (I take only even nuclides in the comparison).

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