Sciencemadness Discussion Board

Nuclear stability and a pattern on the periodic table

j_sum1 - 27-4-2015 at 20:54

Hopefully a simple question. Something I noticed a couple of weeks ago and having seen it, I am surprised that I have never noticed it before.

I was looking at a periodic table such as this one where La and Ac are positioned directly below Sc and Y. (There are a large number of variant positions which opens up other questions but those are not for right now.)

I noticed looking down the Mn group some interesting things happening.
Tc is the first synthetic element with a no stable isotopes.
Two rows down and Pm is the only radioactive lanthanide.
Below that, Np begins the transuranic elements which are all synthetic.
Thus, one column of the table has three significant unstable elements: three elements that are "unstable early" for want of a better term.
It also begs the question of why Re misses out on all of the action.

I was wondering if this pattern was coincidental or whether there was something more to it. I know that odd numbered elements tend to be less stable than evens. I know that an odd number of nucleons is inherently less stable than an even number. I have also heard tell of nuclear shells analogous to electron shells but know nothing more on that.

Paging careysub and neptunium. I think you guys are the gurus on this stuff.

Chemosynthesis - 27-4-2015 at 21:39

I am interested in what others will say on this. There are trends with the periodic table, but nothing as simple as electronegativity or more classical periodic trends. You can look at the "Magic Numbers" like you refer to and see proton rich nuclei are beta positive decay or electron capture unstable, which can influence the amount of daughter nuclides seen from a parent element, which might hint at an answer to your question. Neutron rich elements are beta minus decay unstable. For the shell, you have weaknesses in actinides and excited states, according to some old notes of mine. Katz's rule and Nordheim's/Brennan-Bernstein's rules are one way to model or predict this.
I have attached "Predicting Nuclear Stability Using the Periodic Table."

Attachment: ed066p757.pdf (1.1MB)
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Pasrules - 29-4-2015 at 14:26

I was thinking of writing up a periodic table with each elements stable isotopes as a number to look for trends. If I actually get around to doing it i'll make sure to post it on this thread, however I have a tetradentate prep to make first.

I'll use this chart as a guide line should anybody want to beat me to it.

diddi - 29-4-2015 at 17:16

whilst Predicting Nuclear Stability Using the Periodic Table is an interesting read, it is basically observational summary and does not seek to explain WHY and of the observations are true.

It is an interesting question to which I have not ever really giving much time to contemplate. Why Tc is so anomalous I have no idea, but in the great scheme of things there must be an explanation.

Metacelsus - 29-4-2015 at 18:41

https://en.wikipedia.org/wiki/Nuclear_shell_model

does a pretty good job of predicting stabilities.

Chemosynthesis - 29-4-2015 at 19:31

Quote: Originally posted by Pasrules  
I was thinking of writing up a periodic table with each elements stable isotopes as a number to look for trends. If I actually get around to doing it i'll make sure to post it on this thread, however I have a tetradentate prep to make first. I'll use this chart as a guide line should anybody want to beat me to it.
I believe NIST has that in interactive form already, rather than just the belt of stability visualization.
Quote: Originally posted by diddi  
whilst Predicting Nuclear Stability Using the Periodic Table is an interesting read, it is basically observational summary and does not seek to explain WHY and of the observations are true.

It isn't supposed to. Neither is the liquid drop of the nucleus model. If you want to get into more presumptive why, wetting your lips with shell theory as I mentioned is a decent start. Field theories, which I can't do justice, also have various explanations, but not really within the scope of Beginnings.

j_sum1 - 29-4-2015 at 19:59

So, the consensus so far seems to be that there is no direct correlation between the classical periodic table (which is based on electron shells) and the nucleus stability.
The second thing to note is that there have been observations made of the stability of various isotopes and patterns observed which have some predictive power.
The third consensus is that there has been (complex) work done on configuration of nucleons and various theories proposed. These theories are as yet incomplete and they do not currently fully correlate with observations although they are close. Predictive power is yet to have its major test which will come when/if an island of stability is discovered. The various theories differ as to the location or nature of this island.


With regards to my original question, the pattern I noticed seems to be incidental. (I was going to say coincidental but that connotes an element of chance whereas the properties observed are undoubtedly deterministic.)
If anyone is interested, this is the periodic table that alerted me to this pattern. Look at group 7. Now that I have seen it, I can't un-see it.

neptunium - 29-4-2015 at 20:33

I see what you mean in this periodic table, the Lanthanides and Actinides are presented in that way when they should all fit with La and Ac... (I have seen other presentations where the lanthanides and actinides are on the side )
Quantum physics, the strong and weak nuclear forces gives part of the answer for the stability of isotopes but as far as the predictability I do remember the liquid drop model somewhat predicting a periodicity.
all of Tc isotopes are kind of stuck between two worlds of decay (too many proton B+ and too many neutron B- not heavy enough for alpha) it does have a long enough half life for analysis though. as for Np and beyond ,has to do with the strong and weak nuclear forces , a different value (even small ) of either or both and half lives changes dramatically...that's just the way it is in this universe! maybe another parallel one has Plutonium stable and another were gold is the heavier stable atom...

blogfast25 - 30-4-2015 at 09:12

Quote: Originally posted by j_sum1  
If anyone is interested, this is the periodic table that alerted me to this pattern. Look at group 7. Now that I have seen it, I can't un-see it.


I could show you a pattern generated entirely by random numbers and you would 'see it'. Even if I later revealed that it was entirely due to chance, you would still not be able to 'un-see' it but that wouldn't change the fact that it was random, like a 'Jesus-on-a-piece-of-toast'.

The pattern you see relies entirely on that particular arrangement of the periodic table and ignores the many unstable elements (Ra, Rn, At, Fr, Po for instance) that are scattered over the rest of the PT.

The nucleus is governed by quantum physics of course but finding periodicity of nuclear properties based on Z only appears far more difficult than for electron clouds.

j_sum1 - 30-4-2015 at 14:45

@ blogfast
Well, both electron configuration and nuclear stability are deterministic and so randomness does not come into it.
That periodic table is not entirely illogical in the way it has positioned the f block elements and a pattern of sorts is apparent.

My question was not really whether there was some kind of causative link between the two -- I would seriously doubt that. Rather it was one of the overlap of periodicity of the two phenomena.

At worst, I have found some kind of mnemonic. I am not going to forget that Np is below Pm.
But it seems better than that -- there has been a bit of discussion promoted and some interesting points raised which require some follow up reading. For me, it is an interesting question and I would venture that the observation I made has more validity than portraits of Jesus in the grilled cheese.

blogfast25 - 30-4-2015 at 15:45

Cheese I find is better for Mona Lisas. That cheesy smile...

neptunium - 30-4-2015 at 19:58

we cant loose sight of the fact that the periodic table did not always look that way (1871) but it is organized around the electron cloud and not the nucleus. any similitude must be regarded as pure coincidence .
However , curiosity is always a good thing in science and we have all been surprised by nature many times before.
the unexpected Tc and Pm smack in the middle of it all always raises questions and that's a good thing. the answers aren't always easy.