Energy From Thorium Discussion Forum

Questions about abundance of the elements...
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Author:  Cyril R [ Jan 27, 2014 12:36 pm ]
Post subject:  Questions about abundance of the elements...

Looking at a graph of the relative abundance of elements in the universe, ... _LARGE.png

We can see some interesting and mostly logical things.

Li, Be, and B are underrepresented. That's to be expected with these elements; they are not very stable in a nuclear sense, they fission (if you can call it that), fuse or otherwise transmute away.

Oxygen is overrepresented; makes sense, very stable nuclide, few nuclear reactions. Iron and nickel have the highest binding energy per nuclide so makes sense as well. Lead is also overrepresented, considering its the last stable element its abundance increases as unstable nuclides decay into it.

But some things don't make a lot of sense at first blush. Why is fluorine so underrepresented? It seems very stable.

What's up with Scandium? And why is there a bump up in the Sn, Te, Xe, Ba sequence? And later again on the noble metals sequence?

Why is there so much Xe about? Fission product?

Author:  Gilliam [ Jan 27, 2014 4:25 pm ]
Post subject:  Re: Questions about abundance of the elements...

Fluorine has more neutrons than protons, which seems to make it less common. This hypothesis is somewhat confirmed by Li, Be and B (assuming the most common isotopes) because they all have one neutron more than they have protons. Carbon, Oxygen and Nitrogen on the other hand have an even amount, which fits the bill because they're more abundant.

Scandium seems to be one of the few elements which has less neutrons than protons, somewhat explaining the lack of it. Iron and Nickel have an awesome amount of neutrons compared to protons.

The whole thing seems to be based around the ratio between N and Z, at first glimpse. You'll definitely find the skewed line on the graph comparing both interesting.

Author:  Titanium48 [ Jan 27, 2014 7:46 pm ]
Post subject:  Re: Questions about abundance of the elements...

Fluorine is nearly as fragile as Li, Be and B, and it only has one stable isotope. An overview of stellar nucleosynthesis of fluorine can be found here:

Li and B both have stable isotopes where neutrons = protons. The only stable isotopes where protons exceed neutrons are 1H and 3He. Stable scandium is 100% 45Sc, with 21 protons and 24 neutrons. The relative rarity of scandium may be related to being in between the elements produced by fusion of oxygen (Si, P, S) and the alpha process peak at Fe and Ni. Being monoisotopic doesn't help either. The tin to barium peak may be related to large number of stable isotopes of these elements, although that does not explain why there is more monoisotopic iodine than neighboring Z-odd elements.

Author:  KitemanSA [ Jan 27, 2014 11:22 pm ]
Post subject:  Re: Questions about abundance of the elements...

Those that are most common have "magic numbers" of nucleons.

Wikipedia wrote:
In nuclear physics, a magic number is a number of nucleons (either protons or neutrons) such that they are arranged into complete shells within the atomic nucleus. The seven most widely recognised magic numbers as of 2007 are 2, 8, 20, 28, 50, 82, and 126 (sequence A018226 in OEIS). Atomic nuclei consisting of such a magic number of nucleons have a higher average binding energy per nucleon than one would expect based upon predictions such as the semi-empirical mass formula and are hence more stable against nuclear decay.

Helium = 2 + 2
Oxygen = 8 + 8
Ca-40 (20+20) would normally be quite radioactive, but instead it is observationally stable (half life expected to be ~6E21 years).

PS: it appears that some of the LEAST common ones have one more proton than a magic number.

Author:  Titanium48 [ Jan 28, 2014 1:28 am ]
Post subject:  Re: Questions about abundance of the elements...

Of course, I forgot about the magic numbers. In addition to explaining the weakly bound nature of the last proton and subsequent fragility of some of the n+1 elements (Li, F, Sc, In), they can also help explain the xenon and platinum group peaks. Neutron capture cross sections become very small at neutron magic numbers, so supernova r-process nucleosynthesis tends to stall at neutron numbers of 82 and 126. Beta decay of the initial extremely neutron rich isotopes back to the region of stability produces the observed abundance peaks in elements with a somewhat fewer neutrons than the magic numbers.

Author:  rc1111 [ Jan 28, 2014 10:55 pm ]
Post subject:  Re: Questions about abundance of the elements...

I still think nucleosynthesis has some further work to go. The r- and s- processes are thought to be pretty well understood and the p-processes as well. But the cross sections for reactions are not all very precise. I am surprised at the need for better cross-sections even today. I understand that Los Alamos is generating new data so additional simulations can be run. This will be useful for reactor engineering as well.

N.B: when only looking at Z vs. N count remember the coulomb force is repulsive as too many protons are added.

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