<p><strong>TL;DR:</strong> Nature favors packing nucleons in pairs of anti-parallel spins. (electrons as well!)</p>
<p>The zig-zag is a <a href="https://en.wikipedia.org/wiki/Nucleosynthesis" rel="noreferrer">nucleosynthesis</a> artifact.</p>
<p>H, He and Li are &quot;primordial&quot; (made out of the abundant neutrons in the first hours after the Big Bang.)</p>
<p>Everything else is made in the stars.</p>
<p>There are a few main processes that make heavier elements (oversimplified!):</p>
<ol>
<li>Hydrogen burning (makes helium as we already don't have great deal of it)</li>
<li>Helium burning (makes C and O)</li>
<li>Helium (even element itself) + any even element (makes even elements up to Fe)</li>
<li>Slowly adding neutrons to everything and waiting for it to beta-decay (called <a href="https://en.wikipedia.org/wiki/S-process" rel="noreferrer">s-process</a>, makes odd elements out of even elements and elements heavier than Fe, up to some point)</li>
<li>Rapidly adding neutrons without waiting much anything to beta-decay before adding the next neutron (<a href="https://en.wikipedia.org/wiki/R-process" rel="noreferrer">r-process</a>, makes all elements up to uranium out of existing elements from previous processes)</li>
</ol>
<p>Every one of these processes gets its material from the previous ones and has less yield.
That's why we have heavier elements less than lighter elements, even elements more than odd elements and r-process elements less than s-process elements.</p>
<p>Edit: Both s- and r-processes, as well as other ones not listed here, leave ultimately more even elements as they are in general more stable against both beta-decay and capturing the next neutron. Why they are more stable? Because both protons and neutrons fill their very own shell-like orbitals inside the nucleus and <strong>nature favors packing nucleons in pairs of anti-parallel spins.</strong> (electrons as well!) See <a href="https://en.wikipedia.org/wiki/Nuclear_shell_model" rel="noreferrer">Nuclear shell model</a></p>

CC BY-SA 4.0
2020-10-28T11:51:01.630Z
Nice answer, but Big Nucleosynthesis lasted for about 17 minutes (from t=3 m to t=20 m). It produced some beryllium, mostly Be-7, but that decays with a half-life of 56 days to Li-7. Most beryllium in the modern cosmos is Be-9, which was mostly produced long after the BB, via [cosmic ray spallation](https://en.wikipedia.org/wiki/Cosmic_ray_spallation).
PM 2Ring
CC BY-SA 4.0
2020-10-28T11:56:50.000Z
@PM2Ring Agree for Be. Free neutrons have half-life ~ 10' so after the first hour we could still have ~1% of them fooling around.
fraxinus
CC BY-SA 4.0
2020-10-28T12:14:18.757Z
True, but they aren't doing very much, apart from decaying. ;) I guess a few of them would have enough energy to convert Be-7 to Be-8 (which immediately decays to He-4) and He-3 to He-4. Nothing else in existence at that stage has a significant cross section for thermal neutrons.
PM 2Ring
CC BY-SA 4.0
2020-10-28T14:29:40.123Z
Neutron decaying is still a nucleosyntesis. They make hydrogen.
fraxinus
CC BY-SA 4.0
2020-10-28T21:28:48.437Z
Nitpick: Ne through Si are made through symmetric C+C, Ne+Ne, and O+O fusion, _not_ through alpha-process reactions.  Symmetric fusion is also partly responsible (alongside the alpha process) for both even and odd elements from P through Ca.
Vikki
CC BY-SA 4.0
2020-10-29T02:31:16.140Z
I hope you don't mind the edit, please feel free to modify further.
uhoh
CC BY-SA 4.0
2020-10-29T08:42:35.650Z
It's ok, thank you.
fraxinus
CC BY-SA 4.0
2020-10-29T10:40:27.860Z
@Sean It's complicated. There *are* [alpha process ladder reactions](https://en.wikipedia.org/wiki/Alpha_process) as well as symmetric fusion. Certainly, in [carbon burning](https://en.wikipedia.org/wiki/Carbon-burning_process), symmetric fusion is dominant. But [neon burning](https://en.wikipedia.org/wiki/Neon-burning_process) is predominantly alpha process, and at that temperature photodisintegration becomes significant, so some neon nuclei decompose to oxygen + alpha. [Oxygen burning](https://en.wikipedia.org/wiki/Oxygen-burning_process) is primarily symmetric fusion.
PM 2Ring
CC BY-SA 4.0
2020-10-29T14:50:35.177Z
Hmm, no mention of the [CNO cycle](https://en.wikipedia.org/wiki/CNO_cycle)? I would've thought that accounted for the unexpectedly high abundance of nitrogen-14, despite it being an odd-odd isotope.
Ilmari Karonen
CC BY-SA 4.0
2020-10-29T14:55:29.073Z
@IlmariKaronen the question is about the zigzag. A lot of other things simplified away.
fraxinus
CC BY-SA 4.0
2020-10-28T10:34:46.517Z
2020-10-29T11:05:03.083Z
2020-10-29T11:05:03.083Z
fraxinus
<p>A very informative, short, <a href="https://www.youtube.com/watch?v=0-b1C0zJq3I" rel="noreferrer">You Tube video</a> discusses this - see the 4:20 mark.</p>
<p>Apart from hydrogen, helium, lithium and beryllium, all the elements were formed as the products of fusion in stars.</p>
<p>If you look at the scale for the vertical axis, it's logarithmic. Also, the elements on top of the zig-zag have even atomic numbers and those on the bottom have odd atomic numbers. The significance of this is the elements with even atomic numbers are about ten times more abundant than those with odd atomic numbers.</p>
<p>The reason for the zig-zag pattern is the processes of fusion. Fusion involving helium (which has two protons and two neutrons) is more likely than fusion involving hydrogen (which has one proton). When helium fuses with an even numbered atomic nucleus another even numbered atomic nucleus is produced, but when hydrogen fuses with an atomic nucleus it produces the element next in the periodic table of elements.</p>

CC BY-SA 4.0
2020-10-28T08:04:28.643Z
This doesn't answer the question. You are just repeating that the production of even-numbered nuclei is more likely, without explaining why.
ProfRob
CC BY-SA 4.0
2020-10-31T01:27:40.587Z
Comment only: Iron is "star ash" (nicely stable) - with an abundance about 2 magnitudes higher than might otherwise be expected from a somewhat linear zig-zag. Yes?
Russell McMahon
CC BY-SA 4.0
2020-10-31T16:52:03.717Z
In addition, how would this apply beyond iron, since these elements aren't formed in fusion reactions...
ProfRob
CC BY-SA 4.0
2020-10-28T04:02:30.273Z
2020-10-28T04:02:30.273Z
Fred
<p>This is complementary to other answers and not intended as a full answer in its own right.</p>
<p>I saved the diagram below to disk in March 2009 - source not (yet) known.<br />
<strong>Update:</strong> Wikipedia :-) - with an excellent discussion of the reasons for the variations in the terrestrial versus universe wide distributions <a href="https://wiki2.org/en/Abundance_of_elements_in_Earth%27s_crust" rel="nofollow noreferrer">here</a></p>
<p>This shows elemental abundance in the earth's elemental crust rather than in the universe.<br />
It will be seen that the clear &quot;zig-zag&quot; in the original diagram is not as clear cut here and that some relative ratios are substantially different.<br />
The reasons given for the zig zag shape remain the same but having a partial sample of conditions at the bottom of a tectonically stirred, atmosphere and ocean covered gravity well probably go a long way toward explaining most of the differences.</p>
<p>The thin shell of biosphere probably help account for some of the differences in eg O &amp; H ratio and abundance.</p>
<p>Pb : Bi: Th : U  are much the same, but (sadly) the rarest metals are 2 to 3 orders of magnitude down in abundance  - but this suggests that asteroid mining may yet prove 'interesting'.</p>
<p><a href="https://i.sstatic.net/Y3fBe.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Y3fBe.jpg" alt="enter image description here" /></a></p>
<p>Versus original from above:</p>
<p><a href="https://i.sstatic.net/uvcfD.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/uvcfD.png" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2020-10-31T11:57:18.943Z
2020-10-31T11:57:18.943Z
Russell McMahon
<p>The zig-zig pattern might be related to <a href="https://en.wikipedia.org/wiki/Oddo%E2%80%93Harkins_rule" rel="nofollow noreferrer">Oddo-Harkins rule</a>. It states:</p>
<blockquote>
<p>An element with an even atomic number is more abundant than both
elements with the adjacently larger and smaller odd atomic numbers.</p>
</blockquote>
<p>The basis of this abundance pattern might have something to with nucleon stability. The rule argues that elements with even atomic numbers have their protons paired, with each member of the pair balancing the spin of the other thus enhances nucleon stability while elements with odd atomic numbers have one unpaired proton, thus not stable.</p>
<p>But there are many big exception to this rule. One of the exception is hydrogen-helium-lithium-beryllium.</p>

CC BY-SA 4.0
2020-10-28T03:41:39.187Z
2020-10-28T03:53:22.933Z
2020-10-28T03:53:22.933Z
Nilay Ghosh
<p>In <a href="https://astronomy.stackexchange.com/a/39553/2896">this answer</a> to <em>Is lithium considered a metal in astronomy?</em> this image from Wikimedia of the abundances of elements in the universe was shared:</p>
<p><a href="https://i.sstatic.net/XxlNT.png" rel="noreferrer"><img src="https://i.sstatic.net/XxlNT.png" alt="A log graph of atomic abundances in the universe" /></a>
<br><sup>Credit: <a href="https://commons.wikimedia.org/wiki/File:SolarSystemAbundances.png" rel="noreferrer">Wikimedia Commons user 28bytes, under C.C.-by-S.A.-3.0</a>.</sup></p>
<p>I thought it was fascinating that there is a near-constant zig-zag of abundancies.</p>
<p>Are these values accurate, and if so, what would cause this zig-zag?</p>

CC BY-SA 4.0
2020-10-28T01:41:09.533Z
Pretty sure this is because of nuclear stability - note the "peaks" are all atoms with even atomic numbers, which are inherently more stable than those with odd atomic numbers.
Allure
CC BY-SA 4.0
2020-10-28T02:04:03.027Z
I'd guessed something like that, or alternatively that it results in their emissions being stronger somehow.
curiousdannii
CC BY-SA 4.0
2020-10-28T03:43:32.157Z
I suspect that the most common synthesis events involve a helium-4 nucleus and something heavier, so stepping up by two atomic numbers is common. Beryllium is rare because Be-8 (which would come about from the fusion of two He-4 nuclei) is unstable, and only the Be-8 that gets turned into C-12 survives. Someone more learned than I will have to fill in the gaps.
EvilSnack
CC BY-SA 4.0
2020-10-28T21:06:01.967Z
There should actually be **constant** zig-zags, but some of the elements are missing from the chart (43 Tc, 61 Pm, lots after Bi). The only exception seems to be the first four elements.
BlueRaja - Danny Pflughoeft
CC BY-SA 4.0
2020-10-29T12:31:31.473Z
THAT'S AMAZING. Taking a step back, I'm in awe. Something fundamental we know about how _atoms_ work, and it just naturally pops out of data like this. Sorry for low-quality comment, but that plot is just great.
Joel Reid
<elemental-abundances>
Why is there a zig-zag in elemental abundances?
curiousdannii
# Why is there a zig-zag in elemental abundances?

(Asked by: username_0 on 2020-10-28)

In [this answer](https://astronomy.stackexchange.com/a/39553/2896) to *Is lithium considered a metal in astronomy?* this image from Wikimedia of the abundances of elements in the universe was shared:

[![A log graph of atomic abundances in the universe](https://i.sstatic.net/XxlNT.png)](https://i.sstatic.net/XxlNT.png)
  
Credit: [Wikimedia Commons user 28bytes, under C.C.-by-S.A.-3.0](https://commons.wikimedia.org/wiki/File:SolarSystemAbundances.png).

I thought it was fascinating that there is a near-constant zig-zag of abundancies.

Are these values accurate, and if so, what would cause this zig-zag?

**Tags:** elemental-abundances

> **Comments:**
> **username_1**: Pretty sure this is because of nuclear stability - note the "peaks" are all atoms with even atomic numbers, which are inherently more stable than those with odd atomic numbers.  
> **username_0**: I'd guessed something like that, or alternatively that it results in their emissions being stronger somehow.  
> **username_2**: I suspect that the most common synthesis events involve a helium-4 nucleus and something heavier, so stepping up by two atomic numbers is common. Beryllium is rare because Be-8 (which would come about from the fusion of two He-4 nuclei) is unstable, and only the Be-8 that gets turned into C-12 survives. Someone more learned than I will have to fill in the gaps.  
> **username_3**: There should actually be **constant** zig-zags, but some of the elements are missing from the chart (43 Tc, 61 Pm, lots after Bi). The only exception seems to be the first four elements.  
> **username_4**: THAT'S AMAZING. Taking a step back, I'm in awe. Something fundamental we know about how _atoms_ work, and it just naturally pops out of data like this. Sorry for low-quality comment, but that plot is just great.  

---

## 4 answers

### Answer by username_5 on 2020-10-28. Score: 25

**TL;DR:** Nature favors packing nucleons in pairs of anti-parallel spins. (electrons as well!)

The zig-zag is a [nucleosynthesis](https://en.wikipedia.org/wiki/Nucleosynthesis) artifact.

H, He and Li are "primordial" (made out of the abundant neutrons in the first hours after the Big Bang.)

Everything else is made in the stars.

There are a few main processes that make heavier elements (oversimplified!):

1. Hydrogen burning (makes helium as we already don't have great deal of it)
2. Helium burning (makes C and O)
3. Helium (even element itself) + any even element (makes even elements up to Fe)
4. Slowly adding neutrons to everything and waiting for it to beta-decay (called [s-process](https://en.wikipedia.org/wiki/S-process), makes odd elements out of even elements and elements heavier than Fe, up to some point)
5. Rapidly adding neutrons without waiting much anything to beta-decay before adding the next neutron ([r-process](https://en.wikipedia.org/wiki/R-process), makes all elements up to uranium out of existing elements from previous processes)

Every one of these processes gets its material from the previous ones and has less yield.
That's why we have heavier elements less than lighter elements, even elements more than odd elements and r-process elements less than s-process elements.

Edit: Both s- and r-processes, as well as other ones not listed here, leave ultimately more even elements as they are in general more stable against both beta-decay and capturing the next neutron. Why they are more stable? Because both protons and neutrons fill their very own shell-like orbitals inside the nucleus and **nature favors packing nucleons in pairs of anti-parallel spins.** (electrons as well!) See [Nuclear shell model](https://en.wikipedia.org/wiki/Nuclear_shell_model)

> **Comments:**
> **username_6**: Nice answer, but Big Nucleosynthesis lasted for about 17 minutes (from t=3 m to t=20 m). It produced some beryllium, mostly Be-7, but that decays with a half-life of 56 days to Li-7. Most beryllium in the modern cosmos is Be-9, which was mostly produced long after the BB, via [cosmic ray spallation](https://en.wikipedia.org/wiki/Cosmic_ray_spallation).  
> **username_5**: @username_6 Agree for Be. Free neutrons have half-life ~ 10' so after the first hour we could still have ~1% of them fooling around.  
> **username_6**: True, but they aren't doing very much, apart from decaying. ;) I guess a few of them would have enough energy to convert Be-7 to Be-8 (which immediately decays to He-4) and He-3 to He-4. Nothing else in existence at that stage has a significant cross section for thermal neutrons.  
> **username_5**: Neutron decaying is still a nucleosyntesis. They make hydrogen.  
> **username_7**: Nitpick: Ne through Si are made through symmetric C+C, Ne+Ne, and O+O fusion, _not_ through alpha-process reactions.  Symmetric fusion is also partly responsible (alongside the alpha process) for both even and odd elements from P through Ca.  
> **username_8**: I hope you don't mind the edit, please feel free to modify further.  
> **username_5**: It's ok, thank you.  
> **username_6**: @Sean It's complicated. There *are* [alpha process ladder reactions](https://en.wikipedia.org/wiki/Alpha_process) as well as symmetric fusion. Certainly, in [carbon burning](https://en.wikipedia.org/wiki/Carbon-burning_process), symmetric fusion is dominant. But [neon burning](https://en.wikipedia.org/wiki/Neon-burning_process) is predominantly alpha process, and at that temperature photodisintegration becomes significant, so some neon nuclei decompose to oxygen + alpha. [Oxygen burning](https://en.wikipedia.org/wiki/Oxygen-burning_process) is primarily symmetric fusion.  
> **username_9**: Hmm, no mention of the [CNO cycle](https://en.wikipedia.org/wiki/CNO_cycle)? I would've thought that accounted for the unexpectedly high abundance of nitrogen-14, despite it being an odd-odd isotope.  
> **username_5**: @username_9 the question is about the zigzag. A lot of other things simplified away.  

---

### Answer by username_10 on 2020-10-28. Score: 9

A very informative, short, [You Tube video](https://www.youtube.com/watch?v=0-b1C0zJq3I) discusses this - see the 4:20 mark.

Apart from hydrogen, helium, lithium and beryllium, all the elements were formed as the products of fusion in stars.

If you look at the scale for the vertical axis, it's logarithmic. Also, the elements on top of the zig-zag have even atomic numbers and those on the bottom have odd atomic numbers. The significance of this is the elements with even atomic numbers are about ten times more abundant than those with odd atomic numbers.

The reason for the zig-zag pattern is the processes of fusion. Fusion involving helium (which has two protons and two neutrons) is more likely than fusion involving hydrogen (which has one proton). When helium fuses with an even numbered atomic nucleus another even numbered atomic nucleus is produced, but when hydrogen fuses with an atomic nucleus it produces the element next in the periodic table of elements.

> **Comments:**
> **username_11**: This doesn't answer the question. You are just repeating that the production of even-numbered nuclei is more likely, without explaining why.  
> **username_12**: Comment only: Iron is "star ash" (nicely stable) - with an abundance about 2 magnitudes higher than might otherwise be expected from a somewhat linear zig-zag. Yes?  
> **username_11**: In addition, how would this apply beyond iron, since these elements aren't formed in fusion reactions...  

---

### Answer by username_12 on 2020-10-31. Score: 2

This is complementary to other answers and not intended as a full answer in its own right.

I saved the diagram below to disk in March 2009 - source not (yet) known.  
**Update:** Wikipedia :-) - with an excellent discussion of the reasons for the variations in the terrestrial versus universe wide distributions [here](https://wiki2.org/en/Abundance_of_elements_in_Earth%27s_crust)

This shows elemental abundance in the earth's elemental crust rather than in the universe.  
It will be seen that the clear "zig-zag" in the original diagram is not as clear cut here and that some relative ratios are substantially different.  
The reasons given for the zig zag shape remain the same but having a partial sample of conditions at the bottom of a tectonically stirred, atmosphere and ocean covered gravity well probably go a long way toward explaining most of the differences.

The thin shell of biosphere probably help account for some of the differences in eg O & H ratio and abundance.

Pb : Bi: Th : U are much the same, but (sadly) the rarest metals are 2 to 3 orders of magnitude down in abundance - but this suggests that asteroid mining may yet prove 'interesting'.

[![enter image description here](https://i.sstatic.net/Y3fBe.jpg)](https://i.sstatic.net/Y3fBe.jpg)

Versus original from above:

[![enter image description here](https://i.sstatic.net/uvcfD.png)](https://i.sstatic.net/uvcfD.png)


---

### Answer by username_13 on 2020-10-28. Score: 2

The zig-zig pattern might be related to [Oddo-Harkins rule](https://en.wikipedia.org/wiki/Oddo%E2%80%93Harkins_rule). It states:

> An element with an even atomic number is more abundant than both
> elements with the adjacently larger and smaller odd atomic numbers.

The basis of this abundance pattern might have something to with nucleon stability. The rule argues that elements with even atomic numbers have their protons paired, with each member of the pair balancing the spin of the other thus enhances nucleon stability while elements with odd atomic numbers have one unpaired proton, thus not stable.

But there are many big exception to this rule. One of the exception is hydrogen-helium-lithium-beryllium.


---

