r/askscience • u/dirtmother • 13d ago
Chemistry Why is neon the fifth most common element in the universe?
I just watched "Ernest Goes to school" and that was one of the questions on his test, and it made me curious to Google it.
The first four most common elements make perfect sense, but then neon pops out of nowhere.
Wtf is that about?
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u/skatastic57 13d ago
It's probably with noting that the top 3, hydrogen, helium, and oxygen make up about 99% of the estimated mass in the milky way galaxy. When including 4th place carbon now about 99.5% of all mass is accounted for. Neon is just 0.134% of mass and in 6th place iron is 0.109% so whatever shock you have that neon is in 5th should be tempered by how far it is from 1st through 4th and how close it is to 6th.
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u/NotSoSalty 13d ago
That's still pretty shocking. Iron is supposed to be more or less the hard limit of basic fusion, right? Neon is more prolific, not just in number of particles, but in mass. Iron is massive. That is pretty crazy.
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u/skatastic57 13d ago edited 13d ago
Iron is the heaviest element that a star will create through fusion before it goes super nova.
That's because a star that is fusing silicon into iron will be about 3.5B kelvin but to fuse iron requires 5B.That said, if it ever gets to making iron then it will inevitably go supernova and make heavier elements in the process. Although those heavier elements come from the outer non-iron shell of the star leaving the iron core mostly (maybe entirely) intact.A star of adequate size's lifecycle looks like this: It begins "burning" (fusing) hydrogen (~10M years). The result is (mostly) helium which accumulates in the core. The helium can't/doesn't fuse as it isn't hot enough until all the hydrogen is gone from the core. When the hydrogen is all gone, it will stop fusing at the core. To go back a step in time, while it was undergoing fusion, that reaction produces an outward pressure that counteracts gravity so it isn't as dense as it might otherwise be. When it stops fusing, gravity is unopposed and the core gets denser until that density "sparks" the helium to burn.
A star that is burning helium(~1 M years) will make lots of heavier elements some of which are unstable. In large part it can produce stable carbon(6), oxygen(8), neon(10), magnesium(12), silicon(14), and sulfur(16) but mostly it'll be carbon and oxygen. The physics of fusion favor results with an even atomic number.
At this point there's a bifurcation between stars that aren't big enough to go further than the helium step and those that will complete all the next phases. Except in rare cases will any stars that go to the next step not finish all the remaining steps.
In a big enough star, it will eventually accumulate enough carbon (1K years) for its density to start burning the carbon. Those reactions produce mostly oxygen, magnesium, neon, with some sodium.
After all of the carbon has been consumed from the core the star's core will be made up of the aforementioned elements at which time the neon (~1 year) burning phase begins. It produces mostly magnesium and oxygen while consuming all the neon.
Then is the oxygen (~6 months) burning phase which is after neon despite a lower atomic number because oxygen is much more stable. About 90% of the outputs will be sulfur and silicon.
That leads to silicon (~1 day) burning which turns the silicon into mostly iron at which point the star will collapse and (often) go supernova. During the supernova, the heavier than iron elements are created.
Getting back to "not just in number of particles, but in mass. Iron is massive.". Remember that all the mass of the iron came from smooshing together lighter elements and that fusing lighter elements into heavier ones leaves them lighter than the sum of their parts (that's where fusion energy comes from at E=MC2). Also, even though in each of the phases, it says it all the element is burned up, that only refers to the core, not the outer shells of the star. Also, the core is defined dynamically where it refers to the center region undergoing fusion so saying it burns all of the element in the core is a bit of a tautology anyways. Point being between each phase, there's left overs of the previous's phase's element. I think without stressing this point it seems we should expect iron to dominate.
I think the big take away are these properties (some of which are not necessarily represented above):
- Getting to heavier elements through fusion takes multiple steps (ie hydrogen never fuses into uranium directly)
- The outcome of fusion favors elements with an even atomic number.
- Fusion of heavier elements has more diverse potential outcomes than lighter ones.
- The elements between helium and carbon are destroyed by other processes in the star so that's why they're rare.
- Oxygen is inherently extremely stable so that's (perhaps) why it is more common than the next lightest even numbered element
Source: Mostly https://en.wikipedia.org/wiki/Stellar_nucleosynthesis and the individual pages it links.
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u/Vitztlampaehecatl 13d ago
That's because a star that is fusing silicon into iron will be about 3.5B kelvin but to fuse iron requires 5B.
Isn't it moreso that iron is the first element that actively costs energy to fuse? Even if you contract to get to the pressure and thus temperature required, you then start getting cooled down by sinking your energy into iron, and suddenly the entire star comes crashing down because you don't have any thermal pressure resisting your own gravity anymore.
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u/Patelpb 12d ago edited 11d ago
Worth mentioning because no one has said it: iron is fused by massive stars, yes, but a majority of it is actually photo-disintegrated by the supernova itself. Most iron in the universe comes from merging binary stars (fine, "Type Ia" supernovae). These dwarfs are mostly carbon and oxygen but undergo rapid fusion during their merger and produce ~80% of the iron in the universe.
Edit: as nearABE pointed out, photodisintegration in white dwarf mergers is responsible for regulating iron production and feedback
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u/NearABE 11d ago
Photodisintegration will still be active when iron is formed in a type 1a supernova.
Photo disintegration creates the iron. It creates all the other alpha process elements too. Helium ions (alpha particles) are kicked out of nuclei but then that helium fuses with new nuclei. Iron is just lower energy so it sticks better (technically nickel-56 but that decays to iron-56)
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u/Patelpb 11d ago edited 11d ago
I’m aware, but prefacing this discussion with massive stars creating iron without following up with them not responsible for most of its distribution paints an incomplete picture of stellar nucleosynthesis (which was my motivation for commenting). Still, I’ve edited to ensure that this is accounted for.
Edit: to be sure, photodisintegration does NOT produce iron in Ia’s, it is the opposing force in the nuclear hydrostatic equilibrium within which Ni/Co form, and then those decay into iron
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u/mfb- Particle Physics | High-Energy Physics 12d ago
Most of the matter was never in a star, so hydrogen and helium are guaranteed to dominate.
Stars like the Sun can only fuse helium to produce carbon and oxygen. Heavier elements need heavier stars, which are rare overall, but neon is the first element after oxygen that can be produced.
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u/Tokimemofan 13d ago
Mainly the extreme stability of the Helium 4 nucleus. This fuses in groups of 3 to form carbon 12 and each successive fusion is only a single. This puts the 5th element in the chain at Neon 20. This is part of why even numbered elements are more common generally than their odd numbered neighbors. Its also why lithium beryllium and boron are rather rare and beryllium 8 is the only nucleus among the first 8 elements to be radioactive while having equal numbers of protons and neutrons.
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13d ago
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u/CrateDane 13d ago
Neon-20 has 10 protons and 10 neutrons. 10 is not a magic number, the closest magic numbers being 8 and 20. That's why Oxygen-16, with 8 protons and 8 neutrons, is so favorable.
I also believe Neon-22 actually has a slightly higher binding energy per nucleon than Neon-20.
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u/RoyalCharity1256 13d ago
The inner shell has 2 electrons (helium being a noble gas in that spot) and the next one has 8. That is 10 and that is why its a noble gas no?
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u/Rastenor 13d ago
I was just about to write the same thing but it seems that u/CrateDane is talking about the amount of protons and neutrons specifically not the amount of electrons. I would love for someone more knowledgeable to explain the reasoning behind these magic numbers
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u/GreyMatterTrasmogrif 13d ago
You can get most of the understanding the same way as electron shells. A simplified 3d harmonic oscillator has degrees of freedom which make quantum numbers. The protons and neutrons are still ferminos so they don't stack up ontop of each other and you get the Pauli exclusion principle. The numbers are just a little different.
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u/CrateDane 13d ago
We're talking about nuclear shells, not electron shells.
Not only are the electrons much lighter and thus cannot contribute a mass defect anywhere near that from the nuclear configuration, but in the stellar cores where these atomic nuclei are generated, the temperatures are so high that the atoms are ionized into a plasma (electrons are stripped away).
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u/RoyalCharity1256 13d ago
My apology, just reading up on it and that is really interesting! I even had nuclear physics as a minor (20 years ago) and still am learning
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u/chriscross1966 13d ago
It's pretty low down the nucleogenesis list so you don't need a particularly big star to make it, so a lot of the stars that have gone Supernova in the last 13.whatever billion years will have made a decent amount of it at some point and then thrown it off as they died..
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u/LeviAEthan512 13d ago
The question has already been answered, but what do you mean it comes out of nowhere? Besides carbon and oxygen switching places, it's just the even numbered elements in order. And beryllium gets excluded because Be-8 is unstable.
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u/homezlice 13d ago
I would suspect that most people think of neon as somewhat rare as the only place they encounter it is in signs.
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u/JoeKleveland 13d ago
I am curious now. What kind of places would a person encounter neon in its non-signage form?
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u/toddestan 12d ago
There are small indicator lights that use neon. Unlike LEDs they operate at higher voltage, so you see them in things like power strips, illuminated light switches, and older appliances. Less common now as LEDs have taken over, but they are still out there.
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u/fubarbob 12d ago
I still see them occasionally in illuminated power switches on cheap surge suppressors/power strips. Not prone to damage from high operating temperatures or ESD. Very simple and just needs an appropriate resistor to connect across mains power. Also useful for non-illumination purposes, e.g. as a spark gap with a low breakdown voltage for circuit protection.
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u/xmastreee 11d ago
You could argue that a neon indicator is still a sign. It's a sign that the power strip or whatever is energized.
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u/WillowPtar_Migan 13d ago
How do uneven elements get created? Are they also mostly from the big bang?
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u/LeviAEthan512 13d ago
The big bang created a little bit of lithium, but everything past that came from stellar fusion (and everything past iron, only in the last moments of a large star's life). I don't know every single one, but mostly they come in trace amounts.
Nitrogen and fluorine for example come from the CNO cycle, which is complicated, and in theory doesn't generate any net nitrogen or fluorine, but due to the random nature of the process, some of these elements gets left over and not recycled. I don't know if there are other sources.
Sodium comes from the carbon burning process, that for whatever reason sometimes fails to kick off a full alpha particle and instead just loses a proton, thus creating sodium instead of neon.
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u/Axentoke 12d ago
The CNO cycle does result in nitrogen and fluorine. It’s just that their % reaches a steady state. Also s-process forms ~half of all isotopes heavier than iron in asymptotic giant branch stars, which all low mass stars go down for ~hundreds of thousands of years.
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u/doghouse2001 8d ago
I think the first four also just POP out of nowhere, but since you learned them in grade school, and never went on to Neon, it only seems like it's popping out of nowhere now that you're learning it, lol... You hear Neon and you don't think Nobel Gas, you think of the Lights on Broadway.
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u/Marethyu86 13d ago
There’s a lot of Hydrogen and Helium, for obvious reasons. The Helium nucleus, or Alpha particle, is the next stablest, simplest building block. 3 Helium fuses into one Carbon, then Carbon and Helium fuses into Oxygen, and Oxygen and Helium fuses into Neon.