Is Uranium the Heaviest Natural Element?

Posted by Ethan on April 3, 2009
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Last night, I was watching the Daily Show, and they had Tom Zoellner on, talking about his new book: Uranium: War, Energy, and the Rock that shaped the world.

There are certainly a lot of interesting things about Uranium in culture, particularly in terms of energy (hey, we can use this thing to power the world) and in terms of war (the nuclear arms race). If you look at what we’ve done with all the enriched Uranium (U-235, the fissionable type), the US and the Soviets, from the 1940s to the 1980s, basically took nearly all of it and stockpiled it.

This is the only element on Earth we’ve ever done that to, and that makes it incredibly interesting from a cultural point of view.

But Tom Zoellner said something interesting during the interview about Uranium, that I’d like for you to think about with me:

[Uranium] is one of these things… it’s the heaviest naturally occurring mineral in the periodic table. I mean, the table goes up higher, but this is where it ends. This stuff seems to hit some invisible wall of nature where it has this really big nucleus, this nucleus of 92 protons, and the center can’t hold.

Now, this is mostly true. It is the heaviest naturally occurring element on Earth. We don’t find any elements with 93 protons (Neptunium), 94 protons (Plutonium), or higher numbers except the ones we’ve made in laboratories. Here’s a version of the periodic table of naturally occurring elements for you:

But — here’s a great question for you — why? Why is Uranium the heaviest element on Earth? Well, there are two reasons: one is obvious, and one is very subtle. The obvious reason is that the elements shown above in light grey, 43-Tc, 61-Pm, and everything above and including 93-Np are unstable. They get created in stars and supernovae, just like all the other elements, but they radioactively decay into the other, stable elements (1-42, 44-60, and 62-92).

But hang on a second. Isn’t Uranium, element 92, also unstable? Its most stable isotope, Uranium 238 (92 protons + 146 neutrons), has a half-life of about 4.5 billion years, or the age of the Earth. Well, that explains why there’s still so much Uranium around: it’s only had enough time for about 50% of the atoms to decay. The fissionable type of Uranium, Uranium 235 (3 fewer neutrons), is less stable, with a half-life of 700 million years, is still around, too. Much more of it — about 98.8% — is gone by now. However, there’s still enough left that there’s plenty on Earth.

But every element heavier than lead (element number 82) is unstable, and will eventually decay! Of all the unstable elements that aren’t on Earth, though, I’m curious about why there isn’t any Plutonium? Plutonium-244, with 94 protons and 150 neutrons, has a half-life of 83 million years, which means that there should still be small amounts of it left today. Plutonium is more stable than Protactinium and Actinium, which are found in Nature. Only one study, whose results are not universally accepted, claims to have detected natural Plutonium. So, now for the more subtle point: why isn’t there any Plutonium-244 on Earth?

The only way you produce elements heavier than Iron-56 is in Supernovae: when massive, dying stars collapse and explode. These heavy elements then get spread out over a large area and recollapse, forming new stars and planets. That’s where all of the elements on Earth came from. But the supernova that gave rise to us must not have been powerful enough to make huge amounts of Plutonium! Since there are supernovae that are up to 100 times more powerful than normal ones, there are explosions out there in the Universe that will litter it with Plutonium. We saw one in 2006 that probably did just that:

Well, the planets that get created from the dust of this explosion will not see Uranium (element 92) as their “wall of nature,” they’ll see Plutonium (element 94) as the heaviest, or possibly even Curium-247 (element 96)! There are people searching for this as well. And, on a lighter note, there will surely be planets out there where the heaviest element is lighter than Uranium, possibly even lighter than lead! So yes, Uranium is the heaviest natural element that we find on Earth, but there are certainly heavier ones out there!

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  1. #1 Richard Helmich
    April 4, 2009

    Great explanation!

  2. #2 Chris
    April 6, 2009

    I’ll say it’s a great explanation. Now, how do I role this one out in casual conversation?

  3. #3 LtStorm
    April 6, 2009

    One thing I’ve been curious about is whether any of the elements in the Island of Stability theorized to exist far out on the Periodic Table past what we’ve ever seen before are ever created in Hypernova or some such. I don’t know if it’d be possible, but it does make me curious as to whether anyone has ever looked before. I mean, if it was really delved into the Periodic Table should give some idea of what spectral emissions we’d see (It’s the friggin’ Periodic Table, afterall….), but I don’t know if anyone ever has done this before.

  4. #4 rob
    April 6, 2009

    great. just what we need. aliens visiting us from a planet with naturally occuring plutonium. they won’t have tin-foil hats, they will have plutonium foil hats!

    welcome to SB

  5. #5 Glen Davidson
    April 6, 2009

    The problem with the blogpost is that both neptunium and plutonium are found in very small quantities in the earth–that is to say, naturally-occurring isotopes of those elements are.

    The best I can tell is that uranium is held to be the heaviest “naturally-occurring element” on earth because of tradition, convention, and the very small quantities of heavier isotopes being found. It’s just traces of neptunium and plutonium that are found, after all, nothing anybody is going to mine. What is more, even heavier isotopes could be found “in nature,” at least if one could get close enough to recent supernova II ejecta. What is more, the little bit of neptunium and plutonium found are presumably “built upon” uranium, or, perhaps in some cases, thorium.

    The point is that the “heaviest naturally-occurring element” is an arbitrary designation, depending on where you cut off “nature.” Naturally occurring neptunium and plutonium are simply traces that are being constantly produced at very low rates via nuclear interactions, and barely count at all.

    Interestingly, I watched a program recently on National Geographic Channel which did count neptunium and plutonium as the heaviest “naturally occurring elements”–on earth, as I recall. I did not think it the best thing to do without explanation (there was none), since I do consider the term arbitrary, and the mere fact that a californium atom may occasionally settle upon the earth is about as unimportant as the tiny bits of naturally-produced neptunium and plutonium are found on earth.

    That said, we shouldn’t really make the following statement:

    We don’t find any elements with 93 protons (Neptunium), 94 protons (Plutonium), or higher numbers except the ones we’ve made in laboratories.

    Glen D
    http://tinyurl.com/6mb592

  6. #6 Charles Wade
    April 6, 2009

    Thank you very much for that explanation. I watched the very same show and I was puzzled by that answer. I did not think it through but I am glad that you did. I teach high school physics and these are the types of interesting thought questions that get kids engaged.

  7. #7 doctor logic
    April 6, 2009

    Thanks for the post! Quite fascinating.

    What I don’t understand is this. I assumed that in a dying star, the heaviest element at the core will be Iron, and that anything heavier gets created in the supernova explosion. I assumed the energy of the supernova was enough to fuse nuclei together and create heavier elements. Correct me if I’m wrong…

    So it seemed odd to me that the temperature of the supernova explosion happens to lie so finely between the fusion potentials of Uranium and Plutonium. I would have thought it likely that the explosive energy would overcome the fusion potentials for all kinds of nuclei, eventually leaving only stable elements in its wake. Obviously, your observation about Plutonium means my naive assumption was wrong!

    How good are our supernova models? Can we really predict the heaviest nuclei we’ll produce as a function of the star’s mass? Or are we working in reverse and building supernova models from the Plutonium abundance?

  8. #8 scooter
    April 6, 2009

    Well I, for one, am quite pleased with what used to be a lack of plutonium on Planet zero, too bad we didn’t leave it that way.

    Now keep this low key, otherwise it will be yet another proof of the existence of god. I’m trying to get a couple of kids through school before science is outlawed in TX.

  9. #9 doctor logic
    April 6, 2009

    Yeah, I just looked at the periodic table, and Uranium is still around because it has a long-lived isotope. Plutonium doesn’t. I’m skeptical of this idea. A larger supernova would make more stuff, but I don’t see why the relative abundances would be any different.

  10. #10 Tim
    April 6, 2009

    Interesting, but don’t forget that these actinides have had more than 4.5 billion years to decay, that started as soon as they were made and continued as the supernova remnants coalesced and became our favorite stellar system, time enough for a lot of unstable elements to decay into lead. And I never really thought of Hanford and Savannah River as labs, maybe really large production labs?

  11. #11 Ronald Brakels
    April 6, 2009

    In case any heavy metal fans feel slighted, I will mention that this post is discussing atomic weight, not actual density. As any dedicated metal head knows, tungsten, gold, osmium, iridium and so on are all denser than uranium in evironmental conditions found at a typical heavy metal concert. This should always be borne in mind if you are carrying these elements in your pockets and wish to attempt stage diving.

  12. #12 Blaine
    April 7, 2009

    Thanks for the great post. This will be a good topic of discussion with my kids.

  13. #13 Tom
    April 7, 2009

    Fissionable means capable of undergoing fission, which also applies to U-238. What you probably meant was fissile, which are the isotopes which will fission with the mere absorption of a neutron, without the requirement of any kinetic energy.

    U-235 is fissile, while U-238 is not. They are both fissionable.

  14. #14 Ian
    April 8, 2009

    “This is the only element on Earth we’ve ever done that to,”

    You appear to be neglecting gold! That’s stockpiled up the wazoo!

  15. #15 Ethan Siegel
    April 8, 2009

    I didn’t know they had found and confirmed Plutonium on Earth as being natural. I was aware that they hadn’t found it naturally, whereas they had expected it in significant trace amounts.

    Why is there so little Plutonium? Well, 83 million years for a half-life is long, but 4.5 billion years is a long time. We would expect Pu-244 to be around in small but easily measurable abundances. But if it isn’t there, we must have had trouble forming it. (If you want the details, you can look up r-processes of heavy element formation in supernovae, which I am not going to explain here.) But there are plenty of large and small explosions in space, and it’s pretty reasonable that there are stars out there that produce larger abundances of the heaviest elements as well as ones that make smaller abundances.

    Yes, when the Earth was only 1 billion years old, there was probably more Pu-244 than there is now, but there was also less than there would have been if it was produced in the same quantities as Uranium.

    Yes, plenty of other elements are stockpiled, but there is still plenty of gold in the Earth. Uranium? Not so much…

    As for the “island of stability”… show me some evidence for that one; until then, I’ll say it’s an interesting idea with no support.

  16. #16 Glen Davidson
    April 8, 2009

    From what I’ve read (not extensive), it’s only Pu-239 that has been confirmed as occurring naturally. Below is not what I’d call an unimpeachable source, but one sees how a mix of uranium and beryllium could produce Pu-239, as explained there:


    Natural plutonium-containing mineral.

    The sample I have representing plutonium is the naturally occurring mineral muromontite, which is a mixture of uranium and beryllium. Putting beryllium near uranium is generally considered a bad idea because the alpha particles from the decay of uranium are captured by the beryllium atoms, which in turn release neutrons. Neutrons are very unhealthy to be around.

    In the case of this sample, however, the neutrons are in turn re-captured by the uranium, which then undergoes further decay and is transformed into plutonium. The result is that this mineral contains the highest known naturally occurring concentration of plutonium.

    http://theodoregray.com/PeriodicTable/Elements/094/index.html

    He seems not to realize that natural uranium doesn’t produce many alpha particles to begin with, and then only around 30 neutrons per million alpha particles are produced by the beryllium. Nevertheless, a few neutrons would be produced, and uranium would capture those at reasonably good rates.

    Another source I’ve encountered stated that, in their sample, spontaneous fission didn’t seem sufficient to produce the neutrons needed to make the Pu-239 found, and, apparently, the sample was lacking in beryllium, or they hadn’t thought about it. They contended that other sources had to account for their minute quantities of Pu-239, perhaps radioactive gases in the atmosphere, possibly cosmic rays.

    That’s interesting information about Pu-244. I hadn’t heard that it should have persisted in measurable quantities had it been produced at expected rates. Sounds like one for the theoretical physicist, all right.

    Glen D
    http://tinyurl.com/6mb592

  17. #17 Glen Davidson
    April 8, 2009

    Well, I don’t know if my last post is going to appear, but I went out and found one of the sources I mentioned there. I was wrong, it does discuss alpha particles producing neutrons in light elements (not necessarily beryllium) as a source of neutrons causing Pu-239 (and neptunium as well) to form, as well as spontaneous fission supplying the needed neutrons. Here’s the abstract:

    http://pubs.acs.org/doi/abs/10.1021/ja01151a085

    Glen D

  18. #18 Glen Davidson
    April 8, 2009

    Btw, most U-235 mined and enriched has been split in reactors. Stockpiles of both Russian plutonium and highly enriched U-235 have been fed into US reactors.

    The highly enriched uranium was diluted down with unenriched (or depleted?) uranium for commercial reactors. Some Russian highly-enriched uranium has been saved for US subs and for possible future space reactors.

    And by far the most U-235 is still in the ground. Nevertheless, all isotopes of uranium are about 1/10 as common in the entire earth as gold is. The reason it’s relatively common for our purposes is that it is not siderophilic (so not in the core), nor is it a “compatible element” in the mantle. It’s been estimated that 30 to 60 percent of the entire earth’s uranium is in the earth’s crust.

    Glen D
    http://tinyurl.com/6mb592

  19. #19 kirthi
    April 21, 2009

    i need a element which is power full than uranim pls give in this website

  20. #20 stupidkid
    April 26, 2009

    I don’t understand any of this.

  21. #21 henadzi
    July 2, 2009

    please see about it at:

    http://nanochemical.blogspot.com

    thanks

  22. #22 Rob
    December 10, 2009

    83 million years is not very long for plutonium. That means ~99.999999999% of it is gone. Finding any would be a mighty chore.

    There is still plenty of uranium around, which means uranium still decays. When uranium decays, what you have left is an element with fewer protons. Technically new atoms are created that are heavier than lead, but they only form from radioactive decay. That explains why elements that decay faster then plutonium are still around in vast quantities.

    As an earlier commenter mentioned, there are still random atoms lying around which are heavier than uranium. Heck, there might still be an atom somewhere with 200 protons in it that hasn’t decayed yet since decay is somewhat random. Finding one atom is nearly impossible though.

    Also, immediately after a supernova, with that intense amount of energy, who knows how large of an atom is produced, there could have been 1000′s of protons and electrons in some kind of equilibrium or semi-stable state. Perhaps it only existed for .00000000000000000000001 seconds, but it still existed.

  23. #23 Raja
    February 4, 2010

    uranium projects

  24. #24 Anon.
    April 7, 2011

    naturally occurring plutonium is created near the lower layers of the earth where fission occurs with the uranium that’s still buried deep. It is only small amounts though but that’s still naturally occurring plutonium..it’s being created under natural fissure.

  25. #25 Pete
    April 11, 2011

    Heavier elements? Well, according to some astronomical models, neutron stars are considered a single atom – neutrons and protons all squeezed together into a single lump, electrons combined with protons to form more neutrons or squirted out to who knows where. Same for black holes, although the math of what actually goes on inside breaks down. In that picture, there are not islands of stability, where atoms can have a slightly greater weight than the transuranium elements we have managed to synthesize here. Rather, it might be more fitting to speak of an island of instability, between iron as the heaviest stable small atom and the smallest possible neutron star as the lightest large atom, with an area of unstable elements between the two. And of course, there could be islands of stability inside this large region of instability. Pretty extreme cases though, where gravitational attraction is enough to overcome internal nuclear forces, and (for now, anyway) not commercially useful, where a single atom masses more than our entire solar system.

    No telling at the moment, our physics is very much in its infancy with regard to such extreme conditions. Plenty of room yet for people with good ideas and a PhD. or Nobel prize in their sights.

  26. #26 Nathaniel
    Panama city beach,Fl
    October 12, 2012

    we are talking about uranium 235 and i was wandering how long does it take for you to get more uranium 235?