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!