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Periodic Table

Video: What lies at the end of the periodic table?

Chemists explore the table’s heaviest chemistry

by Laura Howes
November 7, 2019 | A version of this story appeared in Volume 97, Issue 44
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Credit: David Vinson/ACS Productions/C&EN

Elements heavier than uranium don’t exist naturally on Earth. Researchers make these massive elements at the end of the periodic table by smashing existing atoms together in particle accelerators. These experiments create just a handful of short-lived atoms, but researchers around the world can still investigate the chemical properties of these superheavy elements. Future experiments might yield longer-lived isotopes to help researchers probe new superheavies, elements that could challenge our knowledge of chemistry and perhaps even the periodic table itself. Join Speaking of Chemistry as we charter a tour toward this possible island of stability to learn how scientists working with elements at the end of the periodic table perform superheavy chemistry and what they hope to find out. Watch the episode at cenm.ag/islandofstability.

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The following is the script for the video. We have edited the interviews within for length and clarity.

Laura Howes: I recently visited these massive magnets at the GSI in Darmstadt, Germany. They’re so big I had a hard time getting them in the frame of my video.

Matt Davenport: Dang. Those are pretty big. And what are they used for?

Laura: Well, GSI operates a particle accelerator, and one of the facility’s goals is to make and study new atoms.

The magnets are part of the Separator for Heavy Ion Reaction Products, or SHIP. Fun fact, the researchers picked SHIP as a nod to the dream of sailing to the island of stability.

So, let me back up for a second. All of the synthetic elements—elements higher up on the periodic table, higher than uranium—are unstable. They are radioactive.

Matt: And most of them don’t live that long before they decay.

Laura: But there is a theory that if we keep making bigger atoms, some of them will be stable. A little cluster, or island, amid the sea of unstable elements.

Matt: We’ve completed seven rows of the periodic table without hitting that island’s shores, but scientists are still looking.

Laura: Right. And one approach is to keep making brand-new elements by adding protons to the nucleus.

But we’re going to look at the other approach, which is adding more neutrons to existing superheavy elements.

Matt: Yep, and that poses some interesting questions. Namely, what are we hoping to learn from and about the superheavies?

Laura: And what are the prospects of making more stable versions to make that research easier?

Matt: And we’ll talk about the rad superheavy chemistry that folks are already doing, when they have just a few atoms that exist for mere moments. So, where should we start, Laura?

Laura: What about with the nucleus?

Matt: Oh, good call. That’s where everything starts, right? If you want to do chemistry on superheavy atoms, you would prefer that they stick around.

Laura: Yep. And that requires a nucleus with some semblance of stability.

Matt: As we alluded to earlier, the superheavies don’t last too long.

Laura: We’re usually talking in the neighborhood of microseconds or milliseconds. But some are longer. Seaborgium, for example, can last for closer to 30 s.

Matt: And that lifetime really depends on the isotope that you’re talking about. Isotopes are versions of the same element with different numbers of neutrons. And some of the yet-to-be made isotopes of superheavies could be super interesting.

I learned about this from Witek Nazarewicz, who does computational studies of nuclei at Michigan State University.

You’re not the only one who got to go on a field trip.

Laura: But did you get to see a large particle accelerator facility?

Matt: I did. It’s just not open yet. But soon.

Laura: Doesn’t count.

Matt: Fine. You win this round. At any rate, nuclear physics is [mind-blowing sound], but let me drop the basics on you.

The nucleus has protons and neutrons, right? You want a healthy balance of both. Too many of either, and your nucleus is going to fall apart. So sayeth nuclear physics.

That’s what these two lines in Witek’s chart tell us. Left of this left line, you’ve got too many protons.

To the right of the right line, you’ve got too many neutrons. But in between is fair game.

Now, let’s look closer at the superheavies. The colored spots are what scientists have made.

Witek Nazarewicz: So this is the limit of current research on superheavy elements.

Matt: All of that territory between the two lines are isotopes waiting to be made. And some of those isotopes could be really long lived.

Witek Nazarewicz: And this is a humbling picture because it shows you that we’re exploring a little forgotten corner of nuclear landscape. The territory of superheavies is much greater than that.

Matt: Here’s where the other shoe drops.

Laura: It is really, really hard to make new isotopes.

Witek Nazarewicz: For theorists, it’s very easy to make such predictions and be happy. For experimentalists, the challenge to go by one unit up or one unit to the right . . . it can be years. But who says that this game is fair?

Matt: We’ve actually made an entire podcast about how hard it is to make these things, right, Kerri?

Kerri Jansen: We sure did, Matt. We’ve got a link in the description of this video.

Matt: But the fact remains, we can synthesize superheavy isotopes.

Laura: Yeah. So that’s one of the things they do at the GSI in Darmstadt. Let’s walk through that science real quick.

To make superheavies, researchers use a particle accelerator. This fires a beam of particles—say, calcium ions—at a heavy-element target. Plutonium or americium, for instance.

But keep in mind, from the nuclear synthesis perspective, that target atom and the ion beam are mostly empty space.

To the average person, though, these targets are thin bits of film that look pretty solid. But on the scale of atoms, I assure you, we’re dealing with mostly empty space.

Matt: So your beam has a super-teensy probability of hitting the target.

Laura: Right. Or you get a glancing blow instead of a head-on collision that you need for the ion and element to fuse, forming a whole new isotope. And when you finally do get them to fuse, the resulting isotope might be so short lived, it’s gone before you can do anything with it.

Matt: That all sounds impossible. How does any superheavy research get done?

Laura: Slowly.

Christoph Düllmann: At the GSI, we typically now conduct experiments that bring you useful results within a time frame of say, 2–4 weeks. In the time frame of roughly 1 month, we expect the observation of several atoms.

Laura: That’s Christoph Düllmann, head of the superheavy-element chemistry department at the GSI.

Matt: To recap, then, they work for like a month to get, let’s say eight atoms to study? Not to put too fine a point on it, but I feel like we should touch on why someone would do superheavy-element chemistry.

Laura: Good point. One of the big things is that the superheavies are drawing focus on how we’ve arranged the periodic table.

Matt: Which is periodically, right?

Laura: About that. It turns out scientists are still debating the best way to arrange everything. Right, Sam?

Sam Lemonick: Totally. You can read more about that in a feature I wrote. We’ll add a link to it in the description.

Laura: And the chemistry of the superheavies is factoring into those discussions. For example, oganesson, element 118, is in the noble-gas group, but it may behave more like a semiconductor, according to recent computational modeling.

Matt: So it’s almost like an identity thing for superheavies? Like, “I’m not like the others in my group. Where do I belong?”

Laura: Well, I’m not sure they really care, but questions about periodicity are a big deal for the periodic table. And Christoph Düllmann says there are exciting questions beyond that as you make more stable elements.

Christoph Düllmann: We can do more benchtop-style experiments and study much more profound properties. And that’s fantastic, and eventually if you make enough of that, you will find applications.

Laura: For example, remember I mentioned that seaborgium can have a longer lifetime? It’s long enough to actually do chemistry. The isotope can merge with ligands to form a hexacarbonyl compound.

Matt: That is wild. But most superheavies don’t live that long, right?

Laura: No, other superheavies only last long enough for gas-solid chromatography, which can measure the reactivity of the elements.

Matt: I’ve heard of chromatography, but the kind I’m thinking about takes longer than a millisecond to run. How does it work with these short-lived isotopes?

Laura: The moment an atom is created, it is fired down an array of detectors.

A postdoc in Christoph’s group called Lotte Lens showed me what those detectors look like, but she didn’t want to be on camera.

Matt: Oh, that’s too bad.

Laura: But she did explain to me that by measuring where the atom adsorbs on the detectors, she can calculate the reactivity of the atom. Researchers can then compare that to theoretical predictions.

Christoph Düllmann: We get good guidance from theory. And this is always helpful. The experiment is designed to answer a specific question.

Laura: And that’s not all. Aside from gas chromatography, scientists are developing different forms of spectroscopy to study the superheavies too.

Matt: It sounds like scientists are working on this problem from a couple different angles, then. It’s not just, “Let’s make more atoms of more stable isotopes.” It’s also, “Let’s make better gear to study what we can already make.”

Laura: Yeah. But let’s not kid ourselves. More atoms of more stable isotopes is the goal. And Christoph thinks that island of stability might still be out there, even if the elements aren’t superstable like those higher up on the periodic table.

Christoph Düllmann: So the picture of the island of stability is, in my opinion, fully correct. We do not know where the center is, we do not know how high the mountain or this peak of stability really is. We’re at the shoreline more or less. And so we start to climb up the hill and find more long-lived isotopes.

Laura: I like to think of these researchers as explorers. So they’re looking for the island of stability, and it might not have superstable elements there, but they might find some interesting things or something different, and I’m excited about that.

Matt: Yeah. And, either way, we got to go look at some pretty sweet equipment.

Laura: And you got to wear a hard hat.

Matt: That may have been the best part of this. Joking aside, we want to know what you’re most excited to learn about the superheavy elements. Let us know in the comments.

Laura: This video is part of C&EN’s celebrations for the International Year of the Periodic Table. Make sure to visit our website to see all of the other awesome content we’ve produced this year.

Matt: Including an awesome story that you wrote with even more superheavy goodness.

Laura: Thanks for watching.

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