Does the periodic table go on forever?

We have four new elements, but the Russians aren’t happy. The International Union of Pure and Applied Chemistry (IUPAC) has just confirmed the existence of numbers 113, 115, 117 and 118 in the periodic table. A team at the Joint Institute for Nuclear Research (JINR) in Dubna, near Moscow, is one of three worldwide who laid claim to discovering them. IUPAC has affirmed the priority of the Dubna team for three of these, in collaboration with scientists at the Lawrence Livermore and Oak Ridge National Laboratories in the United States. But the first to sight element 113 convincingly, IUPAC says, were researchers at the RIKEN Nishina Center for Accelerator-based Science in Japan. The Russians are grumbling about that decision.

None of these new elements occurs naturally: they are too heavy to be made in the nuclear reactions that occur inside large stars and supernovae (exploding stars), which are responsible for most of the elements that scatter the cosmos. Or rather, if these “superheavy” elements do get produced in high-energy cosmic processes, they are so unstable that they would decay radioactively in a matter of seconds. Such decay means that to all intents and purposes the “natural” periodic table stops at around uranium (element 92). But since the early days of research into nuclear fission in the 1940s—the work underlying the first atom bombs—it has been clear that nuclear reactions induced artificially can make new elements heavier than uranium. One of the first, plutonium (element 94), was the substance whose uncontrolled fission destroyed Nagasaki in August 1945.

Several new “artificial” elements were made soon after the war in particle accelerators, where electrically charged atoms (ions) were boosted to high energies and fired at targets made of heavy elements to make even heavier ones. Atomic nuclei have a positive electrical charge, and so they repel one another. But if they collide with a high enough energy, they can smash through this repulsive barrier, whereupon the fundamental forces of the nucleus can pull them together. Elements such as americium (95), berkelium (97) and californium (98) were made this way; it’s not hard to guess where. Others, such as einsteinium (99) and fermium (100), were found among the decay products of nuclear testing. They extended the periodic table well beyond uranium, fleshing out its seventh row.

For a long time this field of superheavy elements was dominated by the Americans and Russians, who invested heavily in nuclear and accelerator technologies during the Cold War. In the 1980s and 1990s a new competitor entered and soon dominated the field: the Laboratory for Heavy Ion Research (GSI) in Darmstadt, Germany. Now Japan too has a state-of-the-art accelerator at the RIKEN Center.

The more massive these elements are, the less stable they are in general. By the time we had reached elements 100 and above, they tended to decay in a matter of a few tens of seconds at best, and usually just fractions of a second. What’s more, the chances of such a superheavy atom being formed in a collision are tiny, and so it becomes a hugely challenging task to make a few atoms of a new element and to detect them, let alone to figure out what kinds of chemical properties they have.

The competition to be the first to make a new element is all the more intense because that traditionally gives you the right to name it (subject to IUPAC’s approval). The arguments were particularly bitter during the Cold War; it wasn’t until 1997 that IUPAC settled a dispute over elements 104 and 105 that had rumbled on since the 1960s—the latter is now dubnium, reminding us who won that battle. Since the late 1990s things have become more collegial, with collaborations between the Dubna and Livermore teams leading to elements 114 and 116. However, that hasn’t diminished the triumphant parochialism: 116 is livermorium, and 114 is flerovium, after Georgy Flerov, the first director of the JINR. Flerov was a highly respectable scientist, but hardly of a calibre to match einsteinium, rutherfordium, bohrium or copernicium.

All the same, the Russians have greeted the awarding of element 113 to the Japanese team with ill grace. They argue that they made it first (in 2003), that the Japanese experiment is hardly repeatable, and that its leader Kosuke Morita “is to a certain extent the trainee of Dubna; here, in JINR he for a quite long time learned the basics of synthesis of new elements.” But IUPAC has spoken, and the Japanese researchers will now probably rub it in by christening element 113 japonicium.

It’s not clear that this proprietary flag-planting is to the public taste. An online petition calls for one of the new superheavy elements—113 and 115 are expected to be metallic, or at least semi-metals—to be named lemmium, after the recently deceased godfather of heavy metal Ian “Lemmy” Kilmister of Motörhead. (118 wouldn’t do because it falls into the column of inert or rare gases such as neon, argon and krypton for which IUPAC guidelines recommend the “-on” ending. Lemmy was not inert, though arguably very rare.) Another movement wants to see 117—a halogen like fluorine and chlorine, and therefore needing an “-ine” ending—named octarine, the eighth, magical colour of the spectrum in the Discworld books of the late Terry Pratchett. No doubt a petition for “bowium” is underway.

Some press coverage of the IUPAC announcement suggested that the new batch of elements “completes the periodic table.” This is meaningless. The elements complete the seventh row of the table, which ends with the column of the inert gases. But there is no reason why researchers could not start to create elements in an eighth row, beginning on the left-hand side with element 119, which would join the alkali metals lithium, sodium, potassium and so forth. In fact, it is more likely element 120 will be made next, since even-numbered elements tend to be more stable.

The teams in the US, Russia and Japan are surely already on the case. But how long can this continue? The extremely superheavy elements of the eighth row are certainly likely to be even less stable and so harder to make—even more collisions will be needed to create each atom. But does the table extend forever? No one knows.

Nuclear scientists can calculate the expected stability and properties of new superheavy elements, but only approximately. The American physicist Richard Feynman suggested that the table would effectively end at element 137, beyond which (he said) no neutral atoms could exist because the innermost electrons would then be of such high energy that they could no longer stay bound to the nucleus.

More detailed calculations have since shown that it is not until element 173 that weird things start to happen with the electron energies and it isn't clear whether this spells doom for stable neutral atoms. Even if it does, there seems no obvious reason why we couldn’t go on making more massive nuclei—they could perhaps survive as ions, the protons outweighing the electrons. Judging from the rate of element synthesis over the past 50 years, however, there seems scarcely any prospect that this issue will be put to the test before all of us have decayed ourselves.