The elusive number 113

Scientists’ creation of a new “superheavy” element—the culmination of a nine-year project—may change chemistry
October 17, 2012

After scientists ran out of "natural" elements to discover, they raced to make "synthetic" atoms

The periodic table of elements just got a new member. At least, maybe it did—it’s hard to tell. Having run out of new elements to discover, scientists have, over the past several decades, been making “synthetic” atoms, too bloated to exist in nature, which survive for just an instant before they splinter in radioactive decay.

But this is increasingly difficult as the atoms get bigger. The new element recently claimed by the Nishina Centre for Accelerator-Based Science, a Japanese research institute in Saitama, near Tokyo, has proven frustratingly elusive. It is known simply as element 113, its serial order in the periodic table, and efforts to create it have been underway since at least 2003.

These artificial elements are made and detected literally an atom at a time. The Japanese researchers claim only to have made three atoms in total of element 113, all of which decayed almost instantly.

If verified, it will add another member to the already substantial list of artificial elements assembled over the past seven decades of nuclear science. With each new addition to these “superheavy” elements, nuclear chemists have a fresh opportunity to test whether the periodic table—the organising scheme for all of chemistry—falls into disarray at such extremes, as some think it might. There is even a possibility that some elements out beyond 113 might be relatively stable, decaying so slowly that large amounts of them could be painstakingly accumulated for exotic applications.

The “natural’” periodic table runs from hydrogen (element 1) to uranium (element 92), each being distinguished from the others by the make-up of its atoms. Every atom has a super-dense core called a nucleus, surrounded by negatively charged particles called electrons. The nucleus contains positively charged particles, or protons, and—with the exception of hydrogen—neutral particles, or neutrons, which help to bind the protons together. It’s the number of protons that defines an element and determines its place in the periodic table.

The table arranges the succession of elements into groups that share similar chemical properties. Uranium has 92 protons, and is the heaviest element occurring on Earth. Elements with numbers of protons higher than that must be created artificially.

This means forcing more protons and neutrons into nuclei already so replete with such particles that they are prone to falling apart by radioactive decay. For any given element, the number of protons is identical in every atom, but the number of neutrons can vary. Atoms of the same element with different numbers of neutrons are called isotopes. Uranium-235, for example, the key ingredient of nuclear reactor fuel, has 92 protons and 143 neutrons, while the much more abundant uranium-238 has 146 neutrons.

While certain isotopes of any element may decay if they have too many or too few neutrons, all the isotopes of elements as heavy as uranium are too stuffed full of particles to remain stable, and eventually decay. As one progresses through the subsequent artificial elements into the “superheavies”, this instability gets ever more pronounced—which is why those like element 113 fall apart almost as soon as they are made.

The Japanese claim isn’t going to pass without challenge, not least because the first group to sight a new element enjoys the privilege of naming it, an added spur to the desire to win. Just as in the golden years of natural element discovery in the 19th century, naming tends to be nationalistic and chauvinistic. No one could begrudge Marie and Pierre Curie their polonium, the element they discovered in 1898 after laboriously sifting tonnes of uranium ore, which they named after Marie’s Polish homeland. But the recent baptism of element 114 as flerovium, after the founder of the Russian institute where it was made, and element 116 as livermorium, after the Lawrence Livermore National Laboratory where it originated, display rather more concern for bragging than for euphony.

Perhaps this is inevitable, given that making new elements began in an atmosphere of torrid, even lethal, international confrontation. The first element heavier than uranium was identified in 1940 at the University of California at Berkeley. This was element 93, christened neptunium after the planet Neptune, given that uranium had been named after Uranus in 1789. Neptunium quickly decays into plutonium, atomic number 94, the discovery of which was kept secret during wartime. By the time it was announced in 1946, enough had been made to obliterate a city: it was the explosive of the Nagasaki atom bomb.

Making nuclei more massive than those of uranium involves firing elementary particles at heavy atoms in the hope that some will temporarily stick. That was how Edwin McMillan first made neptunium at Berkeley in 1939. McMillan didn’t realise what he’d done until the following year, when chemist Philip Abelson helped him to separate the new element from the debris. By then the world was at war, and almost at once both the Allied and German physicists realised an atomic bomb could be made from artificial elements 93 or 94, which would be created by neutron bombardment of uranium inside a nuclear reactor. Only the Americans managed it, of course.

So the race to make new elements, with roots in wartime, took off during the ensuing cold war, with motives as much military as scientific. Some of them were discovered in the fallout of nuclear bomb tests. The Soviet efforts also began in the 1940s, thanks largely to the work of Georg Flerov. In 1957 he was appointed head of the Laboratory of Nuclear Reactions, a part of the Joint Institute of Nuclear Research in Dubna, north of Moscow. Dubna has been at the forefront of element-making ever since; in 1967 the lab claimed to have made element 105, now called dubnium.

That claim exemplifies the ill-tempered history of artificial elements. It was disputed by the rival team at Berkeley, who made 105 in the same year and argued furiously over naming rights. The Soviets wanted, magnanimously but awkwardly, to call it nielsbohrium, after Danish physicist Niels Bohr. The Americans preferred hahnium, after the German nuclear chemist Otto Hahn. Both dug in their heels until the International Union of Pure and Applied Chemistry (IUPAC), the authority on chemical nomenclature, stepped in to resolve the mess in the 1990s. Finally the Russian priority was acknowledged in the name, which after all was a small riposte to the earlier American triumphalism of americium (element 95), berkelium (element 97) and californium (98).

These “superheavy” elements, with atomic numbers reaching into triple figures, are generally made now not by piecemeal addition to uranium but by trying to merge together two smaller but substantial nuclei. One, typically zinc or nickel, is converted into electrically charged ions by having electrons removed and then accelerated in an electric field to immense energy before crashing into a target made of an element like lead.

This is the method used by the Institute for Heavy Ion Research (GSI) in Darmstadt, Germany, which since the 1980s has outpaced both the Americans and the Russians in synthesising new elements. It has claimed priority for all the elements from 107 to 112, and their names reflect this: element 108 is hassium, after the state of Hesse, and element 110 is darmstadtium. But this crowing is a little less strident now: many of the recent elements have instead been named after scientists who pioneered elemental and nuclear studies: bohrium, mendelevium (after Dmitri Mendeleev, the periodic table’s discoverer), rutherfordium (after Ernest Rutherford), meitnerium (Lise Meitner). In 2010, IUPAC approved the GSI team’s proposal for element 112, copernicium, even though Copernicus is not known to have ever set foot in a chemical lab.

The heaviest element so far recognized by IUPAC is 116, and 114 has also been officially verified. So why the fuss over 113? Although the elements get harder to make as they get bigger, the progression isn’t necessarily smooth: some combinations of protons and neutrons are (a little) easier to assemble than others. In 2003, the group at the Nishina Centre began firing zinc ions at bismuth in the hope of creating element 113. The centre, run by the government-funded research organisation RIKEN, was a relative newcomer to element-making but asserted its success just a year later.

It’s precisely because they are unstable that these new elements can be registered individually by detectors: the radioactive decay of a single atom sends out particles— generally an alpha particle—that can be spotted by detectors. Each atom initiates a whole chain of decays into successive elements, and the energies and the release times of the radioactive particles are characteristic fingerprints that allow the decay chain and the elements within it to be identified.

At least, that’s the theory. In practice the decay events must be spotted amidst a welter of nuclear break-ups from other radioactive elements made by the ion collisions. And with so many possible isotopes of these superheavy elements, the decay properties of which are often poorly known, there’s lots of scope for phantoms and false trails—not to mention outright fraud: Bulgarian nuclear scientist Victor Ninov, who worked at Berkeley and GSI, was found guilty of fabricating evidence for the claimed discovery of element 118 at Berkeley in 2001. When you consider the figures, some scepticism is understandable: the Japanese team estimated that only three to six out of every 100 quintillion (ten to the power of 18) zinc ions would produce an atom of 113.

Last year, IUPAC representatives decided the Japanese results weren’t conclusive. But neither were they persuaded by subsequent claims of scientists at Dubna and Berkeley, who have begun collaborating after decades of bitter rivalry. However, on 27th September, the RIKEN team released new data that makes a stronger case. The team leader Kosuke Morita says that he is “really confident” they have element 113 pinned. They’ve only a single decay chain to adduce, starting from a single atom of 113, but some experts now find the case convincing. If so, the name game may get solipsistic again: rikenium and japonium are in the offing.

Given how hard it is to make this stuff, why bother? Plutonium isn’t the only artificial element to find a use. For example, minute amounts of americium are used in some smoke detectors. Yet as the superheavies get ever heavier and less stable, typically decaying in a fraction of a second, it’s harder to imagine how they could be of technological value. But according to calculations, some isotopes of element 114 and others nearby should be especially stable, with half-lives of perhaps several days, years, even millennia. If true, these superheavies could be gradually accumulated atom by atom. But others estimate this “island of stability” won’t appear until element 126; others suspect it may not really exist at all.

There is another, more fundamental, motivation for making new elements. They test to destruction the current theories of nuclear physics: it’s still not fully understood what the properties of these massive nuclei are, although they are expected to do weird things, such as take on very deformed, non-spherical shapes.

Artificial elements also pose a challenge to the periodic table itself. It’s periodic because, as Mendeleev and others realised, similar chemical properties keep reappearing as the elements’ atomic numbers increase: the halogens chlorine (element 17), bromine (35) and iodine (53) all form the same kinds of chemical compounds, for example. That’s because an atom’s electrons, which determine how it reacts with others, are arranged in successive “shells”. Elements in the same group of the periodic table have electrons arranged in similar patterns, and so some of their chemical behaviour is similar.

But a very massive nucleus starts to undermine this tidy progression of shells. The electrons closest to the nucleus feel the very strong electric field created by such a large number of protons, which makes them very energetic: they circulate around, and indeed through, the nucleus at close to the speed of light. This means they feel the effects of special relativity: as Einstein predicted, particles moving that fast gain mass. That alters the electrons’ energies, with knock-on effects in the outer shells, so that the outermost electrons that determine the atom’s chemical behaviour don’t observe the periodic sequence. Then the periodic table loses its rhythm, as such elements deviate from the properties of others in the same group.

Some anomalous properties of natural heavy elements are caused by these “relativistic” effects. They alter the electron energies in gold so that it absorbs blue light, accounting for the yellow tint of the light it reflects. And they weaken the chemical bonds between mercury atoms, giving the metal its low melting point.

Relativistic deviancy is expected for at least some superheavies. To look for it, researchers have to accomplish extraordinarily adroit chemistry: to figure out from just a handful of atoms, each surviving for perhaps seconds to minutes, how the element reacts with others. This could, for example, mean examining whether a particular chemical compound is anomalously volatile or insoluble. The teams at GSI, Dubna and Berkeley have perfected methods of highly sensitive, quick-fire chemical analysis to separate, purify and detect their precious few exotic atoms. That’s enabled them to establish that rutherfordium (element 104) and dubnium buck the trends of the periodic table, whereas seaborgium (106) does not.

As they enter the artificial depths of the periodic table, none of these researchers know what they will find. The Dubna-Livermore collaboration claims to have been making element 115 since 2003, but IUPAC has not yet validated the discovery. IUPAC is still considering the claims for 117 and 118, and both GSI and the RIKEN team are now hunting 119 and 120.

Is there any limit to it? Richard Feynman, the Nobel prize-winning American physicist, once made a back-of-the-envelope calculation showing that nuclei can no longer hold onto electrons beyond an atomic number of 137. More detailed studies, however, show that to be untrue, and some nuclear scientists are confident there is no theoretical limit on nuclear size. Perhaps the question is whether we can think up enough names for them all.