Periodic Discussions

Element 112 was discovered more than a decade ago. Why wasn’t it given a place on the periodic table until now?

Ununbium in the extended periodic table

The periodic table added its 112th official element Wednesday, when scientists in Darmstadt, Germany, announced they had received official approval for ununbium from an international body of chemists. But the discovery of the new element wasn’t news to anyone—it was first announced back in 1996, when the Darmstadt scientists claimed to have created two atoms of the stuff in a 400-foot particle accelerator. It’s just taken 13 years of formal reviews and appeals for their colleagues around the world to believe them. How did the most basic question of science— what are the fundamental materials that make up our universe?—turn into the science equivalent of a Supreme Court decision?

It seems as if the makeup of the periodic table would be as rudimentary as apples falling down, not up. There’s evidence for elements like oxygen, iron, and silicon all around. Heck, you’re made of evidence. But that’s not true for the dimmer corridors of the table that run along its very bottom, where elements like ununbium sit. No one has ever seen element 112 with their own eyes—we’ve only assumed its existence based on a smattering of computer blips stored on a couple of hard drives around the world. How to interpret those blips has become a matter for endless committee meetings and debates over whether it’s OK to add a new square to the most precious real estate in science.

It wasn’t always that way. In the old days, scientists got visible, earthy samples of new elements by sifting through exotic minerals. Marie and Pierre Curie boiled down a few thousand pounds of uranium ore to isolate a few grams of polonium and radium—the latter of which happened to glow in the dark. By 1940, scientists had exhausted all of nature’s easily accessible elements. From then on, they would extend the periodic table only by creating elements—by hurling bits of matter at heavy-element targets. If they stuck—if the nuclei of the smaller bits fused with the nuclei of the targets—they’d have a new, extra-heavy element to add to the list.

But it’s not easy to tell when you’ve created a new element by these means—three-quarters of all elements are gray metals, after all. So to confirm they’d discovered something new, scientists studied its radioactivity. All heavy elements are unstable, and their nuclei spew atomic shrapnel as they decay. Since each one breaks apart in its own unique way, scientists can spot brand-new heavy elements by looking for novel radioactive signatures in their data. Chemists could later double-check the discovery by creating (often after years of work) a larger sample of the potential element that might be quickly washed in chemicals to see whether it reacted like its prospective neighbors on the periodic table. Confirming the presence of berkelium, for example, can be as easy as watching a vial of the element turn yellow-orange in a chemical bath.

That method allowed us to authenticate elements up to an atomic number of around 107 or 108. But elements even heavier than that fall apart too quickly to allow the chemists to do their work. It’s still possible to use radioactivity data, but the further down the periodic table you push, the less reliable that method gets, too: When you’re only dealing with one or two atoms of the new element, and when each was created months or even years apart during messy shoot-and-scatter experiments, it’s almost impossible to separate the signal from the noise using traditional methods.

Scientists now must search for the faint traces of new elements using a combination of old-fashioned physical detection and new-fangled computer filtering. And instead of looking only for an atom of the new element, they trace the “daughter products” of radioactive decay. When element 112 decayed, it turned into element 110 (darmstadtium), which in turn decayed to element 108 (hassium), and so on. Each step in the process leaves its own signature, and the chain can be reconstructed to guess what the original product must have been. The computer software has to sort through reams of spurious data and determine the probability that a promising signal might be true.

That’s where the “hard” science of nuclear chemistry gives way to a campaign of persuasion and appeals to trust and judgment. Given how ambiguous this research can be, new elements aren’t made official until the scientists hand off their data to a committee of the International Union of Pure and Applied Chemistry. * In the case of element 112, the claims of the Darmstadt team had to be weighed alongside claims from teams in Russia, Japan, and Israel—each with its own method of creating and detecting ununbium and its own software and hardware. (No team had been able to find more than a few atoms.)

To sort all this out, the international committee had to travel to each lab and pore over its data with Talmudic precision. Members reviewed the magnetic bits on computer drives that are the only traces left of ununbium. The committee also weighed each team’s reputation for doing good science. The Israeli team, for example, was notorious for making premature “discoveries” of heavy elements that fell apart under closer scrutiny.

Nuclear scientists have come to expect these long delays—they just move on to creating new elements—but the confirmation process for element 112 was especially grueling. The committee first considered ununbium in the late 1990s, but its members disagreed over what, if anything, could be gleaned from the computer data. Then things got even more complicated. In 1999, an investigation by the University of California at Berkeley concluded that one of its heavy-element scientists, Victor Ninov, had probably forged data. (Ninov denies this.) Berkeley had hired Ninov away from the Darmstadt lab, where he’d been a co-discoverer of element 112. In light of the new accusations, the German team scrambled to check its work. At a crucial moment, it was forced to retract its claim for one of the two ununbium atoms that it had supposedly created.

The committee finally ruled against all four claims for ununbium in 2001, saying in its final report that it “would not be much swayed by arguments that depend to a large extent on statistics of speculative interpretations.” The Germans redid their experiments in 2002, and found one more atom, but it took another seven years before the world would agree that the original evidence was acceptable. The delay can partly be explained by the fact that the committee was also working through competing claims for elements 113 through 116 as well as element 118.

It’s possible that we’re nearing the end of these debates, as scientists approach what may be the absolute limit of the periodic table. It took an amazing 10 billion billion (1019) collisions of calcium atoms on a californium target to produce one alleged atom of element 118. Getting past 118 may require orders of magnitude more precision and energy (and patience). Then again, you could argue that the table has already surpassed any reasonable, natural limit. The 1s and 0s by which we can identify ununbium are far more durable than its bundle of protons and neutrons in the real world. The discovery of new elements today comes down to philosophical judgments of what counts as “existence” and why. Obviously, whatever happened on the subatomic level 13 years ago in Germany happened regardless of how we humans interpret it. But no human could observe it directly. Science is a human venture, and our standards of what counts as proof—what counts as the creation of a new element—are debatable.

When the victorious German team gives element 112 its official name this fall (ununbium was just a placeholder), textbooks will be reprinted and schools worldwide will order new periodic tables for their walls. Many of these tables already use color-coded boxes to distinguish different types of matter: For example, yellow boxes for noble gases, blue for semiconductors, peach for alkali metals, and so on. The ultraheavy metals in the bottom row tend to be colored the same as solid, stable transition metals like iron or zinc. But it might help students even more if these boxes were rendered in a faint, almost translucent white—the perfect color for these ghosts of atoms.

Correction, June 15, 2009: The original version switched the order of “Pure” and “Applied” in the full name of IUPAC. (Return  to the corrected sentence.)