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Boring Bohrium Behaves as Expected
21 August 2000 7:00 pm
Washington, D.C.--Hard as it is to make new elements, it's a lot easier than figuring out how they behave chemically. Consider element 107, bohrium. It was first synthesized more than 20 years ago. At the American Chemical Society's summer meeting on 20 August, chemists finally reported its chemical properties. "Bohrium is boring," reports team member Andreas Tuerler of the Paul Scherrer Institute (PSI) in Villigen, Switzerland. But such straight-arrow comportment, he adds, is itself something of a surprise for such a massive element.
To predict the properties of unknown elements, chemists consult the periodic table, a chart that sorts elements into families according to the arrangement of electrons in their reactive outer shells. For the 115 or so known elements, the table works uncannily well. But sooner or later, physicists believe, it is bound to falter. The more massive an element is, they point out, the faster its electrons swarm it. Eventually, the electrons should start to show relativistic effects--changes of mass that will distort the shape of the swarms, giving ultraheavy elements properties that could not be predicted by looking at their lighter kin. The two previous elements, numbers 105 and 106, had shown signs of unruly behavior. Scientists were eager to see if bohrium would mark the breaking point.
To create a bohrium isotope stable enough to last long enough for an experiment, nuclear chemists Tuerler, Heinz Gaeggeler, and their colleagues at PSI smashed a beam of neon atoms into a berkelium target, creating a bohrium isotope, 267Bh, with a half-life of 17 seconds. From its electronic structure, nuclear chemists judged that 267Bh should behave similarly to other elements in group 7 of the periodic table, such as technetium and rhenium. To test that hypothesis, Tuerler's team swept the atoms directly from their production facility into a 1000°C flow chamber, where they met up with hot oxygen and hydrochloric acid (HCl), gases that react readily with technetium and rhenium. What was left then passed through a chromatography column cooled to a comparatively chilly 70° to 180°C. Although the experiment produced just six workable atoms of 267Bh in 6 months, they behaved exactly as expected: They combined with oxygen and HCl to make BhO3Cl, flew through the chemical separator, and left a fingerprint-like decay pattern on detectors.
"This is exceptional work," says Walter Loveland, a nuclear chemist at Oregon State University in Corvallis, calling the Swiss effort "a unique event and a serious advance in chemistry."