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Expanding the periodic table?

Researchers create compounds from small clusters of atoms acting like single atoms

By | January 14, 2005

Whole new classes of compounds can be created from small clusters of atoms acting like single atoms of another element, researchers demonstrate for the first time in work reported in this week's Science. Clusters of 13 aluminum atoms behave like halogens and can combine with them to form halogen compounds with novel properties, the researchers write. And clusters of 14 aluminum atoms act like single alkaline earth atoms capable of forming ionic compounds that are in effect new types of salt.

This provides a compelling argument for redefining the periodic table of elements, according to Shiv Khanna, professor of physics at Virginia Commonwealth University and co-leader of the study. "We call these superatoms because I would classify [the 13-atom aluminum clusters] in the same position as iodine," Khanna told The Scientist. "Since the iodine position is already taken, I add a third dimension where [a 13-atom aluminum cluster] occupies the same row and column as iodine but is above the plane of the periodic table."

There is disagreement within the clustering field over the significance of Khanna's work. "The idea that a molecular fragment has the same properties as an atom, ion, or molecule [of another element] is not new," said Andrew Barron, professor of materials science at Rice University, who said that the idea first emerged in 1976 in a paper on the isolobal principle, describing how clusters could resemble atoms or ions of other elements.

But the study brings together theory and experiment, according to Kit Bowen, professor of chemistry at Johns Hopkins University. "It's showing the chemical generality of these clusters that's important," Bowen told The Scientist. "The significance of the work is that [aluminum cluster] ions can react with lots of things to make molecules. It's one of the first steps toward using clusters to assemble materials."

Khanna was one of the first to identify the potential of such atomic clusters as building blocks for new materials in 1995, according to Bowen, and the idea of adding a third dimension to the periodic table started to gather steam around that time.

Khanna's team also showed that a 14-atom aluminum cluster could combine with iodine to yield iodide salts. But just as there are differences as well as similarities between elements within columns of the periodic table, so the 14-atom cluster is not exactly like any rare earth alkali, and therefore can be used to create compounds with novel properties not found before, Khanna said.

For example, when the 14-atom aluminum cluster combines with three iodine atoms, it creates an ion with overall negative charge which is actually extremely stable. Such novel properties provide the potential for creating radically new nanoscale materials, Khanna said. He noted that the work on clusters containing iodine could have important medical applications, given the element's key role in a number of biochemical pathways.

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