By Alex Stone|Thursday, April 28, 2005

Aluminum superatoms (gold) also contain iodine atoms (purple).

Courtesy of N.O. Jones/ Virginia Commonwealth University

Time to revise the periodic table? Maybe we should, now that we have a new class of chemical building blocks called superatoms—atomic clusters that behave like individual atoms. Motivated by evidence that electrons in groups of aluminum atoms might form closed “shells,” physicists A. Welford Castleman Jr. at Pennsylvania State University and Shiv N. Khanna at Virginia Commonwealth University began searching for stable configurations of these atoms. “We thought if we could figure out some way of taking advantage of this, maybe we could make some new materials,” says Castleman. “Then we found a few magic numbers.”

To create the clusters, Castleman and his colleagues used a process called laser vaporization. A high-energy laser coaxed “seeds” from an aluminum rod into merging by trapping them in a pressurized stream of helium gas. The most interesting result so far is an assemblage of 13 aluminum atoms (plus an extra electron) that could be used to supercharge rocket fuel. Aluminum boosts a fuel’s thrust, but it also degrades quickly in the presence of oxygen, making it difficult to store. The atom cluster, on the other hand, is immune to oxidation. “We found that it did not react at all with oxygen,” says Castleman.

Another cluster, a group of 14 aluminum atoms, might lead to lighter, more efficient conducting materials for building better electronics and optical devices. But the possibilities don’t end there; with continued research, the team hopes to add more members to the superatom family, Castleman says. “We might be able to open up a new kind of chemistry where elements could be simulated with metals—in this case aluminum—of different numbers of atoms.”

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