this computer simulation shows clouds of electrons (blue) forming bonds between two buckyballs (red). each buckyball acts like a single atom in the resulting hydrogen-like molecule.
than concentrated at the ions. Nuclear
physicists use a similar model for atomic
nuclei; they call it the “jellium” model.
Jellium gave the right answer. The
shared electrons orbiting the cluster do
so in different energy levels, or shells,
just as they would in an atom, Cohen figured. Computer calculations confirmed
his guess. Like ordinary atoms, clusters
with unfilled electron shells are chemically reactive. Full shells, with “magic
numbers” of electrons, are not. Sodium
clusters with eight, 20 or 40 atoms are
the analog of helium, neon, and the other
noble gases, which rarely form molecules.
Clusters with non-magic numbers of
atoms tend to lose or gain electrons, making them more likely to also lose or gain
atoms (to get a magic number) through
collisions with other clusters.
A year later, Exxon Corporate Research
Lab chemist Robert Whetten, now at Georgia Tech, and his collaborators noticed that
clusters of six aluminum atoms could split
hydrogen molecules at room temperature,
something smaller clusters couldn’t do.
“Only aluminum- 6 jumped up and shouted
‘Here I am, I can do this!’ ” says Whetten.
And in the late 1980s, Welford Castleman
of Pennsylvania State University in University Park and his colleagues discovered that
clusters of 13, 23 or 37 aluminum atoms,
plus an extra electron, become chemically
inert, even though pure aluminum usually
reacts violently with oxygen.
The researchers realized that Cohen
and Knight’s magic numbers could explain
the perplexing phenomenon. In an alu-
minum cluster, each atom donates three
electrons to the cause. The 13-atom cluster,
or Al , for example, ended up with 39 com-
mon electrons ( 3 x 13), and the extra elec-
tron in the ion Al - was just what the cluster
needed to reach the magic number 40.
Researchers say that chemically stable
forms of aluminum, which can be destabilized and burned when needed, could
someday yield a powerful yet safe-to-handle additive for rocket fuel.
But the team went further. It showed
that the neutral clusters Al , Al and Al
13 23 37
get into similar chemical reactions as do
elements that crave one extra electron.
Those are the elements such as chlorine
or fluorine, which in the periodic table are
the halogens, the column directly to the
left of the noble gases.
Then in 1995, Shiv Khanna and Puru-sottam Jena of Virginia Commonwealth
University in Richmond found a theoretical explanation for Castleman’s discovery.
While Cohen’s calculation could predict
which clusters would be stable, understanding chlorinelike behavior required
calculating the energetics of adding or
removing an electron from the cluster,
which is what Khanna and Jena did. They
proposed the term “super atom” (two
words, originally) for such clusters.
Jena and Khanna then predicted that
Castleman’s aluminum clusters should
form tightly bound ion pairs with elements
such as sodium or potassium, which like to
donate one electron. Ionic bonds are what
occur in sodium chloride (Na+Cl-), also
known as table salt. Aluminum clusters,
Jena and Khanna proposed, could become
part of the first all-metal salts, clusters that
include superatoms of one metal (which
act like halogen atoms) and atoms of the
same or a different metal.
Two years ago, a team led by Bowen
indeed produced K+Al -
potassium-alu-minum) molecules and showed that their
chemical properties resemble those of
molecules like K+Cl- or Na+Cl-.
In the last few years, researchers have
also begun looking for ways to use superatoms to store hydrogen in solid form.
The difficulty of transporting and storing hydrogen at room temperature poses
a formidable obstacle to the much-touted
Several teams are now trying to create
superatom-based salt crystals — something that’s proving trickier than expected,
since once the molecules start aggregating, the superatoms tend to merge with
each other, forming clumps more than
crystals. “When you put them together,
they slag themselves,” Bowen says. One
approach is to coat superatoms with other
kinds of stuff, as Kornberg did. On the
other hand, Castleman hopes that replacing potassium ions with larger molecules
might prevent superatoms from coalescing. “You have a chance of keeping them
away from each other,” he says.
The interest in making crystals out of
superatoms goes beyond pure curiosity.
By adjusting the types, shapes and sizes
of a material’s ingredients, scientists and
engineers could tune physical properties
to their likes. “You would have a way of
making materials with tailored properties,” Bowen says.
For example, a material that can be
transparent typically won’t conduct electricity, and vice versa. But a suitable all-metal salt, say, might be able to do both.
And with a stretch of imagination, all-alu-minum salts could make airplanes with
see-through fuselages possible. Almost as
cool as a hot-fudge sundae. s
s min Feng, Jin Zhao, Hrvoje petek. “atom-
like, Hollow-Core–bound molecular Orbit-
als of C .” Science, 18 april 2008.