The Beast From 20,000 Geeks

Ongoing developments may force a revision of the venerable periodic table of the elements. Designed by Dmitri Mendeleev, it organizes chemical elements in groups based on their characteristics.

But now it may prove inadequate, New Scientist magazine tells us.

According to Mendeleev's roll call, an element's chemistry can be deduced from where it sits in the periodic table. Reactive metals like sodium and calcium occupy the two columns on the left. The inert "noble" gases make up the column on the far right, flanked by typical non-metals such as chlorine and sulphur.

Now this neat picture is being disrupted by superatoms - clusters of atoms of a particular chemical element that can take on the properties of entirely different elements. The chemical behaviour can be altered, sometimes drastically, by the addition of just one extra atom. "We can take one element and have it mimic several different elements in the periodic table," says Welford Castleman, an inorganic chemist at Pennsylvania State University who has studied the chemistry of aluminium superatoms.

Explained as briefly as I can, the electrons in atoms arrange themselves in "shells" around the nucleus. Each such shell has a certain maximum number of electrons it can hold; additional shells get occupied as lower shells get filled. The chemical reactivity of an element - how readily it reacts with others - is dependent on its ability to either take up, release, or "share" electrons. Those elements which have just the right number of electrons to exactly fill its outermost shell are the least likely to interact; those with just one electron too many or too few to do so are the most reactive.

What chemical researchers have been finding is that for at least some elements, clusters of atoms of certain numbers act like one giant atom - a superatom - that can react like a different element. For example, a cluster of 13 aluminum atoms, or Al13, plus one extra electron has the right number of elections to form a superatom with "closed," that is, filled, shells - with the result that it acts more like a noble gas than aluminum. What's more, without the extra electron, Al13, a metal, acts like bromine, a non-metal.

But curiosity aside, what's the point? What can be gained from making a compound with a superatom mimicking an element like bromine, rather than with bromine itself?

One answer is that superatoms could provide entirely new types of material, including "expanded" crystals. In a solid such as sodium chloride, the atoms are stacked together like oranges in a market display. In an expanded crystal, the atoms would be replaced by a stack of giant superatoms.

Such crystals could bring the vision of room-temperature superconductivity closer to realization. And some superatoms offer the promise of more powerful solid-fuel propellants. That's not an altogether good thing because the primary use for such propellants is in missiles, but they do also power things like the space shuttle and other vehicles for space exploration.

Some researchers even look forward to the possibility of creating new materials with tailor-made properties. Transparent aluminum, anyone?