Beryllium-beryllium bond revealed: scientists clarify confusion over puzzling atomic partnership.

Beryllium is one of those self-loathing elements. Like helium or neon, one atom of beryllium should not partner with another, basic chemistry rules say. But new research, published online May 21 in Science, definitively reports the nature of the rare beryllium-beryllium bond.

"People envisioned the beryllium atom as this sphere like a billiard ball that just bounces off another billiard ball," says quantum chemist Rodney Bartlett of the University of Florida in Gainesville. "They're happy with the electrons the way they are--there's no tendency to form a bond."

Element 4 on the periodic table, the strong, lightweight metal has two electrons in its outer shell, which could hold up to eight. So beryllium should happily team up with other elements, and it does. Beryllium is a component of emeralds and bonds with copper, but beryllium should be repulsed by another atom of its kind.

Yet since the 1930s, theorists and experimentalists have reported that sometimes beryllium may bond to beryllium. Studies have yielded wildly divergent results about the nature of the hookup. Different teams have calculated different bond lengths. And there has been debate over what forces made the bond possible: Some chemists said the [Be.sub.2] attraction is so weak that it shouldn't be called a bond at all.

"It is a very peculiar molecule," says physical chemist Michael Heaven of Emory University in Atlanta, who led the new work. Calculations to describe its electronic properties "seem like something you can do with a paper and pencil," he says. "But it turns out to be something where you need a supercomputer."

Heaven and his colleagues forced beryllium to bond with itself by blasting it with a stream of helium gas. Then the researchers zapped the [Be.sub.2] molecule, a dimer, with a laser that bumped an outer electron into a highly excited state. With a second laser they drove the electron back down to its original lower level. This descent released energy, producing a spectrum that revealed the forces acting between the atoms.

Scientists first caught beryllium in the act of bonding to itself in 1984. But the spectra were incomplete and the researchers had to extrapolate what was really going on between the two atoms.

If you are in a chicken coop and glimpse only the feet and tail of a bird, inferring that it is a chicken is a solid bet. But in beryllium's case, the equations missed the mark, incorrectly predicting the length of the bond at particular energies.

Based on the whole bird, the beryllium-beryllium bond is real, but delicate and not fully developed. The electrons of the two atoms swirl in a complex dance that minimizes repulsion. The new measurements for bond lengths at higher energies agree with theory. And for the bond to form, the new findings suggest, the electrons actually take advantage of the next orbital out, which is typically empty. For the electrons to keep their appropriate distances, this outer orbital hybridizes with the orbital that usually just holds beryllium's two outer electrons.