A New Type of Chemical Bond Takes Hold
By Adrian Cho
ScienceNOW Daily News
23 April 2009
For decades, scientists have known of three ways for two atoms to bind and form a molecule. Now, researchers have discovered a fourth. Most likely, the advance won’t lead to new materials or technologies: The molecules last for about 1/100,000 of a second and can be made only at temperatures a few millionths of a degree above absolute zero. However, their mere existence confirms a surprising prediction and stretches the conceptual boundaries of chemistry.Binding two atoms into a molecule is like conceiving a child: There aren’t that many ways to do it. In the first two strategies, two atoms bind when their orbitals–the cloudlike distributions of electrons that hover above the atomic nucleus–overlap and merge so that the atoms share one or more electrons. If the atoms are of the same element, they will share an electron equally, producing a so-called covalent bond. If the atoms are of different elements–say sodium and chlorine–then one may hog the shared electron in what’s called an ionic bond. In a third type of bond, called a van der Waals bond, the atoms don’t actually share their electrons; instead, tiny fluctuations make one atom momentarily more positively charged on one side than on the other. This fleeting “polarization” induces similar fluctuations in the other atom, pulling the two atoms together.
Vera Bendkowsky, Tilman Pfau, and colleagues at the University of Stuttgart in Germany have demonstrated a fourth way to bind two atoms. The researchers started out with a gas of ultracold rubidium atoms. Using a carefully tuned laser, they then “excited” an electron in some of the atoms to a very high-energy orbital. That orbital is so large that the electron hovers as much as 100 nanometers from the nucleus, which is about 400 times the radius of a normal rubidium atom. If another, unexcited rubidium atom happens to be about that close, it can bind to the excited one, settling into the outer reaches of the electron cloud to make a gigantic two-atom molecule that’s bigger than some viruses.
Way out. This plot shows the probability for finding the electron at a different distance from the nucleus in a highly excited state of rubidium. In the molecule, the unexcited atom gets stuck in one of the outer rings.
CREDIT: VERA BENDKOWSKY/UNIVERSITY OF STUTTGART