Atom Versus Superatom

The concept of superatoms—molecules or collections of atoms that behave like a single atom—has received a boost from a new photoelectron imaging spectroscopy study (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0911240107). A. Welford Castleman Jr., Samuel J. Peppernick, and K. D. Dasitha Gunaratne of Pennsylvania State University compared the electron angular distributions and electron-binding energies of several diatomic molecules and their isoelectronic atomic counterparts—titanium monoxide versus nickel, zirconium monoxide versus palladium, and tungsten carbide versus platinum. They found the electronic properties are almost identical in each case, suggesting that an atom’s corresponding superatoms can be predicted by simply looking at the periodic table and finding combinations that collectively have the same number of outer-shell electrons as the single atom, Castleman says. Although the study confirms the electronic similarity of some transition-metal atoms and their corresponding superatoms, the researchers are now working to determine if the similarities extend to the rest of the periodic table and to atom-superatom chemical reactivities. A periodic table of superatoms could have many uses, the researchers say. For example, tungsten carbide has been shown to exhibit reactivity similar to the much more expensive workhorse catalyst platinum.