Many of us learned in high school chemistry that the electrons around an atomic nucleus occupy different energy levels. The low-energy levels are known as the inner electron shells, and the highest-energy level forms the outer shell. Chemical bonds, we were told, form only when atoms share or exchange electrons in their outermost shells.

But a chemist may have found a loophole in that familiar rule of bonding. Under very high pressures, it appears, electrons in the atom's inner shells can also take part in chemical bonds.

“It breaks our doctrine that the inner-shell electrons never react, never enter the chemistry domain,” says Mao-sheng Miao, a chemist at the University of California, Santa Barbara, and the Beijing Computational Science Research Center in China. Miao's calculations show that under extreme pressures cesium and fluorine atoms can form exotic molecules with inner-shell bonds.

Ordinarily the atoms form relatively simple bonds. Cesium, an alkali metal, has a lone, so-called valence electron in its outer shell. The halogen gas fluorine, on the other hand, is one electron short of a full outer shell—a perfect match for an atom such as cesium that has an electron to give.

But Miao identified two molecules that, at high pressure, would involve cesium's inner electrons as well. To form cesium trifluoride (CsF3), a cesium atom would share its single valence electron and two inner-shell electrons with three fluorine atoms. Four inner electrons would go into making cesium pentafluoride (CsF5). “That forms a very beautiful molecule, like a starfish,” Miao says. He reported his findings in Nature Chemistry. (Scientific American is part of Nature Publishing Group.) Both the shape of the resulting molecules and the possibility of their formation are “very surprising,” says Nobel Prize–winning chemist Roald Hoffmann, a professor emeritus at Cornell University.