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.