A study suggests atoms can bond not only with electrons in their outer shells, but also via those in their supposedly sacrosanct inner shells
Inside atoms, electrons are organized into energy levels, called shells, which can be thought of as buckets of increasing size that can each hold only a fixed number of electrons. Atoms prefer to have filled buckets, so if their outer shell is missing just one or two electrons, they are eager borrow form another atom that might have one or two to spare. But sometimes, a new study suggests, atoms can be incited to share not just their outer valence electrons, but those from their full inner shells. “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 predicted such bonds using so-called first-principles calculations, which rely purely on the known laws of physics, and reported his findings in a paper published September 23, 2013, in Nature Chemistry. Such bonding has yet to be demonstrated in a lab. Nevertheless, “I’m very confident that this is real,” he says.
His calculations show that two possible molecules could form between cesium and fluorine atoms under extremely high pressure—about 30 gigapascals (higher than the pressure at the bottom of the ocean, but less than at Earth’s center). Cesium, all the way on the left side of the periodic table, has one superfluous electron in its outer, or sixth shell. Fluorine, on the other hand, is toward the far right of the table, just next to the column of noble gases with completely full shells (which is why noble gases are notoriously unreactive—they have little incentive to gain or lose electrons) and is one electron short of a full outer shell. “Under normal pressure, cesium gives an electron completely to fluorine and they bind together,” Miao says. “But under high pressure, the electrons from cesium’s inner shells start to form molecules with fluorine.”
Miao identified two compounds that could form and remain stable up to very high pressures: cesium trifluoride (CsF3), where cesium has shared its one valence electron and two from an inner shell with three fluorine atoms, and cesium pentafluoride (CsF5), where cesium shares its valence electron and four inner-shell electrons to five fluorine atoms. “That forms a very beautiful molecule, like a starfish,” Miao says. Both the shape of the resulting molecules and the possibility of their formation are “very surprising,” says chemist Roald Hoffmann, a professor emeritus at Cornell University, who was not involved in the calculations. “This is the first clear case of an alkali metal not only losing its single easily ionized valence electron in bonding, but also ‘breaking into the core’ in its bonding with several fluorines.”