Cs in high oxidation states and as a p-block element

The major chemical feature of an element is the number of electrons available for forming chemical bonds. A doctrine rooted in the atomic shell model states that the atoms will maintain a complete inner shell while interacting with other atoms. Therefore, group IA elements, for example, are invariably stable in the +1 charge state because the p electrons of their inner shells do not react with other chemical species. This general rule governs our understanding of the structures and reactions of matter and has never been challenged. In this work, we show for the first time that while mixing with fluorine under pressure, Cs atoms will share their 5p electrons and become oxidized to a higher charge state. The formal oxidation state can be as high as +5 within the pressure range of our study (<200 GPa) and stable Cs and Cs compounds can form at lower pressures. While sharing its 5p electrons, Cs behaves like a p-block element forming compounds with molecular, covalent, ionic and metallic features. Considering the pressure range required for the CsFn compounds, the inner shell electrons in other group IA and IIA elements may also bond with F or other chemical species under higher pressure.