  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 Cs2+ and Cs3+ 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.
