The change of basic chemical behavior of elements under high pressure

The chemistry at ambient condition has implicit boundaries rooted in the atomic shell structure: The inner-shell electrons and the unoccupied outer-shell orbitals do not involve as major component in chemical reactions and in chemical bonds. The chemical properties of atoms are determined by the electrons in the outermost shell; hence, these electrons are called valence electrons. These general rules govern our understanding of chemical structures and reactions.
Using first principles calculations, we demonstrate that under high pressure, the above doctrines can be broken. Our previous studies showed that both the inner shell electrons and the outer shell empty orbitals of Cs and other elements can involve in chemical reactions. The formation of CsF_n (n > 1) compounds demonstrates that Cs atom can behave like a p-block element under high pressure.[1] Besides, we found that Hg in Hg-F compounds transfers charge from the d orbitals to F atoms, thus behaving as a transition metal. We have extended the above study and found that the involvement of core electrons in forming chemical bonds is a common phenomena while the external pressure is high enough.
[1] Nat. Chem. 2013, 5, 846.