  Abundant evidence has shown the emergence of exotic chemical phenomena under
pressure, including the formation of unexpected compounds and strange crystal
structures. In many cases, there is no convincing explanation for these
phenomena and there are virtually no chemical rules or models capable of
predicting or even rationalizing these phenomena. Here we calculate, as a
function of pressure, two central chemical properties of atoms,
electronegativity and chemical hardness, which can be seen as the first and
second-order chemical potentials. Mulliken electronegativity, which equals
minus the chemical potential of the electron relative to the vacuum, is
appropriately modified - instead of taking the vacuum (impossible under high
pressure), we take the homogeneous electron gas as reference. We find that for
most elements, chemical hardness and electronegativity decrease with pressure,
consistent with pressure-induced metallization. Furthermore, we discover that
pressure-induced s-d orbital transfer makes Ni, Pd and Pt "pseudo-noble-gas"
atoms with a closed d-shell configuration, and the elements preceding them (Fe
and especially Co, Rh, Ir) electron acceptors, while the elements right after
them (Cu, Ag, Zn, Cd, for example) become highly electropositive. We show the
explicative and predictive power of our electronegativity and chemical hardness
scales under pressure.
