This is a useful feature: for example, an atom with a negative electronegativity would lose its electron in favor of the electron gas. Our electronegativities have the meaning of minus the chemical potential of the electron in the atom, relative to the electron gas at the same pressure. Different reservoirs of the electron could be used, but we chose the homogeneous electron gas, the idealized metallic system with an exactly known enthalpy as a function of pressure ( 29, 30). At nonzero pressure, there is no vacuum, and the electron will have to be in some reservoir in which it will have finite volume and the corresponding PV contribution. The standard definition of the ionization potential E i is AHe 107 → AHe 107 + + e, and the electron affinity E a is defined as minus the energy of the reaction AHe 107 + e → AHe 107 −-in both cases, “e” denotes an isolated electron in the vacuum. Mulliken’s definition is inapplicable at high pressure and must be appropriately modified, introducing a proper reservoir of electrons and using the relevant thermodynamic potential (enthalpy instead of energy). Here, we reassess electronegativities of all elements from H to Cm as a function of pressure, properly taking all pressure-related effects (including the PV term) into account and also calculating the chemical hardnesses of the elements under pressure, which was not done before. Recently, an attempt ( 23) was made to calculate atomic electronegativities as a function of pressure using the single atom Hamiltonian with the eXtreme Pressure Polarizable Continuum Model (XP-PCM), but electronegativities therein were based on the energies (rather than enthalpies) and did not contain the essential PV term(where P is the pressure and V is the volume). As for 3, electronegativity and chemical hardness can be expected to be highly nontrivial. For 2, atomic sizes (volumes) decrease and can be easily tabulated at any pressure. For 1, it is well known that, under pressure, the orbitals with higher angular momentum become favorable-hence, atoms typically undergo s- p and s- d transitions ( 1, 22). The most important properties are 1) the electronic configuration, 2) size, and 3) electronegativity and chemical hardness. To put these cases of dramatic changes of chemistry into a general and predictive system, the simplest approach is to determine how the essential chemical properties of the atoms change under pressure. Such compounds with stoichiometries that cannot be anticipated from atomic valences become ubiquitous under pressure and include the highest-temperature superconductors known to date, such as LaH 10 ( 12– 15), H 3S ( 16, 17), ThH 10 and ThH 9 ( 18, 19), and YH 6 ( 20, 21). Furthermore, under pressure, unexpected sodium chlorides, such as Na 3Cl and NaCl 3, become stable ( 11). Caesium displays multivalent states (such as Cs III and Cs V) in the predicted pressure-stabilized CsF n ( n > 1) compounds ( 9, 10). For example, pressure increases the reactivity of noble gases. Recent theoretical and experimental investigations have established that pressure greatly affects chemical properties of the elements ( 1). We show the explicative and predictive power of our electronegativity and chemical hardness scales. 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, and Ir) electron acceptors, while the elements right after them (Cu, Ag, Zn, and Cd) become highly electropositive. We find that for most elements, chemical hardness and electronegativity decrease with pressure, consistent with pressure-induced metallization. Mulliken electronegativity, which is the negative of the chemical potential of the electron in a given atom relative to the vacuum, is appropriately modified instead of taking the vacuum (impossible under high pressure), we take the homogeneous electron gas as reference. 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. 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. Abundant evidence has shown the emergence of exotic chemical phenomena under pressure, including the formation of unexpected compounds and strange crystal structures.
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