Recommended Questions on the Road towards a Scientific Explanation of the Periodic System of Chemical Elements with the Help of the Concepts of Quantum Physics

Abstract

Periodic tables (PTs) are the 'ultimate paper tools' of general and inorganic chemistry. There are three fields of open questions concerning the relation between PTs and physics: (i) the relation between the chemical facts and the concept of a periodic system (PS) of chemical elements (CEs) as represented by PTs; (ii) the internal structure of the PS; (iii) The relation between the PS and atomistic quantum chemistry. The main open questions refer to (i). The fuzziness of the concepts of chemical properties and of chemical similarities of the CE and their compounds guarantees the autonomy of chemistry. We distinguish between CEs, Elemental Stuffs and Elemental Atoms. We comment on the basic properties of the basic elements. Concerning (ii), two sharp physical numbers (nuclear charge and number of valence electrons) and two coarse fuzzy ranges (ranges of energies and of spatial extensions of the atomic orbitals, AOs) characterize the atoms of the CEs and determine the two-dimensional structure of the PS. Concerning (iii), some relevant 'facts' about and from quantum chemistry are reviewed and compared with common 'textbook facts'. What counts in chemistry is the whole set of nondiffuse orbitals in low-energy average configurations of chemically bonded atoms. Decisive for the periodicity are the energy gaps between the core and valence shells. Diffuse Rydberg orbitals and minute spin-orbit splittings are important in spectroscopy and for philosophers, but less so in chemical science and for the PS.

Probing the (empirical) quantum structure embedded in the periodic table with an effective Bohr model

“…The two main equivalent modifications of the Bohr model usually considered to describe the energy of the polyelectronic atoms are: the effective charge (Z eff ) and the quantum defect (δ) [1][2][3]10 or effective quantum defect (u), discussed below. The correspondent energy for a neutral atom can be given by 2 , where n(Z) is the principal quantum number of the ionized sub-shell valence of the atomic ground-state, as presented on the PT. n(Z) is a discrete function of Z, which increases with atomic shell number.…”

“…It is possible to qualitatively (or semi-quantitatively) describe the (quasi-) PP of the array of elements, which depend intrinsically on the individual atoms, 2 with the hydrogen-like Bohr model with an extra parameter. (The effective quantum defect also depends on the orbitals, 2 consequently, a smooth function of Z, with only one parameter, can only describe an average behavior. This average behavior must agree qualitatively or semi-quantitatively with the experimental values.…”

“…Similarly to the IP property, the ratio of the atomic radius can be given by r(Z) = r H (n(Z) − u(Z)) 2 , where r H is the hydrogen's atomic radii for ground-state. If these variables are summed up, the adjusted atomic radiusr (Z) is…”

“…The study of the PP is an active area of research and source of Chemical and Quantum insights. [2][3][4][5][6][7][8][9] Notably, the PT could be added to the list of "fundamental experimental facts".…”

Icon of Chemistry: The Periodic System of Chemical Elements in the New Century

“…However, the difference between free atoms in a physical vacuum and chemically bound atoms in an environment of ligands is unexpectedly large (see Section 4.1). [11,12] …”