On the Quantum Origin of the Periodic Table - The Role of Electron Dynamics and Energy Degeneracy in Atomic Reactivity and Chemical Bonding

Abstract: A perspective is provided which illustrates the central role of quantum mechanics in the structure and periodically varying properties of atoms and in the formation of molecules. Despite having only about one hundred basic building blocks for all substances, chemistry is a subtle discipline due to its fundamentally quantum mechanical origin. The atoms, and their behavior as systematized in the Periodic Table, intimately reflect mechanisms natural to quantum mechanics but foreign to classical mechanics. Given their very small mass, the electrons form atomic structures only predictable by the use of quantum mechanics. Thereby the foundation of chemistry is totally dominated by quantum effects which are generally known only empirically, e.g. as in the Periodic Table. Recently these quantum effects have increasingly become known computationally, but not necessarily understood physically and therefore still challenge students and users of chemistry. In order to provide physical insight we extract here, from known facts and relations of quantum mechanics, but without reliance on detailed mathematics or numerical analysis, a simplest, yet fully consistent, physical explanation of atomic structure and periodically variable reactivity. Atomic reactivity is found to arise due to dynamical constraints on electron motion which lead to degeneracy in the atomic energy spectrum. There is a strong correlation between energy degeneracy and reactivity with the inert gas atoms found to display no degeneracy. Chemical bonding follows as a means of atomic reactivity relaxation and reduction of energy degeneracy, achieved by permanent electron rearrangement (ionic bonding) or by oscillatory electron motion between atoms (covalent bonding). Ionic bonding is directly related to the variable stability of atomic structures which extends from neutral atoms to atomic ions. These stable ions then form molecular structures by electrostatic attraction in neutral ionic clusters. The covalent bonding mechanism is fundamentally dynamical and interatomic since it depends directly on the facility with which electrons can move between atomic centers in a molecule. Both the ionic and the covalent mechanism, interatomic displacement and oscillation of electrons, are active in any bond between atoms of different species.