Quantum origin of atoms and molecules – role of electron dynamics and energy degeneracy in atomic reactivity and chemical bonding

Abstract A simple physical model of atomic structure and reactivity is extracted from quantum mechanics using widely known facts and relations but no expert knowledge of quantum chemistry. The Aufbau model is included and extended to account for the role of electron dynamics and energy degeneracy of the atomic ground states. The periodically variable reactivity is found to arise due to dynamical constraints on electron motion which lead to degeneracy (coincidence or near-coincidence) in the atomic energy spectrum. Verification is found in a strong correlation between energy degeneracy and reactivity with the inert gas atoms found to display least degeneracy. Chemical bonding follows as a means of relaxing atomic reactivity and reducing energy degeneracy. This can be achieved by permanent electron rearrangement among atomic centers (ionic bonding) or by oscillatory electron motion between atoms (covalent bonding). Ionic bonding is directly related to the variable stability of atomic structures, not only of neutral atoms but also of atomic ions formed by permanent electron redistribution. The most stable ions then form nondegenerate molecular structures by electrostatic interaction in neutral ionic clusters. The covalent bonding mechanism is instead fundamentally dynamical and interatomic since it depends directly on the facility (rate and spatial extent) with which electrons can move between atomic centers in a molecule. Energy degeneracy is found to imply dynamical constraint on electron motion which is reduced by covalent bonding, normally, but not always, to form a nondegenerate molecular ground state. Facilitation of interatomic electron motion, and the corresponding decrease in degeneracy of the ground and lowest energy excited states, thus become the covalent bonding mechanism. Equilibrium molecular geometries optimally balance this attraction against interatomic repulsions of electrostatic and steric (Pauli) nature which dominate at short range. The fundamental role of electron dynamics in atomic properties and bonding is reflected in a corresponding importance of not only ground state energy but the spectrum of low energy states.