Atoms in the anionic domain, Z < N

TL;DR

This paper investigates the stability and ionization properties of atoms in the anionic regime (Z < N) using a simple model to estimate the critical nuclear charge Z_c and ionization potentials, revealing that neutral atoms are closer to instability than to the large Z regime.

Contribution

It introduces a simple model to estimate the anionic instability threshold and ionization potentials, connecting these properties with the large Z behavior.

Findings

Estimated Z_c aligns with previous numerical results.

Ionization potentials are qualitatively correct and closer to the instability threshold.

A regularized series fits both large Z and Z→Z_c regimes.

Abstract

We study atoms with N electrons, and nuclear charge Z. It is well known that the cationic regime, Z > N is qualitatively described by Thomas-Fermi theory. The anionic regime, Z < N, on the other hand, is characterized by an instability threshold at Z_c <~ N-1, below which the atom spontaneously emits an electron. We compute the slope of the energy curve at Z=N-1 by means of a simple model that depends on the electron affinity and the covalent radius of the neutral atom with N-1 electrons. This slope is used in order to estimate Z_c, which is compared with previous numerical results. Extrapolation of the linear behaviour in the opposite direction, up to Z=N, allows us to estimate the ionization potential of the atom with N electrons. The fact that the obtained ionization potentials are qualitatively correct is an indication that, with regard to certain properties, neutral atoms are closer to the anionic instability threshold than they are to the Thomas-Fermi, large Z, regime. A regularized series is written for the ionization potential that fits both, the large Z and Z\to Z_c regimes.