  We study the stability and universal behaviour of the ionization energy of
N-electron atoms with nuclear charge Z in the anionic domain (Z<N), considering
the nuclear charge Z as an arbitrary (non-integral) parameter. HF and CISD
ground state energy calculations were performed for systems with N and N-1
electrons to compute the ionization energies for nuclear charges ranging from
the neutral atom region to the anionic instability threshold. As testing
systems we choose inert gases (He-like, Ne-like and Ar-like isoelectronic
sequences) and alkali metals (Li-like, Na-like, K-like sequences). From the
results, it is apparent that, for inert gases case, the stability relation with
N is completely inverted in the singly-charged anion region (Z=N-1) with
respect to the neutral atom region (Z=N), i.e. larger systems are more stable
than the smaller ones. We devised a semi-analytical model (inspired by the
zero-range forces theory) which lead us to establish the ionization energy
dependence on the nuclear charge near the threshold. This dependence is well
observed in our numerical computations. Finally, we were able to describe
qualitatively two universality classes for systems similar to inert gases and
alkali metals, respectively.
