Electronic Configuration of Actinoids

Unlike the lanthanoid series, where the electron-filling pattern across the 4f sub-shell is, with only the two well-documented exceptions at gadolinium and lutetium, fairly clean and predictable, the electronic configuration of the actinoid series is genuinely LESS well-defined and considerably more irregular element to element. The underlying reason is that, for the actinoids, the three relevant orbital sets -- 5f, 6d and 7s -- lie even MORE closely spaced in energy relative to one another than the corresponding 4f, 5d and 6s sets do for the lanthanoids, so which specific orbital an additional electron enters at each step becomes considerably more sensitive to small energetic differences, and correspondingly less uniform across the series.

Despite this greater irregularity, a general valence-shell electronic configuration can still be written for the 5f (actinoid) series as a whole: [Rn]5f⁰⁻¹⁴6d⁰⁻²7s² -- directly analogous in form to the lanthanoid series' general formula [Xe]4f¹⁻¹⁴5d⁰⁻¹6s² (section 4.5.2), but now allowing for a somewhat WIDER range of possible 6d occupation (0, 1 or 2 electrons, rather than just 0 or 1 as in the lanthanoid case), reflecting the closer 5f/6d/7s energy spacing just described. …