# Why is Thorium (At. no. 90) an f-block element at all?

Being an exception of the Aufbau principle, Thorium has an electronic configuration of $$\ce{[Rn]}\mathrm{7s^25f^06d^2}$$ instead of the expected $$\ce{[Rn]}\mathrm{7s^2 5f^2 6d^0}$$.

Two other elements, Lanthanum and Actinium, also showed such an anomaly as $$\mathrm{6s^2 4f^0 5d^1}$$ and $$\mathrm{7s^2 5f^0 6d^1}$$, respectively. But we placed them both in the d-block of the periodic table.

Since, even in Thorium, the last electrons occupied d-orbitals only (disobeying Aufbau), then why is it placed in the f-block of the periodic table. I mean, it should be placed in the d-block, shouldn't it?

Edit: Note that here, I don't intend to ask or know how or why the 2 electrons disobeyed the Aufbau, and occupied the higher energy 5d instead of lower energy 4f.

Rather, I want to know only that why it is kept in the f-block when the last orbitals which the electrons occupied were of d-subshell.

• I don't think it violates the Aufbau principle, but instead it fails the generalised prediction patterns that have been derived from observation, like $n + \ell$ 'rule'. About the placement in d-/f-block, I don't think there is consensus on that either. There recently have been different news about how the elements should be displayed, even turning the PT upside down doi.org/10.1038/s41557-019-0253-6 – Martin - マーチン Sep 3 '19 at 13:24
• …even turning the PT upside down doi.org/10.1038/s41557-019-0253-6”: this would be a decent article for a design journal, but how on Earth did this pave its way to Nature (judging from the name of the first author I think I know the answer, so this is more like a rhetorical question)? – andselisk Sep 3 '19 at 13:37
• Related (or probably duplicates): Anomalous Electronic Configuration of Thorium; Outermost electronic configuration of f block elements. – andselisk Sep 3 '19 at 13:42
• As one gets more and more electrons, their interactions really start messing with simplistic expectations (e.g. Aufbau) based on hydrogen-like energy levels. But, really, to what purpose does one call something an 'f-block' vs a 'd-block' element - you look at the actual filled levels and nearby levels and try to figure out the chemistry from there. For crystal solids, Thorium is just at the edge of some really weird crystal structures popping up because of all those electrons (like Pu with the first 3 allotropes being specific to it). – Jon Custer Sep 3 '19 at 13:54
• @andselisk I edited my question. The first link you related my question to, is demanding the reason why/how it happened, which I'm not. So not a duplicate or related. The second link, which was also my doubt but I later accepted it as one of the abundant errors of chemistry and thus decided not to include in my doubt, is also neither a duplicate of nor related to my question. – user82401 Sep 4 '19 at 11:00