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I was taught in school that the rare earth metals were the $\mathrm{f}$-orbital group. Additionally, the Aufbau principle states that the order of orbitals based on energy levels is $\mathrm{6s4f5d}$ and $\mathrm{7s5f6d}$ for the lanthanides and actinides, respectively. However, upon inspection, both periods have 15 elements whereas the $\mathrm{f}$-orbitals can only hold 14 electrons. I was able to resolve this by assigning one electron to the $\mathrm{5d/6d}$-orbital, which is correct according to chemistry libretexts. Why does this occur for the rare earth metals?

Furthermore, according to libretexts, only some of the rare earth metals have a $\mathrm{5d/6d}$-orbital, whereas others have the $\mathrm{6s/7s}$-orbital and proceed to only fill their outermost $\mathrm{f}$-orbital. Why is this the case, and is there a way to determine which elements are like this short of simple memorization?

Mathew Mahindaratne
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convertedquorauser
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  • There are 14 elements in the lathanide/actinide series, excluding Lanthanum and Actinium. Also, the aufbau principle pretty much breaks down after period 4, don't use it for anything other than main group elements. – Aniruddha Deb Sep 20 '20 at 02:16
  • https://chemistry.stackexchange.com/q/47542/16683 https://chemistry.stackexchange.com/questions/74880/going-from-la-to-ce-why-is-the-extra-electron-added-into-the-4f-orbital – Mithoron Sep 20 '20 at 16:38
  • Yep, all of these help out a lot. Thanks! – convertedquorauser Sep 21 '20 at 12:38

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