# Why do the lanthanides and actinides have a 5d and 6d orbital, respectively? [duplicate]

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?

• 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 at 2:16
• – Mithoron Sep 20 at 16:38
• Yep, all of these help out a lot. Thanks! – convertedquorauser Sep 21 at 12:38