I’ve been looking at the periodic table and began to wonder - why is lutetium part of the lanthanide series? Or, for that matter, why is lawrencium part of the actinide series?

This isn’t readily apparent when looking at the traditional periodic table because the lanthanides and actinides are separate and below the main table. However, on the extended table, the two elements don’t appear that they should be in the groups that they are in: lutetium contains the $5d$ electron shell whereas the lanthanides do not, and lawrencium contains the $6d$ electron shell whereas the actinides do not.

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    $\begingroup$ Lawrencium actually has an electron configuration of $\mathrm{[Rn] (7s^2) (5f^{14}) (7p^1)}$. There are multiple anomalies in the electron configurations of heavy elements and it is not simple to classify them into groups by their electron configuration. In fact it is not always simple to put them into groups at all! $\endgroup$ – bon Sep 13 '15 at 18:34
  • $\begingroup$ @bon So the boundaries for the groups are actually fuzzy and not strict as taught? Or I could ignore them entirely? $\endgroup$ – Status Sep 13 '15 at 19:14
  • $\begingroup$ For heavy elements, and particularly transition/rare earth metals, there is much less similarity between the reactions of elements in the same column of the periodic table. It's not like the halogens, or alkali metals, which basically all have the same reactions. $\endgroup$ – bon Sep 13 '15 at 19:55
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    $\begingroup$ This video might interest you. $\endgroup$ – Martin - マーチン Sep 14 '15 at 17:52
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    $\begingroup$ It's not just heavy elements having "anomalous" electron configurations. This article suggests that we are getting to a point where the whole electron shell theory breaks down. The Periodic Table, like the atomic structure on which it's based, dissolves into a fog. $\endgroup$ – Oscar Lanzi Apr 29 '18 at 20:55

The assignment of elements to groups or series in the periodic table is not based exclusively on the electron configurations concerned. Helium for example is an s-block element yet is placed above Ne in the p-block due its noble gas properties.

Lutetium is part of the lanthanide series, notwithstanding its electron configuration, because its properties and those of its compounds are similar to those of the other lanthanide elements.

Lawrencium is assigned to the actinides for basically the same reason. Experiments have so far failed to find any chemistry-based evidence for Lr having a p electron. While it may have a p electron in the gas phase it probably has a d electron in the condensed phase.

Whether either Lu and Lr, or La and Ac are assigned to group 3 will not make any difference to the composition of the lanthanides or actinides, since both of these are horizontal series rather than vertical groups.

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    $\begingroup$ Why is Mars a "planet" while our Moon and Pluto are not? Mars has a certain combination of properties, internally and within its orbit, that our Moon and Pluto do not. Astronomers now label that combination of properties with the word "planet". Same thing here, basically, so +1. $\endgroup$ – Oscar Lanzi Apr 29 '18 at 10:08

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