The f-block valence electrons understandably include the outermost two from 6s. But why do the f-block elements have an oxidation number of +3, pulling an electron from an inner shell, not even a d-block electron?


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    $\begingroup$ It called f-block nowadays not "inner transition". Also they tend to have +3 cations so what is it with "1 or 2"? $\endgroup$ – Mithoron Feb 17 at 23:15
  • $\begingroup$ I apologize. I have since researched this question a bit and found out that the oxidation state is indeed 3+ because of the two 6s electrons, which are easily removed and a 4f electron, which supposedly is easier to remove than a 5d. But I'm still questioning why the 4f electron is more easily removed than the 5d? Judging from the nonresponse here, this is a very obvious or unknown answer. Anyone? $\endgroup$ – suse Feb 19 at 0:57
  • $\begingroup$ Would be good if you edited you question then, to make it mare clear and accurate; also it bumps to homepage. $\endgroup$ – Mithoron Feb 19 at 1:02
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    $\begingroup$ Related: Why don't we see these lanthanide species? $\endgroup$ – orthocresol Feb 22 at 1:08

First of all, lets define which elements we're discussing as f-block. Generally the lanthanides are treated as f-block elements, though this introduces some ambiguity as La (which starts the lanthanides) has 1 d electron and no f electrons while Lu (which ends the series in some definitions) has 1 d-electron and all 14 4f electrons. So one of these two is a d-block element but its not entirely clear which one. Recall that in the f-block there can only be 14 elements due to the 14 electrons that fill f orbitals (just as the d-block can only have 10 elements).

Not all of the lanthanides have 5d electrons, only La, Ce, Gd and Lu do! The rest fill the 6s and 4f only. The 6s are heavily shielded and easily lost. Only one 4f electron is lost because the 4f closer to the nucleus and more effected by losing electrons, ie once one is removed its difficult to remove another because the rest are far more attracted to the nucleus.

So the real question should be why are La, Ce and Gd exceptions to the filling order? For La and Ce this is because they have a lower Z effective, they're not splitting the energy of the 4f and 5d as effectively. For these two the 5d is lower energy and fills first, then the 4f fills. For the later Ln with a higher Z effective the 4f (being in a lower shell) is pulled to significantly lower energies and fills first.

The reasons for Gd having a different filling order is slightly more complex but can be related to whats seen in the transition metals, recall that Rh and Ru don't actually fill the 5s despite not being at a half-shell closure, they fill one 5s and the rest in 4d. Lu is not an exception.

  • $\begingroup$ How are there 14 lanthanides if you include lanthanum? The lanthanides are supposed to end with lutetium so starting with lanthanum gives 15. Then do you mean 3 out of 15 or 2 out of 14 manganese having d electrons? $\endgroup$ – Oscar Lanzi Feb 21 at 2:20
  • $\begingroup$ @Oscar Lanzi The number of lanthanides depends on the classification system, but by an orbital perspective (which is what we're discussing here) there can only be 14 elements in the f block (like there is only 10 d block elements) because there are only 14 electrons in the 4f. Both La and Lu have 1 5d electron, but Lu has the 4f completely filled and La has the 4f completely empty, so which is truly a 4f elements is depends on the perspective. La is an exception to the filling order which introduces this ambiguity which is not present in d-block elements. $\endgroup$ – Mecury-197 Feb 21 at 18:04
  • $\begingroup$ Then not 14 lantanides, but 14 f block elements in Period 6. $\endgroup$ – Oscar Lanzi Feb 21 at 23:10
  • $\begingroup$ @Oscar Lanzi As a whole people often conflate "lathanides" with "f-block" though the two are not really the same. Some people count La and Lu (making 15 lathanides), but some systems don't count Lu (making 14), though it seems more common now to count all 15. It's really a matter a semantics only $\endgroup$ – Mecury-197 Feb 21 at 23:25

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