It seems to me that *f*-orbitals for lanthanide metals are treated as 'core-like' when a certain number of electrons have been removed. 

Or, as [Radiochemistry](http://www.radiochemistry.org/periodictable/la_series/L5.html) puts it,

> The *4f* binding energy is so great that the **remaining** *4f*-electrons are regarded as "core-like". [Italic and emphasis added]

How is this justified exactly?

**Effort**

I am aware that

 - each electron will be harder to remove as Culonic attraction increases with positive ionic charge,

and probably

 - lathanide contraction is a part of the explanation.

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You have to check out some of these other threads:

 - [Why don't we see these lanthanide species?](https://chemistry.stackexchange.com/questions/53712/why-dont-we-see-these-lanthanide-species)
 - [What is meant by 'electrons of like/unlike rotation'?](https://chemistry.stackexchange.com/questions/53714/what-is-meant-by-electrons-of-like-unlike-rotation)
 - [What's up with this quarter / three-quarter rule?](https://chemistry.stackexchange.com/questions/53715/whats-up-with-this-quarter-three-quarter-rule)

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Original topic: [Predominance of III oxidation state for lanthanides](https://chemistry.stackexchange.com/questions/53605/predominance-of-iii-oxidation-state-for-lanthanides)