# Two different electronic configurations for cerium

Depending on the textbook there are two different electronic configurations stated for cerium. On the one hand

$$[\ce{Xe}]\mathrm{4f^15d^16s^2}\quad$$

and on the other hand

$$[\ce{Xe}]\mathrm{4f^26s^2}$$

Are both configurations possible? If not, why so?

• There is no need to waste your time in memorizing the differences. Yes, you will hear and read "stories" of this electron configuration and that electron configurations without saying why so. I have yet to find a spectroscopist (asked so many leading ones) who can tell me how these electron configurations were obtained from either experiment or by theory. I bet your teacher will have no clue either. So the question you should ask is how would we experimentally distinguish your first configuration from the other. Feb 5 '20 at 0:29

The filling of electrons follows Aufbau principle. However, introduction of $$\mathrm{d}$$ and $$\mathrm{f}$$ orbitals and resulting poor shielding affects predicted energy order, and it consists last 2 or three shells incompletely filled. The energy associated with this subshells is nearly same, and one of the subshell of outer shell is bigger in size than subshell of inner shell. So, to perform minimum repulsion between electrons, one or two electrons jump to a bigger subshell (with similar energy) so this exceptions occur.
In case of cerium, energy of $$\mathrm{4f}$$ and $$\mathrm{5d}$$ subshells are quite similar and $$\mathrm{5d}$$ subshell is bigger than $$\mathrm{4f}$$ subshell. So 1 electron in $$\mathrm{4f}$$ jump to $$\mathrm{5d}$$ subshell.