How is Palladium able to cram all its electrons into it’s first 4 valence shells, but the elements both above AND below it are forced to use their 5th shell?

Question

I understand the basics of valence shells but not enough to figure out why palladium is unique. I expected the elements immediately left of it (rhodium, ruthenium, technetium) to have fewer electrons in their 4th shell–and they do–but they also always have at least one electron in the 5th shell.

Also here’s some neat diagrams of this if anyone’s interested: https://valenceelectrons.com/category/electron-configuration/page/8/

In: 12

Answer 1

Transition metals have overlapping energy states between orbital levels, in the case of going from Rhodium to Palladium it takes less energy to keep an unpaired electron in the S5 orbital in Rhodium. If you add one more you have a pair, which is more stable and can they settle down into the D4 orbital. in the other sections of the table the jump between levels is much higher energy than the jump between orbital types so they fill more regularly. S->P->D->F, then back to S in the next level up.

Side note: when talking about the periodic table, if you say elements below, it means down the columns of the table. Platinum and Darmsadtium are below Palladium, Rhodium is before it.