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

The only reason is that it is simply more energetically favourable.

If you know about orbitals, Hund’s rule provides a reason as to why Palladium has only 4 shells. In summary, the state with the 4th shell completely filled is of lower energy, and thus more stable, than that with the electrons on the 5th shell. You see this in copper too, when the electron configuration shifts from 2.8.16.2 to 2.8.18.1.

But Hund’s rule is merely a rule of observation, as far as I know we have molecules that violate Hund’s rule.