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

Most of these observations are done in gas phase. In solid state, inorganic compounds and organometallic complexes they don’t behave this way. What I mean is that shells are less relevant and subshells become more important. In this sense, none of the elements you mention have electrons in the 5th shell.