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/
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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.
Because the orbitals involved can be in an inner electron shell and do not all correspond to the same electron shell or principal quantum number n in a given element, but they are all at similar distances from the nucleus.
https://en.wikipedia.org/wiki/Valence_electron#Valence_shell
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.
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.
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The 5s orbitals have lower energy than the 4d orbitals, despite being a higher quantum number. So for the other elements you listed, it is more energetically favorable to have electrons in the 5s. Palladium differ in that it has enough electrons to fill the 4d orbitals entirely. A filled 4d has lower energy than a 5s, so instead of 4d9,5s1 it becomes 4d10,5s0
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