Question

How many transition metals actually have used more than 8 valence electrons? Does `"Ag"` have 11 valence electrons or 1 or what?

Answer 1

In terms of actual valence electrons, there is literally only one example. You can consider that their oxidation states have never gotten higher than `+8`, except for Iridium:

https://en.wikipedia.org/wiki/List_of_oxidation_states_of_the_elements

In terms of electrons listed after the noble gas shorthand, note that that's not a good indication of how many valence electrons there would be in a particular transition metal.

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SILVER

No, silver does not have any more than `1` valence electron.

We only count valence electrons as those electrons that are important in chemical bonding (here, the one `5s` electron):

`[Kr] 4d^10 5s^1`

We never see silver with an oxidation state higher than `+1` (which would in principle require transferring one valence electron completely).

This is because the `4d` orbitals are lower in energy by about `"5.2 eV"``""^([1])` which is significant (about `"501.7 kJ/mol"`'s worth of energy, about five times the average strength of a chemical covalent bond!).

Hence, it only uses one valence electron in most if not all cases.

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IRIDIUM

Iridium is a strange example that has `bb9` valence electrons, and can use ALL `9` of them, such as in the iridium (IX) oxide cation,

`[stackrel(color(blue)(+9))"Ir"stackrel(" "color(blue)(-2))("O"_4)]^(+)`.

The atomic electron configuration of `"Ir"` is actually:

`[Xe] 6s^color(blue)(2) 4f^14 5d^color(blue)(7)`

Don't get fooled --- there are not `23` valence electrons here; its `4f` orbital energies are quite core-like.

However, its `6s` and `5d` energies are very close together (about `"1.39 eV"` apart, or only `"134.11 kJ/mol"`!)`""^([1])`, with the `6s` higher in energy, so the `6s` and `5d` orbitals hold the `9` valence electrons.

Using all `9` is a little hard, as it normally maxes out at `8`, but `9` is possible`""^([2])`.

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Endnotes

`""^([1])` http://media.pearsoncmg.com/bc/bc_0media_chem/adv_chem/pdf/11054_appB_ts.pdf (Appendix B.9)

`""^([2])` https://www.chemistryworld.com/news/iridium-forms-compound-in-9-oxidation-state-/7889.article