[Schwarz107]

I browsed through the periodic table, looking for the first element with five shells, finding Rubidium. I was curious to know what the electron configuration was.

This is what it says for Rubidium.
Electron configuration   [Kr] 5s1
Electrons per shell 2, 8, 18, 8, 1

How come there's a link directly to Krypton in the electron configuration of Rubidium?

[Schwarz107]

Think I got it. It's shorthand for Rubidium's core electrons is equal to Krypton's electron configuration, and after that bracket we have Rubidium's valence electrons.

[Schwarz107]

Only half got it, I suppose, as with silver I find this:

[Kr] 4d10 5s1

So for silver, and all other atoms Wikipedia writes out any electrons that aren't part of a 'noble gas core' of a given element. So what we see after the noble gas bracket [Kr] isn't just the valence electrons, i.e. the outermost shell, but also the second outermost shell.

So to find the valence electrons, one would look at the highest principal quantum number for what's written out, and for silver that's "5", so in [Kr] 4d10 5s1 the valence electron is simply a single electron in a s-subshell which is found in the fifth and outermost shell.

[Enthalpy]

The subshells can have energies close to an other. For instance at carbon, 2s and 2p can both provide electrons to bonds, and accordingly, carbon can have the valences 2 and 4 usually.

p⁶ uses to be stable and not to participate in bonds. Note that it's not a full shell: it would have been [Ne]3s²3p⁶3d¹⁰ which doesn't even exist, while Ne [Ne]3s²3p⁶ is a noble gas. And I write "uses to" because heavy noble gases make stable compounds with F for instance.

The varied shells have subshells with energies interleaved, and for instance 4s fills at K and Ca before 3d at Sc. The "Aufbau" predicts the filling sequence with some success by ordering the subshells by energy, with exceptions like Cr and Cu. But as the energies are close, and the electrons interact, several subshells can participate in bonds, and they give an element many different valences.

Ag is rather simple, but the transition elements are a mess because of that.