[Schwarz107]

So, we get a new electron shell with each new row in the periodic table. So in column one we have these elements.

1st period. Hydrogen. 1 electron shell.
2nd period. Lithium. 2 electron shells.
3rd period. Sodium. 3 electron shells.
4th period. Potassium. 4 electron shells.
5th period. Rubidium. 5 electron shells.
6th period. Caesium. 6 electron shells.
7th period. Francium. 7 electron shells.

But then comes this.


Okay. We need a fifth shell in order to have a 5g subshell. No 5g subshell elements are known. But in the periodic table there are plenty of elements with fifth shells, sixth shells, there's even seventh shells. So what's up this?

[Schwarz107]

I might have worked it out. Seems it's about how electrons are stacked. Once we enter the fifth period, we get a 5s1 electron. Okay, good. But we're looking for a 5g1 electron.

But before getting to 5g, the electrons are stacked in:
6s, 6p, 6d, 7s, 7p, 8s.

So in the ground-state, we'd need an element in the eight period (!) in order to begin filling the 5g subshell.

[chenbeier]

You got it. Check Aufbau principle

[Schwarz107]

Thanks, chenbeier. The Aufbau principle looks crystal clear.



But as I looked into it, it became cloudy again. I tried to see if I'd understood it by doing a test on Silver. Silver has 47 electrons, and should fill the atomic orbitals of lowest energy levels first, before trying to occupy higher levels. So going by the graph on the left that would be:

1s 2 electrons
2s 2 electrons
2p 6 electrons
3s 2 electrons
3p 6 electrons
4s 2 electrons
3d 10 electrons
4p 6 electrons (bingo, we've got krypton!)
5s 2 electrons
4d we've got room for 10 electrons, but there's only 9 electrons left

So Silver's electron configuration according to the chart would then be [Kr] 5s2 4d9. But alas, that isn't what it really is.

[chenbeier]

It is more stable to have a full 4d¹⁰ shell as a 4⁹ 5s² configuration. So one electron from 5s orbital is transferred into the last d orbital. For example this you can see also at chromium of the half shell.