Electron Configuration: Understanding and Calculating

[Denver Dang]

A quick question...

When you have to decide the electron configuration for an atom you find the number of electrons in the shell as 2n^2 and the subshell by 2(2l+1).

If I'm not mistaken "the shell" is the total electrons in an atom, right ?
So for n = 1 it makes sense that there can be a total of 2 electrons. But for n = 2 it becomes 8, which makes sense because there is 8 atoms, leading to 8 electrons, in the 2nd row. But if you take for n = 3 you get 18. But in the 3rd row, there is only 8 new atoms as well. So I can only get 18 by adding the electrons from n = 1, n = 2 and n = 3. But if that is correct, why didn't I add n = 1 when I looked at n = 2 ? If you ask me it should have been 10 :/

So, if you know what I'm talking about, am I doing wrong here ? :)

I'm just confused :/

 

[BillX]

8 atoms leads to 8 electrons? typo or am I missing something? :)

I think shells in this case means the energy level (or orbits in a bohr-rutherford diagram) and the forumla, 2n^2, explains the total number of elections possible in the n shell.

But if I understand your question correctly, by the 3rd period (or 3rd energy level), there are three available sublevels s, p and d.
(Btw, if you didn't learn already, there could be 2 electrons for the s sublevel, 6 for the p sublevel and 10 for the d orbital.)
Since there could be a d sublevel in the third period (n=3), the maximum would be 2+6+10=18

 

[Borek]

Shell is not total electrons. Shell is usually all electrons sharing the same principal quantum number.

 

[Denver Dang]

But the atomic number Z is usually a number for how many electrons there is in an atom.
So Z = 1 = Hydrogen = 1 electron and so on...

So again, if you take for n = 1, you get a maximum of 2 electrons in the 1s level.
For n = 2 you get 2 electrons for the 2s level, and 6 for the 2p level.
For n = 3 you get 2 electrons for the 3s level, 6 for 3p level, and 10 for 3d level.

So everything fits so far. But I don't understand where the 3d level comes from ? When I look at my "Periodic Table" level 3 looks exactly like level 2 (With different atoms of course). So in my head there should only be 3s and 3p in level 3. I don't see where the 3d comes from :S

 

[Borek]

Trick is, while technically 3d could be in third period, orbitals are filled according to their energy. And energy of 3d orbital happens to be higher than 3s, 3p and 4s, so it accepts electrons AFTER 4s has been filled.

 

[Denver Dang]

Ahhh, I see now :)
So even though there aren't any electrons in 3d for atoms up to Z = 18, the 3d orbital is actually still there, but won't be filled until we reach Scandium at Z = 21 ?

 

[Borek]

Exactly. Just like every other orbital - say, 9s - "exists" in every atom.

 

[Denver Dang]

Ok...

Thank you very much :)

 

[alxm]

Well, to be pedantic, it's a bit trickier than that. You can always debate whether orbitals exist at all (in most meaningful senses, I'd say yes) There's certainly an orbital in the sense that there's an state with that set of quantum numbers.

But it gets pretty tricky with unoccupied orbitals, because they're basically incorrect. Since every electron interacts with every other one, once you move an electron to a higher orbital (or add an electron), then every orbital will change slightly due to this. (Except for hydrogen of course, since it only has one electron) So, it's not necessarily the case that the lowest unoccupied orbital in one atom will be the same orbital as the highest-occupied orbital after an electron is added.

The periodic table and electron configurations really just gets more and more complicated the closer you look :P

 

[Borek]

The periodic table and electron configurations really just gets more and more complicated the closer you look :P

Same can be said about almost everything in chemistry. When you look from the distance, there is a nice picture. When you get closer, it is a jigsaw puzzle of many wrongly cut pieces that don't fit.