# Electron shells

repugno
Something is confusing me again. In one of my textbooks it clearly states that: generally there are only a certain number of electrons which are allowed in a shell.

1st:2 2nd:8 3rd:8 etc …

I believe this is known as the octet rule.

If I move on to my other textbook it states that the 3rd shell can now hold 18 electrons.
Are they trying to confuse me or is there a reasonable explanation to this?

Staff Emeritus
Gold Member
:) here is the explanation::

Subshell electron capacities:

Code:
s  2
p  6
d  10
f  14

s+p+d = 18

Staff Emeritus
Gold Member
Just remember the aufbau process, the way the electron shells are actually filled up:

Code:
1s
2s  2p
3s  3p  3d
4s  4p  4d  4f
5s  5p  5d  5f
6s  6p  6d
7s  7p
Draw a 45° angle from NE to SW, so that the shells are filled: 1s, 2s, 2p, 3s, 3p, 4s etc.

repugno
It makes a bit more sense now, so when 4s is filled, 3d starts filling. Another problem, why is it that Cu has only one electron in 4s but a full shell of 3d?

Staff Emeritus
Gold Member
Originally posted by repugno
It makes a bit more sense now, so when 4s is filled, 3d starts filling.
No, I don't think so. The second textbook example you gave just says that the first three subshells can hold 18 electrons. But when you look at the aufbau process that I posted, you will see that that an electron will go to 4s before it goes to 3d.

Another problem, why is it that Cu has only one electron in 4s but a full shell of 3d?
Like I just said , again look at the aufbau process, this is how the subshells are filled::

1s, 2s, 2p, 3s, 3p, 4s, 3d that helps?

repugno
Originally posted by Monique

Like I just said , again look at the aufbau process, this is how the subshells are filled::

1s, 2s, 2p, 3s, 3p, 4s, 3d that helps? [/B]

You have successfully made me understand the aufbau principle, thank you very much.
Now, when I look at my textbook it shows a table with the electron configurations of the first 36 elements in terms of the subshells. If I look at copper there is inconsistency.

Cu

1s22s22p63s23p64s13d10

If 4s fills before 3d, then why has 4s only got one electron given that, to fill an “s” subshell there must be two electrons in it?, and here the 3d subshell is completely filled.

Staff Emeritus
Gold Member
Very interesting and well noted!

There is actually something called the Hund's rule: half filled and filled sub-levels have unusual stability.

In fact, the 4s subshell is only slightly lower in energy than the 3d one. So the stabilization that it gets by either half filling or filling the higher subshell is greater than the energy necessary to take the electron out of 4s..

Here are a few other examples::

Code:
Cr = 1s2   2s2   2p6 3s2   3p6   4s1 3d5
Cu = 1s2   2s2   2p6 3s2   3p6   4s1 3d10
Ta = 1s2   2s2   2p6 3s2   3p6   4s2 3d10 4p6   5s1 4d4
Pd = 1s2   2s2   2p6 3s2   3p6   4s2 3d10 4p6   5s0 4d10

Last edited:
eagleone