Electron config

[bennyp85]

Sl.No   Electron Shell   
Maximum Capacity
1   K Shell   2 electrons
2   L Shell   8 electrons
3   M shell   18 electrons
4   N shell   32 electrons

If we write calcium shown below

 Calcium    Ca    20    2,8,8,2

How do i write Coppers configuration ?

[DisaFear]

The 4s subshell is lower in energy than the 3d subshell (Take a look at this diagram)
Hence, the 4s subshell will fill before the 3d subshell

Converting to your terminology, where K=1, L=2, M=3, N=4
The 4s of the 'N' shell fills before the M



(Never seen this KLMN stuff before!)

[bennyp85]

Errr okay
Thanks for the help

[DisaFear]

Oh right, you were asking for Coppers configuration, using Calcium as an example
Copper is a weird one. Copper has 29 electrons right. You'd expect it to have 2 electrons in the 4s subshell and 9 in the 3d subshell

But having the d subshell either half-full (like Chromium) or full (in this case, Copper) is more stable, i.e, lower energy

Hence, we find that 1 electron from the 4s subshell goes to the 3d subshell, filling it up to 10. Hence, 3d10 4s1

Hopefully, im not making a complete fool of myself 

(Chromium has a 3d5 4s1, half-full)

[Blues Fan]

Cu: 1s^2, 2s^2, 2p^6, 3s^s, 3p^6, 4s^2, 3d^9
Rewritten as 1s^2, 2s^2, 2p^6, 3s^s, 3p^6, 3d^9, 4s^2 this is because of the number order.
Also, the 3d^9 is unstable and would rather be full, so then it would be:
1s^2, 2s^2, 2p^6, 3s^s, 3p^6,3d^10. 4s^1

I hope I'm right and that I helped.