How come the order of filling the atomic shells is so strange?

How come the order of filling the atomic shells is so strange? For example, why does it jump from Ar (1s^2 2s^2 2p^6 3s^2 3p^6) to 1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 for K? Why doesn't it first fill the 3d orbital? Also, from V (1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^3) I notice that it jumps to (1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^5). Why does one of the electrons just leave the 4s orbital and jump into the 3d orbital. Is it more stable that way or something? I don't see how it could be though... I mean, isn't a full orbital (i.e. 4s^2) more stable than a half-full one (i.e. 4s^1)?



Thanks for any help!!!!!!!

How come the order of filling the atomic shells is so strange?
As you noted, it is a matter of stability. Electrons will create the most stable arrangement possible.



At the "d' orbital, there are pseudostability points. For example, having 5 electrons at the "d" level is more stable than having 6 or 4. So an atom may find it more stable to have one less electron in the 4S orbital in order to retain 5 in the "d'.