There are two main exceptions to electron configuration: chromium and copper. In these cases, a completely full or half full d sub-level is more stable than a partially filled d sub-level, so an electron from the 4s orbital is excited and rises to a 3d orbital.
Okay alright so let's talk about the exceptions you're going to see when you're dealing with electron configuration there's going to be a few around but we're going to talk about the main ones you're probably going to see in class. Alright so let's talk about chromium. Chromium is a transition metal and it has 24 electrons and here is the orbital diagram. If we're going to make this short hand and make the electron configuration for this we would make this 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d4 okay from now on every time you see 3d4 you're going to change it, we do not like 3d4. The d sublevel is more stable when its either half full all the orbitals are filled with at least 1 electron or completely filled. Right now we're 1 electron short of it being halfway filled. So what we're going to do, we're going to, that should be a 2 there, sorry, and we're going to take one of the electrons in the 4s orbital and move it over to the 3d orbital. So instead we're going to write actually we're going to make this argon as we noted before in the noble gas configuration and we're going to make this 4s1, 3d5 this is halfway filled which is pretty stable and this is much more stable being half way filled rather than being the d4. You might see this is the diagram or the electron configuration that you're going to see and this is actually higher in energy than this. So this is talking about the order of energy you might see it also looking like this, this is just doing it in order of number 3 comes before 4 but they're exactly the same thing, they're depicting the exact same thing, nothing different about them. So you might see either one but they're the same.
Down here the other exception that you're going to see is copper, or anything along copper which we'll talk about. So let's make the electron configuration for that 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d9 from here on out anytime you see d9 we're going to change it. Okay so anytime that it ends in d9 we're going to fix it up a little bit, same exact way we're going to take an electron from the 4s and we're going to move it over to the 3d. That way this d orbital or d sublevel be completely filled which is very stable versus the s orbital will be halfway filled. So, again, I'm going to make this argon just make it for myself 4s1, 3d10 and again you might see it as argon 3d10, 4s1 same exact thing. So those are the exceptions the main exceptions that you'll see when dealing with lifetime configuration.