Exceptions for Electron Configuartions

[Betty Wolkeba section 1L]

Can someone explain why specifically Chromium and Copper are exceptions? I know what their electron configurations are, but I was wondering why those two elements had different configurations?

[Liliana Rosales 1E]

Chromium and copper are exceptions because they are most stable when their d sub-levels are half or completely full rather than partially full.

[Garrett Dahn 1I]

The half-full stability creates a symmetry that is desirable for atoms such that having a half-filled orbital, like, say, Nitrogen, is more stable than having a lone valence pair. This means that the ionization energy (the energy required to remove an electron from the atom -- literally the energy to create an ion)is higher for atoms with half-full orbitals.
For example, between Carbon (C), Oxygen (O), Nitrogen (N), and Fluorine (F), the ionization energy, in order of greatest to least, is FNOC. In this case, Nitrogen has a greater ionization energy than Oxygen because its half-filled 2p shell is symmetric and semi-stable, so an electron is less likely to be removed. Thus, Nitrogen is a sort of exception to the Periodic Table trend where ionization energy increases as you move toward the upper right of the Periodic Table.
It is important to note that half-shell stability is not nearly as stable as full-shell stability (which is why noble gases are nonreactive), but half-shell stability is more stable than having a lone pair of electrons in the outer shell.
Hopefully this helps!

[Sophia Diaz - Dis 1B]

Follow up question: I don't understand where the electrons are coming from when you write the correct configurations.

For example: If Cu is [Ar]3d^9 4s^1, what was the original configuration? Is it [Ar]3d^8 4s^2 or [Ar]3d^10?

Also, if 3d orbitals are supposed to fill up before 4s orbitals, why would there be electrons in 4s if 3d wasn't already filled up all the way?

I'm really confused on how these exceptions work so any explanation would be appreciated. Thanks!

[Shimran Kumar 1C]

Okay so 4s fills before 3d because before the 3d subshell has any electrons shoved into it, 4s actually has a lower energy level than 3d. This is actually pretty cool because it allows us to simply read across the period when writing the electron configuration.

For example, you stated the noble gas core of Cu is [Ar]. So then we go to the next period to count for all the extra electrons not included in Argon. We count straight across until we reach Cu: 4s², 3d⁹. Since 4s has a higher energy than 3d, the original electron configuration is written [Ar] 3d⁹ 4s². However, Cu is an exception. Cu is one away from having its whole inner d subshell full and would rather have that, so it takes an electron from the outer 4s subshell. So we get the final configuration of [Ar] 3d¹⁰ 4s¹.

[Vienna Cimetta 1L]

Also it's important to note that this trend is the same for the next periods down in the d-block. So Ag and Au will do the same (nd10 ns1).