Chemistry electron configuration help

Why is the electron configuration of Cu+ [Ar] 3d^10 and not [Ar] 3d^9 4s^1?

First and foremost, you have to know that the electron configuration of copper (when it has NOT lost any electrons) is [Ar] 3d^10 4s^1 (because the 3d subshell is most stable when it is full, so copper moves one of its 4s electrons to the 3d subshell to complete it)

The 4s orbital loses always its electron(s) before any of the 3d electrons are removed. This is just a trend that is observed to be the case for all of the first row transition metals.

Oh yes thank you I was just confused because I thought 4s sub-shell loses all of its electrons first and I thought it was 4s^2. Do I just need to remember that the 4s electron will move to the 3d sub-shell? Is copper the only one I need to know that's like this?

You need to remember that copper and chromium give up one of their 4s electrons in order to fill out the 3d subshells (fully in the case of Cu and half way in the case of Cr).

So the simplified electron configurations of Cu and Cr are:

[Ar] 4s^1 3d^10 (Cu)

[Ar] 4s^1 3d^5 (Cr)