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So Cu is one of the d-block exceptions, where it would be more stable (symmetric distribution of electrons) to fill all of the 3d block (as 4s is technically higher in energy) first, before moving on to the next energy level.
For ions, it's easiest to first write out the regular electron configuration and then add or take away electrons depending on the charge. Since full and half filled d shells are more stable, those should be filled before the 4s. For Cu, the electron configuration is [Ar] 3d104s1, and when the charge is added to make it Cu+, you should remove the 4s1 which leaves you with Cu+ = [Ar] 3d10
The 3d subshell is high in energy compared to the 4s subshell when the 4s subshell is unfilled with electrons. Once the 4s subshell has been filled, however, the 4s subshell will become higher in energy, and the next subshell to be filled will be the 3d. Even though 3d subshell had higher energy initially which resulted in electrons filling the lower energy subshell 4s, after the 4s subshell has been filled, whenever the atom loses an electron, the electron will be removed from the 4s subshell as it is the one with the higher energy now and will be the valence shell. There is a sort of exception to the 3d configuration rules as well. Spherical symmetry dictates that 3d^5 and 3d^10 are lower in energy and therefore favorable conditions. In this case, the reason that we do not write [Ar]3d10 instead of [Ar]3d84s2 is because when the 4s subshell had been filled in the Cu atom, the valence shell was 4s because it had higher energy. Therefore, when we removed an electron, that electron was removed from the 4s subshell, and we were left with [Ar]3d10. [Ar]3d84s2 does not happen because there is no spherical symmetry to account for that could make it favorable to take electrons away from the 3d subshell to 4s subshell or vice versa. Had the 3d8 been 3d5 then taking an electron from the 4s subshell could have been favorable as 3d5 is lower in energy and is more stable. This is seen in the example of Cr atom with the electron configuration [Ar]3d54s1. In this case, spherical symmetry is established which makes it available for an electron from the 4s subshell to move to the 3d subshell.
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