Cu = [Ar]3d104s1 this configuration does not follow the n+l rule. in other words if you use box notation to map out the electron configuration of Cu2+ you dont fill the 4s orbital first.
so,if:Cu=[Ar]3d104s1=29=atomic number=nu-mber of electrons
Then, Cu2+ indicates that 2 electrons have been given up by the copper atom and the electron configuration is therefore:
Cu2+= [Ar]3d9 =27= atomic number = number of electrons