1. The problem statement, all variables and given/known data Which is the electron configuration of the Ni+ ion? 2. Relevant equations Ni : [Ar]4s23d8 3. The attempt at a solution Ni+: [Ar]4s13d8 Apparently this solution is wrong and should be Ni+: [Ar]3d9 but I do not know why. As far as I know 4s has less energy than 3d so, why does one electron promote from 4s to 3d after one ionization?