How do the electron configurations of transition metals differ from those of other elements?
Many transition metals will have electron configurations slightly different than those predicted by using the Aufbau principle, Pauli exclusion principle and Hund's rule.
For example, using these rules would produce an electron configuration for copper of [Ar] 4s2 3d9. The actual electron configuration for copper will move one electron from the 4s sublevel into the 3d sublevel resulting in [Ar] 4s1 3d10. The copper 1+ ion will have an electron configuration of [Ar] 4s0 3d10 and the copper 2+ ion will have an electron configuration of [Ar] 4s0 3d9. Many other transition elements will show movement of electrons from the highest energy s sublevel into the d sublevel one energy below.