The relationship between the electron configurations of transition-metal elements and their ions is complex. The electron configuration of a neutral cobalt atom is written as follows. The discussion of the relative energies of the atomic orbitals suggests that the 4 s orbital has a lower energy than the 3 d orbitals. Thus, we might expect cobalt to lose electrons from the higher energy 3 d orbitals, but this is not what is observed. Because the valence electrons in transition-metal ions are concentrated in d orbitals, these ions are often described as having d n configurations.
Most transition metals form more than one oxidation state.
Some oxidation states, however, are more common than others. The most common oxidation states of the first series of transition metals are given in the table below. Efforts to explain the apparent pattern in this table ultimately fail for a combination of reasons. Some of these oxidation states are common because they are relatively stable. Others describe compounds that are not necessarily stable but which react slowly.