Ions of Transition Metals
The transition elements are metals that form cations when they react. However, the situation is a bit more complex. Many transition elements are able to form more than one cation because they have a partially filled d subshell that is just slightly lower in energy than the outer s subshell. When a transition metal forms a positive ion, it always loses electrons from its outer s subshell first. Once these are gone, any further electron loss takes place from the partially filled d subshell. Iron is a typical example. Its electron configuration is
Fe [Ar]3d64s2
When iron reacts, it loses its 4s electrons fairly easily to give Fe2+. But because the 3d subshell is close in energy to the 4s, it is not very difficult to remove still another electron to give Fe3+.
Fe3+ [Ar]3d5
Because so many transition elements are able to form ions in a similar way, the ability to form more than one positive ion is usually cited as one of the characteristic properties of the transition elements. Frequently, one of the ions formed has a 2+ charge, which arises from the loss of the outer two s electrons. Ions with larger positive charges result when additional d electrons are lost.