Chemistry – The Periodic Table - Transition elements | e-Consult

Transition elements are characterized by their ability to form ions with multiple oxidation states. This variability arises from the relatively small energy difference between the (n-1)d and ns orbitals. The electrons in these orbitals are readily lost to form cations with different positive charges. For example, iron (Fe) has an electronic configuration of [Ar] 3d⁶ 4s². The loss of electrons from the 4s orbital and/or the 3d orbitals results in different oxidation states.

Iron(II) (Fe²⁺) is formed when two electrons are removed from the 3d orbitals and one from the 4s orbital. This leaves a half-filled 3d subshell, which is a relatively stable configuration. The electronic configuration of Fe²⁺ is [Ar] 3d⁶.

Iron(III) (Fe³⁺) is formed when two electrons are removed from the 4s orbital and three electrons are removed from the 3d orbitals. This leaves a completely empty 3d subshell. The electronic configuration of Fe³⁺ is [Ar] 3d⁵. The ease with which electrons can be removed from the 4s and 3d orbitals explains the variable oxidation states of iron. The stability of the resulting ion is also a factor, with half-filled and completely filled subshells being particularly stable.