We have learned that lanthanides are the f-block elements that are also referred to as the inner transition metals. The terminology used for this group of metals is sometimes confusing. Some people call them lanthanoids while some name them as lanthanides. Let us clear this confusion. When we talk about the inner transition metals from atomic number 57 to 71 (15 elements, including lanthanum) then they are called as lanthanoids. If we consider only the elements of this group without including lanthanum (atomic number 57) then the remaining elements are referred to as lanthanides. They are, sometimes also called as rare earth metals, although they are present on the Earth in ample quantity their abundance is low when compared with other metals. Our subject of interest for this article is oxidation state of lanthanides.
Oxidation State of Lanthanides
All the elements in lanthanide series show an oxidation state of +3.Earlier it was believed that some of the metals (samarium, europium, and ytterbium) also show +2 oxidation states. Further studies on these metals and their compounds have revealed that all the metals in lanthanide series exhibit +2 oxidation state in their complexes in solutions. A few metals in the lanthanide series occasionally show +4 oxidation states. This uneven distribution of oxidation state among the metals is attributed to the high stability of empty, half-filled or fully filled f-subshells. The stability of f-subshell affects the oxidation state of lanthanides in such a way that the +4 oxidation state of cerium is favoured as it acquires a noble gas configuration but it reverts to a +3 oxidation state and thus acts as a strong oxidant and can even oxidize water, although the reaction will be slow.
The +4 oxidation state is also exhibited by the oxides of:
Europium (atomic number 63) has the electronic configuration [Xe] 4f7 6s2, it loses two electrons from 6s energy level and attains the highly stable, half-filled 4f7 configuration and hence it readily forms Eu2+ ion. Eu2+ then changes to the common oxidation states of lanthanides (+3) and forms Eu3+, acting as a strong reducing agent. Ytterbium (atomic number 70) also has similar reasons for being a strong reducing agent, in Yb2+ state; it has a fully filled f-orbital.
The presence of f-subshell has a great influence on the oxidation state exhibited by these metals and their properties. New developments and findings continue to add information on lanthanides.