Intorduction. The Lanthanide Elements.

The lanthanide group consist of following elements: Lan­thanum - [La], Cerium - [Ce], Praseo­dymium - [Pr], Neo­dymium - [Nd], Prome­thium - [Pm], Sama­rium - [Sm], Europ­ium - [Eu], Gadolin­ium - [Gd], Ter­bium - [Tb], Dyspro­sium - [Dy], Hol­mium - [Ho], Erbium - [Er], Thulium - [Tm], Ytter­bium - [Yb], Lute­tium - [Lu].

Not least of the confusions associated with this group of elements is that of terminology. The name “rare earth” was originally used to describe almost any naturally occurring but unfamiliar oxide and even until about 1920 generally included both ThO2 and ZrO2. About that time the name began to be applied to the elements themselves rather than their oxides, and also to be restricted to that group of elements which could only be separated from each other with great difficulty. On the basis of their separability it was convenient to divide these elements into the “cerium group” or “light earths” (La to about Eu) and the “yttrium group” or “heavy earths” (Gd to Lu plus Y which, though much lighter than the others, has a comparable ionic radius and is consequently found in the same ores, usually as the major component). It is now accepted that the “rare-earth elements” comprise the fourteen elements from 58Ce to 71Lu, but are commonly taken to include 57La and sometimes Sc and Y as well.

The lanthanides comprise the largest naturally occurring group in the periodic table. Their properties are so similar that from 1794, when J. Gadolin isolated "yttria" which he thought was the oxide of a single new element, until 1907 when lutetium was discovered, nearly a hundred claims were made for the discovery of elements belonging to this group. In view of the absence at that time of a conclusive test to determine whether or not a mixture was involved, this is not surprising. Indeed, there was a general lack of understanding of the large number of elements involved since the periodic table of the time could accommodate only one element, namely La. Not until 1913, as a result of H. G. J. Moseley’s work on atomic numbers, was it realized that there were just fourteen elements between La and Hf, and in 1918 Niels Bohr interpreted this as an expansion of the fourth quantum group from 18 to 32 electrons.