ABSTRACT
The term "rare earth metals" is usually applied to the group of chemical elements with
atomic numbers from 57 to 71. A l l of them occupy the only cell in the common Periodic
Table of the Elements belonging to sixth period and IIIA subgroup of it. However, as a rule,
only the first of them, lanthanum with atomic number 57, is shown in the cel l . The others
of the group are shown out of the general table as a row of chemical elements arranged in
order of successive increase of their atomic numbers. According to the rules suggested by
International Union of Pure and Appl ied Chemistry [10], two other elements of the IIIA
subgroup of the Periodic Table are also recommended to be called rare earth metals. They
are scandium and yttrium belonging to the fourth and fifth periods of the Periodic Table,
respectively. Nevertheless, sometimes the term "rare earth metals" is used only for the
elements of atomic numbers from 57 to 71 with yttrium and scandium being noted
separately. Thorium is also an element of IIIA subgroup of the Periodic Table, but it was
never considered to belong to the rare earth metals. A l l rare earth metals are listed in Table
1 where their symbols, atomic numbers and atomic masses are also presented.
