It has been noted several times previously that the Rare Earths (RE), a
sequence of elements with atomic numbers in the range from 58 (Ce) to 71
(Lu), are neither earths nor particularly rare. They are metals, whose
ores are often found together with oxides of the alkaline earths (Ca,
Mg), staples of the building industry, th while Cerium, for example, is
the 25 most abundant element in the Earth s crust. However, the chemical
similarity of all REs to each other and to Lanthanum, reflected in their
alternative descriptor, Lanthanoids, made extraction of the separate
elements difficult until technical advances in the 1960s kick-started
the modern era of RE science. The most widespread commercial use of RE
metals at present is in the prod- tion of super-strong permanent
magnets, containing Neodymium: check your refrigerator door for an
example. RE ferromagnetism arises from the angular momentum of electrons
in partially filled 4f atomic shells. In chemical compounds of RE with
non-metals, the 4f shell is surrounded by filled 5s and 5p orbitals, 1 2
while bonding involves the outerlying 5d and 6s electrons, resulting
(usually) in 3+ 3+ a RE ion that is chemically similar to La . (RE may
also be found in a divalent charge state, with an extra electron in the
5d shell. ) Hence the sequence of 3+ 3+ trivalent ions from Ce to Yb is
characterised by a 4f shell occupation that rises from 1 to 13
electrons."
