Modern inorganic chemistry

THE LANTHANIDES AND ACTINIDES 441
Reference has been made already to the existence of a set of "inner
transition' elements, following lanthanum, in which the quantum
level being filled is neither the outer quantum level nor the penulti-
mate level, but the next inner. These elements, together with yttrium
(a transition metal), were called the 'rare earths', since they occurred
in uncommon mixtures of what were believed to be "earths' or oxides.
With the recognition of their special structure, the elements from
lanthanum to lutetium were re-named the^4 lanthanons' or lanth-
anides. They resemble one another very closely, so much so that
their separation presented a major problem, since all their com-
pounds are very much alike. They exhibit oxidation state + 3 and
show in this state predominantly ionic characteristics—the ions,
L3+ (L = lanthanide), are indeed similar to the ions of the alkaline
earth metals, except that they are tripositive, not dipositive.
Originally, general methods of separation were based on small
differences in the solubilities of their salts, for examples the nitrates,
and a laborious series of fractional crystallisations had to be carried
out to obtain the pure salts. In a few cases, individual lanthanides
could be separated because they yielded oxidation states other than
three. Thus the commonest lanthanide, cerium, exhibits oxidation
states of +3 and +4; hence oxidation of a mixture of lanthanide
salts in alkaline solution with chlorine yields the soluble chlorates(I)
of all the -I-3 lanthanides (which are not oxidised) but gives a
precipitate of cerium(IV) hydroxide, Ce(OH) 4 , since this is too weak
a base to form a chlorate(I). In some cases also, preferential reduction
to the metal by sodium amalgam could be used to separate out
individual lanthanides.
When the products of nuclear fission reactions came to be
investigated, it was found that the lanthanides frequently occurred
among the products. (The lanthanide of atomic number 61, pro-
methium, for instance, probably does not occur naturally and was
not discovered until nuclear fission produced it.) Hence it became
necessary to devise more effective procedures to separate lanth-
anides, both from the fission products and from one another. One
method used with great success is that of ion exchange chroma-
tography; a mixture of (say) lanthanide salts in solution is run into a
cation-exchange resin, which takes up the lanthanide ions by
exchange. A solution containing negative ions which form complexes
with the lanthanide ions (ammonium citrate is used) is then passed
into the column and the column is washed Celuted') with this
solution until complexes of the lanthanides begin to emerge. It is
found that those of the highest atomic number emerge first, and that
the kzone' of concentration of each lanthanide is separated from that
of its neighbour. Some examples are shown in Figure 15.1.