Modern inorganic chemistry

442 THE LANTHANIDES AND ACTINIDES

The appearance of a peak between those for neodymium (60)

and samarium (62) was then strong evidence for the existence of

promethium (61).

The reason why lanthanides of high atomic number emerge first

is that the stability of a lanthanide ion-citrate ion complex increases

with the atomic number. Since these complexes are formed by ions,

this must mean that the ion-ligand attraction also increases with

atomic number, i.e. that the ionic radius decreases (inverse square

law). It is a characteristic of the lanthanides that the ionic radius

Peaks due to lanthanides of

higher atomic number occur

here Ho Dy Tb Gd

Time or volume of eluting solution passed through

Figure 15,1. Ion-exchange graph for lanthanides

does decrease slightly as the atomic number increases. This effect,
called the lanthanide contraction, occurs because the nuclear charge
rises with rise of atomic number, whereas the two outer electron
levels (which largely determine the ionic radius) remain unchanged;
hence the ionic radius decreases as the increasing nuclear charge
"pulls in' the outer electrons to an increasing extent.
Another characteristic change across the lanthanide series is that
of the paramagnetism of the ions; this rises to a maximum at
neodymium, then falls to samarium, then rises to a second maximum
at gadolinium before falling finally to zero at the end of the series.
Before it was known that elements beyond uranium were capable
of existence, the heaviest known natural elements, thorium, pro-
tactinium and uranium, were placed in a sixth period of the periodic
classification, corresponding to the elements hafnium, tantalum and
tungsten in the preceding period. It was therefore implied that these
elements were the beginning of a new, fourth transition series, with
filling of the penultimate n = 6 level (just as the penultimate n = 5