THE LANTHANIDES AND ACTINIDES 443level was being filled for hafnium, tantalum and tungsten). The
discovery of many elements beyond uranium (the 'transurankr
elements) and a study of their properties, show that, in fact, a new
inner transition series is being built up, starting after actinium. Hence
the elements beyond actinium are now called the actinides.
Initially, the only means of obtaining elements higher than
uranium was by a-particle bombardment of uranium in the cyclo-
tron, and it was by this means that the first, exceedingly minute
amounts of neptunium and plutonium were obtained. The separa-
tion of these elements from other products and from uranium was
difficult; methods were devised involving co-precipitation of the
minute amounts of their salts on a larger amount of a precipitate
with a similar crystal structure (the "carrier^1 ). The properties were
studied, using quantities of the order of 10~^6 g in volumes of
solution of the order of 10"^3 cm^3. Measurements of concentration
could, fortunately, be made by counting the radioactive emissions—
a very sensitive method. However, much of the chemistry of
plutonium was established on this scale before nuclear fission
reactions yielded larger quantities of plutonium, and also yielded
the first amounts of americium and curium. It soon became apparent
that the ion-exchange chromatography method could be used in the
separation of these new elements in just the same way as for the
lanthanides. The fact that, when this was done, a series of concentra-
tion peaks was obtained exactly similar to those shown in Figure
15 J, is in itself strong evidence that the actinides and lanthanides
are similar series of elements.
The use of larger particles in the cyclotron, for example carbon,
nitrogen or oxygen ions, enabled elements of several units of
atomic number beyond uranium to be synthesised. Einsteinium and
fermium were obtained by this method and separated by ion-
exchange, and indeed first identified by the appearance of their
concentration peaks on the elution graph at the places expected for
atomic numbers 99 and 100. The concentrations available when this
was done were measured not in gcm^3 but in 'atoms cm^3 \ The
same elements became available in greater quantity when the first
hydrogen bomb was exploded, when they were found in the fission
products. Element 101, mendelevium, was made by a-particle
bombardment of einsteinium, and nobelium (102) by fusion of
curium and the carbon-13 isotope.
Evidence other than that of ion-exchange favours the view of the
new elements as an inner transition series. The magnetic properties
of their ions are very similar to those of the lanthanides; whatever
range of oxidation states the actinides display, they always have 4- 3
as one of them. Moreover, in the lanthanides, the element gado-