248 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
tope, Pu2*, makes this one of the most dangerous known poisons. The 
announcement of its discovery to the world was through the atomic 
bomb that fell on Nagasaki. Plutonium, of course, has an important 
future in nuclear power. The fissionable isotope Pu’® makes its 
source isotope U**, which is not fissionable with slow neutrons, a 
potential nuclear fuel. The fertile U** is “burned” by going through 
the intermediate fissionable Pu**®. 
The discovery of the first transuranium element followed a false 
start 6 years earlier. When Enrico Fermi and his coworkers first 
bombarded uranium with slow neutrons in 1934 they found that a 
number of artificially radioactive species were produced, and in the 
immediately following years many more such substances were ob- 
served. Most of these were thought to be transuranium elements. 
Chemical investigation, however, led to the discovery of the fission 
process rather than to the discovery of transuranium elements. Sub- 
sequent work has shown that practically all the radioactive species 
believed to be transuranium elements were in fact fission products 
of uranium. In 1940 EK. M. McMillan and P. H. Abelson discovered 
the first transuranium element. This was neptunium, with atomic 
number 93. In the following years, many more transuranium ele- 
ments—plutonium (94), americium (95), curium (96), berkelium 
(97), californium (98), einsteinium (99), fermium (100), mendel- 
evium (101), and element 102—were synthesized and identified. The 
elements up to and including einsteinium have isotopes sufficiently 
long lived to be isolated in macroscopic, that is weighable, quantities, 
but this does not seem to be true beyond einsteinium. 
The transuranium elements are, for all practical purposes, synthetic 
in origin and must be produced by transmutation, starting, in the 
first instance, with uranium. However, two of them, neptunium and 
plutonium, are present in trace concentrations in uranium ores as 
the result of the action of the neutrons which are present. Investiga- 
tion of these new elements has resulted in the contribution of much 
information to inorganic chemistry, since they have a rich and varied 
chemical behavior, form unusual compounds, and in some cases dis- 
play an extraordinary complexity in solution. The relationship of 
these elements to each other and to the other elements is now within 
our understanding. Problems inherent in the study of these elements, 
such as those of handling quantities of material so small as to be 
unweighable, of working in safety with radioactive materials, and 
of preparing and identifying elements of ever-increasing atomic 
number, are being solved. 
POSITION IN THE PERIODIC TABLE 
Ideas on the position in the periodic table (table 1) of the heaviest 
elements have varied considerably over the years. Until the last war, 
