250 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
thorium, protoactinium, and uranium were commonly placed imme- 
diately below the elements hafnium, tantalum, and tungsten, which 
are members of a transition series in which the 5d electron shell is 
being filled. This placing was based on the assumption that the three 
heavy elements were members of a Gd electron transition series. The 
appearance of N. Bohr’s paper on the quantized nuclear atom in 1913 
led to suggestions that a 5f electron transition series should start in 
the region thorium to element 95 inclusive, before the completion of 
the 6d electron shell. With the discovery of neptunium and then 
plutonium, the boundaries of the periodic table were transcended, and 
as knowledge of the first transuranium elements accumulated, it 
became evident that a whole new family of elements, some known and 
some still to be discovered, existed in this region of the periodic table. 
The fact that the transuranium elements are members of a transition 
series similar to the rare earth, or lanthanide, series is useful in pre- 
dicting the chemical properties of these elements before they are 
actually detected. This particular pattern of similarity, recognized 
by the author in 1944 on the basis of the chemical properties of 
neptunium and plutonium, was the key to the discovery of elements 
95 and 96 (americium and curium) and has been essential to the dis- 
covery of the transcurium elements. Since all the elements beyond 
actinium seem to belong to the actinide group (a name chosen by 
analogy with the lanthanide group), the elements thorium, protoac- 
tinium, and uranium have been removed from the positions they 
occupied in the periodic table before 1939 and placed in this transition 
family; as we shall see, elements 104, 105, and 106 will presumably 
take over the positions previously held by thorium, protoactinium, 
and uranium. Thus we have the interesting result that the new- 
comers have affected the face of the periodic table, and a change has 
been made after many years during which it seemed to have assumed 
its final form. 
NEPTUNIUM 
The discovery of the first transuranium element, neptunium, resulted 
from McMillan’s investigation of the fission process. In measuring 
the energies of the two main fragments from the neutron-induced 
fission of uranium, he found that there was another radioactive prod- 
uct of the reaction, one which did not recoil sufficiently to escape from 
the thin layer of uranium undergoing fission. He suspected that this 
was a product formed by neutron capture in the uranium. McMillan 
and Abelson were able to show on the basis of their chemical work that 
this product was an isotope of element 93 (Np**), arising by beta 
decay of U?** formed by neutron capture in U**. 
It was not obvious what the electronic configuration and chemical 
properties of neptunium might be. Uranium was known to have 
