258 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
them often gives indirect information about the chemical properties 
of another. Thus certain anomalous elution sequences among the 
transplutonium elements have made it possible to make deductions 
about the chemical properties of plutonium in relation to its electronic 
structure. 
There are important differences between the actinide and lanthanide 
elements, however, due largely to the lower binding energies and less 
effective shielding by outer electrons of 5f (as compared to 4f) elec- 
trons. It appears that the first 5f electron is not present in thorium. 
Evidence to date indicates that uranium possesses three 5f electrons. 
The additional 5f electrons apparently are added to the succeeding 
elements in a regular fashion, proceeding through curium with its 
half-filled shell to the as yet undiscovered element 103 which pre- 
sumably will have 14 5f electrons. In the early members of the 
actinide group particularly, the lower binding energies of the 5f elec- 
trons compared to the 4f electrons tend to make higher oxidation states 
more accessible. For these lighter actinide elements, the problem of 
assignment of electrons to 5f or 6d orbitals is difficult, since here the 
energy separations apparently lie within the range of chemical bind- 
ing energies. It may not be possible to establish from the configura- 
tion of the gaseous atom the electronic structure of the compounds or 
of hydrated ions in aqueous solution. In the case of the lanthanide 
elements, the configuration of the gaseous atom includes, in general, 
only two electrons (beyond the xenon structure) outside the 4f shell, 
although the predominant oxidation state in aqueous solution is the 
trivalent state. It may also be noted that for a given element in the 
actinide group there is a stabilization of 5f compared to 6d electrons 
with increasing oxidation state. 
However, the differences between the actinide and lanthanide groups 
give opportunities for the investigation of important new chemical 
phenomena. For example, their energetic position and larger spatial 
extension make the 5f orbitals available in bond hybridization; this 
leads to some very interesting complex ions. Similarly, the exposure 
of the 5f electrons can lead to field-splitting effects which can affect 
ionic entropies in manners which have not been observed in the lighter 
elements. 
FUTURE DEVELOPMENTS 
The discovery of further new transuranium elements seems possible. 
Studies of the known isotopes of the transuranium elements have 
made possible the prediction of the decay properties of new isotopes. 
For decay by both alpha-particle emission and spontaneous fission, the 
regularities have been found to be greatest for nuclei which contain 
an even number of neutrons and an even number of protons, thus 
