TRANSURANIUM ELEMENTS—SEABORG 251 
some similarity to tungsten, and it was thought that element 93 might 
resemble rhenium, the next element beyond tungsten. There was the 
possibility, however, that neptunium might be a member of some new 
type of transition series among the heavy elements. McMillan and 
Abelson’s investigation of neptunium showed that it resembles ura- 
nium, not rhenium, in its chemical properties. This was the first 
definite evidence that the 5f electron shell is filled in the transuranium 
region. 
The early investigation of neptunium, as of all the transuranium 
elements, was made by the tracer technique. In this method, an ele- 
ment having chemical properties similar to those of the element being 
studied is used to follow the behavior of the radioactive element, 
which is present in amounts as small as 10° g., or even less. The 
element is followed in the various reactions by means of its radio- 
activity rather than by chemical analysis. In spite of the smallness 
of the quantities present, much can be deduced about the chemical 
properties of an element—for example, the solubility of its com- 
pounds, its oxidation-reduction potentials, and its formation of 
complexions—by the use of such methods. 
PLUTONIUM 
Plutonium was next to be discovered. By bombarding uranium 
with deuterons, E. M. McMillan, J. W. Kennedy, A. C. Wahl, and the 
author, in late 1940, succeeded in preparing a new isotope of nep- 
tunium, Np***, which decayed to Pu***. The half-life of this isotope 
was found to be sufficiently long to permit detection and to make pos- 
sible our obtaining considerable chemical information about it by 
tracer studies. Armed with this information about the new elements, 
J. W. Kennedy, E. Segré, A. C. Wahl, and the author in 1941 identi- 
fied the most important plutonium isotope, Pu, as the decay product 
of Np*® and we were able to prove that Pu*® undergoes fission with 
slow neutrons. 
The realization that plutonium, as Pu**, could serve as a nuclear 
weapon and that it might be created in quantity in a nuclear chain 
reactor made it imperative to carry out chemical investigations of 
plutonium with microgram quantities. In August 1942, B. B. 
Cunningham and L. B. Werner succeeded in isolating about a micro- 
gram of Pu®® which had been prepared by cyclotron irradiations. 
Thus plutonium was the first manmade element to be obtained in visi- 
ble quantity. 
A background of manipulative techniques for this ultramicrochem- 
ical work was provided by the pioneer investigations of P. L. Kirk 
and A. A. Benedetti-Pichler. If extremely small volumes are used, 
even microgram quantities of material can give relatively high con- 
