208 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



is particularly true since tlie whole group of heaviest elements, includ- 

 ing uranium and those immediately below uranium as well as the four 

 known transuranium elements and a number of as yet undiscovered 

 elements beyond curium, are members of a transition group. This 

 makes the chemical and physical properties of each of the elements of 

 the whole group of comparable interest. The electron structure of 

 these elements, in which the inner "5f shell" of electrons is being filled, 

 with the stable structure containing seven 5f electrons occurring with 

 tripositive curium, has been described in a previous discussion and 

 will not be further discussed here. 



Also, the radioactive properties of the new isotopes in this region 

 contribute greatly to our knowledge of the properties of heavy radio- 

 active isotopes, and the knowledge of the nature of the regularities of 

 these properties contributes to the understanding of nuclear structure. 



The present discussion will be concerned with the four known trans- 

 uranium elements, neptunium, plutoniura, americium, and curium. 



NEPTUNIUM 



Neptunium was the first transuranium element to be discovered. 

 Using the neutrons from the Berkeley cyclotron, E. M. McMillan 

 and P. H. Abelson in 1940 were able to show, with the help of their 

 chemical work, that the irradiation of uranium leads to the produc- 

 tion of the isotope 93^^^. This isotope, which has a half -life of 2.3 

 days, is the decay product of the 23-minute U^^° formed by radiative 

 neutron capture in U^^®. Their experiments on the tracer scale of 

 investigation enabled them to show that neptunium is similar in chemi- 

 cal properties to uranium. This was probably the most significant 

 first evidence that the heavy elements do not have electron structures 

 analogous to the elements immediately above them in the periodic 

 table in which the 5d electron shell is being filled. The similarity of 

 neptunium to uranium in chemical properties and its great dissimi- 

 larity to rhenium, immediately above it in the periodic table, was the 

 first convincing evidence that it is the 5f electron shell which is being 

 filled in the heavy-element region. 



Another isotope of neptmiium, Np^^^, was discovered early in 1942 

 by bombarding uranium with fast neutrons, using the Berkeley cyclo- 

 tron. This isotope is the decay product of the previously known 7-day 

 y5-particle emitting U^" which is formed as the result of an (n,2n) 

 reaction on U^^^. The isotope, Np^^'', is of particular importance 

 because it has a very long life, emitting a-particles with a half -life 

 of 2.25 X 10^ years, and because it is available in weighable amounts. 

 This isotope is produced during the operation of the large uranium 

 chain-reacting units, a very fortunate circumstance, for otherwise 

 it is probable that the element neptunium would not be available for 

 study in the macroscopic state. 



