250 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



95 was discovered by a group of investgators engaged on the Plu- 

 tonium Project and designated americium (Am), by analogy with 

 europium, number 63, which is a member of the rare-earth series. 

 Theoretically, this new element could be synthesized by bombarding 

 plutonium with deuterons. Finally, early in 1950, it was announced 

 by a group associated with Dr. Seaborg, working at the University 

 of California, in Berkeley, that two elements of still higher atomic 

 numbers had been synthesized. Element 97 was produced by bom- 

 barding americium, number 95, with 30- to 35-million-volt energy 

 helium ions. To this element the name berkelium (Bk) has been 

 assigned. Element 98 was synthesized, in the same manner, by bom- 

 barding curium (number 96) with high-speed helium ions, and this 

 new transmutation product has been designated californium (Cf). 

 A list of the elements discovered since 1901 is given in the accom- 

 panying table.^ 



FAemenls discovered since 1901 



Name 



Date of 

 discov- 

 ery 



Lutetium 



Protactinium 



Hafnium 



Rhenium 



Promethium. 



Astatine 



Francium 



Neptunium.- 



Plutonium 



Curium 



Americium __ 



Berkelium 



Californium. 



1907-08 

 1917-18 

 1922 

 1925 

 1937 

 1940 

 1940 

 1940 

 1940 

 1944 

 1945 

 1950 

 1950 



In the periodic arrangement of elements (fig. 1) it will be observed 

 that between Ba (atomic number 56) and Hf (atomic number 72) 

 there is interposed a group of elements, which are chemically very 

 similar and are known as the "rare earths" or lanthanide series. 

 The reason for the occurrence of 15 such chemically similar elements 

 was deduced from a study of the permissible electron configurations 

 of the elements. The same reasoning also leads to the prediction 

 of the existence of the actinide series consisting of elements of atomic 

 numbers 89 to 103, inclusive. That is, we should discover elements 

 chemically similar to Bk and Cf of still higher atomic number. 



* From Fundamentals of Atomic Physics, loc. clt. 



