NEW CHEMICAL ELEMENTS — DUSHMAN 249 



particles or doubly-charged particles are used as bombarding pro- 

 jectiles. That is, an atom of the bombarded element (number X) is 

 transmitted into an atom of a totally different chemical element, of 

 higher atomic number. 



It is in this manner that the four elements previously missing below 

 uranium, and also elements of higher atomic number than uranium, 

 have been synthesized in recent years. 



The first of the elements to be synthesized in this manner (in 1937) 

 was number 43. Molybdenum is number 42, and by bombarding this 

 element in a cyclotron with deuterons, the new element, number 43, 

 was obtained. Since it was the first element produced artificially 

 or technically, it was designated technetium (Tc). Chemical tests 

 showed it to be an element chemically similar to manganese, as had 

 been expected. 



In 1938, element number 61 was produced by bombarding neo- 

 dymium, element number CO, with deuterons. It corresponds to a 

 long-sought- for element, hitherto missing in the series of 15 rare- 

 earth elements which occur in the periodic arrangement between 

 barium (number 56) and hafnium (number 72). This newly dis- 

 covered element has been designated promethium (Pm). 



In 1940, element 85 was produced by bombardment of bismuth, 

 which is number 83, with high-speed helium ions. The new element 

 was identified as a member of the same chemical group as chlorine, 

 bromine, and iodine. Accordingly, it was designated astatine (At), 

 signifying "unstable." 



Francium (Fr, atomic number 87) was discovered in 1939, as a short- 

 lived radioactive form that occurs in the decay of other radioactive 

 elements such as uranium and radium. It corresponds, chemically, 

 to the long-sought-for element "eka-cesium," which belongs to the 

 same alkali group as cesium. 



With the synthesis of these four elements all the 92 places in the 

 periodic arrangement of the elements, beginning with hydrogen and 

 ending with uranium, were completed. But in the course of the 

 investigations on the fission of uranium, four new transuranic elements 

 (that is, elements beyond number 92) were discovered. The first of 

 these, number 93, was produced in 1940 by irradiating uranium with 

 deuterons and was designated neptunium (Np), by analogy with the 

 planet Neptune, which is beyond Uranus. 



Element 94 was produced, later in 1940, by bombarding uranium 

 with a particles. Again, by analogy with Pluto, the outermost of 

 the planets, this new element was designated plutonium (Pu). Both 

 neptunium and plutonium are produced in the atomic pile as a result 

 of the emission of neutrons by the isotope of uranium, of mass 235. 



By irradiating plutonium with helium ions, element 96 was syn- 

 thesized in 1944 and designated curium (Cm). In 1945 the element 



