An Element of Controversy
- PDF / 130,878 Bytes
- 1 Pages / 576 x 783 pts Page_size
- 36 Downloads / 204 Views
An Element of Controversy By 1924, all but five of the spaces in the Periodic Table of the Elements had been filled. Two missing elements were in the same column, just under manganese: atomic numbers 43 (now known as technetium, Tc) and 75 (rhenium, Re). The others were 61 (promethium), 85 (astatine), and 87 (francium). The prolonged search for the two elements in the manganese column included many false claims, mostly as a result of impure samples. In 1877, Russian chemist Serge Kern reported that he had found element 43 and named it “davyum,” after the chemist Sir Humphrey Davy. In 1908, the Japanese scientist Masataka Ogawa erroneously announced that he had discovered element 43, which he named “nipponim” (as a tribute to his country). However, none of these early “discoveries” held up under scientific scrutiny. The search for both elements appeared to have ended in 1925. German chemists Walter Noddack; Ida Tacke, later Ida Tacke Noddack; and Otto Berg identified element 75 in a sample of gadolinite and named it rhenium, after the Latin word Rhenus for the Rhine River. Their ability to isolate and analyze a pure sample of rhenium was essential in verifying their discovery. The same could not be said of element 43, which they had named masurium after Masuria, a region in Prussia. Their evidence in this case was an x-ray spectrograph of a sample of columbite showing a faint peak that they interpreted from theory to belong to element 43. The facts that they could not isolate the element and that their contemporaries could not reproduce the x-ray spectrograph, however, left their claim open to dispute. Noddack, Tacke, Berg, and their predecessors were searching for elements the way it had always been done—looking for naturally occurring sources of the material. Little did they know that it would take something akin to alchemy to produce the sample they were seeking. The alchemy came from nuclear reactions made possible by Ernest O. Lawrence’s cyclotron at the University of California, Berkeley. In 1936, physicist Emilio Segrè of the University of Palermo in Sicily visited Lawrence to see the cyclotron first hand; in February 1937, Lawrence sent him a molybdenum foil from the cyclotron’s deflector. “I suspected at once that it might contain element 43,” Segrè wrote. “The simple reason was that deuteron bombardment of molybdenum (atomic number 42) should give isotopes of element 43 through well-established nuclear reac-
tions.” With the help of Carlo Perrier, he succeeded in chemically separating two isotopes. Careful chemical analysis on these minute samples showed that they were indeed element 43. “In this work we had discovered the first chemical element created by man,” Segrè concluded. Respecting Noddack, Tacke, and Berg’s existing claim, Segrè and Perrier made no attempt to rename the element just then. Instead, Segrè visited Noddack’s laboratory in Freiburg in 1937 to confer with him about masurium. Noddack said that the x-ray plates containing the evidence for the existence of masurium in columbite had been d
Data Loading...
