Once the predictive power of the periodic system became fully acknowledged by chemists, some elements were identified as �missing� for there was a box to be filled. Element 43 was one of them. Since it was a radioelement, it escaped, for a while, the isolation strategies used before the use of nuclear methods, such as the cyclotron. Indeed, 43 was claimed to be discovered many times, one of the last chapters in this saga being the claim made by Walter and Ida Noddack.The Noddacks' claim was taken seriously at first, since they also had isolated element 75, which was indeed confirmed and called rhenium. Yet, masurium, as they named 43, could not be observed by other chemists, and only after Emilio Segr� and Carlo Perrier had produced the first known isotopes of 43, was technetium eventually accepted to fill in the gap below manganese.The Noddacks however did not despair of one day isolating what they called �natural element 43.� This paper will show how they situated their own discovery and the later failure, based on original material found in their papers kept in Leuven, Belgium. In particular, Walter and Ida Noddack, and other chemists as well, had a slightly different conception of what an element is, compared to insights from the new emerging field of nuclear chemistry. A later attempt to isolate another unknown element will shed light on their practice and beliefs.

Found, lost, and found again? The story of element 43

248th National Meeting (2014)

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