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Why were early claims to discover element 61 not accepted?

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A line graph showing electron probability distributions for gadolinium, with separate curves for 4f, 5s, 5p, 5d, and 6s orbitals. The x-axis shows distance from the nucleus in atomic units and the y-axis shows probability P2(r). — rare-earth element Electron probabilities, P²*(r)*, for the 4f, 5s, 5p, 5d, and 6s electrons of gadolinium.

Early claims to the discovery of element 61 were not accepted primarily because they could not be independently verified.1

In the 1920s, research groups at the University of Florence and the University of Illinois claimed to have discovered the element, naming it "florentium" and "illinium," respectively. However, these assertions were premature and lacked the rigorous evidence required by the scientific community.12

The difficulty in identifying the element stemmed from the extreme chemical similarity of the rare-earth elements, which made their separation and purification a complex and time-consuming process. It was not until 1945 that conclusive proof was obtained by American chemists who isolated promethium from uranium fission products. This identification was firmly established using ion-exchange chromatography, a method that provided the necessary precision to distinguish the element from its neighbors in the lanthanide series.12