Buzz
Barry Gehm:
Indeed, didymium, or Di, was once considered an element. It was first identified by Carl Mosander in 1841, who named it didymium after the Greek word didymos, meaning twin, due to its striking similarity to lanthanum. In 1879, a French chemist revealed that Mosander’s didymium actually contained samarium and another previously unidentified element. By 1885, Carl von Weisbach discovered that the unknown element was actually two separate elements, which he named praseodidymium and neodidymium (though the di part was eventually dropped). Ironically, the twin element turned out to be a pair of twins.
The term didymium filter is still used to refer to welding glasses that are tinted with a mixture of neodymium and praseodymium oxides.
Other instances often cited include various claims of having created or discovered synthetic elements. One of the most notable examples is masurium (element 43), which a group of German chemists claimed to have identified in 1925 from columbium ore (now called niobium). While this claim stirred controversy and could not be replicated by others, some literature from the time did list masurium among the elements.
In 1936, Emilio Segrè and Carlo Perrier successfully isolated element 43 from molybdenum foil subjected to cyclotron bombardment and named it technetium. Despite being the longest-lived isotopes, technetium’s half-life is short by geological standards (measured in millions of years), and it has only been detected in nature in trace amounts as a byproduct of uranium fission. For this reason, the initial discovery claim of masurium is now almost universally considered mistaken.
As far as I am aware, no synthetic element has ever been produced in a quantity large enough to see and weigh, and later proven not to be an element, unlike the case of didymium. For instance, in the case of masurium, the only evidence was a faint x-ray signal at a very specific wavelength.
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