The existence of promethium had first been …

Years: 1926 - 1926

The existence of promethium had first been predicted by Bohuslav Brauner in 1902, by which time all of the 14 rare-earth elements, or lanthanides, had been discovered, save element 61, of which no stable isotopes exist in nature; this prediction had been supported in 1914 by Henry Moseley who, having discovered that atomic number was an experimentally measurable property of elements, found that no known element had atomic number 61.

In 1926, American chemist B. S. Hopkins and his colleagues, claiming to have discovered the element in naturally occurring rare earths, propose the name illinium, after the state of Illinois.

About the same time in Italy, …

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