The actinide element group was unknown in…
1912 CE to 1923 CE
The actinide element group was unknown in 1871, when Dmitri Mendeleev had predicted the existence of an element between thorium and uranium.
Uranium was therefore positioned in his periodic table below tungsten, and thorium below zirconium, leaving the space below tantalum empty and, until the 1950s, periodic tables will be published with this structure.
For a long time chemists had searched for eka-tantalum as an element with similar chemical properties to tantalum, making a discovery of protactinium nearly impossible.
The element (originally called protoactinium) is first identified in 1913 by Kasimir Fajans and Oswald Helmuth Göhring during their studies of the decay chains of uranium-238 and named brevium because of the short half-life of the specific isotope studied, namely protactinium-234.
In 1917/18, two groups of scientists, Otto Hahn and Lise Meitner of Germany and Frederick Soddy and John Cranston of Great Britain, independently discover another isotope of protactinium, 231Pa, having a much longer half-life of about 32,000 years.
Hahn and Meitner choose the name proto-actinium.
(The IUPAC will name it finally protactinium in 1949 and confirm Hahn and Meitner as discoverers.
The new name means "parent of actinium" and reflects the fact that actinium is a product of radioactive decay of protactinium.)
The discovery of protactinium completes the last gap in the early versions of the periodic table, proposed by Mendeleev in 1869, and it brings to fame the involved scientists.