Ida Noddack and the missing elements

The search for the missing elements, which were predicted by Dmitri Mendeleev in his Periodic Table, was intensive between 1869 and 1891, especially when his prediction proved to be right after the discovery of gallium, scandium, and germanium between 1875 and 1886. These discoveries were made easier because Mendeleev had been able to describe the properties of these elements, which he called eka-boron, eka-aluminum, and eka-silicon, with a fair degree of accuracy. 

In 1901 Bohuslav Brauner in Prague predicted the existence of an element between neodymium and samarium, which was confirmed in 1914 by Henry Moseley using x-rays, and became known as element 61. In 1918 Otto Hahn and Lise Meitner in Berlin discovered protactinium and in 1923 Georg von Hevesy and Dirk Coster in Copenhagen discovered hafnium. One year earlier Niels Bohr had confirmed that lutetium is a rare earth and not a member of Group IV of the Periodic Table as was originally thought, thus proposing the search for a missing element in zirconium minerals.

Manganese, the ninth most abundant metal in Nature, was in Group 7 of Mendeleev's Table. Mendeleev had left two gaps below it, which he had marked eka-manganese (Em) and dvi-manganese (Dm) which he predicted would fill the gaps. For both eka-manganese and dvi-manganese, he was unable to predict much of their properties because they were the last two members of the Group. However, he did predict an atomic weight of 100 for Em and 190 for Dm - values that are very near to the actual values of 98 and 182.2, respectively. He also predicted that their compounds would be coloured and there would be a series of oxides corresponding to the oxides of manganese. These gaps remained unfilled for more than 50 years.