With the discovery of periodic law for chemical elements that can fit periodic table according to its atomic numbers and chemical properties Mendeleev also predicted the discovery of yet unknown but essential and consequent elements table required. He had published the first edition of the table with the gaps for these elements that were really discovered later.
The four predicted elements lighter than the rare earth elements, ekaboron (Eb), ekaaluminium (Ea), ekamanganese (Em), and ekasilicon (Es), proved to be good predictors of the properties of scandium (discovered in 1879), gallium (discovered in 1875), technetium (discovered in 1937) and germanium (discovered in 1886) respectively, which each fill the spot in the periodic table assigned by Mendeleev. Also the existence of an element between thorium and uranium was predicted by Mendeleev in 1871. In 1900 William Crookes isolated protactinium as a radioactive material from uranium which he could not identify.
Dmitri Mendeleev , article in the journal of Russian Chemical Society