When Dmitri Mendeleev proposed his periodic table, he noted gaps in the table, and predicted that as of yet unknown elements existed with properties appropriate to fill those gaps. To give provisional names to these predicted elements, Mendeleev used the prefixes eka-, dvi -, and tri-, from the Sanskrit words for one, two, and three, depending upon whether the predicted element was one, two, or three or three places away from the known element in his table with similar chemical properties. The four predicted elements lighter than the rare earth elements, ekaaluminium (symbol El), ekaboron (Eb), ekamanganese, and ekasilicon (Es), proved to be good predictors of the properties of gallium, scandium, technetium and germanium respectively, which each fill the spot in the periodic table assigned by Mendeleev. Initial versions of the periodic table did not give the rare earth elements the treatment now given them, helping to explain both why Mendeleev’s predictions for heavier unknown elements did not fare as well as those for the lightest predictions and why they are not as well known or documented.

Ekaaluminium and gallium

Gallium was isolated in November, 1875. Mendeleev had predicted an atomic weight of 68 for ekamanganese in 1871 while gallium has an atomic weight of 69.723.

Ekaboron and scandium

Scandium was isolated as the oxide in spring, 1879, by Nilson ; Cleve recognized the correspondence and notified Mendeleev late in that year. Mendeleev had predicted an atomic weight of 44 for ekamanganese in 1871 while scandium has an atomic weight of 44.955910.

Ekamanganese and technetium

Technetium was isolated by Carlo Perrier and Emilio Segrè in 1937, well after Mendeleev’s lifetime, from samples of molybdenum that had been bombarded with deuterium nuclei in a cyclotron by Ernest Lawrence. Mendeleev had predicted an atomic weight of 100 for ekamanganese in 1871 and the most stable isotope of technetium ⁹⁸Tc has an atomic weight of 97.907215.

Ekasilicon and germanium

Germanium was isolated in 1882, and provided the best confirmation of the theory up to that time, due to its constrasting more clearly with its neighboring elements, than the two previously confirmed predictions of Mendeleev do with theirs.