Mendeleev's predicted elements
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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 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.
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Ekaaluminium and gallium
Gallium was isolated in November, 1875. Mendeleev had predicted an atomic mass of 68 for ekaaluminium in 1871 while gallium has an atomic mass of 69.723.
Ekaboron and scandium
Scandium was isolated as the oxide in spring, 1879, by Nilson; Per Teodor Cleve recognized the correspondence and notified Mendeleev late in that year. Mendeleev had predicted an atomic mass of 44 for ekaboron in 1871 while scandium has an atomic mass 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 mass of 100 for ekamanganese in 1871 and the most stable isotope of technetium 98Tc has an atomic mass 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.
| Property | Ekasilicon | Germanium |
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| atomic mass | 72 | 72.59 |
| density (g/cm3) | 5.5 | 5.35 |
| melting point (°C) | high | 947 |
| color | gray | gray |