Professor Dmitri Mendeleev published the first Periodic Table of the Atomic Elements in 1869 based on properties which appeared with some regularity as he laid out the elements from lightest to heaviest. When 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 his 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 down from the known element in his table with similar chemical properties.
For example germanium was called ekasilicon until its discovery, and rhenium was called dwi-manganese before its discovery in 1926.
It has been speculated that the similarity between the tabular structure commonly used to present the Sanskrit abugida and the periodic table is what led to Mendeleev choosing to use Sanskrit as the basis of these prefixes.
Sometimes the eka- prefix is used to refer to some transuranic elements, for example eka-lead for ununquadium and eka-radon for ununoctium.
Nowadays, the prefix eka- (and, more rarely, dvi-) is sometimes used in discussions about undiscovered elements, such as untriennium, also known as eka-actinium or dvi-lanthanum.
Current official IUPAC practice is to use a systematic element name based on the atomic number of the element as the provisional name, instead of being based on its position in the periodic table as these prefixes require.
Original predictions from 1870
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, gallium, 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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