Dmitri Mendeleev's predicted elements

Dmitri Mendeleev published the first periodic table of the chemical 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-then-unknown elements existed with properties appropriate to fill those gaps.

To give provisional names to his predicted elements, Mendeleev used the prefixes -, -, and -, from the Sanskrit names of digits 1, 2, and 3, depending upon whether the predicted element was one, two, or three places down from the known element of the same group in his table. For example, germanium was called eka-silicon until its discovery in 1886, and rhenium was called dvi-manganese before its discovery in 1926.

The eka- prefix was used by other theorists, and not only in Mendeleev's own predictions. Before the discovery, francium was referred to as eka-caesium and astatine as eka-iodine. Sometimes, eka- is still used to refer to some of the transuranic elements, for example eka-actinium (or dvi-lanthanum) for unbiunium. But 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.

The four predicted elements lighter than the rare earth elements, eka-boron (Eb), eka-aluminium (Ea), eka-manganese (Em), and eka-silicon (Es), proved to be good predictors of the properties of scandium, gallium, technetium and germanium respectively, which 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 lighter ones and why they are not as well known or documented.

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