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Mendeleev's predicted elements
Professor 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-yet-unknown elements existed with properties appropriate to fill those gaps.
== Prefixes ==
To give provisional names to his predicted elements, Mendeleev used the prefixes ''eka''-, ''dvi''-, and ''tri''-, 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 ekasilicon 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-radon'' for ununoctium and ''eka-actinium'' (or ''dvi-lanthanum'') for untriennium. 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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