periodic table , n :
(chemistry) A tabular chart of the chemical elements according to their atomic numbers so that elements with similar properties are in the same group (column). On this day 150 years ago in 1869, Russian chemist Dmitri Mendeleev presented the first periodic table of elements to the Russian Chemical Society. In commemoration of this achievement, the United Nations General Assembly and UNESCO have proclaimed 2019 the International Year of the Periodic Table of Chemical Elements.
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