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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.

The Wikipedia article of the day for January 8, 2018 is Periodic table . The periodic table is a chart of the chemical elements, ordered by their atomic number and electron configurations. The elements in each group (column) often have similar chemical properties. The table also shows four rectangular blocks with some similarities in physical and chemical properties. Six groups have generally accepted names, including the halogens of group 17 and the noble gases of group 18. The table provides a framework for analyzing chemical behaviour, and is extensively used in chemistry and other sciences. The Russian chemist Dmitri Mendeleev published the first widely recognized periodic table in 1869, and correctly predicted some properties of then-unknown elements that would be expected to fill in the gaps. Mendeleev's periodic table has been expanded and refined over time; elements 1–94 have all been found to occur naturally, and elements 95–118 have been synthesized in nuclear reactors o