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PERIODIC TABLE :                       The periodic table is a tabular arrangement of the chemical elements, organized on the basis of their atomic number, electron configurations, and recurring chemical properties. The rows of the table, called periods, generally have metals on the left and nonmetals on the right. The columns, called groups, contain elements with similar chemical properties.                     The periodic table was first developed by Russian chemist Dmitri Mendeleev in 1869. Mendeleev arranged the elements by atomic mass, and he noticed that elements with similar properties fell into groups. He left gaps in his table, predicting that new elements would be discovered with the properties he expected.                     The modern periodic table is based on atomic number, which is the number of protons in the nucleus of an atom. The atomic number determines the chemical properties of an element. Elements with the same number of valence electrons, which are the electro