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The Law of Mass- "Mass is neither created nor destroyed in chemical reactions. In other words, the mass of any one element at the beginning of a reaction will equal the mass of that element at the end of the reaction."
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In 1817, Johann Wolfgang Döbereiner began to formulate one of the earliest attempts to classify the elements.
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Definition of Triad law:-"Chemically analogous elements arranged in increasing order of their atomic weights formed well marked groups of three called Triads in which the atomic weight of the middle element was found to be generally the {Arthmetic mean} of the atomic weights of the other two elements"
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John Newlands classifies all 62 known elements into 8 groups
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Lothar publishes his personal Periodic Table but by valence, not atomic weight
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Mendleev publishes his periodic table in a Russian Journal.
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Glenn Seaborg changes and adds the actinide series to Mendeleev's Periodic Table adding elements 89-103