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In 1829 Dobereiner proposed the Law of Triads: Middle element in the triad had atomic weight that was the average of the other two members.
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in 1860 the first 60 elements found
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the first periodic table was made by proffesor Alexandre Beguyer de Chancourtois
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Dimitri Mendeleev proposed arranging elements by atomic weights
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In 1871 Mendeleev revised the 17-group table with eight columns (the eighth group consisted of transition elements).
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In 1895 Rayleigh reported the discovery of a new gaseous element named argon.
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Soon after Rutherford's landmark experiment of discovering the proton in 1911, Henry Moseley subjected known elements to x-rays.
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Starting with plutonium in 1940, Seaborg discovered transuranium elements 94 to 102 and reconfigured the periodic table by placing the lanthanide/actinide series at the bottom of the table.
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In 1951 Seaborg was awarded the Nobel Prize in chemistry and element 106 was later named seaborgium (Sg) in his honor.