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The middle element in the triad had atomic weight that was the average of the other two members.
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There were about 60 elements which had already been discovered by 1860
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alexandre Beguyer de Chancourtois published a list of all the known elements. The list was constructed as a helical graph wrapped around a cylinder. Elements with similar properties occupied positions on the same vertical line of cylinder.
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Elements exhibit similar behavior to the eighth element following it in the table.
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Dmitri Mendeleev decided to stay at home to work on a way to arrange all the chemical elements so instead of going to visit a factory he proceeded to dicover the periodic table. first he ordered it in rows horisontally but found it easier to understand when ordered in columns.
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after the discovery of argon in 1895, William Ramsey decided that a new group should be established in the periodic table because argon did not fit into any of the other categories.
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moseley arranged the elements in increasing atomic numbers instead of atomic mases
