Historical Development  1800's - began to notice relationships among known elements Prout (1815) -    atomic masses seem to be multiples of hydrogen Dobereiner (1829) -    arranged elements into similar groups of three - triads Newlands (1864) -    arranged elements into eight similar groups - octaves deChancourtois (1864) -    arranged elements into a three dimensional spiral corkscrew Meyer (1864) -    arranged elements by increasing atomic mass Mendeleev (1869) -    given credit for first periodic table       arranged elements by increasing atomic mass       left blank spaces in table where elements seemed out of place       predicted discovery of several elements because of blank spaces       there were several mass discrepancies Thomsen (1895) -    modern format of groups and periods Moseley (1913) -    arrangement by atomic number       discovered during X-ray studies       arrangement discrepancies disappeared

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