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Antiquity to Middle Ages (14 elements): unrecorded discoveries up into the Middle Ages
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Middle Ages – 1800 (20 elements): discoveries during the age of enlightenment
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1800–1849 (24 elements): scientific and industrial revolutions
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1850–1899 (26 elements): the age of classifying elements; application of spectrum analysis techniques: Boisbaudran, Bunsen, Crookes, Kirchhoff, and others "hunting emission line signatures"
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1900–1949 (13 elements): development of old quantum theory and quantum mechanics
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In 1914 Henry Moseley found a relationship between an element's X-ray wavelength and its atomic number (Z), and therefore resequenced the table by nuclear charge rather than atomic weight. Before this discovery, atomic numbers were just sequential numbers
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During his Manhattan Project research in 1943 Glenn T. Seaborg experienced unexpected difficulty isolating Americium (95) and Curium (96). He began wondering if these elements more properly belonged to a different series which would explain why the expect
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1950–1999 (16 elements): post Manhattan project; synthesis of atomic numbers 98 and above (colliders, bombardment techniques)
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Since 2000 (5 elements): recent synthesis