Antiquity to Middle Ages (14 elements): unrecorded discoveries up into the Middle Ages
Middle Ages – 1800 (20 elements): discoveries during the age of enlightenment
1800–1849 (24 elements): scientific and industrial revolutions
1850–1899 (26 elements): the age of classifying elements; application of spectrum analysis techniques: Boisbaudran, Bunsen, Crookes, Kirchhoff, and others "hunting emission line signatures"
1900–1949 (13 elements): development of old quantum theory and quantum mechanics
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
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
1950–1999 (16 elements): post Manhattan project; synthesis of atomic numbers 98 and above (colliders, bombardment techniques)