The Periodic Table Of Elements

Elements!

Dmitri Mendeleev Russian chemist Wrote own textbook in Organized elements, noting their properties repeated (periodicity) when organized by atomic mass

Dmitri Mendeleev Some 'gaps' seemed to exist in his table of approximately 60 elements Predicted new elements would be discovered to fill the gaps

Mendeleev's Predictions Mendeleev predicted the properties and existance of elements missing from the 'gaps'

Mendeleev's Predictions Mendeleev predicted the properties and existance of elements missing from the 'gaps' The discovery of gallium in 1875, of scandium in 1879, and of germanium in 1886 supported the idea underlying Mendeleev's table (and matched his predicted properties well)‏

Problems Some elements did not line up correctly when the atomic mass was used

Problems Some elements did not line up correctly when the atomic mass was used Reason for periodicity not understood in late 1800s

Henry Gwyn Jeffreys Moseley Found relationship between X-ray wavelength and atomic number in 1913 (Moseley's Law)‏ This allowed calculation of atomic number, and re- organization of elements

Henry Gwyn Jeffreys Moseley Predicted existance of technetium and promethium Furthered understanding of Rare Earth Elements (Lanthanide series)‏ Periodic table reorganized by atomic number, and problem elements were resolved

Modern Periodic Table