what was Henry Moseley's contribution to the periodic table?
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 based on an element's atomic weight. Moseley's discovery showed that atomic numbers had an experimentally measurable basis.
placed argon (Z=18) before potassium (Z=19) based on their X-ray wavelengths, despite the fact that argon has a greater atomic weight (39.9) than potassium (39.1). The new order agrees with the chemical properties of these elements, since argon is a noble gas and potassium an alkali metal. Similarly, Moseley placed cobalt before nickel, and was able to explain that tellurium occurs before iodine without revising the experimental atomic weight of tellurium (127.6) as proposed by Mendeleev.
Moseley's research also showed that there were gaps in his table at atomic numbers 43 and 61 which are now known to be Technetium and Promethium, respectively, both radioactive and not naturally occurring. Following in the footsteps of Dmitri Mendeleev, Henry Moseley also predicted new elements. ]
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