<P> In 1914, a year before he was killed in action at Gallipoli, the English physicist Henry Moseley found a relationship between the X-ray wavelength of an element and its atomic number . He was then able to re-sequence the periodic table by nuclear charge, rather than by atomic weight . Before this discovery, atomic numbers were sequential numbers based on an element's atomic weight . Moseley's discovery showed that atomic numbers were in fact based upon experimental measurements . </P> <P> Using information about their X-ray wavelengths, Moseley placed argon (with an atomic number Z = 18) before potassium (Z = 19), despite the fact that argon's atomic weight of 39.9 is greater than the atomic weight of potassium (39.1). The new order was in agreement with the chemical properties of these elements, since argon is a noble gas and potassium is 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, as had been proposed by Mendeleev . </P> <P> Moseley's research showed that there were gaps in the periodic table at atomic numbers 43 and 61, which are now known to be occupied by technetium and promethium respectively . </P> <P> During his Manhattan Project research in 1943, Glenn T. Seaborg experienced unexpected difficulties in isolating the elements americium and curium . Seaborg wondered if these elements belonged to a different series, which would explain why their chemical properties were different from what was expected . In 1945, against the advice of colleagues, he proposed a significant change to Mendeleev's table: the actinide series . </P>

Who arranged the periodic table by chemical and physical properties