<P> In 1864, the English chemist William Odling also drew up a table that was remarkably similar to the table produced by Mendeleev . Odling overcame the tellurium - iodine problem and even managed to get thallium, lead, mercury and platinum into the right groups, which is something that Mendeleev failed to do at his first attempt . Odling failed to achieve recognition, however, since it is suspected that he, as Secretary of the Chemical Society of London, was instrumental in discrediting Newlands' earlier work on the periodic table . </P> <P> By 1912 almost 50 different radioactive elements were found, too many for the periodic table . Frederick Soddy in 1913 found that although they emitted different radiation, many elements were alike in their chemical characteristics so shared the same place on the table . They became known as isotopes, from the Greek eisos topos ("same place"). </P> <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>

Who redefined the periodic table as being organized by atomic number not atomic weight