What is the only element of the periodic table that starts with the letter X?
Extended periodic table due to its much higher atomic weight. Unseptbium should be a strong Lewis acid, forming fluorides and oxides, similarly to its lighter congener xenon. Because of this analogy of elements 165–172 to periods 2 and 3, Fricke et al. considered them to form a ninth period of the periodic table, while the eighth period was taken by them to end at the noble metal element 164. This ninth and final period would be similar to the second and third period in that it should have no transition metals. Immediately after element 172 (unseptbium, the last period 8 element), the first
History of the periodic table known as isotopes, from the Greek "" ("same place"). 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. Using information about their X-ray wavelengths, Moseley placed argon (with an atomic number Z=18) before potassium