Moseley and Seaborg were instrumental in making changes to Mendeleev's table.
Henry Moseley (1887-1915) made a systematic study of the X-ray lines produced by thirty-eight elements distributed throughout the periodic table. He found an orderliness in the shift towards shorter wavelengths as one passed from one element to others of higher atomic weight, but there were some irregularities. To get over the difficulty posed by the irregularities, he assigned a number to each element, specifying its position in the periodic table. Then he could assign a relation between the frequency of X-ray lines and the atomic number - a relation known as Moseley's law.
When the elements were arranged according to the atomic numbers assigned by Moseley, some inconsistencies apparent in the Mendeleev table were removed. Thus Moseley was the first to arrange the elements in order of atomic number, rather than atomic weight, so he can be considered to be responsible for the present-day arrangement of the elements.
Moseley's paper, The High Frequency Spectra of the Elements, was published in the Philosophical Magazine in 1913, and further in 1914.
Moseley is considered to have been an extremely able experimenter, producing work of great significance in a very short time.
Glenn Seaborg (1912-1999) shared the 1951 Nobel prize for Chemistry for his work on the transuranium elements.
In a 1997 interview, Seaborg discusses his role in the discovery of plutonium in 1941, and in investigating the properties of this element. He talks about its significance as the only other useful fissionable material (besides U-235) and thus as a useful energy source. At the time, it was thought that this element, number 94, was the ultimate element, and they considered naming it 'extremium' or 'ultimium'. But up till now, 20 transuranium elements have been discovered, plutonium being only the second of these.
A comprehensive Seaborg site can be accessed here.
