Dmitri Mendeleev produced a table based on atomic weights but arranged 'periodically' with elements with similar properties under each other. His Periodic Table included the 66 known elements organized by atomic weights.
William Ramsay discovered the Noble Gases.
Marie and Pierre Curie isolated radium and polonium from pitchblende.
Ernest Rutherford discovered the source of radioactivity as decaying atoms
Henry Moseley determined the atomic number of each of the elements and modified the 'Periodic Law'.
Edwin McMillan and Philip H. Abelson identify neptunium, the lightest and first synthesized transuranium element, found in the products of uranium fission.
Glenn Seaborg synthesised transuranic elements (the elements after uranium in the periodic table)