Antoine-Laurent Lavoisier established the Law of Combustion, that chemical reactions involve a chemical he named 'oxygen' and helped create chemical nomenclature (the system used to name elements). He also gave new names to substances, most of which are still used today.
Johann Wolfgang Döbereiner is best known for his work on the law of triads. Each of Döbereiner's triads was a group of three elements. The appearance and reactions of the elements in each triad were similar to each other. He discovered that the relative atomic mass of the middle element in each triad was close to the average of the relative atomic masses of the other two elements. This showed that relative atomic masses were important when you were arranging the elements.
Dimitri Mendeleev produced a table based on atomic weights arranged 'periodically' with elements with similar properties under each other. His Periodic Table included the sixty-six known elements organized by atomic weights.
Julius Lothar Meyer developed an early version of the periodic table, with twenty-eight elements organized by valence. He realised that when elements are sorted into their atomic weights, they fell into groups of similar chemical and physical properties.
John Newlands created the first periodic table that was arranged in order of atomic masses. He published his 'Law of Octaves', which stated that 'any given element will exhibit similar behaviour to the eighth element following it in the table.' He arranged the known elements into seven groups of eight, starting with Hydrogen and ending with Thorium.
Glenn T. Seaborg co-discovered plutonium and all further transuranium elements through Nobelium. Through his works, it was possible to predict the radioactive characteristics of many isotopic elements yet to be found.