Scientists of The Periodic Table By evelynritchie 1869 Dimitri Mendeleev started the development of the Periodic Table, aranging chemicals by atomic mass. 1894 Sir William Ramsey discovered nobel gases 1897 J. J. Thomson first discovered electrons. 1903 Rutherford announced that radioactivity is caused by the breakdown of atoms 1911 Rutherford discovered electrons orbit the nucleus of the atom 1914 Rutherford identified protons in the atomic nucleus 1932 James Chadwick discovered neutrons, isotopes were indentified 1945 Glenn Seaborg identified lanthanides and actinides