Atoms By Jessruge14 442 Democritis Democritis's theory was that all matter was made up of atoms. Jan 1, 1803 John Dalton His theory was that all matter was made up of atoms from different weights Jan 1, 1898 J.J Thomson J.J. Thomson-1898---J.J Thomson was the first to discover the electron. He was the first scientist to discover electrons Jan 1, 1903 Philipp Lenard Lenard did a lot of work with cathode rays but his contribution to the periodic table was that he proposed a model of an atom to have both a positive and negative charge bound. Jan 1, 1909 Ernest Rutherford His theory of the structure of atoms was to do with nuclear Jan 1, 1913 Neils Bohr He made alot of contributions to the quantum mechaincs and atomic structure. Jan 1, 1932 James Chadwick He discovered neutrons.