Atomic models By levilevi1212 Period: 5 BCE to 1804 Democritus proposed the existence of atoms. Period: 1804 to 1894 Dalton developed his theory of atoms and matter. Period: 1897 to 1911 JJ Thomson discovered the electron. Period: 1911 to 1913 Rutherford developed the planetary model of the atom. Period: 1913 to 1919 Bohr developed a model of the atom. Period: 1919 to 1932 The proton was discovered. Period: 1932 to 1932 The neutron was discovered.