Democritus Idea

400 B.C. - 399 B.C.

Matter is made of small indivisible particles. Democritus called these particles atoms.

Dalton Theory

1803 - 1804

Developed an atomic theory stating that spherical atoms had measurable properties of mass.

J.J. Thomson

1897 - 1898

Discovers the electron and used cathode ray tubes to determine the charge to mass ratio. Found canal rays were associated with a proton

Ernest Rutherford

1899 - 1900

Discovers alpha and beta rays emitting from radium.

Albert Einstein

1905 - 1906

Publishes theory on special relativity and states that matter can be converted into energy.

Ernest Rutherford

1911 - 1912

Performed the gold foil experiment to determine that the nucleus is a small, dense, and positively charged part of the atom, based on the assumption that electrons are on the outside of the nucleus. Develops the plum pudding model of the atom.

Neils Bohr

1913 - 1914

Publishes paper regarding combining nuclear and quantum theories.

Louis Victor de Broglie

1923 - 1924

Electrons are similar to both particles and waves.

Werner Heisenberg, Max Born and Erwin Schodinger

1925 - 1926

Developed quantum mechanics

Werner Heisenberg

1927 - 1928

Used the frequencies based on spectra lines to describe atoms.

Erwin Schrodinger

1930 - 1931

Saw electrons as a continuous cloud and developed wave mechanics to be a mathematical model for the atom.

James Chadwick

1932 - 1933

Discovered neutral atomic particle with similar mass to a proton, the neutron by using alpha particles.