A Timeline on Atomic Structure

400 B.C. Democritus’ atomic theory posited that all matter is made up small indestructible units he called atoms.

1704      Isaac Newton theorized a mechanical universe with small, solid masses in motion.

1803      John Dalton proposed that elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined together.

1832      Michael Faraday  developed the two laws of electrochemistry.

1859     J. Plucker built one of the first cathode-ray tubes.

1869     Dmitri Mendeleev created the periodic table.

1873     James Clerk Maxwell proposed the theory of electromagnetism and made the connection between light and electromagnetic waves.

1874   G.J. Stoney theorized that electricity was comprised of negative particles he called electrons.

1879      Sir William Crookes’ experiments with cathode-ray tubes led him to confirm the work of earlier scientists by definitively demonstrating that cathode-rays have a negative charge.

1886      E. Goldstein discovered canal rays, which have a positive charge equal to an electron.

1895      Wilhelm Roentgen discovered x-rays.

1896     Henri Becquerel discovered radiation by studying the effects of x-rays on photographic film.

1897     J.J. Thomson determined the charge to mass ratio of electrons.

1898     Rutherford discovered alpha, beta, and gamma rays in radiation.

1898     Marie Sklodowska Curie discovered radium and polonium and coined the term radioactivity after studying the decay process of uranium and thorium.

1900      Max Planck proposed the idea of quantization to explain how a hot, glowing object emitted light.

1900   Frederick Soddy came up with the term "isotope" to explain the unintentional breakdown of radioactive elements.

1903    Hantaro Nagaoka proposed an atomic model called the Saturnian Model to describe the structure of an atom.

1904      Richard Abegg found that inert gases have a “stable electron configuration.”

1906     Hans Geiger invented a device that could detect alpha particles.

1914     H.G.J. Moseley discovered that the number of protons in an element determines its atomic number. 

1919     Francis William Aston used a mass spectrograph to identify 212 isotopes.

1922      Niels Bohr proposed an atomic structure theory that stated the outer orbit of an atom could hold more electrons than the inner orbit.

1923     Louis de Broglie proposed that electrons have a wave/particle duality.

1929   Cockcroft / Walton created the first nuclear reaction, producing alpha particles

1930     Paul Dirac proposed the existence of anti-particles.

1932     James Chadwick discovered neutrons, particles whose mass was close to that of a proton.

1938     Lise Meitner, Hahn, Strassman discovered nuclear fission.

1941-51 Glenn Seaborg discovered eight transuranium elements.

1942      Enrico Fermi created the first man-made nuclear reactor.