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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.