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400 B.C. Democritus’ atomic theory posited that all matter is made up small indestructible units he called atoms. "by convention bitter, by convention sweet, but in reality atoms and void"
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1704 Isaac Newton theorized a mechanical universe with small, solid masses in motion.
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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.
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J. Plucker Built one of the first gas discharge tubes ("cathode ray tube").
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Dmitri Mendeleev created the periodic table.
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1873 James Clerk Maxwell proposed the theory of electromagnetism and made the connection between light and electromagnetic waves.
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G.J. Stoney theorized that electricity was comprised of negative particles he called electrons.
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experiments with cathode-ray tubes led him to confirm the work of earlier scientists by definitively demonstrating that cathode-rays have a negative charge.
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E. Goldstein discovered canal rays, which have a positive charge equal to an electron.
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Wilhelm Roentgen discovered x-rays.
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Henri Becquerel discovered radiation by studying the effects of x-rays on photographic film.
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1897 J.J. Thomson determined the charge to mass ratio of electrons.
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Rutherford discovered alpha, beta, and gamma rays in radiation.
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Marie Sklodowska Curie discovered radium and polonium and coined the term radioactivity after studying the decay process of uranium and thorium.
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Max Planck proposed the idea of quantization to explain how a hot, glowing object emitted light.
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1900 Frederick Soddy came up with the term "isotope" to explain the unintentional breakdown of radioactive elements.
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In 1945 Glenn Seaborg identified lanthanides and actinides (atomic number >92), which are usually placed below the periodic table.
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1903 Hantaro Nagaoka proposed an atomic model called the Saturnian Model to describe the structure of an atom.
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1904 Richard Abegg found that inert gases have a “stable electron configuration.”
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1906 Hans Geiger invented a device that could detect alpha particles.
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H.G.J. Moseley discovered that the number of protons in an element determines its atomic number.
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1832 Michael Faraday developed the two laws of electrochemistry