• 322

    Aristotle felt that regardless of the number of times you cut a form of matter in half, you would always have a smaller piece of that matter.

  • 460

    Democritus First proposed the existence of an ultimate particle. Used the word "atomos" to describe this particle.

  • Period: 460 to

    Atom Timeline

  • John Dalton Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass.

  • Henri Becquerel while studying the effect of x-rays on photographic film, he discovered some chemicals spontaneously decompose and give off very pentrating rays.

  • JJ Thomson discovers electron

  • Marie and Pierre Curie Studied radioactivity which turned out to be the spontaneous splitting of unstable atoms.

  • Robert Millikan discoverd that the elcectron is negatively charged

  • Ernest Rutherford established that the nucleus was: very dense,very small and positively charged. He also assumed that the electrons were located outside the nucleus.

  • Niels Bohr: Bohr Model, The correct theory of the atom is called quantum mechanics; the Bohr Model is an approximation to quantum mechanics that has the virtue of being much simpler.

  • Louis de Broglie suggested that an electron moves with a wavelength that completes a circle, which would explain the different energy levels as Bohr’s model did but would also explain the wave diffraction pattern

  • James Chadwick discovered a previously unknown particle in the atomic nucleus. This particle became known as the neutron because of its lack of electric charge