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History of the Atom

History of the Atom. The beginning of time (400 BC)

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History of the Atom

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  1. History of the Atom • The beginning of time (400 BC) • Democritus stated that matter was composed of small particles called "atoms" that could be divided no further. These atoms differed by their size, shape and weight; thud resulting in different properties. Unfortunately, his ideas were largely ignored for the next 2000 years.

  2. 1803 • John Dalton stated the following • Matter is made of tiny particles called atoms • All atoms of a given element are identical • Atoms of one element are different from atoms of another • Atoms cannot be created, divided, or destroyed • Different atoms combine in whole number ratios to form compounds.

  3. 1890’s • JJ Thomson states that atoms contain a particle called the electron

  4. 1909 • Robert Millikan determined that the electron was negatively charged. • Thomson then states that atoms are made of positive “matter” with evenly distributed electrons. Creating the plum pudding model.

  5. 1911 • Ernest Rutherford • Shot alpha particles at a piece of gold foil to find that while most went straight through some where slightly thrown off course and a few bounced back. • Concluded that atoms are mostly empty space with a dense center called a nucleus

  6. 1913 • James Chadwick found the neutron in the nucleus

  7. 1932 • Niels Bohr created the quantum model of the atom that proposed electrons occupied orbitals.

  8. Today • Nucleus with protons and neutrons with an electron cloud sectioned into energy levels, shells, and subshells where electrons are located.

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