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Introducing Atoms Atomic Models • The Greeks were the first people to think about the nature of matter. • Democritus proposed the idea that matter is formed of small pieces that could not be cut into smaller parts. • Atomos (uncuttable) or Atoms is what he came up with. John Dalton • John Dalton: an English chemist, inferred that atoms had certain characteristics. • He came up with the Atomic Theory Atomic Theory • All elements are composed of atoms that cannot be divided. • All atoms of the same element are exactly alike and have the same mass. Atoms of different elements are different and have different masses. • An atom of one element cannot be changed into an atom of different elements. Atoms cannot be created or destroyed in any chemical change, only rearranged. Atomic Theory • Every compound is composed of atoms of different elements, combined in a specific ratio. JJ Thomson (1897) • Found that atoms contain negatively charged particles. • Scientists knew that atoms themselves had no electrical charge. • So, Thomas reasoned that atoms must also contain a positive charge. • Discovered the electron (-) f-3 P. 103 Earnest Rutherford (1911) • He was a student of JJ Thomas. • Found evidence that countered Thomson’s model. • Said that an atoms positive (+) charge must be clustered in a tiny region in the center called the nucleus. • Protons contain a positive (+) charge Bohr’s Model (1913) • Student of both Thomson and Rutherford. • His model said that electrons could have specific amounts of energy, leading them to move in certain orbits. • Resemble the solar system. Cloud of Electrons • In the 1920’s the atomic model changed again. • Scientists determined that electrons do not orbit the the nucleus like a planet. • Electrons can be found anywhere in a cloud like region around the nucleus. • An electrons movement is related to its energy level- specific amount of energy it has.