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History of Atomic Theory Review of In-Class Research Tuesday, September 2015 th 15 , Ancient Philosophy Who: Aristotle, Democritus, Leucippus When: More than 2000 years agoears ago Where: Greece What: Aristotle believed in 4 elements: Earth, Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atomos”. Why: Aristotle and Democritus used observation and inference to explain the existence of everything. What did an Atom Look Like to the Greeks (Democritus)? This was Democritus’ atomic model. It was simply a round sphere with no electrons, protons, or neutrons. Democritus created the first atomic model. His contribution helped people to understand the idea of an atom, then other scientists looked further into the science of the atom and its general makeup. Dalton’s Particle Theory of Matter • • • • Who: John Dalton When: 1808 Where: England What: Described atoms as tiny particles that could not be divided. Thought each element was made of its own kind of atom. • Why: Building on the ideas of Democritus in ancient Greece. What did an Atom look like to Dalton? Discovery of Electrons • Who: J. J. Thomson • When: 1897 • Where: England • What: Thomson discovered that electrons were smaller particles of an atom and were negatively charged. • Why: Thomson knew atoms were neutrally charged, but couldn’t find the positive particle. Thomson’s Cathode Ray Tube Understanding Thomson’s Experiment What did an Atom look like to Thomson? Bunch of negative charged particles (raisins) in a sphere of positive charge (plum pudding) Discovery of Nucleus • • • • Who: Ernest Rutherford When: 1911 Where: England What: Conducted an experiment that resulted in discovery of atom’s nucleus. Decided that the atoms were mostly empty space, but had a dense central core. • Why: He knew that atoms had positive and negative particles, but could not decide how they were arranged. Rutherford’s Gold Foil Experiment What did an Atom Look Like to Rutherford? Bohr’s Model of the Atom • • • • Who: Niels Bohr When: 1913 Where: England What: Proposed that electrons traveled in fixed paths around the nucleus. Scientists still use the Bohr model. • Why: Bohr was trying to show why each element emitted its own unique frequency of energy as electrons jumped orbits. Bohr’s Model of the Atom - The Electron Cloud Our Current Model - Bohr said electrons orbit the nucleus (like the planets around the Sun) - The electron cloud model says that we can't know exactly where an electron is at any given time, but the electrons are more likely to be in specific areas. - The Electron Cloud Our Current Model - Bohr assigned electrons to different shells - Using quantum mechanics, the electron cloud model assigns electrons to different atomic orbitals. ** These are not all spheres For the quiz/test • You should know the main scientists and their experiments/contributions. • You should be able to draw their models of the atom and explain differences.