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Atomic Theory The Atom Recall the atom is the smallest particle making up an element Evolution of the Atom Our ideas about the nature of atoms have progressed over the last two centuries (and continue to develop today). There are a few scientists who’s works have contributed to our current knowledge of the atom. JJ Thomson MODEL THOMSON (1900) DIAGRAM Year 1900 Discovered the electron Thomson proposed raisin bun model… Described the atom as positively charged sphere of matter with negative electrons dispersed throughout electrons=raisins, positive mass=dough Raisin Bun Ernest Rutherford MODEL RUTHERFORD (1910) DIAGRAM Year 1910 Rutherford proposed the nuclear model…. Described the atom as having a central positive nucleus (core) surrounded by negative orbiting electrons Nuclear Model Proof? Rutherford’s Famous Gold Foil Experiment A beam of positively charged alpha particles are aimed at a thin sheet of gold foil (source radioactive radium) Rutherford expected that the positive particles would pass through the sheet of gold foil since years earlier Thomson proposed that mass of the atom was evenly dispersed throughout the atom. MOST of the alpha particles did go straight through the gold foil with NO deflection. 1. However a small hand full of particles were deflected from original path. This could only happen if the the particles hit something that had substantial mass and size and then bounced off at an angle. Rutherford’s experiment PROVED the presence of a dense, positively charged core called the nucleus which was surrounded by mostly empty space and orbiting electrons. Niels Bohr MODEL BOHR (1911) DIAGRAM Year 1911 Bohr proposed the planetary model…. Nucleus (core) contains protons and neutrons but electrons are located in specific regions around the nucleus 1. Solar System Like planets are at a defined distance from Sun In each of the 3 boxes on the lower half of your sheet, make the following headings: location charge symbol mass Electron Location Charge Symbol Mass orbitals negative e- 1/1800 amu Proton Location Charge Symbol Mass nucleus positive p+ 1 amu 1 atomic mass unit (amu) = 1.67 x -24 g 1 amu = 0.00000000000000000000000000000167 g Proton Location Charge Symbol Mass nucleus neutron n0 1 amu 1 atomic mass unit (amu) = 1.67 x -24 g
