Atomic Theory and Structure The Philosophers

1. Atomic Theory and StructureThe Philosophers Democritus = atoms Aristotle = earth, air, fire, water

2. John Dalton • Father of the modern atomic theory • Published the first modern atomic theory in 1803 • Analyzed the results of many experiments to… • Explain what atoms were • Explain why they had to exist • Debunk the earth/air/fire/water idea • Thought of the atom as a small, indivisible sphere

3. Dalton’s Postulates (1803) • Elements are composed of atoms, which are very small, indivisible, indestructible particles. • All atoms of the same element are identical. • Atoms of different elements have different properties. • Atoms combine in small, whole-number ratios to form compounds. • Atoms of one element cannot change into an atom of a different element during a chemical reaction.

4. Cathode Rays – Waves or Particles? No electric or magnetic field Magnetic Field applied (+ on top) Electric Field applied (+ on top; only works for particles)

5. J. J. Thomson and the Electron • 1897 – discovers the negatively charged electron through experiments with cathode ray tubes • 1904 – published his atomic theory, now known as the “Plum Pudding” Model. He thought electrons moved randomly in a positively charged, amorphous “pudding” or cloud.

6. Ernest Rutherford and the Gold Foil Experiment

7. Rutherford’s Atomic Model • 1909 – Gold Foil Experiment performed • 1911 – Rutherford’s Atomic Theory published: • The atom has a dense, positively charged core which contains most of the mass. • Most of the atom is empty space. • The electrons are somehow moving around in the empty space, possibly like planets around the sun.

8. Niels Bohr and the Bohr Model • A.k.a. the Planetary Model • States that electrons circle the nucleus in specific, stationary orbits, much like planets around the sun.

9. Other Discoveries • 1927 – Erwin Schrödinger showed that a special mathematical equation could be used to predict the area in which an electron could be found • 1932 – James Chadwick discovers the neutron, a neutral particle in the nucleus

10. Modern Atomic Theory • aka Wave Mechanical Model or Quantum Mechanical Model • Protons and neutrons are in the nucleus, which is in the center of the atom and contains most of the mass. • Electrons are in the electron cloud, which surround the nucleus. Electrons move randomly in specific areas called “orbitals.” The shapes of the orbitals are predicted by the Schrödinger Equation.

11. Modern Atomic Theory Pics Electron CloudOrbital Shapes

12. Summary of Subatomic Particles