Development of Atomic Theory

1. Development of Atomic Theory The Scientific Method in Action

2. Democritus vs. Aristotle • Matter is composed of discrete particles. Democritus named these “atoms”. Matter is continuous. You can break it into smaller and smaller particles indefinitly.

3. John Dalton - Early 1800’s • Conservation of Matter • Mass is conserved in a reaction. The amount you begin with is equal to the amount you end up with.

4. John Dalton - Early 1800’s • Conservation of Matter • Law of Definite Proportions • Types of Matter combine in given ratios. For example: H2 + O2 -------> H2O 100g + 793g ----> 893g 50g + 793g ----> 445g Water

5. John Dalton - Early 1800’s • Conservation of Matter • Law of Definite Proportions • Law of Multiple Proportions Matter combines in Whole Number Ratios C + O2 -------> CO2 2C + O2 --------> CO

6. Dalton’s TheoryThe “Pool Ball” Model • All matter is made up of atoms. • Atoms are tiny, indivisible, indestructible, fundamental particles. • Atoms cannot be created or destroyed. • Atoms of a particular element are alike. • Atoms of different elements are different. • A chemical change involves the union or separation of individual atoms.

7. Problems with Pool Ball Model • Doesn’t explain bonding. • Doesn’t explain ions Michael Farraday demonstrated that some matter can be charged or can carry a charge.

8. Michael Farraday • When certain substances are dissolved in water, they conduct electricity. • Certain compounds decompose into their elements when current is passed through them. Atoms of some elements are then attracted to the negative electrode, others to the positive electrode.

9. J. J. Thompson’s “Plum Pudding” Model • Cathode Ray Tube Experiments • A charge was applied across a tube filled with various gases or a vacuum. Tiny charges flowed out of the cathode (-) and traveled towards the anode(+). These particles were negatively charged. Where were they coming from?

10. J.J.Thompson’s “Plum Pudding” Model • Tiny, negatively charged particles which Thompson called “corpuscles” and are now known as electrons were imbedded in a positive mass, like raisins in a plum pudding. - - - - - - - -

11. Rutherford’s Scattering Experiment • Rutherford shot alpha particles (+ charged, 7000x the mass of an electron) at gold foil. • Prediction - They would go straight through. • Results - The particles were scattered, sometimes at wide angles.

12. Rutherford wrote - • “-----Scattered! It was as if you fired a 15 inch shell at a piece of tissue paper, and the shell bounced back and hit you.”

13. Rutherford’s Planetary Model • Something small and massive inside the atom. • Rutherford called it the “nucleus” • Nucleus contains positive protons • Negative electrons orbit around

14. The Neutron • Discovered by James Chadwick in 1932 • About the same mass as a proton • No charge (neutral) + Proton Neutron Electron(-)

15. Line Spectra of Elements • Gas discharge tubes filled with gas of one element produce light. • Light contains discrete lines

16. Bohr’s Planetary Model • Electrons exist in quantized orbitals • Certain wavelengths of light result when electrons jump or fall from one level to another.

17. Quantum Mechanic Model Electrons can be written as an equation Solving equation gives orbitals These orbitals are areas where electrons can exist