2. A good theory/model should :  Explain observed (empirical) evidence  Provide a reasonable, logical, theoretical explanation  Predict the results of experiments not yet performed Nature of Science: When observed evidence contradicts the theory:  The theory must be discarded or altered to take into account the new evidence

3. John Dalton’s BILLARD BALL MODEL 1808

4. Description: -All Matter consists of indivisible particles (called “atoms”) that can be combined in specific ratios to form compounds

5. Innovation of the model  Provided the first correct descriptive picture of an atom, without any experimental evidence  Dalton’s Model also explained Law of Conservation of Mass and Law of Multiple Proportions. Mass of water Mass of Hydrogen Mass of Oxygen % Hydrogen % Oxygen 31 g3.46927.5311.1988.81 8 g0.89517.10511.1988.81

6. Problems with the model as concluded by many cathode ray experiments:  There are smaller positive and negative charges exist in atoms Negative particles have less mass than positive particles Each negative particles always have the same charge and mass in every atom

7. Cathode Ray experiments led to the billard ball’s replacement:

8. J.J. Thomson’s PLUM PUDDING MODEL 1898

9. Description of Model  Atoms contain – charged particles called electrons (e - )  e - are scattered around in a positively charged sphere (like raisins in a bun)

10. Plum Pudding Explained  The presence of smaller, charged particles in atoms  the presence of e - which were identical in all types of atom  e - can be lost or gained

11. Thomson’s Innovation  Atoms can be subdivided  Atoms are formed from electrically charged particles  Electrons can be separated from atoms and are identical for all atoms

12. Problems with Thomson’s model suggested by Rutherford’s Gold Foil Experiment α (alpha) particle: a Helium nucleus that is often emitted by radioactive substances. An α (alpha) particle acts as a positively charged “bullet”

13. Rutherford’s Gold Foil Experiment Problem: If mass and positive charge are uniformly spread out in a gold atom (as in Thomson’s plum pudding model), what should all the particles do after striking the gold foil?

14. Experimental Results:  Most α particles when straight through the foil.  Surprise! A few α particles bounce back

15. Conclusion: -Mass and positive charge is concentrated into a tiny dense core; -Most of the atom is “empty space”

16. Ernest Rutherford’s NUCLEAR MODEL 1911

17. Rutherford Model Description  Most mass and all of the +ve charge of the atom is concentrated into a tiny core called the nucleus  Small, low mass, - charged e- orbits like a planet

18. Rutherford Model Explained  Why a few α particles deflected at large angles  They were headed close to the dense, positive core  Why most α particles passed straight through  They passed through empty space

19. Rutherford’s Innovation  There is a strong nuclear force that tightly binds the +ve charges in the nucleus together  The nuclear force overcomes the repulsive force of the +ve charges  This makes matter stable  e - occupy most space in an atom “like a few flies in a cathedral”

20. The problem with Rutherford’s model But in reality… most atoms are relatively stable! According to classical physics: -e in orbit around a nucleus continually accelerate in a curved path Thus, they should continuously emit electromagnetic radiation (photons) The e- then would lose energy, slow down and spiral into the nucleus in a very short time (i.e. atoms cannot exist)

21. Classical Physics gave Rutherford’s model a rough ride…

22. Max Planck’s Quantum Theory  According to Classical Physics, energy was thought to be continuous. (i.e. could be subdivided into smaller and smaller pieces, indefinitely)

23. Max Planck’s Quantum Theory  Planck proposed that instead, radiant energy emitted by atoms occurred in small bundles called a quantum (plural: quanta) A quanta of light energy is called a “photon”.

24. Max Planck’s Quantum Theory  Albert Einstein extended the idea and proposed that atoms could only absorb radiant energy as one or multiples of one quanta.

25. Max Planck’s Quantum Theory  The Quantum Theory revolutionized Classical Physics. A quanta is a very small amount of energy, so the “step-wise” emission or absorption of energy in macroscopic objects and the “quantum effect” is not noticeable.