1. Early Atomic Theory LG: I understand how the understanding of the atom has changed over time and the experiments that lead to these changes

2. 400 BCE - Greece Democritus – Matter is composed of tiny indivisible particles that are in constant motion and separated by empty space Aristotle – Proposed that all matter is composed of 4 essential elements: earth, air, fire, water (ideas stood for 2000 years)

3. Alchemy: 1 st Century – 17 th Century Alchemists searched for: – Elixir of life (immortality) – A process to turn common metals into gold – Developed a great deal of equipment, glassware, and procedures for various chemical processes still in use today – Also developed symbols for many elements

4. 1808 – John Dalton Dalton’s Atomic Theory:  All matter is made of tiny indivisible particles called atoms.  Atoms of the same element are identical, those of different atoms are different.  Atoms of different elements combine in whole number ratios to form compounds  Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

5. 1897 – J.J. Thomson Thomson questioned Dalton’s model because it did not explain how things acquired an electric charge. Experiments with a cathode-ray tube determine presence of electrons (couldn’t find positive part) “Plum-pudding” model was proposed Mmmm!

6. Thomson’s Experiment Voltage source +- Vacuum tube Metal Disks

7. Thomson’s Experiment Voltage source +-

8. Thomson’s Experiment Voltage source +-

9. Thomson’s Experiment Voltage source +-

10. n Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source +-

11. n Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source +-

12. n Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source +-

13. n Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment Voltage source +-

14. Thomson’s Experiment By adding an electric field

15. Voltage source Thomson’s Experiment n By adding an electric field + -

16. Voltage source Thomson’s Experiment n By adding an electric field + -

17. Voltage source Thomson’s Experiment n By adding an electric field + -

18. Voltage source Thomson’s Experiment n By adding an electric field + -

19. Voltage source Thomson’s Experiment n By adding an electric field + -

20. Voltage source Thomson’s Experiment n By adding an electric field he found that the moving pieces were negative + -

21. 1909 – Ernest Rutherford Believed in Thomson’s model He had previously discovered 3 types of radiation (α, β, γ) at McGill University Performed ‘gold-foil experiment’ to further investigate ‘alpha scattering’ and consequently discovered the “nucleus” Rutherford’s ‘nuclear model” was born

22. Gold Foil Experiment

23. 1932 – James Chadwick Performed many calculations to determine that energy possessed by protons would not be enough to keep electrons in orbit Proposed presence of neutral particle in the nucleus; a.k.a. “neutrons” Proposition was confirmed by experiments that produced neutral radiation

24. Homework Complete Reading for tomorrow’s class: Old Book: Section 1.3 New Book: Section 1.4