1. Foundations of the Atomic Theory

2. Don’t forget how we got here, thanks to Democ., Aristotle, and the Particle Theory of Matter

3. Chemistry in the late 1700s Element – a substance that can not be further broken down by chemical means. Law of conservation of mass – mass can neither be created nor destroyed. Law of definite proportions/law of constant composition - a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size or source of the sample.

4. Antoine Lavoisier A French noblemen Wealthy family in Paris…late 1700’s Lost his head! accused of selling watered -down tobacco, and of other crimes

5. Antoine Lavoiser Father of Modern chemistry Although matter may change its form from shape to shape it still retained the same mass. Law of ??? Extensive list of elements Recognized and named oxygen and hydrogen http://science.discovery.com/videos/100-greatest-discoveries- shorts-discovering-oxygen.html

6. Law of multiple proportions When two elements form a series of compounds, the ratios of the masses of the second element combined with 1 gram of the first element can be reduced to small whole numbers.

7. For Example… 1. Water mass ratio (H:O)= 1:8 2. Hydrogen peroxide Mass ratio (H:O)= 1:16 Mass ratio of second element = 8:16 Reduces to 1:2 …”small whole numbers”

8. Dalton’s Atomic Theory In the early 19 th century John Dalton (a school teacher) explained why the laws of conservation, definite proportions and multiple proportions were true by proposing that elements were composed of atoms

9. Atoms and molecules as shown by using Dalton’s new system of chemical philosophy

10. Dalton’s Atomic Theory 1.All matter is composed of extremely small particles called atoms. 2.Atoms of a given element are identical in terms of size mass and other properties… 3.Atoms cannot be subdivided, created or destroyed. 4.Atoms of different elements combine in simple whole-number ratios to form compounds 5.In chemical reactions, atoms are not subdivided, created, nor destroyed.

11. Correct? Are all the pieces of Dalton’s atomic theory correct today? Should any be modified?

12. Discovering the Atomic Structure JJ Thompson (1897) – Cathode ray tube experiments. Investigations into the relationship b/w electricity and matter

13. What are your observations? Can we use a magnet?? Diagram the CRT.

14. Thompson’s Observations 1.Object in tube between cathode and anode casts a shadow. 2.Paddle wheel spins and rolls along the rails (cathode to anode). 3.Cathode rays are deflected by magnetic fields (same as wire carrying electric current). 4.Cathode rays were deflected away from negative charge.

15. Thompson’s Conclusions The cathode rays are composed of particles found in all matter… These particles have ______ charge They were later named electrons.

16. Plum Pudding Model

17. How does the plum pudding model fit Thompson’s observations? Electrons are negatively charged. Electrons are held loosely by positive charged ‘pudding’.

18. Robert Millikan 1909 Oil drop experiment

19. Conclusions Measured the charge of an electron Charge = 1.60 x 10 -19 C Mass = 9.11 x 10 -28 g

20. Ernest Rutherford 1911 Shot alpha particles at thin gold foil. 2 Properties of Alpha particles –fast moving –Positively charged.

22. Let’s recreate Rutherford’s Gold Foil Experiment I need 6 volunteers What are your observations?

23. Observations Almost all of the alpha particles pass through the gold foil unaffected. Small number of particles deflected. VERY small number bounced back towards the source.

24. Conclusions Why do most alpha particles pass through? They don’t hit anything …Most of the volume of matter is empty space. Why are a few deflected or bounced back? There must be a small, dense, positively charged center – Nucleus (contains protons and neutrons)

25. How did our model of the atom change over time? (Late 1700’s – early 1900’s)

27. Rutherford’s Nuclear Atom

28. Consider the following: If the nucleus were the size of a basketball the electrons would be the size of grains of sand and scattered around the county. Matter is mostly empty space!!!! – Void Democ. was right!!!