1. Section 4.2 Defining the atom

2. The Atom What we know now: Definition: Atoms are the smallest particle of an element that retains the properties of that element.

3. Just how small is an atom? If atoms were the size of an orange, how big would an orange be?

4. New Model What we know: An atom is made of a nucleus containing protons and neutrons; electrons move around the nucleus. How have things changed since Dalton’s work?

5. Cathode Ray Tube AKA CRT Voltage source Cathode (-) Anode (+) - +

6. Cathode Ray Tubes Scientists discovered Cathode rays were a stream of charged particles Particles carry a negative charge Value of the charge was unknown Ray was produced no matter what material the cathode and anode were What can you conclude from the last point?

7. J. J. Thomson (1856 – 1940) Wanted to know more about electrons Electron – negatively charged particles found in all types of matter Experimented with CRTs to find the mass-to-charge ratio Found that the mass was much smaller than the mass of hydrogen What conclusion can we determine based on this finding? What did this change about Dalton’s theory?

8. Mass and Charge of the Electron J. J. Thomson determined the mass, but not the charge Robert Millikan (1868 – 1953) built on his work

9. Millikan’s Oil Drop Experiment What was the conclusion of these results?

10. Plum Pudding Model

11. The Nucleus Ernest Rutherford (1871 – 1937) Studied the interaction between charged alpha particles and solid matter

12. Rutherford’s Gold Foil Experiment: Expectations If you shot a cannon at a piece of tissue paper, what would you expect to happen?

13. Rutherford’s Gold Foil Experiment

14. Rutherford’s Gold Foil Experiment: Results Some deflected at a small angle Some deflected at a large angle Most passed through with little to no deflection What can we conclude from these results?

15. Rutherford’s Model of the Atom