Quantum Theory, Part 1, Day 2

1. Quantum Theory, Part 1, Day 2

2. Is There Something Inside of the Atom?!?!?

3. Cathode Rays • Cathode rays are the carriers of electric current from cathode to anode inside a vacuumed tube. • Cathode rays have the following characteristics: • Emit from the cathode when electricity is passed through an evacuated tube. • Emit in a direction perpendicular to the cathode surface. • Travel in straight lines. • Cause glass and other materials to fluoresce. • Deflect in a magnetic field similarly to negatively charged particles.

4. Crookes Tube William Crookes Crookes tube (Cathode ray tube) Glow Cathode (-) Anode (+) Mask holder Mask holder

5. High voltage The Effect of an Obstruction on Cathode Rays shadow source of high voltage cathode yellow-green fluorescence

6. Joseph John Thomson 1897 J. J. Thomson • English • Credited with the discovery of the electron. • His model of the atom featured negatively charged electrons embedded in a ball of positive charge. • “Plum-Pudding” model • Awarded the Nobel prize in 1909 for calculating the charge/mass ratio of the electron. • me /e • The value is determined to be –5.686 X 10-12 kg/C

7. Thomson’s Model

8. Source of Electrical Potential Stream of negative particles (electrons) Metal Plate Gas-filled glass tube Metal plate A Cathode Ray Tube

9. Thomson’s Experiment voltage source - + Cathode Anode vacuum tube metal disks

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

11. + - Thomson’s Experiment voltage source ON - OFF + By adding an electric field… he found that the moving pieces were negative.

12. magnet Crooke’s Tube voltage source William Crookes - + vacuum tube metal disks

13. Cathode Ray Experiment - Displacement Volts Anodes / collimators Cathode + Deflection region Drift region

14. Conclusions • He compared the value with the mass/charge ratio for the lightest charged particle. • By comparison, Thomson estimated that the cathode ray particle weighed 1/1000 as much as hydrogen, the lightest atom. • He concluded that atoms do contain subatomic particles - atoms are divisible into smaller particles. • This conclusion contradicted Dalton’s postulate and was not widely accepted by fellow physicists and chemists of his day. • Since any electrode material produces an identical ray, cathode ray particles are present in all types of matter - a universal negatively charged subatomic particle later named the electron.

15. J.J. Thomson • He proved that atoms of any element can be made to emit tiny negative particles. • From this he concluded that ALL atoms must contain these negative particles. • He knew that atoms did not have a net negative charge and so there must be balancing the negative charge. J.J. Thomson

16. Lord Rutherford’s Gold Foil Experiment (1909) • Worked with Bohr, Geiger, and Marsden in order to prove Thomson’s model. • Used Polonium to produce alpha particles (He+2). • Aimed alpha particles at gold foil by drilling hole in lead block. • Since the mass is evenly distributed in gold atoms, alpha particles should go straight through. • Used gold foil because it could be made only a few atoms thick.

17. Rutherford’s Apparatus beam of alpha particles radioactive substance fluorescent screen circular - ZnS coated gold foil

18. Fluorescent Screen, ZnS Po Pbblock Au Foil

19. What he expected…

20. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Because, he thought the mass was evenly distributed in the atom Alone, the subatomic particles were not enough to stop the alpha particles.

21. What he got… richocheting alpha particles

22. The Predicted Result: expected path expected marks on screen Observed Result: mark on screen likely alpha particle path

23. Atom is mostly empty space. Small dense, positive pieceat center (core). Contains most of the atom’s mass. Alpha particles are deflected by it if they get close enough. Disproved Thomson’s Model This is called the Nuclear Model + Rutherford’s Model of the Atom How he explained it:

24. +

25. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper, and it came back to hit you. • "On consideration, I realized that this scattering backwards must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive center carrying a charge."

26. Scale of the atom. While an atom is tiny, the nucleus is ten thousand times smaller than the atom and the quarks and electrons are at least ten thousand times smaller than that. We don't know exactly how small quarks and electrons are; they are definitely smaller than 10-18 meters, and they might literally be points, but we do not know. It is also possible that quarks and electrons are not fundamental after all, and will turn out to be made up of other, more fundamental particles. (Oh, will this madness ever end?) Website “The Particle Adventure”

27. Problem with Rutherford’s Model?!?! • We know that unlike charges attract (i.e. positive and negative), therefore what should happen between the positive nucleus and negative electrons?

28. Robert Millikan’s Oil Drop Experiment (1911) • American • Measured the charge of an electron

29. Millikan’s Experiment (1911)

30. Telescope Oil Drop Experiment oil droplets Robert Millikan (1909) . . . . . . . . . . . . . . . . . . . . . . . . Charged plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Atomizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . + Oil Small hole oil droplet under observation - Charged plate X-rays give oil drops a charge, by transferring electrons to them from the air Mass was calculated using charge to mass ratio (9.1093 x 10-28 g). Balancing electrical and gravitational forces allowed the electron charge to be determined.

31. The charge of each drop was always a multiple of the same smaller charge. 1.60 x 10-19 C