1. Discovery of the Electron
Atomic Structure
Discovery of the Electron

2. What is the relationship between electrical charge and the structure of atoms?
Historical Background –
Early 1800’s matter is composed of atoms
19th century – what is the nature of electricity and magnetism?

3. What is the relationship between electrical charge and the structure of atoms?
Historical Background –
Early 1800’s matter is composed of atoms
19th century – what is the nature of electricity and magnetism?

4. Electricity and Electrical Charges
Link to Balloon Phet with charges
Matter is generally electrically neutral (no overall or net charge).
Like charges repel; opposites attract
Electrical charges are carried through certain objects (like Cu wire).

5. The Mysterious Cathode Ray
Link to intro clip
What could it be made of ?
Light?
Stream of Particles?

6. Link to video clip

7. What does it tell us if the cathode ray is deflected by a magnet?
Is it made of ordinary light or particles?
Ordinary light is not deflected by a magnet. Electrically charged particles are attracted to a magnet.

8. Thomson ~ First Experiment
Charged plates above and below; beam bent away from negative plate and towards the positive plate!
Conclusion: charge of particles in beam must be _____________.

9. Thomson ~ First Experiment
Charged plates above and below; beam bent away from negative plate and towards the positive plate!
Conclusion: charge of particles in beam must be NEGATIVE!

10. What do we know about cathode rays so far?
Particle, not ordinary light
Negatively Charged
What is a property that we could measure that could help identify a particle that is too small to see?

11. Rolling Spheres Demonstration

12. Rolling Spheres Demonstration

13. Rolling Spheres Demonstration

14. Rolling Spheres Demonstration

15. 1-5, #5
5) Which sphere was not deflected at all? Why did it not deflect?
6) Why did the other spheres deflect?
7) What determines the degree of deflection? Which spheres experience the greatest degree of deflection?
Plastic sphere. Plastic is not attracted to a magnet.
All of the other spheres were made of steel which is mostly made of iron. Iron is strongly attracted by a magnet.
The degree of deflection is determined by the mass. THE LIGHTER THE MASS THE GREATER THE DEFLECTION!

16. Rolling Spheres Demonstration
2nd Experiment – how does the mass of the cathode ray particle compare to the mass of a atom of the lightest known element?
Is the cathode ray particle an atom of an element? How do you know?

17. Thomson ~ Second Experiment
8) H is unique: it’s the lightest of all elements.
Cathode ray particles were deflected much more than the H+ ions.
Deflection indicates: cathode ray particles are about times lighter than H ions.
Conclusion: CR particle is a part of an atom!

18. Thomson ~ Third Experiment
Evacuated and filled tube with many elements: all generated cathode ray beams with identical properties.
Conclusion: all elements contain cathode ray particles
9) Today cathode rays are called ELECTRONS!

19. Thomson expt video
Summary experiment animation

20. 9) What direction will a stream of + charged particles bend toward + or – charged plates?

21. 10) What direction will a stream of + charged particles bend toward + or – charged plates?
Ans: towards - plate

22. 11) What subatomic particle was discovered by experiments similar to those described in question #10?
ANS: The proton.

23. 12) Draw the deflection of Na+1 ions with a mass of 23 and K+1 ions with a mass of 39.

24. 12) Draw the deflection of Na+1 ions with a mass of 23 and K+1 ions with a mass of 39.
ANS: Both have same charge, but Na+1 is lighter and therefore will be deflected more.
+ plate
Na +1
K+1
- plate

25. 12b) S-34 -2 vs O -16 -2 + plate - plate
Same charge but O is lighter, therefore deflected more
- plate

26. 12b) S vs O
+ plate
Same charge but O is lighter, therefore deflected more
- plate

27. 12C) Cr+2 vs Cr+5
+ plate
- plate

28. 12C) Cr+2 vs Cr+5
+ plate
Same mass: greater charge = greater deflection
- plate

29. 12D) 37Cl-1 vs 37K +1
+ plate
- plate

30. 12D) 37Cl-1 vs 37K +1
+ plate
Equal mass same deflection angle: opposite charges, opposite directions
- plate