1. Ch. 3-2 Structure of the Nuclear Atom
Atomic Structure

2. POINT > Explain Thomson’s Cathode Ray Tube experiments: the discovery of electrons
POINT > Identify the rationale for inferring protons
POINT > Explain Rutherford’s Gold Foil experiments: the discovery of the nucleus
POINT > Describe atomic size

3. Models through Time…(so far)
Dalton

4. POINT > Explain Thomson’s Cathode Ray Tube experiments: the discovery of electrons
J.J. Thomson (1856 – 1940)
Cathode Ray Tube (CRT) experiment
Cathode ray: a stream of negatively charged particles that move from cathode (-) to anode (+) due to an applied voltage

5. POINT > Explain Thomson’s Cathode Ray Tube experiments: the discovery of electrons
The ray travels from (-) to (+), and is bent by a magnetic or electric field

6. Cathode Ray Tube experiment
POINT > Explain Thomson’s Cathode Ray Tube experiments: the discovery of electrons
Cathode Ray Tube experiment

7. POINT > Explain Thomson’s Cathode Ray Tube experiments: the discovery of electrons
Cathode Ray Tube Experiment: 1. All types of metal electrodes produced the same results 2. The particles carried a negative charge 3. The particles were much smaller than any atom So… negative charged, sub-atomic particles had been discovered These were named electrons

8. POINT > Explain Thomson’s Cathode Ray Tube experiments: the discovery of electrons
Electrons were the first subatomic particles discovered Negatively charged Mass of an electron was found to be 1/1840 the mass of a hydrogen atom

9. WB CHECK:
What is the charge of the particles in a cathode ray?
What are the particles in a cathode ray?

10. WB CHECK: Compared to hydrogen atom, the mass of an electron is
Almost identical
Much larger
Much smaller
Still unknown

11. Models through Time…(so far)
Thomson’s “Plum pudding” model: electrons embedded in a mass of positive charge

12. POINT > Explain Rutherford’s Gold Foil experiments: the discovery of the nucleus
Ernest Rutherford (1871 – 1937)
Gold-Foil experiment: Used (+) charged alpha particle beam

13. POINT > Explain Rutherford’s Gold Foil experiments: the discovery of the nucleus
The plum pudding model predicts that the alpha particles would all go straight through the foil Instead, a small fraction were deflected backwards, indicating a sizeable mass of positive charge

15. POINT > Explain Rutherford’s Gold Foil experiments: the discovery of the nucleus
Gold-Foil Experiment interpretation:
All the positive charge, and most of an atom’s mass, is concentrated in a small region: the nucleus

16. POINT > Explain Rutherford’s Gold Foil experiments: the discovery of the nucleus

17. POINT > Identify the rationale for inferring protons
Atoms are electrically neutral. If e- are negatively charged, there had to be positively charged particles to balance the electron charge: the proton
Proton mass was found to be 1840 times greater than an electron

18. Neutrons were later deduced as subatomic particles with no charge
POINT > Identify the rationale for inferring protons
Neutrons were later deduced as subatomic particles with no charge
Neutron mass was found to be nearly equal to that of a proton

19. WB CHECK: Compared to a neutron, the mass of an electron is
almost identical
much larger
much smaller
still unknown

20. WB CHECK: Compared to a neutron, the mass of a proton is
almost identical
much larger
much smaller
still unknown

21. Particle
Symbol
Relative Charge
Relative Mass
Actual Mass (g)
Electron e- 1-
1/1840
9.11 x 10-28
Proton p+ 1+ 1
1.67 x 1o-24
Neutron n0
1.67 x 10-24

22. POINT > Explain Rutherford’s Gold Foil experiments: the discovery of the nucleus
Rutherford’s model of the atom replaced Plum Pudding

23. POINT > Explain Rutherford’s Gold Foil experiments: the discovery of the nucleus
Rutherford’s model:
1. Atoms are mostly empty space
2. Protons & neutrons form the nucleus: a tiny central core containing most of the mass:
3. Electrons are distributed around the nucleus, taking up most of the volume

24. WB CHECK:
What is the charge on an alpha particle?
Most of the volume of an atom is
a) taken up by the nucleus
b) positively charged
c) empty space with tiny electrons
d) water

25. WB CHECK:
The deflection of alpha particles in the Gold Foil experiment was caused by
collision with a cluster of positively charged protons
interactions with other electrons
collisions with dispersed protons
collisions with neutrons

26. Models through Time…(so far)
Rutherford

27. POINT > Describe atomic size
Most atoms have a radius ranging from 5 x m to 2 x m
A penny-sized amount of copper has 2.4 x 1022 atoms
(To contrast, human population just passed 7 x 109)

28. Homework: Read pages 68-72 F. A
Homework: Read pages F.A. #1-5 page 72 Ready for Quiz at any time!