1. Chapter 3 Experiments of Thomson and Millikan
Mr. Mills
Chemistry

2. Charges: Pre-Assessment
What is the charge of an electron?
What is the charge of a proton?
Do electrons attract or repel other electrons?
Do protons attract or repel other protons?
Do electrons attract or repel protons?
What is the charge of a neutron? Does it attract or repel any subatomic particle?

3. Atoms J.J. Thomson Studied electrical discharges in cathode-ray tubes
When voltage was applied to the tube, a cathode ray was produced
Cathode ray came from the negative electrode – an ‘electron’ gun

4. Atoms

5. Atoms
Cathode ray was repelled by the negative pole of an applied electric field
Concluded the ray was a stream of negatively charged particles.
Thomson measured the deflection of the beam of electrons in a magnetic field to determine the charge-to-mass ratio of an electron

6. Atoms Charge-to-mass ratio of an electron:
e is the charge of an electron in coulombs (C) and m represents electron mass in grams

7. Atoms
Thomson reasoned that electrons could be produced from electrodes made of various types of metals
Therefore, ALL atoms must contain electrons
Since atoms are electrically neutral, Thomson hypothesized that atoms must contain some positive charge

8. Atoms
Thomson concluded that an atom consisted of a cloud of positive charge with negatively charged electrons embedded randomly in it
“plum pudding” model
“chocolate-chip cookie” model

9. Check for Understanding
Do like charges attract or repel?
Do opposite charges attract or repel?
Who was the first scientist to show that atoms have negatively charged particles?
Does the cathode ray travel from cathode to anode or from anode to cathode?
What pole does the cathode ray bend toward in the electromagnetic field, the positive pole or the negative pole?
What is the charge of the cathode ray?
What ratio did Thomson determine using his data?
What is the name of the model of the atom that Thomson developed?
In Thomson’s model of the atom, ? particles are embedded in a “sea” of ? charge.
Individual units of ? charge were not discovered until later and were not included in Thomson’s model of the atom.

10. Atoms
Robert Millikan performed experiments involving charged oil drops
Allowed him to determine the size of the electron charge
Electron charge = x C
C = coulomb (unit of charge)

11. 9.11 x 10-31 kilograms Atoms (itty bitty)
With the magnitude of the electron charge and the charge-to-mass ratio determined by Thomson, Millikan calculated the mass of the electron
9.11 x kilograms
(itty bitty)

12. Radioactivity Uranium (U) Spontaneous emission of radiation
3 types of radioactive emissions
 - carries 2+ charge (2 protons)
 - high speed electron
 - high energy light

13. Rutherford’s Experiment
Ernest Rutherford tested Thomson’s plum pudding model
-particles were shot at a thin sheet of gold foil
If Thomson’s model was correct, -particles should go straight through the atoms due to their low density

15. Rutherford’s Experiment
Although most passed straight through, a few particles bounced off at large angles
Some went back to the alpha particle gun and never hit the detector
Atoms cannot be of uniform density
Plum pudding model disproved

16. Atoms

17. Rutherford’s Conclusion
Rutherford concluded that an atom has a nucleus
The nucleus is dense, small, and positively charged with electrons moving around it
Electrons occupy a large amount of space around the nucleus

18. Check for Understanding
What did Millikan use to conduct his experiment?
What did Millikan determine to be the electron’s charge?
What did Millikan determine to be the mass of an electron?
What type of radioactive emission did Rutherford use in his experiment?
What did Rutherford shoot at gold foil?
What surrounded Rutherford’s piece of gold foil?
What did most of the radioactive particles do when they hit the gold foil and why did they do this?
What did a very few of the radioactive particles do when they hit the gold foil and why did they do this?
What did Rutherford discover about the atom from his famous experiment?
Did Rutherford depend on direct or indirect evidence to make his discovery?