1. Chemistry 4.1
Section 4.1-2
Defining the Atom
Part II

2. Defining the Atom
4.1-2
Cathode-ray tubes are found in TVs, computer monitors, and many other devices with electronic displays.

3. Models of the Atom – II: Michael Faraday
4.1-2
Models of the Atom – II: Michael Faraday
1839: English Chemist Michael Faraday
Suggested that the structure of atoms was somehow related to electricity. He proved that atoms contain particles that have electrical charges.
Questions about electricity and charges on particles were answered in a few short years using a device called a Cathode Ray Tube or CRT.

4. Models of the Atom – II: Eugen Goldstein
4.1-2
Models of the Atom – II: Eugen Goldstein
1886: German Physicist Eugen Goldstein
In 1886, Eugen Goldstein observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays. He concluded that they were composed of positive particles.
Such positively charged subatomic particles are called protons. 4

5. Models of the Atom – II: JJ Thomson
4.1-2
Models of the Atom – II: JJ Thomson
1897: English Physicist J. J. Thomson
In 1897, the English physicist J.J. Thomson (1856–1940) discovered the electron using a cathode ray tube (CRT). Electrons are negatively charged subatomic particles. 5

6. Models of the Atom – II: J. J. Thomson
4.1-2
Models of the Atom – II: J. J. Thomson
Thomson performed experiments that involved passing electric current through gases at low pressure.
The result was a glowing beam, or cathode ray, that traveled from the cathode to the anode. 6

7. Models of the Atom – II: J. J. Thomson
4.1-2
Models of the Atom – II: J. J. Thomson
Cathode Ray Tube 7

8. Models of the Atom – II: J. J. Thomson
4.1-2
Models of the Atom – II: J. J. Thomson
A cathode ray is deflected by a magnet. 8

9. Models of the Atom – II: J. J. Thomson
4.1-2
Models of the Atom – II: J. J. Thomson
A cathode ray is deflected by electrically charged plates. 9

10. Models of the Atom – II: J. J. Thomson
4.1-2
Models of the Atom – II: J. J. Thomson
Using a CRT, Thomson discovered the following:
A cathode ray is a stream of electrons.
Electrons are parts of the atoms of all elements.
Both magnetic and electrical fields deflect the ray’s path in a CRT in a mathematically predictable way.
Changing the gas or the material that the plates were made of effected his results. 10

11. Models of the Atom – II: J. J. Thomson
4.1-2
Models of the Atom – II: J. J. Thomson
From this, Thomson proposed the following:
Cathode rays are made up of “-” charged particles.
Particles in a CRT came from the cathode plate.
Atoms are not invisible balls, but had structure.
Called these negative particles “Electrons”.
Calculated the ratio of the electron’s charge to its mass.
Developed the atomic model called “Plum Pudding”. 11

12. Models of the Atom – II: J. J. Thomson
4.1-2
Models of the Atom – II: J. J. Thomson
Thomson’s Plum Pudding Model:
J. J. Thomson’s model of the atom, he called “Plum Pudding” (that’s what it reminded him of) had a “sea of positive charge” called the nucleus with electrons scattered throughout the atom’s positively charged interior.
However, this model did not last very long. 12

13. Models of the Atom – II: Henri Becquerel
4.1-2
Models of the Atom – II: Henri Becquerel
1896: French Physicist Henri Becquerel
While working with Marie Curie and her husband, Pierre, accidentally discovered radiation. He accidentally placed a piece of potassium uranyl sulfate (radioactive material) on an unexposed film plate. He went to use the film plate, and when he developed the plate, all he saw was the outline of the material. 13

14. Models of the Atom – II: Henri Becquerel
4.1-2
Models of the Atom – II: Henri Becquerel
Because of his work, along with the work of Marie and Pierre Curie, all three received the Nobel Prize in Physics.
Photographic Plate showing Radioactivity 14

15. Models of the Atom – III: Robert Millikan
4.1-2
Models of the Atom – III: Robert Millikan
1909: American Physicist Robert Millikan
Performed the famous “Oil Drop” experiment in order to measure the electrical charge on an electron.
Unlike his colleagues before him, Robert Millikan was able to determine the charge of an electron using his famous “Oil Drop” experiment.

16. Models of the Atom – III: Robert Millikan
4.1-2
Models of the Atom – III: Robert Millikan
Robert Millikan’s Oil Drop Experiment 16

17. Models of the Atom – III: Robert Millikan
4.1-2
Models of the Atom – III: Robert Millikan
The oil drop experiment was an experiment performed by Robert Millikan and Harvey Fletcher in 1909 to measure the electrical charge of an electron.
The experiment entailed balancing the downward gravitational force with the upward bouyant and electric (charged) forces on tiny charged droplets of oil suspended between two metal electrically charged plates or electrodes.
By repeating the experiment for many droplets, they were able to confirm that the charges were all multiples of some fundamental value, and calculated it to be the charge of a single electron. 17

18. That’s All Folks ! !