Using the process of science to discover the properties of electrons.

1. Process of Science
How did scientists discover the
properties of electrons?
Dr. Mark A. McGinley
Professor, Head of Science Unit
Core Curriculum and General Education Office
Lingnan University

2. How Did They Learn About the Existence and
Characteristics of Electrons?
• Need to understand the level of knowledge at that time

3. John Dalton
• English (1766 – 1844)
• Teacher and public lecturer
• Interested in Meteorology
• Kept detailed weather records
• Described “color blindness”
• Which he had
• Developed a theory about the
structure of atoms from studying
behavior of gasses
http://www.chemheritage.org/discover/online-resources/chemistry-in-history/themes/the-path-to-the-periodic-table/dalton.aspx

4. Dalton’s Atomic Laws
• 1) All matter is made of atoms.
• 2) All atoms of a given element are identical in mass and properties
• 3) Atoms of different elements have different masses
• 4) Atoms only combine in small whole number ratios such as 1:1, 1:2, 2:3. etc.
http://www.chemteam.info/AtomicStructure/Dalton.html

5. J J Thomson
• English Chemist- 1897
• His work with cathode ray tubes
led to his discovery and
characterization of electrons

6. Cathode Ray Tube
• Glass tube with gas removed
(vacuum)
• Turn on electrical charge
• Creates negatively charge cathode
and positively charge anode
• When there was a hole in the anode
he noticed that something was
moving from cathode towards the
anode
• Detected because the screen
fluoresced (light up) when something
struck it
• Moving objects known as cathode
rays
• Cathode ray tubes used in old tvs
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7. J.J. Thomson
• Proposed that what was moving
in the cathode ray tube were
particles that broke off of atoms
in the cathode and moved
towards the anode
• Called these particles
“corpuscles”
• Objections to this idea
• At that time most scientists
followed Dalton’s idea that atoms
were indivisible particles.
• Other scientists thought that
cathode rays were not particles
• “ethereal disturbances”
• Previous experiments had shown
that cathode rays could pass
through thin strips of gold so they
could not be particles

8. Do These Particles Have a Charge?
• Now we can apply the scientific method.
• First we need to generate hypotheses
• (only 4 possibilities…. I think)

9. Do These Particles Have a Charge?
• Now we can apply the scientific method.
• Generate hypotheses
• Hypothesis 1. All particles have no charge
• Hypothesis 2. All particles are negatively charged
• Hypothesis 3. All particles are positively charged
• Hypothesis 4. Some particles are positively charged and others are negatively
charged

10. Thomson’s Experiment
• Passed cathode rays through an
a region surrounded by an
electric field that had a positive
charge on one side and a
negative charge on the other
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11. Time to Make Predictions
• One at a time we will imagine that each of the hypotheses are true
and ask “what will happen” if the particles described in that
hypothesis pass through a region where one side of the electric field
is positive and the other is negative
• Why should the charge of a particle passing through an electrical field
matter?
• Try to draw your predictions on a diagram that looks like this-
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12. Process of Science
• Now all we have to do is to compare the actual observations during
the experiment with your predictions
• If then the observations are different from the prediction of a
particular hypothesis then we can conclude that hypothesis is wrong
• If the observation is the same as predicted by only one hypothesis
then we can conclude that hypothesis is possibly correct

13. Question for you-
• What do we conclude and what do we do if the observations are
consistent with more than one of the hypotheses?

14. Thomson’s Observations
• What do you conclude based on
these results?

15. Thomson’s Observations
• Because the particles travelled
away from the negative side and
towards the positive side then
Thomson concluded that the
particles were negatively
charged.

16. Local Perspective
• Most of the scientists we will be discussing when exploring the
history of chemistry are from Europe because that was the center of
scientific discovery at that time.
• That does not mean that there was a long scientific tradition in Asian
and the Middle East
• To learn more about the history of science in China you can check
out-
• https://en.wikipedia.org/wiki/History_of_science_and_technology_in_China
• http://www.crystalinks.com/chinascience.html
• http://www.ch.ic.ac.uk/local/projects/gondhia/history.html

17. Further Reading and Viewing
• http://www-
outreach.phy.cam.ac.uk/camphy/electron/electron1_1.htm
• http://padakshep.org/otp/subjects/chemistry/physical-
chemistry/discovery-of-electrons-protons-and-neutrons/
• https://www.youtube.com/watch?v=Rb6MguN0Uj4
• https://www.youtube.com/watch?v=kBgIMRV895w

18. Intended Learning Outcomes
• 1. Discuss the contributions of Dalton and Thomson on the
development of our understanding of the atom and atomic structure.
• 2. Briefly define “cathode rays” and the functioning of a cathode ray
tube.
• 3. Describe the thought process and experiments that led Thomson to
conclude that electrons were negatively charged.
• - you should be able to explain this to a friend, roommate, etc. who is not in
this class.