How Did Scientists discover the properties of electrons
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J.J. Thomson discovered electrons through his experiments with cathode ray tubes in 1897. He observed that something was moving from the cathode to the anode, even through a vacuum. To determine if these particles carried an electrical charge, Thomson passed cathode rays through an electric field. He found that the particles were deflected towards the positive plate, indicating they were negatively charged particles. This led Thomson to conclude that the particles, which he named "corpuscles," were subatomic constituents within atoms that could be broken off, contradicting prevailing theories that atoms were indivisible. His discovery of electrons and their negative charge was a major breakthrough in understanding atomic structure.
How Did Scientists 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 - +
- 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 + -
- 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- + -
- 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.