Thomson's Atomic Theory

Did you know that older-style television sets use cathode ray tubes to create the image?



© Wikimedia Commons
B2.11 J. J. Thomson
The next key development in the atomic theory occurred in the late 1890s. British physicist J. J. Thomson was investigating cathode rays. Cathode rays are created in a cathode ray tube, which is a glass tube with almost all of the air removed. The tube also contains two pieces of metal, one at each end. When an electric current is applied to one of the pieces of metal, a ray or beam can be seen traveling through the tube. Initially, scientists did not understand what these rays were. To try to gain a better understanding of cathode rays, Thomson tested them by placing negative and positive plates along the sides of the cathode ray tube. The cathode ray was repelled by the negative plate and attracted by the positive plate. This indicated that the ray was composed of negatively charged particles. (Recall the law of charges; Opposite charges attract and like charges repel.) Thomson repeated his experiments using different metals and found that the properties of the cathode ray remained constant no matter what cathode material they originated from. He concluded that these subatomic particles must be found within atoms of all elements and that they are negatively charged.

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 B2.12 Cathode ray tube

  Did You Know?


B2.14 Scale

Thomson also measured the mass of the particles he identified. He did this by determining how much the cathode rays bent when he varied the voltage. He found that the mass of the particles was 2,000 times smaller than the mass of the smallest atom—the hydrogen atom.

This disproved Dalton’s theory that atoms are the smallest particles of matter. Thomson stated that the structure of an atom contained randomly distributed negative particles that he called corpuscles (later renamed electrons). Since he knew that atoms were neutral, he theorized that the remainder of the atom was a positively charged sphere. He called his atom “the plum pudding model.” In this analogy, the pudding part is the positive part of the atom and the embedded raisins are negatively charged electrons.
© Wikimedia Commons
 B2.13 Thomson model of the atom
© Wikimedia Commons
 B2.14 plum pudding

  Virtual Lab

Cathode Ray Tube © The Concord Consortium


Please review this simulation to help you visualize the experimental evidence that Thomson collected that helped him develop his plum pudding model of the atom.

Click on the procedure tab to continue.

  1. Click on the play icon to open the virtual lab.The lab can also be found at https://quick.adlc.ca/cathode
  2. Change “Adjust charge on horizontal plates” to “None.”
  3. Leave electrode material as “silver.”
  4. Check the “TURN ON” box to start the simulation.
  5. Select “Display beam.”
  6. Observe.
  7. Select “Display particles.”
  8. Change “Adjust the charge on horizontal plates” to “Very high +/–.”
  9. Which way do the particles move?

    Down, toward the positive plate
  10. What does this indicate about the charge on the particles?

    Must be negative if it is attracted to a positive charge.
  11. Select “Display beam.”
  12. Change “Adjust the charge on horizontal plates” to “Very high –/+.”
  13. Which way does the beam bend?

    Up, toward the positive plate
  14. What does this indicate about the charge of the beam?

    Must be negative if it is attracted to a positive charge.


  Read This

Please read pages 22 and 23 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on J. J. Thomson’s experiment and his atomic model. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. Describe the evidence that J. J. Thomson collected in the cathode ray tube experiment.

    Particles flew out of metal when a current was applied. When charged plates were brought close to this beam or ray of particles, they deflected toward the positive plate.
  2. Describe how Thomson interpreted the evidence he collected in the cathode ray tube experiment.

    There were two key interpretations.

    1. Smaller particles than atoms existed.

    2. The beam of particles that were emitted through the cathode ray tube bent toward the positive plate, and since opposite charges attract, the beam must consist of negative particles.
  3. Describe how Thomson modified Dalton’s model of the atom in light of the evidence gathered during the cathode ray tube experiment.

    The atom could no longer be a solid sphere; it had to contain subatomic particles that were negatively charged. To maintain the neutrality of the atom, the remainder of the atom must be positively charged.
  4. How would you describe J. J. Thomson’s model of the atom? What analogy would you use for J. J. Thomson’s model of the atom? Use your own words!