J.J. Thomson Teacher Sheet

J.J. Thomson Teacher Sheet By Nobel foundation [Public domain], via Wikimedia Commons


Your students should continue their study of the history of the atom by reviewing what they have learned in the previous lessons in the series and studying a timeline. They should then move on to consider J.J. Thomson's work and his discovery of the electron. Use the questions and answers on this sheet to help guide students in their exploration.

The Discovery of the electron

Who was J.J. Thomson, and what was the nature of his work?
He was a British physicist who attempted to investigate the interior of atoms by experimenting with electrical currents inside glass tubes. More specifically, he sought to determine the mystery behind “cathode rays.” 

What did his experiments with cathode rays cause him to conclude?
He made the bold hypothesis that cathode rays were “streams of particles much smaller than atoms” and were, in fact, “miniscule pieces of atoms.” He called these particles “corpuscles”—later to be called “electrons”—and asserted that they comprise all matter within atoms and are “the substance from which all the chemical elements are built up.”

Briefly describe the experimentation process that led him to this conclusion.
In short, to prove that “atoms have parts,” Thomson and his team conducted three experiments—based on Jean Perrin’s work—where they attempted to bend cathode rays separating their electrical charge from the ray itself. In his first experiment, he determined that the rays have a negative charge and that both are stuck together. In the second, he succeeded in separating the charge from the ray using an electric field, a conductor, and by extracting all gas from the tube. From this, he concluded that cathode rays are “charges of negative electricity carried by particles of matter.” In his third experiment, he was able the calculate the m/e ratio of the particle, bringing him to the unbelievable conclusion that cathode rays were much smaller than atoms, and either had extremely high charges or were extremely light compared to their charge. Over the next two years, Philipp Lenard and others would confirm these findings that particles had much smaller masses than atoms.

How would you describe Thomson? In what sense does he represent the best of human scientific endeavor?
Accept all reasonable answers; among them, Thomson was a bold, adventurous, flexible, innovative, and astute scientist.

Now, years later, how have Thomson’s assertions turned out?
His daring assertions turned out to be correct, with the exception that other types of particles have since been discovered to exist within atoms.

What kind of impact did these findings about the inner workings of atoms have on technology?
The discovery of the electron helped to spur technological development—televisions and computers, in particular—and arguably triggered a fundamental shift bringing science and technology closer together.

What, in your opinion, is extraordinary about Thomson’s work?
Accept all reasonable answers. In short, Thomson’s adventurous work and proposals gave the world its first critical view inside the atom. The applications of his findings have gone on to improve the quality of life for the world through technological development.


This teacher sheet is a part of the The History of the Atom 4: J.J. Thomson lesson.

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