1. Module 7

1.7. Page 5

Lesson 1: Page 5

Module 7—Investigating the Nature of the Atom

 

Lesson Summary

 

In this lesson you focused on the following questions: 

  • How did the cathode ray contribute to the development of atomic models?
  • How did J. J. Thomson determine the charge-to-mass ratio of an electron?
  • How did Thomson’s experiment contribute to science and technology?

Early work with vacuum tubes and electric potential led to the discovery of the cathode ray, which served as a vehicle for investigations into the nature of the particles that produced it. Experimentation and observations of cathode rays indicated that they were negatively charged particles capable of being deflected by magnetic and electric fields, and that they possessed the particle characteristics of kinetic energy and momentum.

 

Using the cathode ray, J. J. Thomson determined the charge-to-mass ratio of the particles by first measuring their speed with perpendicular electric and magnetic fields, and then by using only a magnetic field to produce uniform circular motion. Knowing the strength of the magnetic field, the velocity of the particles and the radius of the circular path they followed in the magnetic field, Thomson concluded that the unique charge-to-mass ratio for all cathode ray particles is 1.76 × 1011 C/kg, a ratio thousands of times larger than that for other common particles, such as the hydrogen ion.

 

Thomson’s discovery of the electron’s charge-to-mass ratio meant that the atom was indeed divisible into much smaller parts. Using this idea, he proposed the raisin-bun model of the atom in which negative charges are embedded in a blob of positive charge. Although this model is now known to be incorrect, it was the first model that identified positive and negative charges as divisible parts of the atom.

 

The concepts and theories used in Thomson’s original experiment are now commonly applied in mass spectrometer technology. This technology can be used to identify unknown chemicals by comparing the unique charge-to-mass ratio of the unknown to the charge-to-mass ratio of other known compounds.

 

Lesson Glossary

 

cathode ray: a free electron emitted by a negative electrode in a low-pressure environment