Charge To Mass Ratio Of Electron


The history of the atomic structure and quantum mechanics dates back to the times of Democritus, the man who first proposed that matter is composed of atoms. These theories could not gain much importance due to the lack of technology. The experiments conducted during the nineteenth century and early twentieth century revealed that even an atom is not the ultimate particle. The continued efforts of the scientists led to the discovery of subatomic particles like electrons, protons, and neutrons.

In the nineteenth century, J.J Thomson proposed Thomson’s Atomic Model discovered the electron to mark inception to the world of subatomic particles. Once the electron was discovered, he continued his experiments to calculate the charge and the mass of the electron. With the help of his experiments, he derived a formula for the calculation of charge to mass ratio of the electron.

Experimental setup for the determination of charge to mass ratio of electron

charge to mass ratio of electron

Diagramatic representation of charge to mass ratio of electron

While carrying out the discharge tube experiment, Thomson observed that the particles of the cathode deviate from their path. He noticed the amount of deviation in the presence of an electrical or magnetic field depends on various related parameters. They are:

  1. Particles with a greater magnitude of the charge experienced greater interaction with the electric or magnetic field. Thus, they exhibited greater deflection.
  2. Lighter particle experienced greater deflection. Thus, deflection is inversely proportional to the mass of the particle.
  3. Deflection of the particle from their path is directly proportional to the strength of the electrical and the magnetic field present.

Let us now understand this with the help of his experimental observations.

  • The electrons deviated from their path and hit the cathode ray tube at point ‘x’ in the presence of a lone electric field.
  • Similarly, electron strikes the discharge tube at point ‘z’ when only the magnetic field was present.
  • Thus, to make electrons continue on the same path we need to balance the electric and magnetic field acting on them.
  • Finally, based on the deflection of the electron, Thomson calculated the value of charge to mass ratio of the electron.

Charge to mass ratio of electron

The charge to mass ratio of the electron is given by :

e/m = 1.758820 × 1011 C/kg

m = mass of an electron in kg = 9.10938356 × 10-31 kilograms.
e = magnitude of the charge of an electron in coulombs = 1.602 x 10-19 coulombs.

To learn more about the charge to mass ratio of the electron, J J Thomsons Atomic Model and its limitations and more topics of chemistry, register with BYJU’S.

Frequently asked questions

1. Which scientist discovered charge to mass ratio of an electron?

Ans: J. J. Thomson

2. In which year was the charge to mass ratio of an electron discovered?

Ans: In the year 1897 charge to mass ratio of an electron was discovered.

3. Name the particle that has constant charge to mass ratio for all matter?

Ans: Electron.

4. What is the charge of an electron?

Ans: – 1.6 x 10-19 Coul.

5. What is the mass of an electron?

Ans: 9.1 x 10-31 Kg.

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