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Discovery of electron, charge of electron

https://www.khanacademy.org/science/chemistry/electronic-structure-of-atoms/history-of-atomic-structure/a/discovery-of-the-electron-and-nucleus

Key points

  • J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons.
  • Thomson's plum pudding model of the atom had negatively-charged electrons embedded within a positively-charged "soup."

J.J. Thomson and the discovery of the electron

In the late physicist J.J. Thomson began experimenting with cathode ray tubes. Cathode ray tubes are sealed glass tubes from which most of the air has been evacuated. A high voltage is applied across two electrodes at one end of the tube, which causes a beam of particles to flow from the cathode (the negatively-charged electrode) to the anode (the positively-charged electrode). The tubes are called cathode ray tubes because the particle beam or "cathode ray" originates at the cathode. The ray can be detected by painting a material known as phosphors onto the far end of the tube beyond the anode. The phosphors spark, or emit light, when impacted by the cathode ray.
 
A diagram of a cathode ray tube.
A diagram of J.J. Thomson's cathode ray tube. The ray originates at the cathode and passes through a slit in the anode. The cathode ray is deflected away from the negatively-charged electric plate, and towards the positively-charged electric plate. The amount by which the ray was deflected by a magnetic field helped Thomson determine the mass-to-charge ratio of the particles. Image from Openstax, CC BY 4.0.
 
To test the properties of the particles, Thomson placed two oppositely-charged electric plates around the cathode ray. The cathode ray was deflected away from the negatively-charged electric plate and towards the positively-charged plate. This indicated that the cathode ray was composed of negatively-charged particles.
Thomson also placed two magnets on either side of the tube, and observed that this magnetic field also deflected the cathode ray. The results of these experiments helped Thomson determine the mass-to-charge ratio of the cathode ray particles, which led to a fascinating discoveryminusthe mass of each particle was much, much smaller than that of any known atom. Thomson repeated his experiments using different metals as electrode materials, and found that the properties of the cathode ray remained constant no matter what cathode material they originated from. From this evidence, Thomson made the following conclusions:
  • The cathode ray is composed of negatively-charged particles.
  • The particles must exist as part of the atom, since the mass of each particle is only 1/2000 the mass of a hydrogen atom.
  • These subatomic particles can be found within atoms of all elements.
While controversial at first, Thomson's discoveries were gradually accepted by scientists. Eventually, his cathode ray particles were given a more familiar name: electrons. The discovery of the electron disproved the part of Dalton's atomic theory that assumed atoms were indivisible. In order to account for the existence of the electrons, an entirely new atomic model was needed.
 
Concept check: Why did Thomson conclude that electrons could be found in atoms of all elements?
 
 
 

Milikans Oil drop experiment

 

https://chemistrytalk.org/millikan-oil-drop-experiment/

Devised by Robert A. Millikan and Harvey Fletcher, the Millikan Oil Drop Experiment is conducted in a chamber and is a method of measuring the electric charge of a single electron.

To elaborate, this chamber contains an atomizer, a microscope, a light source, and two parallel metal plates. These metal plates obtain a negative and a positive charge when an electric current would pass through them.

Experiment chamber for the Millikan Oil Drop Experiment
Experiment Chamber


 

The Procedure

First, the atomizer was to release a fine mist of oil that would drift within the chamber. While drifting, the droplets of oil would make their way into the bottom half of the chamber (between the metal plates) due to a gravitational pull. Here, the oil droplets would be ionized into being negatively charged. Thereafter, while these negatively charged droplets are being pulled down by gravity, the external power-dial would be used to add a charge to the two metal plates (above and below the droplets). Specifically speaking, the top plate would cultivate a positive charge, and a negative charge would be cultivated on the bottom plate.

This creates a situation in which the oppositely charged (positive) metal plate is pulling the negatively charged droplet upwards, while gravity is pulling the droplet downwards. Or in other words, the electrostatic and gravitational forces are now controlling the direction in which the droplet is flowing. Now, if the electrostatic force is greater, then the droplet would rise towards the positively charged plate. Likewise, if the gravitational force is greater than the electrostatic force, then the droplet would be pulled down.

Observations and Conclusion

The purpose of this experiment was to balance these two electrostatic and gravitational forces – which would cause the droplets to halt midair. By doing this, the droplet’s mass, gravitational force, and electrostatic force could be measured, revealing the charge of the electron. Furthermore, by doing these final calculations, Millikan was able to reveal that the charge of an electron would be multiples of 1.602 × 10−19 C.

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