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Structure of atom

Subatomic Particles : Discovery and Characteristics

Subatomic Particles 

Electrons, protons and neutrons are the three main subatomic particles that form an atom.

Discovery of Electron (Michael Faraday’s Cathode Ray Discharge Tube Experiment)

          Experimental Setup: 

  • Glass tube is partially evacuated (low pressure inside the tube).

  • Very high voltage is applied across the electrodes.

    Observation:

      Stream of particles move from the cathode (−ve) to the anode (+ ve). These particles are known as cathode ray particles.

          Results:

  • Cathode rays move from the cathode to the anode.

  • Cathode rays are not visible; they can be observed with the help of phosphorescent or fluorescent materials (such as zinc sulphide).

  • These rays travel in a straight line in the absence of an electric or magnetic field.

  • The behaviour of cathode rays is similar to that of the negatively charged particles (electrons) in the presence of an electrical or magnetic field.

  • Characteristics of cathode rays do not depend upon the material of the electrodes and the nature of the gas present in the tube.
     
    Conclusions:

  • Cathode rays consist of electrons.

  • Electrons are the basic units of all atoms.

Charge to Mass Ratio of Electrons (J. J. Thomson’s Experiment)

  • J. J. Thomson measured the ratio of charge (e) to the mass of an electron (me) by using the following apparatus.

  • He determined  by applying electric and magnetic fields perpendicular to each other as well as to the path of the electrons.

  • The amount of deviation of the particles from their path in the presence of an electric or magnetic field depends upon:

    1. the magnitude of the negative charge on the particle (greater the magnitude on the particle, greater the deflection)

    2. the mass of the particle (lig…

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